Processing of somatosensory input

Question 1

The somatosensory system and its modalities?

Answer 1

Mediates all sensations that are not visual, auditory, gustatory, olfactory or the vestibular sense of equilibrium; special sensory.

Includes the sensory modalities of:
* Touch, pressure and vibration (mechanosensation)
* Joint and muscle position sense (proprioception)
* Temperature (thermosensation)
* Pain (nociception)
* Itch (pruriception)

Question 2

The —– of sensory neurons transduce a stimulus (e.g. mechanical) into electrical activity.
Stimulus (e.g. mechanical) opens cation-selective ion channels in nerve terminal to elicit a —– receptor potential.
The receptor potential is —- and proportional to stimulus —–
Local current flow triggers ‘—–’ action potentials at a frequency proportional to the —— of the receptor potential

Answer 2

terminals
depolarising
graded
intensity
all or none
amplitude

Question 3

what are the Sensory receptive/unit properties of primary afferent neurons?

Answer 3

Modality
Threshold
Adaptation
Conduction velocity
Site and extent of peripheral termination [receptive field (RF)]

Question 4

Modality?

Answer 4

The principal type of adequate stimulus that causes a signal

Question 5

Threshold?

Answer 5

• what intensity of the stimulus is required for excitation of the sensory receptor?
  
  • e.g. low threshold mechanoreceptors mediate fine discriminatory touch, high threshold (HT) (nociceptor) mechanoreceptors respond to high intensity mechanical stimuli; Information from HT units can be perceived as painful, but is subject to modulation.

Question 6

Adaption?

Answer 6

• Adaption rate - does the sensory unit discharge action potentials continuously during the stimulus, or does it respond preferentially to a changing stimulus?
  
  • Slowly adapting/tonic response - continuous information to CNS while terminal deformed e.g. stretch receptors
  • Fast adaptic/phasic response - detects changes in stimulus strength, number of impulses proportional to rate of change of stimulus e.g. some muscle spindle afferents
  • Very fast adapting response - responds only to very fast movement such as rapid vibration e.g. Pacinian corpuscle

Question 7

Conduction velocity?

Answer 7

• how rapidly does the sensory unit conduct action potentials along its axon?
  
  • Depends on diameter of axon and whether the axon is myelinated

Question 8

Site and extent of peripheral termination

Answer 8

where is the peripheral terminal of the sensory unit located, do its sensory receptors have a small or large anatomical distribution?
- The peripheral termination of cutaneous afferent fibres branch into many fine processes, the tips of which can be free nerve endings or associated with specialised structures
- Receptor field (RF): target territory from which a sensory unit that can be excited
- Sensory acuity correlates inversely with RF size

Question 9

Two-point discrimination (or spatial acuity)

Answer 9

• Two-point discrimunation is an important measure of somatosensory function
• Clinically tested by applying simultaneously two sharp point stimuli, separated by a variable distance, at different sites on the body surface
• Subject reports on whether one point, are two, are sensed and a threshold distance between the two is established
• Two-point thresholds match the diameter of the corresponding RF, regions with the highest discriminative capacity have the smallest RFs

Question 10

Cutaneous receptors

Answer 10

Question 11

Cutaneous and subcutaneous receptors, associated nerve fibres and sensory modality

Answer 11