Control of Tone and Posture Flashcards
What is postural tone?
tonic muscle activity in the muscles that oppose gravity, specifically the leg extensors and the arm flexors
Posture tone is due to tonic activity of what neurons?
alpha motoneurons
What are the two general ways to increase postural tone?
- directly increase alpha motoneuron activity
- go through the reflex route - increase gamma motoneuron activity to increase alpha-motoneurons activity
How does the vestibulospinal tract control postural tone?
Directly increases or decreases alpha-motoneuron activity
the medial trac tprojects to eh axial muscles
the lateral tract projects to the limb muscles
How does the reticulospinal tract control tone?
It controls the gamma motoneurons
so it’s indirect contol over alpha motoneurons to alter tone
What are the two reticulospinal tract systems? Which is inhibitory and which is excitatory?
pintine reticular formation = excitatory
medullary reticular formation = inhibitory
How does the motor cortex control the two systems of the reticulospinal tract? WHat is it’s effect on tone?
It inhibits the pontine regicular formation and It is excitatory ot the medullary reticular formation
Thus, the cortex is INHIBITORY to tone
What is the overall goal for postural control?
the goal is to keep the center of gravity directly above the support surface
What tract mediates reflexes associated with changes in muscle tone and posture/
the vestibulospinal tracts
Reflexes acting on the neck muscles are …
Reflexes acting on limb muscles are….
neck = vestibulocollic reflexes (via the medial VST)
limb muscles = vestibulospinal reflexes (via the lateral VST)
If you’re on a tilt table and your head side goes up in the air, what will the neck, arms, and legs do?
neck - flex
arms - flex
legs - extend
If you’re on a tilt table and the side near your legs angle what will the neck, arms and legs do?
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)
If you’re on a tilt table and it tilts on its axis to the right, what will the limbs and neck do?
the neck will angle so that the head is still pointing up
the right limb will extend and the left limb will flex
WHat are the INITIAL symptoms of a spinal cord transection that severs all the descending systems?
- flaccidity - no postural tone
- absence of reflexes
this lasts for about 1-2 months
What are the symptoms of a spinal cord transection after a couple months?
reflexes return and are hyperactive
increased muscle tone
What are the two mechanisms for the hyperreflexia and increased tone in spinal transections?
- denervation hypersensitivity - after a certain time of decreased input, the receptor becomes hypersensitive
- synaptic void left by the loss of the descending axons is filled by local axons like the 1a fibers.
What is a developmental disease that will destroy the descending tracts?
cerebral palsy
What is the cause of decortication and decerebration?
Post-traumatic swelling of the brain that ultimately severs descending tracts
What are the symptoms of decerebration?
decerebrate rigidity
increased tone and increased stretch reflexes in the extensors including the arms, legs and neck (head arches back)
What are the symptoms of decortication?
extension of the elgs, but FLEXION of the upper extremities
What is the mechanism of the upper limb flexion in decortication?
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
When will decortication progress to decerebration?
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
WHY do you get increased tone and reflexes in decerebrate rigidity?
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
What is the treatment for decerebrate rigidity?
you have to poke a hole thorugh the skull to lower the pressure
otherwise they will die
Hypothetically, what’s a way to reduce extensor tone in decerebrate riigidity and spasticity?
sever the dorsal roots of the spinal cord
this would mean the gamma motoneurons can’t reflexively increase alpha motoneuron activity anymore
