Somatic sensation

Question 1

What is sensation?

Answer 1

Sensation is a conscious or sub-conscious awareness of an external or internal stimulus

Question 2

What are the different types of senses?

Answer 2

General senses:

• Somatic: From body
  
  • Tactile (touch, pressure, vibration)
  • Thermal
  • Pain
  • Proprioception
• Visceral: Internal organs

Special senses
(Smell, taste, vision, hearing, balance)

Question 3

Describe the general organisation of the somatosensory system

Answer 3

First order neurone
(AKA primary afferent neurone)

Second order neurone

Third order neurone

Question 4

Give some examples of sensory receptors of 1st order neurones.

Answer 4

Free nerve ending (e.g. cold stimulus)
Encapsulated nerve ending (e.g. pressure stimulus)
With specialised cell (e.g. gustatory (taste) receptor)

Question 5

What are some different stimulus modalities?

Answer 5

Light
Touch 
Temperature
Chemical changes (e.g. taste)
etc...

Question 6

What is meant by quality of a sense?

Answer 6

A subdivision of modality, e.g. taste can be subdivided into sweet, sour, salt or temperature can be subdivided into hot or cold.

Question 7

Are sensory receptors modality specific?

Answer 7

Yes (to a point)
They only respond to one type of stimulus
BUT if you hit a receptor hard enough with a stimulus it can cause a response regardless of a modality
(E.g. if you press your eye hard enough you see white light even though there is no light)

Question 8

What are proprioceptors?

Answer 8

Sensory receptors of muscles and joints providing information on body position, i.e. proprioception

Question 9

How do we detect changes in our environment?

Answer 9

1. A stimulus evokes a change in the permeability to ions of the receptor membrane
2. Receptor potential (movement of ion across membrane)
3. Generator potential
4. Action potentials
5. Propagate into CNS

Called sensory transduction

Question 10

How do we detect the strength of a stimulus?

Answer 10

Strength is determined by rate of action potential stimulus (frequency coding)
A stronger stimulus also activates neighbouring cells (but to a lesser degree)

Question 11

How long can a stimulus last? (different mechanisms)

Answer 11

Adaptation

Slowly adapting (tonic) receptors may keep firing as long as the stimulus lasts, e.g. joint and pain receptors

Rapidly adapting (phasic) respond maximally and briefly to a stimulus, e.g. touch receptors

Question 12

What is sensory acuity?

Answer 12

The precision by which a stimulus can be located

Question 13

What is sensory acuity determined by?

Answer 13

Lateral inhibition in the CNS
Two point discrimination
Synaptic convergence and divergence

Question 14

Describe lateral inhibition in terms of sensory acuity.

Answer 14

When you activate a first order neurone it also has inhibitory neurones that inhibit the 2nd order neurones adjacent to it.

This works to pinpoint where the stimulus is coming from, and not have just a general area of sensation

Question 15

What is meant by two point discrimination in terms of sensory acuity?

Answer 15

Minimal interstimulus distance required to perceive two simultaneously applied skin indentation

Fingertips - 2mm apart
Forearm - 40mm apart

This is due to receptive fields:

• Vary in size and density
• Overlap with neighbouring receptive fields

And so two point discrimination is determined by:
- Density of sensory receptors (3-4 times greater in
fingertips)
- Size of neuronal receptive fields (fingertips = 1-2mm,
palm= 5-10mm)
- Psychological factors can affect this

Question 16

What are convergence and divergence in terms of sensory acuity?

Answer 16

Convergence decreases acuity
(multiple 1st order neurones converge on one 2nd order neurone)

Divergence amplifies the signal
(One first order neurone diverges onto multiple 2nd order neurones)

Question 17

What is significant about the thalamic level in regards to feeling a sense?

Answer 17

It is where there is crude localization and discrimination of stimuli
Highly organized projections to cortex
(the fibres are organised in the thalamus to perceive more precise information)

Thalamic lesions (e.g. stroke) can create thalamic overreaction

Question 18

Tell me about the somatosensory cortex.

Answer 18

Located at the post-central gyrus

Sharp localisation and full recognition of qualities of modalities

Cortical columns

Somatotopic representation - each body area has specific cortical representation

Relays to other cortical and sub-cortical areas
Choice to respond to stimulus taken at cortical level

Question 19

What is the sensory homunculus?

Answer 19

Contralateral representation

The relative size of each area is reflective of the degree of sensory acuity associated with that body area

Question 20

What is meant by perception?

Answer 20

Perception is our awareness of stimuli and our ability to discriminate between different types of stimuli

Question 21

What happens in lesion of sensory cortex?

Answer 21

E.g. in repeated epileptic events

Loss of two point discrimination
Astereognosis (loss of 3-dimensional sensation interpretation)