Neural Control
of Exercising Muscle
CHAPTER 3 Overview
• Overview of nervous system
• Structure and function of nervous system
• Central nervous system
• Peripheral nervous system
• Sensory-motor integration
• Motor response
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
Figure 3.1
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
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
Figure 3.2
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
Resting Membrane Potential
• Difference in electrical charges between
outside and inside of cell
• −70 mV
• Caused by uneven separation of charged
ions
• Polarized
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
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
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
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
Action Potentials
• Rapid, substantial depolarization
• Last ~1 ms
• Begin as GPs
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
Figure 3.3
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)
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
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
Figure 3.4
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
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
Figure 3.5
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
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
Central Nervous System
• Brain
–
–
–
–
Cerebrum
Diencephalon
Cerebellum
Brain stem
• Spinal cord
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)
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
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)
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
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
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
Figure 3.6
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
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
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
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
Motor Division
• Transmits information from brain to
periphery
• Two divisions
– Autonomic: regulates visceral activity
– Somatic: stimulates skeletal muscle activity
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
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
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
Table 3.1
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
Figure 3.7
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)
Figure 3.8
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 
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
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
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
Figure 3.9
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