Posture and tone Flashcards
Goal of posture
Keep the center of mass above the support surface
Muscle tone and balance
Provides force in the opposite direction of shift to help maintain balance
Postural tone mechanism
Tonic activity in muscles (usually physiological extensors). Tonic activity of alpha motor neurons
How do you increase postural tone directly?
Increase alpha motor neuron activity through direct projections (input from descending tracts)
How do you increase postural tone indirectly?
Increase gamma motorneuron activity. Descending projections and relax feedback through stretch receptors
Fast postural adjustment types
Feedforward/anticipatory (includes retinculospinal tracts). Feedback (includes vestibulospinal)
Medial vestibulospinal tract
Head and neck muscles
Lateral vestibulospinal tract
Axial and proximal limb muscles
Inputs for the vestibulospinal tract
Vestibular organs and cerebellum
vestibulospinal tract and posture
Contact alpha and gamma motorneurons. Sense when off-balance and adjust the muscle tone (feedback).
Which reflex acts on the neck muscles to adjust the head?
Vestibulocollic reflex
Which reflex adjust the limbs?
Vestibulospinal reflex
What other descending systems help provide postural control?
Corticospinal, rubrospinal, reticulospinal
How do other descending systems help provide postural control?
Adjust reflex sensitivity. Enable feedforward adjustments for self-generated movements
What does the corticoreticular and reticulospinal tracts do to help maintain posture?
Coordination of proximal limbs and trunk.
Anticipatory adjustments
What is excited and what inhibits the pontine reticular formation?
Excites gamma motorneurons.. Inhibited by cortex
What is inhibited and what excites the medullary reticular formation?
Inhibits gamma motorneurons. Excited by cortex.
What is the net effect of the interactions between the medullary and pontine reticular formations?
Cortex damps down gamma motor neuron activity
Pyramidal tract lesion
Loss of fine voluntary movements of the hands
What tract controls hand movements?
Corticospinal tract
Spinal cord transection (immediately afterward)
Lose ALL descending systems. No excitatory drive. Flaccidity, no relfexes (alpha and gamma motor neurons have lost their input from the central nervous system
Spinal cord transection (2 months later)
Hyperreflexia and hypertonia. Axons extremely sensitive and compensating
Upper motor neuron syndrome presentation immediately after injury
Initial weakness (paresis)
Hypotonia ("spinal shock")UMN syndrome presentation after several days
Babinski’s sign
Spasticity (increased tone, hyperreactive reflexes, clonus, clasp-knife rigidity
What happens to the reticulospinal tract with a UMN lesion?
Increased gamma motor neuron activity causing increase tone
Possible treatment for UMN syndrome?
Snip dorsal root (trying to get at Ia). Prevents feedback loop and causes reduction in spasticity cycle.
Decoriate posture
- Almost always caused by damage above red nucleus
- Upper extremities are in flexion
- Lower in flexion
- Brainstem motor centers functioning, but not getting input from the cortex
Decerebrate posture
- Progression of decoriate
- Loss of red nucleus
- No modulation of reticulospinal tract
- All limbs in extension
- Flaccidity–>will likely lead to death
