Midterm Flashcards
What are the three basic purposes that neural systems serve?
1) Sensory systems report information about the state of the organism and its environment
2) Motor systems organize and generate actions
3) Associational systems provide ‘higher-order’ brain functions such as perception, attention, memory, language & thinking
How is the Human Nervous System organized?
1) Central nervous system (CNS)
- Brain
- Spinal cord
2) Peripheral nervous system (PNS)
- Sensory neurons
- Somatic motor division
- Visceral/autonomic motor division
What is the reticular theory?
Scientist Golgi, supported the ‘reticular theory’ that all neurons formed a single continuously connected network.
How did Ramon y Cajal use Golgi’s method?
He used it to reconstruct neurons and argued for the ‘neuron doctrine’ that neurons communicate at specialized contact points rather than through physical continuity.
It was identified that these points of communication were ‘synapses.’ Ultimate proof was discovered when there was an electron microscopy to visualize synapses and confirm that neurons are discrete entities.
What are the two basic cell types?
-> Neurons and glia are the primary cells of the brain
What are the function of neurons?
- Process information
- Sense environmental changes
- Communicate changes to other neurons via electrical signalling
- Control bodily responses
What are the functions of Glia?
- support the signalling functions of neurons
- insulate, nourish, repair neurons (probably more also)
-maintaining the ionic milieu of neurons - modulating the rate of action potential propagation
- modulating synaptic transmission by regulating neurotransmitter uptake & metabolism at the synaptic cleft
- regulating recovery from neural injury
- interface between brain & immune system
- facilitating flow of interstitial fluid through the brain during sleep
- complexes processes extending from their cell bodies but these serve different functions than neuronal processes
- Glia is Greek for ‘glue’ - long through that glia’s primary purpose was to hold neurons together
Dendrites
- primary target for synaptic input from axon terminals of other neurons
- extensive branching that differs greatly between neuron types
- complexity of dendritic arbour depends on number of inputs a neuron recieves
- arbour complexity dictates capacity to integrate information from many sources
axon
- signal transduction from cell body; reads out information
- most neurons have only one that extends for a long distance
- some branching
- site of output to other neurons
action potential
- electrical event that carries signals
- also called ‘spikes’ or ‘units’ are ‘all or nothing’ changes in electrical potential across the neuronal cell membrane
pre-synaptic terminal
where molecules are secreted into synaptic cleft
post-synaptic specialization
contains receptors where molecules bind
synaptic cleft
space between pre- and post-synaptic terminals
How are neurons specialized for long-distance electrical signaling?
- Extensive branching:
-> dendrites
-> axons
What happens to information conveyed by synapses on the dendrites?
It is integrated and converted to an electrical signal, the action potential, at the origin of the axon.
How far does the axon extend from from the neuronal cell body?
- It may travel a few hundred micrometers or even further.
eg. local interneurons have very short axons
eg. axons from the human spinal cord to the foot are a meter long - axons can branch to innervate multiple post-synaptic sites on multiple neurons.
What is the axon terminal of the presynaptic neuron immediately adjacent to?
- the postsynaptic area on the target cell.
neurotransmitters
they are specialized molecules that are released from the presynaptic terminal, cross the synaptic cleft, and bind receptors in the postsynaptic density
nodes of ranvier
gaps in the myelination of axons where action potentials are generated (regenerated)
astrocytes
- type of glia
- restricted to brain & spinal cord
- major function is to maintain the appropriate chemical environment for neuronal signalling, including formation of the blood-brain barrier
- recent evidence suggests astrocytes secrete substances to influence construction of new synaptic connections
oligodendrocytes
- type of glia
- restricted to brain & spinal cord
- lay down myelin around axons, regulating speed of transmission of action potentials
schwann cells
- type of glia
- provide myelin in the peripheral nervous system
microglia
- type of glia
- primarily scavenger cells that remove cellular debris from sites of injury or cell turnover
- secrete signalling molecules, particularly cytokines (immune signalling molecules)
glial stem cells
- type of glia
- cells that retain the capacity to proliferate and generate additional precursor cells or differentiated glia or neurons
What is a neural circuit?
- neurons do not act alone
- diverse subsets of neurons are organized into ensembles called neural circuits that process specific types of information
- specific arrangement varies with function
What are basic components of neural circuits?
1) Afferent neurons carry information toward CNS
2) Efferent neurons carry information away from CNS
3) Interneurons participate in local aspects of circuit function
What defines all neural circuits?
- direction of information flow
knee-jerk response
- simple reflex circuit
1) Hammer tap stretches tendon, which in turn, stretches sensory receptors in leg extensor muscle
2) Sensory neuron synapses with and excites motor neuron in the spinal cord. Sensory neuron also excites spinal cord interneuron. Interneuron synapse inhibits motor neuron to flexor muscles.
3) Motor neuron conducts action potential to synapses on extensor muscle fibres, causing contraction. Flexor muscle relaxes because the activity if its motor neurons has been inhibited.
4) leg extends.
divergent circuits
spread information
one presynaptic neuron branches to affect a larger number of postsynaptic neurons
convergent circuits
integrate information
many presynaptic neurons converge to influence a smaller number of postsynaptic neurons
Electrophysiological recordings are used to study neural circuits. How?
Electrophysiological recordings:
- Classically, the primary technique for probing neural circuit function.
- > Extracellular recording:
- An electrode is placed near a neuron. Useful for detecting temporal patterns of action potential activity
-> Intracellular:
- An electrode is placed inside the neuron
- Can detect smaller, graded changes in electrical potential that trigger action potentials
- Assess communication among neurons within a circuit
Calcium Imaging is used to study neural circuits. How?
- record transient changes in the concentration of calcium ions that are associated with action potential firing to infer changes in neural activity.
Optogenetic mechanisms are used to study neural circuits. How?
- Optogenetic mechanisms can assess the physiology of neural circuits based on the activation of neuronal populations.
- Bacterial channels referred as opsins
transduce light energy into chemical signal that activates channel proteins.
Describe the building blocks: Neuron, Circuit & System
Diverse subsets of neurons constitute ensembles called neural circuits which are primary components of neural systems that process specific types of information.
sensory systems
acquire & process information from the internal and external environment
motor systems
respond to information (e.g sensory) by generating movements
association systems
lie between input & output systems
Neural Systems are characterized by:
-> Unity of function
- a system is defined by the neurons and connections dedicated to a function. (visual system defined by all neurons and connections dedicated to vision)
- components of a system are often distributed throughout the body and brain. (sensory systems include peripheral sensory specializations e.g. ear, eye, skin, nose) (motor systems include peripheral motor nerves and target muscles)
-> orderly representation of specific information at various levels
-> division of the function of the system into subsystems that are relayed and processes in parallel
- information from sub-modalities is processed separately but in parallel.
e.g. frequency and volume of an auditory signal, colour & motion of a visual stimulus
Topographic maps
- they reflect a point-to-point correspondence between the sensory periphery and neurons within the CNS
- systems that distinguish differences between neighboring points (e.g. vision, in visual field or touch, on the body’s surface) represent information topographically.
computational maps
- systems like smell and taste use these maps to compare, assess, & integrate multiple stimulus attributes to extract essential information about stimuli
do the higher order systems such as language and emotion go by a specific map?
no, they are less well understood and may not follow the neat organization of sensory & motor systems.
nervous system
collection of neurons
gray matter
cell bodies in the brain, appear grey in freshly dissected brain
cortex
thin sheet of neurons, usually at the brain’s surface
nucleus
clearly distinguishable mass of neurons, usually deep in the brain (nucleus is Lain for “nut”)
substantia
related neurons, but with less distinct boundaries than a nucleus
locus
s (pl: loci): small, well-defined group of cells
ganglion
(pl: ganglia): collection of neurons in the PNS (ganglion is Greek for “knot”). Only the basal ganglia in the CNS.
nerve
a bundle of axons in the PNS. Only nerve in the CNS is the optic nerve
white matter
generic term for collection of axons; appear white from myelination
tract
collection of CNS axons having common origin and destination
bundle
collection of axons that run together but do not necessarily have a
common origin/destination
capsule
axon collection that connects cerebrum with brainstem
commisure
axon collection that connects one side of the brain to the other
lemniscus
a tract that meanders through the brain like a ribbon
Describe genetic analysis of neural system
- genetic variation shapes structure & function of the nervous system
- in humans, this has been studied by:
-> genetic analysis in families affected by inherited diseases
-> look for genetic variation between affected & unaffected individuals
-> genome wide associated studies (GWAS) - once candidate genes have been identified in humans, these can be studied in cell models or animal models to understand the biological function of these genes
genome wide associated studies (GWAS)
Large scale population
studies that assesses
statistical correlation
between genetic variation
and frequency of
clinically diagnoses
conditions to identify ‘risk
locus’ for a particular
condition
describe the structural analysis of neural systems: lesion studies.
- Inferences of functional location made
by correlating post-mortem
observation of gross brain damage
with functional deficits observed in life - e.g. Henry Molaison (HM) lost the
ability to form new lasting
autobiographical memory. - Inherent limitations include
-> Biased recall of functional changes
-> Uncontrolled damage - Animal studies experimentally induce
lesions but still limited
What does tract tracing permit?
- detailed assessment of connections
between brain regions
-> retrograde (cells that send connections to the injection site)
-> anterograde (regions that receive connections from the injection site)
Historically, what were the most widely used methods to do functional analysis of neural systems?
- electrophysiological recording
- functional brain imaging
what are non-inasive techniques of functional analysis of neural systems?
- EEG
- transcranial magnetic stimulation
- CT
- fMRI
Anatomical references for humans
rostral = forehead
caudal = back of head
dorsal = top of brain
ventral = bottom of brain
superior = above head
inferior = below
anterior = in front of
posterior = behind
For the brain stem & spinal cord:
- dorsal is to the back
- rostral is towards the top of the head
For the forebrain:
- dorsal is toward the top of the head
- rostral is towards the face
What are the 3 different planes when cutting the brain?
1) Sagittal
2) Horizontal (axial)
3) Coronal (frontal)
When we cut the brain along the midline, we observe the brain has bilateral symmetry.
contralateral
on opposite sides of the midline
ipsilateral
on the same side of the midline
What are the 7 basic parts that the CNS has?
- Spinal cord
- Medulla
- Pons
- Midbrain
- Cerebellum
- Diencephalon
- Cerebrum
brainstem
- medulla
- pons
- midbrain
forebrain
diencephalon and cerebral hemispheres (cerebrum)
cerebrum
generally receives and sends information to the contralateral
side of the body
cerebellum
- ‘tiny brain’
- contains as many neurons as cerebrum
- Movement control
brain stem
- Relay between cerebrum/cerebellum and spinal cord
- Basic vital functions e.g. breathing
Describe the spinal cord - external anatomy
-> peripheral nerves that innervate much of the body arise from the spinal nerves
-> sensory information carried by afferent axons of the spinal nerves enters the cord via the dorsal roots
-> motor commands carried by the efferent axons leave the spinal cord via the ventral roots
-> once the dorsal and ventral roots join sensory and motor axons usually travel together in the spinal nerves
Describe the spinal cord - Internal Anatomy
- Interior of the cord is formed by gray matter, surrounded by white matter
- cervical and lumbosacral enlargements accommodate the greater number of nerve cells and connections required to process information from upper and lower limbs
- the white matter of the spinal cord is divided into dorsal, lateral and ventral columns
- dorsal columns carry ascending sensory information from somatic mechanoreceptors
- lateral columns include axons that travel from the cerebral cortex to interneurons and motor neurons in the ventral horns (‘lateral corticospinal tract’)
- ventral columns carry both ascending information about pain & temperature, and descending motor information from the brainstem & motor cortex
- In transverse sections we can identify the dorsal and ventral horns in the gray matter
- neurons of the dorsal horns receive sensory information that enters via the dorsal roots of the spinal nerves
- the ventral horns contain cell bodies of motor neurons that send axons via the ventral roots of spinal nerves to striated muscles
Brainstem & Cranial Nerves
- Midbrain, pons, medulla
- located between diencephalon and spinal cord
What are the 3 fundamental functions of the brainstem?
- target and source for cranial nerves that deal with sensory and motor function of head and neck
- A ‘throughway’ for ascending sensory tracts from spinal cord, sensory tracts for head and neck, descending motor tracts from forebrain, local pathways linking eye movement centers.
- regulating levels of consciousness through extensive forebrain projections.
are brainstem structures tightly packed
yes
what are cranial nerve nuclei in the brainstem a target of
they are the target of cranial sensory nerves and the source of cranial motor nerves
what is there a seperation of in the brainstem?
- there is a separation of sensory & motor nuclei in the brainstem
- sensory nuclei are found laterally
- motor nuclei are found medially
what does the lateral fissure separate?
temporal lobe from frontal & parietal lobe
what does the central sulcus separate
frontal and parietal lobes
what does the parieto-occipital sulcus seperate
parietal and occipital
what does the precentral gyrus locate
motor cortex
what does the postcentral gyrus locate
somatic sensory cortex
what is the insula cortex hidden beneath
frontal and temporal lobes
corpus callosum
bridges the two hemispheres, carrying axons originating from neurons in cerebral cortex of each hemisphere to contact target neurons in the opposite cortical region
what does the calcarine sulcus locate
the primary visual cortex
what is the cingulate gyrus part of
the limbic system
what are the components of diencephalon
thalamus
hypothalamus
thalamus
- relay of sensory and motor signal to relevant primary cortical cortex and also distributer of high order signals from one part of cortical area to another
- the sensory pathways from the eye, ear, and skin all relay in the thalamus before terminating in the cerebral cortex
- around 50 subdivisions maintain distinct inputs & outputs
- receives input from throughout brain and spinal cord
- sends axons to different cortical areas
- sends information back to brain stem via internal capsule and basal ganglia
hypothalamus
homeostatic and reproductive functions
where is the amygdala located
infront of the hippocampus
What are the parts of the basal ganglia
caudate, putamen & globus pallidus
what does the anterior commissure axon tract connect
it connects the two hemispheres
what is the internal capsule
major pathway linking cerebral cortex to brain & spinal cord
what does damage to major blood vessels by trauma or stoke result in
combinations of functional defects caused by local cell death, disruption of axons passing through area of vascular damage.
what are neruons highly sensitive to?
oxygen (& glucose) deprivation because they have high metabolic rate
What does brief loss of blood suply (ischemia) caue
cellular changes that may end in cell death
what does prolonged loss of blood supply lead to
cell death and degeneration
stroke
the death or dysfunction of brain tissue that follows compromised blood supply
What is the blood-brain barrier? (BBB)
- BBB protects the brain from toxins & fluctuations in ionic milieu
- Interface between walls of capillaries and surrounding tissue are observed throughout the body
- In the brain, tight junctions form between capillary endothelial cells that are not seen elsewhere in the body
How do molecules enter the brain?
They must move through endothelial membranes:
- lipid soluble
- Actively transported e.g. glucose
Meninges
- Greek for “covering”
- membranes protecting the brain and spinal cord preventing direct contact with skull or bone
dura mater
outermost, latin for “hard mother”
Arachnoid membrane
- middle layer with a web-like consistency, from the Greek for “spider”
- blood vessels pass between the dura and arachnoid membranes-ruptures to these subdural hematomas
-fluid build-up here is dangerous because is puts pressure on the CNS
pia mater
- inner layer
- adheres closely to the brain and included many blood vessels, latin for “gentle mother”
- the pia is separated from the arachnoid by the subarachnoid space which contains cerebrospinal fluid (CSF)
ventricular system
- ventricles are canals through the brain filled with cerebrospinal fluid (CSF)
- provide useful anatomical landmarks in the brain
what is CSF produced by? where does it flow?
- produced by the choroid plexus, special tissue lining the ventricles of the brain
-> produces 500 mL CSF/day
-> normal volume in ventricular system is 150 mL
-> CSF turnover multiple times daily - CSF flows through the ventricles and exits the CNS into the subarachnoid space by small openings along the dorsal midline of the forebrain, where it is absorbed by subarachnoid vilii into the blood
glymphatic system of the brain
- The brains waste clearance system
- CSF passes from arterial perivascular
space through the substance of the
brain - The CSF rinses metabolic waste and
discarded proteins - The waste-carrying CSF passes out of
the brain via the perivascular space
surrounding veins - CSF flow increases during sleep when
extracellular spaces expand
Sensation
Sensation entails the ability to transduce, encode, and ultimately perceive information generated by stimuli arising from both external and internal environments.
What are the 5 basic senses
1) Somatic - pressure, temperature, vibration and pain
2) Vision - light waves
3) Audition - sound waves
4) Taste - chemical
5) Smell or Chemical sense
What is the sixth sense
proprioception (sense of where your body is)
All senses provide very different information. But they follow similar basic rules for sensation. What are they?
- specialized cells (receptors) covert energy (mechanical forces - light) into afferent sensory signals - conveys information to the brain.
- signals convey information about:
Modality (touch versus pain: type of touch - sharp versus dull)
where it is (location)
intensity
time course
Why is understanding deficits in sensory processing very important?
Diagnosing various neurological problems
What is the function of the somatic sensory system?
Mediates a range of sensations e.g. touch, pressure, limb position, temperature, pain
what are the three sub-systems of the somatic sensory system?
1) fine touch, vibration, pressure
- cutaneous mechanoreceptors
2) Proprioception: sense of relative position of our body parts in space
- specialized receptors associated with muscles, tendons & joints
3) Temperature, pain and non-discriminative (sensual) touch
Transmission of somatic sensory information
1) Receptors (nerve endings) in skin/muscles/joints
2) Afferent nerve fiber (axons)
3) Afferent cell body (dorsal root ganglia or cranial nerve ganglia-trigeminal ganglia)
4) Central nervous system circuits
What is the external anatomy of the spinal cord?
- the peripheral nerves that innervate much of the body arise from the spinal nerves (sensory - afferent AND motor - efferent)
- sensory information carried by afferent axons of the spinal nerves enters the cord via the dorsal roots
Where are cell bodies of afferent nerve fibres located?
- in the ganglia adjacent to the spinal cord & brain stem
dorsal root ganglia: body
trigeminal ganglia : head
What is a characteristic of neurons of dorsal root ganglia?
they are pseudounipolar - no synapse before entering the spinal cord
where is the first synaptic terminals?
within the grey matter of the spinal cord
what does the PNS neurons supply the CNS with?
information about sensory events in the periphery
‘Pseudounipoalr’ sensory neurons vs bipolar neurons
- Bipolar neuron (axon, dendrites)
- pseudounipolar neurons
-> one axon with two branches, no true dendrites
central: cell body to spinal cord
peripheral: cell body to peirphery
where are ‘pseudounipolar’ sensory neurons found?
- they are found in the dorsal root ganglia:
cell body in DRG
Axon exits DRG, splits into 2 branches
Central branch to dorsal horn of spinal cord ‘peripheral branch travels through the spinal nerve to skin, join, muscle
also found in sensory ganglia of cranial nerves
How does sensory transduction convert energy from a stimulus into an electrical signal?
1) sensory stimulus produces a depolarization current in the afferent nerve endings called a receptor potential
2) upon reaching a threshold, action potentials are generated in the afferent fibre
3) APs then travel along the peripheral axon past the cell body in the dorsal root ganglion & along the central axon to reach the synaptic terminals in spinal cord
What are some characteristics of afferent fiber terminals? what can they be?
They can be:
- encapsulated by specialized receptor cells ‘mechanoreceptors’ which usually tune in to a particular feature, or lower threshold - more sensitive
What are free terminals (pain)?
- higher threshold
- same stimuli in higher intensities will produce ‘pain’
How is a ‘receptor potential’ (depolarizing current) generated?
- a stimulis (physical) changes the permeability of cation channels in the afferent nerve endings
(the same basic mechanisms mediate sensory transduction in all somatic sensory efferents)
Explain: sensory transduction: converting physical stimuli to electrical signals.
- if the stimulus is sufficient, the receptor potential reaches the threshold to generate an action potential in the afferent fiber
- the rate of action potential firing is proportional to the magnitude of depolarisation
- because somatic sensory neurons are pseudounipolar, the electrical activity does not need to be conducted through the cell body membrane, but rather travels along the continuous peripheral and central axon
What are piezo mechanotransduction channels?
Piezo1 and Piezo2 are essential components of distinct mechanically activated cation channels
What do distinct functional properties of somatic sensory afferents define?
distinct classes of afferents with specialized mechanoreceptors which convey unique sensory information
what are sensory afferents often encapsulated by?
specialized receptor cells that tune the fibre to specific stimulation
are free nerve endings important in pain sensation?
yes
What are for key properties characterize sensory afferents?
- axon diameter
- receptive field
- temporal dynamics
- quality of somatic sensory stimulation
what does axon diameter determine?
conduction speed of conduction of action potential (larger, faster)
What is the receptive field of sensory afferent?
It is the area of skin surface which stimulation results in a significant change in the rate of action potentials
does receptive field size vary in different parts of the body?
yes
what is the size of the receptive field largely determined by?
- the branching of the sensory afferents in the skin: smaller arborization -> smaller receptive field
- density of afferent innervation, more afferents -> smaller receptive field
what does receptive field size determine?
- spatial accuracy with which tactile stimulaiton can be sensed
- two point discrimination measures the minimum distance between two simultaneously applied stimuli that is perceived as two distinct stimuli
- discrimination varies dramatically
fingertips: 2 mm
forearm: 40 mm
What is the two-point discrimination threshold?
- two-point discrimination varies throughout the body
- somatic acuity is much higher in fingers, toes and face than in arms, legs, toso
- this is the result of differences in receptive field size
- species specific
How do temporal dynamics affect sensory afferents?
- sensory afferents respond to the same stimulus with different temporal dynamics
- rapidly adapting afferents: fire upon the initiation of stimulation, quickly become quiescent if stimulation is maintained, may fire again on termination
- slowly adapting efferents continue to fire with sustained stimulation
- rapidly adapting afferents may be important for conveying information about changes in ongoing stimulation e.g. movement
- slowly adapting afferents may convey information about spatial attributes of a stimulus e.g. size, shape
- adaptation characteristics are determined in parts by properties of mechanoreceptors
Different classes of sensory afferents respond only to a restricted set of sitmuli. e.g. stretch, temperature. what is this determined by?
This is determined by:
differences in properties of channels
filter properties of mechanoreceptors
these different afferents are parallel pathways and remain segregated (even if they travel together at first)
What are the three sub-systems of the somatic sensory system?
fine touch, vibration, pressure
cutaneous mechanoreceptors
proprioception
sense of relative position of our body parts in space, specialized receptors associated with muscles, tendons & joints
Touch/cutaneous sensation
best understood for glabrous skin (i.e. palm, fingertrips) - specialized for high- definition neural image of manipulated objects.
depends on specialized end organs surrounding the nerve terminal: mechanoreceptors
haptics
active touching - interpretation of complex spatiotemporal patterns of stimuli - active many classes of mechanoreceptors
stegnosis
capacity to identify an object by manipulating it with the hand
Experiments recording from individual sensory afferents in nerves have identified the specific contributions of 4 distinct mechanoreceptors to somatic sensation. What are they?
1) Merkel cell-neurite complex
2) Meissner corpuscle
3) Pacinian corpuscle
4) Ruffini corpuscle
What is a mechanoreceptor?
it is a receptor that provides an organism with information about mechanical changes in its environment. virtually all mechanoreceptors have specialized end organs surrounding the nerve terminal.
mechanoreceptor afferents for touch: what is the sensitivity to mechanical displacement a property of?
the sensitivity to a mechanical displacement is a property of both the nerve terminal membrane and the specialized capsule
Merkel’s disks (aka merkel cell-neurite complex)
a merkel cell (a specialized epithelial cell) closely associated with an enlarged nerve terminal
meissner’s corpuscles
globluar, fluid-filled structure that encloses a stack or flattened epithelial cells; the sensory nerve terminal is entwined between the various layers
ruffini endings
nerve fibers that are often but not always associated with a fibrous capsule
pacinian’s corpuscles
consists of many modified fibroblasts to make lamellae like an onion
each is connected to a sensory neuron
merkel cell afferents
- slow adapting - static aspect of touch 25% of mechanosensory afferents in hand
- highest spatial resolution of all sensory afferents
- enriched in fingertips
- from merkel cells and the sensory afferents express Piezo2
- highly sensitive to points, edges, and curvature and suited to processing information about form & texture
- often afferents sampling information from the epidermis
meissner afferents
- express Piezo2
- rapidly adapting
- high spatial resolution
- 40% of mechanosensors in hand
- skin indentation deforms the corpuscle to trigger receptor potentials
- removal of stimulus relaxes the corpuscle to resting position also generating receptor potentials
- meissner corpuscles are formed by a connective tissue capsule of flattened cells derived from schwann cells with the center of each capsulse containing 2-6 afferent nerve fibres
what is the difference of meissner afferents to merkle?
- they are closer to the skin surface
- more sensitive to skin deformation
- larger receptive field -> reduced spatial resolution
- sensitive to vibration of objects moving across skin
- detect slippage between the skin and the object held in hand, essential feedback information for the efficient control of grip.
pacinian afferents
- rapidly adapting
- 10-15% of mechnosensors in hand
- pacinian corpuscles located deep in dermis or subcutaneous tissue. concentric layers of membranes around a single fiber (like an onion)
- laminar structure of pacinian corpuscle filters out all but high frequencies.
- lower response threshold tha meissner corpuscles
- can respond to skin displacements as small as 10 nm
- large receptive fields
- detect vibrations transmitted through objects in touch with the hand
- may be important in tool use
ruffini afferents
- slow adapting
- 20% of mechanoreceptors in hand
- ruffini corpuscles are elongated, spindle-shaped capsules in dermis and also found in ligaments and tendons
- long axis of corpuscle lies parallel to stretch lines in skin making them sensitive to cutaneous stretching with digit or limb movement
- contribute, along with muscle receptors to sensation of finger position and hand conformation
The different kinds of information that sensory afferents conveyed to central structures was first illustrated in?
Experiments conducted by K. O Johnson and colleagues, who compared the responses of different afferents as a fingertip was moved across a row of raised brailled letters
Proprioception
- information about mechanical forces arising within the body itself.
- proprioceptors (low threshold mechanoreceptors) provide continuous detailed information about the position of the limbs and other body parts in space
-> muscle spindle, gogli tendon organ, joint receptors - essential for accurate performance of complex movements
- in the case of position of the head, integration with vestibular system.
muscle spindles
- found in striated (skeletal) muscle
- consist of 4-8 specialized intrafusal muscle fibers surrounded by capsule of connective tissue, distributed in parallel with extrafusal fibers
- sensory afferents coil around the centra part of the intrafusal spindle. when the muscle is stretched, tension of intrafusal fibers activates mechanically gated ion channels, triggering action potentials.
muscle spindle is innervated by?
two types of fibers:
1) primary endings
2) secondary endings
also innervated by efferent y motor neurons in the ventral horn of the spinal cord, which change intrafusal fiber tension and increase sensitivity of the afferents to changes in muscles length.
primary endings that innervate muscle spindles
- group Ia afferents (largest myelinated sensory axons)
- rapidly adapting responses to changes in muscle length
- transmit information about limb dynamics
secondary endings that innervate muscle spindles
- group II afferents
- sustained responses to constant muscle lengths
- information about static limb position
How does density of spindles in human muscle vary with function?
- muscles that generate coarse movement have fewer spindles than muscles that generate very fine movements
- more precise movement requires more refined sensory input (eyes, hand, neck)
artificial stimulation of spindles by vibration produces..?
- sensory illusions of altered limb position in stationary limbs
- illusion only produced if visual input is prevented
- in normal conditions, properioception is achieved by integration of somatic and visual cues
what does joint angle perception arise from?
from integration of afferent signals from muscle spindles and efferent motor commands
golgi tendon organs
- low-threshold mechanoreceptors in tendons
- senses changes in muscle tension
- distributed along collagen fibers that form tendons
- arranged in series with extrafusal muscle fibers
- innervated by branches of group Ib afferents
- contribute less to conscious sensation of muscle activity
- important role in reflex circuits protecting muscle from injury
what central pathways convey tactile information?
- tactile afferents, enter through dorsal horn of spinal cord
- the main ascending branches (direct projections) extend ipsilaterally through the dorsal columns (also called the posterior funiculi) of the cord to the lower medulla, where they synapse on neurons in the dorsal column nuclei.
what central pathways convey proprioceptive information?
- proprioceptive afferents enter through dorsal horn of spinal cords
- many fibres then bifurcate to form both ascending and descending branches, collaterals synapse on neurons of the dorsal and ventral horn
- proprioceptive information also reaches cerebellum where it is required in control of voluntary movement
Spinocerebellar tract
first order neuron collaterals from lower body synapse in Clarke’s nucleus. Neurons in Clarke’s nucleus send their axons via the dorsal spinocerebellar tract to the cerebellum, with a collateral to the dorsal column nuclei.
where do the proprioceptive afferents for the upper body ascend via?
via the dorsal column to the dorsal column nuclei; the cuneate nucleus, in turn, relays signals to the cerebellum.
Compare the central pathways for conveying proprioceptive and tactile information? (first order neuron - ipsilateral)
Proprioceptive:
Dorsal Root ganglion / Trigeminal ganglion
Tactile:
Dorsal column (ascend ipsilaterally) Cuneatus,
gracile tracts //Trigeminal trac
Compare the central pathways for conveying proprioceptive and tactile information? (second order neuron - ipsilateral)
Proprioceptive:
Gracile, Cuneiform nuclei / Trigeminal nucleus
Tactile:
Medial Lemniscus // Trigeminal Lemniscus **Axons
cross the midline
Compare the central pathways for conveying proprioceptive and tactile information? (third order neuron - contralateral)
Proprioceptive:
Ventral posterior complex / Medial Thalamic and
Parabrachial (nuclei in the thalamus)
Tactile:
Internal capsule
Central pathway conveying proprioceptive and tactile information (cerebral cortex)
Cerebral cortex – Primary somatosensory cortex
– postcentral gyrus – parietal lobe // Anterior
cingulate and insula. Also, secondary
somatosensory cortex.
somatic sensory in the thalamus
- ascending somatic sensory pathways from the spinal cord and brainstem converge in the ventral posterior complex of the thalamus in a highly organized manner
- afferents terminate in somatotopic representation of the body and head
-> VP lateral: relay from body (via medial lemniscus)
-> VP medial: relays from face (via trigeminal lemniscus) - parallel pathways (body and head)
- inputs carrying different types of somato-sensory information terminate on seperate populations of relay cells
- information from distinct somatosensory receptor types remains segregated in passage to cortex
where do most neurons from VP thalamus project?
- to layer 4 of primary somatic sensory cortex (SI)
- SI is located in post central gyrus of the paritetal lobe and has 4 regions (brodmann’s area 3a, 3b, 1 and 2)
- each region contains a complete somatotopic map of the body in a medial to lateral arrangement
somatotopic maps in SI
- foot, leg, trunk, forelimbs and face are represented in a medial to lateral arrangement
- do not represent the body in its actual proportions
-> bigger area (and number of neurons) for more richly innervated regions
-> homunculus - grossly enlarged representation of face & hands - the proportionality of representation reflects the neural circuitry required to govern the associated functions (e.g. facial expressions, speech, manual manipulation of objects)
for a somatopic map for a naked mole rate what dominates?
the teeth
Despite similar topography
across SI areas, functional
properties of neurons in each
region are distinct, reflecting
segregated, parallel inputs
from VP thalamus. What do neurons in 3b &1, 3a, and 2 respond to?
3b & 1: respond primarily to cutaneous stimulation
3a: respond primarily to proprioceptive stimulation
2: respond to both tactile & proprioceptive stimuli
What is an example of neurons in SI forming functionally distinct columns?
- For example, neurons with
responses to rapidly and slowly
adapting mechanoreceptors
cluster into separate zones
within the representation of a
single finger - This modular organization is a
fundamental feature of cortical
organization but the functional
significance is still being
determined
describe the functional hierachy with 3b as an obligatory first step in cortical processing
- 3b receives largest input from VP
thalamus & sends dense projections
to 1 & 2 - Lesions of area 3b in non-human
primates - deficits in all forms of
tactile sensations mediated by
cutaneous mechanoreceptors - lesions limited to areas 1 or 2 -
partial deficits to discriminate either
the texture of objects (area 1 deficit)
or the size and shape of objects
(area 2 deficit)
What are substantial connections between SI areas called?
corticocortial connections
all regions of SI project to?
secondary somatosensory cortex
also projections to pariteal areas for motron integration & limbic areas for learning and memory
SII sends projections to?
limbic structures such as the amygdala and hippocampus - tactile learning and memory
where does SI project to?
parietal areas 5a and 7b
these areas supply inputs to neurons in the motor and premotor areas of the frontal lobe.
thats how proprioceptive afferents signalling the current state of muscle contraction gains access to circuits that initiate voluntary movements - sensorimotor integration.
do descending projections outnumber ascending projections?
yes
descending projections to thalamus, brainstem & spinal cord
their function is not well understood but assumed they modulate sensory information flow in thalamus and brainstem
plasticity in the adult cerebral cortex
- Primary somatosensory cortex
responses adapt to differences in
stimulation
-Lesioning an input
-> Initial lack of response in
corresponding cortical area
-> Gradual increase in responding to
stimulation of neighboring regions - Changes in cortical
representation also induced
by less drastic changes in
sensory or motor
experience
–> e.g. Training a monkey to
use specific fingers to
perform a task expands
associated cortical
representation
–> e.g. local anesthetic induces
temporary remapping of
receptive fields
–> Rapid plasticity suggests
likely reflect changes in
synaptic strength of existing
synapses
What is sound?
- pressure waves generated by vibrating air molecules
- like ripples spreading across water but in 3D
what are the 4 features of sound?
- amplitude (dB: loudness)
- frequency (Hz; pitch)
- waveform (amplitude across time)
- phase
what is sound?
displacements of air molecules
- condensation
-rarefraction
what is example of complex sounds?
- birdsong and speech contain highly periodic elements
- can be modeled as the sum of sinusoidal waves of varying amplitudes, frequencies and phases
- environmental sounds e.g. wind lack periodic structure
fourier transform
- decomposes a function of time in its constituent frequencies - inner ear
the audible spectrum
- species specific:
different species emphasize the frequency of their vocalizations
also, echolocation (high frequency), predation (low frequency)
humans: 20Hz to 20kHz
loss of high frequency with age (max 15-17kHz adult)
auditory function
- auditory system transforms sound (air vibration patterns) into neural activity (mechanoelectric transduction)
- external and middle ears collect and amplify sound waves and transmit to the fluid-filled choclea of the inner ear
- in the inner ear, biomechanical processes allow hair cells transduce frequency, amplitude and phase of the signal into electrical signals
- acoustical decomposition results in systematic representation of sound frequency along the length of the cochlea (tonotopy)
What is the anatomy of the external ear?
- Pinna
- Concha
- Auditory meatus
what is the function of the auditory meatus?
gathers sound energy and focuses it on the tympanic membrane (ear drum)
what is the function of the pinna & concha?
filter differ sound frequencies to provide cues about elevation of sound source
what does the human auditory meatus selective boost?
it boosts sound pressure 30-100x for frequencies around 3kHz which is directly related to speech perception
what is the function of the middle ear?
- Boosts the pressure of the
sound’s energy from the
tympanic membrane to the inner
ear by ~200x - This is necessary to carry
airborne sound (low-impedance) to aqueous environment of inner
ear (high-impedance) - Impedance describes a medium’s
(i.e. air, water) resistance to
movement
In the middle ear, how many mechanical processes boost pressure?
- two mechanical processes
- focus force from the large surface tympanic membrane to small diameter of the oval window (where bones connect to inner ear)
- level action of the ossicles:
malleus, incus, stapes
small interconnected bones
what is efficiency of sound transmission in the inner ear regulated by?
- the tensor tympani & stapedius muscles innervated by cranial nerve V and VII
- in response to loud noise, these muscles contract to counteract the movement of the ossicles and limit transmission of sound energy.
-> paralysis of these muscles - bell’s pasly - can generate hyperacusis.
sensory transduction
The cochlea transforms sonically
generated pressure waves into
neural impulses carried by the
auditory nerve
( inner ear)
mechanical
frequency analyzer
- inner ear
- decomposing acoustical
waveforms into their elements -
tonotopy
what is the structure of the inner ear
- The cochlea is a small
coiled tube structure
(like a snail shell) - Oval window & round
window are at the basal
end of the tube
describe the cochlea in cross-section
- Cochlear partition runs from basal
end almost to apex - A flexible structure that supports the
basilar membrane & tectorial
membrane - Fluid-filled (perilymph) chambers
on either side of the partition n Scala vestibuli
-> Scala tympani
-> Scala media runs within the partition - Cochlear partition runs from basal
end almost to apex – opens at
helicotrema where scala vestibuli
and scala tympani join
what is the effect of the siffness of the basilar membrane along the length
making it differentially
sensitive to different
frequencies (Tonotopy)
Travelling wave in cochlea
growing in amplitude until a
point of maximum
displacement. Basilar
membrane is tuned to high
frequencies while apex
tuned to low frequencies.
where are hair cells located?
- located between basilar membrane and tectorial membranes: sound wave, transmitted through oval window, causes motion between these two membranes
- hair cells have ‘stereocilia’
- inner hair cells receive afferents from cranial nerve VIII - carry the impulses towards the CNS
- outer hair cells receive mostly efferent innervation - amplifier/otoacoustic emmsions
In hair cells, which ones are the sensory receptors?
inner hair cells
hair cells
are epithelial cells with a bundle of hair-like processes that protrude into the scala media
each bundle has 30 to a few hundred stereocilia
what are tip-links of hair cells
connect the tips of
adjacent stereocilia & translate hair
bundle movement into a receptor potential
what strecthes the tip links?
- displacement of the hair bundle in the direction of the tallest stereocilia (kinocilium) stretches the tip-links
- this directly opens cation-selective channels that are located at the end of the link, allowing K+ to enter
- this depolarizes the hair cell
what does depolarization of the hair cell lead to?
- opening of voltage-gated
Ca2+ channels - Ca2+ influx
- Neurotransmitter release
onto auditory nerve
endings
what compresses the tip-links?
Movement in the opposite
direction compresses the tip- links, closes the channels and
hyperpolarizes the cell
Because some channels are
open at rest, the receptor
potential is biphasic
describe the mechanoelectrical transduction of sound waves:
1) movement of the stereocilia back and forth modulate ionic flow to produce a graded receptor potential
2) transmitter release triggers action potential in CN VIII following the up and down vibration of the basilar membrane
- hair transduction is fast and sensitive - 10 microseconds - essential for sound localization
when is mechanical gating of ion channels essential?
for rapid high-resolution signal of hair cell transduction of sound waves
what happens if there is damage to steriocilia? (high intensity sounds)
leads
to irreversible hearing loss
what happens if there is damage to tip-links?
leads to temporary hearing loss as tip links can regenerate within hours
hair cell receptor potentials follow mechanical displacement of?
- of the hair bundle
- The hair cell can
produce a sinusoidal
wave in response to
low frequency
(<3Khz) sinusoidal
stimulation
at high frequencies, what does the hair cell respond with?
- direct current offset but does not faithfully track the frequency
