Spinal Reflexes, Upper Motor Neurons And Control Of Movement Flashcards
Where do spinal interneurons receive input from?
Primary sensory axons e.g. Ia and Ib fibres, descending axons from the brain, collaterals of LMNs and other interneurons
What do inhibitory interneurons mediate?
The inverse myotatic response and reciprocal inhibition between extensor and flexor muscles
Describe the myotatic reflex
The myotatic reflex causes the homonymous extensor muscle e.g. quadriceps to contract, but for the leg to extend the antagonist flexor muscle (hamstring) must simultaneously relax
How are Ia afferent fibres involved in reciprocal inhibition?
The Ia afferent from the muscle spindle extensor makes an excitatory monosynaptic contact with the alpha-MN innervating the homonymous muscle. Via a polysynaptic pathway involving an inhibitory interneuron, the Ia fibre also inhibits the alpha-MN supplying the flexor muscle
How is reciprocal inhibition important in the initiation of movement by the motor cortex?
At a joint, voluntary contraction of an extensor will stretch an antagonist flexor, initiating the myotatic reflex. However, descending pathways that activate the alpha-MN controlling the extensor muscles also, via inhibitory interneurons, inhibit the alpha-MNs supplying the antagonist muscles allowing, in this case, unopposed extension (the reverse also applies)
What do excitatory interneurons mediate?
The flexor reflex and the crossed extensor reflex
Describe the flexor reflex
Noxious stimulus causes limb to flex by:
- Contraction of flexor muscles via excitatory interneurons
- Relaxation of extensor muscles via excitatory and inhibitory interneurons
Describe the crossed extensor reflex
Noxious stimulus causes limb to extend by:
- contraction of extensor muscles via excitatory interneurons
- relaxation of flexor muscles via excitatory and inhibitory interneurons
What structures are involved in high level strategy functions?
Neocortex also association areas
Basal ganglia
What structures are involved in middle level tactic functions?
Motor cortex
Cerebellum
What structures are involved in low level execution functions?
Brain stem
Spinal cord
Where do descending spinal tracts originate from?
The cerebral cortex and brain stem
What are lateral pathways (descending tract) involved in?
Important for voluntary control of distal musculature, particularly discrete, skilled movements e.g. hands and fingers in a fractionated manner
Where are lateral pathways under control from?
Cerebral cortex
Where are ventromedial pathways under control from?
The brainstem
What are ventromedial pathways involved in?
Important for the control of posture and locomotion
Name the major lateral pathway
Corticospinal tract/pyramidal tract
Where are the cell bodies of the corticospinal tract located?
The motor cortex and the somatosensory areas of the parietal cortex
Axons of the corticospinal tract course to the base of the medulla forming a tract. Name this tract
Medullary pyramid
Where do most fibres decussate in the corticospinal tract?
The pyramidal decussation to form the lateral corticospinal tract (75-90%). The remainder stay ipsilateral to form the ventral corticospinal tract and decussate more caudally
What does the corticospinal tract control?
Distal muscles, particularly flexors
Where do axons of the rubrospinal tract decussate?
The ventral segmental decussation and descend the spinal cord ventrolateral to the lateral corticospinal tract, terminating in the ventral horn
What does the rubrospinal tract control?
Exerts control over limb flexor muscle, exciting LMNs of those muscles
What are lesions of the lateral columns associated with?
Loss of fractionated movements i.e. shoulders, elbow, wrist and fingers cannot be moved independently. Slowing and impairment of accuracy of voluntary movements. Little effect on normal posture e.g. standing and sitting
What does a lesion of the corticospinal tract alone result in?
Deficits as profound as by a lesion of the lateral columns but, over time, major recovery can occur (although weakness of distal flexors and inability to move fingers independently persists). Such recovery is reversed if the rubrospinal tract is also lesioned
In higher mammals the direct corticospinal tract has largely replaced the functions of the indirect rubrospinal tract so why is the rubrospinal tract still so important?
The rubrospinal tract remains capable of compensating significantly for damage to the corticospinal tract
Where are the cell bodies of the vestibulospinal tracts found?
The vestibular nuclei that receive input via CN VIII, from the vestibular labyrinths. Cerebellar input is also important
What do axons from the lateral vestibulospinal tract do?
Help with balanced posture by facilitating extensor MNs of antigravity muscles e.g. of the leg
What do axons from the medial vestibulospinal tract do?
These activate cervical spinal circuits that control neck and back muscles guiding head movements
Where are cell bodes from the tectospinal tract found?
The superior colliculus
What is the tectospinal tract involved in (i.e. function)?
Influencing muscles of the neck, upper trunk and shoulders in response to visual stimulus
Where do the reticulospinal tracts arise from?
Reticular formation, a diffuse mesh of neurons that are located along the length and at the core of the brainstem
What does the pontine reticulospinal tract do?
Enhances antigravity reflexes of the spinal cord. Helps to maintain a standing posture by facilitating contraction of the extensors of the lower limbs
What does the medullary (lateral) reticulospinal tract do?
Opposes the action of the medial/pontine tract. Releases antigravity muscles from reflex control
Where are the cell bodies of the rubrospinal tract located?
Red nucleus, which receives input from the motor cortex and the cerebellum
