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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.