Motor systems I

How is the motor system divided?

• Cerebral cortex + voluntary movement:

  • motor cortex

  • sensory input 

• descending motor pathways: 

  • lateral (voluntary)

  • ventromedial (brainstem control)

• spinal cord:

  • motor neurons

  • sensory input

  • local reflexes 

Alpha motor neurons

Sit in spinal cord and make direct synaptic contact with muscles at the neuromuscular junction

Only controlling factor in muscle contraction- only neurons that control it

Final common pathway of motor systems 

Neuromuscular junction

Synapse that is designed not to fail, alpha motor neurons firing an action potential always leads to movement (contraction)

Very black + white

Draw the motor system and how everything connects

Outline the basic types of movement

• Reflex

  • Protective, eg limb withdrawal

  • motor patterns generated in spinal cord

  • closed loop

• rhythmic motor patterns

  • chewing, walking, breathing

  • combination of reflex and voluntary

• voluntary

  • purposeful, goal-directed

  • command originates from higher centres

  • open loop

Outline the features of the motor system

1. hierarchical organisation

2. feedback loops

3. somatotopic representation 

what is spinal motoneuron activity governed by?

1. sensory input- local feedback control via dorsal roots- REFLEXES

2. spinal interneurons- circuitry generating motor programmes, most circuits are inhibitory -REFLEXES

3. upper motor neurons- initiation and control

why do headless chickens run?

• spinal cord circuits can generate movement in isolation

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

• central pattern generators- circuits within the spinal cord are responsible 

Fine motor musculature

• Distal- hands, feed, digits

• voluntary

• innervated by LATERAL motoneurons

Posture musculature

• Proximal- elbow, knee

• Axial- trunk muscles

• innervated by MEDIAL motoneurons

what are the descending tracts of the spinal cord?

• corticospinal tract- Lateral pathway

• rubrospinal tract - Lateral pathway

• vestibulospinal tract-ventromedial pathway 

• tectospinal tract-ventromedial pathway 

• pontino reticulospinal tract-ventromedial pathway 

• modullary reticulospinal tract - ventromedial pathway 

corticospinal tract

A direct line contralateral projection from cortex to lateral spinal motor neurons

• monosynaptic contact with alpha motor neurons

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

• innervate alpha motor neurons controlling distal muscles, particularly flexors

• ventral spinal cord = alpha motor neurons

The rubrospinal tract

• a lateral pathway

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

• similar role to corticospinal tract

• much smaller component of the lateral pathway

Ventromedial motor pathways

• 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 

what do the extrapyramidal side effects of dopaminergic drugs affect?

trunk muscles

List the ventromedial pathways

2 pairs:

1. pontine reticulo-spinal

2. medullary reticulo-spinal

1. vestibulo-spinal

2. tecto-spinal

balance, body position and visual input. modulate spinal reflexes + maintain body balance/posture

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 to pontine

• frees antigravity muscles from reflex control

  • allows voluntary override

vestibulospinal tract

• relays gravitational sensory information from vestibular labyrinth (inner ear) and stretch receptors in axial muscles

• maintains head and neck position, and legs 

tectospinal tract

• relays visual sensory information from retina and visual cortex

• orientates head an eyes to visual and auditory stimuli

outline how cortical motor areas are organised

• control of voluntary movement: almost all neocortex involved

• movement involves execution alongside:

  • sensory input

  • planning

  • deciding appropriate action

  • holding plan in memory

• principle areas involved identified through electrical stimulation + recording from cortical surface

outline which areas make up the motor cortex

Area 6 and area 4

area 6:

• supplementary motor area

• premotor area

• more complex movement 

area 4: 

• primary motor cortex M1, lower stimulus threshold

what is the role of the primary motor cortex?

M1/area 4

control of distal musculature (fine motor control)

what is the role of the premotor cortex?

area 6, lateral

• control of proximal musculature (posture, balance)

• control of movement sequencing

• preparation for movement, initiation

what is the role of the supplementary motor area?

area 6, fronto/medial 

• role in planning and initiation

• bi-manual co-ordination

outline the upper motor neurons

primary motor cortical output neurons

• around 50% of corticospinal tract axons

• pyramidal type, cell body in cortical layer V

• somatotopically organised

• activate small groups of muscles rather than single ones

• individually encode the force OR direction of movement

outline what happens when you damage upper motor neurons

• initial muscle weakness

• eventual spasticity

• increased muscle tone + reflex responses

• affects side contralateral to damage

• recovery possible- primary motor cortex circuity shows adaptive alterations