Motor Control

Question 1

Describe a basic reflexive movement.

Answer 1

The knee-jerk reflex is a 2-neuron loop, one afferent neuron and one efferent neuron.
The afferent neuron is connected to a muscle spindle, which detects stretch in the muscle.
The efferent neuron is the motor neuron that causes the reflexive movement.

Question 2

Describe a basic spinal reflex, using the example of a person stepping on a thumbtack.

Answer 2

The impact on the cells that the tack contacts is translated into a signal that travels up sensory nerve fibres.
In the spinal cord, the information is transmitted to interneurons, some of which synapse on motor neurons that send descending signals to the relevant muscles.
The motor command causes contraction of the relevant muscles and withdrawal of the foot.

Question 3

What do motor tracts do?

Answer 3

Motor tracts transmit signals from the brain to the muscles of the limbs and trunk through the spinal cord.

Question 4

What is the Corticospinal Tract?

Answer 4

Tract from the Primary Motor Cortex to the spinal cord.

Question 5

Outline the path of the axons from the PMC neurons (Corticospinal tract).

Answer 5

Primary motor cortex –> Corona Radiata –> Internal Capsule –> Midbrain –> Pyramid (medulla) –> Decussation –> Spinal Cord

This is a continuous axon which will synapse somewhere depending on which muscle is being moved.

Question 6

Describe where the Corona Radiata and Internal Capsule are in terms of the Corticospinal Tract.

Answer 6

Axons leaving the precentral gyrus (PMC) join a large body of axons that form the corona radiata.
As the axons travel deeper into the hemispheres, they pass between the caudate and putamen. At this point, the axons are called the internal capsule.

Question 7

Describe where the midbrain comes in, in terms of the Corticospinal Tract.

Answer 7

The axons exit the cerebral hemispheres and enter the brainstem at the level of the midbrain. At this point the axons are called the crus cerebri (or cerebral peduncles).

Question 8

Describe where the Pyramids of the Medulla come in, in terms of the Corticospinal Tract.

Answer 8

The axons pass through the pons.
In the medulla, the axons form the pyramids (seen as two ridges running down the ventral aspect of the medulla).
Most of the axons cross the midline (about 75 - 90%).

Question 9

Describe where the Spinal Cord comes in, in terms of the Corticospinal Tract.

Answer 9

Once in the spinal cord, the corticospinal axons synapse on neurons in the ventral horn.
Thus, these neurons originate in the primary motor cortex and terminate in the ventral horn of the spinal cord.
These terminate on lower motor neurons within the ventral horn.

Question 10

How can TMS be used to check the integrity of motor tracts?

Answer 10

Stimulation of cortical neurons in the motor strip (precentral gyrus) can elicit movement on the contralateral side of the body. The movement that occurs reflects the normal function of the cells that were stimulated.

Question 11

What happens if the neurons in the underlying motor cortex die? e.g. Due to a stroke

Answer 11

A stroke affecting the neurons in the precentral gyrus can cause contralesional weakness or paralysis (hemiparesis/hemiplegia)
After neurons in primary motor cortex die, they can no longer innervate the muscles to which they indirectly project.

Question 12

Complex actions involve more than projections from the primary motor cortex. What other area of the brain is closely involved with volitional movements?

Answer 12

Supplementary Motor Area (SMA)

Question 13

In lecture, there was an example of fMRI imaging showing recovery of function after a hemiparetic stroke. Explain what was shown.

Answer 13

The stroke resulted in moderate left hemiplegia (so damage to the right hemisphere), which had fully recovered at the time of the scan.
During right hand squeezing you can see activation within the left hemisphere as you’d expect.
During left hand squeezing you can see activation of the right hemisphere as you’d expect (remember they had fully recovered their movement). However, you also see a whole bunch of activation in the left hemisphere too.
Suggests that the brain recruited brain areas in left hemisphere to support recovery of movement.

Question 14

How do the cerebellum and basal ganglia participate in cortical and subcortical movement?

Answer 14

The cerebellum and basal ganglia participate in important feedback loops in which they project back to the cerebral cortex via the thalamus and do not themselves project to lower motor neurons.

Question 15

How do cortical neurons influence movement?

Answer 15

Cortical neurons modulate the activity of reflex circuits in order to orchestrate coherent goal-directed behaviour.

Question 16

How does damage to cortical neurons influence reflexive responses? Use Babinski’s Sign as an example.

Answer 16

If you scrape along the sole of a baby’s foot (less than 1 year old), the baby may fan its toes upward because babies’ corticospinal tract have not fully developed yet.
The normal response in adults is downward contraction of the toes.
Re-emergence of Babinski’s sign (i.e., upward fanning of the toes) is indicative of damage to the corticospinal tract.

Question 17

What is the visual grasp reflex? Why do we have it? What area is this associated with?

Answer 17

The visual grasp reflex refers to a rapid eye movement triggered by the sudden appearance of a visual signal in the periphery. It serves to align the fovea with the visual signal which is important for survival.
As we get older we have more control over this reflex.
The superior colliculi (on the back of the midbrain) play an important role in generating reflexive eye movements.

Question 18

What does the Frontal Eye Field do, and where is it?

Answer 18

The frontal eye field plays an important role in generating voluntary eye movements.
The frontal eye field is located at the intersection of the superior frontal sulcus and the precentral sulcus.
It projects down to the superior colliculi and allows volitional control (cortical projection to a subcortical area).

Question 19

In terms of the oculomotor system, how do cortical and subcortical cells compare?

Answer 19

Oculomotor behaviour is determined by cells in a number of brain areas at the subcortical and cortical levels.
Subcortical cells mediate more primitive reflexive oculomotor responses.
Phylogenetically newer cortical cells impose control over primitive reflexes via projections to subcortical cells, facilitating them when advantageous and inhibiting them when disadvantageous.

Question 20

Describe the experiment in patients with Frontal Eye Field damage, and what were the results?
What type of task does this involve?

Answer 20

The experiment was about how oculomotor control changes after damage to FEF.
The FEF group were patients with damage to FEF always to one side.
The Control group were age-matched neurologically healthy individuals.
The task was to fixate on the centre of the screen, when stimulus appears in the periphery, move your eyes in the opposite direction and then back to centre. This is an antisaccade task which requires inhibition of the visual grasp reflex.
Patients with unilateral frontal eye field (FEF) damage made abnormally frequent reflexive eye movements toward contralesional targets, but not ipsilesional targets.
The FEF normally imposes inhibitory control over the ipsilesional oculomotor circuitry that generates the visual grasp reflex.

Question 21

What does DTI imaging tell us about the Frontal Eye Field connections?

Answer 21

Using DTI imaging of white matter tracts, we can see fibers linking the antisaccade zone in the FEF to the basal ganglia.

Question 22

What is Alien Hand Syndrome and what issue does it demonstrate?

Answer 22

Alien Hand Syndrome demonstrates top-down control issues.
Patients with lesions of the supplementary motor area may suffer from alien hand syndrome, causing them to unintentionally reach out and grasp objects with the affected arm.
The sight of an object within reaching distance evokes a motor plan to grasp the object; execution of the motor plan reflects reduced internal (top-down) control.