Somatosensory system

Question 1

What are the 3 divisions of the somatosensory system?

Answer 1

Exteroceptive, proprioceptive, enteroceptive

Question 2

The somatosensory system mediates all sensations other than those of the special senses, such as…?

Answer 2

Fine discriminatory touch, stretch, joint and muscle position sense, temperature, pain and itch

Question 3

What does the exteroceptive division do?

Answer 3

Registers information from the surface of the body by numerous receptors (cutaneous receptors)

Question 4

What does the proprioceptive division do?

Answer 4

Monitors posture and movement (sensors in muscle and tendons and joints)

Question 5

What does the enteroceptive division do?

Answer 5

Reports upon the internal state of the body and is closely related to autonomic function

Question 6

Describe the somatosensory pathway in terms of where the neurons are in the body

Answer 6

1st order neuron/primary sensory afferent (in PNS) -> 2nd order neuron/projection neuron (in CNS) -> 3rd order neuron/projection neuron (in CNS) -> somatosensory cortex (in CNS)

Question 7

What is the cell body location of the 1st order neuron in the somatosensory pathway?

Answer 7

Dorsal root ganglia (innervation of limbs, trunk, posterior head) or cranial ganglia (innervation of anterior head)

Question 8

What is the cell body location of the 2nd order neuron in the somatosensory pathway?

Answer 8

Dorsal horn of spinal cord or brainstem nuclei

Question 9

What is the cell body location of the 3rd order neuron in the somatosensory pathway?

Answer 9

Thalamic nuclei

Question 10

Sensory neurone terminals transduce a stimulus into electrical activity. How does the stimulus produce a signal?

Answer 10

Stimulus opens cation selective ion channels in peripheral terminal of primary sensory afferent eliciting a depolarising receptor potential. Amplitude of receptor potential is graded and proportional to stimulus intensity. A supra-threshold receptor potential triggers ‘all or none’ potentials, conducted by the axon, at a frequency proportional to its amplitude. Action potentials arriving at the central terminal cause the graded release of neurotransmitter on to 2nd order neurons

Question 11

Describe the modality of sensory units

Answer 11

Primary afferent neurons are especially tuned to respond to a specific type of energy that normally excites them underlying the sensation that they subserve

Question 12

Describe the threshold of sensory units

Answer 12

Relates to the intensity of a stimulus required to excite a sensory unit Low threshold units respond to low intensity stimuli e.g. fine discriminatory touch, cold, warm and hot High threshold units (nociceptors) respond to high, but not low, intensity stimuli e.g. high intensity mechanical stimuli, extremes of heat or cold, chemicals

Question 13

Describe the adaptation of sensory units

Answer 13

Adaptation is a feature of sensory units that determines whether they change their firing rate only in response to a stimulus of changing intensity, or fire continuously throughout a constant stimulus

Question 14

Describe a slow adapting response/tonic/static response

Answer 14

Continuous information to CNS while terminal deformed. Provides information about position, degree of stretch, or force e.g. stretch receptors

Question 15

Describe a fast adapting response/phasic/dynamic response

Answer 15

Detects changes in stimulus strength e.g. rate of movement. Number of impulses proportional to rate of change of stimulus. Some muscle spindle afferents, hair follicle afferents

Question 16

Describe very fast adapting/very phasic/dynamic response

Answer 16

Responds only to very fast movement, such as rapid vibration e.g. Pacinian corpuscle

Question 17

Describe the conduction velocity of sensory units

Answer 17

Primary sensory afferent fibres differ greatly in their axon diameter, extent of myelination, conduction velocity and associated sensory receptor

Question 18

Describe the receptive field (site of termination) of sensory units

Answer 18

The peripheral terminal of cutaneous afferent fibres branch into many fine processes the tips of which can be free nerve endings or associated with specialised structures. A patch of skin may contain overlapping RFs of sensory units with a different modality

Question 19

Describe two-point discrimination

Answer 19

Two-point discrimination is an important measure of somatosensory function. Clinically tested by applying simultaneously 2 sharp point stimuli, separated by a variable distance, at different sites on the body surface. Subject reports on whether 1 point, or 2, are sensed and a threshold distance between the two is established. Two-point thresholds match the diameter of the corresponding RF. Regions with the highest discriminitive capacity have the smallest RFs

Question 20

What are the subdivisions of skin low threshold mechanoreceptors?

Answer 20

FA1 (fast adapting small receptive field), FA2, SA1, SA2 (slow adapting, wide receptive field)

Question 21

Describe Merkel cell-neurite complexes in terms of type of mechanoreceptor and location

Answer 21

SA1 - formed from many expanded nerve terminals,each closely associated with a Merkel cell as a synapse-like structure. Located in basal epithelium of skin areas with high sensory acuity e.g. fingers. Signal sustained pressure, texture and are extremely sensitive to the edges of objects

Question 22

Describe Ruffini endings

Answer 22

SA2 - collagenous core in which axons branch. Located in dermis of both glabrous and hairy skin. Sensitive to drag/shearing forces

Question 23

Describe Pacinian corpuscles

Answer 23

FA2 - large onion-like structure. Subcutaneous location close to periosteum of bone. Very sensitive to vibration over frequency range higher than Meissner corpuscle. Act as event detectors during manipulation of objects

Question 24

Describe Meissner corpuscles

Answer 24

FA1 - formed from a capsule in which several axons zigzag between modified Schwann cells (teloglia). Located close to basal epithelium of skin areas with high sensory acuity. NOT present in hairy skin. Sensitive to stroking, flutter, low frequency vibration.

Question 25

Which sensory receptors allow for the detection of textured surfaces?

Answer 25

The location of Merkel cell neurite complexes and Meissner corpuscles at, or just below, base of epidermis, together with their small RF and high density, allows for the detection of textured surfaces

Question 26

Which sensory receptors are associated with hairy skin?

Answer 26

Merkel cell neurite complex (SA1), Ruffini endings (SA2) and Pacinian corpuscles (FA2) but not Meissner corpuscles (FA1) that are instead functionally replaced by hair units

Question 27

What is a hair unit?

Answer 27

Comprise follicular (free) nerve endings that either wrap around the follicle (circumferential fibres) or parallel to it (palisade fibres)

Question 28

What is a dermatome?

Answer 28

An area of skin that is innervated by the left and right dorsal roots of a single spinal segment. When mapped, the dermatomes delineate a set of bands on the body surface

Question 29

Which condition highlights dermatomes?

Answer 29

Shingles - due to infection of dorsal root ganglion neurones by Varicella Zoster virus (causes chickenpox in childhood). In adulthood the virus may reactivate revealing, anatomically, as inflamed and blistered skin, the dermatome innervated by that ganlgion

Question 30

The grey matter is divided into dorsal and ventral horns and 10 distinct …?

Answer 30

Laminae of Rexed

Question 31

What is the fibre class and termination site (in the dorsal horn of grey matter) of nociceptors?

Answer 31

Fibre class = Adelta/C Termination site = laminae I and II

Question 32

What is the fibre class and termination site (in the dorsal horn of grey matter) of low threshold mechanoreceptors?

Answer 32

Fibre class - Abeta Termination site - laminae III to VI

Question 33

What is the fibre class and termination site (in the dorsal horn of grey matter) of proprioceptors?

Answer 33

Fibre class - Aalpha Termination site - laminae VII to IX