Download 07-Control of Movement

11/13/2009
The Neurological Control of Movement
Mary ET Boyle, Ph.D.
Department of Cognitive Science
UCSD
Levels of Control of Movement
Movement
• A change in the place or position of the
bodyy or a bodyy part.
p
• When neurological control of movement is
working correctly we can … do anything!
• If not, movement disorders such as
myasthenia gravis, movement apraxia, ALS,
Parkinson’s disease, and Huntington’s
disease.
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Levels of Control of Movement:
Simple to Complex
The simplest movements are reflexive reactions
withdrawing your hand after touching a hot stove or blinking when something
gets in your eye
more complex than reflexes, but less complex than other skills
maintaining posture, sitting, standing, walking, and eye movement
complex movements can be learned
playing the violin, riding a bike, and operating exercise equipment
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Stimulation of Movement
 Most basic level of control
is the spinal cord
(e.g., spinal reflexes, such as
the withdrawal reflex, are
solely controlled by the
spinal cord).
 Next level involves brain
stem structures in the
hindbrain and midbrain
(e.g., visual pursuit of a light
stimulus).
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 Highest level of control
involves the cerebral
cortex and structures such
as the dorsolateral
prefrontal cortex, the
primary and secondary
motor cortex, and the
somatosensory cortex.
Basal Ganglia
(main components: striatum, pallidum, substantia nigra, and subthalamic nucleus)
Influences movement by smoothing out and
refining it
(gets rid of extraneous movement and acts to
ensure that the selected movement occurs with
sufficient, but not excessive, force);
also responsible for muscle tone and
postural adjustments.
The corpus striatum and Huntington’s
disease – leads to substantial
enlargement of the lateral ventricles
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Cerebellum


Plays a central role in
translating
l i uncoordinated
di
d
movements into a skilled
action;
receives feedback from
sensory receptors that
monitor movement and brain
stem structures that initiate
movement.
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Three types of muscle tissue in the body:
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extension
Contraction of
triceps muscle
(extensor)
movement
away from the
body
flexion
Contraction of the
biceps muscle
(flexor)
brings the extended
limb back toward
the body
Skeletal muscle
Muscle fibers
Myofibrils
Myofilaments
y
Myosin
(thick filaments)
Actin
(thin filaments)
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Neural Control of
Muscle Contraction


The motor neurons of
the peripheral nervous
system control the
skeletal muscles.
The cell bodies of motor
neurons are located in
the gray matter of the
ventral horn of the spinal
cord and in different parts
of the brain stem.
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Motor Neurons:
Transmission of a Neural Impulse

Transmission of a neural impulse from motor neuron to
muscle fiber at the neuromuscular junction—similar to
th ttransmission
the
i i off neurall iimpulses
l
bbetween
t
neurons

Motor neuron releases ACh into the synaptic cleft.

ACh binds to receptor proteins on the postsynaptic
membrane (the muscle fiber).

EPSPs are then produced; upon sufficient excitation, an
action potential is generated.
Neuronal
voltage gated
calcium channel
Skeletal muscle
sodium
di
channel
h
l
Potassium
channel
ACH
ACH receptor
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As the action potential travels down the muscle fiber, it increases the permeability of the fiber membrane to Ca2+ ions, which
causes myosin heads
which causes myosin heads to form cross bridges with actin filaments.
The myosin heads pivot, causing the myosin and actin filaments to slide past one another. The Motor Unit
A motor neuron and the
muscle fibers it controls form a
motor unit
Each branch of an axon
synapses with a single muscle
fiber.
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Patellar Reflex
The reflex in which tapping
pp g the tendon of
the knee stretches one of the muscles that
extends the leg, and the resulting muscle
contraction causes the leg to kick
outward.
Muscle spindle—A structure embedded
within an extrafusal muscle fiber than enables the
CNS to contract a muscle to counteract
the stretching of the extrafusal muscle fiber.
Intrafusal muscle fiber— A muscle fiber that
extends the length of the muscle spindle that is
surrounded by annulospiral endings (sensory
receptors in the central part of the muscle fiber).
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Components of the Monosynaptic Stretch Reflex
When the extrafusal muscle fibers are stretched,
so are the intrafusal fibers
stimulates the annulospiral endings, causing
them to fire more rapidly
This increased neural activity travels along the
Ia fibers of the annulospiral endings
entering
i the
h dorsal
d
l roott off the
th spinal
i l cord
d
and synapsing with alpha motor neurons.
The Ia fibers have an excitatory influence on
alpha motor neurons, causing the extrafusal
muscle fibers to contract
A Polysynaptic Reflex
Withdrawal reflex—
The automatic withdrawal
of a limb from a painful
stimulus
the brain can
influence the
execution of
polysynaptic reflexes
the spinal cord can
also inhibit reflexes
to prevent damage to
our muscles
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Golgi Tendon Organs
Receptor located among the fibers of tendons
that measures the total amount of force exerted by
the muscle on the bone to which the tendon is
attached
Enables motor system to control extent of
muscle contraction.
Strength of muscle contraction reflects the force
exerted by the muscle on the bone—the
greater the contraction off the muscle fibers,
f
the greater the force on the bone.
With too much force, Golgi tendon organs
reduce the contraction of extrafusal muscle
fibers, resulting in the muscle exerting less
pressure on the tendon and bone.
Renshaw Cells
An inhibitory
interneuron excited by
an alpha motor neuron that
causes it to stop
p firing,
g,
preventing excessive muscle
contraction.
C b t muscle
Combats
l damage
d
that can result from fatigue,
which results from muscles
contracting often in a
short period of time.
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Gamma Motor Neurons
A neuron that synapses with intrafusal
muscle fibers to produce continuous
muscle tension.
Continuous activity of gamma motor
neurons produces a constant contraction
of extrafusal muscle fibers
(muscle tone)
This muscle tone is maintained at all
times, except during REM sleep.
The gamma motor system also gives us
the ability to anticipate certain
movements and react quickly.
Brain Control of Voluntary Movement
• Hierarchically organized
• Starts at dorsolateral prefrontal cortex.
• Decision maker
• Sensory input primarily integrated by the
posterior parietal cortex.
• Learning changes the locus of control over the
movements.
• Subcortical control. More efficient.
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Dorsolateral Prefrontal Cortex

The top executive in the perception-action cycle in
nonhuman primates and humans.
cells in this area integrate
sensory information
across time with motor
actions needed to deal
with the information
Secondary Motor Cortex

Cortical area consisting of
the supplementary motor
and the premotor areas.
areas
Supplemental motor
area --involved in the
planning and
sequencing of
voluntary movements
(internally generated
stimuli)
Premotor cortex plans and
sequences externally guided
movements.
Receives input mostly from
the
visual cortex
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Supplemental motor area

Part of the secondary motor cortex.
receives input
p from
the posterior
parietal cortex and
the somatosensory
cortex
Because most movements are guided by both intentions and
external stimuli, the connections between the supplementary motor area
and the premotor cortex coordinate movement planning.
Cortical Control of Movement
Primary motor
cortex:
initiates voluntary
movements
Directly involved in
the control of
motor neurons.
Stimulation results
in movements
involving groups of
muscles – not
individual muscles.
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Mirror Neurons

Neurons in the primate premotor cortex that are
activated by performing an action or by watching
another monkey or person performing an action.


Also seems to exist in humans, in Broca’s area and the
primary motor cortex.
Humans learn many actions through the observation and
imitation of the actions of others and the mirror neuron
system provides a possible mechanism through which
observation can be translated into action.
Plasticity of the Primary Motor Cortex


The primary motor cortex shows great plasticity in its
response to sensory and motor changes.
Any body part has multiple and widely distributed
representations in the topography of the primary motor
cortex.
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Plasticity of the Primary Motor Cortex

Though the exact mechanism for this plasticity is not
known, one possibility is long-term potentiation (LTP), a
long-lasting increased excitability in a specific neural
circuit caused by repetitive stimulation.


May represent learning at the cellular level.
D
Dependent
d
on NMDA receptor activation
i i and
d GABAA
receptor inhibition; weakened by the drug scopolamine.
Primary Somatosensory Cortex



The sensory receptors in the muscles and joints send
information about the external environment to the
somatosensory cortex and
d the
h posterior
i parietal
i l
cortex.
It then goes to the dorsolateral prefrontal cortex, the
secondary motor cortex, and then the primary motor
cortex.
The primary motor cortex then becomes aware of the
status off the
h muscles
l that
h must bbe activated
i
d and
d the
h
location of the body parts that must be moved in order
to exert the right amount of force.
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Posterior Parietal Cortex

Posterior parietal cortex—Cortical area that
integrates input from the visual
visual, auditory
auditory, and skin
senses and relays it to the primary motor cortex,
which uses the information to guide our movements.

Damage to this area results in difficulty responding to
visual, auditory, or somatosensory stimuli presented to the
contralateral (opposite) side of the body (contralateral
neglect).
neglect)
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Other Movement Disorders




Apraxia—A movement disorder characterized by
missing or inappropriate actions not caused by paralysis
or any other motor impairment.
impairment
Constructional apraxia—A disorder characterized by
difficulty drawing pictures or assembling objects.
Limb apraxia—An impairment in the voluntary use of a
limb caused by damage to the left parietal lobe or the
corpus callosum.
Apraxia of speech—A disorder characterized by
difficulty speaking clearly, caused by damage limited to
Broca’s area.
Motor Pathways


The primary motor cortex uses information from the
posterior parietal cortex, somatosensory cortex, and
secondary motor cortex to initiate movement.
From the primary motor cortex and other cortical
areas, two fiber tracts travel through the midbrain and
hi db i and
hindbrain
d connect with
i h the
h PNS in
i order
d to
produce movement.
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Tracts Originating in the Motor Cortex

Corticospinal tract—Motor pathway that controls
movements of fingers, hands, arms, trunk, legs, feet.



Lateral corticospinal tract—The axons of the corticospinal
tract that cross over in the medulla to connect to the opposite
side of the spinal cord, controlling the movement of the fingers,
hands, arms, lower legs, and feet on the opposite side of the
b d
body.
Ventral corticospinal tract—The noncrossing axons that
control the movements of the trunk and upper legs on the
same side of the body.
Corticobulbar tract—A motor pathway that controls
movements of the face and tongue.
Tracts
Originating
in the
Primary
M t
Motor
Cortex
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Tracts Originating in the Subcortex



Ventromedial tract—One of four motor pathways
originating in different parts of the subcortex that
control movements of the trunk and limbs.
Vestibulospinal tract—A ventromedial tract that plays a
central role in posture.
T
Tectospinal
l tract—A
A ventromedial
d l tract that
h controls
l
upper trunk (shoulder) and neck movements and
coordinates the visual tracking of stimuli.
Tracts Originating in the Subcortex



Lateral reticulospinal tract—A ventromedial tract that
activates the flexor muscles of the legs.
Medial reticulospinal tract—A ventromedial tract that
activates the extensor muscles of the legs.
Rubrospinal tract—A motor pathway that controls
movements off the
h hhands,
d lower
l
arms, llower legs,
l
and
d
feet.
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Tracts Originating in the Subcortex
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The Cerebellum



The brain area responsible for developing rapid,
coordinated responses or habits.
Located behind and beneath the cerebral cortex; outer
surface is extremely convoluted; represents 10% of the
brain’s mass, but contains more than half of its neurons.
Ballistic movement—A habitual, rapid, well-practiced
movement that does not depend on sensory feedback;
controlled by the cerebellum.
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The Cerebellum



Input to/output from the cerebellum is conveyed by large
bundles of axons called peduncles.
Integrates information about motor activity, balance and
head position, limb position and extent of muscle
contraction and determines whether ongoing movements
are deviating
d i ti from
f
their
th i intended
i t d d course.
If movements begin to deviate, the cerebellum can correct
them by sending signals to other structures, such as the
deep cerebellar nuclei.
The Cerebellum


Purkinje cells–An output cell from the cerebellar cortex,
which has an exclusively inhibitory effect.
B k t cell—A
Basket
ll A cerebellar
b ll neuron th
thatt has
h an inhibitory
i hibit
influence on Purkinje cells.
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Consequences of Cerebellar Damage




Difficulty maintaining a stable posture, making
movements such as walking unsteady, slurred speech, and
uncoordinated eye movements.
Research suggests that the cerebellum plays a significant
role in cognitive behaviors in addition to its role in finetuning motor movements and in motor learning.
Neurons in the cerebellum are sensitive to alcohol;
alcohol intoxication can lead to signs of cerebellar
malfunction.
See Scientific American Spotlight “Probing Cerebellar
Function”
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The Basal Ganglia

Basal Ganglia—Group of structures that integrates
movement and controls postural adjustments and
muscle tone.

Consists of three subcortical nuclei:




Caudate nucleus (part of neostriatum)
Putamen (part of neostriatum)
Globus pallidus (paleostriatum)
Corpus striatum—Part of the basal ganglia consisting of
the caudate nucleus and putamen.
The Basal Ganglia


Integrates movement through interconnections with
the primary motor cortex, the cerebellum, substantia
nigra, red nucleus, and other motor centers in the
brain.
Damage to the basal ganglia results in impairments in
muscle
l tone, posturall instability,
i
bili poorly
l iintegrated
d
movements, and difficulty performing voluntary
movements (e.g., standing and walking).
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The Basal Ganglia
Damage to the Motor System:
Parkinson’s Disease

Parkinson’s disease—A degenerative neurological
disorder characterized by rigidity of the limbs and
muscle tremors.



Bradykinesia—A movement disorder characterized by
slow movement.
Festination—A tendency in a movement disorder to speed
up a walking pace to running.
Other symptoms
y p
include: difficultyy in speaking,
p
g, lack of
facial expression, apathy, lack of spontaneous blinking,
cognitive deficits (e.g., problems with learning, memory,
attention, and judgment).
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Damage to the Motor System:
Parkinson’s Disease




First signs of the disease may include a tremor in one hand
or some stiffness in the muscles of a leg
Over time, tremors and rigidity worsen; movement
becomes increasingly impaired.
Motor disturbances are caused by the degeneration of the
DA-producing cells of the substantia nigra that synapse
with the basal ganglia.
Damage to this nigrostriatal dopaminergic system leads to
the bradykinesia, whereas the rigidity and tremors are
believed to result from excessive activity in a neural loop
extending from the ventrolateral thalamus to the primary
motor cortex.
Damage to the Motor System:
Parkinson’s Disease

Suggested causes:







Genetic basis
E
Encephalitis
h liti
Arteriosclerosis
Carbon monoxide or manganese poisoning
Trauma to the head
Syphilitic damage to the brain
Environmental toxins
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Damage to the Motor System:
Parkinson’s Disease

Possible treatments:

Levodopa—L-dopa; a drug converted to DA in the brain,
increasing levels of DA depleted by the disease.


Effective in decreasing rigidity and improving movement, but less
effective in reducing tremors.
High
g dosages
g can result in schizophrenic-like
p
symptoms.
y p
Damage to the Motor System:
Parkinson’s Disease

Possible treatments:




Thalamotomy—Psychosurgical treatment that relieves
tremors and improves rigidity; does not relieve bradykinesia.
Pallidotomy—Psychosurgical treatment that reduces tremors,
rigidity, and bradykinesia.
Deep brain stimulation
Transplantation of fetal tissue into the corpus striatum (highly
controversial)
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Damage to the Motor System:
Huntington’s Disease

Huntington’s disease—An inherited neurological
disorder characterized by a slow, progressive
deterioration of motor control, cognition, and
emotion.


Caused by a dominant, defective gene on the short arm of
chromosome 4.
4
Symptoms usually begin between the ages of 30 and 50,
usually with a decline in physical activity and loss of
interest in activities (apathy).
Damage to the Motor System:
Huntington’s Disease


Other symptoms include: involuntary movements of
whole limbs or parts of a limb, interference of voluntary
movements like walking, writing, swallowing, and speaking,
and cognitive deficits (e.g., impaired storage and retrieval
of information, poor abstract reasoning, and diminished
cognitive flexibility).
flexibility)
Symptoms worsen over 15 years or so, and death
eventually results from a loss of muscle control.
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Damage to the Motor System:
Huntington’s Disease


Inability to control voluntary movements is caused by
atrophy of the cerebral cortex and basal ganglia.
The death of neurons in the basal ganglia decreases
GABA and ACh levels, which increases activity in the
nigrostriatal dopaminergic pathway. This causes the
appearance off the
h involuntary
i l
movements that
h
characterize the disease.
Terms




Opposing action of skeletal muscles can be illustrated in
the extension
e tensi n and flexion
fle i n off a limb
limb.
Contracting an extensor muscle produces extension of
the limb (movement away from the body).
Contracting a flexor muscle causes flexion (brings the
extended limb back toward the body).
Muscles that work in opposition to each other (e
(e.g.,
g
biceps and triceps) are called antagonistic muscles;
muscles whose contraction results in the same movement
are called synergistic muscles.
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
Muscle fibers—One of the units comprising a skeletal
muscle; about 10 to 100 micrometers in diameter.
Myofibrils—One of the units comprising a muscle fiber;
cylindrical structures about 1 to 2 micrometers across.
Myofilament—A component of a myofibril.

There are two kinds of myofilament in myofibrils:





Myosin—The protein component of thick myofilaments.
Actin—The protein component of thin myofilaments.
Sarcomere—The functional unit of a myofibril, consisting
of overlapping bands of thick myosin filaments and thin
actin filaments.

This gives skeletal muscles a striped appearance, which is why
they are sometimes called striated muscles.
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
Alpha motor neurons—Motor neurons with a long
axon that leaves the ventral root of the spinal cord or
b i stem and
brain
d synapse with
i h individual
i di id l muscle
l fibers.
fib




Their axons conduct information rapidly (220 m/second).
Extrafusal muscle fiber—A muscle fiber controlled by
an alpha motor neuron.
Neuromuscular junction—A specialized synapse
between an alpha motor neuron and an extrafusal
muscle fiber.
f
Motor end plate—The flattened area of an extrafusal
muscle fiber where a motor neuron and the muscle
fiber synapse.
The Motor Unit




Each branch of an axon synapses with a single muscle
fiber.
Collectively, a motor neuron and the muscle fibers it
controls form a motor unit (an alpha motor neuron
and the muscle fibers it controls).
When the axon of an alpha motor neuron has few
branches and controls only a few muscle fibers, fine
motor control is possible.
When the axon has many branches and controls many
muscle fibers, gross limb movement is possible.
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Muscle Adaptation

Different types of muscles facilitate diverse abilities:




Slow-twitch muscle—A muscle fiber that contracts and fatigues
slowly; produces slower contractions that can be maintained for
long periods of time without fatiguing.
Fast-twitch muscle—A muscle fiber that contracts and fatigues
quickly; produces rapid contractions that tire quickly.
I t
Intermediate-twitch
di t t it h muscle—A
l A muscle
l fiber
fib that
th t contracts
t t att a
lower rate than fast-twitch and a higher rate than slow-twitch
muscles; produces contractions of moderate speed and duration.
We use primarily slow or intermediate-twitch muscles when we
stand or walk; fast-twitch muscles when we run.
Reflex Control of Movement



Reflex—A simple, automatic response to a sensory
stimulus.
Patellar reflex—A reflex in which tapping the tendon
of the knee stretches one of the muscles that extends
the leg, and the resulting muscle contraction causes
the
h leg
l to kick
ki k outward.
d
Most reflexes are produced by the spinal cord
without involvement of the brain.
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Monosynaptic Stretch Reflex

Monosynaptic stretch reflex—A spinal reflex with a
single synapse between the sensory receptor and the
muscle effector.


Muscle spindle—A structure embedded within an
extrafusal muscle fiber than enables the CNS to contract a
muscle to counteract the stretching of the extrafusal
muscle
l fib
fiber.
Intrafusal muscle fiber—A muscle fiber that extends the
length of the muscle spindle that is surrounded by
annulospiral endings (sensory receptors in the central part
of the muscle fiber).
Monosynaptic Stretch Reflex



When the extrafusal muscle fibers are stretched, so are
the intrafusal fibers; stimulates the annulospiral endings,
causing them to fire more rapidly.
This increased neural activity travels along the Ia fibers (or
axons) of the annulospiral endings, entering the dorsal
root off the
h spinal
i l cord
d and
d synapsing
i with
i h alpha
l h motor
neurons.
The Ia fibers have an excitatory influence on alpha motor
neurons, causing the extrafusal muscle fibers to contract.
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Primary Motor Cortex: Organization

Organization of the Primary Motor Cortex


The movement of different body parts is elicited by
stimulation of different regions of the primary motor cortex.
There is greater cortical representation of some body parts
than of others.

Body parts that produce precise movements (e
(e.g.,
g hands and
mouth) have greater representation than do parts that produce
gross movements (e.g., arms and legs).
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