Somatosensation 1: Peripheral and Central Processing

Question 1

Subdivisions of Somatosensation

Answer 1

• Cutaneous sensation
  
  • touch
    
    • pressure
    • vibration
    • flutter
  • temperature
    
    • cold
    • warm
• Proprioception
  
  • joint
  • muscle
  • tendon
• Pain
  
  • Slow pain
  • Fast pain
  • Itch

Question 2

Transduction

Answer 2

In periphery transduction results in receptor potentials - generate potential

is stimulus specific:

• role of filters prior to transduction

different gating mechanisms

Question 3

Rapidly acting receptors

Answer 3

Pacinian corpuscles are much more sensitive than Messniers.

Adaptation: responding transiently to a step stimulus

Adapatation arisies:

i) mechanical filtering by corpuscle
ii) ion channels in axon

Question 4

Sensations from single axons

Answer 4

axon gives a specific response no matter how you stimulate it

All myelinated axons = extremely selective for submodalitity to which they respond.

Meissner - flutter

Pacinian - vibration

SA1 (slow acting) - pressure

Question 5

Acuity

Answer 5

constrained by receptor density

cannot be improve

the higher the receptor density, the higher the acuity

Question 6

Cold sensitive fibres

<25oC

Answer 6

Small myelinated

A-delta axons :

• cold-nociceptors (<5oC) are C axons

Question 7

Warm sensitive fibres

>35oC

Answer 7

small, unmyelinated C axons

• Heat-nociceptors: >45oC are A-delta axons

Question 8

Fast pain

Answer 8

depends on small myelinated axons (Aδ),

especially high-threshold mechanoreceptors that respond selectively to cutting or pinching

Question 9

Slow pain

Answer 9

depends on signals in umyelinated axons (C).

Such axons are usually polymodal eg responding to warmth and touch as well as noxious stimuli. Some respond to chemicals released in trauma

Question 10

Proprioception: muscle receptors

Muscle Spindles

Answer 10

lie in parallel with main muscle fibres

• Sensitive to small changes in length: static and dynamic components
• Sensitive range reset by special motor inputs to spindle
• Crucial in regulating motor control of muscle length but also contribution to sense of limb position

Question 11

Proprioception: muscle receptors

Golgi tendon organs

Answer 11

Golgi Tendon Organs lie in series with main muscle fibres

• Sensitive to changes in muscle tension
• May help linearise muscle length tension curve

Question 12

Dorsal Root Projections

Answer 12

Dorsal root ganglion cells are bipolar

• Different classes of afferent behave differently on entering the spinal cord
  
  • Local vs acending projections
  • Lamina differences
  • Decussation patterns

Crudely, the distinction is between tactile, nociceptive and proprioceptive afferents

Question 13

Lateral Inhibition

Answer 13

Elimination of redundancy

signals changes in stimulus profile

Enhances contrast

Lateral inhibition shuts off the middle axon (look at diag) since it has a reduced firing rate due to not being directly stimulated

Question 14

Trigeminal input

Answer 14

Somatosensory info from face enters via CN V

synapses on entering Brainstem

• principle nucleus of trigeminal complex

Decussation in medial lemniscus

Synapses in the thalamus

- Ventroposterior medial nucleus (VPM)

Question 15

Thalamocortical projection

Answer 15

• Dorsal column nuclei (trunk and limbs) project to Ventral Posterior Lateral nucleus (VPL)
• Trigeminal pathway (face), projects to Ventral Posterior Medial nucleus
• VPL and VPM project to the postcentral cortical gyrus
• postcentral gyrus contains four separate cortical areas:
  
  • 3a, 3b, 1 and 2
  • The main thalamic target is area 3b