Intro to sensory Receptors and sensory Neurones, The somatosensory system

Question 1

the somatosensory system

• function?
• includes what sensory modalities? (5)
• somatic sensation can also be classed as what 4 sensations?

Answer 1

-mediates all sensations that are not visual/auditory/gustatory/olfactory or the vestibular sense of equilibrium

-touch, pressure, vibration (mechanosensation)
joint and muscle position sense (proprioception)
temp (thermosensation)
pain (nocicpetion)
Itch (pruriception)

-Exteroceptive sensations (cutaneous senses)
proprioceptive sensations (concerning posture and movement)
visceral sensations (internal organs)
Deep sensations (from fasciae, muscle, bone)

Question 2

Function of sensory neurones?

explain how sensory neurones can bring about their function?

Answer 2

They transduce a stimulus e.g. mechanical, into electrical activity

A stimulus opens cation selective ion channels in nerve terminal to elicit a depolarising receptor potential
the amplitude of generator potential is graded and proportional to stimulus intensity
this local current flow then triggers an all or none AP

Question 3

What 5 things determine the sensory receptive properties of an afferent Neurone?

Answer 3

Modality
Threshold
Adaptation rate
Conduction velocity
Site and extent of peripheral termination

Question 4

What is modality?

Give the somatic modalities and the matching stimuli

Answer 4

the principle type of adequate stimulus that is transduce into an electrical signal by primary afferent neurone

mechanosensation- mechanical forces acting on skin
proprioception- mechanical forces acting on joints or muscles
thermosensation- heat
nociception- strong mechanical force on skin/viscera or heat
pruriception- irritant n skin or mucous membranes

Question 5

Threshold

• explain the function of Low threshold units?
• explain the function of High threshold units?

Answer 5

-respond to low intensity (non damaging) stimuli

-called nociceptors
respond to high (potentially damaging) stimuli. information from HT can be perceives as painful but not necessarily

Question 6

Adaption

• what is it?
• give the 3 different types of adaption

Answer 6

-a feature of sensory neurones that determines whether they change their firing rate only in reposes to a stimulus of changing intensity or fire continuously throughout a constant stimulus

-slowly adapting 
(continuous info to CNS)
rapidly adapting 
(detects change in stimulus strength)
very rapidly adapting
responds only to very fast movement

Question 7

Conduction velocity

• what is it?
• Give the 4 different classes of neurone and describe features (diameter, myelin, CV, sensory receptor)?

Answer 7

-the speed at which an electrochemical impulse propagates down a neural pathway.

-Aa
13-20
thick
80-120
proprioceptors of skeletal muscle 

AB
6-12
moderate 
35-75
Mechanoreceptors of skin

Agamma
1-5
thin
5-30
pain/temp

c
0.2-1.5
None
0.5-2.0
temp, pain, itch

Question 8

Site of termination (receptive field (RF))

• what is it?
• what illustrates the variation in RF size?

Answer 8

-the RF of an afferent neurone is the region that when stimulated with an adequate stimulus causes a reasons in that neurone
in primary afferent neurones the RF is the location of the peripheral terminal
a patch of skin will contain many overlapping RFs innervated by individual primary afferent fibres

-the difference in 2 point discrimination

Question 9

Sensory system, cutaneous receptors

name the sensory receptors located in the skin and their function? (7)

Answer 9

-free nerve endings

Meissner’s corpuscles
abundant where 2 point discrimination is highest, not hairy skin

Merkel’s discs
as for ^ but present in moderate numbers in hairy skin, grouped into igoo domes

Hair-end organs

Krause end bulbs
at the border of dry skin and mucous membranes

Ruffini endings
in dermis and joint capsules

Pacinian corpuscles
in dermis and fascia

Question 10

difference between Pacinian and Meissner’s corpuscles?

Answer 10

Pacinian have a much larger receptive field than Meissner’s

Question 11

segmental organisation of the spinal cord

• how do peripheral nerves communicate with the CNS?
• what forms the spinal segments?
• What is a dermatome?
• anterior head innervated by what?

Answer 11

• via the spinal cord
• paired dorsal and ventral roots giving rise to the spinal nerves
• area of skin innervated by the L and Dorsal roots of a single segment
• trigeminal system

Question 12

Sensory input into spinal cord

-how is the spinal cord organised?

Answer 12

grey matter
(cell bodies and sensory afferent terminals)
White matter
(fibre tracts)

grey matter is subdivided into dorsal and ventral horns and 10 distinct laminae of Rexed