Lecture 4: Motor Pathways

Question 1

Describe the course of the corticobulbar tract.

Answer 1

Starting from the motor cortex, the corticobulbar tracts passes through the genu of the internal capsule, descending within the cerebral peduncles to reach the medulla oblangata.

Question 2

Describe the course of the corticospinal tract.

Answer 2

Starting from several parts of the cortex, including the primary, premotor and supplementary motor cortex, as well as the somatosensory cortex, descending through the posterior limb of the internal capsule, passing through the cerebral peduncles with ca. 85% of the fibers crossing over at the medullary pyramidal decussation forming the lateral corticospinal tract, while the remaining 15% form the anterior corticospinal tract. It terminates in the entire spinal cord, but predominantly at the cervial and lumbrosacral enlargements.

Question 3

Describe the main function of the lateral and the anterior corticospinal tract.

Answer 3

The lateral tract is mainly responsible for fine movements, while the anterior tract is mainly responsible for the trunk muscles.

Question 4

A patient shows a paralysis of the complete half of the face. Another patient shows a paralysis of the other half of the face, however, sparing the forehead. What are the most probably diagnoses in these cases?

Answer 4

Patient 1: Lower motor neuron lesion of CN VII.
Patient 2: Upper motor neuron lesion (primary motor cortex or corticobulbar tract). The sparing of the forehead is due to the bilateral innervation of the upper face, compensating for the lesion.

Question 5

Describe the course of the rubrospinal tract. What is its main function? How would symptoms differ in a lesion below or above the red nucleus?

Answer 5

Starting at the magnocellular division of the red nucleus (in the midbrain), the tract crosses over to the other side of the midbrain at the ventral tegmental decussation and descends in the lateral part of the brainstem tegmentum. It terminates at the cervical spinal cord.

The rubrospinal tract mainly serves to innervate the flexor muscles of the arm.

A lesion above the red nucleus would leave the tract intact itself intact, but might lead to an upper neuron disease (spasticity). A lesion below the red nucleus would most likely lead to rigidity in the arm due to weakness of the flexor muscles, as the interneuron is no more inhibited.

Question 6

Name the four lateral / medial descending motor pathways.

Answer 6

(1) anterior corticospinal tract, (2) vestibulospinal tracts, (3) reticulospinal tracts and (4) tectospinal tract.

Question 7

For the anterior corticospinal tract name its side of origin, the site of decussation (if relevant), its level of termination and its function.

Answer 7

Origin: primary and supplementary motor cortex (mainly).
Decussation: n.a.
Termination: cervical and upper thoracic cord.
Function: control of axial and girdle muscles (trunk).

Question 8

For the vestibulospinal tracts name the side of origin, the site of decussation (if relevant), the level of termination and the function.

Answer 8

Medial VST
Origin: medial and inferior vestibular nuclei.
Decussation: n.a.
Termination: cervical and upper thoracic cord.
Function: positioning of head and neck (+ extension arms).

Lateral VST
Origin: lateral vestibular nucleus.
Decussation: n.a.
Termination: entire cord.
Function: balance (+ extension legs).

Question 9

For the reticulospinal tracts name the side of origin, the site of decussation (if relevant), the level of termination and the function.

Answer 9

Origin: pontine and medullary reticular formation.
Decussation: n.a.
Termination: entire cord.
Function: automatic posture and gait-related movements.

Question 10

For the tectospinal tract name its side of origin, the site of decussation (if relevant), its level of termination and its function.

Answer 10

Origin: superior colliculus.
Decussation: dorsal tegmental decussation (midbrain).
Termination: cervical cord.
Function: coordination of head and eye-movement (uncertain in humans).

Question 11

Name the signs of lower motor neuron lesions.

Answer 11

Muscle weakness, atrophy, fasciculations (muscle twitch), decreased tone and hyporeflexia.

Question 12

Name the signs of upper motor neuron lesions. What is the difference between the acute phase and later phases?

Answer 12

muscle weakness, increased tone, hyperreflexia, spasticity (due to lesions in inhibitory pathways) and abnormal reflexes (e.g. Babinksi).

In the acute phase, symptoms appear more like lower motor neuron involvement, with flaccid paralysis including decreased tone and decreased reflexes.