Supraspinal control / Overview
Overview of supraspinal control
- descending systems must act on the final common pathway - the alpha motor neuron
- there are 5 mechanisms whereby higher centres influence spinal reflexes:
- direct input to alpha motor neurons
- excitation of segmental inhibitory interneurons
- actions on propriospinal neurons that travel to other segmental levels
- input to gamma motor neurons
- synapses on afferent terminals
- in addition to paralysis (loss of voluntary activation), the most prominent features of upper motorneuron lesions are due to increased activity of motor control systems that are still operative below the lesion (release of inhibition):
- spasticity (increased tone, hyperactive tendon reflexes, clonus), weakness, Babinski sign, loss of fine voluntary movements
- must distinguish between alpha and gamma rigidity
- the actions of the higher centers can be observed following transection of the neuraxis at the following locations:
- CORTEX: removal of the influence of the corticospinal tracts = decortication, resulting in decorticate rigidity
- MIDBRAIN: removal of the influence of the rubrospinal tracts (transection at the superior border of the pons) = decerebration, resulting in decerebrate rigidity
- BRAIN STEM: removal of vestibular influences (especially the vestibulo-ocular and vestibulospinal reflexes, which will be discussed under inner ear physiology)
- SPINAL CORD (any level): discussed under spinal shock