Motor control How is the motor system organized at the neural level?

Motor control

How is the motor system organized at the neural level?

Organization of muscles• Muscles are organized in the body by

antagonistic pairs - one extensor and one flexor.

• Muscles are bundles of fibers that contract in response to the presence of the neurotransmitter acetylcholine.

• Muscles are infused with two types of neurons - alpha neurons and sensory neurons.

Alpha motor neurons

• Alpha motor neurons are the neurons that project from the spinal cord to the muscles.

• All alpha neurons have their dendrites and cell body in the spinal cord and an axon that terminates on a muscle.

• An action potential down the axon of an alpha motor neuron releases acetylcholine.

• Every muscle in the body is controlled by an alpha neuron.

Sensory (proprioceptive) neurons• Sensory neurons start in the muscles and terminate

in the spinal cord.• By and large, what they detect is the stretching of

muscles.• Part of a sensory neuron’s axon terminates on the

alpha neuron that controls the same muscle.• This leads to the stretch reflex, best seen when the

doctor hits your knee with a hammer.

Spinal cord motor schemas• There exists evidence that, even if the spinal cord

is severed, certain types of coordinated motions are still preserved.

• Brown (1911) severed the spinal cord of cats and put them on a treadmill where they demonstrated relatively normal walking motions.

• Certain neurons in the spinal cord - central pattern generators - seem to provide the ability for simple sets of coordinated actions.

• It is possible, then, that more complex actions are simply combinations or modifications of central pattern generators.

Sensory feedback• The sensory feedback we get from proprioceptive neurons

has been show to be very important for sustained motion and action.

• If you sever the proprioceptive neurons from one limb, an animal won’t use that limb.

• When you sever them from the other limb, the animal will then start to use both limbs again.

• People with neuropathy can still make complex, coordinated movements.

• However, once they start to make small errors, those errors compound quickly.

• Furthermore, their motions lack precision.

Medulla and brainstem• Control involuntary muscle movements,

mainly breathing and heart beat.

• Also serve as a switching station for the cortico-spinal motor pathways in that the medulla is where these pathways cross over to the contra-lateral side of the spinal cord.

The Cerebellum• Three functional divisions:

• Vestibulocerebellum: Balance and eye movements. Input comes from the semicircular canals and vistibular nuclei; outputs go to vestibular nuclei; also receives inputs from visual system (both V1 and superior colliculus)

• Spinocerebellum: Does proprioceptive processing to help control and correct movements; inputs come from spinal cord; outputs go to deep cerebellar nuclei

• Cerebrocerebellum: Movement planning and monitoring; inputs come from cerebral cortex (esp. parietal lobe); outputs go to thalamus

Cerebellar deep nuclei• Cerebellar outputs come almost exclusively from

four deep nuclei– Dentate– Emboliform– Globose– Fastigal

• By and large, these correspond to analogous areas of the cerebellar cortex

• The cerebellar cortex largely seems to exist to mediate the flow of information between the deep nuclei.

Cerebellar function• It is clear that the cerebellum is important

for coordinated motor actions. Just how it accomplishes this is in dispute:– One theory argues the cerebellum regulates the

timing of movements so that the can be coordinated.

– The tensor network theory argues that the cerebellum allows for the mathematical transformation of spatial coordinates derived from sensation into spatial coordinates that are useful for motion.

Basal ganglia• A collection of structures buried between the

cerebral hemispheres (near the thalamus and hippocampus).

• Primary role seems to be to mediate between the cerebral cortex and the thalamus. Inputs come from the cortex through the striatum and outputs project almost exclusively to the thalamus.

• Unclear exactly what the basal ganlia do, but damage is linked to numerous disorders, including cerebral palsy, ADHD, Parkinson’s disease, OCD, Tourette’s syndrome, and stuttering

Cortical motor areas• Primary motor cortex (also called the motor strip or M1)

- Runs almost the entire length of the central sulcus in the most posterior part of the frontal lobe. Most cortico-spinal fibers start here and project directly into the spinal cord.

• Supplementary motor area (SMA) - The medial section immediately anterior of the motor strip.

• Premotor cortex (PMC) - the lateral section immediately anterior of the motor strip

• Frontal Eye Field (FEF) - A small area immediately anterior of the SMA on the dorsal side of the frontal lobe (Broadman’s area 8); specialized for controlling eye movements.

Cortical motor representations

• Each cortical area is laid out in an ordered representation from toe (medial surface) to head (lateral surface).

• What do you think determines the amount of brain area devoted to controlling a part of the body?

Motor plans• Cognitively, there is a debate over whether

motor plans are distance based (move our muscles a certain distance) or location based (move our muscles to a certain location in space).

• Evidence from deafferented monkeys seems to clearly indicate plans are location based.

Hierarchical motor planning• Conceptual (goal) level

• Response level

• Implementation level

Neural representations of action• Georgopoulos (1995) demonstrated that neurons

in motor cortex are directionally selective, similar to neurons in MT.

• We get an overall representation of where we want to move a limb by computing the population vector - a function of the responding of a number of directionally sensitive cells.

• Population vectors can be used to analyze motion not just in the cortex, but in the basal ganglia and cerebellum as well.

Goal-based representations• Why all this redundancy? That doesn’t make a lot of

sense for a hierarchical system.• It seems to be the case that we develop our motor

plans in reverse order of the motions necessary to achieve a goal. In other words, our motor planning is goal based rather than direction based.

• This would seem to imply that different parts of the system may be planning different movements at different points in time.

• There are also neurons that, while the are directionally selective, also seem selective for particular motor actions, such as reaching vs. grasping vs. manipulating

Putting the pieces together• PET scans indicate that different brain areas

become active for different types of motion.– Simple, repetitive motion activates just M1

(and somatic cortex)– When motion becomes more complex, such as

and order sequence of movements, SMA and prefrontal cortex also become active

– When we are imagine the same complex action, just SMA becomes active.

Two types of motor plans• Externally guided: Movements guided by

external stimuli, such as vision (e.g., catching, grasping, blocking, etc.)– Utilize the external loop, which includes the

cerebellum, parietal lobe and PMC

• Internally guided: Self-guided, voluntary movements. Automatic processes probably fall into this category– Utilize the internal loop, which includes SMA,

Prefrontal cortex, and the basal ganglia

Movement disorders• Apraxia: Disorder of coordinated movement

where simple actions and muscle strength are intact. These patients tend to fail at miming, but have limited success when the object is actually in front of them.– Ideomotor apraxia: Patients seem to understand what

they need to do, but aren’t able to do it.– Ideational apraxia: Patient’s knowledge of appropriate

actions is severely disrupted. They might still make the right motion, but with the wrong object or goal.

• Generally associated with damage to the left parietal cortex.