Lecture 14: The Neurobiology of Motor Control Flashcards
What is the anatomy of motor control?
Motor control, the ability to regulate movement, involves a complex interplay of brain regions, including the motor cortex, basal ganglia, cerebellum, and spinal cord, working together to plan, execute, and refine movements
What is the function of the spinal cord?
Provides direct control of the muscles via motor neurons and interneurons.
What is the length of axons in a motor neuron?
Up to a metre in length
What is the function of the preomotor/SMA and parietal cortex?
Action plans and goals
What is the function of the primary motor cortex and subcortical brain region?
Translate the motor plans and goals into specific actions
What are muscles composed of?
Elastic fibres that can change in length and tension
How are muscles arranged?
In antagonist pairs e.g. biceps and triceps
What is motor control carried out by?
Muscles, so when the biceps contracts, the triceps relaxes which enables flexion of the elbow, and if the triceps contracts and the biceps contracts this enables extension of the elbow.
What are muscles controlled by?
Motor neurons in the spinal cord
Where do motor neurons originate?
In the spinal cord and exit through the ventral root and terminate in the muscle fibres
What does action potential in the motor neuron trigger?
The release of acetylcholine - a neurotransmitter that makes muscle fibres contract
What determines the force that the muscles can generate?
The number and frequency of action potential and number of muscle fibres
How many cranial nerves control essential reflexes which keep us alive?
12
What to extrapyramidal tracts?
Send direct pathways down the spinal cord to exert indirect control over posture, muscle tone and movement speed
What does the motor cortex regulate?
Activity of spinal motor neurons
What is the function of corticospinal (pyramidal tract)?
Has axons that project directly form the cortex to the spinal cord
Where do the cerebral hemispheres control movement?
On the opposite side of the body
What are somatotopic organisations?
Different regions represent different body parts. The representation looks something like this, with larger areas dedicated to parts of the body involved in movement such as the hands and face.
Can elicit predictable twitches in the different regions using TMS
What are the premotor and SMA involved in?
Planning and the control of movement, either sensory guided or internally guided.
What do lesions result in?
Apraxia, which affects movement of limbs and also speech
What is the association motor areas involved in?
Representation of space, attention, sensorimotor integration
What do lesions of parietal cortex produce?
Apraxia but also more general problems with attention, particularly spatial attention (nelgect)
What are Central Pattern Generators (CPGs)?
Neurons in the spinal cord that can generate rhythmic, patterned movements required for complex motor acts without input from higher brain regions.
What experiment did Sherrington conduct to study motor control?
He severed the spinal cord of cats and placed them on a treadmill, observing that they could still produce rhythmic limb movements for walking.
What did Sherrington’s experiment reveal about CPGs?
Even without descending commands from the brain, the spinal cord could independently generate rhythmic movements, proving the existence of CPGs.
How do CPGs contribute to the hierarchical nature of motor control?
Higher-level brain regions can send simple signals to trigger CPGs instead of holding the entire representation of complex movements themselves.
What did Georgopoulos’ experiment investigate?
How neurons represent the direction of movement in the brain.
What task did Georgopoulos require monkeys to perform?
Move a lever from a center position to one of eight targets.
What was the key finding from Georgopoulos’ experiment?
Neurons showed a preference for a specific movement direction, regardless of the starting position or target location.
What does Georgopoulos’ experiment suggest about motor control?
The brain encodes movement direction rather than absolute position, indicating a more abstract representation of movement.
What is the concept of a population vector in motor control?
A population vector is the sum of individual neuron vectors, which helps predict the direction of movement.
How does a single neuron’s preferred direction contribute to movement prediction?
Each neuron has a preferred direction, but since neurons respond to multiple directions, a single neuron is not enough to predict movement accurately.
How does the population vector improve movement prediction?
By summing the vectors of multiple neurons, the population vector provides an accurate prediction of movement direction, even 300 ms before movement initiation.
How is the concept of population vectors used in brain-machine interfaces?
Population vector output is used to predict movement direction, forming the basis for brain-machine interfaces.
What pioneering experiment did Chapin conduct on brain-machine interfaces?
He trained rats to press a lever for a reward while recording multiple neuron responses in the motor cortex.
How were neural networks used in Chapin’s experiment?
They learned patterns of neuronal activation to predict the forces exerted on the lever.
What was observed in the population vector responses in Chapin’s experiment?
The output of around 40 neurons showed complex patterns of activation during different stages of lever pressing.
What does the population vector response reveal about motor control?
It shows how groups of neurons coordinate to generate movement and how their combined activity can predict force and direction.
How did Chapin modify the experiment to test brain-machine interface learning?
He switched the input to the reward system from the lever to the neuronal population vector.
What happened when the reward was no longer linked to the lever press?
Mice stopped pressing the lever as they learned the force exerted was no longer correlated with the reward.
What surprising behavior did the mice exhibit after the switch?
They continued generating the cortical signals necessary to move the lever, even without physical movement.
How can adaptation to sensorimotor mismatches be studied in the lab?
Using the “hidden hand” paradigm, where a mismatch is created between actual hand movement and perceived movement, leading to adaptation over time.
What happens in the brain during the adaptation phase?
Increased activation across many different motor regions.
How do lesions in motor areas affect visuomotor adaptation?
Patients with lesions in motor control areas show deficits in visuomotor adaptation, suggesting these regions are critical for the process.
What method did Galea et al. use to investigate visuomotor adaptation?
Transcranial direct current stimulation (tDCS), which applies electrical currents to the scalp to change neuronal excitability.
What was the effect of tDCS on the cerebellum in Galea et al.’s study?
It led to a faster rate of adaptation.
What was the effect of tDCS on the primary motor cortex (M1) in Galea et al.’s study?
It led to increased retention of adaptation, causing more error for a longer period after adaptation ended.
What role does the cerebellum play in visuomotor adaptation?
It is involved in learning new movement mappings, possibly by generating forward models.
What role does M1 play in visuomotor adaptation?
It consolidates newly learned mappings, playing a less flexible, instructive role by passing motor plans to spinal cord motor neurons.
What experiment did Miall and colleagues conduct to investigate the cerebellum’s role in forward models?
They had subjects move their arm to the right, then make a movement to a visual target after hearing a tone, requiring a prediction of where the hand would be due to a delay between the tone and the movement initiation.
Why is a prediction of the hand’s location necessary in Miall and colleagues’ experiment?
Because there is a delay between hearing the tone and initiating the movement, and without a prediction, the motor command would be out of date, causing the subject to miss the target.
What did Miall and colleagues find under normal conditions in their experiment?
Subjects were generally accurate in hitting the target, suggesting they were predicting the hand’s future location.
What happened when TMS (Transcranial Magnetic Stimulation) was applied in Miall and colleagues’ experiment?
The path of the hand matched what would have been expected if the motor command had been issued 138 ms earlier than when the movement was initiated.
What does the finding from Miall and colleagues’ experiment suggest about the cerebellum?
It suggests that the cerebellum plays a crucial role in generating forward models by predicting the future location of the hand and adjusting the motor command accordingly.
