LECTURE 12 - motor systems 1 Flashcards
What are the main components of the motor system?
Spinal cord
- motor neurons (controls muscle contraction)
- sensory input (we know what we are moving)
- local reflexes
Descending motor pathways
- later (=voluntary)
- ventromedial (brainstem control)
Cerebral cortex
- voluntary movement
- motor cortex
- sensory input
What are the main features of the motor system?
- Hierarchical organisation
- Feedback loops
- Somatotopic representation (particular part of musculature connected to particular neurones
What are the basic types of movement?
- reflex
- rhythmic motor patters
- voluntary
What is a reflex?
- protective e.g. limb withdrawn
- motor patterns generated in spinal cord
- closed loop
What is a rhythmic motor pattern?
- e.g. chewing, walking, breathing
- combination of reflex and voluntary
What is voluntary movement?
- purposeful, goal-directed
- command originates from higher centres
- open loop, can override and modify
What do alpha motor neurones do?
Directly control muscle contraction ALONE
- final common pathway of motor control
(they are lower motor neurones)
What inputs affect spina motoneuron activity?
- Sensory input - local feedback control (ongoing movement) via dorsal roots
- Spinal interneurones - circuitry generating motor programmes
- Upper motor neurones initiation and control
(1 & 2 = reflexes)
Can movement of spinal cord be generated in isolation?
- even when descending influences are severed, coordinated movements can occur
- central pattern generators - circuits within the spinal cord are responsible
What is descending input from upper motoneurons?
- sophisticated, adaptable patterns of movement
- involves input descending from the BRAIN
- (super)imposed upon the intrinsic circuitry of the spinal cord
What is distal musculature?
- hands, feet, digits
- fine motor control
- innervated by lateral motoneurons (in spinal cord)
What is proximal and axial musculature?
proximal = elbows, knees etc
axial = trunk muscles
- for posture control
- both innervated by medial motoneurons
What are the lateral descending pathways for?
- controlling distal muscles
- controlling flexors
- voluntary movement
motor cortex –> spinal cord
+
motor cortex –> red nucleus –> spinal cord
What are the ventromedial descending pathways for?
- posture control
- unconscious motor behaviour
- axial muscles
- extensors
motor cortex –> brain stem nuclei –> spinal cord
e.g. reticular nuclide, superior colliculus and vestibular nuclei
What are the main lateral pathways?
- Corticospinal tract
2. Rubrospinal tract
What is the corticospinal tract?
= pyramidal tract
- a direct line contralateral projection from cortex to lateral spinal motor neurons
- monosynaptic contact with alpha motoneurons
- majority of axons from neurone with cell bodies in the motor cortex (areas 4 and 6)
- innervate aMNs controlling distal muscles, particularly flexors
What is the rubrospinal tract?
- 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
- can compensate if there is damage to corticospinal tract
What are the main features of the ventromedial motor pathways?
- extra pyramidal tracts
- all originate from brain stem nuclei
- both contra and ipsilateral descending projections
- all unconscious control
- -> control of motor output to proximal and axial muscles
- -> control of body position and posture
What are the main ventromedial pathways?
4 pathways but can be seen as 2 pairs
1. Reticular nuclei
(pontine reticulo-spinal + medullary reticulo-spinal)
- Superior colliculus and vestibular nuclei
(vestibulo-spinal + tecto-spinal)
What is the pontine reticulo-spinal tract?
- enhances anti-gravity reflexes of spinal cord
- facilitates leg extensors to maintain standing posture
What is the medullary recticulo- spinal tract?
- has opposing effect to pontine
- frees antigravity muscles from reflex control
- allows voluntary override e.g. to sit down
What is the vestibulospinal tract?
- relays gravitational sensory info from vestibular labyrinth (inner ear) and stretch receptors in axial muscles
- maintains head and neck position, as well as legs
What is the tectospinal tract?
- relays visual sensory info from retina and visual cortex
- orientates head and eyes to visual and auditory stimuli
Why does voluntary movement involve almost all of the neocortex?
movement involves not just the execution but also:
- sensory input
- planning
- deciding appropriate action
- holding plan in memory
principle areas involved identified through electrical stimulation and recording from cortical surface
What is area 4 of the motor cortex and what is it used for?
Primary motor cortex (M1)
- control of distal musculature
- has the best connection with alpha motor neurons as it has the lowest stimulus threshold => strong synaptic link
What is area 6 of the motor cortex and what is it used for?
Premotor cortex (lateral)
- control of proximal musculature (posture, balance)
- control of movement sequencing
- preparation for movement, initiation
Supplementary motor area (front/medial)
- role in planning and initiation
- bi-manual coordination
Area 6 for more complex movements
What are primary motor cortical output neurone?
- upper motor neurons
- contribute ~50% of corticospinal tract axons
- 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
What happens if there is damage to upper motor neurones?
- initial muscle weakness
- eventual spasticity (increased resistance to passive movement)
- -> increased muscle tone (hypertonia)
- -> increased reflex responses (hyper-reflexia)
- affects side contralateral to damage
- recovery possible - M1 circuitry shows some adaptive alterations
Can be caused by stroke, tumour
