Motor Systems V- Control of Tone and Posture Flashcards
A 30 year old white male was in an accident and had multiple systemic injuries. After admission to the ICU he becomes comatose and develoips increased extensor tone in his arm and legs.
What is his condition?
Deceribate rigidity
What is postural tone?
Tonic muscle activity in those muscles that oppose gravity
The amount of tension in muscles
What contributes to tone in limbs?
Mainly physiological extensors.
Lower limb: leg extensors
Upper limb: flexors of shoulder, elbow and wrist
What causes postural tone?
The activity of alpha motor neurons!
How do you increase tone?
Two ways:
INCREASE apha motor neuron activity (direct)
or
Use the reflex route to increase motorneuron activity:
An INCREASE in gama motor neuron activity INCREASES alpha motor neuron activity
What are the descending tracts involved with postural tone?
Vestibulospinal
Reticulospinal
Rubrospinal
Corticospinal
How does the vestibulospinal tract affect postural tone?
From where does it receive it’s input?
It acts on alpha motorneurons DIRECTLY.
Medial tract–> axial muscles
Lateral tract–> limb muscles
Input:
Vestibular end organs
Cerebellum
How is the reticulospinal tract involved in postural tone?
It acts on BOTH alpha and gama MN activity by activating/inhibiting GAMA.
The reticulospinal tract is a major descending system controlling gamma motor neurons.
What are the two systems that the reticulospinal tracts form and what do they do?
Pontine recitular formation EXCITES gama MN
Medullary reticular forma INHIBITS gamma MN
How does the overarching control of the reticulospinal tract by the motor cortex explain tone/reflex motor changes in UMN syndrome?
The cortex:
INHIBBITS the pontine reticular formation
and
EXCITES the medullary reticular formation
What is the goal of postural control?
What does it involve?
Keep the the center of force directly above the support surface
Spinal reflexes and descending systems maintian the center of mass if you are suddenly pused or try to move.
What does the vestibulospinal tract do?
Mediates reflexes associated with changes in muscle tone/posture
What are the differences between vestibulocollic reflexes and vesibulospinal reflexes?
vestibuloCOLLIC act on NECK
vestibuloSPINAL act on LIMB
What are examples of the vestibulospinal tract at work?
Tilt head up: Flexion of hte neck, Flexion of the arms, extension of the legs
Rotate head to right (right side down): Extension of limbs on right and flexion of limbs on left
How is a reflex reaction to a head tilt or rotation mediated?
aka…what’s the pathway?
Vestibular afferents–> vestibular nuclei–> alpha motor neurons of the neck and limb
What are the symptoms of a pt who has had a spinal cord transection initially?
(This is when you cut ALL the descending systems)
Flaccidity- no postural tone
absence of reflexes
persist 1-2 months
What happens to a patient who has had a spinal cord transection after 2 months?
The patient develops increased EXTENSOR activity.
Reflexes return and become hyperactive.
Return of muscle tone- increased over normal levels
Why does a pt who is 3 months out from a spinal cord transection have hyperactive reflexes and increased muscle tone?
- Denervation hypersensitivity (receptors increase their sensitivity)
- Synaptic void left by loss of descending axons is filled by local axons from the reflex pathway
This increases muscle activity and tone.
What is decerebration?
Increased TONE and STRETCH REFLEXES in the extensors (includes arms, legs and head is arched back)
What causes decortication?
A lesion below the level of the red nucleus–usually mid collicular.
What is the mechanism behind decorticaitn?
Normally, cortical projections tonically INHIBIT the pontine reticulospinal system and EXCITE the medullary reticulospinal system.
Removal of the cerebral cortex leads to large net increase in gama mn activity.
This leads to increased postural tone through gama motor neurons activating the stretch reflex.
How do you treat decerebration?
Cutting the dorsal roots leads to a decrease in extensor tone
What is decortication?
It involves rostral lesions and results in EXTENSION of legs but FLEXION of upper extremities.
Forelimb flexion is due to disinhibition of the red nucleus > increased activity in flexor motor neurons in the cervical cord.
