3

Neural Control of Exercising Muscle

chapter

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. Resting state2. Depolarization3. Propagation of an action potential4. Repolarization5. 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.

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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.

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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 PointsPeripheral 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.

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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 PointsThe 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).