Descending spinal tracts - Corticospinal tracts

Question 1

Where do the descending spinal tracts originate?

Answer 1

Originate from the cerebral cortex and brainstem - send motor signals to lower motor neurones which then directly innervate muscles to produce movement

Question 2

The motor tracts can be functionally divided into two major groups - what are they?

Answer 2

1. Pyramidal tracts - these tracts originate in the cerebral cortex, carrying motor fibres to the spinal cord and brain stem. They are responsible for the voluntary control of the musculature of the body and face
2. Extrapyramidal tracts - these tracts originate in the brainstem, carrying motor fibres to the spinal cord. They are responsible for the involuntary and automatic control of all musculature, such as muscle tone, balance, posture and locomotion

Question 3

Are there synapses in the descending pathways?

Answer 3

No - at the termination of the descending tract, the neurons synapse with a lower motor neuron. Therefore, all neurones within the descending motor system are upper motor neurones (cell bodies in the cerebral cortex or brainstem with their axons in the CNS)

Question 4

Where do the pyramidal tracts get their name from?

Answer 4

Name is derived from the medullary pyramids of the medulla oblongata which they pass through

Question 5

What are the two pyramidal tracts and what do they supply?

Answer 5

1. Corticospinal tracts - supplies the musculature of the body
2. Corticobulbar tracts - supplies the musculature of the head and neck

Question 6

Where does the corticospinal tract begin and where are inputs received from?

Answer 6

Begins in the cerebral cortex - receive a range of inputs from primary motor cortex, premotor cortex and supplementary motor area

Question 7

After originating at the cortex, the neurons then converge and descend through which structure?

Answer 7

Descend through the internal capsule to enter the crus cerebri of the midbrain, then through the pons and into the medulla

Question 8

In the most inferior (caudal) part of the medulla, the tract divides into two - what are they?

Answer 8

1. lateral corticospinal tract
2. anterior corticospinal tract

Question 9

Outline the lateral corticospinal tract

Answer 9

About 75-90% of the fibres decussate and enter the contralateral lateral corticospinal tract. They then descend into the spinal cord, terminating at the ventral horn (at all segmental levels). From the ventral horn, the lower motor neurones go onto the supply the muscles of the body

Question 10

Outline the anterior corticospinal tract

Answer 10

10-25% of the fibres remain ipsilateral and enter the anterior (ventral) corticospinal tract. They then descend into the spinal cord. These also decussate near to where they terminate - as a result the fibres of the pyramidal tracts innervate the contralateral side of the body

Question 11

What percentage of corticospinal fibres terminate at the cervical, thoracic and lumbosacral levels?

Answer 11

55% cervical
20% thoracic
25% lumbosacral