somatic sensory systems

Question 1

somatic sensory system - functions

Answer 1

representation of body in the brain
2 major inputs:
- mechanical stimuli (light touch, vibration, pressure)
- painful stimuli and temperature

input and interpretation of these allow you to:
- identify shape and texture of objects
- monitor internal and external forces acting on the body
- detect potentially harmful circumstances
- sense of self within environment

Question 2

sensory receptors - cellular (2 groups, 5 types)

Answer 2

encapsulated nerve endings - act as a filter to give specific frequencies:
- Meissner (tactile) corpuscles
- Pacinan (lamellated) corpuscles
- Ruffini corpuscles

unencapsulated nerve endings:
- Merkel (tactile) disks
- free nerve endings

other:
- hair follicles

Question 3

sensory receptors - cellular - encapsulated nerve endings (3)

Answer 3

Meissner (tactile) corpuscles:
- dermal papillae of skin - esp palms, eyelids, lips, tongue
- sense light touch, texture, movement
- sensitive to 30-50Hz - rapidly adapting

Pacinan (lamellated) corpuscles:
- dermis, join capsules, viscera
- deep pressure, stretch, tickle, vibration
- sensitive to 250-350Hz - rapidly adapting

Ruffini corpuscles:
- dermis, subcutaneous tissue, joint capsules
- heavy touch, pressure, skin stretch, joint movement - proprioceptor
- slow adapting

Question 4

sensory receptors - cellular - unencapsulated nerve endings (2)

Answer 4

Merkel (tactile) discs:
- superficial skin (epidermis)
- light touch, texture, edges, shapes
- slow adapting

free nerve endings:
- widespread in epithelia and connective tissue
- pain, heat, cold

Question 5

sensory receptors - cellular - hair follicles

Answer 5

in epithelia
varied function according to type
rapid and slow adapting types

Question 6

cellular vs molecular receptors

Answer 6

cellular = device made of cells that detects change in body or environment

molecular = molecule, usually on surface of cell with transmembrane linkage to cytoplasm, detects change in molecular environment e.g. growth factors, hormones

Question 7

non-neuronal sensory receptors

Answer 7

many examples the sensory neurons are the receptors
other cells are specialised as sensory receptors e.g. photoreceptors, auditory and vestibular hair cells, taste receptors
these cells synapse with a primary afferent neuron to relay sensory info to CNS

Question 8

2 classes of mechanoreceptor response

Answer 8

phasic = rapid adapting = info about changes in stimulus e.g. Pacinian corpuscles
tonic = slow adapting = respond as long as stimulus is present e.g. Ruffini corpuscles

Question 9

primary afferent axon subtypes

Answer 9

classified by conduction velocity - widely reflects diameter as wider = faster

axons coming from skin = letters - ABC where A is fastest/widest and C is slowest/smallest
A group is broken down with Greek characters - alpha = fastest, then beta, then gamma = slowest
e.g. A(beta) = Meissner, Pacinian, Ruffini etc.

axons coming from muscles = roman numerals I - IV where I is largest, IV = smallest
I group broken down further - Ia = fastest, Ib is slower etc.
e.g. Ia = muscle spindles, GTOs

Question 10

organisation of sensory info

Answer 10

organised starting with layers in dorsal horn of spinal cord
cell bodies of different classes of sensory neuron are grouped in DRG - projections organised to different layers of dorsal horn e.g. hair follicles classes are in different layers

Question 11

routes into brain from sensory receptor cells (2) and orders of neurons

Answer 11

medial lemniscal tract:
mechanoreceptive and proprioceptive signals to the thalamus

spinothalamic tract:
pain and temperature signals to thalamus

sensory info travels through 3 neurons to reach higher centres:
1. first-order neurons = detect stimulus and transmit to spinal cord
2. second-order neurons = relay signal to thalamus - gateway to cortex - crosses the midline in both pathways (commissural)
3. third-order neurons = thalamus to cortex

Question 12

medial lemniscal system and neurons topology

Answer 12

axons are topologically organised

1st order from upper body follow lateral pathway and synapse to 2nd order in cuneate nucleus
1st order from lower body (below T6) follow medial pathway and synapse to 2nd order in gracile nucleus
— cuneate and gracile nuclei = dorsal column nuclei

2nd order axons cross midline and ascend to medial lemniscus
— crossing midline reverses topology relative to midline i.e. medial becomes lateral and vv

3rd order axons reverse topology again so lower body are more medial and upper map onto lateral cortex (topology same as at the start)

Question 13

topological vs topographical

Answer 13

topological = way in which parts are interrelated or arranged

topographical = representation of physical distribution of parts or features on surface or within organs

Question 14

dermatone

Answer 14

each sensory ganglion (DRG) innervates a specific domain of the body (dermatone) - in embryo DRGs are associated with specific somites and innervate tissues arising from that somite
dermis of each region comes from somite
somite = embryonic structure which gives rise to underlying musculature and skeleton

Question 15

receptive fields

Answer 15

each sensory neuron has a receptive field - size varied depending on location in body
large field = low discrimination e.g. legs, arms
small field = high discrimination e.g. fingers
therefore more cortex dedicated to areas with small receptive fields - larger number of axons per unit area represented in cortex
related to behavioural significance of area

Question 16

function of Brodmann areas of postcentral gyrus (1, 2, 3a, 3b)

Answer 16

1 = texture
2 = finger co-ordination, shape and size discrimination
3a = proprioception, sensory input from muscles
3b = texture and shape discrimination

Question 17

plasticity of cortical map

Answer 17

very plastic - adapts to changes in inputs

cutting off fingers of monkeys shows disappearance of area normally used for these fingers and increased area for neighbouring fingers

increasing stimulation of specific monkey fingers increases area for these fingers and area for neighbouring fingers decreases