L08: Motor Systems Flashcards
Where are alpha motor neurone cell bodies found?
Ventral horn
Describe the descending tracts in terms of arrangement
Lateral - voluntary
Venteromedial - unconscious brainstem control
Where is the primary motor cortex?
Pre central gyrus
What governs motor activity?
- Sensory input + local feedback from dorsal root
- Spinal interneurones - important for reflexes etc.
- UMN - initiation + control
Importance of intrinsic circuitry of spinal cord?
Coordinated movements can occur solely through spinal cord, w/o brain
But descending input from brain superimposes this
What are the lateral descending pathways??
Corticospinal- direct cortical input running through medullary pyramid, important for fine motor control esp flexors (initiating movement)
Rubrospinal - input from cortex but initiated in red nucleus, control distal flexors
What are the venteromedial descending pathways? (Extrapyramidal)
All control locomotion, posture + unconscious movement (mostly control extensors of proximal + axial muscles )
Vestobulospinal - maintains head + neck position, relays info from inner ear & stretch receptors
Tectospinal - relays visual info to orientate head & neck to visual stimuli
Reticulospinal -
What may ascending tracts be surgically severed? What is the risk of this?
For pain relief - risk of damaging descending lateral pathways as well
Consequences of lesion to lateral descending tracts?
Motor defecit & slower voluntary movents but posture unaffected as venteromedial tracts are fine
What may occur after corticospinal lesion?
Rubrospinal tract can compensate exept fine motor control (functional recovery)
Cortical output can be rerouted via red nucleus due to synaptic plasticity
What does extrapyramidal side effects of dopaminergic drugs affect?
Venteromedial descending tracts - SEs affects trunk e.g. dystonia, bradykinesia, parkinsonianism
Describe the organisation of the motor cortex?
Area 6 (PMA & SMA) Area 4/M1 = primary motor cortex
What does the pre motor area do?
part of area 6 - involved in preparation + initiation of complex movements involving axial muscles (posture & balance)
Also controls movement sequencing for smooth movement
What does the supplementary motor area do?
Part of area 6 - involved in planning & initiation, bimanual coordination by connecting to other side
What does M1, the primary cortex do?
Directly link to alpha (lower) motoneurones, requires smallest stimulus to cause movement (shows strongest synaptic link to muscles), controls distal musculature
What are alpha motoneurones?
Large, lower motorneurones w cell body in ventral horn innervating extrafusal muscle fibres
Describe primary motor cortical output UMNs (betz cells)
From cortex –> spinal cord, making up 50% of corticospinal tract axons
Each innervates small group of muscles (motor unit), not individual muscles
Arranged somatotropically in cortical columns that recieve sensory info from proprioceptors (of changes occuring due to the movement)
What could lead to damage on UMN?
Stroke, tumour
Consequences of UMN lesion?
Initial muscle weakness followed by hypertonia, hyperreflexia & spasticity (increased resistance to movement)
Why does an UMN lesion lead to hyper-reflexia?
Intrinsic spinal circuitry –> increased tone & hyper-reflexia but normally this is overriden by cortical influences
What are Betz cells?
Giant pyramidal UMNs w cell body in 5th layer of the grey matter of M1
Each Betz cell sends signal coordinating EITHER force or direction of movement (hence 2 cells work together to determine movement)
What is the importance of feedback to primary cortical neurones within columns?
Allows intra-cortical reflexes - important for motor skills acquisiton e.g. learning musical instrument
Movements eventually become unconscious
Learned via repetition
