chapter 3 Neural Control of Exercising Muscle

Learning Objectives Learn the basic structures of the nervous system Follow the pathways of nerve impulses from initiation to muscle action Discover how neurons communicate with one another and learn the role of neurotransmitters in this communication (continued)

Learning Objectives (continued) Understand the functional organization of the central nervous system Become familiar with the roles of the sensory and motor divisions of the peripheral nervous system Learn how a sensory stimulus gives rise to a motor response Consider how individual motor units respond and how they are recruited in an orderly manner depending on the required force

Organization of the Nervous System

Neurons Insert Figure 3.2 b

Nerve Impulse A nerve impulse—an electrical charge—is the signal that passes from one neuron to the next and finally to an end organ, such as a group of muscle fibers, or back to the CNS

Resting Membrane Potential (RMP) Difference between the electrical charges inside and outside a cell, caused by separation of charges across the cell membrane High concentration of K+ inside of the neuron and Na+ on the outside of the neuron Cell is more permeable to K+, thus K+ ions can move more freely In an attempt to establish equilibrium, K+ will move outside the cell Sodium-potassium pump actively transports K+ into and Na+ out of the cell to maintain the RMP RMP is maintained at –70mV

Changes in Membrane Potential Depolarization occurs when inside of cell becomes less negative relative to outside and is caused by a change in the membrane’s Na+ permeability (>–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 depolarizations of the cell membrane (–70 mV → +30 mV → –70 mV in 1 ms)

What Is an Action Potential? All action potentials begin as graded potentials Requires depolarization greater than the threshold value of 15 mV to 20 mV to be initiated The membrane voltage at which a graded potential becomes an action potential is called the depolarization threshold Once threshold is met or exceeded, the all-or-none principle applies and an action potential results

Refractory Periods不反應期 Absolute refractory period When a given segment of an axon is generating an action potential, its sodium gates are open and it is unable to respond to another stimulus Relative refractory period When the sodium gates are closed, the potassium gates are open, and repolarization is occurring, the segment of the axon can respond to a new stimulus, but the stimulus must be substantially greater to evoke an action potential

Events During an Action Potential Resting state Depolarization Propagation of an action potential Repolarization Return to the resting state with the help of the sodium-potassium pump

Voltage and Ion Permeability Changes During an Action Potential Fig. 8.9, p. 259 from HUMAN PHYSIOLOGY, 4th ed. By Dee Unglaub Silverthorn. Copyright © 2007 by Pearson Education, Inc. Reprinted by permission.

The Velocity of an Action Potential Myelinated fibers Saltatory conduction—action potential travels quickly from one node of Ranvier to the next Action potential is faster in myelinated fibers than in unmyelinated fibers Diameter of the neuron Larger-diameter neurons conduct nerve impulses faster due to less resistance to the current flow

The Nerve Impulse Key Points A neuron’s RMP of –70 mV results from a separation of Na+ and K+ ions and is actively 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 - Depolarization (membrane potential becomes less negative) - Hyperpolarization (membrane potential becomes more negative) If the membrane potential depolarizes by 15 mV to 20 mV, the threshold is reached, resulting in an action potential (continued)

The Nerve Impulse (continued) Key Points In myelinated neurons, the impulse travels through the axon by jumping between nodes of Ranvier in a process called saltatory conduction Nerve impulses travel faster in myelinated axons and in neurons with larger diameters

The Synapse A synapse is the site of an impulse transmission from one neuron to another An impulse travels to a presynaptic axon terminal, where it causes synaptic vesicles on the terminal to release neurotransmitters into the synaptic cleft Neurotransmitters bind to postsynaptic receptors on the postsynaptic neuron

A Chemical Synapse

The Neuromuscular Junction The site where an -motor neuron communicates with a muscle fiber Axon terminal releases neurotransmitters which travel across a synaptic cleft and bind to receptors on a muscle fiber’s plasmalemma Neurotransmitter binding causes depolarization, and once a threshold is reached, an action potential occurs 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

Neurotransmitters Categories of Neurotransmitters Small molecule, rapid-acting Neuropeptide, slow-acting Common Neurotransmitters Acetylcholine is the primary neurotransmitter for the motor neurons that innervate skeletal muscle and most parasympathetic nerve endings Norepinephrine is the neurotransmitter for most sympathetic neurons

Synapses Key Points Neurons communicate with one another by releasing neurotransmitters across synapses Synapses involve a presynaptic axon terminal, neurotransmitters, a postsynaptic receptor, and the synaptic cleft Once sufficient amounts of neurotransmitter bind to the receptors, depolarization (excitation) or hyperpolarization occurs, depending on the specific neurotransmitter inhibition the site to which it binds Neurotransmitters are destroyed by enzymes, removed by reuptake into the presynaptic terminal, or diffused away from the synapse

Neuromuscular Junctions Key Points Neurons communicate with muscle cells at neuromuscular junctions A neuromuscular junction involves presynaptic axon terminals, the synaptic cleft, and motor end-plate receptors on the plasmalemma (plasma membrane) The neurotransmitters most important in regulating exercise are acetylcholine and norepinephrine

The Postsynaptic Response Excitatory postsynaptic potentials (EPSPs) are depolarizations of the postsynaptic membrane Inhibitory postsynaptic potentials (IPSPs) are hyperpolarizations of the membrane A summation of impulses is necessary to generate an action potential and is monitored at the axon hillock

Central Nervous System Brain: 4 Major Regions Cerebrum is the site of the mind and intellect Diencephalon is composed of the thalamus and hypothalamus and is the site of sensory integration and regulation of homeostasis Cerebellum plays a crucial role in coordinating movement Brain stem is composed of the midbrain, pons橋, and the medulla oblongata延 and connects brain to spinal cord; it contains regulators of the respiratory and cardiovascular systems

Central Nervous System Spinal cord Afferent fiber carry neural signals from sensory receptor, such as those in the skin, muscle, and joints, to the upper levels of the CNS. Motor (efferent) fibers from the brain and upper spinal cord transmit action potentials to end organ (eg., muscle and glands) 26

Four Major Regions of the Brain and Four Outer Lobes of the Cerebrum

Peripheral Nervous System Sensory division carries sensory information from the body via afferent fibers to the CNS Motor division transmits information from CNS via efferent fibers to target organs (fig 3.1)

Peripheral Nervous System: Sensory Division 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

Muscle and Joint Nerve Endings 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

Peripheral Nervous System: Motor Division Autonomic Nervous System Sympathetic Nervous System Parasympathetic Nervous System The effects of the two systems are often antagonistic, but the systems always function together

Sympathetic Nervous System Fight-or-flight response prepares the body to face crisis and sustains its function during that crisis Effects of the SNS Increases heart rate and strength of heart contraction Increases blood supply to the heart and active muscles Increases vasoconstriction to inactive vascular beds Increases metabolic rate Increases glucose release from the liver Increases blood pressure Causes bronchodilation支氣管擴大作用 to improve gas exchange Improves mental activity and quickness of response Other functions not directly needed are slowed

Parasympathetic Nervous System Housekeeping: digestion, urination, glandular腺體 secretion, and energy conservation Actions oppose those of the sympathetic system: Decreases heart rate Constricts coronary vessels Bronchoconstriction in the lungs支氣管收縮

Peripheral Nervous System Key Points The peripheral nervous system contains 43 pairs of nerves: 12 cranial and 31 spinal Sensory Motor (includes autonomic) The sensory division carries information from the sensory receptors to the CNS The motor division carries motor impulses from the CNS to the muscles, organs, and other tissues The autonomic nervous system includes Sympathetic (fight or flight) Parasympathetic (housekeeping)

Sensory Motor Integration A sensory stimulus is received by sensory receptors The sensory action potential is transmitted along sensory neurons to the CNS The CNS interprets the incoming sensory information and determines the most appropriate reflex response The action potentials for the response are transmitted from the CNS along a-motor neurons The motor action potential is transmitted to a muscle, and the response occurs

Integration Centers整合中心 Spinal cord controls simple motor reflexes Lower brain stem controls more complex subconscious motor reactions Cerebellum governs subconscious control of movement Thalamus governs conscious distinction among sensations Cerebral cortex maintains conscious awareness of a signal and the location of the signal within the body

The Sequence of Events in Sensory-Motor Integration

Sensory Receptors and Their Pathways Back to the Spinal Cord and Brain

Motor Control Motor responses can originate from any one of three levels Spinal cord Lower regions of the brain Motor areas of the cerebral cortex Motor responses for more complex movement patterns typically originate in the motor cortex A motor reflex is integrated by the spinal cord without conscious thought

Muscle Spindles Lie between 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, and the rate at which the length is changing Reflexive muscle contraction is triggered to resist further stretching

Golgi Tendon Organs (GTOs) Encapsulated sensory organs through which a small bundle of muscle tendon fibers pass Located proximal to the tendon’s attachment to the muscle Sensitive to changes in tension Inhibit contracting (agonist) muscles and excite antagonist muscles to prevent injury (muscle contraction)

Muscle Spindles and GTOs (a) A muscle belly, (b) a muscle spindle, and (c) a Golgi tendon organ Page 368 from HUMAN PHYSIOLOGY, 4th ed. By Dee Unglaub Silverthorn. Copyright © 2007 by Pearson Education, Inc. Reprinted by permission.

Higher Brain Centers Primary motor cortex: controls fine and discrete不連接的muscle movement ( fig 3.6 frontal lobe) Premotor cortex: controls learned motor skills of a repetitious pattern Basal ganglia大腦白質區: important in initiating movement of a sustained and repetitive nature (walking and running) Cerebellum: integration system that controls rapid and complex muscular activity and facilitates movement patterns by smoothing out the movement Receives visual and equilibrium input

Sensory-Motor Integration Key Points Sensory-motor integration is the process by which the PNS relays sensory input to the CNS; the CNS interprets this information and then sends out an appropriate motor signal to elicit the desired motor response Sensory input can terminate at various levels of the CNS (Spinal cord, Lower regions of the brain, Motor areas of the cerebral cortex) Reflexes are the simplest form of motor control (not conscious responses) (continued)

Sensory-Motor Integration (continued) Key Points Muscle spindles trigger reflexive muscle action when the muscle spindle is stretched Golgi tendon organs trigger a reflex that inhibits contraction if the tendon fibers are stretched from high muscle tension The primary motor cortex, located in the frontal lobe, is the center of conscious motor control The basal ganglia大腦白質區help initiate some movement and help control posture and muscle tone The cerebellum is an integration center that is involved in all rapid and complex movement processes

Control of Small vs. Large Motor Responses 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) finger 1 neuron for 4 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).