Neural Control of Exercising Muscle Chapter 3 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

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 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 Cerebral cortex 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 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

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