Somatosensory System

Question 1

What is the somatosensory system?

Answer 1

the representation of the body in the brain

Question 2

what are the 2 major inputs components?

Answer 2

• mechanical stimuli(eg, touch, pressure, vibration etc)

- painful stimuli and temperature

Question 3

input and interpretation of the 2 major inputs allows

Answer 3

allows us to:

• identify the shape and textures of objects
• to monitor the internal and external forces acting on the body
• to detect potentially harmful circumstances
• to have a sense of ourselves within our environment, so we can plan our actions accordingly

Question 4

Sensory receptors:
encapsulated?
unencapsulated?

Answer 4

encapsulated:
ruffini copuscles
Meissner (tactile) discs
Pacinian Corpuscles

unencapsulated:
free nerve endings
hair follicles
Merkel (tactile) dics

Different levels of encapsulation filter different frequencies

Question 5

Cellular receptors have:

Answer 5

merkels discs, meissners corpuscles, free nerve endings

Question 6

Classes of mechanoreceptor responses:

Answer 6

Rapidly adapting = phasic - gives info about changes in stimulus
Slowly adapting = tonic - continue to respond as long as stimulus is present( gives info about presence)

Question 7

Primary afferent axon subtpes?

Answer 7

classified according to axon velocity= broadly reflects diameter (faster= larger diameter)
-Axons from skin =designated by letters (ABC)
A= fastest/ largest
(A group further broken down into, α-fastest-, β-medium-, δ-slowest-)
Aα - I - 13-20um - proprioceptors of skeletal muscle eg.
muscle spindles
Aβ - II - 6-12 um - mechanoreceptors of skin eg.
encapsulated recpetors
Aδ - III - 1 - 5um - pain, temperature
C - IV - 0.2 - 1.5um - temp, pain, itch
Axons from muscles are designated by roman numerals eg I, II, III, IV (I = largest)

Question 8

How is info from primary afferent axon subtypes relayed to brain?

Answer 8

sensory info is organised into layers in the spinal cord dorsal horn
- cell bodies of sensory neurons are grouped in dorsal root ganglion(DRG) and their projections are organised to different layers of dorsal horn eg. info from different classes of hair follicle is represented in different layers
(this applies to different sensory modalities)
- sensory info = spatially organised as its carried into brain by different pathways

Question 9

Somatosensory projections:

info gets from sensory subtypes to the brain by either:

Answer 9

1) The Medial Lemniscal Tracts
- they carry mechanoreceptive and proprioceptive signals to the thalamus
2) The spinothalamic Tracts
- carries signals about pain and temperature to thalamus

Question 10

3 neurons sensory info typicaly travels through to reach higher centres?

Answer 10

first-order neurons- detect stimulus and transmit to spinal cord
second-order neurons -relay signal to thalamus (gateway to cortex)
third-order neurons - carry signal from thalamus to cortex
(second order cross the midline = ‘commissural’)

Question 11

Medial Lemniscal system
Upper body neurons follow which pathway and synapse where?
Lower body neurons follow which pathway and synapse where?
What is the Dorsal Column Nuclei?

Answer 11

axons are topographically organised
– 1st order axons(from the UPPER body) follow the LATERAL pathway and synapse on 2nd order neurons in the cuneate nucleus
– 1st order axons(from the LOWER body) follow the MEDIAL pathway and synapse on 2nd order neurons in the gracile nucleus
Gracile nucleus and Cuneate nucleus = ‘Dorsal Column Nuclei’
-2nd order axons cross the midline and ascend in the medias lemniscus (because of this = topology is reversed)
SO,
-upper body axons end up more medial on reaching thalamus
-lower body axons end up more lateral on reaching thalamus

Question 12

Somatopic order in the brain

Answer 12

The result of topographic projection = map of body in cortex

–> shows each DRG innervates a specific domain of the body known as ‘dermatomes’

Question 13

Dermatomes?

Answer 13

Each sensory ganglion innervates a specific region of skin (dermatome)
-these regions arise because the dermis of each region is derived from a specific embryonic structure called the ‘somite’
Somite= iterated structures that give rise to the underlying musculature and skeleton
In embryo = each sensory ganglion is associated with specific somite, so, subsequently innervates tissues arising from that somite
(topographic organisation is preserved in spinal cords and the somatosensory projections)

Question 14

Fine topographic map and proportion

Answer 14

the area occupied by different regions in the cortex are NOT proportional to actual physical size = somatosensory homunculus
Why are they not proportional? because..
-each sensory neuron has a receptive field
- 2 point discrimination used to measure
small receptive field
= high discrimination (explains why more of cortex is dedicated to small receptive fields)
= no. of receptors -nerve endings- may be similar, but no. of nerve endings from different neurons per unit area is higher= more info
(no. of sensory neurons innervating area = related to behavioural significance)
–> cortical representation is determined by behavioural signficance

Question 15

Sensory modality is represented in the cortex. How?

Answer 15

somatotopic map = preserved in coronal plane throughout postcentral gyrus
different sensory modalities are localised along sagittal axis
different regions receive different sensory inputs
cortical map plasticity = cortex can adapt to changes in input