1. Neural Control of Exercising Muscle
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
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 Cerebral cortex
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 Hypothalamus 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

51. Motor Response • a-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