Somatosensation pt 1

Question 1

What is the somatosensory system?

Answer 1

The somatosensory system conveys sensations from the body.
•	Touch
•	Proprioception 
•	Heat, cold
•	Pain, itch

Question 2

Peripheral nerves: spinal and cranial

Answer 2

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 are the sensory receptors of the somatosensory system

Answer 4

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 depends on afferent fibre type

Answer 5

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

Tactile afferents (discriminative touch)

Free nerve endings (low-resolution tactile, temperature, pain)

Question 7

Proprioception

Answer 7

• A-α afferents: large diameter, myelinated, fastest conducting (≤100 m/s)
Muscle spindles

Question 8

Tactile afferents (discriminative touch)

Answer 8

•	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 9

Free nerve endings (low-resolution tactile, temperature, pain)

Answer 9

• A delta fibres: small diameter, thinly myelinated, moderate conduction velocity (≤30 m/s)
• C fibres: small diameter, unmyelinated, slow conducting (≤1 m/s)

Question 10

Receptive fields

Answer 10

Ability to localize depends on sensory receptive fields

Question 11

Two major central pathways of the somatosensory system

Answer 11

• Dorsal column – medial lemniscal system (DCML)

* Spinothalamic tract (STT, also known as anterolateral system)

Question 12

Dorsal column – medial lemniscal system (DCML)

Answer 12

• mediates discriminative touch, vibration, proprioception

* Inputs from A-β and A-α afferent fibres

Question 13

Spinothalamic tract (STT, also known as anterolateral system)

Answer 13

• coarse touch, temperature, pain

* Inputs from A-δ and C fibres

Question 14

Regional variation in cortical cytoarchitecture

Answer 14

• 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