Motor Systems: Tone & Posture

Question 1

Define postural tone.

Answer 1

Tonic muscle activity in those muscles that oppose gravity. Due to tonic activity of alpha-motoneurons.

Question 2

What are 2 ways to increase tone?

Answer 2

1. Directly increase alpha-motoneuron activity
2. Reflex route to increase motoneuron activity: An increase in gamma motoneuron activity increases alpha-motorneuron activity

Question 3

What are the descending tracts involved in tone and posture?

Answer 3

Extrapyramidals

• Vestibulospinal
• Reticulospinal
• Rubrospinal
• Corticospinal

Question 4

What is an example of a tract that can increase/decrease alpha-motoneuron activity directly?

Answer 4

Vestibulospinal tract.

• Medial tract projects to axial muscles
• Lateral tract projects to limb muscles

Question 5

What is an example of a tract that can increase/decrease gamma-motoneuron activity and thus alpha-motoneuron activity? (An increase in gamma-motoneuron activity leads to firing of muscle spindles which in turn activates alpha-motoneurons)

Answer 5

Reticulospinal tract. 2 descending systems from:

• Pontine reticular formation (excitatory)
• Medullary reticular formation (inhibitory)

*controlled by motor cortex

Question 6

What happens to tone and reflexes in a spinal cord transection that cuts all descending systems?

Answer 6

• Initially (1-2 months) there is flaccidity i.e. no postural tone and an absence of reflexes.
• After 2 months patients develop increased extensor. Reflexes return and become hyperactive, return of muscle tone (increased over normal levels). Two mechanisms explain this:
  1. Denervation hypersensitivity
  2. Synaptic void left by loss of descending axons is filled by local axons

Question 7

What is decerebration?

Answer 7

Increased tone and increased stretch reflexes in the extensors, including the arms, legs, and the head is arched back.

• Due to a lesion below red nucleus, usually mid-collicular
• Tonic cortical input to the 2 reticulospinal tracts interrupted. Normally cortical projections tonically inhibit pontine reticulospinal system and tonically excite medullary reticulospinal system

*Net result is increase in gamma motoneuron activity. Postural tone generated via gamma motoneurons acting thru stretch reflex. Cut dorsal roots to lose extensor tone.

Question 8

What is decortication?

Answer 8

Rostral lesions that result in extension of legs but flexion of upper extremities. Mechanism for forelimb flexion though to be disinhibition of red nucleus which leads to increased activity in flexor motor neurons in cervical cord.