chapter
3
Neural Control of
Exercising Muscle
Nerve Impulse
An electrical charge that passes from one neuron to
the next and finally to an end organ, such as a group
of muscle fibers.
Resting Membrane Potential (RMP)
• Difference between the electrical charges inside and
outside a cell, caused by separation of charges across
a membrane.
• High concentration of K+ inside the neuron and Na+
outside the neuron.
• K+ ions can move freely, even outside the cell, to help
maintain imbalance.
• Sodium-potassium pump actively transports K+ and Na+
ions to maintain imbalance.
• The constant imbalance keeps the RMP at –70mV.
RESTING STATE
Changes in Membrane Potential
Depolarization occurs when inside of cell becomes
less negative relative to outside (>–70 mV).
Hyperpolarization occurs when inside of cell becomes
more negative relative to outside (<–70 mV).
Graded potentials are localized changes in membrane
potential (either depolarization or hyperpolarization).
Action potentials are rapid, substantial depolarization
of the membrane (–70 mV to +30 mV to –70 mV all in
1 ms).
What Is an Action Potential?
• Starts as a graded potential.
• Requires depolarization greater than the threshold
value of 15 mV to 20 mV.
• Once threshold is met or exceeded, the all-or-none
principle applies.
Events During an Action Potential
1.
2.
3.
4.
5.
Resting state
Depolarization
Propagation of an action potential
Repolarization
Return to the resting state with the help of the
sodium-potassium pump
ACTION POTENTIAL
The Velocity of an Action Potential
Myelinated fibers
• Saltatory conduction—potential travels quickly from one
break in myelin to the next.
• Action potential is slower in unmyelinated fibers than in
myelinated fibers.
Diameter of the neuron
• Larger-diameter neurons conduct nerve impulses
faster.
• Larger-diameter neurons present less resistance to
current flow.
Key Points
The Nerve Impulse
• A neuron’s RMP of –70 mV is maintained by the
sodium-potassium pump.
• Changes in membrane potential occur when ion gates
in the membrane open, permitting ions to move from
one side to the other.
• If the membrane potential depolarizes by 15 mV to 20
mV, the threshold is reached, resulting in an action
potential.
(continued)
Key Points (continued)
The Nerve Impulse
• Impulses travel faster in myelinated axons and in
neurons with larger diameters.
• Saltatory conduction refers to an impulse traveling
along a myelinated fiber by jumping from one node of
Ranvier to the next.
The Synapse
• A synapse is the site of an impulse transmission
between two neurons.
• An impulse travels to a presynaptic axon terminal,
where it causes synaptic vesicles on the terminal to
release chemicals into the synaptic cleft.
• Chemicals are picked up by postsynaptic receptors on
an adjacent neuron.
A Chemical Synapse
The Neuromuscular Junction
• The junction is a site where a motor neuron
communicates with a muscle fiber.
• Axon terminal releases neurotransmitters (such as
acetylcholine or epinephrine), which travel across a
synaptic cleft and bind to receptors on a muscle fiber.
• This binding causes depolarization, possibly causing an
action potential.
• The action potential spreads across the sarcolemma,
causing the muscle fiber to contract.
The Neuromuscular Junction
Refractory Period
• Period of repolarization.
• The muscle fiber is unable to respond to any further
stimulation.
• The refractory period limits a motor unit’s firing
frequency.
Key Points
Synapses
• Neurons communicate with one another by releasing
neurotransmitters across synapses.
• Synapses involve a presynaptic axon terminal, a
postsynaptic receptor, neurotransmitters, and the
space between them.
• Neurotransmitters bind to the receptors and cause
depolarization (excitation) or hyperpolarization
(inhibition), depending on the specific neurotransmitter
and the site to which it binds.
(continued)
Key Points (continued)
Neuromuscular Junctions
• Neurons communicate with muscle cells at
neuromuscular junctions, which function much like a
neural synapse.
• The refractory period is the time it takes the muscle
fiber to repolarize before the fiber can respond to
another stimulus.
• Acetylcholine and epinephrine are the
neurotransmitters most important in regulating
exercise.
(continued)
Key Points (continued)
The Postsynaptic Response
• Binding of a neurotransmitter causes a graded action
potential in the postsynaptic membrane.
• An excitatory impulse causes hyperpolarization or
depolarization.
• An inhibitory impulse causes hyperpolarization.
• The axon hillock keeps a running total of the neuron’s
responses to incoming impulses.
• A summation of impulses is necessary to generate an
action potential.
Central Nervous System
Brain
• Cerebrum is the site of the mind and intellect.
• Diencephalon is the site of sensory integration and
regulation of homeostasis.
• Cerebellum plays a crucial role in coordinating
movement.
• Brain stem connects brain to spinal cord; contains
regulators of respiratory and cardiovascular systems.
Peripheral Nervous System
Spinal Cord
• 12 cranial nerves connected with the brain.
• 31 spinal nerves connected with the spinal cord.
• Sensory division carries sensory info from body via
afferent fibers to the CNS.
• Motor division transmits information from CNS via
afferent fibers to target organs.
• Autonomic nervous system controls involuntary internal
functions.
Four Major Regions of the Brain and Four
Outer Lobes of the Cerebrum
The Nervous Systems
Types of Sensory Receptors
Mechanoreceptors respond to mechanical forces
such as pressure, touch, vibrations, and stretch.
Thermoreceptors respond to changes in
temperature.
Nociceptors respond to painful stimuli.
Photoreceptors respond to light to allow vision.
Chemoreceptors respond to chemical stimuli from
foods, odors, and changes in blood concentrations.
SENSORY RECEPTORS AND PATHWAYS
Muscle and Joint Nerve Endings
• Joint kinesthetic receptors in joint capsules sense
the position and movement of joints.
• Muscle spindles sense how much a muscle is
stretched.
• Golgi tendon organs detect the tension of a muscle
on its tendon, providing information about the
strength of muscle contraction.
Sympathetic Nervous System
Fight-or-flight prepares you for acute stress or
physical activity.
It facilitates your motor response with increases in
• heart rate and strength of heart contraction,
• blood supply to the heart and active muscles,
• metabolic rate and release of glucose by the liver,
• blood pressure,
• rate of gas exchange between lungs and blood, and
• mental activity and quickness of response.
Parasympathetic Nervous System
Housekeeping: digestion, urination, glandular
secretion, and energy conservation
Actions oppose those of the sympathetic system:
• Decreases heart rate
• Constricts coronary vessels
• Constricts tissues in the lungs
Key Points
Peripheral Nervous System
• The peripheral nervous system contains 43 pairs of
nerves and is divided into sensory and motor
divisions.
• The sensory division carries information from the
sensory receptors to the CNS.
• The motor division includes the autonomic nervous
system.
• The motor division carries impulses from the CNS to
the muscles or target organs.
(continued)
Key Points (continued)
Peripheral Nervous System
• The autonomic nervous system includes the
sympathetic and parasympathetic nervous systems.
• The sympathetic nervous system prepares the body
for an acute response.
• The parasympathetic nervous system carries out
processes such as digestion and urination.
• The sympathetic and parasympathetic systems are
opposing systems that function together.
Integration Centers
Spinal cord controls simple motor reflexes such as
pulling your hand away after touching something hot.
Lower brain stem controls more complex
subconscious motor reactions such as postural control.
Cerebellum governs subconscious control of
movement, such as that needed to coordinate multiple
movements.
Thalamus governs conscious distinction among
sensations such as feeling hot and cold.
Cerebral cortex maintains conscious awareness of a
signal and the location of the signal within the body.
SENSORY-MOTOR INTEGRATION
Motor Control
• Sensory impulses evoke a response through a
motor neuron.
• The closer to the brain the impulse stops, the more
complex the motor reaction.
• A motor reflex is a preprogrammed response that is
integrated by the spinal cord without conscious
thought.
MUSCLE BODY, MUSCLE SPINDLE,
AND GTO
Muscle Spindles
• Lie between and are connected to regular skeletal
muscle fibers.
• The middle of the spindle cannot contract but can
stretch.
• When muscles attached to the spindle are
stretched, neurons on the spindle transmit
information to the CNS about the muscle’s length.
• Reflexive muscle contraction is triggered to resist
further stretching.
Golgi Tendon Organs (GTOs)
• Encapsulated sensory organs through which
muscle tendon fibers pass
• Located close to the tendon’s attachment to the
muscle
• Sense small changes in tension
• Inhibit contracting (agonist) muscles and excite
antagonist muscles to prevent injury
Conscious Control of Movement
• Neurons in the primary motor cortex control
voluntary muscle movement.
• Clusters of nerve cells in the basal ganglia initiate
sustained and repetitive movements—walking,
running, maintaining posture, and muscle tone.
• The cerebellum controls fast and complex muscular
activity.
Key Points
Sensory-Motor Integration
• Sensory-motor integration is the process by
which the PNS relays sensory input to the CNS,
which processes the input and response with the
appropriate motor signal.
• Sensory input may be integrated at the spinal
cord, in the brain stem, or in the brain, depending
on its complexity.
• Reflexes are automatic responses to a given
stimulus.
(continued)
Key Points (continued)
Sensory-Motor Integration
• Muscle spindles and Golgi tendon organs trigger
reflexes to protect the muscles from being
overstretched.
• The primary motor cortex, basal ganglia, and
cerebellum all integrate sensory input for voluntary
muscle action.
• Engrams are memorized motor patterns stored in
the brain.
Did You Know . . . ?
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).
Key Points
The Motor Response
• Each muscle fiber is innervated by only one neuron,
but one neuron may innervate up to several
thousand muscle fibers.
• All muscle fibers within a motor unit are of the same
fiber type.
• Motor units are recruited in an orderly manner.
Thus, specific units are called on each time a
specific activity is performed; the more force
needed, the more units recruited.
• Motor units with smaller neurons (ST units) are
called on before those with larger neurons (FT
units).