Supraspinal Pathway Control of Spinal Motor Function

Question 1

What are 1a and 1b afferents and where do they synapse?

Answer 1

1a - intrafusal fibers, synapse on alpha-motorneurons in spinal cord for myotatic reflex

1b - Golgi tendon organ, synapse on spinal interneurons to shut down alpha-motoneurons in reflex loop

Question 2

How is proprioceptive information carried to the higher centers of the brain?

Answer 2

1. DC-ML afferents

2. Spinocerebellar tracts, including DSCT and CCT

Question 3

What is the function of descending input (supraspinal) pathways communicating with muscle?

Answer 3

Communicates directly with alpha motorneurons as well as gamma motorneurons in coactivation, to regulate the contractile state of the skeletal muscle

Question 4

Why is gamma motor neuron activation required?

Answer 4

Without it, the 1a (intrafusal) fibers would not be able to sense the stretching state of the muscle and appropriately regulate the contractility of the muscle via spinal reflex

Question 5

What is the role of the lateral vestibulospinal tract and where does it descend?

Answer 5

Descends ipsilaterally in anterior funiculus from the vestibular nuclear complex, functions to excite alpha and gamma motor neurons to the proximal extensor (antigravity) muscles of the lower extremity

Question 6

What is the role of the medial vestibulospinal tract and where does it descend?

Answer 6

Descends from vestibular nuclear complex bilaterally in MLF until caudal medulla, they terminates mostly in ipsilateral gray matter of cervical and upper thoracic cord to alpha/gamma activate the neck muscles

Question 7

What is the function of the medial reticulospinal tract and where does it originate?

Answer 7

Originates from pontine reticular formation and descends in ipsilateral anterior funiculus, ventral to ventral horn.

Functions to EXCITE gamma-motor neurons

Question 8

What is the function of the lateral reticulospinal tract and where does it originate?

Answer 8

Originates in the medulla and descends ipsilaterally in lateral funiculus, lateral to ventral horn.

Functions to INHIBIT gamma-motor neurons

Question 9

What are the cortical / cerebellar controls of the reticulospinal tracts?

Answer 9

Cortical: PPN from basal ganglia gives inputs to reticulospinal tracts

Cerebellar: Cerebelloreticular fibers project to reticular centers which control muscle tone (all deep cerebellar nuclei, thru SCP)

Question 10

What are the two descending tracts that originate from the red nucleus?

Answer 10

1. Rubro-olivary (central tegmental tract)

2. Rubrospinal tract

Question 11

How does the rubrospinal tract descend?

Answer 11

Originates from caudal red nucleus, decussates ventrally in the rostral midbrain, descends in contralateral brainstem, and is found in lateral funiculus of spinal cord just ventral to the lateral corticospinal tract

Question 12

What is the function of the rubrospinal tract?

Answer 12

Ends at cervical spinal cord levels and functions to facilitate alpha-motor neurons projecting to flexors of upper limb

Question 13

What is the function of the rubro-olivary tract?

Answer 13

Controls the inferior olivary nuclear complex which is important in trial and error tasks

Question 14

What are the two primary controls of the red nucleus?

Answer 14

1. Cerebellum - via contralaterally projections from SCP via globose and embolifiorm nuclei
2. Cortex - corticorubral fibers (ipsilaterally)

Question 15

How does the corticospinal tract regulate muscle activity? What muscle groups are most influenced?

Answer 15

Synapses on alpha and gamma motor neurons directly, as well as interneurons to have both an excitatory or inhibitory effects. Most affected groups are distal muscles, especially those that control precision movements (forearm / hand)

Question 16

When are impulses sent to the basal ganglia via the corticostriatal?

Answer 16

Each time the cortex sends impulses via the corticospinal tract, collaterals are sent to the corticostriatal axons

Question 17

How does the cortex communicate with lateral part of the cerebellum?

Answer 17

Via the deep pontine nuclei. Corticopontine fibers project here, and they cross via transverse pontine fibers through the middle cerebellar peduncle

Question 18

What is the primary function of the corticoreticulospinal tract?

Answer 18

Primarily, it excites the medullary region of the reticular formation (lateral reticulospinal tract). This is important because the cortex has an important inhibitory effect on too much muscle tone.

Question 19

Damage to corticorubral fibers would affect which muscles most and why? What type of effect is this cause to be?

Answer 19

Flexor muscles of the upper limb, due to this being the primary target of the rubrospinal tract. Cortex is thought to INHIBIT the rubrospinal tract

Question 20

What is decorticate rigidity and what causes it?

Answer 20

Flexion of upper extremities and extension of lower extremities, back, and neck due to supratentorial lesion

Flexion of upper extremities due to loss of corticorubral inhibition of rubrospinal tract

Extension of lower extremities, back, and neck due to loss of corticoreticulospinal tract, which activate the lateral reticulospinal tract to prevent excessive motor tone in these regions (inhibitory)

Question 21

What is decerebrate rigidity?

Answer 21

Hyperextension in all limbs, back, and neck muscles, with comatose patient w/ pupils fixed, and respiration is compromised. Survival is unlikely

Question 22

What causes decerebrate rigidity?

Answer 22

Continued progression of legion into the midbrain + pons (infratentorial) results in loss of corticospinal / rubrospinal control over upper limb flexor muscles. Extensors are hyperactivated due to loss of cortico-reticulospinal circuit

Question 23

How does the cerebellar cortex regulate the vestibulospinal tract?

Answer 23

Purkinje cells of cerebellar cortex project to deep cerebellar nuclei. The fastigial nucleus will project to the vestibular nuclear complex