C H A P T E R 2

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C H A P T E R 2
NEUROLOGICAL CONTROL OF MOVEMENT
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Learning Objectives
w Learn the basic structures of the nervous
system.
w Follow the pathways of nerve impulses from
initiation to muscle action.
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Learning Objectives
w Understand the functional organization of the
central nervous system.
w Become familiar with the roles of the sensory
and motor divisions of the peripheral nervous
system.
w Learn how a sensory stimulus gives rise to a
motor response.
w Consider how individual motor units respond
and how they are recruited in an orderly
manner depending on the required force.
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ORGANIZATION OF THE NERVOUS SYSTEM
Skeletal Muscles
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STRUCTURE OF A NEURON (NERVE CELL)
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Alpha-Motoneuron/Motor Unit
Brooks G, et al., Exercise Physiology, Mayfield,
2000.
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Action Potential (Impulse)
An electrical event that passes down an excitable
cell membrane by ionic fluxes and from one neuron
to the next via chemical transmission and finally
to an end organ via chemical transmission, such
as a group of muscle fibers.
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Resting Membrane Potential (RMP)
w Difference between the electrical charges
(ions) inside and outside a cell, caused by
separation of charges across a membrane
w High concentration of potassium ions (K)
inside the neuron and sodium ions (Na) outside
the neuron
w K ions can move more freely through the
membrane, i.e., the membrane is more permeable to
this ion thus some K ions slip out through
the membrane, making the outside more positive
w Sodium-potassium pump actively transports K in
and Na out through the membrane to maintain
imbalance
w The constant imbalance keeps the RMP at 70 mV
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Changes in Membrane Potential
Depolarizationinside of cell becomes less
negative relative to outside (gt 70 mV)
Hyperpolarizationinside of cell becomes more
negative relative to outside (lt 70 mV)
Graded potentialslocalized changes in membrane
potential (either depolarization or
hyperpolarization)
Action potentialsrapid, substantial
depolarization of the membrane (70 mV to 30 mV
to 70 mV all in 1 ms)
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What Is an Action Potential?
w Starts as a graded potential
w Requires depolarization greater than the
threshold value (e.g., 50 to 55 mV)
w Once threshold is met or exceeded, the
all-or-none principle applies
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Action Potentials
w Once threshold is met or exceeded, the
all-or-none principle applies the action
potential is propagated down the neuron or muscle
fiber membrane
Threshold
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Events During an Action Potential
1. The resting state
2. Depolarization
3. Propagation of an action potential
4. Repolarization
5. Return to the resting state via the
sodium-potassium pump
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RESTING STATE
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ACTION POTENTIAL
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Velocity of an Action Potential
Alpha-motoneurons and the sensory neurons from
muscle spindles and Golgi tendon organs are the
largest, fastest neurons in the PNS (100 m/s)
an example of small, relatively slow unmyelinated
neurons are the sensory neurons from some
nociceptors (dull, diffuse aching sensation) (15
m/s)
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Synapse
w A synapse is the site of impulse transmission
between two neurons.
w An action potential travels to a presynaptic
axon terminal where it causes synaptic vesicles
on the terminal to release chemicals
(neurotransmitters) into the synaptic cleft (gap).
w The neurotransmitters bind to postsynaptic
receptors on an adjacent neuron, usually on the
dendrites (80-95), which results in increased
permeability to sodium.
w Neural impulses can only be transmitted from
the dendrite or cell body through the axon to the
adjacent neuron since the neurotransmitters are
released only from the terminal end of the axon.
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CHEMICAL SYNAPSE
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Neuromuscular Junction
w The junction is a site where a motor neuron
communicates with a muscle fiber.
w Motor axon terminal releases acetylcholine
(ACh) which diffuses across the cleft and binds
to receptors on the muscle fiber membrane,
increasing permeability to sodium ions.
w This causes depolarization, leading to
development of an action potential on the muscle
fiber.
w The action potential spreads across the
sarcolemma into the t-tubules causing the muscle
fiber to contract.
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NEUROMUSCULAR JUNCTION
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Refractory Period
w Period of repolarization of a neuron or muscle
fiber.
w The cell is unable to respond to any further
stimulation during the refractory period.
w The refractory period limits a motor unit's
firing frequency.
Nonetheless, fast-twitch motor units may fire at
frequencies as high as 100 Hz (Hertz), or 100
action potentials per second.
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Thought Question
Where in the body are the longest
alpha-motoneurons located? At 100 m/s, about how
long would it take an action potential to get
from the CNS to a motor unit in this location?
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Key Points
Neural Synapses
w Neurotransmitters released by neurons bind to
the post-synaptic receptors and cause
depolarization (excitation) or hyperpolarization
(inhibition) depending on the specific
neurotransmitter and the specific receptor to
which it binds.
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Key Points
Postsynaptic Response
w An excitatory presynaptic impulse causes
depolarization.
w An inhibitory presynaptic impulse causes
hyperpolarization.
w Whether or not the membrane reaches threshold
and generates an action potential depends on the
balance of the hundreds of excitatory and
inhibitory inputs.
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Central Nervous System
Spinal cord
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REGIONS OF THE BRAIN
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Peripheral Nervous System
w 12 pairs of cranial nerves connected with the
brain.
w 31 pairs of spinal nerves connected with the
spinal cord.
w Sensory division carries sensory information
from sensory receptors in the body by afferent
fibers to the CNS.
w Motor division transmits information from CNS
via efferent fibers to target organs (e.g.,
muscles).
w Autonomic nervous system controls involuntary
internal functions sympathetic and
parasympathetic components.
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Sympathetic Nervous System
Fight-or-flightprepares you for acute stress or
physical activity norepinephrine is the
neurotransmitter also stimulates release of
catecholamines (norepinephrine and epinephrine)
from the adrenal medulla glands
Facilitates the motor response with increases in
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Parasympathetic Nervous System
Internal Housekeeping controls digestion,
urination, glandular secretion, and energy
conservation ACh is the primary neurotransmitter
Actions oppose those of the sympathetic system
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SENSORY RECEPTORS AND PATHWAYS
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Types of Sensory Receptors
Mechanoreceptorsrespond to mechanical forces
such as pressure, touch, vibration, or stretch.
Thermoreceptorsrespond to changes in temperature.
Nociceptorsrespond to painful stimuli.
Photoreceptorsrespond to light to allow vision.
Chemoreceptorsrespond to chemical stimuli from
foods, odors, and changes in blood concentrations
of gases and substances.
Proprioceptorssensitive to bodily movement and
position
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Muscle and Joint Proprioceptors
w Kinesthetic receptors in joint capsules sense
the position and movement of joints.
w Muscle spindles sense muscle length and rate of
length change.
w Golgi tendon organs (GTOs) detect the tension
of a muscle on its tendon, providing information
about the strength of muscle contraction.
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Muscle Spindles
w A group of 4 to 20 small muscle fibers
(intrafusal) with sensory and motor nerve
endings, covered by a connective tissue sheath,
and connected in parallel with extrafusal (or
regular) muscle fibers.
w The middle of the spindle can stretch, but
cannot contract as it contains little or no actin
and myosin.
w When extrafusal fibers attached to the spindle
are stretched, sensory neurons on the spindle
transmit information to the CNS about the
muscles length.
w Reflexive muscle contraction is triggered
through the alpha motoneuron to resist further
stretching.
w Gamma motoneurons activate intrafusal fibers,
causing the middle of the spindle to stretch,
making the spindle sensitive to small degrees of
stretch.
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Golgi Tendon Organs (GTOs)
w Encapsulated sensory organs through which
muscle tendon fibers pass
w Located close to the tendon's attachment to the
muscle
w Sense small changes in tension
w Inhibit contracting (agonist) muscles and
excite antagonist muscles to prevent injury
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MUSCLE BODY, MUSCLE SPINDLE, AND GTO
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MUSCLE SPINDLE
Eccles, JC. The Understanding of the Brain,
McGraw-Hill, 1977.
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SENSORY-MOTOR INTEGRATION
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Integration Centers
Spinal cordsimple motor reflexes such as pulling
your hand away after touching something hot.
Lower brain stemmore complex subconscious motor
reactions such as postural control.
Cerebellumsubconscious control of movement such
as that needed to coordinate multiple movements.
Thalamusconscious distinction among sensations
such as feeling hot or cold.
Cerebral cortexconscious awareness of a
sensation and the location within body where the
sensation originates.
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Motor Control
w Sensory impulses evoke a response through a
motoneuron.
w The closer to the brain the impulse stops, the
more complex the motor reaction.
w A motor reflex is a preprogrammed response that
is integrated by the spinal cord or brain
without conscious thought.
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MOTOR PATHWAYS
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Conscious Control of Movement
w Neurons originating in the primary motor cortex
control voluntary muscle movement (cerebro-spinal
motoneurons, or upper motoneurons).
w Clusters of nerve cells in the basal ganglia
initiate sustained and repetitive
movementswalking, running, maintaining posture,
and muscle tone.
w The cerebellum compares the actual movement
with the intended movement. For this reason it
is called a comparator.
Engrams are learned motor patterns stored in the
cortex.
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Thought Question
How might the cerebellum (comparator) use
information from the muscle spindles to monitor
the muscle actions? If the movement is not going
as intended, what would the cerebellum do to
correct the movement?
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Motor unit size/function
Muscles controlling fine movements, such as those
controlling the eyes, have a small number of
muscle fibers per motor neuron (about 1 neuron
for every 15 muscle fibers). Muscles with more
general function, such as those controlling the
calf muscle in the leg, have many fibers per
motor neuron (about 1 neuron for every 2,000
muscle fibers).