Motor control 2: Basal ganglia and cerebellum 27.02.23

Question 1

What are the stages of a descending control system?

Answer 1

• Association cortex
• Motor cortex
• Brainstem circuits
• Spinal circuits
• Motor unit

Lots of ascending feedback from cerebellum and basal ganglia

Question 2

How is the vagus nerve connected to speech?

Answer 2

• Branches of vagus nerve project to larynx to control speech which also controls units of respiration
• This circuit is richly interconnected with cerebellum and other brainstem sensorimotor systems

Question 3

How does the motor cortex function?

Answer 3

• Exerts direct, top-down muscle control
• As few as one synapse (in spine) between neuron and innervation of muscle cell

Question 4

Describe where the projections of UMN from the cortical motor areas are?

Answer 4

• Motor command originates in motor cortex pyramidal cells
• These are UMN

Question 5

How do UMN project and synapse with LMN?

Answer 5

• Pyramidal cell axons project directly or indirectly (via brainstem) to the spinal cord
• Here they synapse with LMN
• The axons of the UMNs form the pyramidal tract

Question 6

How do cortical projections mostly innervate? (ipsilateral or contralateral)

Answer 6

to contralateral motor units

Question 7

How are excitatory and inhibitory inputs to the motor cortex controlled?

Answer 7

• The cerebellum controls excitatory inputs
• The basal ganglia controls inhibitory inputs
• However, they are not opposing each other they are both modulating the motor cortex together

Question 8

How do the basal ganglia and cerebellum transmit information through the motor cortex?

Answer 8

• They input directly into the motor cortex
• Which then directly transmits information down the pyramidal tracts via UMNs etc….

Question 9

What are the two descending projections (tracts) from the motor cortex down the spinal column?

Answer 9

-The dorsolateral tracts
- The ventromedial tracts

Question 10

What does the dorsolateral corticospinal tract control?

Answer 10

• Projects to contralateral distal limb muscles
• More fine control
• White matter tract

Question 11

What does the ventromedial corticospinal tract control?

Answer 11

• Projects to the trunk and more proximal (midline) muscles
• White matter tract
• Controls more large limb movements and posture

Question 12

What are the indirect routes via brainstem nuclei of the dorsolateral and ventromedial tracts?

Answer 12

Dorsolateral: via the red nucleus
Ventromedial: via tectum, vestibular nuclei and cranial nerve nuclei

Question 13

How do the dorsolateral and ventromedial tracts differ in innervation?

Answer 13

Dorsolateral: Innervates contralateral side of one segment of spinal cord
but sometimes projects directly to alpha motor neuron

Ventromedial: Diffuse innervation projecting to both sides and multiple segments of spinal cord

Question 14

What is the basal ganglia?

Answer 14

• A group of structures beneath the cortex that act as a ‘gatekeeper’ for control of the motor system
• Group of nuclei

Question 15

What are the 5 principle nuclei in the basal ganglia?

Answer 15

• Substantia Nigra (pars compacta and pars reticulata
• Caudate and putamen (striatum)
• Globus Pallidus (internal and external)
• subthalamic nucleus

Question 16

Describe the direct pathway of how the basal ganglia inputs information to the cortex?

Answer 16

1. Starts at cortex and projects to striatum (excitatory)
2. Neurons from striatum send axons to globus pallidus and substantia nigra (reticula) (inhibitory GABA)
3. Then send the axons to thalamus (inhibitory)
4. From thalamus excitatory pathways to the cortex which increases movement

Question 17

How can the basal ganglia control whether the motor cortex is excited or not?

Answer 17

• At rest: striatum inactive, globus pallidus active, thalamus inhibited so motor cortex has reduced excitation
• When the striatum is active, globus pallidus is transiently inhibited (only in waves), so thalamus is disinhibited (excitable) and the motor cortex has an increased excitation

Question 18

What is the cerebellum?

Answer 18

A large brain structure that acts as a ‘parallel processor’ enabling smooth, co-ordinated movements

Question 19

How does the cerebellum project information?

Answer 19

Like the basal ganglia, no direct projection to LMNs. Instead modulate activity of UMNs

Question 20

How many CNS neurons are in cerebellum?

Answer 20

• Contains half total number of CNS neurons
• Projects to almost all UMN
• Dense folding and very compact

Question 21

What are the three inputs of the cerebellum?

Answer 21

1. Spinal cord (info about limb position and movement)
2. Cerebral cortex via pons (somatosensory and visual areas)
3. Vestibular system (rotational and head movements)

Question 22

What is the output of the cerebellum?

Answer 22

Thalamus

Question 23

What is the function of the cerebellar?

Answer 23

Computes motor error and adjusts motor commands accordingly as it knows about body position