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Chapter 10
Lecture Outline*
Control of Body Movement
Eric P. Widmaier
Boston University
Hershel Raff
Medical College of Wisconsin
Kevin T. Strang
University of Wisconsin - Madison
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figures and tables pre-inserted into
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1
Motor Control Hierarchy
Fig. 10-1
2
Cerebellum
Fig. 10-2a
3
Subcortical and Brainstem Nuclei
Fig. 10-2b
4
Voluntary and Involuntary Actions
• Voluntary movements are accompanied by a
conscious awareness of what we are doing,
and our attention is directed toward the action
or its purpose.
• Involuntary movements are often characterized
as unconscious, automatic or a reflex.
• Most motor behavior is neither purely
voluntary nor purely involuntary.
5
Local Control of Motor Neurons
• Local control systems receive instructions
from higher brain centers and make
adjustments based on information received
from sensory receptors in the muscles,
tendons, and joints of the body part to be
moved.
6
Interneurons
Fig. 10-3
7
Local Afferent Input
Fig. 10-4
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Fig. 10-5 9
Fig. 10-6 10
The Withdrawal Reflex
• Painful stimulation of the skin, as occurs from stepping on a tack,
activates the flexor muscles and inhibits the extensor muscles of
the ipsilateral (on the same side of the body) leg.
• The resulting action moves the affected limb away from the
harmful stimulus, and is thus known as a withdrawal reflex.
• The same stimulus causes just the opposite response in the
contralateral leg (on the opposite side of the body from the
stimulus).
• Motor neurons to the extensors are activated while the flexor
muscle motor neurons are inhibited. This crossed-extensor reflex
enables the contralateral leg to support the body’s weight as the
injured foot is lifted by flexion.
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The Withdrawal Reflex
Fig. 10-9 12
Cerebral Cortex
• The cerebral cortex plays a critical role in both the planning and
ongoing control of voluntary movements, functioning in both the
highest and middle levels of the motor control hierarchy.
• The term sensorimotor cortex is used to include all those parts of the
cerebral cortex that act together to control muscle movement.
• A large number of neurons that give rise to descending pathways for
motor control come from two areas of sensorimotor cortex on the
posterior part of the frontal lobe: the primary motor cortex
(sometimes called simply the motor cortex) and the premotor area.
• The neurons of the motor cortex that control muscle groups in
various parts of the body are arranged anatomically into a
somatotopic map.
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Cerebral Cortex
Fig. 10-10
Fig. 10-11
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Cerebral Cortex
• Other areas of sensorimotor cortex include the supplementary
motor cortex, which lies mostly on the surface on the frontal
lobe where the cortex folds down between the two
hemispheres, the somatosensory cortex, and parts of the
parietal-lobe association cortex .
• Although these areas are anatomically and functionally
distinct, they are heavily interconnected, and individual
muscles or movements are represented at multiple sites.
• The cortical neurons that control movement form a neural
network, meaning that many neurons participate in each single
movement.
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Cerebral Cortex
• The interactions of the neurons within the networks are
flexible so that the neurons are capable of responding
differently under different circumstances.
• This adaptability enhances the possibility of integrating
incoming neural signals from diverse sources and the final
coordination of many parts into a smooth, purposeful
movement.
• It probably also accounts for the remarkable variety of ways in
which we can approach a goal. For example, you can comb
your hair with the right hand or the left, starting at the back of
your head or the front. This same adaptability also accounts for
some of the learning that occurs in all aspects of motor
behavior.
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Cerebral Cortex
• Additional brain areas are also involved in the initiation of
intentional movements, such as the association cortices and
areas involved in emotion and motivation.
• Association areas of the cerebral cortex also play other roles in
motor control. For example, neurons of the parietal association
cortex are important in the visual control of reaching and
grasping.
• These neurons play an important role in matching motor
signals concerning the pattern of hand action with signals from
the visual system concerning the three-dimensional features of
the objects to be grasped.
17
Subcortical and Brainstem Nuclei
• Numerous highly interconnected structures lie in the brainstem and
within the cerebrum beneath the cortex, where they interact with
the cortex to control movements.
• Their influence is transmitted indirectly to the motor neurons both
by pathways that ascend to the cerebral cortex and by pathways
that descend from some of the brainstem nuclei.
• It is not known to what extent, if any, these structures are involved
in initiating movements.
• Their role is to establish the programs that determine the specific
sequence of movements needed to accomplish a desired action.
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Subcortical and Brainstem Nuclei
• Subcortical and brainstem nuclei are also important in learning
skilled movements.
• Prominent among the subcortical nuclei are the paired basal
nuclei.
• This explains why brain damage to subcortical nuclei
following a stroke or trauma can result in either hypercontracted muscles, or flaccid paralysis—it depends on which
specific circuits are damaged.
19
Parkinson Disease
• The input to the basal nuclei is diminished, the interplay of the
facilitory and inhibitory circuits is unbalanced, and activation of
the motor cortex is reduced.
• Clinically, Parkinson disease is characterized by a reduced amount
of movement (akinesia), slow movements (bradykinesia),
muscular rigidity, and a tremor at rest.
• Other motor and nonmotor abnormalities may also be present. For
example, a common set of symptoms includes a change in facial
expression resulting in a masklike, unemotional appearance, a
shuffling gait with loss of arm swing, and a stooped and unstable
posture.
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Parkinson Disease
• Although the symptoms of Parkinson disease reflect
inadequate functioning of the basal nuclei, a major part of
the initial defect arises in neurons of the substantia nigra.
These neurons normally project to the basal nuclei, where
they release dopamine from their axon terminals.
• The substantia nigra neurons degenerate in Parkinson
disease, and the amount of dopamine they deliver to the
basal nuclei is reduced. This decreases the subsequent
activation of the sensorimotor cortex.
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Parkinson Disease
• It is not currently known what causes the degeneration of neurons
of the substantia nigra and the development of Parkinson disease.
It may be an inherited condition, exposure to environmental toxins
such as manganese, carbon monoxide, and some pesticides may
play a role.
• The drugs used to treat Parkinson disease are all designed to
restore dopamine activity in the basal nuclei, and fall into three
main categories:
1. Agonists (stimulators) of dopamine receptors
2. Inhibitors of the enzymes that metabolize dopamine at synapses
3. Precursors of dopamine itself (ex. - Levodopa, also known as
L-dopa)
22
Cerebellum
Fig. 10-2a
23
Cerebellum
• Is involved in posture and movement indirectly by means of input
to brainstem nuclei and (by way of the thalamus) to regions of the
sensorimotor cortex that give rise to pathways that descend to the
motor neurons.
• The cerebellum receives information both from the sensorimotor
cortex (relayed via brainstem nuclei) and from the vestibular
system, eyes, skin, muscles, joints, and tendons.
• One role of the cerebellum in motor functioning is to provide
timing signals for precise execution of the different phases of a
motor program, in particular the timing of the agonist/antagonist
components of a movement. It also helps coordinate movements
and is involved in “muscle memory”.
24
Cerebellum
• The cerebellum also participates in planning movements—
integrating information about the nature of an intended movement
with information about the surrounding space.
• During movement, the cerebellum compares information about
what the muscles should be doing with information about what
they actually are doing and can send correction signals if needed.
• People with cerebellar disease have uncoordinated movements,
cannot start or stop movements quickly or easily, and they cannot
combine the movements of several joints into a single smooth,
coordinated motion (have trouble walking).
25
Descending Pathways
• The influence exerted by the various brain regions on
posture and movement occurs via descending pathways
to the motor neurons and the interneurons that affect
them.
• The pathways are of two types: the corticospinal
pathways, which, as their name implies, originate in the
cerebral cortex; and a second group we will refer to as the
brainstem pathways, which originate in the brainstem.
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Descending Pathways
Fig. 10-12
27
Corticospinal Pathway
• The nerve fibers of the corticospinal pathways have their cell
bodies in the sensorimotor cortex and terminate in the spinal cord.
• The corticospinal pathways are also called the pyramidal tracts or
pyramidal system because of their triangular shape as they pass
along the ventral surface of the medulla oblongata.
• In the medulla oblongata near the junction of the spinal cord and
brainstem, most of the corticospinal fibers cross to descend on the
opposite side. So skeletal muscles on the left side of the body are
controlled largely by neurons in the right half of the brain, and
vice versa.
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Brainstem Pathways
• Axons from neurons in the brainstem also form pathways
that descend into the spinal cord to influence motor neurons.
These pathways are sometimes referred to as the
extrapyramidal system.
• Axons of most of the brainstem pathways remain uncrossed
and affect muscles on the same side of the body (a minority
do cross over to contralateral muscles).
• The brainstem pathways are especially important in
controlling muscles of the trunk for upright posture, balance,
and walking.
29
Muscle Tone
• Muscle tone is the resistance to stretch
exhibited by a relaxed muscle.
• It is due to passive elastic forces and to a state
of partial contraction due to a slight degree of
motor neuron activity.
30
Abnormal Muscle Tone
• High muscle is called hypertonia, and It is due to a
greater-than-normal level of motor neuron activity.
– Hypertonia is accompanied by either spasticity, in which
the excess tone diminishes as the muscles are stretched, or
rigidity, in which the excess tone is continual.
• Low muscle tone is called hypotonia, and it is due to
disorders of motor neurons, neuromuscular junctions,
or the muscles themselves.
– Hypotonia is accompanied by weakness and atrophy.
31
Maintenance of Upright Posture and Balance
• Your body needs the skeleton and muscles to work against
gravity to keep a you upright.
• Added to the problem of maintaining upright posture is that of
maintaining balance. For stability, the center of gravity must be
kept within the base of support the feet provide .
• Once the center of gravity has moved beyond this base, the
body will fall unless one foot is shifted to broaden the base of
support. Yet, people can operate under conditions of unstable
equilibrium because complex interacting postural reflexes
maintain their balance.
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Maintenance of Upright Posture and Balance
Fig. 10-14
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