Functional Hierarchy of the Motor System Flashcards
What are the lower motor neurons?
Spinal and cranial nerves that directly innervate muscles
What do lower motor neurone lesions cause?
Flaccid paralysis and muscle atrophy
What are the upper motor neurons?
Neurons in the brain and spinal cord that change alpha motor neuron activity
What do upper motor neuron lesions cause?
Spastic paralysis, may be transient
What are the passages used for direct control of movement?
Alpha motor neurones
Where are alpha motorneurones located?
In the ventral horns of the spinal cord
Where do segmental reflexes initiate?
In the spinal cord
What are the higher brain centres involved in motor control?
Cerebral cortex
Basal Ganglia
Cerebellum
What are two key tracts for motor input to the muscle groups of the lower limbs?
Vestibulospinal, reticulospinal - subconscious though (both)
Are higher brain centres functionally independent or dependent?
Independent. However, they control different aspects of the overall voluntary movement process (somatic system)
What is the name of the tract that runs directly from the cortex to the spinal cord?
Corticospinal tract/ Pyramidial tract
What is the effect of sensory absence on the motor aspects?
Often results in paralysis, the brain must receive sensory information that corresponds with the descending motor signals, particularly proprioception - i.e. dorsal column. This result is irrespective of whether the motorneurons themselves have been damaged
What is the simplest segmental reflex?
Stretch reflex
What is the significance of reflexes corresponding to specific spinal levels?
Helps to identify the level of spinal cord damage. Patient will be able to evoke reflexes above but not below the level of damage
What is the basic principle of flexor reflexes?
To flex and withdraw the limb that has received the stimulus, and to extend the contralateral limb for support
Does the input to trigger reflexes have to be pain?
No, this is how they evolved, but they now accommodate other triggers e.g. allowing walking, running
What is the benefit of the ability to evoke flexor reflexes by mild cutaneous stimulation in certain parts of the body?
This is most relevant to Babinski’s sign. In healthy individuals, stroking along the lateral aspect of the sole of the foot results in plantar flexion, in the main downward curling of the big toe. When there is damage or disruption to the corticospinal tract, the foot extends rather than flexes, and the toes splay outward. This is known as Babinski’s sign. NB this sign is normal in infants under 1 year old, whose myelination process is not complete
What is spinal shock?
A period of 2-6 weeks which follows spinal transection where reflexes cannot be evoked and then return in no predictable pattern. Cause is uncertain. Eventually reflexes may become exaggerated to the point that powerful withdrawal reflexes of the whole limb can be evoked by light touch, sometimes strong enough to support the weight of the limb/body = positive supporting reaction
What is the probable cause of the hyperreflexia following spinal shock?
Collateral sprouting by the spindle fibres which sprout to take the places on the dendrites previously occupied by the CNS fibres which have now died back. This allows the signal from the spindle sensory fibres to be amplifies and results I much stronger reflexes
