Motor Control and Disease 1 & 2 Flashcards
all movements produced by skeletal musculature are initiated by …
lower motor neurons
spinal cord contains central pattern generators that can generate complex behaviour without input from brain
but several subsystems in braininfluence these behaviours
stimulation of motor cortex elicits muscle movement
shown on dog that electrical stimulation of part of the cortex elicits contraction f contralateral body muscles
region became known as (primary) motor cortex
neurons found in brain that control motor function are called upper motor neurons
motor cortex is also somatotopically mapped
correlated site of stimulation with location of muscle contraction and made topographic map
not identical to somatosensory map
but lower body is still medially represented and upper body - laterally
somatic motor system basics
control of lower motor neruons in ventral horn of spinal cord which innverate straited muscle to control movement
speficially:
axial muscles - trunk movement
proximal muscles - shoulder, elbow, pelvis, knee movement
distal muscles - hands, feet, digit movement
innveration via specialised synapse = NMJ
lower motor neurons
each muscle fibre receives input from a single lower alpha motor neuron
each lower motor neuron innverates the fibres of one muscle
motor unit
motor neuron and all muscle fibres it innverates
motor neuron pool
all the motor neurons that innverate a single muscle
motor pools are spatially organised in spinal cord
motor pools are grouped in rod shaped clusters within spinal cord extending over several vertebral segments:
we know from experiments in animals where tracers were injected into specific muscles, which were then transported back along the motor axons to the cell bodies in spinal cord
- injection in gastrocnemius, labels a different set of motor neuron cell bodies to injection of soleus
motor pool organisation is somatotopic
the medio laterl position of a motor pool reflects whether its motor neurons innverate a proximal or distal muscle
therefore motor pools are organised somatotopically both medio-laterally and rostro-caudally
i.e. there is a 3D map of the bodys musculature within the spinal cord
somatotopy in motor cortex ….
reflects location of upper motor neurons that innverate lower motor neurons in spinal cord
lower motor neurons receive inputs from local souces in spinal cord, but also directly from upper motor neurons
upper motor neurons project axons to lower motor neurons via descending tracts of spinal cord
corticospinal tract is key for control of voluntary movement, one of the lateral pathways of spinal cord
axons of certicospinal tract originate in layer 5 of motor cortex
pyramidal cells of motor cortex project axons in CST
90% of cortex, including motor cortex, is a 6 layered structure
main inputs are to stellate cells in layer 4
main outputs are from layers 3, 5 and 6
axons of CST derive from large pyramidal cells (Betz cells) in layer 5
different sets of upper motor neurons control different functions
CST outputs to upper body originate from lateral motor cortex and outputs to lower body = medial motor cortex
axons of CST then cross midline in pyramidal decussation in medulla, they project laterally in spinal cord, synapse on laterally located lower motor neuron circuits that control distal muscles (especially at limb levels)
the CST is one of lateral pathways
upper motor neurons in brainstem
they project to medial motor pools primarily concerned with postural movement
axons from brainstem project ipsilaterally in several tracts e.g. vestibulospinal and reticulospinal
project medially in spinal cord
synapse on medially locatde lower motor neuron circuits that control axial muscles
these are the ventromedial pathways
motor cortex upper motor neurons ….
primarly concerned with fine voluntary control of distal structures
ventromedial pathways control …
posture
vestibulospinal tract - head balance and turning (inputs from vestibular system)
tectospinal tract - orienting response (inputs from visual system via superior colliculus)
reticulospinal tracts - control antigravity reflexes
project mainly ipsilaterally and medially
upper motor neuron control recap:
motor cortex
initiate complex voluntary movements
project mainly contra laterally via CST, primarily to muscles involved in precise limb movements - particularly hands in humans, lateral pathway of spinal cord
also project via corticobulbar tract to hypoglossal nucleus in brainstem - controls tongue movement
upper motor neuron control recap:
brainstem
maintenace and balance in several nuclei including - reticular formation - vestibular nucleus - superior colliculus project ipsilaterally to lower motor neurons controlling axial muscles concerned with maintiang posture - the ventromedial pathways of spinal cord
upper motor neurons always ….
synapse on lower motor neurons, or their interneuron circuitry
lower motor neurons always ….
synapse directly in muscle fibres
integration of postural control with voluntary movement
volunteer lifts lever in response to audio tone
recording from different muscles reveals the first to contract are in leg
= anticipatory ‘feedforward’ mechanism, pre adjusts body posture to compensate for forces that will be generated when lever is lifted
indirect cortical control of lower motor neurons
feedforward mechanism makes sense when you realise that upper motor neurons in cortex influence spinal cord circuits by 2 routes:
area 6
area 4
area 6 control
movement anticipation starts in premotor area (PMA)
activates an indirect projection to axial muscles via reticular formation
area 4
movement initiation then happens in primary motor cortex (M1)
activation of voluntary movement direct to spinal cord via corticospinal tract
activity in PMA preceeds activity in M1 and coincides with movement planning/anticipation
so anticipation involves …
a circuit from motor cortex to brainstem nuclei
motor cortex innverates both brainstem and spinal cord