The Motor Cortex Flashcards
Name the different cortical motor areas involved in movement.
Supplemental motor area, premotor cortex (area 6)
M1 (area 4)
Posterior parietal cortex (area 5+7)
S1
Function of the posterior parietal cortex
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
What can lesions of the posterior parietal cortex cause?
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
What are the inputs to m1?
Come via the thalamus
Basal ganglia (via ventroanterior and lateral thalamus)
Cerebellum (Ventrolateral thalamus)
Somatosensory afferents (ventral posteriorolateral thalamus)
What is the purpose of somatosensory afferents to M1/
Forms a feedback system which can allow for a reflex. I.e. M1 neurones will fire in response to muscle spindle input
Describe the experiments which show somatosensory afferents project to M1 and elicit a response.
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.
What are the outputs of M1?
Corticobulbar
Corticospinal
Corticotectal
Outputs to the red nucleus
What is the homoculus?
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
When do neurons in M1 fire?
100ms before movement
What does a cortical motor neuron do?
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
How does a CMN select a task group?
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
What does a single CMN synapsing on several interneurons networks and spinal motor neuron pools allow for?
Activate different combinations of agonist and antagonist
I.e. Prime mover will be activated aswell as synergists by same neuron
What is movement kinetics?
Term which describes the force of movement
What is kinematics
Term to describe the speed, amplitude and direction of movement
What does M1 function allow for> Kinectics is or kinematics/
Kinetics (force)
It does this via the size and rate code principle
What is the size and rate code principle?
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
What has Ohms law got to do with the size principle of recruitment?
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
Describe the experiment done to work out M1 controls movement kinectics.
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
What is grip force?
Forced needed to grip an object
What is load force?
Forced need to lift object against gravity
What is rTMS?
Repetitive transcranial magnetic stimulation reduces cortical excitability (temporary disruption of function of neurons for a brief period of time)
What occurs in the preload phase of lifting an object?
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
What occurs in the load phase?
The object is lifted against gravity and grip force is adjusted so we can hold the object
What dos rTMS of s1 do to the preload phase?
It increases it which means S1 is important for integrating grip information (done via sensory feedback from hands
What occurs with an M1 lesion?
Load force is adjusted prior to grip force which means anticipatory scaling of grip force to load force occurs upstream of M1
Why is grip force adjusted prior to load force?
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.
In terms of a flexion extension movement of elbow what will rTMS of M1 do?
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.
