Somatosensory system 1

Question 1

Describe the somatosensory system and what it can be classified into

Answer 1

This system has receptors throughout the body

It includes the sensory modalities of:

• Touch, pressure and vibration (mechanosensation)
• Joint and muscle position sense (proprioception)
• Temperature (thermosensation)
• Pain (nociception)
• Itch (pruriception)

It can also be classified as: (these diff areas where sensations come from make up the somatosensory system)

1. Exteroceptive sensations (cutaneous senses) from the surface of the body
2. Proprioceptive sensations concerning posture and movement (sensors in muscle and tendons and joints)
3. Visceral sensations specifically from the internal organs
4. Deep sensations from fasciae, muscles and bone

Question 2

What happens in sensory neurons in response to a stimulus (sensation)?

Answer 2

• Stimulus (mechanical, thermal, or chemical) opens ion channels (cation selective) in nerve terminal to elicit a depolarizing receptor (or generator) potential - due to influx of Na+
• Amplitude of generator potential is graded and proportional to stimulus intensity

Question 3

What are the 2 main classes of sensory receptors and in general what sensations do they sense?

Answer 3

• Low threshold units - respond to low intensity (non-damaging and non-painful) stimuli
• High threshold units - respond to high (noxious, potentially damaging), but not (normally) low, intensity stimuli. Information from HT units may be perceived as painful

Question 4

What are the main types of low threshold sensory units

Answer 4

• Low threshold mechanoreceptors (LTMs): mediate touch, vibration, pressure
• Low threshold thermoreceptors: mediate cold, cool, indifferent, warm and hot

Question 5

What are the main types of high threshold sensory units ?

Answer 5

• High threshold mechanoreceptors (HTMs) (also called mechanical nociceptors) respond to high intensity mechanical stimuli
• Thermal nociceptors respond to extreme degrees of heat (>45°C), or cold < 10-15°C)
• Chemical nociceptors respond to substances in tissue (as found in inflammation) e.g. prostaglandins, bradykinin, serotonin, histamine, K+, H+ and ATP and many others
• Polymodal nociceptors respond to at least two of the above

Question 6

Define what the receptive feild (RF) of an afferent neuron (sensory) is

Answer 6

It is the region that when stimulated with an adequate stimulus causes a response in that neurone

Question 7

Do receptive fields from different primary afferent neurons overlap?

Answer 7

Yes - a patch of skin contains many overlapping RFs innervated by individual primary afferent fibres

Question 8

Define two point discrimination

Answer 8

• It is the ability to discern that two nearby objects touching the skin are truly two distinct points, not one.
• It is often tested with two sharp points during a neurological examination and is assumed to reflect how finely innervated an area of skin is.

Question 9

The skin has many sensory receptors in it, for each of the following sensory receptors in the skin state what they sense:

1. Free nerve endings
2. Meissners corpuscles
3. Merkles discs
4. Hair end organs
5. Ruffini endings
6. Pacinian corpuscles

Answer 9

1. Free nerve endings - pain, heat, cold
2. Meissners corpuscles - light touch
3. Merkles discs - touch, pressure
4. Hair end organs - touch
5. Ruffini endings - pressure
6. Pacinian corpuscles - vibration, pressure

Question 10

Just go over and appreciate this

Question 11

Why if a single dorsal root is cut, does the corresponding dermatome innervated by that root not loose all sensation?

Answer 11

Because there is a degree of overlap with the region supplied by adjacent dorsal roots