The Cyprus International Institute for the Environment and Public Health In collaboration with the Harvard School of Public Health Somatic Nervous System

Lecture 5 • Consists of axons of motor neurons • Originate in spinal cord or brain stem and end on skeletal muscle • Motor neuron releases neurotransmitter, ACh Neuromuscular Physiology • Stimulates muscle contraction (240-249, 253-267,270-286,288-297) • Motor neurons = final common pathway • Various regions of CNS exert control over skeletal muscle activity Excluded: muscle length, tension, contraction and velocity, • Spinal cord, motor regions of cortex, basal phosphorylation of myosin nuclei, cerebellum, and brain stem • Pathologies • Polio virus destroys the cell bodies of motor neurons • Amyotrophic Lateral Sclerosis (ALS) Constantinos Pitris, MD, PhD • A.k.a. Lou Gehrig’s Disease • Most common motor neuron disease Assistant Professor, University of Cyprus • Gradual degeneration of motor neurons [email protected] • Unknown cause http://www.eng.ucy.ac.cy/cpitris/courses/CIIPhys/ 2

Somatic Nervous System Muscle

• Comprises largest group of tissues in body • Skeletal (30-40% BW), smooth and cardiac (10% BW) Striated Unstriated • Controlled muscle contraction allows muscle muscle • Purposeful movement of the whole body or parts of the body • Manipulation of external objects • Propulsion of contents through various hollow internal organs • Emptying of contents of certain organs to external environment Skeletal Cardiac Smooth • Three types of muscle muscle muscle muscle • Skeletal muscle • Make up muscular system • Cardiac muscle • Found only in the heart • Smooth muscle • Appears throughout the body systems as Voluntary Involuntary components of hollow organs and tubes muscle muscle • Classified in two different ways • Striated or unstriated • Voluntary or involuntary 3 4 Structure of Skeletal Muscle Neuromuscular Junction

• Muscle consists a number of • Axon terminal of motor Tendon muscle fibers lying parallel to Muscle neuron forms

one another and held together Connective neuromuscular junction tissue by connective tissue Muscle fiber with a single muscle cell (a single • Single skeletal muscle cell is muscle cell) • Terminal button (of neuron) known as a muscle fiber • Motor End Plate (of muscle • Multinucleated cell) • Large, elongated, and cylindrically shaped • Fibers usually extend entire length of muscle Muscle fiber Dark A band Light I band

Myofibril 5 6

Neuromuscular Junction Neuromuscular Junction

• Signals are passed between • Acetylcholinesterase nerve terminal and muscle fiber by means of neurotransmitter Action potential • On the chemically-gated cation ACh propagation Myelin sheath channels of the end plate in motor neuron • AP in motor neuron reaches Voltage-gated • Inactivates ACh (as ACh terminal calcium channel Axon 2+ terminal • Voltage-gated Ca channels open Vesicle of acetylcholine molecules attaches and • ACh is released by exocytosis detaches from the receptors) Terminal button • ACh diffuses across the space and • Ends end-plate potential and the binds to receptor sites on motor end Action potential propagation action potential plate of muscle cell membrane in muscle fiber • Binding triggers opening of cation • Ensures prompt termination of channels in motor end plate contraction •Na+ movements (larger than K+ movements) depolarize motor end Voltage-gated plate, producing end-plate potential Na+ channel • Local current flow between depolarized end plate and adjacent Acetycholinesterase Motor end plate muscle cell membrane brings Acetylcholine adjacent areas to threshold receptor site Contractile elements • Action potential is initiated and Chemically gated within muscle fiber propagated throughout muscle fiber cation channel 7 8 Neuromuscular Junction Structure of Skeletal Muscle

Z line A band I band • Neuromuscular junction is vulnerable to chemical agents and diseases • Myofibrils Portion • Black widow spider venom • Contractile elements of muscle fiber of myofibril • Causes explosive release of ACh • Viewed microscopically myofibril • Prolonged depolarization keeps Na+ channels at inactive state displays alternating dark (the A M line H zone • Respiratory failure from diaphragm paralysis bands) and light bands (the I bands) Thick filament Sarcomere • Botulism toxin giving appearance of striations Thin filament A band I band • From food infected with Clostridium Botulinum Æ Botulism • Blocks release of ACh • Regular arrangement of thick and • Respiratory failure from inability to contract diaphragm thin filaments • Thick filaments – myosin (protein) • Curare Cross M line H zone Z line • Poisonous arrowheads • Thin filaments – actin (protein) bridges • Binds at ACh receptor sites but has no activity and is not degrated • Organophosphates • Sarcomere • Pesticide and military nerve gases • Functional unit of skeletal muscle • Prevent inactivation of Ach by inhibiting AChE Myosin Actin • Found between two Z lines • Effect similar to Black widow spider venom Thick filament Thin filament • Z lines connect thin filaments of two • Myasthenia gravis inactivates ACh receptor sites Cross bridge adjoining sarcomeres • Autoimmune condition (Antibodies against ACh receptors) • ACh is degraded before it can act. • Antidote is neostigmine (inhibits AChE and prolongs ACh action) Thick filament 9 10 Thin filament

Structure of Skeletal Muscle Myosin

Z line A band I band • Titin • Component of thick filament Myosin molecule Portion • Several hundred of them • Giant, highly elastic protein of myofibril Actin binding site • Protein molecule consisting of Myosin ATPase site • Largest protein in body M line H zone two identical subunits shaped Heads • Extends in both directions from Thick filament Sarcomere somewhat like a golf club Tail along length of thick filament to Thin filament A band I band • Tail ends are intertwined around Z lines at opposite ends of each other sarcomere • Globular heads project out at one end • Two important roles: Cross M line H zone Z line • Tails oriented toward center of bridges Thick filament • Helps stabilize position of thick filament and globular heads filaments in relation to thin protrude outward at regular Cross bridge filaments intervals Myosin molecule • Greatly augments muscle’s Myosin Actin • Heads form cross bridges between elasticity by acting like a spring Thick filament Thin filament thick and thin filaments • Cross bridge has two important sites Cross bridge critical to contractile process • An actin-binding site • A myosin ATPase (ATP-splitting) site Thick filament 11 Thin filament 12 Actin Tropomyosin and Troponin

• Primary structural component of • Often called regulatory thin filaments proteins • Spherical in shape Binding site for Binding site for attachment • Tropomyosin attachment • Thin filament also has two other with myosin with myosin proteins Actin molecules cross bridge • Thread-like molecules that lie Actin molecules cross bridge • Tropomyosin and troponin end to end alongside groove of • Each actin molecule has special actin spiral

binding site for attachment with Actin helix • In this position, covers actin Actin helix myosin cross bridge sites blocking interaction that + + • Binding results in contraction of leads to muscle contraction muscle fiber • Troponin • Actin and myosin are often called contractile proteins. Neither Tropomyosin Troponin • Made of three polypeptide units Tropomyosin Troponin actually contracts. • One binds to tropomyosin • Actin and myosin are not unique • One binds to actin to muscle cells, but are more • One can bind with Ca2+ abundant and more highly organized in muscle cells. Thin filament Thin filament 13 14

Tropomyosin and Troponin Sliding Filament Mechanism

• Troponin • Thin filaments on each side of • When not bound to Ca2+, sarcomere slide inward troponin stabilizes tropomyosin Thin filament • Over stationary thick filaments in blocking position over actin’s • Toward center of A band Tropomyosin cross-bridge binding sites Cross-bridge binding sites • They pull Z lines closer together Actin • When Ca2+ binds to troponin, • Sarcomere shortens Relaxed tropomyosin moves away from Cross-bridge binding site • All sarcomeres throughout muscle Tropomyosin blocking position Actin binding site fiber’s length shorten simultaneously Myosin cross bridge Excited Sarcomere • Contraction is accomplished by thin A band • With tropomyosin out of way, Z line H zone I band Z line actin and myosin bind, interact Troponin filaments from opposite sides of each sarcomere sliding closer Relaxed at cross-bridges together between thick filaments • Cross-bridge interaction 2+ • Ca plays a key role A band H zone I band same between actin and myosin 2+ shorter shorter • Increase in Ca starts filament width brings about muscle contraction sliding by means of the sliding filament • Decrease in Ca2+ turns off sliding Contracted mechanism process

Sarcomere shorter Thick filament Thin filament 15 16 Power Stroke Power Stroke

• Activated cross bridge bends • Contraction toward center of thick filament, continues if ATP is 2+ “rowing” in thin filament to which available and Ca it is attached level in sarcoplasm is high • Sarcoplasmic reticulum releases Ca2+ into sarcoplasm • Myosin head remains Energized Resting • Myosin heads bind to actin attached until ATP • Myosin heads swivel toward center binds Æ rigor mortis of sarcomere (power stroke) •Ca2+ released • ADP released • ATP quickly depleted Detachment Binding • ATP binds to myosin head and • Onset in a few hours detaches it from actin • Can last 1-2 days • Hydrolysis of ATP transfers energy to myosin head and reorients it • Ca2+ stores are in the Bending (power stroke) • Energy expended in the form of sarcoplasmic ATP reticulum

Rigor complex 17 18

Sarcoplasmic Reticulum Sarcoplasmic Reticulum

• Sarcoplasmic Reticulum (SR) 2+ • Release of Ca Lateral sac • Modified endoplasmic reticulum of sarcoplasmic • Foot proteins reticulum • Consists of fine network of Surface membrane of muscle fiber T tubule interconnected compartments that • Cover the lateral sacs of the Foot protein surround each myofibril Dihydropyridine Segments of sarcoplasmic reticulum • Not continuous but encircles receptor sarcoplasmic • Span the gap between the SR myofibril throughout its length reticulum • Segments are wrapped around each and T tubules as well as SR A band and each I band Myofibrils membrane • Ends of segments expand to form • Half interlock (“zipped”) with saclike regions – lateral sacs Lateral (terminal cisternae) sacs Dihydropyridine (DHP) • T tubules Transverse receptors on T tubules • Run perpendicularly from surface of (T) tubule • Dihydropyridine (DHP) muscle cell membrane into central receptors portions of the muscle fiber • Voltage sensors • Since membrane is continuous with Cytoplasm I band A band I band surface membrane – action potential • Depolarization from AP opens on surface membrane also spreads Ca2+ channels of attached foot Ca2+ down into T-tubule proteins • Spread of action potential down a T 2+ tubule triggers release of Ca from •Ca2+ release opens the SR into cytosol remaining foot proteins 19 20 Excitation-Contraction Coupling Sarcoplasmic Reticulum Excitation-Contraction Coupling

• Relaxation - Reuptake of Ca2+ • AChE degrates ACh at the endplate • Electrical activity stops • On-going activite of Ca2+- ATPase pump returns the Ca2+ to the SR • Trponin-tropomyosin complex returns to blocking position • No interaction between actin and myosin • Muscle fiber passively relaxes

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Excitation-Contraction Coupling Skeletal Muscle Mechanics

• Contractile activity Latent • Muscle consists of groups of muscle period fibers bundled together and attached to • AP is very short (1-2 msec) Contraction Relaxation bones • Contraction does not start until time time • Connective tissue covering muscle divides muscle internally into bundles enough Ca2+ is released Muscle twitch • Connective tissue extends beyond ends of • Latent period muscle to form tendons • Contraction process requires • Tendons attach muscle to bone Contractile • Muscle Contraction response time to complete • Contractions of whole muscle can be of • Contraction time (~50 msec) varying strength • Twitch – Contraction of single muscle fiber • Relaxation also requires time to from single AP complete • Brief, weak contraction • Produced from single action potential • Relaxation time (~50 msec) Action potential • Too short and too weak to be useful • Normally does not take place in body • Two primary factors which can be adjusted to accomplish gradation of whole-muscle tension • Number of muscle fibers contracting within a Stimulation muscle • Tension developed by each contracting fiber 23 24 Motor Unit Recruitment Factors Influencing Tension

• Motor unit • Factors influencing • One motor neuron and the muscle fibers it innervates extent to which tension • Number of muscle fibers varies can be developed among different motor units • Varying from contraction to • Number of muscle fibers per motor contraction unit and number of motor units per muscle vary widely • Frequency of stimulation • Muscles that produce precise, delicate • Length of fiber at onset of movements contain fewer fibers per contraction motor unit • Muscles performing powerful, coarsely Spinal cord • Permanent or long term controlled movement have larger number adaptation of fibers per motor unit = Motor unit 1 • Extent of fatigue • Asynchronous recruitment of motor units helps delay or prevent fatigue = Motor unit 2 • Thickness of fiber • Muscle fibers which fatigue easily are = Motor unit 3 recruited later • Can engage in endurance activities for a long time but can only deliver full force for brief periods of time

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Frequency of Stimulation Lever Systems

• Twitch summation • Bones, muscles, and joints • Individual twitches are interact to form lever systems Tetanus summed • Bones function as levers • AP much sorter in time than Stimulation contraction Æ Multiple APs Contractile activity ceases or • Joints function as fulcrums Twitch fatigue can be delivered Single summation begins Fulcrum twitch • Skeletal muscles provide force • Results from sustained to move bones elevation of cytosolic calcium • Muscles usually exert more • Tetanus force than actual weight of • Occurs if muscle fiber is stimulated so rapidly that it load! Action potentials does not have a chance to Muscle fiber Muscle fiber is Muscle fiber is stimulated • Advantages: higher speed, relax between stimuli restimulated after it restimulated before so rapidly that it does not has completely it has completely more distance relaxed have an opportunity to relax Biceps • Contraction is usually three to relaxed at all between stimuli Insertion of biceps four times stronger than a

single twitch Fulcrum for lever • Do not confuse with the disease of the same name!

27 28 Skeletal Muscle Metabolism Skeletal Muscle Metabolism

• Contraction-Relaxation Steps Requiring • Transfer of high-energy ATP • Splitting of ATP by myosin ATPase phosphate from creatine provides energy for power stroke of cross phosphate to ADP bridge • Binding of fresh molecule of ATP to myosin • First energy storehouse tapped lets bridge detach from actin filament at end at onset of contractile activity of power stroke so cycle can be repeated • Reversible reaction • Active transport of Ca2+ back into sarcoplasmic reticulum during relaxation • Stores Creatine Phosphate depends on energy derived from when ATP ↑ breakdown of ATP • Contributes ATP when ATP ↓ • Energy Sources for Contraction • Transfer of high-energy phosphate from • Short duration or bursts of creatine phosphate to ADP exercise • First energy storehouse tapped at onset of contractile activity • ~160g or 12.5 kcal • Oxidative phosphorylation (citric acid cycle • E.g. 100 m running and electron transport system) • Takes place within muscle mitochondria if sufficient O2 is present • Glycolysis • Supports anaerobic or high-intensity exercise 29 30

Skeletal Muscle Metabolism Skeletal Muscle Metabolism

• Oxidative phosphorylation (citric acid cycle and electron transport system) • Takes place within muscle mitochondria • Anaerobic or high-intensity • Moderate exercise exercise • Sufficient O2 must be present • Aerobic or endurance-type exercise • Limit to the amount of O2 that can • Deeper, faster breathing be delivered • ↑ Heart rate and contraction • Respiratory and cardiac maxima • Dilation of blood vessels • Myoglobin • Muscle contraction constricts the • Similar to hemoglobin blood vessels • Increase the transfer and store O2 in muscle cells Anaerobic conditions • Uses glucose or fatty acids • Glycolysis No O2 • Glucose derived from muscle glycogen (chains of glucose) stores Glycolysis available • Limited (~ 150g or 600 kcal) • Supports anaerobic or high-intensity Glucose Pyruvic acid Lactic acid • Athletes can store more (2000 kcal for marathon runners) exercise • Glucose derived from liver glycogen stores • Limited (~80-200g or 320-800 kcal) • Less efficient but much faster than 2 ATP • Fatty acids derived from lipolisis oxidative phosphorylation • Plenty of these! (~15kg or 135.000 kcal) • Quickly depletes glycogen supplies Mitochondrial outer Aerobic conditions } and inner membranes • Lactic acid is produced Glycolysis Glucose • Soreness that occurs during the time O2 available or Pyruvic acid 34 ATP + CO + H O Fatty acids 2 2 (not after) intense exercise Citric acid Electron transport 2 ATP cycle chain • Energy depletion and ↓ pH contribute to muscle fatique Cytosol 31 Mitochondrion 32 Skeletal Muscle Metabolism Skeletal Muscle Metabolism

Sport Creatine & Glycolysis Oxidative Glycolisis & Oxidative Golf swing 95 5 Sprints 90 10 Volleyball 80 5 15 Gymnastics 80 15 5 Tennis 70 20 10 Basketball 60 20 20 Soccer 50 20 30 Skiing 33 33 33 Rowing 20 30 50 Distance running 10 20 70 Swimming 1.5km 10 20 70

Table adapted from Fox E. L. et al, The Physiological Basis for Exercise and Sport, 1993

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Fatigue Major Types of Muscle Fibers

• Contractile activity can not be sustained indefinitely • Classified based on differences in Æ Fatigue • Muscle Fatigue speed of contraction and ATP • Occurs when exercising muscle can no longer respond to hydrolysis and synthesis stimulation with same degree of contractile activity • Defense mechanism that protects muscle from reaching • Three major types point at which it can no longer produce ATP • Slow-oxidative (type I) fibers • Underlying causes of muscle fatigue are unclear. Implicated • ADP increase (interferes with cross-bridges and Ca2+ uptake in • Low intensity contractions for long the SR) periods of time (e.g. back) • Lactic acid accumulation (may interfere with key enzymes in energy-producing pathways) • Fast-oxidative (type IIa) fibers • Accumulation of extracellular K+ (decrease in membrane potential) • High intensity for medium periods • Depletion of glycogen (e.g. limbs) • Central Fatigue • Occurs when CNS no longer adequately activates motor • Fast-glycolytic (type IIx) fibers neurons supplying working muscles • Rapid forceful movements (e.g. • Often psychologically based arms) • Discomfort, boredom or tiredness • Mechanisms involved in central fatigue are poorly • Fast fibers can contract ~ 10 x understood faster • Recovery

• Excess postexercise O2 consumption (EPOC) helps • Oxidative fibers contain more • Restore Creatine Phosphate (few minutes) •Replenish ATP mitochondria and myoglobin and • Convert Lactic acid to pyruvate for oxidative ATP generation have a richer blood supply Æ red • Cover increased general O2 demand because of higher temperature meat • Nutrient replenishment (1-2 days after a marathon) 35 36 Muscle Adaptation & Repair Control of Motor Movement

• Muscle has a high degree of plasticity Corticospinal tracts • Input to motor-neurons Primary • Improvement of oxidative capacity • Input from afferent neurons motor • From regular aerobic exercise cortex • Capillaries and mitochondria increase • Input from primary motor cortex • Hypertrophy • From anaerobic high intensity exercise • Muscle fiber diameter increases (more actin and myosin) • Input from afferent neurons • Mainly fast-glycolytic fibers • Usually through intervening • Testosterone and other steroids increase Interneurons the synthesis of actin and myosin interneurons • Steroid abuse • Responsible for spinal reflexes • Fast muscle fibers are interconvertible (e.g. withdrawal) Afferent pathway •Oxidative ↔ glycolytic • Input from the primary motor Efferent •But NOTfast ↔ slow pathway • Muscle atrophy cortex Muscles • Disuse atrophy (e.g. space exploration) • Corticospinal motor system • Denervation atrophy (e.g. paralysis) beginning from the motor cortex • Muscle has limited repair cababilities • Responsible for fine voluntary • Satellite cells can create a few myoblasts Receptor which fuse and create a few muscle fibers movement Response Stimulus 37 38

Control of Motor Movement Control of Motor Movement

• Afferent sensory neuron • Three levels of control and provide continues feedback coordination • Three levels of control and • The Segmental Level coordination • Spinal cord circuits including central pattern generators • The Segmental Level (CPGs) • The Projection Level • The Precommand Level

39 40 Control of Motor Movement Control of Motor Movement

• Three levels of control and • Three levels of control and coordination coordination • The Projection Level • The Precommand Level • Premotor and Primary motor • Cerebellum cortex • Coordination of movement • Plan and execute voluntary • Maintenance of balance movements • Control of eye movements •Brain stem • Basal Nuclei • Multineural reflexes • Inhibit muscle tone • Regulation of involuntary • Select and maintain control of body posture purposeful motor activity while suppressing unwanted patterns of movement • Monitor and coordinate slow and sustained contractions

41 42

Muscle Receptors Muscle Receptors

• Receptors are necessary to • Muscle Spindles • Consist of collections of plan and control complicated Capsule specialized muscle fibers movement and balance known as intrafusal fibers Alpha motor • Lie within spindle-shaped neuron axon Intrafusal (spindle) • The brain receives information muscle fibers connective tissue capsules from all muscles and joints in parallel to extrafusal fibers Gamma motor • Have contractile ends and a neuron axon Contractile end portions the body Æ proprioception of intrafusal fiber non-contractile central portion

• Two types of muscle • Each spindle has its own Noncontractile private nerve supply Secondary (flower-spray) central portion receptors endings of afferent of intrafusal • Plays key role in stretch fibers fiber • Muscle spindles reflex

• Monitor muscle length and • Efferent Primary (annulospiral) tension • Gamma motor neurons* Extrafusal (“ordinary”) endings of afferent fibers • Afferent muscle fibers • Golgi tendon organs • Primary (annulospiral) • Monitor whole muscle tension endings (in the central portion) • Secondary (flower-spray) endings (at the end segments) * Efferent neurons to extrafusal fibers are called alpha motor neurons 43 44 Muscle Receptors Muscle Receptors

• Stretch Reflex • Coactivation of alpha • Primary purpose • Resist tendency for passive stretch of extensor muscles by gravitational forces and gamma motor Extrafusal skeletal when person is standing upright neurons muscle fiber • Classic example is patellar tendon, or knee-jerk reflex • Spindle coactivation Extensor muscle of knee Muscle (quadriceps femoris) spindle during muscle contraction Spinal γ Intrafusal cord muscle • Spindle contracted to spindle fiber reduce length α • With no coactivation Patellar tendon • Slackened spindle Alpha motor • Not sensitive to neuron stretch • Adjustment to keep muscle spindles sensitive to stretch

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Muscle Receptors Smooth Muscle

• Found in walls of hollow organs and • Gogli Tendon Organs tubes • Provide necessary feedback for • No striations overall muscle tension • Filaments do not form myofibrils • Not arranged in sarcomere pattern found • Integrates all factors which in skeletal muscle influence tension • Spindle-shaped cells with single • Specialized nerve fibers nucleus • Cells usually arranged in sheets embedded in the tendons within muscle • Stretch of tendons exerts force • Cell has three types of filaments on nerve endings • Thick myosin filaments Relaxed • Longer than those in skeletal muscle • Increase firing rate • Thin actin filaments • Contain tropomyosin but lack troponin • Part of this information reaches • Filaments of intermediate size conscious awareness • Do not directly participate in contraction • Form part of cytoskeletal framework that • We are aware of tension (but supports cell shape not of length) of muscles • No sacromeres • Have dense bodies containing same protein found in Z lines Contracted 47 48 Smooth Muscle Smooth Muscle

• Two major types • Single-unit Smooth Muscle

• Multiunit smooth muscle • Most smooth muscle Pacemaker smooth muscle cell Spontaneous action potential • Single-unit smooth muscle • Also called visceral smooth muscle induced by pacemaker potential • Multiunit Smooth Muscle • Self-excitable (does not require nervous stimulation for contraction) Action potential spread • Neurogenic (nerve initiated) to nonpacemaker cell • Fibers become excited and contract • Consists of discrete units that as single unit function independently of one • Cells electrically linked by gap another junctions • Units must be separately • Can also be described as a stimulated by nerves to contract functional syncytium Nonpacemaker • Found • Contraction is slow and energy- Gap junction smooth muscle cell • In walls of large blood vessels efficient • In large airways to lungs • Slow cross-bridge cycling • In muscle of eye that adjusts • Cross-bridges “latch-on” the thin filaments Æ muscle maintains lens for near or far vision tension • In iris of eye • Well suited for forming walls of • At base of hair follicles distensible, hollow organs

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Smooth Muscle Smooth Muscle

• Single-unit Smooth Muscle • Smooth muscle activity Excitation • Gradation • Myogenic activity (contraction • All muscle fibers are contracting • Tension can be modified by Mitochondrion initiated by the muscle itself) 2+ varying the intracellular Ca Vesicle containing • Certain non-contractile cells can •Ca2+ from the ECF (no SR) neurotransmitter Varicosity initiate their own APs (self-excitable) Pacemaker potentials • Tone Axon of postganglionic • Small number distributed at key • Many single-unit smooth muscle autonomic locations cells maintain a low level of neuron tension (tone) even in the Varicosities • Electrical activity of two types absence of APs • Pacemaker potentials • Effect of autonomic nervous • Cells depolarize and initiate APs system spontaneously and regularly • Typically innervated by both • Slow-wave potentials branches Smooth • Cells depolarize and hyperpoplarize muscle Slow-wave potentials • Does not initiate APs but can cell rhythmically but not always initiate modify the activity (rate and APs Neurotransmitter strength of contraction) • Once initiated the action potential • Enhancement or inhibition spreads to the contractile cells • Smooth muscle cells interact with more than one neurons

51 52 Smooth Muscle Cardiac Muscle

• Smooth muscle activity • Found only in walls of the • Tension-Length relationship heart • Increased tension when • Combines features of skeletal stretched and smooth muscle • Can produce near-maximal tension at lengths 2.5 x the • Striated normal length • Cells are interconnected by gap • When stretched, smooth junctions muscle has the ability to relax • Fibers are joined in branching • These two properties are very network important for hollow organs • Can accommodate varying • Innervated by autonomic volumes while being able to nervous system produce adequate contractile force • You will learn more about cardiac muscle in Cardiac Physiology

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