Transcript Control of Movement

Carlson (7e)
Chapter 8: Control of Movement
Skeletal Muscle

Movements of our body are accomplished by contraction
of the skeletal muscles
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Flexion: contraction of a flexor muscle draws in a limb
Extension: contraction of extensor muscle
Skeletal muscle fibers have a striated appearance
Skeletal muscle is composed of two fiber types:
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Extrafusal: innervated by alpha-motoneurons from the spinal
cord: exert force
Intrafusal: sensory fibers that detect stretch of the muscle
Afferent fibers: report length of intrafusal: when stretched, the fibers
stimulate the alpha-neuron that innervates the muscle fiber: maintains
muscle tone
 Efferent fibers: contraction adjusts sensitivity of afferent fibers.
8.2
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Skeletal Muscle Anatomy

Each muscle fiber consists
of a bundle of myofibrils
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Each myofibril is made up of
overlapping strands of actin
and myosin
During a muscle twitch, the
myosin filaments move
relative to the actin filaments,
thereby shortening the
muscle fiber
8.3
Neuromuscular Junction
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The neuromuscular junction is the synapse formed
between an alpha motor neuron axon and a muscle fiber
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Each axon can form synapses with several muscle fibers
(forming a motor unit)
The precision of muscle control is related to motor unit size
 Small:
precise movements of the hand (e.g., fingers, 1:<10)
 Large: movements of the leg (e.g., 1:>300)
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ACh is the neuromuscular junction neurotransmitter
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Release of ACh produces a large endplate potential
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Voltage changes open CA++ channels
CA++ entry triggers myosin-actin interaction (rowing action)
Movement of myosin bridges shortens muscle fiber
8.4
Smooth and Cardiac Muscle
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Smooth muscle is controlled by the autonomic nervous
system
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Multiunit smooth muscle is normally inactive
Located in large arteries, around hair and in the eye
 Responds to neural or hormonal stimulation
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Single-unit smooth muscle exhibits rhythmic contraction
 Muscle
fibers produce spontaneous pacemaker potentials that elicit
action potentials in adjacent smooth muscle fibers
 Single-unit muscle is found in gastrointestinal tract, uterus, small
blood vessels
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Cardiac muscle fibers resemble striated muscle in
appearance, but exhibit rhythmic contractions like that
of single-unit smooth muscle
8.5
Muscle Sensory Feedback
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Striated muscle contraction is governed by sensory
feedback
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Intrafusal fibers are in parallel with extrafusal fibers
Intrafusal receptors fire when the extrafusal muscle fibers
lengthen (load on muscle)
 Intrafusal fibers
activate agonist muscle fibers and inhibit antagonist
muscle fibers
 Extrafusal contraction eliminates intrafusal firing
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Golgi tendon organ (GTO) receptors are located within tendons
 Sense
degree of stretch on muscle
 GTO activation inhibits the agonist muscle (via release of glycine onto
alpha-motoneuron
 GTO receptors function to prevent over-contraction of striated muscle
8.6
Spinal Cord Anatomy
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Spinal cord is organized
into dorsal and ventral
aspects
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Dorsal horn receives
incoming sensory
information
Ventral horn issues efferent
fibers (alpha-motoneurons)
that innervate extrafusal
fibers
Fig 3.23
8.7
Spinal Cord Reflexes
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Monosynaptic reflexes involve a single synapse between
a sensory fiber from a muscle and an alpha-motor neuron
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Sensory fiber activation quickly activates the alpha motor
neuron which contracts muscle fibers
Patellar reflex
 Monosynaptic stretch stretch (posture)
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Polysynaptic reflexes involve multiple synapses between
sensory axons, interneurons, and motor neurons
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Axons from the afferent muscle spindles can synapse onto
Alpha motoneuron connected to the agonist muscle
 An inhibitory interneuron connected to the antagonist muscle
 Signals from the muscle spindle activate the agonist and inhibit the
antagonist muscle
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8.8
Polysynaptic Reflex
8.9
Motor Cortex
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Multiple motor systems control body movements
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Walking, talking, postural, arm and finger movements
Primary motor cortex is located on the precentral gyrus
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Motor cortex is somatotopically organized (motor homunculus)
Motor cortex receives input from
Premotor cortex
 Supplemental motor area
 Frontal association cortex
 Primary somatosensory cortex
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Planning of movements involves the premotor cortex and the
supplemental motor area which influence the primary motor
cortex
8.10
Motor “Homunculus”
8.11
Cortical Control of Movement
8.12
Descending Motor Pathways
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Axons from primary motor cortex descend to the
spinal cord via two groups
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Lateral group: controls independent limb movements
 Corticospinal
tract: hand/finger movements
 Corticobulbar tract: movements of face, neck, tongue, eye
 Rubrospinal tract: fore- and hind-limb muscles
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Ventromedial group control gross limb movements
 Vestibulospinal
tract: control of posture
 Tectospinal tract: coordinate eye and head/trunk movements
 Reticulospinal tract: walking, sneezing, muscle tone
 Ventral corticospinal tract: muscles of upper leg/trunk
8.13
Corticospinal Tract
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Neurons of the corticospinal tract terminate on motor
neurons within the gray matter of the spinal cord
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Corticospinal tract starts in layer 5 of primary motor cortex
Passes through the cerebral peduncles of the midbrain
Corticospinal neurons decussate (crossover ) in the medulla
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80% become the lat. corticospinal tract
20% become the ventral corticospinal tract
Terminate onto internuncial neurons or alpha-motoneurons of
ventral horn
Corticospinal tracts control fine movements
Destruction: loss of muscle strength, reduced dexterity of hands and
fingers
 No effect of corticospinal lesions on posture or use of limbs for
reaching
8.14
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The Apraxias
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Apraxia refers to an inability to properly execute a learned
skilled movement following brain damage
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Limb apraxia involves movement of the wrong portion of a limb,
incorrect movement of the correct limb part, or an incorrect
sequence of movements
 Callosal
apraxia: person cannot perform movement of left hand to a verbal
request (anterior callosum interruption prevents information from reaching
right hemisphere)
 Sympathetic apraxia: damage to anterior left hemisphere causes apraxia of
the left arm (as well as paralysis of right arm and hand)
 Left parietal apraxia: difficulty in initiating movements to verbal request
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Constructional apraxia is caused by right parietal lobe damage
 Person
has difficulty with drawing pictures or assembling objects
8.15
The Basal Ganglia
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Basal ganglia consist of the caudate nucleus, the putamen
and the globus pallidus
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Input to the basal ganglia is from the primary motor cortex and
the substantia nigra
Output of the basal ganglia is to
 Primary
motor cortex, supplemental motor area, premotor cortex
 Brainstem motor nuclei (ventromedial pathways)
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Cortical-basal ganglia loop
 Frontal,
parietal, temporal cortex send axons to caudate/putamen
 Caudate/putamen projects to the globus pallidus
 Globus pallidus projects back to motor cortex via thalamic nuclei
8.16
Anatomy of the Basal Ganglia
8.17
Parkinson’s Disease
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Parkinson’s disease (PD) involves muscle rigidity,
resting tremor, slow movements
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Parkinson’s results from damage to dopamine neurons within
the nigrostriatal bundle (projects to caudate and putamen)
Slow movements and postural problems result from
Loss of excitatory input to the direct circuit (caudate-Gpi-VA/VL
thalamus-motor cortex)
 Loss of output from the indirect circuit (which is overall an excitatory
circuit for motor behavior)
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Neurological treatments for PD:
Transplants of dopamine-secreting neurons (fetal subtantia nigra cells or
cells from the carotid body)
 Stereotaxic lesions of the globus pallidus (internal division) alleviates
some symptoms of Parkinson’s disease
8.18
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Huntington’s Disease
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Huntington’s disease (HD) involves uncontrollable, jerky
movements of the limbs
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HD is caused by degeneration of the caudate nucleus and putamen
Cell loss involves GABA-secreting axons that innervate the
external division of the globus pallidus (GPe)
The GPe cells increase their activity, which inhibits the activity of
the subthalamic nucleus, which reduces the activity level of the
GPi, resulting in excessive movements
HD is a hereditary disorder caused by a dominant gene on
chromosome 4
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This gene produces a faulty version of the protein huntingtin
8.19
The Cerebellum
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Cerebellum consists of two hemispheres with associated
deep nuclei
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Flocculonodular lobe is located at the caudal aspect of the
cerebellum
 This
lobe has inputs and outputs to the vestibular system
 Involved in control of posture
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Vermis is located on the midline of the cerebellum
 Receives
auditory and visual information from the tectum and cutaneous
information from the spinal cord
 Vermis projects to the fastigial nucleus which in turn projects to the
vestibular nucleus and to brainstem motor nuclei
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Damage to the cerebellum generally results in jerky, erratic
and uncoordinated movements
8.20