Chapter 9 Muscular System Three Types of Muscle Tissues Skeletal Muscle usually attached to bones under conscious control striated Cardiac Muscle wall of heart not under conscious control striated Smooth Muscle walls of most viscera, blood vessels, skin not under conscious control not striated

Muscle Tissues Skeletal Muscle Smooth Muscle Cardiac Muscle

Major Skeletal Muscles

Major Skeletal Muscles

Structure of a Skeletal Muscle organ of the muscular system - skeletal muscle tissue - nervous tissue - blood - connective tissues fascia tendons aponeuroses

Connective tissue coverings Fascia (dense connective tissue) covers each skeletal muscle. - may project beyond end of muscle to form a cordlike tendon (muscle to bone) Other connective tissues form broad fibrous sheets (aponeuroses) which may attach to coverings of adjacent muscles. Epimysium - layer that closely surrounds skeletal muscle

Perimysium - layer that extends inward and separates muscle tissue into small sections Fascicles - bundles of skeletal muscle fiber in the small sections Endomysium - thin covering than surrounds each muscle fiber within a fascicle Layers of connective tissue enclose and separate all parts of skeletal muscle allowing somewhat independent movement.

Fascia is a complex network that extends throughout the body Deep fascia - surrounds each muscle Subcutaneous fascia - lies just beneath skin Subserous fascia - covers serous membranes

Connective Tissue Coverings

Skeletal Muscle Fibers

Skeletal muscle fibers Each skeletal muscle fiber is a single muscle cell. Fibers are cylindrical with rounded ends that are attached to connective tissues Each cell has many nuclei Muscle cell membrane - sarcolemma

Cytoplasm = sarcoplasm The sarcoplasm contains: nuclei - control center mitochondria - site of cellular respiration sarcoplasmic reticulum - membraneous channels that activate muscle contraction when stimulated

cisternae - enlarged portion of sarcoplasmic reticulum transverse tubules - membraneous channels that activate myofibrils (threadlike protein filaments) actin -thin filaments myosin - thick filaments

Actin and Myosin arrangement of the filaments causes striations: I bands(light) - thin actin Z lines - I bands attach to it A bands( dark) - thick myosin overlapping thin actin myosin attached to Z lines by titin (protein) H zone - central region of only thick filaments (myosin) M line - thickening area on H zone Sarcomere - extends from one Z line to the next Z line

Sarcomere I bands A bands H zone Z lines M line

Myofilaments

Neuromuscular Junction Each muscle fiber is connected to an extension of a motor neuron extends from the brain or spinal cord Motor neurons stimulate muscle fibers to contract Nerve fiber and muscle fiber meet at the neuromuscular junction The motor end plate lies on one side of a neuromuscular junction

Synaptic cleft separates the nerve fiber and muscle fiber In response to a nerve impulse, the end of a motor nerve fiber secretes a neurotransmitter which diffuses across the junction and stimulates the muscle fiber to contract.

Neuromuscular Junction

Motor units Made of one motor neuron and the muscle fibers associated with it Number of muscle fibers varies Fewer muscle fibers = finer movements Ex: eyes More muscle fibers = coarse movements Ex: large muscle in the back

Motor Unit single motor neuron all muscle fibers controlled by motor neuron

Role of myosin and actin Muscle fiber contraction results from a sliding movement of actin and myosin filaments. About 2/3 of protein in skeletal muscle is myosin Myosin consists of 2 twisted protein strands with globular cross bridges projecting outward

If Ca2+ present cross-bridges of myosin filaments form linkages with actin filaments. Actin - myosin interactions provides the basis for contraction in all 3 muscle types. Actin is about 1/4 of the protein in skeletal muscle Many actin molecules aggregate into double twisted strand

When a fiber is at rest, proteins troponin and tropomyosin molecules interfere with linkage formation. Ca ions remove the inhibition, binding to troponin and altering the position of tropomyosin exposing the binding sites on actin.

Sliding filament theory When stimulated to contract the head of a myosin cross bridge attaches to actin binding site, pulling the actin filament toward the center of the sarcomere, shortening the muscle ATP converted to ADP releasing energy

Stimulus for contraction Muscle fiber is usually stimulated by acetylcholine (ACh) released from the end of a motor nerve fiber. Ach is synthesized in cytoplasm of a motor neuron ACh combines with receptors in the sarcolemma and stimulates it

A muscle impulse travels through the transverse tubules to the sarcoplasmic reticulum (SR) which has a high concentration of Ca2+ cisternae membranes become more permeable to Ca2+ diffuse out of cisternae into the sarcoplasm causing linkages to form between actin and myosin which leads to muscle contraction

Contraction continues as long as ATP and ACh are present Cessation of nerve impulse leads to muscle relaxation ACh is decomposed by acetylcholinesterase Stimulus to sarcolemma ceases, Ca2+ goes back into SR - Cross bridge linkages break and recock Muscle relaxes

Stimulus for Contraction

Sliding Filament Model of Muscle Contraction

myosin cross-bridge pulls thin filament

Major Events of Muscle Contraction and Relaxation

Energy sources for contraction • ATP supplies the energy for muscle fiber contraction Muscles only have enough ATP to contract briefly ATP must be regenerated creatine phosphate stores energy that can be used to synthesize ATP as it is decomposed

The amount of ATP and creatine phosphate in a muscle can sustain contraction for only a few seconds Active muscles depend upon cellular respiration of glucose for energy Muscles store glucose in the form of glycogen

Oxygen Supply and Cellular Respiration

Anaerobic respiration yields few ATP molecules (2), whereas aerobic respiration provides many ATP's (34) Hemoglobin in red blood cells carries oxygen from the lungs to body cells

Myoglobin(pigment) in muscle cells stores some oxygen temporarily reducing a the need for a continuous blood supply during contraction Contracting muscle fibers may compress blood vessels

Oxygen debt During rest or moderate exercise, oxygen is sufficient to support aerobic respiration During strenuous exercise, an oxygen deficiency may develop, and lactic acid may accumulate as a result of anaerobic respiration Lactic acid is carried by blood to the liver where it can be converted to glucose

The amount of oxygen needed to convert accumulated lactic acid to glucose and to restore supplies of ATP and creatine phosphate is called oxygen debt Several hours may be needed to repay an oxygen debt after strenuous exercise

Oxygen Debt

Muscle fatigue A fatigued muscle loses its ability to contract Usually due to the effects of accumulation of lactic acid Lactic acid lowers the pH and prevents muscle fibers from responding to stimulation

Muscle cramps are due to a lack of ATP ATP moves Ca2+ into SR and breaks the linkages between actin and myosin so the muscle fibers can relax In a cramp part of the muscle may contract uncontrollably and other parts may be rigid

Physical training promotes new capillary growth and more mitochondria within muscles Athletes usually produce less lactic acid than nonathletes because of their increased ability to supply oxygen and nutrients to muscles

Fast and slow muscles The speed of contraction is related to a muscle's specific function Slow-contracting(slow twitch) muscles contain oxygen storing myoglobin (red pigment) and many mitochondria - can generate ATP fast enough to keep up with ATP breakdown and can contract for long periods Ex: long back muscles

Fast-contracting(fast twitch) muscles have less myoglobin, a poorer blood supply and fewer mitochondria - have reduced ability to carry on aerobic respiration and tend to fatigue relatively rapidly Ex: hand and eye muscles Most skeletal muscles have both types

Heat production By-product of cellular respiration Active muscles release large amounts of heat Blood transports heat throughout the body

Muscular Responses Threshold stimulus Minimal strength of stimulation needed to cause a muscular contraction . All-or-none response A muscle fiber does not partially contract If a muscle fiber contracts at all, it will contract completely.

Recording a muscle contraction A myogram is a recording of an electrically stimulated isolated muscle pulling a lever.

A twitch is a single contraction lasting a fraction of a second The latent period is the time between stimulus and responding muscle contraction (0.01 second in a frog) The refractory period immediately follows a contraction - a muscle cannot respond (very brief)

Muscular Responses

Staircase effect An inactive muscle undergoes a series of contractions of increasing strength when subjected to a series of stimuli The staircase effect seems to be due to failure to remove calcium ions from the SR rapidly enough A muscle fiber contraction is otherwise an all-or-none response

Summation A muscle fiber exposed to a series of increasing stimuli reaches a point where it can't completely relax before the next stimuli Force of individual twitches combines and the contraction is sustained • Forceful, sustained contraction without relaxation is a tetanic contraction (tetanus)

Summation process by which individual twitches combine produces sustained contractions can lead to tetanic contractions

Recruitment of motor units Each motor unit is controlled by a single motor neuron all muscle fibers in a motor unit are stimulated at the same time (all-or-none) A whole muscle is made of many motor units controlled by different motor neurons

At low intensity of stimulation, relatively small numbers of motor units contract At increasing intensities of stimulation, other motor units are recruited until the muscle contracts with maximal tension (recruitment)

Sustained contractions Smaller motor units (finer fibers) respond earlier to stimuli Larger units ( thicker fibers) respond later and more forcefully Product of small and larger motor units is a sustained contraction of increasing strength (multiple motor unit summation)

Tetanic contractions occur in skeletal muscles Muscle tone is a continuous state of partial contraction even when the muscle appears to be at rest - nerve impulse goes to a few muscle fibers Ex: Maintaining posture

Types of contractions Isotonic - a muscle contracts and its ends are pulled closer together Ex: lifting objects Isometric - a muscle contracts but its attachments do not move and it does not shorten. Ex: push against a wall

Isokinetic - the force a muscle generates is less than that required to move or lift an object, lengthening the muscle Ex: laying an object down on a table Most body movements involve both isometric and isotonic contractions.

Smooth Muscles The contractile mechanisms of smooth and cardiac muscles are similar to those of skeletal muscle. • Shorter than the fibers of skeletal muscle and have single centrally located nuclei Cells are elongated with tapering ends and contain thinner filaments of actin and myosin - lack striations

They lack transverse tubules, and the SR is not well developed. • Two major types multiunit smooth muscle: fibers occur separate rather than in sheets - contraction occurs after stimulation by motor nerve impulses or certain hormones Ex: Iris and walls of blood vessels

visceral smooth muscle: sheets of spindle shaped cells - most common - found in walls of hollow organs (stomach, intestines, bladder, uterus) -fibers can stimulate each other -can display pattern of repeated spontaneous contractions (rhythmicity) -wavelike motion called peristalsis (alternating contraction and relaxation) that moves materials through digestive system.

Smooth muscle contraction Smooth muscles lack troponin In smooth muscles, calmodulin (protein) binds to calcium ions and activates the contraction mechanism. Both ACh and norepinephrine are neurotransmitters for smooth muscles. Hormones stimulate or inhibit contraction Ex: oxytocin stimulates uterus to contract during childbirth

Stretching smooth muscle can stimulate contractions Ex: food stretches wall of intetines. Smooth muscle can maintain a contraction for a longer time than skeletal muscle. Smooth muscles can change lengths without changing tautness Ex: walls can stretch as the organ fills up Smooth muscles are slower to contract and relax than skeletal muscle

Smooth Muscle

Cardiac Muscle Appears only in the heart Composed of striated cells joined end to end Fibers are interconnected in branching 3-D networks Each cell has a single nucleus Contains actin and myosin similar to skeletal muscle Has a well developed SR and many mitochondria

Larger transverse tubules supply more Ca ions in response to an impulse Ca ions come from fluid outside the muscle fiber Contracts for a longer time than skeletal muscle because transverse tubules supply extra Ca ions. Intercalated disks connect the ends of adjacent cardiac muscle cells and hold the cells together.

A network of fibers contracts as a unit and respond in an all-or­none manner. Cardiac muscle is self-exciting, rhythmic, and remains refractory until a contraction is completed (no tetanic contractions).

Cardiac Muscle

Skeletal Muscle

Skeletal Muscle Actions Origin and insertion One end of skeletal muscle is attached to an immovable part (origin: and the other end is attached to a movable end (insertion) A muscle is pulled toward its origin when it contracts Some muscles have more than one origin or insertion Ex: biceps (2 heads) brachii

Skeletal Muscle Actions

Interaction of skeletal muscles Skeletal muscles function in groups. A prime mover is responsible for most of a movement Synergists aid prime movers

Antagonists can resist movement of a prime mover and cause movement in the opposite direction. Smooth movements depend upon antagonists giving way to the actions of prime movers.

Characteristics of Muscle Tissue

Life-Span Changes myoglobin, ATP, and creatine phosphate decline by age 80, half of muscle mass has atrophied adipose cells and connective tissues replace muscle tissue exercise helps to maintain muscle mass and function