Transcript Chapter 8 Control of Movement

Chapter 8
Control of Movement
Muscles
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3 types of muscles:
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Skeletal muscles – move us (and our bones) around; attached to
bones, fastened via tendons, which are sting bands of connective
tissue
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Flexion – contraction of a flexor muscle, bending the joints
Extension – contraction of an extensor muscle, straitening the joints
Anatomy of skeletal muscle
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Extrafusal muscle fibers – responsible for force exerted by contraction of
skeletal muscle; served by axons of alpha motor neurons
Intrafusal muscle fibers (a.k.a. muscle spindles) – contain sensory endings
sensitive to stretch; served by axons of gamma motor neurons
An alpha motor neuron, its axon, and the several extrafusal muscles fibers
it innervates constitute a motor unit
A single muscle fiber consist of bundle of myofibrils, each containing
the proteins actin and myosin, which serve to contract the muscle;
where these protein filaments overlap, the muscle appears to be
striated, or striped, thus referred to as striated muscle
Muscles
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Skeletal muscles (con’t)
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The physical basis of muscular contraction
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The synapse b/t the axon of an efferent neuron and the muscle fiber it
innervates is called the neuromuscular junction
The terminal buttons synapse onto motor endplates, which are located
in grooves on the muscle fibers
ACh is released into the neuromuscular junction to depolarize the
postsynaptic membrane – this is called an endplate potential (much
larger than EPSPs; always causes muscle fiber to fire, causing a
“twitch”)
Depolarization causes the actin and myosin filaments to work together
(see “rowing movement” figure in book, Fig 8.2) to contract or shorten
the muscle fiber
A single impulse from motor neuron produces a single twitch of muscle
fiber, with the strength of the contraction determined by rate of firing of
motor units
Muscles
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Skeletal muscles (con’t)
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Sensory feedback from muscles
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The afferent stretch receptors of the intrafusal muscle fibers serve to
detect muscle length
Located within the tendon, in the Golgi tendon organ, and encode the
degree of stretch by rate of firing
The Golgi tendon organs detect the strength of the muscle contraction,
and thus fire in proportion to the stress on the muscle
Smooth muscle
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Nonstriated muscle innervated by the ANS
Found in blood vessel walls, reproductive tracts, in sphincters,
within eye, in gut, around hair follicles
Multiunit smooth muscles contract in response to neural
stimulation or hormones; Single-unit smooth muscles normally
contract in a rhythmic pattern
Muscles
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Cardiac muscle
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Found in the heart, responsible for contractions
Looks striated, but acts like single-unit smooth muscle
Neural activity and certain hormones serve to modulate heart
contraction rate
Reflex control of movement
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The spinal cord can work autonomously from the brain in
certain situations
Monosynaptic stretch reflex
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A reflex in which a muscle contracts in response to being quickly
stretched
e.g. knee tap, leg kick reflex
Too short for brain involvement (i.e. sensory info travel up, motor
command travel back down)
Involves one sensory and one motor neuron, with one synapse in
between
Begins at muscle spindle, synapsing on an alpha motor neuron,
and innervating the extrafusal muscle fibers of that same muscle
Reflex control of movement
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Polysynaptic reflexes
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All other spinal reflexes besides the previously mentioned
monosynaptic reflex are polysynaptic
The Golgi tendon organs have two types of afferent axons that
detect muscle stretch: highly sensitive axon that sends info to
brain about the degree of the stretch; less sensitive axon that
synapse on interneurons in the gray matter of the SC, which then
inhibit the alpha motor neurons of that same muscle
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This functions to decrease the strength of the muscle contraction to
prevent damage to tendons or bones attached
Reflex control of movement
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Polysynaptic reflexes (con’t)
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This inhibitory reflex provided early evidence for neural inhibition,
even before most was known about synaptic mechanisms
In a decerebrate (where brain stem is transected from the rest of
the brain) animal, we see an extension of the muscles caused by
excitation of neurons located in brain stem, which is normally
inhibited by neurons rostral to the transection (decerebrate rigidity)
Stretch reflexes excite the agonist (produces movement) muscle
and inhibiting the antagonist (resists movement) muscle
Control of movement by the brain
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Organization of motor cortex
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Somatotopic organization – topographically organized map of the
different parts of the body represented in a certain area of the
brain
Control of movement by the brain
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Organization of motor cortex (con’t)
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Main input to primary motor cortex is frontal association cortex:
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Supplementary motor cortex – located medial and rostral to primary
motor cortex
Premotor cortex – located laterally and rostral to primary motor cortex
Both involved in planning movements, and receive info from
association areas in parietal and temporal cortices
Also receives info from primary somatosensory cortex about
sensory stimuli
Control of movement by the brain
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Cortical control of movement: Descending pathways
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Neurons in primary motor cortex control movements by 2 groups
of descending tracts:
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Lateral group – control independent limb movements (i.e. not
coordinated)
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Corticospinal tract
Corticobulbar tract
Rubrospinal tract
Ventromedial group – controls automatic movements such as gross
movements of trunk, posture and locomotion
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Vestibulospinal tract
Tectospinal tract
Reticulospinal tract
Ventral corticospinal tract
Lateral group (L) & Ventromedial group (R)
Control of movement by the brain
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Deficits of verbally controlled movements: The Apraxias
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Difficulty in carrying out purposeful movements, in the absence of
paralysis or muscular weakness
Limb Apraxia
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characterized by movement of the wrong part of the limb, incorrect
movement of the correct part, or correct movements but the wrong
sequence in response to a verbal command
Can be caused by:
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Callosal apraxia – apraxia of the left hand from by damage to anterior
corpus callosum
Sympathetic apraxia – disorder of left hand by damage to left frontal lobe
Left parietal apraxia – caused by damage to left parietal lobe; produces
difficulty in performing movement sequences by verbal command or
imitating others’ movements
Constructional apraxia
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Caused by damage to right parietal lobe; produces difficulty in drawing
pictures or diagrams
Control of movement by the brain
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The basal ganglia
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Anatomy and function
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Caudate nucleus, putamen & globus pallidus – motor nuclei
Receive input from cortex (esp. primary motor and somatosensory)
and substantia nigra
2 primary outputs:
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To primary motor, supplementary and premotor cortices via the thalamus
To motor nuclei in brain stem as part of the ventromedial pathway
2 thalamic nuclei associated with BG:
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Ventral anterior nucleus & ventrolateral nucleus
Control of movement by the brain
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The basal ganglia (con’t)
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Direct pathway – includes
caudate and putamen,
internal globus pallidus and
thalamic nuclei; excitatory
effect on movement
Indirect pathway – includes
caudate and putamen,
external and internal globus
pallidus, subthalamic
nucleus & thalamic nuclei;
inhibitory effect on
movement
Parkinson’s disease
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Primary symptoms: muscular rigidity, movement slowness,
resting tremor, postural instability
Caused by lack of inhibitory control (lack of DA in indirect
pathway of BG) to balance out excitatory (direct pathway)
Standard treatment: admin of L-DOPA, precursor for DA; allows
more DA to be created in BG
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Side effects: dyskinesias ( involuntary movements) and dystonias
(involuntary postural movements) caused by too much DA in BG
Deep brain stimulation
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Permanent electrode in the subthalamic nucleus; allows patient to
induce electrical stimulation
Huntington’s disease
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Caused by degeneration of caudate and putamen, specifically
of GABA and ACh neurons
Symptoms: uncontrollable movements especially jerky limb
movements
Hereditary disorder, usually appears after age 30-40
Progressive and eventually fatal
The cerebellum
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Outputs project to every major motor structure in brain
Consists of 2 hemispheres that contain deep cerebellar nuclei
surrounded by the cortex
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Flocculonodular lobe – caudal region; control of postural reflexes
Vermis – midline; receives auditory, somatosensory & visual info
Deep cerebellar nuclei – control of descending motor tracts
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Fastigial nucleus
Interposed nuclei
Dentate nucleus
Reticular formation
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Consists of large number of nuclei located in core of medulla,
pons, and midbrain
Controls activity of gamma motor system and thus regulates
muscle tonus
Mesencephalic locomotor region – region of RF of midbrain;
stimulation of this region causes alternating movements of
limbs normally seen during locomotion