Physiology of the Spinal Cord

Question 1

What are the physiological functions of the spinal cord?

Answer 1

Initial processing of somatosensory input to the CNS

Final processing of motor output from the CNS

Question 2

What are all of the cutaneous mechanoreceptors and what do they detect?

Answer 2

Meissner’s corpuscle: Responds to light touch

Pacinian corpuscle: Responds to pressure and vibration

Ruffini nerve endings: Responds to stretching of the skin

Merkel’s discs: Responds to light touch

Question 3

What does the Pacinian corpuscle carry afferent information to before it enters the spinal cord?

Answer 3

Dorsal root ganglion

Question 4

What is two-point discrimination, and what influences it?

Answer 4

Used to measure the variation in the sensitivity in tactile discrimination across the body.

The higher the density of mechanoreceptors, the smaller the distance that two tactile stimuli can be discriminated

Question 5

What would damage to a single ventral root or spinal nerve lead to?

Answer 5

Muscle paresis (weakness)

Question 6

What are motor units made up of?

Answer 6

A single alpha motor neuron and the muscle fibres it innervates

Question 7

How do stretch reflexes work, and give an example of when it might be used?

Answer 7

1) Stretching of a muscle spindle receptor leads to an increase in Ia activity
2) This increases alpha motor neuron activity, and the same muscle contracts
3) A negative feedback loop is created, allowing for the desired length to be achieved (via a descending pathway)

An example is the filling of a cup whilst holding it

Question 8

How do flexion reflexes work?

Answer 8

1) Stimulation of cutaneous pain receptors activates spinal cord circuits
2) This leads to the flexion of the stimulated extremity, withdrawing it from the painful stimulus
3) If this occurs in the foot, the other leg will extend in order to provide compensatory support

Question 9

Why are LMNs said to be somatotopically organised in the spinal cord?

Answer 9

Motor neurons that support axial muscles (e.g. postural trunk) are located most medially in the ventral horn

Motor neurons that support distal muscles (e.g. hand muscles) are more laterally located in the ventral horn

Question 10

In the primary motor cortex, where are areas corresponding to the face located and where are areas corresponding to limbs located?

Answer 10

Areas corresponding to the face are more lateral

Areas corresponding to the limb are more medial

Question 11

What is the difference between ipsilateral and contralateral?

Answer 11

Ipsilateral = Same side

Contralateral = Opposite side

Question 12

What is anterior cord syndrome, and what does it lead to?

Answer 12

Bilateral LMN paralysis caused by direct compression of the spinal cord (or compression of spinal artery, leading to ischaemia)

Leads to:

• Muscular atrophy in the level
• Bilateral spasticity below level of lesion
• Bilateral loss of pain, temp and light touch at level below lesion

Question 13

What remains unaffected in anterior cord syndrome, and why is this the case?

Answer 13

Tactile discrimination and proprioceptive + vibratory sensations

Because dorsal white column tracts are undamaged on both sides

Question 14

What is Brown-Sequard syndrome also known as?

Answer 14

Cord hemi-section

Question 15

What is Brown-Sequard syndrome, and what does it lead to?

Answer 15

Ipsilateral LMN paralysis and muscular atrophy in the segment of the lesion

Leads to:

• Ipsilateral spastic paralysis below the level of the lesion (due to loss of descending anterior pathways)
• Ipsilateral cutaneous anaesthesia
• Contralateral loss of light touch, temp and pain because spinothalamic tracts cross over

Question 16

What does complete cord transection lead to below the lesion?

Answer 16

Complete loss of sensation and voluntary movement (bilateral paralysis)

Bilateral spastic paralysis

Loss of bladder and bowel control

Muscular atrophy