motor control 2

Question 1

motor cortex

Answer 1

Primary motor cortex exerts quite direct, top down control over muscular activity, with as few as one synapse (in the spine) between a cortical neuron and innervation of muscle cells

Question 2

Descending projections from cortical motor areas

Answer 2

• Motor command originates in motor cortex pyramidal cells (in layer 5-6, grey matter).
• These are the upper motor neurons.
• Pyramidal cell axons project directly or indirectly (e.g. via brainstem) to spinal cord, where they synapse with lower motor neurons.
• The axons of these upper motor (pyramidal) neurons form the pyramidal tract
• Most cortical projections innervate contralateral motor units

Question 3

dorsolateral tracts

Answer 3

distal limbs
divide into: dorsolateral corticospinal tract (cortex → spine) and dorsolateral corticorubrospinal tract (cortex → red nucleus → spine, innervates distal limbs and also facial muscles due to nuclei of cranial nerve, synapse – UMN → LMN

Question 4

ventromedial tracts

Answer 4

proximal limb muscles
divide into ventromedial corticospinal tract and ventromedial cortico-brainstem-spinal tract (synapses with various parts of midbrain and brain stem, indirect pathway)
• both pathways project on both sides

Question 5

basal ganglia

Answer 5

A group of structures beneath the cortex that act as a ‘gate-keeper’ for control of the motor system (muscles)
• The basal ganglia are a group of nuclei lying deep within cerebral hemispheres
• Role in motor control not fully understood
• Basal ganglia dysfunction implicated in many disorders
• Multicomponent system of structures

Question 6

how basal ganglia work

Answer 6

• Receives excitatory input from many areas of cortex (Glutamate)
• Output goes back to cortex via the thalamus
• Output is mainly inhibitory (GABA)
• Complex internal connectivity involving 5 principle nuclei:
o Substantia Nigra
o Caudate & Putamen
o Globus Pallidus
o Subthalamic Nucleus
Disinhibitory – increases level of excitation of cortex – increase success of motor commands getting to muscles

Question 7

the selection problem

Answer 7

• Multiple command systems
• Spatially distributed
• Processing in parallel
• All act through final common motor path [Cannot do more then one thing (well) at a time]
• How do you resolve the competition?

Question 8

the cerebellum

Answer 8

The cerebellum is a large brain structure that acts as a ‘parallel processor’, enabling smooth, co-ordinated movements. It may also be very important in a range of cognitive tasks.
• Like basal ganglia, no direct projection to the lower motor neurons – instead modulate activity of upper motor neurons
• Projects to almost all upper motor neurons
inputs form spinal cord, cortex and vestibular system

Question 9

cerebellar function

Answer 9

• It knows what the current motor command is
• It knows about actual body position and movement
• It projects back to motor cortex
• Computes motor error and adjusts cortical motor commands accordingly

Question 10

agency of action

Answer 10

• As adults we take ‘sense of agency’ for granted
• As such, violations of sense of agency give us a perceptual shock
• Evidence suggests that sense of agency is retrospectively created
• Connections between frontal areas that develop motor plans for voluntary action and the parietal (association) areas that monitor outcomes play a key part in computing sense of agency
• The delay between motor command, and perceived outcome is crucial – too short or too long can disrupt sense of agency