Somatosensory System

Question 1

What is sensation

Answer 1

Conscious/sub-conscious awareness of an external/internal stimulus

Question 2

What are the types of sensory systems

Answer 2

General:
Somatic (conscious)
Visceral (sub-conscious)

Special

Question 3

Rules of somatosensory system

Answer 3

1. It is a chain of 3 sensory neurones
2. First order neurone has its cell body in dorsal root ganglion
3. Second order neurone crosses the midline
4. Third order neurone projects from Thalamus to cortex

Question 4

What is modality

Answer 4

Type of stimulus

Each receptor type responds to specific modality

Question 5

What are the types of somatosensory modalities

Answer 5

Temperature
Pain
Pressure
Vibration 
Stretch
Proprioception
2 point discrimination

Question 6

What is proprioception

Answer 6

Sense of relative position of ones own body parts and strength being employed in movement
2 receptors involved: muscle spindle, Golgi tendon organ

Question 7

What is 2 point discrimination

Answer 7

Ability to sense 2 nearby objects as 2 distinct objects

Question 8

What is Quality of sensation

Answer 8

Subdivision of modality
Depends on receptor subtype
E.g. temperature - hot, cold

Question 9

Describe the process of sensory transduction

Answer 9

1. Stimulus detected by receptor and generator potential produced
2. If GP reaches threshold, AP generated
3. AP travels down to second order neurone, etc

Question 10

How does AP encode info about intensity + duration of a stimulus

Answer 10

Frequency coding: stronger stimulus will increase frequency of AP; stimulus intensity is proportional to change in membrane potential

Activation of neighbouring neurones: stronger stimulus will activate neighbouring neurones to greater degree

Question 11

What are tonic receptors

Answer 11

Receptors that slowly adapt to stimulus and continue to fire AP over presence of stimulus
Frequency of AP is constant
Constant awareness of sensation
E.g. pain receptors

Question 12

What are phasic receptors

Answer 12

Receptors that rapidly adapt to stimulus
Frequency of AP decreases during maintained stimulus
Loss of sensation over time
E.g. touch receptors

Question 13

What is acuity

Answer 13

Precision by which a stimulus can be located

Question 14

What are the factors that determine acuity

Answer 14

2 point discrimination
Lateral inhibition
Convergence + divergence

Question 15

What is lateral inhibition? Describe its process

Answer 15

Primary afferent fibre whose receptive field is closest to point of stimulation produces more AP than those at periphery

1. PAF closest to stimulus produces most AP
2. PAF synapses with adjacent secondary AF via inhibitory interneurones
3. SAF with receptive field at periphery of stimulus are strongly inhibited
4. SAF at periphery produce fewer APs than SAN w receptive field at point of stimulation

Question 16

What is 2 point discrimination? What does it depend on?

Answer 16

Minimal distance required to distinguish 2 simultaneously applied skin indentations

Density of receptors
Receptive field of PAF: area of skin covered by nerve ending of PAF; larger the receptive field, poorer the 2pd

Question 17

What is convergence and divergence

Answer 17

Convergence: multiple PAF synapse with one SAF
Poorer acuity

Divergence: one PAF synapse with multiple SAF
Amplification

Question 18

What brain regions are involved in feeling the sensation

Answer 18

Thalamus

Somatosensory cortex - post-central gyrus

Question 19

What does the Thalamus do in sensation

Answer 19

Crude localisation

Produces organised projections to somatosensory cortex

Question 20

What does the cortex do

Answer 20

Localisation of stimulus
Recognise Quality of stimulus
Then relay information to other areas and allow choice to the response to stimulus to be taken

Question 21

What is somatotopic representation

Answer 21

Point-for-point correspondence of area of body to specific point on somatosensory cortex (post-central gyrus)