Introduction to Sensory Systems Flashcards
What are the four main functions of sensory information?
Sensory information is used to drive behaviour
1. Perception:
- Computational systems (visual system that allows you to see the world.
- Our sensory experiences feel like something.
2. Control of movement
- constantly interacting with the sensory system
3. Regulation of the function of internal organs.
- the sensory system monitors what is happening inside our body.
4. Maintenance of arousal
- sensory system essential to stay awake/alert.
What are the common properties of sensations?
- Modality (quality/ our senses: vision, sense of touch, audition…)
- Submodality: within a modality there are sub categories (e.g. fine touch, warmth, heat…)
- Intensity (dim, light, soft vs strong touch)
- Duration
- Location (location of pressure, usually localized at one specidic area on skin).
Somatic Sensory Systems
The somatic sensory system encodes sensory information about the skin surface (e.g. touch, vibration, temperature, pain) and sensory information from inside the body (e.g. visceral pain).
What are the 2 parallel pathways that convey sensory information to the brain?
- Dorsal column-medial lemniscal pathway (fine touch, proprioception): how we interact with our environment. Sensory feedback from muscles and joints goes up this pathway. Cerebral cortex is the final destination for sensory processing.
- Anterolateral pathway (Pain and temperature): Monitors how your body is doing - tells you about the state of the body.
The dorsal column-medial leminiscal and anterolateral pathways convey different information and have morphological differences.
Primary Somatic Sensory Afferents (Aα, Aβ, Aδ, and C fibers)
Primary Somatic Sensory Afferents(Aα, Aβ, Aδ, and C fibers) detect stimuli on the
skin surface and convey somatic sensory information to the central nervous
system.
Dorsal column-medial lemiscal pathway
- The Aα and Aβ fibers are the fine touch pathway. These fibers start in the skin surface and enter in the spinal cord.
- They stay on the same side as they enter. The 1st synapse for these fibers is in the brainstem.
- The medial leminiscus fibers cross from one side to the other after the 1st synapse then synapse again in the thalamus (2nd synapse).
- The last synapse is in the cortex (stays on same side).
Anterolateral Pathway
- The Aδ and C fibers enter the spinal cord and immediately make a synapse with a 2nd neuron in pathway on the same side.
- It then crosses from the 1st synapse and goes all the way up to the brainstem, thalamus and cerebral cortext on the same side (making a synapse at each).
Morphology of Primary somatic sensory afferents
- They have only an axon - no dendrites.
- The ending of the axon is specialized to transform sensory input into an AP (this is equivalent to the dendrites).
- The sensory receptor propagates the AP through the lenght of the axon.
- The cell body sticks off the side of the axon.
What is the 1st step of sensory processing
- The first step in sensory processing is sensory transduction, which transforms sensory information from the external (or internal) environment into opening (or closing) of ion channels in receptor cells.
- Ion channels are specialized to open under special conditions (i.e. ion channels that open by change of pressure, change of temperature, activated by photons of light…)
- For example, indentation of the skin causes opening of stretch-activated ion channels in low-threshold mechanoreceptors.
- Ion channels open by stretching of skin = flow of Na into channels = trigger AP.
- No stretching of skin = no flow of Na into channel as channel is closed.
- Transformation of pressure on skin to AP.
Transformation of sensory event to action potential
- Pressure on the skin surface is encoded as action potentials in primary sensory fiber.
- Action potential is the currency (all or none, info has to be conveyed in terms of frequency of AP).
- The intensity is conveyed in the frequency of the action potential.
Different types of sensory receptors
Sensory receptors (e.g., the endings of primary somatic sensory fibers) act as filters, extracting specific forms of sensory information and ignoring others.
- Merkel cells (SA1): slowly adapting, fire as long as pressure is applied, stops when no pressure applied. keeps on firing to enable you to get constant info. Fades out over time = desensitazation.
- Ruffini endings (SA2)
- Meissner corpuscle (RA1): Fire at beginning and when end of pressure, not during constant pressure. Function: detecting low frequency vibrations on skin surface and texture on surface.
- Pacinian corpuscle (RA2): detecting high frequency vibration.
- Free nerve ending: Bare axon endings in the skin surface. Function: pain and temperature.
What are the 3 different types of fibers
- slow adapting
- rapidly adapting
- free endings
Labeled Line
- Each submodality is mediated by a specific
receptor/fiber type and a specific labeled line (labelled line = seperate pathway). - Therefore, each type of ending corresponds to one labelled line (ie. pascinian corpuscles only respond to high frequency vibrations so all the pascinian corpuscles in your body are the labelled line for high frequency vibrations).
- Seperate labelled lines for each sensory submodality.
- Seperate endings for all modalities: There is one set of axons for each of these.
- detecting constant pressure
- conveying vibration
- conveying warmth
- conveying painfully HOT
- conveying cool
- conveying COLD
- conveying itch
- Example: there is a subset of neurons that respond to hight heat. Fiber endings are activated when temp goes over a certain threshold.
- this same labelled line can be activated chemically. ie: chilli peppers. Meaning it does not matter how you activate a labelled line, you will feel the same sensation regardless.
Combinatorial Processing
- Perceived sensations (e.g., wetness, color perception, the smell and taste of food) are typically caused by activation of multiple receptor types and integration of multiple parallel channels by the brain, i.e., they involve combinatorial processing.
- Combinatorial processing is combining inputs from multiple labelled lines. Specific sensations we perceive are caused by activating different combinations of labelled lines at the same time.
Terms:
Afferent
Efferent
Ascending
Descending
Ipsilateral
Contralateral
Afferent – incoming information (e.g. primary somatic sensory afferents).
Efferent – outgoing information (e.g. efferent motor neurons).
Ascending – heading up to higher levels of the nervous system (e.g. ascending sensory afferents).
Descending – heading down from higher levels of the nervous system (e.g. descending corticospinal projections).
Ipsilateral – on the same side of the body.
Contralateral – on the opposite side of the body.
Classification of primary somatic sensory afferents
- Large diameter, well-myelinated fibers (Aa, Ab) correspond to specialized sensory endings (e.g. Pacinian corpuscles) and are responsible for fine touch and proprioception.
- Small diameter, thinly myelinated or unmyelinated fibers (Aδ, C) correspond to free nerve endings and are responsible for pain and temperature sensation (pain messages do not need it to be fast - do not need it to coordinate movements).
Receptive fields
The primary somatic sensory neurons innervate a specific region of the body surface. A stimulus within this receptive field will excite the cell.
* Each ending of the branches ends in a merkel cell and each ending innervates a part of the skin surface.
* The primary afferent will make a synapse with neurons in the brainstem. Therefore, the 2nd neuron in the pathway has receptive fields because it will also fire when the 1st afferent fires.
Receptive field size varies.
Receptive fields in high-acuity regions of the body surface (e.g., the fingertips) are small, compared to receptive fields in low-acuity regions (e.g., the torso).
* There are way more neurons in the cerebral cortex for your finger tips than your back (the neurons are conserved all the way up to the cerebral cortex).
Medial and Lateral
Dorsal, Caudal, Rostral, Ventral
dorsal, rostral, caudal, and ventral in humans
What happens when someone has a lesion on one side of the spinal cord?
Spinal Cord
- The human spinal cord is 42 to 45 cm
long and < 1 cm in diameter. - The spinal cord comprises 31 segments
(8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal), each of which
corresponds to the entry point of a pair of spinal nerves. - Cervical and lumbar enlargements correspond to the segments that innervate the limbs.
- The cord is shorter than the vertebral canal, ending at around the 1st lumbar vertebra. The spinal nerves below this level project downward through the
lumbar cistern(where you would stick a needle in to not damage spinal cord) to exit at the appropriate vertebrae, forming the cauda equina (horse’s tail –> where the nerves enter the canal). - The cord enters the skull through the
foramen magnum to form the medulla, the most caudal region of the
brainstem. Spinal cord ends where it enters into the skull (foramen magnum). - Spinal cord does not go all the way down, the rest is filled with CSF. Sensory axons enter the spinal cord.
Cross section view of spinal cord
- The spinal cord comprises central gray matter
surrounded by white matter.
Definition of:
- Gray matter
- White matter
Gray matter – regions of the nervous system consisting mainly of neuronal cell bodies, dendrites and synapses.
White matter – bundles of myelinated axons.
General Cross-Sectional Anatomy of the spinal cord
Primary somatic afferents organization in the spinal cord
- Primary somatic sensory afferents enter the dorsal spinal cord through the dorsal roots.
- Motor neurons project to skeletal muscle fibers through the ventral roots.
Definitions:
- Granglion (ganglia)
- Nucleus (Nuclei)
Ganglion (Ganglia) – An organized cluster of neuronal cell bodies in the peripheral nervous system
Nucleus (Nuclei) – An organized cluster of neuronal cell bodies in the central nervous system.
How many pairs of spinal nerves running down the length of the spinal cord?
There are 31 pairs of spinal nerves running down the length of the spinal cord.
Dermatone
Each pair of spinal nerves innervates a region of the body surface called a dermatome.
- Each pair of nerves is innervatinf a specific sensory and motor in the body.
Dorsal horn, ventral horn, intermediate region in spinal cord
- The dorsal horn of the spinal cord gray matter is somatic sensory.
- The ventral horn is motor.
- The intermediate region contains interneurons and preganglionic autonomic neurons.
Primary afferents for fine touch and pain/temperature
- Primary afferents for fine touch enter the dorsal columns and ascend to the dorsal column nuclei in the brainstem.
- Primary pain and temperature afferents synapse in the dorsal horn of the spnal cord gray matter.
Somatotopical organization of the spinal cord
The dorsal columns and the anteriolateral columns are somatotopically organized (map of the body in the dorsal column).
- As you move up, more fibers come in, they will get layerd.
Sensory-motor hierarchy
The spinal cord is part of a sensory-motor hierarchy that includes the
brainstem and higher brain regions including the cerebral cortex, the cerebellum and the basal ganglia.
- ie, the highest level is the cerebral cortex, which itself is hierarchichally organized.
- the nervous system is hierarchichally organized
What are the simplest goal-directed movements?
The simplest goal-directed movements are spinal reflexes.
* Spinal reflexes are involuntary coordinated patterns of muscle contraction and relaxation. They are elicited by peripheral stimuli.
* Sensory stimuli come from receptors in muscle, joints and skin.
* They produce complex movements that serve protective and postural functions – e.g., withdrawal reflex, stretch reflex.
Basic circuit of the stretch reflex
- It is controlled entirely at the level of the spinal cord.
- Why do we have these types of reflexes? used daily –> hold limb at particular position with changing load
Two types of reflexes
- Withdrawal reflex: remove hand from hot stove.
- Stretch reflex: knee jerk
How are reflexes modulated?
- Discrete stimulus can produce large contraction of multiple muscles,
suggesting divergence of sensory signal. Ie, same sensory input can lead to different motor output (same sensory input can cause contraction of diff muscles). - Both extent and force of muscle contraction depend on stimulus
intensity. Thus, reflexes are modulated by properties of the stimulus. - Reflexes are regulated and modified by descending inputs from higher
brain areas. - Complex voluntary movements utilize existing reflex circuits.
- Reflexes are under control of higher brain areas. We know this because the brain can gate reflexes. Reflexes can be modulated by the brain even though they operate at the level of the spinal cord.
Central pattern generators
The intrinsic circuitry of the spinal cord forms central pattern generators
responsible for rhythmic movements like walking, scratching and the rhythmic
motions of swimming fish.
* There is circuitry that is like a program and once activated it starts the rhytmic motion.
* ie, Walking = message coming from brain when brain decides to walk –> movement systems are hierarchichally organized –> activate walking program.
* Low level = program (rhytmic movement of legs )
* Higher level = speed, start,
Brainstem
- The brainstem is the conduit for information flow between the brain and spinal cord.
- The brainstem comprises the medulla, pons and midbrain.
Fiber tract (tract)
Fiber tract (tract) refers to a bundle of axons in the central nervous system. A bundle of axons in the peripheral nervous system is called a nerve. Axons that extend from one region of the nervous system to another are often referred to as projections.
Projections describe the axons that are heading towards something.
Important structures of the brainstem
Here’s another sagittal section, showing some of the important structures of the brainstem, including the fourth ventricle, the cerebral aqueduct, the superior and inferior colliculi and the pineal gland.
Cross section of caudal medulla
- Used a stain that stains white matter (dark regions = bundles of axons).
Cross section a bit further up the medula.
- Dorsal column axons synapse with the 2nd order neurons in the dorsal column nuclei of the medulla. The nucleus gracilis receives input from legs and lower body, whereas the nucleus cuneatus receives input from the arms and upper body.
- Fasciculus gracilis and fasciculus cuneatus are being replaced by the 2nd nucleus.
Cross section of junction between medulla and pons
In the rostral medulla, the axons of the dorsal column nuclei have crossed to the contralateral side of the brainstem to form a fiber tract called the medial lemniscus.
* This is why your right brain is in charge of fine touch on the left side of your body.
* Brainstem no longer looks like the spinal cord (new shape).
* Brainstem is spreading out and a space appears = 4th ventricle.
Cross section of Pons
Ascending somatic sensory tracts
* brainstem starts spreading out even more
* as you move up the brain, the medial leminuscus spreads laterally.
Cross section of Midbrain
ascending somatic sensory tracts
Cranial nerves
- 31 pairs of spinal nerves (sensory info in, movement info out).
- brainstem nerves = cranial nerves = various functions.
Function of each cranial nerve
IMPORTANT
General organization of cranial nerve nuclei
Error on image: Nerve V is not a visceral sensory nerve
Cranial Nerve Nuclei in the Brainstem.
Cross section
- There are some cranial nerves that are carrying sensory information to the nuclei in the brainstem.
- There are some cranial nerves that have their cell body in the axon.
- There are autonomic sensory neurons.
- Parasympathethic output.
- There are nucleis for each of these and they have a pattern of organization.
- Same organization as spinal cord but more spread out
Cranial Nerve nuclei in the brainstem
Straight on view
- Nucleis are spread out to varying degrees along the entire brainstem
Motor nuclei = medial
Autonomic nuclei = in between
Sensory nuclei = lateral
Parasympathetic vs sympathetic
Parasympathetic = visceral motor in brainstem. Parasympathetic is exclusively from the brainstem.
Sympathetic = exclusively from spinal cord. Increases heart rate and the strength of heart contraction.
- The sympathetic and parasympathetic systems tend to have opposed effects on target tissues (fight or flight).
- The parasympathetic stimulation decreases heart rate and contraction (caused by activation of vagus nerve)
Sympathetic preganglionic neurons
Sympathetic preganglionic neurons release acetylcholine which activate nicotic acetylcholine receptors on postganglionic neurons. Postganglionic neurons release norepinephrine, which activates alpha and beta adrenerguc receptors on the target organs.
Parasympathetic preganglionic neurons
Parasympathetic preganglionic neurons emerge from the brainstem (cranial nerves III, VII, IX and X) and the sacral spinal cord. These preganglionic neurons extend almost all the way to their peripheral targets.
Reticular formation
Caudal medulla and Rostral medulla
The central core of gray matter in the brainstem is reffered to as the reticular formation.
- looks very similar to spinal cord but it is a section through the brainstem at the caudal medulla.
Reticular formation
Pons
Key point: central core of grey matter (reticular formation) is always there.
- Contains lots of neurons and synapses that carry out the functions of the brainstem.
Functions of reticular formation
The reticular formation is involved in numerous integrative functions of the brainstem, e.g.,
* Stereotyped motor responses
* Autonomic functions
* Ascending arousal (reticular activating system) –> system that sends axons up to cortex which is involved in keeping you awake and conscious. Brainstem is required to keep your cerebral cortex awake.
- The cranial nerve nuclei (nuclei = grey matter) are part of the reticular formation. The cranial nerve nuclei are involved in various kinds of movements of the head, eyes, face. They are aslo involved in various kinds of sensory and autonomic functions (controlling visceral organs, dilation of pupils, secretion of saliva).
- All of these autonomic functions are carried out by clusters of nuclei that are embedded withing the reticular formation.
Stereotyped motor responses.
- The brainstem is an organizing center for a variety of stereoptyped motor programs, including facial expressions, chewing, swallowing.
- The sensory motor function/stereotyped responses is hierarchically organized.
Periaqueductal gray (PAG)
The periaqueductal gray (PAG) exemplifies the integrative functions of the reticular formation.
- The PAG is connected to regions of the brainstem involved in autonomic, somatic and behavioral responses and to higher brain regions involved in fear.
- Stimulating the PAG in cats results in increased respiration and cardiac function, piloerection, hissing, and arching of the back. In mice, stimulation results in freezing or escape responses. These are stereotyped behaviours meaning the cat will do them the same way every single time and it is not thinking about it - it just happens automatically.
- These complex sets of behaviours and outputs is controlled at the leavel of the brainstem and is coordinated by a region of the periaqueductal gray.
- The periaqueductal gray isn’t actually causing all of the behaviors, but what its doing is its communicating with other parts of the brainstem that are involved in the specific elements of the response. The different outputs come from other parts of the brainstem but the PAG controls all the outputs.
- Stimulation in humans causes stereotyped defensive behaviors and feelings of anxiety.
- hypothalamus controls PAG, PAG controls outputs of other regions in brainstem (hierarchy of control).
- If you stimulate PAG or use opioids you get analgesia = sensation of pain goes away.
- The brainstem/the PAG can communicate with the spinal cord and vice versa.
- Among the responses that stimulation of the PAG elicits involves activation of the sympathethic nervous system is because the PAG is communicating with the spinal cord.
- All these different regions are not acting in isolation, they have their own functions but they are all connected to each other (brainstem, spinal cord, hypothalamus, cerebral cortex).
- Periaqueductal gray is under control from higher brain centers (hypothalamus and cerebral cortex)
Somatic sensory cranial nerve nuclei
- Multiple nucleis that can be lumped together as one continuous nucleus
- It is getting somatic sensory input from the head (mainly from the trigeminal nerve).
- This nucleus (V) is getting fine touch and proprioceptive input that is coming in from the head and then those sensory axons from the head are coming in and making synapses in this nucleus (V).
- What is the equivalent of this for the spinal nerves = nucleus cuneatus and nucleus gracious.
- Cranial nerve somatic sensory nucleus contains the 2nd order neurons not only for for fine touch in your head but also for pain and temp in your head. The equivalent for this for pain and temp coming in from your body is 2nd order neuron in the dorsal horn of spinal cord.
Loss of fine touch on the right (ipsilateral) foot.
Loss of pain/temperature on the left (contralateral) foot.
If the arms are entering the spinal cord above the level of the lesion then nothing will happen to your arms
Fine touch in spinal cord
Pain and temperature
Ascending Arousal System
- it turns out the reason you are conscious is because of activity in your cerebral cortex (and sub cortical structures interacting). The cerebral cortex is what makes you a conscious being.
- Without the input coming up from the brainstem, you lose consciousness
- Experiments that were done and illustrated the essential role of the brainstem in consciousness and alertness.
- In cats, lesions at the level of the caudal medulla did not affect arousal; however lesions in the rostral pons/caudal midbrain resulted in a persistent sleep-like state. “This suggested an ascending arousal system that started in the brainstem and “activated” the cerebral cortex.
- they used electroencephalogram (EEG) to record the electrical activity in the brain. You are recording average behaviour from thousands of neurons that are in the vicinity of the electrode. Because the neurons are busy doing their own thing - their activity is cancelling out . You end up with low frequency random noise (low amplitude). When you drift off to sleep, the EEG changes it starts to oscillate (rhythmic oscillations). Amplitude gets really big because when you drift off to sleep neurons fire large AP in rhythmic bursts. All firing at the same time so that activity adds up.
There is something in the brainstem that is actively involved in keeping the animals awake and when you cut it off, the animal goes into sleep.
- there was some system that was connecting the brainstem with the higher brain areas and it was invovled in sleep-wake cycle and alertness (ascendinf arousal system).
The ascending arousal system
Ascending projections from the brainstem arousal system comprise a dorsal pathway that goes through the thalamus and a ventral pathway that project directly to the cortex.
- Long axons that start in the brainstem and extend all the way up into cerebral cortex.
- Ventral pathway (blue): axons in the neurons go all the way up the cerebral cortex.
- Dorsal patway: goes up the brainstem to the thalamus and then the neurons in the thalamus project up all the way to the cerebral cortex.
- These 2 ascending systems collectively play this role in sleep, wake cycle.
Norepinephrine neurons
Norepinephrine neurons originating in the locus coeruleus project to virtually the entire brain as well as the spinal cord. The ascending projections are involved in sleep-wake cycles, arousal and attention.
* Norepinephrine is a neuromodulator that is involved in speep wake cycles and attention.
* There is norepinephrine is being released all in your cerebral cortex. The norepinephrine comes from the locus coerulus (2 of these in the brain).
* How can such a small number of neurons influence such a big area. The axons of these neurons go up and when they get to the cortex they branch and keep branching. Small number of neurons but have vastly branching number of axons (norepinephrine is being released globally).
How do the norepinephrine neurons behave?
NE neurons fire during wakefulness,. Firing decreases during slow wave sleep. NE neurons are silent during REM sleep.
- if you suddenly wake up they start firing again.
- these are involved in putting you to sleep and keeping you awake when you are awake
Norepinephrine neurons role in atention.
- vertical spikes on image = action potentials
- At the point where something catches your attention and you need to change your attention suddenly (redirect your focus). There will be a burst of action potential.
- If you are in a situation where you are paying attention to many different things (scanning), the norepinephirene neurons are firing a lot. If they fire too much you are all over the place!
- These neurons are plaing a role in elicited attention to a stimulus.
Serotonin Neurons
Serotonin neurons originate in the brainstem raphe nuclei and project throughout the brain and spinal cord.
- serotonin is involved in regulation of mood (more serotonin = happier).
Dopamine neurons
Dopamine neurons originate in the substantia nigra (parkinsons) and the ventral tegmental area (involved in learning certain associations). They project (from the substantia nigra) to the basal ganglia and (from the VTA) to the cerebral cortex.
Dopamine signaling is involved in cognition, movement and learning. Addictive drugs increase dopamine release.
- important in learning that a certain stimulus is inolved in a certain outcome (ie, compulsive gambling - release of dopamine when arrive at casino).
Acetylcholine Neurons
Acetylcholine neurons originate in the basal forebrain and project to the cerebral cortex, the hippocampus and the amygdala. ACh in the brain acts through ionotropic nicotinic receptors and metabotropic muscarinic receptors. ACh neurons are among the first to die in Alzheimer’s disease.
-acetylcholine is a neuromodulator in the brain (activates a metabotropic receptor - muscaranic receptor).
- Histamine is another neuromodulator involved in sleepwake cycle. It helps keep you awake. Antihistamine blocks the receptor to histamin = feel sleepy
Sensory information moving through brainstem, thalamus, cerebral cortex.
- Sensory information is transformed as it moves through each synaptic level. In other words, the receptive fields of output neurons are different (generally more complex) than the receptive fields of input neurons.
- Information that comes out of the cerebral cortex can be very different than what goes in to the cerebral cortex. The cerebral cortex transforms information (ie, processing complex info to make it coherent).
Convergence vs Divergence
Higher order neurons in the sensory pathway have more complex receptive fields than primary afferents, in part, because of convergence and divergence of lower order inputs.
Convergence: single neuron in brainstem (ie, complex input from many neurons). Receiving sensory info from skin gets input from many sensory neurons.
Divergence: single primary afferent, the axons split and innervate multiple secondary nuclei. This is happening because neurons are combining together info (ie: combined in more complex ways to integrate info so you can assemble coherent sensory information).
Lateral Inhibition
Lateral Inhibition allows a strong central signal to pass through but filters out weak surrounding signals. This mechanism sharpens the contrast of of sensory signals.
Thalamus
- Sensory information passes through the thalamus to reach the cerebral cortex.
- The thalamus is the gateway to the cerebral cortex.
- To get to the cortex, you must go through the thalamus. Any information that wants to get to the cerebral cortex must go through the thalamus first (in almost every case).
Three different planes
The brain can be sectioned on three different planes, called coronal, horizontal and sagittal.
Coronal and Midsagittal section
- When we cut through the mid section, we are sectioning them in two (not cutting through them).
- to identify the thalamus (anatomical detail): the spaces above the thalamus in the coronal section are called the lateral ventricle (black openings) - the thalamus is easy to find in a human brain because it forms the floor of the ventricles.
Horizontal section
Organization of thalamus
The thalamus is organized as numerous discrete nuclei, which can be arranged into three functionally distinct groups: specific relay, association and intralaminar.
* Thalamus = collection of nuclei
* Specific nuclei involved in relaying specific info.
Specific relay for somatic sensory input
ventral posterior lateral (VPL) and ventral posterior medial (VPM) relay somatic sensory input to primary somatic sensory cortex (S1).
* somatic sensory info will converfe VPL and VPM - converge info and synapse on these 2 nuclei.
* VPL = conveying somatic sensory info from body
* VPM = conveying somatic sensory info from head.
* All seing info is passing through to get to the cerebral cortex.
Specific Relay for visual information
e.g. the lateral geniculate nucleus relays visual information to primary visual cortex.
* Able to direct your attention to specific objects by allowing some info to come in and docking info (thalamus as a gate).
Specific relay for auditory information
e.g. the medial geniculate nucleus relays auditory information to primary auditory cortex.
Two regions of brain (subcortical regions) involved in movement
The ventral anterior and ventral lateral nuclei are specific relay nuclei that connect the cerebellum and basal ganglia(collection of nuclei) with motor cortex.
Specific relay nuclei of the thalamus
Association nuclei
- Most of the thalamus is dedicated to different kind of nuclei called the association nuclei.
- two main association nuclei: along the midline (medial dorsal nucleus) and one enormous one called the pulvinar. - The association nuclei are interconnected with regions of association cortex. These two nuclei connect different regions of the cerebral cortex together.
- These neurons can have axons that connect different regions of the cerebral cortex together. There are neurons that send axons from the cerebral cortex to the pulvinar and neurons that send axons from the pulvinar to specific regions of the cerebral cortex.
- The medial dorsal nucleus is connected to the prefrontal cortex, which is responsible for executive functions. The Pulvinar nucleus projects to parietal-temporal association cortex, which integrates sensory information and mediates language.
- Throughout the cortex, there seem to be 2 different ways that different regions of the cortex can communicate together. They can either communicate w each other directly (one region can be directly connected to the other) or they can communicate indirectly (by going down to these association nucleis in the thalamus and then back up to the cerebral cortex).
- Thalamus seems to have this property that it can regulate which inputs gets passed on to the next level.
- Indirect pathway: connections that go through the thalamus: cortex to thalamus back to cortex. Direct pathway: regions are connected together so they communicate together.
- They are called the association nuclei because they send their outputs to the association cortex in the cerebral cortex –> most of the cerebral cortex is involved in higher level and this is called association cortex.
* do not need to know what these connections are for
Association Nuclei of the Thalamus
medial dorsal = connecting different regions in the frontal lobe
Pulvinar = connecting different regions in the back
The intralaminar Nuclei
The intralaminar nuclei project diffusely to many regions of the cortex as well as subcortical structures such as the basal ganglia.
- The internal medullary lamina are clusters of neurons embedded. They are part of the arousal system
Thalamic nuceli and reciprocal feedback
- The thalamic nuclei receive reciprocal feedback from the cortical regions to which they project.
- The axons from the cortex to the thalamus vastly outnumber the projections from the thalamus to the cortex.
- Every single thalamic nucleus is getting input from the cerebral cortex (feedback input)
- Ie: LGN has more projections going from the cerebral cortex to LGN (feedback projections). These are used to modulate the thalamus.
- For every region of the thalamus there are feedforward and feedback projections. There are way more feedback projectuons than feedforward projections.
Feedforward vs Feedback projections
Feedforward: projections go up the sensory pathway. ie: LGN –> visual cortex or Retina –> LGN –> primary visual cortex.
Feedback: projections go back down the sensory pathway. Higher levels regulate/tweak what is happening in lower levels. Ie: regulating attention. Ie: primary visual cortex –> LGN this allows you to focus/zero in on one thing.
Drivers versus Modulator inputs to the thalamus
Cerebral cortex:
- thin sheet of tissue (2-4mm thick)
- divided into 6 layers (layer 1 = outermost, layer 6 = deepest)
Drivers:
feetforward projections that go from relay nucleus to layer 4.
ie: Driver input come from the Retina. If driver inputs (neurons in the optic tract) start firing APs then it will cause neurons in the LGN to fire APs - they are driving AP activity in the thalamus.
Modulators:
feedback projections that come back from layer 6. This enables the cortex to tweak what is going on in the thalamus.
ie: modulator inputs come from layer 6 and go to LGN
When modular neurons are activated they do not cause the other neurons to fire APs. Ie, when neurons in layer 6 are activated, it does not cause the neurons in the LGN to fire APs. It modulates the physiological properties of the neurons in the LGN (ie, changes the AP thershold or the membrane potential).
What are these modulatory inputs doing?
- The Thalamus is a gatekeeper, regulating information flow to the cortex.
- Not all of it will get into the cortex –> some may be altered or changed (regulating what is relevant).
Therefore, 1 hypothesis is that these thalamus and these modulatory inputs are regulating what gets through to the cortex and altering the properties of these neurons.
The connections are always there (ie between cortex and LGN or between VPL/VPM and cortex) but maybe only some of the neurons are actually active because other neurons activity is being supressed.
Thalamic neurons (physiology)
Thalamic neurons switch between two different firing modes:
* During wakefulness they are in a transmission mode, in which they fire single action potentials that faithfully relay sensory inputs (i.e. if the sensory inputs increase their firing rates, the thalamic neurons increase their firing rates).
- when a thalamic neuron is in transmission mode, its output is APs. It mirrors its input. The thalamic neuron is replicating the pattern of its inputs APs. It is faithfully transmitting that pattern of AP firing up to the cerebral cortex.
- this is the mode that the thalamic neurons are using when they are simply relaying info up to the cerebral cortex.
* In the bursting mode, thalamic neurons fire bursts of 3 – 8 action potentials (they are happening so fasts (rate of 100 APs/s) that you cant resolve the individual AP), separated by quiet periods lasting 100s of milliseconds.
- the membrane potential has this complex wave form shape: slow depolarization, reach AP threshold, burst of APs and then after a very short period of time the AP stops and then the neuron slowly hyperpolarizes and stops firing AP. Cycle can repeat itself.
- Bursting mode is involved in the sleep wake cycle, when you go into slow wave sleep, the neurons in your cortex start to fire in rhythmic bursts. The thalamus is doing this too (both thalamic and cortical neurons are bursting in SWS).
- one hypothesis is that when the thalamic neurons are bursting it is blocking the flow of information to the cerebral cortex. blocking info from getting to your brain during sleep.
- The bursting behaviour involves complicated interactions between the cerebral cortex and the thalamus.
- These bursts also happen when you are awake. When you are awake the modulary inputs are thought to be changing different groups of thalamic neurons into either bursting or transmission mode.
What causes neurons to switch between the bursting and transmission modes?
The resting membrane potential
The resting membrane potential determines whether a thalamic neuron is in transmission mode or bursting mode. If the resting potential of the neuron is more positive than ~ -65 mV it is in transmission mode. If it is more negative
than -65 mV it is in bursting mode.
image: experiment where they injected current to either hyperpolarize or depolarize the cell’s resting membrane potential. Start at a fairly negative current (neuron is in burst mode) then inject positive current, neuron switches to transmission mode. Hyperpolarize neuron again and it goes back to the burst mode.
Therefore, by modulating the resting membrane potential of the neuron we can switch the neuron from the bursting mode to the transmission mode.
What do modulators coming from the brain stem do?
Neuromodulators, such as acetylcholine, norepinephrine and histamine, which
are released during wakefulness, depolarize thalamic neurons, shifting them
from burst firing mode to tonic firing mode.
- The modulators coming from brainstem to depolarize the resting membrane potential of the neuron ans shift them into transmission mode.
- ie: norepeniphrine neurons stop firing when you to go sleep, so you are not getting norepeniphrine in the thalamus when you are sleeping meaning the neurons will hyperpolarize and go into the bursting mode. But when you wake up the norepinephrine neurons start firing AP so they release norepinephrine into the thalamus and then depolarize the resting membrane potential of the thalamic neurons shifting them to the transmission mode.
How does the activity in the thalamus get up to the cerebral cortex - what is the pathway that the axons follow to get from the thalamus up to the cerebral cortex?
Information to and from the cerebral cortex travels through a massive fiber tract called the internal capsule.
- internal capsule is an internal landmark that will help us identify different structures inside of the brain.
Internal capsule from horizontal view of the human brain
Internal capsule is like a thin sheet, a narrow point where all these axons have to squeeze through this narrowing and then once they get up to the cerebral cortex, they spread out and they innervate the entire cerebral cortex.
Internal capsule is the main pathway for the cortical structures to get to and from the cerebral cortex.
Internal capsule goes to the brainstem. The internal capsule also contains the axons of neurons in the cerebral cortex that are going out of the cortex and going down to the brainstem and spinal cord to control voluntary movements
Cerebral cortex
The cortex is a thin sheet of tissue
* It ranges from ~ 2 – 4 mm in thickness (thin but has a large surface area)
* It’s basic architecture is nearly identical over a broad range of species (e.g. from rat to human).
- all have the same basic organization (layers)
- fundamental difference is that we have way more cortex (bigger surface area) explains our bigger cognitive abilities - our cortex has expanded).
Sulci and Gyri
The human cortex has many folds, which enable the large cortical surface area to fit inside the skull. The folds are called gyri (sing. gyrus), whereas the grooves between the folds are called sulci (sing. sulcus) or fissures.
The cerebral cortex can show substantial structural variability in different individuals
- These brains dont look the same.
- Different human brains look different.
- in almost all human brains you can identify the different sulcus and gyri. They have a distinct pattern.
Two prominent landmarks
Nevertheless, he typical human brain has identifiable sucli and gyri. Two especially prominent landmarks are the central sulcus
(Rolandic fissure) and the lateral sulcus (Sylvian fissure).
- Cortex forms a “c” shape
- Sulcus divide regions into lobes
- Sulcus are external landmarks.
Lobes of the brain (lateral view)
The lateral surface of the cerebral cortex is divided into four lobes
The 5th lobe and its sulcus
A lot of the cerebral cortex is hidden within the longitudinal fissure. The four lobes are represented within this medial surface, as well as a 5th lobe called the limbic lobe, which is bounded by the cingulate sulcus.
- in order to see this slice of the brain, you must cut along the longitudinal fissure.
The two hemispheres are seperated by…
The two cerebral hemispheres are separated by a deep groove called the longitudinal fissure.
Corpus Callosum
The corpus collosum connects the two cerebral hemispheres –> huge sheet of axons connecting the 2 hemispheres.
How many layers is the cortex made of? name them
The cortex comprises six layers.
Layer IV
- Layer IV is the main recipient of afferent inputs from the thalamus.
- Comprised of small neurons called stellate 4. These are excitatory neurons (release glutamate).
- They do not have very long axons, axons only go to superficial layers.
- Thalamus axons synapse in layer 4. Layer IV receives input from thalamus (ie: LGN axons end in layer IV of primary visual cortex).
Layers II and III
- Layers II and III consist of pyramidal neurons. Layer III is the external pyramidal layer and layer II is the external granule layer (but these are pyramidal neurons too).
- Distinction between layer II and III is that the pyramidal neurons in layer II are smaller. Pyramidal neurons are bigger in layer 3.
- Pyramidal neurons in layers II and III make feedforward projections to other regions of cortex.
- They send feetforward projections - enabling one region of cerebral cortex (low level) to communicate with another region in the cerebral cortex (higher level).
- A lot of the axons are going to go down to the underlying white matter to send feetforward prokections to other parts of the cerebral cortex.
Layer V and VI
- Neurons in layer V and VI project to subcortical targets.
- Neurons in layer V are sending outputs from the cerebral cortex to subcortical structures (ie: neurons in the motor regions of cortex involced in controlling voluntary movement, they have axons that go down to the brainstem and spinal cord to control voluntary movement).
- So when the cortex wants to communicate with the basal ganglia, the brainstem, the spinal cord…all of these outputs are going out from layer 5 pyramidal neurons.
- Feedback projections to the thalamus come from layer VI. Neurons in layer VI provide the feedback projections from the cerebral cortex to the thalamus.
This explains why the association thalamic nuclei get their driver inputs (from layer V) (inputs that drive them to fire AP) from the cerebral cortex and their feedback projections(from layer VI) from the cerebral cortex. The driver inputs from layer V are firing the AP in the thalamic nucleis. The modulator inputs from layer VI are modulating the overall physiology of the thalamic neurons - ie, shift them between bursting mode and transmission mode.
Does the thickness of the layers of the cerebral cortex stay constant from region to region?
The thickness of the various layers varies in different regions of cortex.
* Different layers are more pominent in different structures:
- Primary Visual cortex:
- large layer IV (very prominent) due to it getting a large amount of sensory input.
- not prominant layer III and V
- Primary motor cortex:
- Prominent pyramidal layers which reflex the outputs that's coming out of the primary motor cortex.
- Layer IV is almost completely absent
Primary sensory regions tend to have a large layer IV and smaller layers III and V.
Thickness of the cortex also varies: ie primary visual cortex is thinner than primary motor cortex.
How do axons end in layers?
Neurons in the cerebral cortex: dendrites blue and axons red.
* Even though the neuron is located in layer V, its dendrites span all the layers. The dendrites extend into layer VI and layer 1… so this neuron even though it is in layer V is getting inputs from all the cortical layers.
* The axons of this neuron is heading into layer VI and has a ton of branches so when the neuron fires an AP, the AP qill propage down into the endings of all these axons. Therefore this neuron is making synapses with a whole bunch of other neurons that are all over the cortex (divergence)
* Can see axons coming up from LGN and ending in layer IV. Axon branches to make a cluster of synapses in layer IV.
Axons do not tend to end in a single presynaptic terminal, they end in these clusters of presynaptic terminals. A single axon can have many many brances that extend all over the place.
Columns of the cortex
The cortex is organized into vertical columns, ~ 100 – 500 μM in diameter.
* The neurons in a column are highly interconnected and show similar
response properties.
* The column is the fundamental functional module of the cortex.
* The general organization of the cortex: flat sheet is broken up into functional processors/modules (these columbs)
Occular dominence in primary visual cortex
Example of functional columns in cortex. They are actually in complex shapes. All neurons in white strip respond to another eye.
Orientation columns in primary visual cortex
Neurons that respond to light in particular bands
How are neurons connected together within a singular column
Within a column, neurons in layer IV project to layers II and III. Neurons in these superficial layers, in turn, project to layers V and VI and to other cortical areas.
* There is a basic flow of information within a column in the cerebral cortex (illustrate in the image).
Example of this flow in the primary somatic sensory cortex:
- The inputs are coming in from the thalamus (VPL/VPM nuclei for somatic sensory). These inputs end in in layer IV. They will make excitatory synapses on the little neurons called stellate cells).
- Those neurons in layer IV will send their axons up to the superficial layers (layers II and III).
- The pyramidal neurons in layer II and III will send outputs to other regions of the cerebral cortex (they could send axons that would go to the other side of the brain). These outputs wil go out from the white matter and head off to some other part of the cerebral cortex.
- There will be branches of these axons that will make synapses with the neurons in the deep layers V and VI. The outputs of neurons in layer V will leave the cortex and go back to the thalamus. While the outputs of neurons in layer VI will leave the cortex and go subcortical structures (brainstem,spinal cord…).
General flow of info: up to layer 4 –> up to superficial layers (II and III) –> back down to deep layers (V and VI) –> out of cerebral cortex
Different regions of the cortex are specialized for different functions
- Each sensory modality has a region in the cerebral cortex where information is first sent.
- Projections from thalamus (ie:VPL,VPM) go to the cerebral cortex (specifically in the primary somatic sensory cortex)
- Primary somatic sensory cortex and Primary motor cortex: these 2 regions are constantly communicating.
- Primary visual cortex: axons from LGN go to the primary visual cortex (most of this region is on the medial surface of the brain).
- Primary auditory cortex: axons that leave the medial geniculate nucleus go to the primary auditory cortex.
Primary somatic sensory cortex location
Primary somatic sensory cortex is located in the postcentral gyrus
Somatotopic organization
Somatotopic organization is preserved up to the level of primary somatic sensory cortex.
* Patients are awake during surgery (local anesthethic for the skin)
- find out what happens when you stimulate different regions of cortex.
- led to somatotopic map (not proportional - hand and faces overly represented).
- Because of importance of precision in these somatosensory information. Hands - packed densely - smaller receptive field. Huge number of primary afferents.
Primary somatic cortex - how the body is represented in the primary somatic cortex. Mapping of body parts is conserved all the way up to the cerebral cortex.
* Map of the primary sensory cortex and primary motor cortex are lined up.
Somatic sensory association cortex
- Neurons in SI project to higher order processing areas in cortex, including SII and Area 5 (somatic sensation is processed at higher levels in SII and BA5 - they are processing things in different ways).
- These areas are referred to as somatic sensory association cortex. They are
specialized for more complex somatic sensory processing. - Somatic sensory association cortex (goes to the next higher level in sensory processing).
Primary somatic sensory cortex (S1)–> Secondary somatic sensory cortex (S2) + BA 5 (higher order somatic sensory processing) - these 2 regions are the somatic sensory association cortex.
Unimodal association regions
Unimodal association regions are involved in higher order processing of individual sensory modalities.
* Primary visual cortex is located at very back (grey).
* Vision processing takes up a huge amount of the cortex because we are very visual animals so huge part of cerebral cortex is taken up by visual association cortex.
* Visual system is gradually asembling visual perceptions from fundamental basic building blocks.
* In the somatic sensory system these building blocks are ll the little receptive fields all over your body.
* There are also regions that serve the analogous function in terms of movement, except in movement everything is going in the other direction. Meaning in movement you start with an abstract goal. I want to lift my hand up and then that is transmitted in a series of stages down to the final commands that go out to the muscles in your arm that enable your arm to go up. So its a similar kind of organization but its going in the opposite direction.
Main difference between sensory system and motor/movement system: they work in opposite directions. Sensory system (start with a pixalated view of the world) and movement system (start with abstract goal, think about what you want to do and then does it).
huge amount of cortex is dedicated to vision.
Flow of information between 2 regions of the cerebral cortex
Feedforward projections originate in superficial layers and project to layer IV. Feedback projections originate in deep layers and avoid layer 4.
Primary Visual Cortex (V1) and Secondary visual cortex (V2). Same concept for S1 and S2.
* So sensory information is going into V1and being processed in V1 and then being sent up to V2.
* When one region of the cortex wants to communicate with another, the outputs come from pyramidal cells in layer II and III. So when V1 wants to send information to v2; the pyramidal neurons in v1 have axons that project to layer IV of V2.
* The connectipn between V2 and V3 are going to originate in layers II and IIIof v2 and end in layer IV of V3.
* Generally speaking, layer IV is a input layer that is receiving feetforward projections that are moving up the sensory processing hierarchy.
feetforward and feedback projections
- These projections are going through the white matter (fairly long connections). ie, V1 and V2 are 2 seperate regions of the cerebral cortex.
- Feetforward projections from LGN to layer IV in V1. Feedback projections are coming from layer VI of V1 and going back to LGN
- feetforward projections output from layer 2 and 3 and go to different region in cortex (down to white matter) and synapse in layer 4 (input layer). Layer 4 receives the feetforward projections.
- feedback projections at every single layer in the cortex that enable higher level sensory processing to look back down on lower levels come from deep layers (5and6)
- feedback projections come from deep layers and they feedback to either superficial layers (layer 1,2,3) or to deep layers (layer 6). They avoid layer 4 because that is the input layer.
Receptive fields in the cortex.
- Receptive fields in somatic sensory association cortex are larger than in primary somatic sensory cortex.
- Ie, if you touch the skin, it will stretch the skin at that location, which will activate some sensory neurons that has its ending right in the finger tip and that neuron will fire AP. That little patch in the finger tip is the receptive field for that neuron. And these receptive fields apply all the way up.
In image below:
- found a neuron that if we touch the specific grey region on the finger that neuron in the brain fires APs. Therefore, that little patch on the finger is the receptive field for that neuron in the cortex.
These receptive fields in the cerebral cortex are not the same as the receptive fields of the primary afferents. They change in two ways:
1) they get bigger, as we move up into the cortex, further up the sensory hierarchy, receptive fields get bigger and bigger. ie: a single neuron in primary somatic sensory cortex might have a receptive field that corresponds to a patch on the finger, but that patch might represent a few 100 primary afferents. So this neuron is going to respond to touching this fingertip, but it’s going to respond to a fairly large patch on the fingertip compared to the primary afferents. The receptive fields get bigger because of convergence; the neuron in the primary somatic sensory cortex is receiving convergent informatuon coming in from many other neurons that represent a somewhat larger patch of the skin surface than the primary afferents do. Receptive fields get larger because informatuon that is flowing up converges. So a lot of information from a large patch of skin is assembled together to form a large receptive field for this neuron.
2) the reason the receptive fields get larger is because it enables the neurons at higher levels in the cortex to understand the whole big picture. Because what is coming up is all the pixelated information. So that neurons can understand what the sensory input is in a larger context.
- if we move to higher levels (ie; association cortex), the receptive field gets even bigger. f we were looking at a neuron in some higher level somatic sensory association cortex, that neuron might respond to any stimulus anywhere on the hand, any one of the fingers on, on this hand. Allows the neuron to interpret info coming from the entire hand. Understand how the information is integrated together so that you can fell the object and understand what its shape is.
- enables us to interpret and understand the world.
Neurons that respond to both hands.
This neuron responds to sensory stimulation of either hand. Combine info that is coming in from both hands. Enables you to understand the shape and texture of objects.
Neurons sensitive to stimulus moving in a specific direction
- This neuron is most sensitive to a stimulus moving in a specific direction.
- There are neurons that only fire if the stimulus is moving in particular direction. These are called (direction selective neurons).
- ie; a neuron responds to stimulus anywhere on the hand as long as its moving in the specific direction. Big receptive field but will only respond if hand is stroked in the correct direction.
- This is occuring because of convergence. Smaller, less complex receptive fields are converging on these neurons to create larger and more complex receptive fields.
Dorsal vs ventral association areas
- Dorsal sensory association areas enable sensory information to be used to direct action.
- Ventral association areas are involved in object recognition and memory.
- every modality has a ventral and dorsal region and pathway.
- for somatic sensory information, it is first processed in S1 and then send off to higher order regions (s2 and BA5). So there is a dorsal projection and a ventral projection. Same is true for visual and auditory system.
- 2 flows or pathways for sensory information:
- dorsal pathway: Using sensory information to guide movement and action.
- ventral pathway: use sensory info to understand the world. the “what” pathway. Allows to know what something is. Involved in our conscious perception of things.
Regions of association cortex
1) The part in the front of the brain: frontal association cortex (PFC). Especially involved in executive function (decision making, planning, imagining future, inhibating innapropriate behaviours, making decisions.
2) Posterior multimodal association cortex : involved in combining together the different sensory modalities. Using that info to enable high level cognitive processing (combining the information together that enables us to move our arms and limbs in in purposeful ways that we can interact with the world and enabling us to understand our sensory perceptions),
Thalamic association nuclei are interconnected with association cortex (pulvinar many interaction with posterior multimodal association cortex) of thalamus and medial dorsal nucleus (interacting with frontal association cortex).
Wernicke’s area
- In most people, language is mainly lateralized to the left hemisphere.
- Lesions to left -hemisphere posterior association cortex can result in aphasia – loss of language function.
- Wernicke’s area is a higher order auditory region in the parietal lobe that is involved in liking the soinds of words with their meaning (language comprehension).
- Wernickes aphasia:
- non sensical, fluent language
- uses wrong words or giberish
- cannot identify the meaning of words
- also has difficulty comprehending was is being said.
Broca’s area
- Left-side lesions to more rostral regions of the cortex can result in Broca’s aphasia.
- Broca’s area = language production. Because it is close to movement areas for the face/ mouth.
- Lesions to Broca’s area: difficulty producing language.
- slow and labour intensive
- keeps answers very short, misses linking words (and, the..), can’t say some words.
- hard time understanding complex senstences (more complex=more difficult).
Deficits of split brain patients
- Project something to just right visual field. Image will be in left cerebral hemisphere (language hemisphere). Ask “what do you see?” “hammer”
- Project the same image (hammer) to the left visual field. The person would say “I do not see anything”.
- If you then ask the same person to reach for the object with their left hand (even though they don’t see it), they will grab the hammer.
