3.1. Motor Control I
“Life’s aim is an act, not a thought.” -Charles Sherrington
The aim of all cognition is action.
What are the main motor structures and pathways?
Motor pathway hierarchy
∙ The lowest level of the hierarchy is the spinal cord. The spinal cord is the point of access between the nervous system and muscles(output signals). It’s also in charge of reflexes.
∙ At the top of the hierarchy is the premotor and association areas in the cortex. Processing within these regions is critical for planning an action based on an individual’s current goals, perceptual input, and past experience.
Because the motor system is hierarchical, a lesion at any point in the hierarchy could have different effects on movement.
Spinal cord neurons can generate simple sequences of actions without any external feedback. These are reflexes. Reflexes are independent of higher processing.
Effector: A part of the body that can move. All forms of movement result from stimulation of effectors. Muscles are affected by motor neurons:
∙ Alpha Motor Neurons are responsible for contractions by the release of acetylcholine. They provide a physical basis for turning signals into movements by changing the length and tension of the muscles. We also discuss several other topics like What is the meaning of neocolonialism?
Don't forget about the age old question of What is the meaning of hippocampus?
∙ Gamma motor neurons are responsible for sensing and regulating the length of the muscle fibers
∙ Spinal Interneurons lie within the spinal cord and are innervated by afferent sensory nerves (skin, muscles, joints). Signals to the muscles involve continual integration of sensory feedback with the motor commands from higher centers.
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Muscles are often arranged in antagonistic pairs, one may serve to contract and the other relaxes.
Pyramidal & Extrapyramidal Tracts
Pyramidal Tracts originate in the cortex and project all the way down to the spinal cord. ∙ Corticospinal tracts are responsible for voluntary skilled movement of the hands and fingers.
Extrapyramidal Tracts project from the sub-cortex to the spinal cord.
∙ Causes involuntary reflexes and movement and influences regular movement as well.Don't forget about the age old question of What is planning?
The cerebellum has an ipsilateral orientation because the input to and output tracts from the cortex cross over to the contralateral side: Right side controls right, left side controls left. ∙ vestibulocerebellum: Balance and coordination of eye movements with body movements. ex.vestibulo-ocular reflex (VOR) assures that the eyes stay fixed on an object, regardless of body movements (like typing!)
∙ Spinocerebellum: Receives information from visual and auditory systems. Lesions here can result in unsteady gait and lack of balance. Cells here are sensitive to the effects of alcohol.
∙ Neocerebellum: Heavily connected to frontal and parietal lobes. Lesions here produce Ataxia which is the lack of voluntary control over voluntary muscle movements. Basal Ganglia
Caudate nucleus, putamen, substantia nigra, globus pallidus, subthalamic nucleus. ∙ Plays a role in motor control, specifically the selection and initiation of actions. Basal Ganglia as the gatekeeper. It inhibits irrelevant motion and lets through the most potent stimulus.
∙ Huntington’s Disease: Causes excess movement. Basal Ganglia is no longer inhibiting stimuli. Associated with the Indirect Pathway. The brakes are not working. o Indirect Pathway inhibits the Thalamus & cortical motor areas.(Inhibition of inhibitory pathway just results in less control. Signals will get through that you don't want to get through) Don't forget about the age old question of What is unemployed?
∙ Parkinson’s Disease: Too much inhibition causes loss of voluntary movement. The basal ganglia is ALWAYS inhibiting (tremors while at rest). Associated with the Direct Pathway. The brakes are always on.
o Hypokinesia: Reduced voluntary movement
o Bradykinesia: Slowness in movement
o Direct Pathway excites the thalamus. (Inhibition of an excitatory pathway is too rigid. Nothing gets through.)
o Parkinson’s is linked with a loss of dopaminergic cells in the Substantia Nigra. Treatment usually involves L-Dopa injections, lesions to global pallidus or deep brain stimulation.
o Patients w/ Parkinson’s diseases have difficulty shifting between concepts as well as actions.
Direct & Indirect Pathways have different receptors that respond to Dopamine differently (D1 & D2)
Primary Motor Cortex (M1)
M1 is located in the posterior part of the frontal lobe and receives input from nearly all regions involved in motor control. M1 also has a crude somatotopic map.
∙ The representation of the effectors doesn't have to do with their size but rather how important they are to movement, and the level of control required for manipulating it. ex.fingers! If you want to learn more check out What are the calculate components of income statement?
Lesions to M1 typically result in hemiplegia which is the loss of motor control on one side of the body.
Motor Execution vs. Planning
∙ Execution of motor movements only involves the motor cortex
∙ Planning a motor movement involves both M1 and the Supplementary Motor Area (SMA) ∙ Thinking about a motor movement only recruits SMA
If you apply TMS while someone is moving their fingers:
TMS to M1: Result is a temporary loss of coordination. The next response is halted or incorrect. TMS to SMA: Subjects lose track of their movements. The goal was disrupted.’
Internally vs Externally Guided Movements
In the task, Internally guided movements are guided by memory, whereas the externally guided movements are guided by a light.
Internally guided movements recruit SMA
Externally guided movements recruit PMC
3.2. Motor Control II
Apraxia Subtypes: (lesions to secondary motor areas)
∙ Apraxia is characterized by the inability to make skilled movements. Linking gestures to meaningful actions. (NOT to be confused by Alexia or Ataxia).
1. Ideomotor Apraxia: The person has a sense of what the action is, but they cannot execute that action. They have trouble with initiation. Ex. Hitting the hand on the head when prompted with a comb. They understand that the comb is associated with the hair but cannot make the proper action.
2. Ideational Apraxia: The concept of the action is impaired. They have difficulty recognizing and executing such actions. ex. Brushing teeth with a hair brush.
∙ Premotor: Premotor region is responsible for planning and perceptual input. ∙ Motor: Takes the action plan and makes it happen.
Abstract Representation of Action
Action that is independent of a particular muscle group. Ex. Writing a word with multiple body parts.
Trajectory Planning: How do we represent movements?
Trajectory-Based Representation: An action plan specifies the trajectory that will move the limb across a certain distance, along a certain path.
Location Based Representation: Motor representation specifies the desired final position to achieve movement goal.
Monkey Study on Trajectory Planning
Deafferentation: the interruption or destruction of the afferent connections of nerve cells, performed in animal experiments to demonstrate the spontaneity of locomotor movement. Results in a loss of sensory input.
Deafferented monkeys got trained in a simple pointing experiment which would determine whether trajectory based or location based planning seemed to be most valid. Because they were deafferented, they did not know a force was being applied.
∙ The monkeys were tasked to point at a light in a dark room, however the catch was that before they would be before the arm movement was made, an opposing force would restrict the movement of the arm.
∙ The monkeys were NOT trained with the force. So if they were using a trajectory based approach, the surprise force would’ve hindered their plan and they would have undershot their target. ( The monkeys were not aware of the force. Had they known they could’ve changed their motor plan upon noticing the force.)
∙ Results of this show that monkeys did not overshoot the goal so this supports location based representations or endpoint control.
Hierarchical Control of Action
∙ Chunking movements together is possible (like memory can be chunked). ex. Asking someone to dance. The number of responses is big, but once accepted the arms and legs act to stand up and dance.
Neural Coding of Movement
Some neurons tended to have preferred directions.
ex. Monkeys trained on a task to move their hand in the direction of a light stimulus. ∙ Population vectors tended to have better correlation with behavior than the analysis of individual neurons. (Recording from a bunch of different cells and sum their signals together. A population of neurons.)
∙ Population vectors can also predict movement direction. Population vectors are already curving towards the indicated direction before the movement is even made. So it doesn't happen at the same time as the movement.
Brain Machine Interface (BMI)
Allows neural signals to be used to perform a desired action with the aid of a mechanical device outside of the body.
ex. Signals from neurons in motor cortex can be used to move a robotic arm. ∙ First done in rats (water/lever experiment). If they pushed a button, a lever would cause the release of water. They turned a switch so that instead of the button the firing of the neurons would control the lever. The rats picked up on the association and controlled it without touching it.
∙ Quadriplegic woman eating chocolate using a robotic arm. Patient M.N
Making BMI stable
Recorded same neurons for 19 days, and actually the monkeys got better at doing the tasks with the arm. Also when the target directions were changed, the monkeys were able to adapt to the changes after some practice. Shows that motor cortex is plastic to some degree.
Action Goals & Movement Plans
Affordance Competition Hypothesis: At any given time we have multiple opportunities for action that are defined by the environment. The opportunities for action compete against each other, but only one action plan wins out.
∙ Action selection and specification happen simultaneously and they continuously evolve. ∙ Action plans are eventually updated by environmental feedback and internal feedback & drives.
∙ One action wins is selected and executed.
Evidence for AC Hypothesis
Premotor cortex recording in monkeys. Repeated action plans are kept in mind until its clear that one is selected.
Representational Variation in terms of how motor areas represent action selection and planning. Direct brain stimulation of dorsal premotor cortex and posterior parietal cortex has different effects:
∙ Stimulation evoked complex movements without conscious awareness or movement intention.
∙ Plays more of a role in execution
∙ Stimulation evoked intention to move or perception of movement in absence of muscle activity. With enough stimulation the patients would feel as if there was an actual movement, even though there wasn't any muscle activity. ex. “i felt a desire to lick my lips.”
∙ Plays a role in intention.
Repetition Suppression for Movements
Repetition Suppression typically corresponds to less activation for repeated stimuli ∙ Participants shown small movie clips of both novel and repeated movements. They had to see if the type of movement (kinematic) that was made was repeated/novel or if the outcome of the movement was repeated/novel. ROI activation during this was measured. ∙ Less activation for repeated movements was found in left frontal cortex (execution) ∙ Less activation in right parietal cortex for repeated action goals. (Goals/Intentions)
Mirror Neurons allow us to comprehend the actions that other people are doing. ∙ Same neural activity happens when we watch someone doing an action as when we are doing the action. However it is not just about visual properties, it can also be aural or something.
∙ signal goal oriented actions & understanding imitation.
Mirror neurons & Expertise: Level of expertise impacts activation in mirror neurons. Example of dancers. Viewing choreography caused activation in premotor and parietal areas. So mirror neurons were activated for viewing choreography.
How do we learn new motor skills?
Sensorimotor Adaptation: When a learned skill is modified due to a change in the environment.
Ex. Throwing a ball normally vs. throwing a ball with Prism Glasses on.
∙ The prism glasses distort the vision to one side so at first the there is a horizontal displacement between the target and where the ball ends up but after a while the person gets used to the change in visual input and begins to compensate for this deficit and throws the ball at the target. When the glasses are removed they miss the target because they are over compensating (after effect) but after a while they realize they no longer have to.
∙ People with damage to the cerebellum are not able to adapt to changes when they put on the prism glasses. Suggests cerebellum plays a role in sensorimotor adaptation.
Transcranial Direct Current Stimulation (Similar to but NOT the same as TMS) In the study TDCS was either applied to cerebellum or primary motor cortex, and the current excites the neurons and helps along processing. In the experiment people had to make fast reaching movements with a stylus. They had to practice these movements and then the researchers rotated the visual input and they had to learn to adapt to the change in direction
There was a double dissociation found between the two areas with respect to sensorimotor adaptation.
∙ Cerebellar stimulation resulted in faster learning of the movements. People were making fewer errors. Suggests that cerebellum is important in the initial learning that helps with the sensorimotor adaptation
∙ Primary Motor Cortex stimulation resulted in stronger consolidation. The after effect of the change in input lasted longer. Suggests that this is more involved in the retention and consolidation of what is learned.
Role of Dopamine in Motor Cortex
There are dopaminergic projections in the brain from the Ventral Tegmental Area (VTA) in the brainstem, to the Primary Motor Cortex that are necessary for acquiring new motor skills. **Projection tracts typically go from higher and lower brain regions as well as the spinal cord region.
Rat study where rats had to pick up pieces of food with their paws (which is challenging for them). Researchers lesioned the VTA in some rats, cutting off the dopamine projections and then they were given an L-Dopa injection and others with the lesion were given a saline injection.
Control(Sham) Rats = no lesion = no hindrance
Lesioned rats w/ L-Dopa injection = learned only with shot.
Lesioned rates w/ saline injection = did not learn
The sham rats learned the skill fine, but the lesioned rats did not improve unless they were given the L-Dopa injection. So this suggests dopamine plays a key role in the learning of new motor skills.
Role of Cerebellum & Timing
∙ Cerebellar lesions disrupt learning in the context of delay conditioning. ex. Air puff/eye blink. They're trained to start blinking once they hear a tone since it had been paired with a puff to the eye. But when rabbits are lesioned in the nucleus of the cerebellum, rabbits blinked at the air puff but not at the tone. so they did not have an anticipatory response.
∙ When cerebellum is lesioned in other areas, the anticipatory blink occurs but the timing of the blink is off. So cerebellum must play a role in timing our movements. ∙ May generate sensory predictions of what it expects to happen given a specific movement.