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Movement

VIEWS: 12 PAGES: 17

									Movement

Execution of movement is the single
most important function of the
nervous system (Sperry, 1952)
Muscles
Reflexes
Voluntary movements
Muscles
• Skeletal or striate muscle
• Smooth muscle
• Cardiac muscle
Striate skeletal muscle
• Voluntary movement and reflex movement
• Skeletal because usually attached to bone
• Contracting skeletal muscles leads to
  either flexion or extension of limbs.
• Striate muscles consist of
  – extrafusal muscle fibers
  – intrafusal muscle fibers of muscle spindles
  – g afferent fibers
The alpha motor system
• Alpha motor neurons control several
  extrafusal muscle fibers:
  – 10 fibers in fine coordination motor units
  – up to 500 in gross coordination motor units
• Extrafusal muscles fibers provide motive
  force
• Extrafusal muscle fibers are fasciculi of
  fibers, which are bundles of myofibrils
Striate muscle structure
• Myofibrils are composed of overlapping
  strands or myofilaments: thick myosin
  filaments surrounded in a hexagonal
  pattern by thin actin strands.
• Thus, there are four times as many actin
  strands as myosin strands: 64 billion actin
  filaments in a typical muscle fiber.
• The overlap is partial, producing bands of
  color called striations: striate muscle
Arrangement of myofilaments




Cross-section of a portion of myofibril. Large
myosin filaments are surrounded by smaller
actin filaments.
    Z   I      A        H       A       I   Z




                       ATP


The myosin cross-bridges pull the actin filaments
into the H band, contracting the muscle. More ATP
energy is necessary to break actin-myosin bonds.
Reflexes
• Muscle spindle is composed of intrafusal
  muscle fibers and g afferents and efferents
• g efferents cause intrafusal muscle fibers
  to contract along with the a efferent-
  controlled extrafusal muscle fibers
• Monosynaptic stretch reflex: If a muscle
  spindle is stretched, g afferents send
  impulse to a motor neurons to the same
  muscle, strengthening the contraction.
Polysynaptic reflexes

• The g afferents also synapse on
  interneurons which inhibit a motor
  neurons for the antagonist muscle.
• Golgi tendon organ cells synapse on spinal
  cord interneurons, which release glycine to
  inhibit a motor neurons for the same
  muscle: a protective, force-limiting effect.
Polysynaptic reflexes
• Golgi tendon organs also excite a motor
  neurons for the antagonist muscle,
  producing an inverse myostatic reflex.
• Golgi tendon organ stretch reflex acts to
  balance the monosynaptic stretch reflex,
  improving fine motor coordination.
• Spasticity and the clasp knife reflex.
• Flexion reflex: Withdrawal
Infant reflexes
• Grasp reflex and rooting reflex
• Babinski reflex: Extend great toe, fan others
• Infant reflexes return if
  – The relevant inhibitory cortex is damaged
  – The cortex is depressed by alcohol or other
    drugs
• Allied reflexes (eg. Close eyes--close
  mouth--sneeze) also tend to diminish as we
  mature
Voluntary motor control: Cortex
• Primary motor cortex: Initiate movement
• Secondary motor cortex:
  – Supplementary motor area: sequencing
    complex movements, learning phase
  – Premotor cortex: externally guided or timed
    complex movements. Bimodal cells here are
    contralateral-hand centric visual fields
• Cingulate motor areas
Voluntary and involuntary
control
• Some reflexes can be initiated voluntarily--
  within limits. Try swallowing repeatedly.
• Some can be inhibited voluntarily: Try not
  swallowing.
• Walking and arm swinging
• Visual stimuli and walking: Parkinsonism
• Ballistic vs. feedback controlled movement
Sensorimotor association cortex
• Posterior parietal association cortex
  – Damage produces apraxia and contralateral
    neglect
  – Information that locates the body and its parts
    in space (visual, statoacoustic, and
    somatosensory) relayed to secondary motor
    cortex, to frontal eye field, and to
• Dorsolateral prefrontal association cortex
  – Memory fields and pre-decision firing
Voluntary control: Pathways
• Dorsolateral pathways
  – D. corticospinal: decussates in pyramids,
    control arms and hands, legs and feet. Direct
    to a motor neurons.
  – D. corticorubrospinal: Synapse in red nucleus,
    control facial muscles and gross movements
    of limbs independent of trunk movements
• Corticobulbar pathways: Synapse with
  cranial nerves in the medulla, move face
Ventromedial pathways
• V. corticospinal: ipsilateral descent, partial
  decussation at level in spinal cord; direct.
  Control trunk muscles
• V. cortico-brainstem-spinal: Synapse in
  – tectum: coordinate head/eye/trunk movement
  – Vestibular nuclei: control balance
  – Reticular formation: Tonus and species-
    specific movements of breathing/coughing/
    sneezing, and walking/swimming/jumping
Other motor inputs
• Cerebellum: 50% of brain neurons
  – Evaluates, coordinates, and corrects actions
    initiated by the cerebral cortex
  – Role in learning and cognition is controversial
• Basal ganglia: The extrapyramidal motor
  system
  – Smoothness, grace, and tremor: SN, caudate
  – Sensorimotor learning: TANs and conditioning

								
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