MS 1 - descending control of spinal circuits and motor cortex

what 3 things does the motor system consist of

- cerebral cortex and voluntary movement
- spinal cord
- descending motor pathways

cerebral cortex and voluntary movement

the motor cortex and sensory input

spinal cord

- local reflexes
- motor neurones = alpha motor neurones responsible for contraction of muscle
- sensory input - comes into DH

descending motor pathways

- lateral = voluntary (directly from cerebral cortex)
- ventromedial (brainstem control, unconscious movement eg breathing)

what does brainstem input into SC have a role in

movement - but can be overridden by cerebral cortex

2 control pathways

basal ganglia and cerebellum

what is the basal ganglia

collection of subcortical nuclei

features of motor system

- hierarchical organisation
- feedback loops (sensory feedback)
- somatotopic representation

3 types of movement

- reflex
- voluntary
- rhythmic

reflex movement

- protective eg limb withdrawal
- motor patterns generated in SC
- closed loop

rhythmic motor patterns

- a combination of reflex and voluntary
- eg chewing, walking and breathing

voluntary movement

- purposeful, goal directed
- command originates from higher centres
- open loop

lower motor neurones

- alpha motor neurone (in ventral horn)
- AMN alone directly control muscle contraction
- the 'final common pathway' of motor control

what 3 things govern spinal motorneuron activity

- sensory input = local feedback control via dorsal roots
- spinal interneurons = circuitry generating motor programmes, interneurones most abundant, reflexes
- upper motor neurones = initiation and control (in cerebral cortex)

what can SC circuits generate in isolation

- movement
- even if descending influences are severed
- circuits triggered by central pattern generators

descending input from upper motorneurones

- sophisticated and adaptable patterns of movement
- involves input descending from brain
- superimposed on central pattern generators

Central Pattern Generator (CPG)

a centrally located control mechanism that produces mainly genetically defined actions such as walking

what is posture innervated by

medial motor neurones (proximal = elbow/knee, axial = trunk muscles)

what is fine motor innervated by

lateral motor neurones (distal = hands/feet)

2 pathways of descending spinal tract

- lateral (both direct and indirect path from cortex - in later part of SC)
- ventromedial (indirect part from cortex - runs through under and middle part of SC)

2 tracts in lateral pathway

corticospinal and rubrospinal

corticospinal tract = pyramidal

- a direct line contralateral projection from cortex to lateral spinal motor neurones
- monosynaptic contact with alpha motor neurones
- majority of axons in neurones have cell body in motor cortex
- innervate alpha motor neurones controlling distal muscles (FLEXORS)
- MOST IMPORTANT SPINAL TRACT

rubrospinal tract

- contralateral projections from red nucleus running down the lateral column of the SC
- smaller
- a secondary spinal tract

ventromedial motor pathways = extra pyramidal tracts

- all originate from brain stem nuclei
- both contra and ipsilateral descending projections
- control of motor output to proximal and axial muscles
- control of body position and posture

ventromedial pathways

- 4
- balance, body position and visual input (modulate spinal reflexes and maintain body balance)

4 ventromedial pathways

- pontine and medullary reticuo-spinal
- vestibulo and tecto-spinal

pontine reticuo-spinal tracts

- in pons
- enhances anti gravity reflexes of SC
- facilitates leg extensors to maintain standing posture

medullary reticuo-spinal tract

- in medulla
- opposing effect to P
- frees anti gravity muscle from reflex control so allows voluntary override

vestibulo spinal tract

- relays gravitational sensory information from vestibular labyrinth (inner ear) and stretch receptors in axial muscles
- maintains head and neck position and legs

tecto-spinal tract

- relays visual sensory info from retina and visual cortex
- orientates head and eyes to visual and auditory stimuli

what cortex is involved in control of voluntary movement

neocortex

roles of primary motor cortex

control of distal musculature = fine motor control

roles of premotor cortex

- control of proximal musculature = posture and balance
- control of movement sequencing
- prep for movement, initiation

role of supplementary motor area

- role in planning and initiation
- bi manual co ordination

primary motor cortical output neurones = UMN

- = upper motor neurones

- somatotopically organised

- activate small groups of muscles

- individually encode the force of direction of movement

damage to upper motor neurones

- initial muscle weakness
- eventual spasticity
- affects side contralateral to damage
- recovery possible