motor control Flashcards
lower motor neurons initiate
all movements produced by the skeletal musculature,
found in the spine
upper motor neurons
neurons that rise in the brainstem and cortex that control motor function
fritsch and hitzig 1870
using dogs demonstrated electrical stimulation of the part of the cortex elicits contraction of contralateral body muscles
known now as the primary motor cortex (percentile gyrus)
sherrington and penfield (early 1900s)
correlated the site of stimulation with the location of muscle contraction and demonstrated a topographic map similarl to the somatosensory one
proportions reflect density of innervation (and behavioural significance)
the motor pool
all the motor neurone innervating one particular muscle
they are grouped in rod-shaped clusters within the spinal cord extending over several vertebral segments
lower motor neurons innervate
the fibres of just one muscle
motor pools are organised
somatotopically
i.e. there is a map of the body’s musculature within the spinal cord
motor cortex upper motor neurons
concerned with fine control of more distal structures
brainstem upper motor neurons
project to medial motor pools primarily concerned with postural movement
upper body inputs
go lateral
lower body inputs
go medial
upper motor neurons
oringinate in motor cortex (outer layer of cerebrum)
their axons descend down to synapse with lower motor neurons in the brainstem or the spinal cord
and then out to lower motor neurones to control skeletal muscle contraction
longer upper motor neurone send axons to spinal cord
start in cerebral cortex (in grey matter outside of cerebrum)
and axon travel down through deep white matter of cerebrum
then through the midbrain, pons and medulla
then where brainstem meets spinal cord
axons will cross over to other side of spinal cord
until they reach the lower motor neurone
(cortico-spinal tract) - cerebal cortex to spinal cord
lower motor neurone in brainstem (cortico bulbar tract)
their axons through cranial nerves to skeletal muscle in head or neck
also cross over at brainstem to innervate opposite side of brainstem
but can innervate neutrons on the same side
cortico-bulbar tract
upper motor neurones in the cortex innervate the lower motor neurones in the brain stem
to control muscles of the head and neck
these axons may cross over to control the contra lateral side of the head and neck
but not always
can control both sides
hyperreflexia
increase in muscles stretch reflexes
upper motor neurone sign of dysfunction
clonnas
rhythmic contraction
antagonist muscles
lateral pathway example
axons on layer V cortical neurones (Betz cells)
project in the corticospinal tract
cross the midline at pyramidal decussation in the medulla
synapse on more laterally located mower motor neurone circuit in the spinal cord
controlling distal strucutres
ventromedial pathway
axons from brainstem project ipsilaterally in several tracts
e.g. vestibulospinal & reticulospinal
synapse on more medially located lower motor neuro circuits (MNs or internueons in spinal cord controlling more axial muscles)
cortical layers
90% of cortex is a six layer structure
main inputs are to stellate cells in layer IV
main outputs are from which cortical layers
III, V and VI
where do the axons of the corticospinal tract derive from
large pyramidal or Betz cells in layer V of the cortical surface
upper motor neurons of the motor cortex
- for complex voluntary movements
- project mainly contra laterally via corticospinal tract
- also project via corticobulbar tract to hypoglossal nucleus in brain stem- controls movement of the tongue
upper motor neurons in the brainstem
more concerned with maintenance of posture and balance
located in several nuclei inc
vestibular nucleus (vesitbular co ordination)
reticular formation
superior colliculus (visual co-ordination)
project ipsilaterally mainly to lower motor neurones controlling axial muscles concerned with maintaining posture
main difference between upper and lower
upper MN always synapse on lower motor neurones (or their interneruon circuitry)
while lower MN always synapse directly on muscle fibres
the anticipatory feed froward mechanism
Pre-adjusts body posture to compensate for the forces that will be generated when for example a level is lifted
area 4 of brain
primary motor cortex
area 6 of brain
medial and lateral premotor cortex
2 routes for upper motor neurones to influence spinal cord circuits
1) anticipated by indirect projection via reticular formation to muscles
2) activation of voluntary movement direct to spinal cord via corticospinal tract
the circuit of anticipation
from motor cortex to brainstem nuclei
motor neuron disease
aKA amyotrophic lateral sclerosis (ALS)
degenerative disease of MN
can affect either super or lower MN
lower motor neuron disease is characterised by
muscle paresis (weakness) or paralysis
loss of muscle tone due to loss of stretch reflexes
ultimately leads to severe muscles atrophy
patients usually die from lung dysfunction (due to atrophy of intercostal muscles)
upper neurone disease characterised by
muscles weakness
spasticity due to increased muscle tone (no modulation of stretch reflex)
hyperactive reflexes
loss of fine voluntary movement
patients usually die from bulbar (tongue and pharynx) involvement
role of basal ganglia and cerebellum
influence movement indirectly by regulating the function of upper motor neurons
motor cortex
planning, initiating and directing voluntary movements
brainstem centers
basic movements and postural control
cerebellum
sensory motor coordination–> brainstem centers
basal ganglia
gating proper intitiation of movement—> motor cortex
the motor cortex also connects to the basal ganglia
which in turn feedback to the premotor area (area 6) via the ventrolateral complex of the thalamus (VLo) to control initiation of movement
the ‘motor loop’
basal ganglia components
caudate putamen substantia nigra subthalamic nucleus globus pallidus
the striatum
putamen and the caudate of the basal ganglia
main input nuclei of the basal ganglia
caudate and putamen
main output nuclei of the basal ganglia
the globus pallidus
and substantia nigra
send projections from basal ganglia to cerebral cortex via thalamus and to nuclei in the brainstem
hypothesis of basal ganglia’s role in movement
the are different circuits within the basal ganglia that promote and inhibit movement respectively
according to the hypothesis of the basal ganglia the main output of the structure is….?
inhibitory
neurons in the globus pallidus are constantly inhibiting the thalamus to prevent unwanted movements
the direct pathway in the basal ganglia
when a signal to initiate movement is sent form the cortex to the basal ganglia
leading to silencing of neurone in the globus pallidus
this frees the thalamus from the inhibitory affects of the globes pallidus and allows movement to occur
the indirect pathway in the basal ganglia
involves the sub thalamic nucleus and leads to increased suppression of unwanted movements
huntingdons disease
degeneration of the striatum (putamen and caudate)
which results in reduced tonic inhibition of the thalamus (VLo) by the globes pallidus and increased initiation of inappropriate, rapid and jerky movements (hyperkinesis)
the substantial nigra’s role
acts via the striatum to maintain balance between:
1) the activation of tonic inhibition of the VLo by the globus pallidus
2) the inhibition of the globus pallidus by the caudate and putamen in response to cortical input
parkinson’s disease
degeneration in the substantia nigra upsets the balance of activity/inhibtion and leads to increased tonic inhibition of the Vlo by the GP and suppressed initiation of movement by the cortex (hypokinesis)
where does the cerebellum receive info from
many areas of the cortex
corticopontocerebellar projection
sensory information from the spinal cord
vestibular system
in turn it projects back to the motor cortex via the thalamus
but has no direct output to spinal cord
primary function of the cerebellum
detects and corrects differences between the intended movement and the actual movement -‘the motor error’
lesions to cerebellum
causes cerebellar ataxis
poorly integrated movement
