L08: Motor Systems

Question 1

Where are alpha motor neurone cell bodies found?

Answer 1

Ventral horn

Question 2

Describe the descending tracts in terms of arrangement

Answer 2

Lateral - voluntary

Venteromedial - unconscious brainstem control

Question 3

Where is the primary motor cortex?

Answer 3

Pre central gyrus

Question 4

What governs motor activity?

Answer 4

1. Sensory input + local feedback from dorsal root
2. Spinal interneurones - important for reflexes etc.
3. UMN - initiation + control

Question 5

Importance of intrinsic circuitry of spinal cord?

Answer 5

Coordinated movements can occur solely through spinal cord, w/o brain
But descending input from brain superimposes this

Question 6

What are the lateral descending pathways??

Answer 6

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

Question 7

What are the venteromedial descending pathways? (Extrapyramidal)

Answer 7

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 -

Question 8

What may ascending tracts be surgically severed? What is the risk of this?

Answer 8

For pain relief - risk of damaging descending lateral pathways as well

Question 9

Consequences of lesion to lateral descending tracts?

Answer 9

Motor defecit & slower voluntary movents but posture unaffected as venteromedial tracts are fine

Question 10

What may occur after corticospinal lesion?

Answer 10

Rubrospinal tract can compensate exept fine motor control (functional recovery)
Cortical output can be rerouted via red nucleus due to synaptic plasticity

Question 11

What does extrapyramidal side effects of dopaminergic drugs affect?

Answer 11

Venteromedial descending tracts - SEs affects trunk e.g. dystonia, bradykinesia, parkinsonianism

Question 12

Describe the organisation of the motor cortex?

Answer 12

Area 6 (PMA & SMA)
Area 4/M1 = primary motor cortex

Question 13

What does the pre motor area do?

Answer 13

part of area 6 - involved in preparation + initiation of complex movements involving axial muscles (posture & balance)
Also controls movement sequencing for smooth movement

Question 14

What does the supplementary motor area do?

Answer 14

Part of area 6 - involved in planning & initiation, bimanual coordination by connecting to other side

Question 15

What does M1, the primary cortex do?

Answer 15

Directly link to alpha (lower) motoneurones, requires smallest stimulus to cause movement (shows strongest synaptic link to muscles), controls distal musculature

Question 16

What are alpha motoneurones?

Answer 16

Large, lower motorneurones w cell body in ventral horn innervating extrafusal muscle fibres

Question 17

Describe primary motor cortical output UMNs (betz cells)

Answer 17

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)

Question 18

What could lead to damage on UMN?

Answer 18

Stroke, tumour

Question 19

Consequences of UMN lesion?

Answer 19

Initial muscle weakness followed by hypertonia, hyperreflexia & spasticity (increased resistance to movement)

Question 20

Why does an UMN lesion lead to hyper-reflexia?

Answer 20

Intrinsic spinal circuitry –> increased tone & hyper-reflexia but normally this is overriden by cortical influences

Question 21

What are Betz cells?

Answer 21

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)

Question 22

What is the importance of feedback to primary cortical neurones within columns?

Answer 22

Allows intra-cortical reflexes - important for motor skills acquisiton e.g. learning musical instrument
Movements eventually become unconscious
Learned via repetition