Module B: Spinal Cord and Sensory Processing

Question 1

Common Features of Somatosensory pathways

Answer 1

• Somatosensory receptors that sense environment - 1°, 2° and 3° afferent neurons - Decussation - Include a thalamic nucleus

Question 2

1° afferent neuron

Answer 2

• psuedounipolar neuron - peripheral axon that innervates one receptor - central process that synapses with a 2° afferent neuron

Question 3

2° afferent neuron

Answer 3

synapses with 3° afferent neuron in thalamus

Question 4

3° afferent neuron

Answer 4

synapses with neurons in the cerebral cortex

Question 5

Decussation

Answer 5

• occurs in spinal cord or brain stem - allows better seperation of tracts (more robust against wiring errors than same-sided schemes)

Question 6

Mechanoreceptor

Answer 6

• is a sensory receptor that responds to mechanical pressure/distortion - leads to pressure sensitive action potential - different types allow perception of different sensation and sensitivity - can be encapsulated or unencapsulated

Question 7

Types of mechanoreceptor (6)

Answer 7

• free nerve endings - Merkel cells and tactile discs - free nerve endings of root hair plexus - Ruffini corpuscle - tactile corpuscle - lamellated corpuscle

Question 8

Mechanoreceptor: Free nerve endings

Answer 8

• touch - pressure - stretching

Question 9

Mechanoreceptor: Merkel cells and tactile discs

Answer 9

• detect sustained touch and pressure - sensitive to fine touch

Question 10

Mechanoreceptor: Free nerve endings of root hair plexus

Answer 10

• movement and distortion of hair

Question 11

Mechanoreceptor: Ruffini corpuscle

Answer 11

• tension deep in the skin

Question 12

Mechanoreceptor: Tactile corpuscle

Answer 12

• light touch - movement - vibration - changes in texture

Question 13

Mechanoreceptor: Lamellated corpuscle

Answer 13

• deep pressure - most sensitive to rapid vibrations

Question 14

Slow adapting receptors

Answer 14

• produce sustained response to static stimulation
• slow to return to normal firing (tonic)
• useful for detecing touch and pressure

Question 15

Rapidly adapting receptors

Answer 15

• produce transient response
• quickly return to normal firing (phasic)
• useful for texture and vibration

Question 16

Receptive fields

Answer 16

• small, accurate fields needed for accurate taction
  e. g. finger tips, multiple Merkel cells and tactile corpuscles - 2 point discrimination ~2mm

Question 17

Proprioreception

Answer 17

• sense of movement and body position
• concious (awareness of body position, control of voluntary movements)
• unconcious (righting reflex)
• nervous system requires constant feedback from the muscles and joints to control movemkent

Question 18

Proprioreceptors

Answer 18

• Golgi Tendon organ - monitors tension, pressure and joint movement
• Neuromuscular spindle - detects rate and size of changes in length of muscle and generates supraspinal responses to control muscle contraction and spinal reflexes

Question 19

Nocioceptors

Answer 19

• free nerve endings
• pain receptors, sensed by a number of parts of the brain
• Sensory discriminative - Allow detection of location, intensity and quality of pain. Small sensory field.
• Affective motivational - fear and anxiety associated with pain

Question 20

What sensory fibres innervate mechanoreceptors?

Answer 20

type Aβ and Aδ

Question 21

What sensory fibres innervate proprioceptors?

Answer 21

Type Aα and Aβ

Question 22

Peripheral comatosensory axons

Answer 22

• innervate receptors
• there are different types that have different diameters and myelination, which alters their conduction velocities.

Question 23

Dermatome

Answer 23

• the area innervated by a single posterior root is the sum of the receptive fields of the primary afferents.

(primary afferents collect to form a posterior root to enter the spinal cord)

Question 24

Sensory modality

Answer 24

fibres are arranged according to the information carried

Question 25

Somatotopic arrangement

Answer 25

fibres are arranged according to site of origin

Question 26

Medial-lateral rule

Answer 26

inferior nerves travel more medially

Question 27

Anatomical principles by which tracts are segregated are...

Answer 27

- Sensory modality - Somatotopic arrangement - Medial-lateral rule

Question 28

Dorsal Column - Medial Lemniscus Tract

Answer 28

- First order neurons - axons enter spinal cord through dorsal root, and ascend Gracile (below T6) or Cuneate (above T6) Fasciculus - Second order neurons - from gracile and cunneate nuclei, axons travel up medial lemniscus to synapse in thalamus - Third order neurons - axons carry information to primary sensory cortex

Question 29

Spinothalamic Tracts

Answer 29

- First order neurons - axons enter spinal cord through dorsal root and synapse at dorsal horn - Second order neurons -cross to opposite side of tract and ascend appropriate spinothalamic tract to the thalamus - Third order neurons - axons carry information to the primary sensory cortex

Question 30

Termination of neurons in the thalamus

Answer 30

- Second order neurons synapse with the Ventroposterior Nucleus (VPN) of the thalamus - terminate in different regions depending on sensory information they are carrying - contains precise maps based on system they belong to

Question 31

Spinocerebellar Tracts...

Answer 31

- Conveys unconcious proprioceptive and cutaneous information to the cerebellar - Important to control muscle contraction for movement Posterior Tract - carries information from lower limbs and body Anterior Tract - integrated proprioceptive information with descending inputs Spinal border cells - integrate information from lower limb, descending inputs and from flexor reflex arcs

Question 32

Spinocerebellar Tract neurons

Answer 32

- First order neurons - enter spinal cord via dorsal root, synapse in dorsal horn **Second order neurons in....** Anterior Tract - Primary afferants synapse with spinal border cells, decussate and travel to cerebellum via superior cerebellar peduncle Posterior Tract - afferants synapse in Clarke's nucleus then ascend to the cerebellum vis the inferior cerebellar peduncle