1. 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. Figure 3.2

8. 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

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

10. 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

11. 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

12. 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

13. 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

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

15. 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

16. Figure 3.3

17. 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)

18. 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

19. 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

20. Figure 3.4

21. 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

22. 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

23. Figure 3.5

24. 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

25. 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

26. Central Nervous System Brain –Cerebrum –Diencephalon –Cerebellum –Brain stem Spinal cord

27. 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)

28. 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

29. 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)

30. 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

31. 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

32. 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  -endorphin release with exercise

33. Figure 3.6

34. 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

35. 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

36. 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

37. 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

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

39. 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

40. 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

41. 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

42. Table 3.1

43. 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  -motor neurons 5. Motor AP arrives at skeletal muscle, response occurs

44. Figure 3.7

45. 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)

46. Figure 3.8

47. 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 

48. 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

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

50. 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

51. Figure 3.9

52. Motor Response  -Motor neuron carries AP to muscle AP spreads to muscle fibers of motor unit –Fine motor control: fewer fibers per motor unit –Gross motor control: more fibers per motor unit Homogeneity of motor units –Fiber types not mixed within a given motor unit –Either type I fibers or type II fibers –Motor neuron may actually determine fiber type