1. chapter 3
Neural Control of Exercising Muscle

2. 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)

3. 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

4. Organization of the Nervous System

5. Neurons
Insert Figure 3.2 b

6. 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

7. 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

8. 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)

9. 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

10. 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

11. 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

12. 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.

13. 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

14. 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)

15. 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

16. 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

17. A Chemical Synapse

18. 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

19. The Neuromuscular Junction

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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

26. 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

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

28. 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)

29. 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

30. 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

31. 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

32. 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

33. 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

35. 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)

36. 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

37. 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

38. The Sequence of Events in Sensory-Motor Integration

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

40. 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

41. 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

42. 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)

43. 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.

44. 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

45. 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)

46. 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

47. 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).