Flashcards in Neurobiology Deck (206):
What are the basic properties of neurons?
Sensory neurons detect incoming sensory info.
Interneurons = neurons in CNS
Axons allow sensory neurons to contact the interneurons
Neurons interact at synapses
CNS interneurons synapse with motor neurons which synapse with muscles.
What type of signals do neurons need?
- Electrical signals for speed over distance + also chemicals over short distances (e.g. in synapses
What maintains the Resting potential of neurons?
inside negative relative to outside: . Lipid bilayer bridged by protein pores (ion channels)
o most let K+ through
o too small for Cl-, Ca+ (or Na2+) (more Cl- ions outside than inside)
- Salts exist as ions in aqueous solution
- Charged ions move from high to low concentration (move down conc gradient)
- Therefore K+ leaves cell through pores í K+ leaving makes inside -ve (anions remain inside).
What does the active pumping across a neurone do?
IT DOES NOT maintain resting potential
Separation of ions between inside + outside: All cells have high [K+] (400mMi, 20mM o) inside & high [Na+]outside (450Mm O; 50 mM i)
3. Active pumping maintains concentrations (2k+ into cell for each 3Na+ it pumps out í imbalance in conc + electrical charge across the membrane; also other effects that change in the LT (ATPí ADP).
What is the equilibrium potential?
where the concentration + charge of K+ or Na+ are equal so there is no net movement of ions in our out of the cell
What is the Soma?
the cell body of the neuron
What did Hodgkin + Huxley win the Nobel prize in the 1963?
They worked worked on large axons in squid + squeezed out cytoplasm and measured ion concentrations as well as making electrical recodtings
How can the electrical potential be measured across an axon?
- Axon isolated + out in physiological saline to mimic external environment of neuron
- Electrode pushed into axon which contains KCl to conduct electricity
- Electrode connected to amplifier
- This measures electrical charge on inside compared to outside
- There is a reference electrode on the outside (in the saline solution)
How are Na+ ions invlolved in the resting potential?
Na+]o > [Na+]i so Na+ moves in by concentration and charge
o Na+ would enter until inside is sufficiently (+) to balance concentration (equilibrium, can calculate with Nernst equation)
when reduce Na+ conc outside of cell, from (460 to 230) o í Resting potential becomes more negative by ~ 1 mV (sodium does contribute much to RP, so halving Na+ outside moves RP closer to Ek)
How many mechanoreceptors in a crab?
What is a Proprioceptor?
sensory receptor which receives stimuli from within the body, especially one that responds to position and movement eg. stretch under cuticle (crab)
What is sensory adaptation?
Neural adaptation or sensory adaptation is a change over time in the responsiveness of the sensory system to a constant stimulus
How is a receptor potential created?
A receptor potential is often produced by sensory transduction. It is generally a depolarising event resulting from inward current flow (Na ions into the cell through now open ion channels)
Experimental evidence for the involvement of Na ions when generating a receptor potential
o Reduce outside concentration of Na ions: should reduce the amplitude of receptor potential (directly involves Na ions)
o Show that receptor potential is not blocked by TTX (toxin that blocks Na channels): doesn't have effect on Receptor potential
What are ommatidia?
optical units that make up the compound eye of an insect. (eg. honey bee).
What is a rhabdom?
a translucent cylinder forming part of the light-sensitive receptor in the eye of an arthropod
What causes the release of Na ions into the reticular cells (depolarisation)?
photons are absorbed by photopigments: activates cell signalling pathway which results in opening of Na ion channels
how does the escape system work in the squid?
If a stimulus comes into animal, goes to brain + the signal travels to stellate ganglia. These signals excite impulses in giant axons that run down muscles in the mantle, this makes the muscles contract rapidly + squirt water from the mantle cavity through the siphon which propels the animal away. These giant axons are part of the escape response.
how can you measure the current across the membrane?
how did H+H see that sodium was involved in the squid action potentials ?
It was already known that if you replaced sodium outside of the cell with coline (won't go through NA+ channels) , you reduce / block the AP. í start of AP carried by Na+. with this replacement (wne no NA+ outside): no inward current but left the outward current.
what are the 4 terms of basic H+H equations?
the leak of K+ (to a lesser extent Na+); the opening of Na+ channels; opening of K+ channels; inactivation of the Na+ channels.
- At the peak of Na current: NA+ channels stay open but are inactivated. - this inactivation has to be removed before another AP can be generated.
- Opening of channels are voltage dependent
what does TEA block?
What do action potentials?
APs carry signals over distance in axons without getting smaller: continually regenerated as they pass along the axon
Where are action potentials found?
- Cardiac muscle cells
- Cynops pyrrhogaster
- Mimosa pudica - some plants.
- In the skin of amphibian embryos, carrying info through the skin
How do electric organs work?
- Stacks of modified muscle cells called ELECTROPLAQUEs.
- The masses of these electroplaques are surrounded by lipid to insulate them. There are openings to the surface of the body through pores.
- The electric signals come in pulses, each lasting 2-3ms means they are like impulse activity.
- Electroplaque cells have a RP (-ve on the inside with respect to the outside), as well as voltage gated ion channels (needed for impulses), but only in the front face of each cell.
What happens when the electroplaque cells become active? (in electric organs of fish)
the front face generates an AP (inside +ve with respect to outside), but back face doesn't so there is a potential difference across the whole cell
- RP relatively high (-84mV)
- Across the front face on the inside: +67mV
- í across the whole cell there is a potential difference of ~150mV.
- These cells are found in stacks of 5k-10k: combining them means the electroplaques can briefly generate pulses of 4-700v.
- The stacks are in parallel so the current generated is much higher than you'd get across single stacks.
How do electroplaque cells all fire the impulses simultaneously?
- For this to work, all the electroplaque cells have to fire the impulses simultaneously.
- Like other motorcells they are controlled by motorneurons, and the motorneuron input is highly synchronised and muscles generate the impulses at the same time.
what is the structure of a typical synapse?
cleft of around 20-30 nm between two neurons (pre-synaptic + post -synaptic
- In the pre-synaptic we have vesicles containing transmitter (30-100nm).
What are the evidence for synaptic transmission?
1. Delay between impulse in MN + PSP (~0.7ms) í no direct electrical current flow between pre + post synaptic cell: something else was happening.
2. Drug - curare í made PSP smaller + you could see the shape - therefore a chemical is involved (the drug had weakened the PSP). í now know it is Ach. (curare blocks transmission involving Ach)
3. Calcium - ↓ [Ca2+] in saline around the recording í↓ PSP, therefore Ca2+ needed
4. Miniature PSPs seen quite often even if there was no impulses (spontaneous): either the same size or whole number multiples of the same size: quanta release- vesicles contain packets of transmitter (quanta)
5. E.M Heuser: used electron microscopy - known that you could see vesicles fusing with the pre-synaptic membrane + exocytosing to release content.
6. Record pre and post - clear delay
7. Calcium - Ca2+-sensitive dye or Ca2+ injection
How can you see if the vesicles fuse with pre synaptic membrane is in response to an AP?
Freeze slamming to freeze at time of impulse + FREEZE FRACTURE: split the s
split the synapse along the plane of the cleft of the synapse. í evidence of vesicles opening in the membrane at exactly the time the impulse occurred.
How can you prove calcium is involved in an AP?
a. Calcium sensitive dye into the presynaptic terminal to show there is an ↑ in calcium at the time of transmitter release + Ca2+ injection to stimulate transmitter release.
What is the function of a synapse?
Impulse arrives at nerve terminal and initiates the process of transmitter release
What is a PSP?
movement of ions through ion channels in the post synaptic membrane
steps in mechanoreception?
external mechanical energy
trandsuction in the sensory cells (or impulses)
depolarisation at synapse
signal passed on to next neuron
What is the purpose of sensory adaptation?
Allows NS to focus on changing stimuli + avoid being swamped by unnecessary info
what are the 2 different possible ways that adaptation can be produced?
in impulse generation e.g. in sensillum campaniform
mechanical nature: - The end of the axon of this kind of cell is wrapped in layers of membrane
- And as long as that wrapping is intact you see that there is a rapidly adapting response to a sustained stimulus
what are some problems with animal sensory systems?
o Huge numbers of sensory cells
o How to get specificity in sensory signals?
o How do NS ignore irrelevant information + concentrate on important info?
what types of sensory nerve endings does mammalian skin have?
1. Info carried in Unmyelinated axons e.g. free nerve endings
2. some sensory endings are associated with myelinated axons (messier corpuscle)
how are three ways in which free nerve endings are excited?
" hairs in the skin, wrapped around the base- response will be amplified by movement of the hair;
" others excited by warming of skin: max rate of impulses comes with ↑ in temp;
" others respond to cooling: max firing rate comes with ↓ in temp. - between these two you can get good temperature discrimination in the skin)
how can human sensory nerve activity be measured?
- Tungsten electrodes inserted into the skin into nerve bundles in the arm to record responses in these sensory cells (from sensory axons)
- Stimuli applied to various parts of the periphery of the hand
What is the receptive field?
the region of the skin with that sensory cell response
why is pacini fast adapting?
only responds (fires impulses) as the stimulus is applied: this means it is fast adapting -
What are two characteristics that are used to distinguish receptor types?
adaptation rate and the receptive field
What types of coding are there?
a frequency code (stronger stimulus í higher freq of impulses) AND a population code (stronger stimulus likely to produce a response in more sensory cells).
Strength of coding: few cells responding, few impulses to many cells responding with many impulses.
why is sensation in mammalian skin is partly based on labelled lines
We can conclude for the mammalian skin that there are diff anatomical types of sensory receptors +each of the different anatomical types responds differently: carry info about diff kinds of mechanical stimuli + the sensation that results from that depends on which kind of receptor has been stimulated ( so dependent on where the stimulus has come from)
í sensation in mammalian skin is partly based on labelled lines.
what are the sensory receptors in arthropods?
they have sensilla: base with sensory neuron with axon, + dendrite which pokes into little extension of the cuticle, where you get a little hair at the surface.
- There are lots of these over the surface of insects, crustaceans etc
Why do arthropods need sensilla?
- Arthropods have a hard exoskeleton and so struggle to detect sensory stimuli
How do sensilla respond to a stimulus?
- Hair of sensillum bent í distort dendrite í opens STRETCH sensitive ion channels in dendrite membrane í Na+ into receptor cell í depolarisation í RP (lecture 2) í travels through cell to axon + if big enough will generate impulses in the axon í impulses carry info up to the CNS
- These sensilla can respond to touch + carry info on touch to the CNS
what are the different variants on the basic pattern of sensiillum?
1.hair plates - groups of sensilla
2.Filiform sensilla on cricket cerci
3. chemoreceptor sensilla on moth antenna
4. Cockroach campaniform sensillum
What are hair plates and how do they work?
Hair plates. - groups of sensilla
o Field of hairs that are found at joints
o signal position of body parts
o as joint moves, hairs are bent which signals the movement.
o Hairs between head and neck give sense of gravity to dancing worker bee
o Facial hair beds in locusts detect wind direction during flight
o Wing base in cockroach
What are Cockroach campaniform sensillum?
Hair has disappeared, just a dome of cuticle at the surface
Respond to stresses in the cuticle - e.g in limb when the insect puts the limb down it takes up load + these kind of receptors can detect this.
What does the escape mechanism of the cockroach from the toad involve?
specialised use of sensilla & mapping of connections
What do cockroach cerci have along the length?
long filifiorm hairs (more than 2000 sensilla on each cercus)
How are long aliform hairs involved in the escape of the cockroach?
detect the air movement produced by the toad shooting out its tongue.
o The air flow when the toad shoots its tongue has a particular pattern of air flow: the acceleration is very high and this is important. (velocity 12mms-1; acceleration 600mms-2)
How do filiform sensilla produce directional responses in the cockroach?
Freq of impulses depends on directions: strength of respond indicated by polygon.
- - the individual sensilla are responsive to air coming from particular directions.
on each segment, there are 9 sensual each of which are max responsive to air from a particular direction.
How are the filiform hairs (sensilla) arranged on the cerci of the cockroach?
each of the cerci is segmented
- -if you look at a single segment, it has 9 sensilla, each of which is max. responsive to air from a particular direction.
what are the directional sensitivities of the giant interneurons of the cockroach?
on each side 2 of them don't show directional sensitivity: they respond generally to the stimulus
- The other five show clear directional sensitivity.
What has given the GIs of a cockroach good directional sensitivity?
Very precise mapping of sensilla onto the GI has given the interneurons a very good directional sensitivity.
The specificity of response can be produced by the accurate mapping of sensory neurons onto neurons in the CNS.
How are giant interneurons involved in controlling the escape of the cockroach?
- Stimulating these GIs individually excited leg-motor-nerve activity
- This comes about because the axons of the GIs go up the ventral nerve cord to the thoracic ganglia which control the working legs (they excite neurons in the thoracic ganglia)
- The way that the neurons in the thoracic ganglia use this info is complex but generally: if more impulses travelling up one side of the animal than the other, the animal will turn in that direction - it is the balance between impulses travelling up the two sides.
Give a summary of how the sensory receptors _ other mechanisms in the cockroach produce a response?
Specialised sensory receptors: filiform sensilla which respond to a specific stimulus (acceleration of air produced by toad's tongue, which gives array of sensilla a directional responsiveness), Then organised, carefully mapped central connections in these neurons and those GIs distribute info to the motor system that actually produces the escape response.
what is important about the cockroach escape response?
This response doesn't involve the brain: the signal will grow to the brain but it isn't necessary for this response - a complex BHV being organised w/out the brain.
what is the structure of the spinal cord?
- grey matter on inside (cell bodies), dendrites of neurons + outside the white matter with axons running the length of the spinal cord.
- Dorsal (upper) = sensory; ventral= motor í dorsal horn is sensory in function; ventral mostly motor.
- Along the length of the spinal cord, segmentally there are dorsal roots in, dorsal ganglia + ventral roots out.
- The dorsal root ganglia are where there are sensory neurons coming into the SC (from skin receptors).
- At the bottom there are motor neurons with axons running out into the muscles.
what are the two types of projection neurons?
one group of which has axons that cross to the opposite side + then travel up to the brain, and the other group have axons on the same side that travel up to the brain.
What does the projection of a neuron mean?
the places of the axons + where they run
Which sensory pathway has info from sensory neurons with unmyelinated axons?
what are the steps of the Spinothalamic pathway?
- Tend to carry information of noxious stimuli (pain, temp + some touch)
- These neurons synapse with the projection neurons that cross to the opposite side. + then travel up the brain í these groups of axons form the Lateral spino-thalamic tract
- Takes info up to forebrain (thalamus) + from the thalamus the info goes out to neurons into the sensory cortex (outer part of the front of the forebrain)
which sensory pathway carries info from sensory neurons with myelinated axons?
what are the steps / features of the Lemniscal pathway?
- Deep touch, pressure, vibration, proprioreception.
- This info goes through a pathway on the same side of the SC: these neurons connect to projection neurons with axons on the same side.
- This set of PNs runs up to structures/ ganglia/ nuclei in the brainstem called the Dorsal column nuclei. E.g. the cuneate + gracile nucleus
- At the DCN there are then synapses to neurons that again cross to the opp side + travel up to the thalamus + form a structure: the medial lemniscus.
- (eventually the pathway crosses + again the info goes to the cortex - outer part of the forebrain).
what can a shows a mapping of where the connections go in the cortex show us?
- Can see that regions of the body surface that are close together map onto regions close together in the cortex
- What we can also see is the density of connections: a lot of sensory info in humans is associated with hands + fingers; face, lips + tongue + things associated with vocalisations.
- The key message: there is huge speciity in the connections as you move from the sensory neurons to where they end up in the sensory cortex of the forebrain - think in terms of labelled lines (lecture 5).
What are the steps of parallel pathways in birds + mammals?
Info is going up to the cortex that can then control motor responses.
in a similar way to the cr, the are important connections at all the levels between sensory info coming in + the motor responses: doesn't all happen at the cortex.
During the course of evo: progressively higher layers have been added in. this has meant that veterbates like us have conscious perception from stimuli
Why are higher centres not always the most important thing in a NS?
with the brain removed frogs can still wipe off a piece of filter paper soaked in vinegar --> o í lower centres in the NS can still control sophisticated behavioural responses.
- Now it is known that other vertebrates like turtles can do the same kind of thing.
what are the receptor cells in the lateral line system and what are their structure?
The receptor hair cells have cilia at the surface embedded in a jelly structure = the cupula which acts like a lever. There is two types of cilia at the top of the hair cells: shorter, stereocilia + a single large kinocilium
how do the cilia in the lateral line system detect movement?
Movement of the cupula + cilia by currents /vibrations sets of the transduction process (this is the receptor organ of the LLS): Movement of these cilia in different directions excites or inhibits activity of the hair cell and ↑ or ↓ release of transmitter from the synapse at the base.
This transmitter release is onto a sensory neuron: an afferent neuron (taking signals to the brain)
How do the cilia in the lateral line system excite the afferent neurons?
If something bends the cupula towards the kinocilium, this ↑ transmitter from the hair cell which excites impulse activity in the sensory neuron so see an ↑ in freq.
If the cupula moves away from the kinocilium, the transmitter release ↓ which ↓ / stops firing in the sensory nerve.
This is an important basic principle: stimuli of different directions can ↑ OR ↓ the response in the sensory nerve. This is because it has a resting rate of firing.
what are the different structures of the lateral line system?
free neuromast organ: simplest; hair cells + jelly cupula are at the skin surface and the skin is slightly raised.
Pit organ - some teleost fish: cupula structure is sunk down into a pit in the body surface
Canal organs - most extreme: hair cells in groups of canals below the surface that connect to the surface by holes (openings). - these are found in cartilaginous fish (sharks, rays, skates).
How do animals with a lateral line system prevent self-stimulation of the nervous system?
efferent nerves are inhibitory: inhibit activity of the hair cells, and become active when the animal is very active so when the animals move they don't respond to their own movement.
what are some of the roles of the lateral line system?
- Allow aquatic vertebrates (fish, some amphibians) to detect objects in water, to orient their bodies in weak currents, helps with schooling behaviour
- Allows small fish + amphibian tadpoles to detect predators + avoid predation
- Can help some detect prey (e.g. insects at the water surface which produce ripples)
maybe cichlids can asses status of others with the LLS
How was it found that dog fish use electro-receptors to detect fish?
when a flatfish was placed in sand the dog fish could find it, and still in a jelly box so no signals from movement.
but with a polythene sheet to insulate electrical activity, the dogfish couldn't detect.
dogfish found electrodes mimicking the flatfish electrical signals --> use electrical signals to locate prey
what is he structure of the ampullary system?
there is an array of canals contains under the skin with openings at the surface + at the ends they connect to nerves leading to the brain. the nerves lead to the ampullae of lorenzini at the base of the structures.
how does the ampullary system detect electrical activity?
- Canals contain jelly that is conductive to electrical signals like sea water: electrical signals at the surface can travel through this jelly medium
- The wall is resistive (insulator) í electrical current has to flow along the canal
- í electrical signals go through the canals
- In the wall of the ampulla, you find modified hair cells (this system has same origin of LLS); hair cells are the receptors and have a single cilium + their release of transmitter depends on electrical signals being detected by the hair cell (they detect the electrical current flowing through the canal).
- The detection of electrical signals controls release of transmitter from the base of the hair cell onto an afferent nerve (a sensory neuron)
How do impulses from the electro-receptor change depending on the stimulus
there is a resting discharge in the afferent nerve (when there are no volts there is a firing rate of about 40Hz - 40 impulses /s), this ↑ or ↓ in size depending on the size and the polarity of the electrical current detected (the electrical stimulus)
How do weakly electric fish use electrical signals?
- they generate long lasting, continuous trains of pulses (~500Hz), the timing of the pulses is controlled by pacemaker cells: control firing in motor neurons that drive high freq of pulses in muscles to generate high freq pulses of electrical signals in the electric organs.
- have pores at the front + back that act like the poles of a battery: these high freq electrical signals can get out of the body + generate an electric field around the fish.
- Then they detect this electrical field using electroreceptors
what kinds of receptors do weakly electric fish use to detect their electric fields?
use some receptors of ampullary (canal) type, but also have another sort of receptor: TUBEROUS RECEPTORS (contained in a bulb),
o Tuberous receptors mainly responsibly in detecting their own electrical field
o They have an array of these receptors over the body surface, particularly over the head
give a summary of how weakly electric fish use electrical signals
Produces the field, monitors the form of the field + can detect changes in the field. (used to detect things in the environment: things that come close to body / obstacles in the environment.
what are the two features of the weakly electric fish electro-receptor system
amplitude + timing of pulses are analysed separately and then recombined.
o Jamming avoidance: o The ability to monitor their own field + the field of other fish + modify their own field avoids problems from jamming between other individuals.
how is jamming avoidance done?
: detect the differences between their own field + a neighbouring field + adjust the frequency of their own field (adjust the Hz ) so the individual that has a slightly lower freq will make it lower and the one with a higher freq adjusts it higher í by doing that + analysing the timing they can separate info about their own field + the field from the other fish.
How do owls locate their prey?
Rustling of leaves (see Roger Payne, 1960s experiments)
At what frequency and how accurate is the optimum sound-localisation of barn owls?
6-9kHZ has the best sound-localisation with an accuracy of 1-2 degrees in both horizontally and vertically
How do owls recognise horizontal and vertical planes?
Owl can use timing and amplitude differences on the horizontal plane (ears on both sides).
Left - Right asymmetry of ears vertical plane
How do owls use tri-dimensional localisation of the sound?
localisation depends on the coding of intensity and time differences.
What are the roles if the magnocellular nucleus - angular nucleus ?
Incoming sound in the ear is separated and processed in one of the two nuclei.
o magnocellular nucleus - processes timing
o angular nucleus - processes intensity
What effect do timing differences ab dintensity differences have on the owls head movements?
Timing differences - horizontal head movements
Intensity differences - vertical head movements
What is the coincidence detector and what does it do?
Coincidence detectors influence neuronal information processing by reducing temporal jitter, reducing spontaneous activity, and forming associations between separate neural events.
--> Ray of neurons that are comparing signals from both sides (ears), where the neurons that respond depend on the relative timing of the stimulus.
What are 'Phase ambiguities' and how are they resolved?
o 'Phase ambiguities' (where cycles don't overlap) resolved by external nucleus of auditory midbrain
how are the photoreceptors in the eye depolarised or hyper polarised.
In dim light, Na+ channels are open + the photoreceptors are depolarised, but in the light Na+ channels closed + photoreceptors relatively hyperpolarised. (see year 1 notes)
what do rods + cones do?
they are photoreceptors: glutamate (transmitter) at the synapses at their base, and there are two kinds of bipolar cells: ones that are excited by the glutamate (depolarised) + ones that are hyperpolarised by the glutamate
o None of these cells have axons + none fire impulses.
what are ganglion cells?
do have axons and do fire impulses. - they provide the output from the retina
o Ganglion cells also have a pattern of resting discharge (at rest they are firing impulses) + so the freq of these impulses can be increased or decreased.
o Ganglion cells have rather little response to diffuse light stimuli.
o Each of the gcell gets input from a small no. of neighbouring photoreceptors + they each have a distinct receptive field: distinct part of the visual field to which it's responses
what are the two kinds of responses from ganglion cells?
on-centre stimulated when centre of RF exposed to light + inhibited when surround exposed to light – little response to light that covers both regions as excitation in centre cancels inhibition from surround = Lalteral inhibition
o off centre have the opposite reaction: (off centre / onsurrond): gets inhibition from light in the centre, and excitation from light in the surround.
what are the two types of bipolar cells?
ones that are excited by the glutamate (depolarised) + ones that are hyperpolarised by the glutamate
why don't ganglion cells respond to diffuse light?
the light in the two regions cancels out the response.
what are the x type ganglion cells?
Most of the ganglion cells in the mammal are X type gcell which project up to the thalamus in the forebrain (important relay of sensory signals), particularly to the lateral geniculate body. From here they are passed onto the visual cortex.
what are y type ganglion cells?
some of which also project to the lateral geniculate but some project to the roof of the midbrain (the tectum of lower vertebrates, here is the Superior colliculus). The signals from the Y type gcells are used for unconscious visual processing + results of that: control of movement (eye position, head movement)
where do neurons in neighbouring Lateral geniculate receive signals from?
neighbouring LG neurons receive input from neighbouring ganglion cells).
what are the simplest kind of retina cells?
what are the simple cells of the retina? how do they work?
simple cells: ones that have "on centre" receptive fields: on centre surrounded by the opposite response.
o these simple cells are combining info from lower down in the visual pathway - this is the simplest type of combination to form the receptive field.
o If there is a vertical bar stimulus, as it crosses the middle part of the receptive field there is a response
o A horizontal bar (Crossing at 90o) wouldn't produce a response.
o So this type of neuron responds to the orientation of stimuli.
- There are also off centre versions of those.
how does the vertebrate visual system detect edges + boundaries?
- There are different simple cells responding to shapes at all angles and orientations.
- They primarily are protecting edges + boundaries which are enhanced by the inhibition which produce the opposite responses just outside the centre of the receptive field.
what do ganglion cells have?
a pattern of resting discharge (at rest they are firing impulses) + so the freq of these impulses can be increased or decreased.
how did Ewert test the discrimination in the to a?
- Tested toads using two kinds of stimuli: horizontal bars (worms) + vertical bars (anti-worm). (same size + shape but moving with a diff orientation)
- First looked at it behaviourally: put a toad inside a cylinder with a transparent front + moved the shapes across infront of the toad's visual field.
- Moved worm, anti-worm and a block for both acorss the visual field + quantified the response
how did the toad respond to stimuli in Ewert's experiment?
o In response to the worm stimulus there were attempts to grasp the object + the attempts ↑ as the size of the object ↑, up until a kind of maximum.
o Antiworm: to begin with it shows some interest but as it gets bigger it stopped responding
o With the square shape: to begin as it gets bigger the toad goes towards it but as it gets even bigger it starts to actively avoid that shape.
o If concentrate on just worm + anti-worm shapes it is clear the toad can discriminate between the two.
what are the visual pathways in the toad?
- Info from retinal ganglion cells go through optic chiasm up into the tectum + also up to the thalamus (forebrain)
o Within the tectum, two types of neuron (type 1 + II)
o There are also connections of the thalamus onto the tectum
o tectal 1 + II: strong response to worm orientation stimulus and a much weaker response, particularly in tectal 2 to the anti-worm stimulus.
in thalamus: also find neurons that respond differently to the two kinds of stimulus: if the thalamus is removed (or if connections from thalamus removed), the tectal type II receptors no longer act as worm detectors: they respond to both anti-worm AND worm configuration
how does the thalamus contribute to the visual pathways in the toad?
o If you look in thalamus: also find neurons that respond differently to the two kinds of stimulus: if the thalamus is removed (or if connections from thalamus removed), the tectal type II receptors no longer act as worm detectors: they respond to both anti-worm AND worm configuration.
o This is because the neurons in the thalamus, which respond more strongly to inhibit the anti-worm, inhibit the response of tectum neurons
" This inhibition / imposes the ability to discriminate worm + anti-worm on the tectal II type cells.
" By inhibiting tectal II cells, the thalamus makes them much better worm receptors.
how do tectal two type neurons produce a movement respond?
o If you connect the tectal II type neurons to a motor system in the medulla of the hindbrain which controls feeding, then the tectal II cells could produce the appropriate feeding response in the presence of a worm type stimulus
o If the tectal II type cells are stimulated they initiate the feeding movements.
what is a sherringtonian reflex ?
something that has the kind of relationship shown in the graph : the motor response - the movement response of a mammal, particuarly at muscle contraction / or the motor neurons that control this contraction.
the key thing is the response grades with stimulus strength + we say that this relationship is a fixed input output relationship.
(input = sensory side, stimulus); output = on the motor side.
How do many muscles exist? give an example
as antagonistic pairs e.g. extensor (opens a joint) + a flexor (closes that joint).
What do you need for a reflex pathway?
sensory element + motor element.
What are the three features of a reflex response?
THE RESPONSE IS GRADED: " Strength of response grades with strength of stimulus
Closed loop reflex: the sensory element directly monitors the result of that reflex response
extra inhibitory pathway, such that when you get a reflex response in the extensor, this is enhanced by inhibition the flexor (inhibiting the antoganist)
How is a reflex response graded?
" Strength of response grades with strength of stimulus, we know how sensory neurons grades the response (Freq codes of impulses + pop code - ↑ no. of neurons responding)
" Increasing stretch in the muscle will produce ↑ response: ↑ freq of impulses + more sensory neurons respond.
" Response is graded in the same way: ↑ in motorneuron responses + ↑ in no. of motor neurons responded.
How does the Crustacean Resistance Reflex work?
The angle of the joint is monitored by a sensory structure called a tonal organ.
Something like a change in posture alters the angle of the joint, the sensory structure produces a reflex contraction of the muscle (here the closer apodeme), to restore the angle of that joint. This is also a resistance reflex: the angle of the joint is restored by reflex contraction of the muscle.
What is the flexor reflex in spinal reflexes?
reflex is to an external signal. A reflex whereby a stimulus to the skin produces a strong contraction of flexor muscles to withdraw a limb from harm.
What are the features of the flexor reflex (spinal reflex)?
This response is also graded: stronger response for stronger stimulus, but this time the reflex is an example of an OPEN LOOP REFLEX: because the sensory element doesn't directly monitor the result to withdraw the limb (isn't directly monitoring the response, although it might do indirectly)
Other features of this reflex:
- Not just the single flexor muscle that is activated, it tends to be a number of flexors in the same limb to make sure the limb gets withdrawn = a more widespread response which affects a number of flexor muscles
- There is a corresponding inhibition of the antagonistic motorneurons (MN of the antagonistic muscle).
How does inhibition in the flexor reflex in spinal reflexes occur?
- There is a corresponding inhibition of the antagonistic motorneurons (MN of the antagonistic muscle).
o The sensory neuron of the 1st excitatory neuron excites an inhibitory neuron that inhibits the extensor motorneurons.
o Again we have this pattern: the motor neurons for one muscle are excited while those for the antagonist are inhibited.
o But in this case, the effects are even more widespread than that: the sensory neuron also affects the motor neuron + muscle on the opposite side (for the opposite limb)
o The connections are the opposite way round, this makes sense if the limb being stimulated is for e.g. what is standing on: if 1 limb is withdrawn, the other is extended to maintain support í flexing of one limb, extension of the opp. Limb.
o Again, there is opposing activity between antagonists within a limb and now between limbs.
What is the vestibular ocular reflex?
Vestibular ocular reflex:
- Open looped
- A reflex signalled in us via the vestibular system + used to control the eyes.
- It is a reflex whereby if the head move, the eyes move in an opposite direction to maintain gaze on an object.
- Sensory = vestibular system; response = muscles in the eye
What does an open loop reflex rely on?
the strength of connections being very accurately determined - (Sensory part of the cell is not monitoring the response).
How do stereotyped responses differ from reflexes?
If you plot motor response against stimulus strength, there is a threshold but above this threshold, the strength of motor response is fixed - independent of the stimulus: doesn't grade in the way a reflex does.
Predictable, repeatable response.
(reflexes responses are graded with the stimulus)
How does the fish startle response (fast-start response) occur?
responds to a waterborne vibration.
1 C FLEX: body goes into the C shape, 2. recovery from this initial bend
- A strong contraction of the trunk contractions on the opposite side of the body to stimulus is what produces the response.
What is the general course of action in impulse responses in the startle response of fish?
Impulse in M cell, muscle potential from trunk muscles as they start to contract, movement
what are Mauthner cells?
involved in the fish startle response
large diameter, axons (~50 micrometers in diameter).
We now know these axons come from a pair of neurons in the hindbrain. And these neurons have crescent-shaped cell bodies. From this cell body, the axon crosses to the opposite side and runs down the opposite side of the spinal cord.
There are only two of these neurons in fish: one on each side.
How does the M cell produce the startle response?
- In the spinal cord, the axons contact motorneurons that control the trunk muscles
- They get incoming signals (input) from the sensory systems that detect vibration which come in trhough the 8th cranial nerve (the auditory nerve). simulating this auditory nerve excited the M cell + made it fire an impulse. impulse travels rapidly down the axon + excites motor neurons on the opp side + the muscles on the opp side contract.
What is an important feature of the fish startle response?
RECIPROCAL INHIBITION: inhibition between opp motor pathways: when one path is active, the other part is inhibited.
This reciprocal arrangement is seen in the SC + also in the hindbrain.
Why is the fish startle response stereotype response + not a reflex?
- Because above the threshold, the response is always the same strength ( the response is not graded)
- 1st you need a stimulus that is strong enough (above the threshold) - here, the threshold is the size of stimulus that will make the M cell fire an impulse.
- The M cell has a negative RP (~-80 mv), so there is a high threshold: you need quite a high stimulus to produce a response (generate an impulse)
- Above this threshold, whatever the stimulus level, the M cell still only fires one impulse. This is because the M cell actually inhibits itself (once it has fired an impulse it inhibits itself so it doesn't fire again): always one M impulse above the threshold, no matter the stimulus strength
How is the fast response produced in the M cells of the fish startle response?
o firstly, the M cell axon is large in diameter + is myelinated so conducts impulses quickly.
o The connections are very quick: from the auditory nerve to the M cell + from the M cell to the motor neuron are very quick because the synapses aren't just chemical synapses, but there are also electrical synapses (this is where there is direct flow of current from one cell to another - they are instant)
What is lateral inhibition?
In neurobiology, lateral inhibition is the capacity of an excited neuron to reduce the activity of its neighbours. Lateral inhibition disables the spreading of action potentials from excited neurons to neighbouring neurons in the lateral direction.
Impulse activity in eccentric cells does what?
Impulse activity in eccentric cells in response to a light stimulus and the boundary between light stimulus and the dark.
--> Pattern of activity is enhancing the boundaries between light and dark
What is 2 point discrimination and what role does lateral inhibition play??
How close 2 stimuli can be before they stop being recognised as separate stimuli; ability to discern that two nearby objects touching the skin are truly two distinct points, not one.
Lateral inhibition sharpens response by inhibiting receptors either side and focuses the receptor field for each of these receptor neurone (frequency discrimination in auditory system)
Neurons at the centre of the stimulus have the strongest stimulus. How do they increase their own activity?.
Increase their own activity by disinhibition (removing inhibition coming in to them)
what is gating ihibition and when is it used?
Ability of neural circuits to gate inputs by either suppressing or facilitating specific synaptic activity.
Related to pain pathways.
Inhibitory neurons reduce or prevent transmission of the pain signals via projection neurons --> gate out transmission of the pain signal (gate-control mechanism).
They do this by releasing the substance Enkephalin (endogenous opiate).
what is Periaqueductal grey and what is its function in the central pathway?
The periaqueductal gray (PAG, also known as the central gray) is the primary control center for descending pain modulation. It has enkephalin-producing cells that suppress pain.
Periaqueductal grey matter (PAG), when stimulated, that stimulation will reduce pain or responses relating to noxious stimuli.
Neuron the PAG connect down to the dorsal horn of the spinal cord through structure called the raphe nucleus. This is done via the release of serotonin (5HT) from the raphe neurons.
Also done indirectly by neurons that excite the inhibitory neurons and release of Enkephalin
What role does gating inhibition play in locomotion/reflexes?
Reflexes being altered to make them appropriate to the behaviour. This involves gates inhibition. (cat paw)
What is anguilliform swimming?
Simple side to side rhythmic moving of the body.
On what species was the activity from isolated spinal cord recorded (Grillner lab.) and to what result?
On lampreys to identofy the pattern of rhythmic activity that results in locomotion.
What are central pattern generators (CPG)?
Central pattern generators are neuronal circuits that when activated can produce rhythmic motor patterns such as walking, breathing, flying, and swimming in the absence of sensory or descending inputs that carry specific timing information.
What did Sherrington believe at the beginning of the 20th century was the origin of locomotor movements?
Experiment of spinalised cat: brain provides basic signal to the spinal cord and reflexes in the spinal cord would then organise the limb movement. But this can't be an explanation for what was recorded in the lamprey: no sensory information going into the lamprey so no reflexes.
Key difference between central pattern generators (CPGs) and reflexes
Key point: the activity does not depend on any sensory information coming back in: can operate without what movement is producing. For reflexes you need sensory information because part of the pathway.
When was CPG first dicovered?
The first modern evidence of the central pattern generator was produced by isolating the locust nervous system and showing that it could produce a rhythmic output in isolation resembling that of the locust in flight. This was discovered by Wilson in 1961.
Since that time, evidence has arisen for the presence of central pattern generators in vertebrate animals, starting with work on the cat in the 1960s by Elzbieta Jankowska in Gothenburg, who provided the first evidence for a spinal cord CPG.
Who is the first one to do research of neural control of locomotion and suugest that A series of coordinated muscles produce locomotor movement?
Thomas Graham Brown undertook seminal experiments on the neural control of locomotion between 1910 and 1915. Although elected to the Royal Society in 1927, his locomotion research was largely ignored until the 1960s when it was championed and extended by the distinguished neuroscientist, Anders Lundberg.
What role does the tonic drive play in the CPG?
- 'tonic drive' (no rhythmic component) is exciting the both half centres.
- Process can continue as long as the tonic drive (background excitation) is still going.
- Tonic drive normally supplied by the brain, when removing brain, top of spinal cord had to be stimulated.
What is the role of fatigue in CPG?
Activity in the first group of neurons (e.g., extensor half-center) would send motor commands to motoneurons (exciting extensors), and would inhibit simultaneously the reciprocal group of neurons (flexor half-center) preventing the excitation of antagonists (silencing flexors). After a period of “depression” (e.g., fatigue, adaptation, post-inhibitory rebound) of the extensor half-center, the flexor half-drive would predominate for a new phase of activity, etc.
What neurotransmitter does the reciprocal inhibition use (in CPG)?
What chemical be used to simulate the tonic excitation in CPG?
What are edge cells?
Edge Cells are mechanosensory neurons in the lamprey spinal cord that are located on the lateral margin of the spinal cord and respond to movement. Edge cells are capable of entraining the swimming motor pattern and receive synaptic input from spinal locomotor neurons.
How is limb coordination controlled in cats?
- The general message: CPG for each limb + simple patterns of coordianation between those to coordinate limb movements
- E.g. in cat: fore + hind L+R limbs: suggested there is a cpg for movements in each of these limbs
o When the animal move, top diagram shows the diff locomotor gaits: time horiztonal, limbs as blocks (when the diff limbs are active)
o During the trot: LF + RH move together + RF +LH move together: this is the pattern of coordination during the trot
- If CPG for LF + RF, LH + RH were all connected by reciprocal inhibition, or coordinated nueurons providing RI, then this would provide a very simple mechanism to produce this pattern
What is exteroception.
In simplest vertebrates, main sensory info for movement comes from interaction with the environment. The main feedback comes from interaction with environment (Avoiding obstacles) - this is
What is PROPRIOCEPTION
- As you move up through the vertebrates, e.g. to birds + mammals, where body can be lifted off the ground, there is more sensory info relating to PROPRIOCEPTION for controlling posture.
How can the activity produced by CPGs be modified?
proprioception + exteroception) can modify activity produced by CPGs.
- This info also travels up pathways to the brain.
How can sensory info be altered to avoid the problem that may arise when walking (the swing PHASE) with reflexes in a limb important in maintaining posture in stance phase
1. Sensory gaiting: at particular times sensory info is prevented from reaching the CNS + here the CPG. - sensory path of reflexes is prevented from producing a response
2. Reflex reversal - examples involve resistance reflexes (Mondays lecture) important in posture, which in during locomotion can reverse to become assistance reflex. - reflex that resist changes in muscle can reverse to assist the change: quite common in limb reflexes.
Not only do sensory signals modify CPG but the CPG through things like sensory gaiting / reflex reversal, they can alter access of sensory signals.
What do CPgs depend on
CPGs don't require sensory info to produce the basic rhythm, they depend on two kinds of things: the properties of the neurons (cellular properties) + the connections between neurons (circuit properties)
What is the features of the cricket song?
relationship between CPGs
The male crickets sing with very stereotyped species specific patterns of song + the females detect songs of conspecifics + home in on them to mate.
(males sing, F detect + the songs are species specific)
How do crickets produce sound?
The male rubs its wings together (don't use legs like grass hoppers) the wings have serrated structures: scraper + File and as the wings close they generate a pulse of sound of ~5kz + these pulses are collected together to form chirps + trills.
now know that the wing movements of song are generated by a CPG: rhythmic wing movements don't require sensory info í driven by CPG.
What is the Sequence of events underlying a single syllable of cricket song?
Sequence shows nerve impulse in the motor neurons, which contact the muscles í muscle impulse í muscle contraction í closes the wings + produces a pulse of sound
In a sense it is a typical motor output
now know that the wing movements of song are generated by a CPG: rhythmic wing movements don't require sensory info í driven by CPG.
which thoracic ganglia are needed for a cricket to sing?
as long as the two first thoracic ganglia were intact, the NS could produce the wing movement + song.
í the first 2 thoracic ganglia contain the neurons + connection needed to move the wings + produce sound.
what is Motoneuron and interneuron activity during stridulation and flight (Hennig) of cricketS?
Hennig found the motor neurons were of two kinds: ones active exclusively in flight + bifunctional ones that were active in flight + stridulation.
interneuon is active in song production but not in flight, but the second kind of interneuron is inactive in stridulation but is active in flight
- He found the interneurons are one of two groups: active in song production or flight
How are crickets / insects able to produce specific songs but general flight?
are two CPGS with their own interneurons: flight CPG + Song CPG
- These control diff motor neurons: wing. Some are bifunctional (controlled by flight CPG + song CPG), whereas some are flight specific - only controlled by flight CPG.
the separation of CPGs for diff responses, but still operating between same motor neurons allows the song CGP to work in the stereotyped, specific way while still allowing flight CGP to be finely tuned by sensory info.
4 main regions of the foregut (crustacean)
o Oesophagus leading in from the mouth
o Cardiac sac (storage structure)
o Gastric Mill (teeth made of chitin chew the food)
o Pylorus (filter and contains exit to the midgut)
STNS Ganglia (4)
o Pair of Commissural ganglia (CoG)
o Single Oesophageal ganglion (OG)
o Single Stomatogastric ganglion (STG)
Why does this stomatogastric system get so much attention?
o Contains only 30 neurons
o All of them can be recorded
o Several can be recorded simulations
o Record from the nerves to record the impulses that would go to the muscles
o In an experiment: researcher would record from each of the neurons in turn to map their position (possible because neurons in the ganglia identifiable as individuals or in a small group)
Photoinactivation (Miller & Selverston) - Lucifer Yellow. What is its role?
--> injected into neurons through the microelectrode. Can shine blue or weak ultraviolet light on it and it will fluoresce: show the anatomy of the neuron. Under stronger UV light; you fluoresce the neurons death: allows you to kill single neurons by shining strong UV light on them. With an UV lase, you can kill only a part of the neuron (like part of its axon)
--> anatomical tool.
What did Heinzel do? (in his experiment with the stomatogastric system?
Combined endoscopy and electrical recording.
Push endoscope up the animal's oesophagus so that he could look at the movement of the teeth in the gastric mill. At the same time, he made recordings from the stomatogastric NS.
What is a neuromodulator?
A neuromodulator is a messenger released from a neuron in the central nervous system, or in the periphery, that affects groups of neurons, or effector cells that have the appropriate receptors (dopamine, serotonin, …)
Neuromodulators have profound effects on the circuit of neurons: control the behaviour of the neuronal circuit.
What happens when the Cardiac sac sytem and the pyloric system are active together and under the influence of neuromodulators?
One of the neurons of the pyloric circuit shifts allegiance and becomes part of the cardiac sac circuit.
If 2 circuits active together, there is potential for conflict in that control. By functionally shifting allegiance of that neuron, those neurons operate together and you don't get conflict in control of that valve.
What difference can there be between circuits/ between neuron?
Circuits can be very specific for some motor tasks (such as song).
Other circuits of neuron can be and need to be much more flexible so that they can control a range of related motor response (activity is altered, by neuromodulators on cell properties and cell connections).
Specific vs Multifunctional neurons
How does the brain control song production in the cricket?
Huber found that by stimulating the brain, the animal could be made to produce diff kinds of song. So, the top example (brain stimulation in one region) inhibits calling song / elsewhere triggers calling song / convert it into courtship song / trigger flight song.
But the brain provides descending commands (descending from the higher centre - the brain, to a lower centre - the nerve cord).
How is the brain connected to the song SPG in the thoracic ganglion?
The brain exerts the descending commands, normally on the basis of input that it receives.
This can be sensory input of various kinds but is also subject to hormonal influences. These inputs are basically signalling the state of the animal - either long term or immediate circumstances.
This happens in various regions in the brain to control the pattern of descending command to select the activity in the thoracic ganglion.
There is also information that ascends (lower centre to higher) - back to the brain: to inform it of what is happening in the thoracic ganglia. (Ascending 'reafference') - a kind of feedback signal
How is the brain involved in motor control?
usually required to switch on + off motor responses (behaviour organised in the SC + elsewhere). Turn on / off behaviour or select diff motor responses. It is also needed to fine tune those motor responses. It plays an increasing role in processing sensory ( + other) info + using this to control motor responses.
What are the main features of the relationship between the brain and the spinal cord?
1. Phylogenetic increase in the complexity (numbers of pathways connecting brain + SC)
2. An increase in the direct control of motor neurons by the brain - this is important because motor neurons are the direct output of muscles
What are the different structures of the brain?
Structures similar throughout vertebrates
Spinal cord at one end, connecting to the hind brain. This hind brain consists of the medulla at the back + the pons at the front.
Ahead of the hindbrain we have the midbrain. The midbrain + hindbrain make up the brainstem.
Ahead of this we have the forebrain.
What make up the brain projection areas? What is characteristic about them?
3 main sets of brain stem nuclei These are symmetrical on the two sides: we have the reticular nuclei, the vestibular nuclei + and in some vertebrates, the red nuclei.
The thing about all these nuclei is that they contain neurons with axons that project down into the spinal cord.
How are the sets of axons from the brain stem nuclei named? what are they?
These sets of axons are named according to where they come from + where they go to: reticular spinal axons (reticular-spinal tracts - groups of axons), vestibular spinal axons / tracts & rubro-spinal axons/ tract from the red nuclei.
These regions give rise to axons running down into the spinal cord.
What do the axons + connections from the brainstem nuclei do?
These brain stem areas are a series of regions with connections down into the spinal cord. They act as a focus for descending control and in general, they are phylogenetically very old + have been conserved right through the vertebrates.
What do the - Vestibular spinal connections do?
present throughout the vertebrates but they are much greater in number as you move through particularly to birds + mammals. Primitively they are associated with signals coming from the lateral line system, but later on they are associated with the vestibular system + the sense of balance (which evolved from the LLS). - concerned with things like postural control in mammals.
what do the reticular connections do?
phylogenetically the oldest - found throughout the vertebreates + throughout them, some of them make connections directly onto motor neurons on the SC. Primitively they were concerned with control of the tail + locomotion - the main propulsive organs. These connections control the excitability of neurons in the cord - they are primarlily excitatory. Among their functions they turn on activitiy in spinal CPGs - they're key source of ascending drive is to spinal CPGs. In a more general sense in mammals, the reticular nuclei make up the "reticular formation", concerned with control of arousal (the likelihood of behavioural responses occur). í really important connections throughout the connections
What are the - Rubro-spinal from the red nuclei?
: only found from reptiles onwards: birds reptiles + mammals. In mammals they have functions like the reticular spinal connections.
- In general, the reticular spinal neurons are needed to drive locomotion + vestibular Spinal connections just make it more stable.
Which region in the midbrain can produce locomotion in cats? How was this discovered?
the Midbrain locomotor region / mesonsophalic (MLR), exists right through the vertebrates. It has been shown right through the mammals: lamprey, fish, amphibians, reptiles, birds, rat, mouse, cat + monkey.
Sikh et al when stimulated, could specifically turn on locomotor behaviour. (this is an animal w/out a forebrain). It wasn't just turning on bhv in general or just increasing arousal: it was actually turning on locomotion.
What is the evidence for the MLR being the same region in all vertebrates?
share common features in all vertebrates
1. Localised: stimulation within the region produces locomotion, stimulation outside doesn't
2. Acts as a gate: this means that while it is being stimulated, locomotion will occur but if you stop stimulating, locomotion stops. -í it's not a trigger in which a stimulus would start locomotion + then carry on.
3. The strength / pattern of locomotion grades with the strength of stimulus: in the cat, the stimulus sterngh ↑, it would go through walking + running + then galloping.
í important region for locomotion in midbrain: MLR + is throughout the vertbrates but how does it influence the SC
- Exerts its influence via one of the brain stem nuclei.
How does the MLR work in mammals? what evidence supports this?
works through part of the reticular formation: the medial reticular formation.
evidence that supports this: anatomically, we know that axons of the MLR neurons terminate in medial reticular formation . if you stimulate in the region where these axons terminate, this can also turn on locomotion. In exps it was shown that if you locally cool the region where the MLR axons terminate (in a mammal this basically anaesthetises that region), the MLR is no longer able to turn on locomotion.
- í MLR acts on the spinal cord through one of the brainstem nuclei.
what is the current thinking about how the MLR itself is controlled?
the MLR neurons are normally inhibited by neurons within the forebrain (the basal ganglia). The cerebral cortex can exert an inhibitory effect on the basal ganglion neurons + if the cortex inhibits the basal ganglion neurons, this will remove the inhibition from the MLR neurons. (the mlr neurons normally inhibited by basal ganglia but if these are inhibited by the cortex, mlr uninhibited + become active) í control from the cortex is through this inhibition.
What is the cerebellum?
- A major area involved in vertebrate motor control. It is found in all vertebrates: an outgrowth of the pons at the front of the hindbrain. Generally, increases in size with phylogeny but there are some particular vertebrates like electric fish that have a particularly large cerebellum
- í the cerebellum processes huge amounts of particularly sensory info, and outputs these in a much simpler form to control motor responses.
What happens if you lose the cerebellum?
if it is removed from an animal, then motor responses can still occur but they become much less precise. So for example, transitions between behaviours are less smooth. More specifically, we can highlight three kinds of issues
o a loss of sensory motor coordination (poor reflexes)
o loss of balance (related to links with the vestibular system)
o a loss of muscle tone (muscles get more floppy).
What are the two features of the cerebellum that allow it to function + process sensory info + output motor responses?
o Firstly, the cerebellum has widespread connections. - in terms of responses (e it has input form the SC + this is from things like muscle receptors, but also info from CPGs. Info from the brainstem about things like balance (vestibular nuclei) + info from the cerebral cortex. (widespread patterns of input) --> The inputs go to the outer part of the cerebellum: the cerebrellar cortex. The output is via the deep neurons.
SPECIAL INTERNAL STRUCTURE: the outer part of the cerebellum, the cerebellar cortex has a clear, layered structure. It contains huge numbers of neurons but only five types of neurons. Because of its v.organised structure, it has been studied in a huge amount of detail.
How does the indirect pathway ni the cerebellum work?
Indirect pathway is through incoming signals from mossy fibres, bringing in signals from SC + Cerebral cortex.
These mossy fibres excite granule cells. These granule cells have axons that run up to the molecular layer + branch to give what are called PARALLEL FIBRES. These parallel fibres distribute info in a v. organised way over the surface of the cerebellar cortex. These parallel fibres encounter dendrites of the purkinje celll + excite them. the purkinje cells act as the output from the cerebellar cortex. This output is inhibitory and it inhibits neurons in these deep cerebellar nuclei.
give an overview of the indiirect pathway though the cerebellum?
fibres (billiokns). which distribute huge amounts of info across the surface of the cerebellar cortex, which is picked up by a smaller number of purkinje cells. (fewer: 7 million). P cells are output of the cerebellar cortex. Their output is inhibitory + they act on deep cerebellar nuclei with connections to the brainstem.
how does the direct [pathway through the cerebellum work?
" Comes through the climbing fibres, and these come from a structure in the brainstem called inferior olive / inferior olivary nucleus (this gets a lot of info from cerebral cortex).
" These connections are onto purkinje cells + are excitatory - they are some of the strongest excitatory connections in the brain.
" Effect activity of purkinje cells with their inhibitory output.
" Summary: masses of excitatory signals into cerebellar cortex, simplified down + output from a smaller number of neurons as inhibition.
What are the inhibotory cells in the cerebellum + what do they do?
- Basket cell + stellate cell : excited by parallel fibres in the granule cells, they inhibit purkinje cells.
- GOLGI cells which are excited by the parreallel fibres but inhibit the granule cells (from which the parallel fibres come - they are the axons of the granule cells).
These inhibitory cells sharpen up responses : sharpen up processing of signals by purkinje cells by lateral inhibition. Golgi cells provide the feedback inhibition that control the firing of the granule cells.
What are the features of the cerebellum ?
- widespread connections
-special internal structure
- modulates descending control
It hinges very much on the deep cerebellar nuclei.
How does the cerebellum modulate descending control?
Incoming signals through mossy + climbing fibres excite the cerebellar cortex + the processed info comes out through the purkinje cells as a process of inhibition. The purkinje cells responsible have a high resting firing rate (of impulseS) which can be ↑ or ↓ and that sets the level of inhibition they cause. The firing of purkinje cells inhibits neurons in the deep nuclei. Those neurons in the deep nuclei also receive excitation from the incoming signals from the mossy / climbing fibres.
Neurons in DCN get a balance of excitation + inhibition which determines their level of activity
what controls the activity of deep cerebellar nuclei?
balancing of incoming inhibition + excitation sets activity of those neurons + controls their output + the way they then act on the brainstem).
How can the processing of signals by the cerebellum affect the brainstem?
: this is because the cerebellum doesn't have direct outputs to things like the SC: all its effects are via the brainstem nuclei. Those effects of the cerebellum are via the deep nuclei (the neurons of the deep nuclei).
what is the Spino-cerebellar loop'?
There is a loop of info from the SC, processes in the cerebellum, influencing the brainstem + then passing down to the SC. This is usually referred to as the Spino-cerebellar loop). It is an important way that motor activity in the spinal cord is controlled.
what are the two ways in which CPGs in the spinal cord send info to the cerebellum?
via two pathways: DSCT + VSCT )DSCT = Dorsal spino-cerebellar tract (muscle receptors etc.)
VSCT = Ventral spino-cerebellar tract (CPG) - names not essentials) - axons going to cerebellum. These carry info from sensory structures like muscle receptors and from locomotor CPGs.
These pathways basically tell the cerebellum, in terms of locomotion, what is happening (muscle + proprio-receptors)+ what should be happening (activity of CPGs).
What is the second main projection area of the brain? where is it found?
FOREBRAIN PROTECTION AREA
second main projection area, found only in animals: the motor cortex.
what are the ways in which the motor cortex cortex influences descending control?
1. extra-pyramidal system Through connectiions onto the various brainstem nuclei. These are phylogenetically the oldest connectiions fom motor cortex to spinal cord (indirect via brainstem nuclei) - known as the EXTRA PHYRAMIDIAL SYSTEM.
2. pyramidal system (cortico-spinal system / tracts): Second system increasingly important as we move through the mammals: an ↑ no. of connections from motor cortex to the SC. From cortex, through cortex to the SC. The pyramidial tracts, at the most extensive, are massive byndles of axons, carrying info from muscles in the motor cortex, directly down into the spinal cord.
what gives the motor cortex direct access to motor output of the spinal cord?
If you look at the back end of the hindbrain, the medulla, you find that most of the axons cross to the opposite side + this means the motor cortex has its main effect on the opp side of the body.
Phylogenetically, as well as an ↑ in the numerbs of the axons, there is an ↑ in the numbers of axons that that make connections directly onto spinal motor neuron
What featre of the motor cortex allow particuarly in primates, is fast + precise control of manipulation + vocalisation?
- In primates: there is a huge amount of control of hands for manipulation + also for the face + regions controlling vocalisation
connections from the MC are very closely mapped
what are the Main regions of the basal ganglia?
what is the kind of loop arrangement of the regions in the basal ganglia?
Input from cortex into the striatum, which then makes connections onto other two regions (Globus pallidus & Substantia nigra) these regions make up the major parts of BG. Then there are connections via the thalamus back up to the motor cortex.
There is another, within the BG from the striatum to the substantia nigra + back up the striatum.
Don't worry about the details, get the esscence of the loop.
what are the outputs of the basal ganglia like?
The BG have outputs to midbrain locomotor region + also the brainstem nuclei. - they lie between cerebral cortex + MLR + brainstem nuclei.
A lot of the output is inhibitory, using the transmitter GABA.