Chapter 11 Physiology of the Muscular System

Introduction Muscular system is responsible for moving the framework of the body In addition to movement, muscle tissue performs various other functions

General Functions Movement of the body as a whole or of its parts Heat production Posture

Function of Skeletal Muscle Tissue Characteristics of skeletal muscle cells Excitability (irritability)—ability to be stimulated Contractility—ability to contract, or shorten, and produce body movement Extensibility—ability to extend, or stretch, allowing muscles to return to their resting length

Overview of the muscle cell Muscle cells are called fibers because of their threadlike shape Sarcolemma—plasma membrane of muscle fibers Sarcoplasmic reticulum (SR) Network of tubules and sacs found within muscle fibers Membrane of the sarcoplasmic reticulum continually pumps calcium ions from the sarcoplasm and stores the ions within its sacs for later release

Overview of the muscle cell Muscle fibers contain many mitochondria and several nuclei Myofibrils—numerous fine fibers packed close together in sarcoplasm Sarcomere Segment of myofibril between two successive Z lines Each myofibril consists of many sarcomeres Contractile unit of muscle fibers

Overview of the muscle cell Striated muscle Dark stripes called A bands; light H zone runs across midsection of each dark A band Light stripes called I bands; dark Z line extends across center of each light I band T tubules Transverse tubules extend across sarcoplasm at right angles to long axis of muscle fiber Formed by inward extensions of sarcolemma Membrane has ion pumps that continually transport Ca++ ions inward from sarcoplasm Allow electrical impulses traveling along sarcolemma to move deeper into cell

Overview of the muscle cell Triad Triplet of tubules; a T tubule sandwiched between two sacs of sarcoplasmic reticulum; allows an electrical impulse traveling along a T tubule to stimulate the membranes of adjacent sacs of the sarcoplasmic reticulum

Myofilaments Each myofibril contains thousands of thick and thin myofilaments Four different kinds of protein molecules make up myofilaments Myosin Makes up almost all the thick filament Myosin “heads” are chemically attracted to actin molecules Myosin “heads” are known as cross bridges when attached to actin Actin—globular protein that forms two fibrous strands that twist around each other to form bulk of thin filament Tropomyosin—protein that blocks the active sites on actin molecules Troponin—protein that holds tropomyosin molecules in place

Myofilaments (cont.) Thin filaments attach to both Z lines (Z disks) of a sarcomere and extend partway toward the center Thick myosin filaments do not attach to the Z lines

The mechanism of contraction Excitation and contraction A skeletal muscle fiber remains at rest until stimulated by a motor neuron Neuromuscular junction—motor neurons connect to sarcolemma at motor endplate (Figure 11-7) Neuromuscular junction is a synapse where neurotransmitter molecules transmit signals

Excitation and contraction Acetylcholine—neurotransmitter released into synaptic cleft that diffuses across gap, stimulates receptors, and initiates impulse in sarcolemma Nerve impulse travels over sarcolemma and inward along T tubules, which triggers release of calcium ions Calcium binds to troponin, causing tropomyosin to shift and expose active sites on actin

Excitation and contraction Sliding filament model When active sites on actin are exposed, myosin heads bind to them Myosin heads bend, pulling the thin filaments past them Each head releases, binds to next active site, and pulls again Entire myofibril shortens

The mechanism of contraction Relaxation Immediately after Ca++ ions are released, sarcoplasmic reticulum begins actively pumping them back into sacs (Figure 11-3) Ca++ ions are removed from troponin molecules, shutting down contraction

Energy sources for muscle contraction Hydrolysis of ATP yields energy required for muscular contraction Adenosine triphosphate (ATP) binds to myosin head and then transfers its energy to myosin head to perform work of pulling thin filament during contraction Muscle fibers continually resynthesize ATP from breakdown of creatine phosphate (CP)

Energy sources for muscle contraction Catabolism by muscle fibers requires glucose and oxygen At rest, excess O2 in the sarcoplasm is bound to myoglobin Red fibers—muscle fibers with high levels of myoglobin White fibers—muscle fibers with little myoglobin Aerobic respiration occurs when adequate O2 is available

Energy sources for muscle contraction Anaerobic respiration occurs when low levels of O2 are available and results in formation of lactic acid Glucose and oxygen supplied to muscle fibers by blood capillaries Skeletal muscle contraction produces waste heat that can be used to help maintain set point body temperature

Twitch contraction A quick jerk of a muscle that is produced as a result of a single, brief threshold stimulus (generally occurs only in experimental situations) The twitch contraction has three phases Latent phase—nerve impulse travels to the sarcoplasmic reticulum to trigger release of Ca++ Contraction phase—Ca++ binds to troponin and sliding of filaments occurs Relaxation phase—sliding of filaments ceases

Treppe—the staircase phenomenon Gradual, steplike increase in the strength of contractions seen in a series of twitch contractions that occur 1 second apart Eventually, the muscle responds with less forceful contractions, and relaxation phase becomes shorter If relaxation phase disappears completely, a contracture occurs

Tetanus—smooth, sustained contractions Multiple wave summation—multiple twitch waves are added together to sustain muscle tension for a longer time Incomplete tetanus—very short periods of relaxation occur between peaks of tension Complete tetanus—the stimulation is such that twitch waves fuse into a single, sustained peak

Muscle tone Tonic contraction—continual, partial contraction of a muscle At any one time, a small number of muscle fibers within a muscle contract, producing a tightness or muscle tone Muscles with less tone than normal are flaccid Muscles with more tone than normal are spastic Muscle tone is maintained by negative feedback mechanisms

Graded strength principle Skeletal muscles contract with varying degrees of strength at different times Factors that contribute to the phenomenon of graded strength Metabolic condition of individual fibers Number of muscle fibers contracting simultaneously; the greater the number of fibers contracting, the stronger the contraction Number of motor units recruited

Isotonic and isometric contractions Isotonic contraction Contraction in which the tone or tension within a muscle remains the same as the length of the muscle changes Concentric—muscle shortens as it contracts Eccentric—muscle lengthens while contracting Isotonic—literally means “same tension” All of the energy of contraction is used to pull on thin myofilaments and thereby change the length of a fiber’s sarcomeres

Isotonic and isometric contractions Contraction in which muscle length remains the same while the muscle tension increases Isometric—literally means “same length” Most body movements occur as a result of both types of contractions

Cardiac Muscle Tissue Cardiac muscle Found only in the heart, forming the bulk of the wall of each chamber Also known as striated involuntary muscle Contracts rhythmically and continuously to provide the pumping action needed to maintain a constant blood flow

Cardiac Muscle Tissue Cardiac muscle resembles skeletal muscle but has specialized features related to its role in continuously pumping blood Each cardiac muscle contains parallel myofibrils Cardiac muscle fibers form strong, electrically coupled junctions (intercalated disks) with other fibers; individual cells also exhibit branching Syncytium—continuous, electrically coupled mass Cardiac muscle fibers form a continuous, contractile band around the heart chambers that conducts a single impulse across a virtually continuous sarcolemma

Cardiac Muscle Cardiac muscle T tubules are larger and form diads with a rather sparse sarcoplasmic reticulum Cardiac muscle sustains each impulse longer than in skeletal muscle; therefore, impulses cannot come rapidly enough to produce tetanus Cardiac muscle does not run low on ATP and does not experience fatigue Cardiac muscle is self-stimulating

Smooth Muscle Tissue Smooth muscle Smooth muscle is composed of small, tapered cells with single nuclei No T tubules are present, and only a loosely organized sarcoplasmic reticulum is present Ca++ comes from outside the cell and binds to calmodulin instead of troponin to trigger a contraction No striations, because thick and thin myofilaments are arranged differently than in skeletal or cardiac muscle fibers; myofilaments are not organized into sarcomeres

Smooth Muscle Tissue Two types of smooth muscle tissue Single-unit (visceral) Gap junctions join smooth muscle fibers into large, continuous sheets Most common type; forms a muscular layer in the walls of hollow structures such as the digestive, urinary, and reproductive tracts Exhibits autorhythmicity, producing peristalsis Multiunit Does not act as a single unit but is composed of many independent cell units Each fiber responds only to nervous input