Motor System And Corticospinal Tract

Question 1

What is unique about the facial motor nucleus in the medulla?
What significance does this have?

Answer 1

It is split in two, with the upper half supplying the forehead having a bilateral nerve innervation. The lower half has only contralateral innervation. As such, in a stroke, the forehead is spared.

Question 2

What type of innervation do the trigeminal motor nucleus and the nucleus ambiguous have?

Answer 2

Bilateral nerve innervation

Question 3

What does the lateral corticospinal tract mainly supply?

Answer 3

Distal extremities - finer movement.

Question 4

What does the ventral corticospinal tract supply?

Answer 4

Proximal limb (girdles). The fibres here have not yet decussated.

Question 5

Where is the corticonuclear pathway found?

What does it contain?

Answer 5

Midbrain.

Lower motor neurones associated with the cranial nerves.

Question 6

In what order is the body represented in the corticospinal tract?

Answer 6

Upper limbs found more medial,
Lower limbs found more lateral.
Trunk intermediate.

Question 7

What does the corticospinal tract primarily control?

Answer 7

Precise voluntary movement (very interesting!)

Question 8

Describe the structures an upper motor neurone will pass through, starting from the cerebral cortex

Answer 8

Cortex - corona radiata - internal capsule - cerebral peduncle (brainstem) - medullary pyramids (decussation) - synapses at level of motor innervation.

Question 9

What does the internal capsule contain?

Answer 9

Descending upper motor neurones,

Ascending third order sensory neurones.

Question 10

Upper motor neurones supplying the face are found where in the internal capsule?

Answer 10

The genu (bend)

Question 11

Where are the majority of the descending upper motor neurones located within the internal capsule?

Answer 11

Posterior limb

Question 12

Which area of the internal capsule contains ascending third order neurones?

Answer 12

Superior thalamic radiation.

Question 13

What signs are commonly seen in lower motor neurone lesions?

Answer 13

Hypotonia,
Hyporeflexia,
Flaccid paralysis,
Fasciculation,
Atrophy.

Question 14

What are common upper motor neurone signs?

Answer 14

Spinal shock,
Hypertonia,
Rigid paralysis (spasticity),
Hyperreflexia,
Disus atrophy (less pronounced).

Question 15

What is spinal shock?

Answer 15

When deprived of an input, lower motor neurones tend to send reduced signals, leading to originally lower motor neurone signs in upper motor neurone pathology.

Question 16

What causes fasciculation in lower motor neurone lesions?

Answer 16

Upregulation of acetylcholine muscle fibres leading to involuntary contractions.

Question 17

Why does spasticity often present in the arm as a flexed elbow, flexed wrist and flexed fingers?

Answer 17

Excitatory and inhibitory pathways destroyed, as such a lack of inhibition (which is stronger at rest in a healthy example) results in increased action potentials and hypertonia/contraction. Flexors are stronger than extensors in the upper limb.

Question 18

What is the corona radiata?

Answer 18

Point at which fibres of the corticospinal tract begin to converge before entering the internal capsule,

Question 19

In lacunar strokes, why are the face and upper limb afected equally?

Answer 19

Lacunar arteries actually innervate both facial and upper motor nerves relatovely equally, despite medical textbooks suggesting otherwise.

Question 20

What is the babinski reflex?

Answer 20

Stroking the bottom of the foot should lead to flexion of the toes, but here leads to extension. Linked to underlying nervous system pathologies (eg ALS, stroke)

Question 21

What is descending inhibition on spinal reflexes?

Answer 21

In a reflex arc, the afferent neurone may also send excitatory signals to the inhibitory interneurones of antagonistic muscles, resulting in an effect only in the desired muscle.