Carlson (7e) Chapter 8: Control of Movement

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Carlson (7e) Chapter 8 Control of Movement

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Skeletal Muscle

• Movements of our body are accomplished by
  contraction of the skeletal muscles
• 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
• 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.

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Skeletal Muscle Anatomy

• Each muscle fiber consists of a bundle of
  myofibrils
• 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

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Neuromuscular Junction

• The neuromuscular junction is the synapse formed
  between an alpha motor neuron axon and a muscle
  fiber
• 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, 1lt10)
• Large movements of the leg (e.g., 1gt300)
• ACh is the neuromuscular junction
  neurotransmitter
• Release of ACh produces a large endplate
  potential
• Voltage changes open CA channels
• CA entry triggers myosin-actin interaction
  (rowing action)
• Movement of myosin bridges shortens muscle fiber

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Smooth and Cardiac Muscle

• Smooth muscle is controlled by the autonomic
  nervous system
• Multiunit smooth muscle is normally inactive
• Located in large arteries, around hair and in the
  eye
• Responds to neural or hormonal stimulation
• 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
• Cardiac muscle fibers resemble striated muscle in
  appearance, but exhibit rhythmic contractions
  like that of single-unit smooth muscle

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Muscle Sensory Feedback

• Striated muscle contraction is governed by
  sensory feedback
• 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
• 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

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Spinal Cord Anatomy

• Spinal cord is organized into dorsal and ventral
  aspects
• Dorsal horn receives incoming sensory information
• Ventral horn issues efferent fibers
  (alpha-motoneurons) that innervate extrafusal
  fibers

Fig 3.23
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Spinal Cord Reflexes

• Monosynaptic reflexes involve a single synapse
  between a sensory fiber from a muscle and an
  alpha-motor neuron
• Sensory fiber activation quickly activates the
  alpha motor neuron which contracts muscle fibers
• Patellar reflex
• Monosynaptic stretch stretch (posture)
• Polysynaptic reflexes involve multiple synapses
  between sensory axons, interneurons, and motor
  neurons
• 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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Polysynaptic Reflex
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Motor Cortex

• Multiple motor systems control body movements
• Walking, talking, postural, arm and finger
  movements
• Primary motor cortex is located on the precentral
  gyrus
• Motor cortex is somatotopically organized (motor
  homunculus)
• Motor cortex receives input from
• Premotor cortex
• Supplemental motor area
• Frontal association cortex
• Primary somatosensory cortex
• Planning of movements involves the premotor
  cortex and the supplemental motor area which
  influence the primary motor cortex

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Motor Homunculus
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Cortical Control of Movement
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Descending Motor Pathways

• Axons from primary motor cortex descend to the
  spinal cord via two groups
• 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
• 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

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Corticospinal Tract

• Neurons of the corticospinal tract terminate on
  motor neurons within the gray matter of the
  spinal cord
• 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
• 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

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The Apraxias

• Apraxia refers to an inability to properly
  execute a learned skilled movement following
  brain damage
• 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
• Constructional apraxia is caused by right
  parietal lobe damage
• Person has difficulty with drawing pictures or
  assembling objects

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The Basal Ganglia

• Basal ganglia consist of the caudate nucleus, the
  putamen and the globus pallidus
• 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)
• 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

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Anatomy of the Basal Ganglia
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Parkinsons Disease

• Parkinsons disease (PD) involves muscle
  rigidity, resting tremor, slow movements
• Parkinsons 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)
• 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
  Parkinsons disease

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Huntingtons Disease

• Huntingtons disease (HD) involves
  uncontrollable, jerky movements of the limbs
• 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
• This gene produces a faulty version of the
  protein huntingtin

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The Cerebellum

• Cerebellum consists of two hemispheres with
  associated deep nuclei
• 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
• 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
• Damage to the cerebellum generally results in
  jerky, erratic and uncoordinated movements

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