Neural Control of Exercising Muscle

CHAPTER 3 CHAPTER 3 OverviewOverview

• Overview of nervous system

• Structure and function of nervous system

• Central nervous system

• Peripheral nervous system

• Sensory-motor integration

• Motor response

Major Divisions Major Divisions of the Nervous Systemof 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 3.1Figure 3.1

Nervous System Structure Nervous System Structure and Function 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 Nervous System Structure and Function 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 3.2Figure 3.2

Nervous System Structure Nervous System Structure and Function: Nerve Impulse 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 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 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 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 PotentialsGraded 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 PotentialsGraded 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 PotentialsAction Potentials

• Rapid, substantial depolarization

• Last ~1 ms

• Begin as GPs

Action Potentials:Action Potentials:Generating an APGenerating 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 3.3Figure 3.3

Action Potentials:Action Potentials:Refractory PeriodsRefractory 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:Action Potentials:Propagation Down AxonPropagation 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:Synapse:Transmitting APsTransmitting 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 3.4Figure 3.4

Synapse:Synapse:Transmitting APsTransmitting 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:Neuromuscular Junction:A Specialized SynapseA 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 3.5Figure 3.5

NeurotransmittersNeurotransmitters

• 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 ResponsePostsynaptic 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 SystemCentral Nervous System

• Brain– Cerebrum– Diencephalon– Cerebellum– Brain stem

• Spinal cord

Brain: CerebrumBrain: 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 LobesCerebrum: 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 Cerebrum: Regions of Interest for Exercise PhysiologyExercise 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: DiencephalonBrain: 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: CerebellumBrain: 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 StemBrain: 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

Figure 3.6Figure 3.6

Spinal CordSpinal 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 SystemPeripheral 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 DivisionSensory 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 Sensory Division: Special Families of Sensory Receptors 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 DivisionMotor Division

• Transmits information from brain to periphery

• Two divisions– Autonomic: regulates visceral activity– Somatic: stimulates skeletal muscle activity

Motor Division: Motor Division: Autonomic Nervous SystemAutonomic 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:Autonomic Nervous System:SympatheticSympathetic

• 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:Autonomic Nervous System:ParasympatheticParasympathetic

• 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 3.1Table 3.1

Sensory-Motor IntegrationSensory-Motor Integration

• Process of communication and interaction between sensory and motor systems

• Five sequential steps1. 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

Figure 3.7Figure 3.7

Sensory-Motor Integration:Sensory-Motor Integration:Sensory InputSensory 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 3.8Figure 3.8

Sensory-Motor Integration:Sensory-Motor Integration:Motor ControlMotor 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:Sensory-Motor Integration:Reflex ActivityReflex 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:Sensory-Motor Integration:Muscle SpindlesMuscle 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

Sensory-Motor Integration:Sensory-Motor Integration:Golgi Tendon OrgansGolgi 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 3.9Figure 3.9

Motor ResponseMotor 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