The Motor Cortex

Question 1

Name the different cortical motor areas involved in movement.

Answer 1

Supplemental motor area, premotor cortex (area 6)
M1 (area 4)
Posterior parietal cortex (area 5+7)
S1

Question 2

Function of the posterior parietal cortex

Answer 2

Forms egocentric maps of world around us based on visual stimuli i.e. Where we are in relation to the objects we see
Also forms global maps of where we are geographically
This representation of object is important for forming desired trajectories of movement

Question 3

What can lesions of the posterior parietal cortex cause?

Answer 3

Apraxia (inability to initiate specific, purposeful movements even through the pathways which execute those movements are fine)
Lesions of the right hemisphere particularly associated with sensory hemineglect syndrome. Patient will ignore everything on contralateral side

Question 4

What are the inputs to m1?

Answer 4

Come via the thalamus
Basal ganglia (via ventroanterior and lateral thalamus)
Cerebellum (Ventrolateral thalamus)
Somatosensory afferents (ventral posteriorolateral thalamus)

Question 5

What is the purpose of somatosensory afferents to M1/

Answer 5

Forms a feedback system which can allow for a reflex. I.e. M1 neurones will fire in response to muscle spindle input

Question 6

Describe the experiments which show somatosensory afferents project to M1 and elicit a response.

Answer 6

Passive movement of elbow and tendon tap causes M1 neurons to fire

Movement of animal was from extended arm to flexed arm. On movement perturbation where a mechanical stimulus briefly and suddenly opposes the direction of movement the firing rate of an M1 neuron will increase as a result from muscle spindle afferents in order to correct movement.

Question 7

What are the outputs of M1?

Answer 7

Corticobulbar
Corticospinal
Corticotectal
Outputs to the red nucleus

Question 8

What is the homoculus?

Answer 8

M1 and S1 are somatotopically organised meaning that each area of the body is represented in a particular area on the cortex. The size of the area represents the precision of control. I.e. The larger the area (such as the hands) the larger the precision of control

Question 9

When do neurons in M1 fire?

Answer 9

100ms before movement

Question 10

What does a cortical motor neuron do?

Answer 10

Selects a ‘task group’ of muscles to activate and inhibit to execute a particular movements. One neuron will synapse on different combinations of spinal interneurons networks and spinal MN pools

Question 11

How does a CMN select a task group?

Answer 11

One neuron will synapse on several different alpha motor neurons located in different muscle motor nuclei in the ventral horn of the spinal cord. These may be direct (~33%) or indirect via interneurons (delayed EPSP on alpha MN compared to direct). They may even occur through inhibitory interneurons which inhibits the muscle

Question 12

What does a single CMN synapsing on several interneurons networks and spinal motor neuron pools allow for?

Answer 12

Activate different combinations of agonist and antagonist

I.e. Prime mover will be activated aswell as synergists by same neuron

Question 13

What is movement kinetics?

Answer 13

Term which describes the force of movement

Question 14

What is kinematics

Answer 14

Term to describe the speed, amplitude and direction of movement

Question 15

What does M1 function allow for> Kinectics is or kinematics/

Answer 15

Kinetics (force)

It does this via the size and rate code principle

Question 16

What is the size and rate code principle?

Answer 16

Firing of a neuron from M1,and therefore the alpha MN, is proportional to the force of movement.
More firing = more force a motor unit will generate (summation of action potentials)
Alpha Motor neurons have different sizes which means they are not recruited at the same time (small before larger). The more the neuron fires the more alpha MN will be recruited

Question 17

What has Ohms law got to do with the size principle of recruitment?

Answer 17

V=IR
Smaller neurons have a larger input resistance compared to larger neurons. Therefore, in order to reach a certain voltage threshold a smaller amount of current is needed. Therefore, smaller neurons are recruited first. More current needed to reach threshold in larger neurons

Question 18

Describe the experiment done to work out M1 controls movement kinectics.

Answer 18

need to eliminate kinematic principles. Therefore, simple extension and flexion at wrist in more direction.

Movement from wrist from straight to flexed.
No load = flexor pyramidal tract neuron will fire just before contraction of flexor 
Flexor load (flexing would lift weight) = PTN will fire are higher frequency to contract flexor against weight
Extensor load = EMG for extensor muscle will show activity before flexing to hold weight stable. On flexing the PTN will not fire as the force of flexion will be produced by the weight

THEREFORE M1 CORTICAL NEURON FIRES TO PRODUCE FORCE

Question 19

What is grip force?

Answer 19

Forced needed to grip an object

Question 20

What is load force?

Answer 20

Forced need to lift object against gravity

Question 21

What is rTMS?

Answer 21

Repetitive transcranial magnetic stimulation reduces cortical excitability (temporary disruption of function of neurons for a brief period of time)

Question 22

What occurs in the preload phase of lifting an object?

Answer 22

We grip object. This is normally an overestimated amount of grip to grip object we have never lifted before as initial grip is anticipatory
On subsequent lifting our grip will be less tight

Question 23

What occurs in the load phase?

Answer 23

The object is lifted against gravity and grip force is adjusted so we can hold the object

Question 24

What dos rTMS of s1 do to the preload phase?

Answer 24

It increases it which means S1 is important for integrating grip information (done via sensory feedback from hands

Question 25

What occurs with an M1 lesion?

Answer 25

Load force is adjusted prior to grip force which means anticipatory scaling of grip force to load force occurs upstream of M1

Question 26

Why is grip force adjusted prior to load force?

Answer 26

Grip force is anticipatory based on previous experience. Without an M1 lesion i will anticipate the increase in grip needed to lift an object. With a M1 lesion this anticipatory mechanism is attenuated…any change in grip force will come from sensory feedback which comes after the initial grip.

Question 27

In terms of a flexion extension movement of elbow what will rTMS of M1 do?

Answer 27

Will not impair the execution of the plan as movement will be executed in same way. However, it will delay the execution of movement. This means the plan is made upstream of M1 and M1 is region where selected neurons fire to execute pre planned movement.