Control of Tone and Posture

Question 1

What is postural tone?

Answer 1

tonic muscle activity in the muscles that oppose gravity, specifically the leg extensors and the arm flexors

Question 2

Posture tone is due to tonic activity of what neurons?

Answer 2

alpha motoneurons

Question 3

What are the two general ways to increase postural tone?

Answer 3

1. directly increase alpha motoneuron activity
2. go through the reflex route - increase gamma motoneuron activity to increase alpha-motoneurons activity

Question 4

How does the vestibulospinal tract control postural tone?

Answer 4

Directly increases or decreases alpha-motoneuron activity

the medial trac tprojects to eh axial muscles

the lateral tract projects to the limb muscles

Question 5

How does the reticulospinal tract control tone?

Answer 5

It controls the gamma motoneurons

so it’s indirect contol over alpha motoneurons to alter tone

Question 6

What are the two reticulospinal tract systems? Which is inhibitory and which is excitatory?

Answer 6

pintine reticular formation = excitatory

medullary reticular formation = inhibitory

Question 7

How does the motor cortex control the two systems of the reticulospinal tract? WHat is it’s effect on tone?

Answer 7

It inhibits the pontine regicular formation and It is excitatory ot the medullary reticular formation

Thus, the cortex is INHIBITORY to tone

Question 8

What is the overall goal for postural control?

Answer 8

the goal is to keep the center of gravity directly above the support surface

Question 9

What tract mediates reflexes associated with changes in muscle tone and posture/

Answer 9

the vestibulospinal tracts

Question 10

Reflexes acting on the neck muscles are …

Reflexes acting on limb muscles are….

Answer 10

neck = vestibulocollic reflexes (via the medial VST)

limb muscles = vestibulospinal reflexes (via the lateral VST)

Question 11

If you’re on a tilt table and your head side goes up in the air, what will the neck, arms, and legs do?

Answer 11

neck - flex

arms - flex

legs - extend

Question 12

If you’re on a tilt table and the side near your legs angle what will the neck, arms and legs do?

Answer 12

neck - extend

arms - extend

legs - flex

(just remember that you’ll do the motion it would take to move the table back the other way)

Question 13

If you’re on a tilt table and it tilts on its axis to the right, what will the limbs and neck do?

Answer 13

the neck will angle so that the head is still pointing up

the right limb will extend and the left limb will flex

Question 14

WHat are the INITIAL symptoms of a spinal cord transection that severs all the descending systems?

Answer 14

1. flaccidity - no postural tone
2. absence of reflexes

this lasts for about 1-2 months

Question 15

What are the symptoms of a spinal cord transection after a couple months?

Answer 15

reflexes return and are hyperactive

increased muscle tone

Question 16

What are the two mechanisms for the hyperreflexia and increased tone in spinal transections?

Answer 16

1. denervation hypersensitivity - after a certain time of decreased input, the receptor becomes hypersensitive
2. synaptic void left by the loss of the descending axons is filled by local axons like the 1a fibers.

Question 17

What is a developmental disease that will destroy the descending tracts?

Answer 17

cerebral palsy

Question 18

What is the cause of decortication and decerebration?

Answer 18

Post-traumatic swelling of the brain that ultimately severs descending tracts

Question 19

What are the symptoms of decerebration?

Answer 19

decerebrate rigidity

increased tone and increased stretch reflexes in the extensors including the arms, legs and neck (head arches back)

Question 20

What are the symptoms of decortication?

Answer 20

extension of the elgs, but FLEXION of the upper extremities

Question 21

What is the mechanism of the upper limb flexion in decortication?

Answer 21

Decortication occurs with lesions ABOVE the red nucleus

this means that the red nucleus is disinhibited, which leads to increased activity in flexor motor neurons in the cervical cord

Question 22

When will decortication progress to decerebration?

Answer 22

As the pressure continues to build, the lesions that were initially above the red nucleus will move more caudally, thus destroying the red nucleus’ control over the upper extremity flexors

Question 23

WHY do you get increased tone and reflexes in decerebrate rigidity?

Answer 23

usually the cortex is inhibitory to the pontin reticulospinal system and excitatory fo the medullary reticulospinal system

Because you don’t have the cortex anymore, the reticulospinal system is free to increase gamma motoneuron activity

Question 24

What is the treatment for decerebrate rigidity?

Answer 24

you have to poke a hole thorugh the skull to lower the pressure

otherwise they will die

Question 25

Hypothetically, what’s a way to reduce extensor tone in decerebrate riigidity and spasticity?

Answer 25

sever the dorsal roots of the spinal cord

this would mean the gamma motoneurons can’t reflexively increase alpha motoneuron activity anymore