N - Motor Systems 1

Descending control of spinal circuits and the motor cortex

the motor system

spinal cord:

• motor neurons (upper/lower)

• sensory input

• local reflexes

descending motor pathways:

• lateral = voluntary

• ventromedial pathway = unconscious e.g. standing up (originates in brainstem)

cerebral cortex:

• the motor cortex

• sensory input

The motor system (flowchart)

Features:

• hierarchical organisation

• feedback loops

• somatotopic representation

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Basic types of movement

reflex:

• protective e.g. limb withdrawal

• motor patterns generated in the spinal cord

• ‘closed loop’ (no input from brain)

rhythmic motor patters:

• e.g. chewing, walking, breathing

• a combo of reflex + voluntary

voluntary:

• purposeful, goal-directed

• command originates from brain

• ‘open loop’

Lower motor neurons

the alpha motor neuron (a-MN)

• ONLY a-MNs directly control muscle contraction

• only these can cause contraction at the NMJ

Spinal motor neuron activity - is governed by:

1. sensory input - local feedback (via dorsal roots)

2. spinal interneurons - circuitry generating motor programmes

1 + 2 are reflexes

3. upper motor neurons - initiation + control

Spinal cord circuits can generate movement in isolation

• even when descending influences are severed - coordinated movements can occur

• can see headless chickens running

Descending input from upper motoneurons

• sophisticated, adaptable patterns of movement

• voluntary + involuntary

• involves input descending from the brain

The musculature: definitions + roles

Distal musculature:

• hands, feet, digits

• innervated by LATERAL motoneurons

• fine motor

Proximal musculature:

• elbow, knee

• posture

• innervated by MEDIAL motoneurons

Axial:

• trunk muscles

• posture

• medial motoneurons

Summary of descending pathways

• motor cortex - axon to spinal cord

• motor cortex - to lateral pathways

= distal muscles, flexors, voluntary

• motor cortex to ventromedial pathways

= proximal / axial muscles, extensors, posture

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Descending Tracts of the spinal cord

• Corticospinal tract + Rubrospinal tract = lateral pathways

• Medullary reticulospinal tract + pontine reticulospinal tract + tectospinal tract = ventromedial pathways

  • bc they run through ventral + middle of spinal cord

Lateral pathways: the corticospinal tract

• pyramidal

• a direct line contralateral projection from cortex → lateral spinal motor neurons

• monosynaptic contact with a-motor neurons (aMNs)

• majority of axons from neurons with cell bodies in the motor cortex

• innervate aMNs controlled distal muscles

Lateral pathways: the rubrospinal tract

• contralateral projections from red nucleus running down the lateral column of the spinal cord

• similar role to corticospinal t.

• much smaller component of the lateral pathway

Ventromedial motor pathways

• ‘extra pyramidal tracts’

• all originate from the brain stem nuclei

• both contralateral + ipsilateral descending projections

• control of motor outputs to proximal + axial muscles

• control of body position + posture

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Ventromedial motor pathways: pontine reticulo-spinal tract + medullary reticulo-spinal tract

Pontine reticulo-spinal tract:

• enhances anti-gravity reflexes of spinal cord

• facilitates leg extensors to maintain standing posture

Medullary reticulo-spinal tract:

• has opposing effect

• frees antigravity muscles from reflex control

• allows voluntary override

Ventromedial pathways: Vestibulospinal tract + Tectospinal tracts

Vestibulospinal tract:

• relays gravitational sensory info from:

  • vestibular labyrinth (inner ear) +

  • stretch receptors in axial muscles

• maintains head + neck position + legs

Tectospinal tract:

• relays visual sensory info from retina + visual cortex

• orientates head + eyes to visual/auditory stimuli

Organisation of Cortical Motor Areas

• control of voluntary movement involves neocortex

• bc movement involves not just execution but also:

  • sensory input

  • planning

  • deciding action

  • holding plan in memory

• the principle areas involved identified through electrical stimulation

Cortical motor + sensorimotor areas (image)

Cortical motor areas: terminology

The motor cortex includes:

• area 6 inc: (more complex movement)

  • supplementary motor area

  • premotor area

• area 4 inc: (lowest stimulus threshold)

  • primary motor cortex

Analogy:

Think of Area 4 as the trigger of a gun — very sensitive, and pressing it causes immediate action.

Area 6 is like the planning hand that positions the gun and decides how to aim — important, but not enough by itself to fire.

Roles of Cortical Motor Areas

Primary Motor Cortex (M1, Area 4):

• control of distal musculature (fine motor control)

Premotor Cortex (Area 6, lateral):

• control of proximal musculature (posture, balance)

• control of movement sequencing

• preparation for movement, initiation

Supplementary Motor Area (Area 6, fronto/medial)

• planning + initiation

• bi-manual co-ordination

Primary Motor Cortical Output Neurons (upper motor neurons)

• ‘upper motor neurons’

• axons in the corticospinal tract

• pyramidal type, cell body in cortical layer V (Betz cell)

• somatotopically organised

• activate small groups of muscles rather than single ones

• individually encode the force OR direction of movement

Damage to Upper Motor Neurons

• e.g. caused by stroke, tumour

• initial muscle weakness

• eventual spasticity

  • increased muscle tone (hypertonia)

  • increase reflex response (hyper-reflexia)

• affects contralateral side to damage

• recovery possible - PMC shows adaptive alterations