The Somatic Sensory System (text book based)

Question 1

What is the somatosensory system?

Answer 1

Mediating a range of sensations—touch, pressure, vibration, limb position,heat, cold, itch, and pain that are transduced by receptors within the skin, muscles, orjoints and conveyed to a variety of CNS targets.

Question 2

What are tactile and proprioceptive subsystems?

Answer 2

The mechanisms responsible for sensations of pain, temperature and coarse sensual touch

Question 3

What are mechanoreceptors?

Answer 3

All receptors trigger sensory transduction– energy of a stimulus is converted electrical signal

Two types of receptor : Rapidly and Slowly adapting – convey information about static and dynamic qualities of a stimulus

Question 4

What is the fundamental mechanism of sensory transduction?

Answer 4

the process of converting the energy of a stimulus into an electrical signal—is similar in all somatosensory afferents: A stimulus alters the permeability of cation channels in the afferent nerve endings, generating a depolarizing current known as a receptor (or generator) potential

Question 5

Where do the cell bodies of afferent fibres reside?

Answer 5

In a series of ganglia that lie alongisde the spinal cord and brain stem and are considered part of the PNS

Question 6

Neurons in the dorsal root ganglia and in the cranial nerve glanglia are critical links for what?

Answer 6

Critical links for supplying CNS circuits with information about sensory events that occur in the periphery

Question 7

Why are neurons in the dorsal root ganglia called pseudounipolar?

Answer 7

Because peripheral and central components of afferent fibers are continuous, attached to the cell body in the ganglia by ONE SINGLE process

Question 8

What are the 5 receptors for tactile stimulation?

Answer 8

Meissner corpuscle
Pacinian corpuscle
Ruffini’s corpuscles
Merkel’s disks
Free nerve endings

Question 9

What are Meissner corpuscle receptors?

Answer 9

*Rapidly adapting
*Most common in smooth hairless skin
*40% innervation of hand
*efficient at transducing information about low-frequency vibrations that occur when textured objects move across the skin
*Detection of slippage between hand and object – important for grip

Question 10

What are Pacinian corpuscle receptors?

Answer 10

*Rapidly adapting
*Mores sensitive than Meissner. Fine textured surfaces
*10-15% innervation of the hand
*Produce sensation of vibration or tickle
*Important for the skilled use of tools

Located deep in the dermis or in the subcutaneous tissue;
their appearance resembles that of asmall onion, with concentric layers of membranes surround-ing a single afferent fiber. This laminar capsule acts as a filter, allowing only transient disturbancesat high frequencies (250–350 Hz) to activate the nerve endings.

Pacinian corpuscles adapt more rapidly than Meissner corpuscles and have a lower response threshold.
The most sensitive Pacinian afferents generate action potentials for displacements of the skin as small as 10 nanometers. Because they are so sensitive, the receptive fields of Pacinianafferents are often large and their boundaries are difficult to define.

Question 11

What are Ruffini’s corpuscles receptors?

Answer 11

*Slowly adapting
* 20% receptors in hand
*Unsure of function
*Something to do with proprioception
*Conformation of the hand in space

The long axis of the corpuscle is usually oriented parallel to the stretch lines in skin; thus, Ruffini corpuscles are particularly sensitive to the cutaneous stretching produced by digit or limb movements

Question 12

What are Merkel’s disks receptors?

Answer 12

*Slowly adapting
* 25% receptors in hand
*Dense in finger tips
*Stimulation produces sensation of light pressure
*Role in detection of shapes, edges and rough textures
*For Braille reading

Question 13

What are free nerve endings?

Answer 13

For pain and temperature

Afferent fibers that lack specialized receptor cells are referred to as free nerve endings and are especially important in the sensation of pain

Question 14

How do somatosensory afferents differ?

Answer 14

AXON DIAMETER
- the largest diameter sensory afferents are those that supply the sensory receptors of the muscles
-slightly smaller diameter afferents are those involved in touch
-even smaller diameter afferents involved in pain and temperature

The diameter of the axon determines the action potential conduction speed and is well matched to the properties of the central circuits and the various behavior demands for which each type of sensory afferent is employed

SIZE OF THE RECEPTIVE FIELD
receptive field=for cutaneous afferents, the area of the skin surface over which stimulation results in a significant change in the rate of action potentials

The size of the receptive field is largely a functionof the branching characteristics of the afferent within theskin; smaller arborizations result in smaller receptive fields
-The receptive fields in regions with dense innervation (fingers, lips, toes) are relatively small compared with those in the forearm or back that are innervated by a smaller number of afferent fibers

Regional differences in receptive field size and inner-vation density are the major factors that limit the spatial accuracy with which tactile stimuli can be sensed. Thus, measures of two-point discrimination—the minimum interstimulus distance required to perceive two simultaneously applied stimuli as distinct—vary dramatically across the skin surface.

In the fingertips, stimuli are perceived as distinct if they are separated by roughly 2 mm, but the same stimuli applied to the upper arm are not perceived as distinct until they are at least 40mm apart

TEMPORAL DYNAMICS OF RESPONSE TO SENSORY STIMULATION
Some afferents fire rapidly when a stimulus is first presented, then fall silent in the presence of continued stimulation; others generate a sustained discharge in the presence of an ongo-ing stimulus

Rapid adapting afferents- are thought to be particularly effective in conveying informa-tion about changes in ongoing stimulation such as those pro-duced by stimulus movement

Slowly adapting afferents- are better suited to provide information about the spatial attributes of the stimulus, such as the size and shape.

DIFFERENT RESPONSES TO DIFFERENT QUALITIES OF SOMATOSENSORY STIMULATION
Due to differences in the properties of the channels expressed in sensory afferents, or to the filter properties of the specialized receptor cells that encapsulate many sensory afferents, generator potentials are produced only by a restricted set of stimuli that impinge on a given afferent fiber
For example, the afferents encapsulated within specialized receptor cells in the skin respond vigorously to mechanical deformation of the skin surface, but not to changes in temperature or to the presence of mechanical forces or chemicals that are known to elicit painful sensations.

Question 15

Though afferents give rise to multiple peripheral branches, the transduction properties of all the branches of a single fibre are identifical and so somatosensory affferents constitute what?

Answer 15

Parallel pathways

These parallel pathways differ in conduction velocity, receptive field size, dynamics, and effective stimulus features

These different pathways remain segregated through sev-eral stages of central processing, and their activity con-tributes in unique ways to the extraction of somatosensoryinformation that is necessary for the appropriate control ofboth goal-oriented and reflexive movements

Question 16

What regions of the skin are best to understand the distinct contribution of afferent pathways to cutaneaous sensation?

Answer 16

The glabrous (hairless) portions of the hand (i.e. the palm and fingertips).
These regions of the skin surface are specialised to generate a high definition neural image of manipulated objects

Question 17

What activates mechanoreceptors?

Answer 17

Active touching or haptics, which involves the interpretation of complex spatiotemporal patterns of stimuli

Question 18

What is it called when manipulating an object with the hand provides sufficient information to identify an object?

Answer 18

Stereognosis

Question 19

What are proprioceptors?

Answer 19

They provide information about mechanical forces arisingwithin the body itself, particularly from the musculoskeletalsystem.

The purpose of proprioceptors is primarily to give detailed and continuous information about the position of the limbs and other body parts inspace.

Low-threshold mechanoreceptors, including musclespindles, Golgi tendon organs, and joint receptors, providethis kind of sensory information, which is essential to the accurate performance of complex movements.

Informationabout the position and motion of the head is particularly important; in this case, proprioceptors are integrated with thehighly specialized vestibular system

Question 20

What the two main types of proprioceptor?

Answer 20

Muscle Spindles : to signal changes in muscle length

Golgi tendon organs : to signal changes in muscle tension

Question 21

How do muscle spindles work?

Answer 21

Muscle spindles consist of 4-8 specialized intrafusal muscle fibers surrounded by connective tissue.
The intrafusal fibers are distributed among and in a parallel arrangement with the extrafusal fibers of skeletal muscle, which are the true force-producing fibers.
Sensory afferents are coiled around the central part of the intrafusal spindle, and when the muscle is stretched, the tension on the intrafusal fibers activates mechanically gated ion channels in the nerve endings, triggering action potentials.
Innervation of the muscle spindle arises from two classes of fibers: primary and secondary endings.
Primary endings arise from the largest myelinated sensory axons, and have rapidly adapting responses to changes in muscle length
Secondary endings produce sustained responses to constant muscle lengths.

Primary endings are thought to transmit information about limb dynamics—the velocity and direction of movement—whereas secondary endings provide information about the static position of limbs.

Piezo2 is expressed by proprioceptors and is required for functional proprioception

The density of spindles in human muscles varies. Large muscles that generate coarse movements have relatively few spindles; in contrast, extraocular muscles and the intrinsicmuscles of the hand and neck are richly supplied with spindles, reflecting the importance of accurate eye movements,the need to manipulate objects with great finesse, and thecontinuous demand for precise positioning of the head.

Question 22

How do golgi tendon organs work?

Answer 22

Each Golgi tendon organ is arranged in series with a small number (10–20) of extrafusal muscle fibers.
Taken together, the population of Golgi tendon organs for a given muscle provides an accurate sample of the tension that exists in the muscle

Remains unclear how proprioceptive afferents contribute to the perception of limb position, movement and force
-> but using vibrators to stimulate the spindlesof specific muscles have provided compelling evidence that the activity of these afferents can give rise to vivid sensations of movement in immobilized limbs

Question 23

What is the central pathway conveying tactile information ?

Answer 23

Dorsal Column Medial-Lemniscal System

The axons of cutaneous mechanosensory afferents enter the spinal cord through the dorsal roots, where they bifurcate into ascending and descending branches.
Then project to gray matter of the spinal cord across several segments, terminating in the deeper layers of the dorsal horn.

Main ascending branches extend ipsilaterally through the dorsal columns to the lower medulla, where they synapse on neurons in the dorsal column nuclei

The dorsal columns of the spinal cord are organized such that the fibers conveying information from lower limbs lie most medial and travel in a bundle known as the fasciculus gracilis
Those fibers that convey information from the upper limbs, trunk, and neck lie in a lateral bundle known as the fasciculus cuneatus

Fibers in these two tracts end in different subdivisions of the dorsal column nuclei: a medial subdivision, the nucleus gracilis, and a lateral subdivision, the nucleus cuneatus

The second-order neurons in the dorsal column nuclei send their axons to the somatosensory portion of the thalamus. The axons exiting from dorsal column nuclei are identified as the internal arcuate fibers.The internal arcuate fibers subsequently cross the midline and then form a dorsoventrally elongated tract known as the medial lemniscus.
the medial lemniscal axons carrying informationfrom the lower limbs are located ventrally, whereas the axons related to the upper limbs are located dorsally.
As the medial lemniscus ascends through the pons and midbrain, it rotates 90 degrees laterally, so that the fibers representing the upper body are eventually located in the medial portion of the tract and those representing the lower body are in the lateral portion. The axons of the medial lemniscus synapse with thalamic neurons located in the ventral posterior lateral nucleus (VPL). Thus, the VPL receives input from contralateral dorsal column nuclei

Third-order neurons in the VPL send their axons via the internal capsule to terminate in the ipsilateral postcentral gyrusof the cerebral cortex, a region known as the primary somatosensory cortex. Neurons in the VPLalso send axons to the secondary somatosensory cortex, a smaller region that lies in the upper bank of the lateral sulcus. Thus, the somatosensory cortex represents mechanosensory signals first generated in the cutaneous surfaces of the contralateral body

Question 24

What pathway is conveying tactile information about the face?

Answer 24

The Trigemothalamic System

Cutaneous mechanoreceptor information from the face is conveyed centrally by a separate set of first-order neurons that are located in the trigeminal ganglion

The peripheral processes of these neurons form the three main subdivisions of the trigeminal nerve (the ophthalmic, maxillary, and mandibular branches).
Each branch innervates a well-defined territory on the face and head, including the teeth and the mucosa of the oral and nasal cavities. The central processes of trigeminal ganglion cells form the sensory roots of the trigeminal nerve; they enter the brainstem at the level of the pons to terminate on neurons in the trigeminal brainstem complex.The trigeminal complex has two major components: the principal nucleus and the spinal nucleus.

Most of the afferents conveying information from low-threshold cutaneous mechanoreceptors terminate in the principal nucleus. In effect, this nucleus corresponds to the dorsal column nuclei that relay mechanosensory information from the rest of the body.

Trigeminal neurons that are sensitive topain, temperature, and non-discriminative touch do not proj-ect to the principal nucleus; they project to the spinal nucleusof the trigeminal complex.

The second-order neurons of the trigeminal brainstem nuclei give off axons that cross the midline and ascend to the ventral posterior medial (VPM) nucleus of the thalamus by way of thetrigeminal lemniscus. Neurons in the VPM send their axons to ipsilateral cortical areas SI and SII

Question 25

Central pathways conveying proprioceptive information from the body

Answer 25

The axons of proprioceptive afferents enter the spinal cord through the dorsal roots, and many of the fibers from proprioceptive afferents also bifurcate into ascending and descending branches, which in turn send collateral branches to several spinal segments.
Some collateral branches penetrate the dorsal horn of the spinal cord and synapse on neurons located there, as well as on neurons in the ventral horn. These synapses mediate, among other things, segmental reflexes, such as the knee-jerk
The ascending branches of proprioceptive axons travel with the axons conveying cutaneous mechanosensory information through the dorsal column. However, proprioceptive information reaches higher cortical circuits as branches of pathways that are also targeting the cerebellum, and some of these axons run through spinal cord tracts whose names reflect their association with this structure.

First-order proprioceptive afferents that enter the spinal cord between the mid-lumbar and thoracic levels(L2–T1) synapse on neurons in Clarke’s nucleus, located in the medial aspect of the dorsal horn. Afferents that enter below this level ascend through the dorsal column and then synapse with neurons in Clarke’s nucleus.

Second-order neurons in Clarke’s nucleus send their axons into the ipsilateral posterior lateral column of the spinal cord, where they travel up to the level of the medulla in the dorsal spinocerebellar tract. These axons continue into the cerebellum, but in their course, give off collaterals that synapse with neurons lying just outside the nucleus gracilis

Axons of these third-order neurons decussate and join the medial lemniscus, accompanying the fibers from cutaneous mechanoreceptors in their course to the VPL of the thalamus

Question 26

Central pathways conveying proprioceptive information from the face

Answer 26

Proprioceptive information from the face is conveyed through the trigeminal nerve.
However, the cell bodies of the first- order proprioceptive neurons for the face have an unusual location: they are found within the CNS, in the mesencephalic trigeminal nucleus, a well-defined array of neurons lying at the lateral extent of the periaqueductal gray matter of the dorsal midbrain.

These pseudounipolar neurons have peripheral processes that innervate muscle spindles and Golgi tendon organs associated with facial musculature (especially the jaw muscles) and central processes that include projections to brainstem nuclei responsible for reflex control of facial muscles. Although the exact route is not clear, information from proprioceptive afferents in the mesencephalic trigeminal nucleus also reaches the thalamus and is represented in somatosensory cortex.

Question 27

Somatosensory components of the thalamus

Answer 27

Each of the several ascending somatosensory pathways originating in the spinal cord and brainstem converges on the ventral posterior complex of the thalamus and terminates in an organized fashion.

One of the organizational features of this complex instantiated by the pattern of afferent terminations is a complete and orderly somatotopic representation of the body and head.

The more laterally located ventral posterior lateral nucleus (VPL) receives projections from the medial lemniscus carrying somatosensory information from the body and posteriorhead, whereas the more medially located ventral posteriormedial nucleus (VPM) receives axons from the trigeminal lemniscus conveying somatosensory information from the face.

Inputs carrying different types of somatosensory information terminate on separate populations of relay cells within the ventral posterior complex. Thus, the information supplied by different somatosensory receptors remains segregated in its passage to cortical circuits

Question 28

Axons projecting from the neurons in the ventral posterior complex of the thalamus project to where?

Answer 28

Cortical neurons located in layer 4 of the primary somatosensory cortex

Question 29

What is the primary somatosensory cortex?

Answer 29

is located in the postcentral gyrus of the parietal lobe and comprises four distinct re-gions, or fields, known as Brodmann’s areas 3a, 3b, 1, and 2.

Question 30

What have mapping studies shown us?

Answer 30

Mapping studies in humans and other primates show that each of these four cortical areas contains a separate and complete representation of the body.

In these somatotopic maps, the foot, leg, trunk, forelimbs, and face are represented in a medial to lateral arrangement.

Question 31

What is a salient feature of somatotopic maps?

Answer 31

A salient feature of somatotopic maps, recognized soon after their discovery, is their failure to represent the human body in its actual proportions. When neurosurgeons determined the representation of the human body in the primary sensory (and motor) cortex, the homunculus (“little man”) defined by such mapping procedures had a grossly enlarged face and hands compared with the torso and proximal limbs.

These anomalies arise because manipulation, facial expression, and speech are extraordinarily important for humans and require agreat deal of circuitry, both central and peripheral, to govern them.

Thus, in humans the cervical spinal cord is enlarged to accommodate the extra circuitry related to the hand and upper limb, and the density of receptors is greater in regions such as the hands and lip.

Such distortions are also apparent when topographical maps are compared across species

Question 32

What are somatotopic maps like in other animal brains?

Answer 32

In the rat brain, an inordinate amount of the somatosensory cortex is devoted to representing the large facial whiskers that are key components of the somatosensory input for rats and mice

While raccoons overrepresent their paws and the platypus its bill.

In short, the sensory input (or motor output) that is particularly significant to a given species gets relatively more cortical representation

Question 33

How are the functional properties of neurons in each somatosensory region distinct?

Answer 33

Experiments carried out in non-human primates indicate that neurons in areas 3b and 1 respond primarily to cutaneous stimuli, whereas neurons in 3a respond mainly to stimulation of proprioceptors and area 2 neurons process both tactile and proprioceptive stimuli.
These differences inresponse properties reflect, at least in part, parallel sets ofinputs from functionally distinct classes of neurons in theventral posterior complex.

Area 3b receives the bulk of the input from the ventralposterior complex and provides a particularly dense pro-jection to areas 1 and 2. This arrangement of connec-tions establishes a functional hierarchy in which area 3bserves as an obligatory first step in cortical processingof somatosensory information

Question 34

What have lesions in area 3b of non-human primates found?

Answer 34

Deficits in all forms of tactile sensations mediated by cutaneous mechanoreceptors

Question 35

What have lesions in areas 1 or 2 found?

Answer 35

Lesions in areas 1 or 2 result in partial deficits and an inability to use tactile information to discriminate either the texture of objects (area 1 deficit) or the size and shape of an object (area 2 deficit).

Question 36

What did Mountcastle find in his analysis of electrode penetrations in primary somatosensory cortex?

Answer 36

That neurons with similar response properties might be clustered together into functionally distinct “columns” that traverse the depth of the cortex.
Subsequent studies of finely spaced electrode penetrations in area 3b provided strong evidence in support of this idea, demonstrating that neurons with rapidly and slowly adapting properties were clustered into separate zones within the representation of a single digit.

Previously, it was assumed that the rapidly and slowly adapting cortical neurons receive segregated inputs from rapidly and slowly adapting mechanoreceptors, respectively. However, the cortical slowly adapting neurons all show a large touch-OFF response in addition to the sustained firing during contact.
Such OFF responses are signaled only by rapidly adapting afferents in fingers.
Furthermore, the rapidly adapting cortical neurons sometimes show sustained firing in response to stimuli of preferred directions.
Thus, the cortical rapidly and slowly adapting columns reflect differential processing of convergent inputs from different peripheral receptors, rather than the strict segregation of afferent inputs that convey distinct physiological signals.

This columnar organization of cortical areas, a fundamental feature of cortical organization throughout the neocortex, is especially pronounced in visualcortical areas in primates.
Slowly and rapidly adapting columns in somatosensory cortex are therefore more analogous to orientation columns in the visual cortex (reflecting cortical computations derived from converging input) than ocular dominance columns (which reflect strictly segregated thalamocortical inputs).