Muscular System

Characteristics of Muscle Tissue Irritability Ability to respond to a stimulus (either external or internal) Stimulation produces a muscle contraction Contractibility Ability to shorten or contract Extensibility Ability to stretch or extend Elasticity Ability of muscle to return to its original shape after contraction or extension

Types of Muscles Skeletal – striated and voluntary Smooth – involuntary Cardiac – heart

Skeletal Muscle Characteristics Most attach to bones by tendon Cells are multinucleate Striated—have visible binding Voluntary Cells surrounded & bundled by connective tissue

Smooth Muscle Characteristics Has no striations Spindle-shaped cells Single Nucleus Involuntary—no conscious control Found mainly in the walls of hollow organs

Characteristics of Cardiac Muscle Has striations Usually has a single nucleus Joined to another cardiac muscle cell Involuntary Found only in the heart Made to work continuously

Muscles and Muscle Fibers Each muscle is an organ comprised of skeletal muscle tissue, connective tissues, nervous tissue, and blood Muscles are composed of many fibers that are arranged into bundles called fascicles Muscle fiber = muscle cell

Connective Tissue Coverings Fascia – strong, dense connective tissue that surround and separates each muscle Tendon – connective tissue that goes beyond the muscle to join with the periosteum of the bone Aponeuroses – broad sheets of connective tissue

Connective Tissue Coverings Epimysium – layer of connective tissue around each whole muscle Perimysium – layer of connective tissue around each bundle of muscle fibers Endomysium – layer of connective tissue around each muscle cell

Muscle Fibers Each muscle fiber membrane is the sarcolemma; muscle cytoplasm is called sarcoplasm Within the sarcoplasm are many fibers called myofibrils; myofibrils are individual muscle fibers made up of myofilaments These filaments are found in two types: myosin (thick) and actin (thin). The organization of these filaments produces striations found in skeletal muscle

Muscle Fibers A bands (dark bands) are made up of overlapping thick and thin filaments. In the center of A bands is an H zone, consisting of myosin filaments only. I bands (light bands) are made up of actin filaments anchored to Z lines Sacromere – extends from Z line to Z line Z lines – lines across the muscle fiber

Transverse tubules – passageways for the nerve impulse to travel into the muscle fiber Sacroplasmic reticulum – passageway that stores and releases Ca+ Calcium ions – Ca+ binds with troponin and tropomyosin which allows the actin and myosin to bind together

It is important to remember the heirarchy myosin myofibrils fasicles myofilaments actin

It is important to remember the heirarchy fasicles myofibrils myofilaments actin myosin

muscle fiber myofilament myofibrils epimysium muscle sarcomere

myofilament muscle sarcomere epimysium myofibrils muscle fiber

Neuromuscular Junction The site where the motor neuron and muscle fiber meet Activates the muscle Also called the motor unit

1.  Neuron         2.  Sarcolemma   (or motor end plate)        3.  Vesicle      4.  Synapse        5.  Mitochondria Motor Unit or Neuromuscular Junction

Neurotransmitters Acetylcholine – this neurotransmitter released from the end of the neuron which relays impulses to the muscle cell Acetylcholinesterase – enzyme that quickly breaks down and removes acetylcholine

Sliding Filament Theory The theory of how muscle contracts The contraction of a muscle occurs as the thin filament slides past the thick filaments Involves five different molecules plus calcium ions Myosin Actin Tropomyosin Troponin ATP

Video of sliding filament theory

Other Terms Threshold stimulus – minimal strength required to cause a contraction All-or-None response – fibers do not contract partially, either they do or they don’t Motor unit – the muscle fiber + the motor neuron Recruitment – more and more fibers contract as the intensity of the stimulus increases Muscle tone – sustained contraction of individual fibers, even when muscle is at rest Hypertrophy – muscles enlarge Atrophy – muscles become small and weak due to disuse Muscle fatigue – muscle loses ability to contract after prolonged exercise or strain Muscle cramp – a sustained involuntary contraction