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Neural Control of Exercising Muscle: Nervous System Structure and Function

This chapter provides an overview of the nervous system, including its structure and function, major divisions, and the generation and transmission of nerve impulses. It also covers the role of graded and action potentials in nervous system function, as well as the propagation of action potentials down the axon and across synapses. Additionally, it explores the specialized synapse at the neuromuscular junction.

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Neural Control of Exercising Muscle: Nervous System Structure and Function

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  1. Chapter 3 • Neural Control of Exercising Muscle

  2. Chapter 3 Overview • Overview of nervous system • Structure and function of nervous system • Central nervous system • Peripheral nervous system • Sensory-motor integration • Motor response

  3. Major Divisions of the Nervous System • Central nervous system: brain, spinal cord • Peripheral nervous system • Sensory (afferent): incoming • Motor (efferent): outgoing • Somatic: voluntary, to skeletal muscles • Autonomic: involuntary, to viscera • Sympathetic • Parasympathetic

  4. Figure 3.1

  5. Nervous System Structure and Function • Neuron • Basic structural unit of nervous system • Has same basic structure everywhere in body • Has three major regions • Cell body (soma) • Dendrites • Axon

  6. Nervous System Structure and Function • Cell body • Contains nucleus • Cell processes radiate out • Dendrites • Receiver cell processes • Carry impulse toward cell body • Axon • Sender cell process, starts at axon hillock • End branches, axon terminals, neurotransmitters

  7. Nervous System Structure and Function: Nerve Impulse • Electrical signal for communication between periphery and brain • Must be generated by a stimulus • Must be propagated down an axon • Must be transmitted to next cell in line

  8. Resting Membrane Potential • Difference in electrical charges between outside and inside of cell • −70 mV • Caused by uneven separation of charged ions • Polarized

  9. Resting Membrane Potential • Why −70 mV? • High [Na+] outside cell, medium [K+] inside cell • Inside more negative relative to outside • Na+ channels closed • Na+ wants to enter cell but can’t • Electrical and concentration gradients • K+ channels open • K+ leaves cell (concentration gradient) • Offset by Na+−K+ pumps

  10. Depolarization and Hyperpolarization • Depolarization • Occurs when inside of cell becomes less negative, -70 mV  0 mV • More Na+ channels open, Na+ enters cell • Required for nerve impulse to arise and travel • Hyperpolarization • Occurs when inside of cell becomes more negative, -70 mV  −90 mV • More K+ channels open, K+ leaves cell • Makes it more difficult for nerve impulse to arise

  11. Graded and Action Potentials • Depolarization and hyperpolarization contribute to nervous system function via • Graded potentials (GPs) • Help cell body decide whether to pass signal to axon • Can excite or inhibit a neuron • Action potentials (APs) • Pass signal down axon • Only excitatory

  12. Graded Potentials • Localized changes in membrane potential • Generated by incoming signals from dendrites • Inhibitory signal = K+ efflux = hyperpolarization • Excitatory signal = Na+ influx = depolarization • Strong GP  AP • How strong? Must depolarize to threshold mV • AP will be propagated down axon • AP will be transmitted to next cell

  13. Action Potentials • Rapid, substantial depolarization • Last ~1 ms • Begin as GPs

  14. Action Potentials:Generating an AP • If GP reaches threshold mV, AP will occur • ~−55 mV • Threshold mV not reached = no action potential • All-or-none principle • − 70 mV  +30 mV  − 70 mV again • − 70 to −55 mV: depolarizing GP, Na+ influx • − 55 to +30 mV: depolarizing AP, Na+ influx • +30 to −70 mV: repolarizing AP, K+ efflux

  15. Figure 3.3

  16. Action Potentials:Refractory Periods • Absolute refractory period • During depolarization • Neuron unable to respond to another stimulus • Na+ channels already open, can’t open more • Relative refractory period • During repolarization • Neuron responds only to very strong stimulus • K+ channels open (Na+ closed, could open again)

  17. Action Potentials:Propagation Down Axon • Myelin: speeds up propagation • Fatty sheath around axon (Schwann cells) • Not continuous (nodes of Ranvier) • Saltatory conduction • Multiple sclerosis: degeneration of myelin • Axon diameter: larger = faster

  18. Synapse:Transmitting APs • Junction or gap between neurons • Site of neuron-to-neuron communication • AP must jump across synapse • Axon  synapse  dendrites • Presynaptic cell  synaptic cleft  postsynaptic cell • Signal changes form across synapse • Electrical  chemical  electrical

  19. Figure 3.4

  20. Synapse:Transmitting APs • AP can only move in one direction • Axon terminals contain neurotransmitters • Chemical messengers • Carry electrical AP signal across synaptic cleft • Bind to receptor on postsynaptic surface • Stimulate GPs in postsynaptic neuron

  21. Neuromuscular Junction:A Specialized Synapse • Site of neuron-to-muscle communication • Uses acetylcholine (ACh) as its neurotransmitter • Excitatory: passes AP along to muscle • Postsynaptic cell = muscle fiber • ACh binds to receptor at motor end plate • Causes depolarization • AP moves along plasmalemma, down T-tubules • Repolarization, refractory period

  22. Figure 3.5

  23. Neurotransmitters • 50+ known or suspected • Two major categories • Small molecule, rapid acting • Large molecule neuropeptides, slow acting • ACh and norepinephrine (NE) govern exercise • ACh stimulates skeletal muscle contraction, mediates parasympathetic nervous system effects • NE mediates sympathetic nervous system effects

  24. Postsynaptic Response • Neurotransmitters trigger GPs on new cell • Excitatory postsynaptic potential (EPSP) • Depolarizing, excitatory, promotes AP • Summation: multiple EPSPs = more depolarizing • Reach threshold depolarization  AP will occur • Inhibitory postsynaptic potential (IPSP) • Hyperpolarizing, inhibitory, prevents AP • Summation: multiple IPSPs = more hyperpolarizing

  25. Central Nervous System • Brain • Cerebrum • Diencephalon • Cerebellum • Brain stem • Spinal cord

  26. Brain: Cerebrum • Left and right hemispheres • Connected by corpus callosum, which allows interhemisphere communication • Cerebral cortex • Outermost layer of cerebrum • Gray matter (nonmyelinated) • Conscious brain (mind, intellect, awareness)

  27. Cerebrum: Five Lobes • Four superficial (outer) lobes • Frontal: general intellect, motor control • Temporal: auditory input, interpretation • Parietal: general sensory input, interpretation • Occipital: visual input, interpretation • One central (deep) lobe • Insular: emotion, self-perception

  28. Cerebrum: Regions of Interest for Exercise Physiology • Primary motor cortex (frontal lobe) • Conscious control of skeletal muscle movement • Pyramidal cells  corticospinal tract  spinal cord • Basal ganglia (cerebral white matter) • Clusters of cell bodies deep in cerebral cortex • Help initiate sustained or repetitive movements • Walking, running, posture, muscle tone • Primary sensory cortex (parietal lobe)

  29. Brain: Diencephalon • Thalamus • Major sensory relay center • Determines what we are consciously aware of • Hypothalamus • Maintains homeostasis, regulates internal environment • Neuroendocrine control • Appetite, food intake, thirst/fluid balance, sleep • Blood pressure, heart rate, breathing, body temperature

  30. Brain: Cerebellum • Controls rapid, complex movements • Coordinates timing, sequence of movements • Compares actual to intended movements and initiates correction • Accounts for body position, muscle status • Receives input from primary motor cortex, helps execute and refine movements

  31. Brain: Brain Stem • Relays information between brain and spinal cord • Midbrain, pons, medulla oblongata • Reticular formation • Coordinates skeletal muscle function and tone • Controls cardiovascular and respiratory function • Analgesia system • Opioid substances modulate pain here - b-endorphin release with exercise

  32. Figure 3.6

  33. Spinal Cord • Continuous with medulla oblongata • Tracts of nerve fibers permit two-way conduction of nerve impulses • Ascending afferent (sensory) fibers • Descending efferent (motor) fibers

  34. Peripheral Nervous System • Connects to brain and spinal cord via • 12 pairs of cranial nerves (connect to brain) • 31 pairs of spinal nerves (connect to spinal cord) • Both types directly supply skeletal muscles • Two major divisions • Sensory (afferent) division • Motor (efferent) division

  35. Sensory Division • Transmits information from periphery to brain • Major families of sensory receptors • Mechanoreceptors: physical forces • Thermoreceptors: temperature • Nociceptors: pain • Photoreceptors: light • Chemoreceptors: chemical stimuli

  36. Sensory Division: Special Families of Sensory Receptors • Joint kinesthetic receptors • Sensitive to joint angles, rate of angle change • Sense joint position, movement • Muscle spindles • Sensitive to muscle length, rate of length change • Sense muscle stretch • Golgi tendon organs • Sensitive to tension in tendon • Sense strength of contraction

  37. Motor Division • Transmits information from brain to periphery • Two divisions • Autonomic: regulates visceral activity • Somatic: stimulates skeletal muscle activity

  38. Motor Division: Autonomic Nervous System • Controls involuntary internal functions • Exercise-related autonomic regulation • Heart rate, blood pressure • Lung function • Two complementary divisions • Sympathetic nervous system • Parasympathetic nervous system

  39. Autonomic Nervous System:Sympathetic • Fight or flight: Prepares body for exercise • Sympathetic stimulation –  Heart rate, blood pressure –  Blood flow to muscles –  Airway diameter (bronchodilation) –  Metabolic rate, glucose levels, FFA levels –  Mental activity

  40. Autonomic Nervous System:Parasympathetic • Rest and digest • Active at rest • Opposes sympathetic effects • Parasympathetic stimulation includes –  Digestion, urination • Conservation of energy –  Heart rate –  Diameter of vessels and airways

  41. Table 3.1

  42. Sensory-Motor Integration • Process of communication and interaction between sensory and motor systems • Five sequential steps 1. Stimulus sensed by sensory receptor 2. Sensory AP sent on sensory neurons to CNS 3. CNS interprets sensory information, sends out response 4. Motor AP sent out on a-motor neurons 5. Motor AP arrives at skeletal muscle, response occurs

  43. Figure 3.7

  44. Sensory-Motor Integration:Sensory Input • Can be integrated at many points in CNS • Complexity of integration increases with ascent through CNS • Spinal cord • Lower brain stem • Cerebellum • Thalamus • Cerebral cortex (primary sensory cortex)

  45. Figure 3.8

  46. Sensory-Motor Integration:Motor Control • Sensory input can evoke motor response regardless of point of integration • Spinal cord • Lower region of brain • Motor area of cerebral cortex • As level of control moves from spinal cord to cerebral cortex, movement complexity 

  47. Sensory-Motor Integration:Reflex Activity • Motor reflex • Instant, preprogrammed response to a given stimulus • Response to stimulus identical each time • Occurs before conscious awareness • Impulse integrated at lower, simple levels

  48. Sensory-Motor Integration:Muscle Spindles • Specialized intrafusal muscle fibers • Different from normal (extrafusal) muscle fibers • Innervated by g-motor neurons • Sensory receptors for muscle fiber stretch • When stretched, muscle spindle sensory neuron • Synapses in spinal cord with an a-motor neuron • Triggers reflex muscle contraction • Prevents further (damaging) stretch • Stretch reflex

  49. Sensory-Motor Integration:Golgi Tendon Organs • Sensory receptor embedded in tendon • Associated with 5 to 25 muscle fibers • Sensitive to tension in tendon (strain gauge) • When stimulated by excessive tension, Golgi tendon organs • Inhibit agonists, excite antagonists • Prevent excessive tension in muscle/tendon • Reduce potential for injury

  50. Figure 3.9

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