Somatosensation l (neuro)

Question 1

Somatosensory system

Answer 1

• the somatosensory system conveys sensations from the body
• sensations such as:
  • Touch
  • Proprioception
  • Heat, cold
  • Pain, itch

Question 2

Peripheral nerves

Answer 2

• spinal and cranial peripheral nerves

- The CNS (brain and spinal cord) is connected to the body via spinal (31 pairs) and cranial nerves

Question 3

Peripheral nerve structure

Answer 3

• Think of a nerve as a bundle of axons ensheathed in connective tissue
• Epineurium is the connective tissue ensheathing the whole nerve
• Within the nerve axon bundles may be in separate fascicles surrounded by perineurium connective tissue sheath

Question 4

Dorsal root ganglion cells

Answer 4

• Dorsal root ganglion cells are the sensory receptors of the somatosensory system
• Broadly, two anatomically and functionally distinct systems
  • Large fibres (large diameter, myelinated, fast conduction): tactile and proprioceptive
  • Small fibres (small diameter, thinly-myelinated or unmyelinated, medium or slow conducting): temperature, pain, itch, crude touch

Question 5

Quality of sensation

Answer 5

• Quality of sensation depends on afferent fibre type
• Specificity e.g., mechanosensitive fibre insensitive to thermal stimulation
• Thermosensitive fibres sensitive to warming or cooling
• Example of cold receptor responding to skin cooling from 34 to 26 °C (A) and warming back to 34°C (B)

Question 6

Receptors of the somatosensory

system

Answer 6

• Proprioception:
  • A-α afferents: large diameter, myelinated, fastest conducting (≤100 m/s), Muscle spindles
• Tactile afferents (discriminative touch)
  • A-β afferents: large diameter, myelinated, 2nd fastest conducting (30-70 m/s).
• They include:
  • superficial:
  Meissner’s corpuscles
  Merkel’s discs
  • deep:
  Ruffinni corpuscles
  Pacinian corpuscles

Question 7

Receptors of the 
somatosensory system (2)

Answer 7

• Free nerve endings (low-resolution tactile, temperature, pain)
  • A delta fibres: small diameter, thinly myelinated, moderate
  conduction velocity (≤30 m/s)
  • C fibres: small diameter, unmyelinated, slow conducting (≤1
  m/s)

Question 8

Receptive fields

Answer 8

• ability to localise depends on sensory receptive fields

Question 9

Major pathways of somatosensory system

Answer 9

• Dorsal column – medial lemniscal system (DCML):
  • mediates discriminative touch, vibration, proprioception
  • Inputs from A-β and A-α afferent fibres
• Spinothalamic tract (STT, also known as
  anterolateral system):
  • coarse touch, temperature, pain
  • Inputs from A-δ and C fibres

Question 10

Regional variation in cortical

cytoarchitecture

Answer 10

• Different areas of cortex have the same basic cell types organized in layers, with the same basic organization
• regional differences can be identified on the basis of relative thickness of the different layers, cell size and density (cytoarchitectural differences)
• Brodmann defined and numbered over 50 areas in human cortex based on subtle cytoarchitectural differences
• Many Brodmann areas now associated with function: Brodmann areas 1,2&3 comprise somatosensory cortex