CHAPTER 10

Sphincter muscles

Muscles that encircle body openings and function as valves, relaxing to allow passage of a substance and contracting to close the passageway.

Muscle tone

The constant low-level contraction of muscles that helps stabilize and strengthen synovial joints.

Striations

Light and dark stripes present in muscle cells, which can be used to characterize different types of muscle tissue.

Skeletal muscle tissue

Muscle tissue located in the skeletal muscles, which attach to and move the skeleton. It is innervated by the voluntary division of the nervous system and is subject to conscious control.

Cardiac muscle tissue

Muscle tissue found only in the wall of the heart. It is striated muscle but its contraction is involuntary.

Smooth muscle tissue

Muscle tissue found in the walls of hollow internal organs other than the heart. It is involuntary and responsible for the movement of substances through these organs.

Contractility

The ability of muscle tissue to contract forcefully.

Excitability

The ability of muscle cells to be excited by nerve signals or other stimuli, leading to contraction.

Extensibility

The ability of muscle tissue to be stretched.

Elasticity

The ability of muscle tissue to passively recoil and return to its resting length after being stretched.

Sarcolemma

The plasma membrane of muscle cells.

Sarcoplasm

The cytoplasm of muscle cells.

Sarcoplasmic reticulum

The endoplasmic reticulum in muscle cells, specialized for the storage of calcium.

Myofilaments

Specific types of microfilaments responsible for the shortening of muscle cells. They include actin and myosin proteins.

Sarcomere

The contractile unit of a muscle fiber, composed of myofilaments made up of contractile proteins.

Thin filament

A myofilament composed of actin molecules and regulator proteins troponin and tropomyosin.

Thick filament

A myofilament composed of bundled myosin molecules.

Elastic filaments

Filaments composed of titin molecules that maintain the organization of the A band and provide for elastic recoil when muscle contraction ends.

Muscle tissue

Tissue that is responsible for movement in the body.

Striated muscle

Muscle tissue that has a striped appearance due to the arrangement of its fibers.

Visceral muscle

Muscle tissue found in the visceral organs, such as the stomach and bladder.

Muscle fibers

Elongated cells that make up muscle tissue.

Smooth muscle

Muscle tissue that lacks striations and is found in the walls of organs.

Cardiac muscle

Muscle tissue found in the heart.

Involuntary division of the nervous system

The part of the nervous system that controls smooth and cardiac muscle.

Epimysium

The outer layer of dense, irregular connective tissue that surrounds a skeletal muscle.

Perimysium

The layer of fibrous connective tissue that surrounds each fascicle within a skeletal muscle.

Endomysium

The fine sheath of loose connective tissue that surrounds each muscle fiber within a fascicle.

Fascicle

A bundle of muscle fibers within a skeletal muscle.

Tendon

The connective tissue structure that joins skeletal muscles to bones.

Origin

The attachment of a muscle on the less movable bone.

Insertion

The attachment of a muscle on the more movable bone.

Nerves and blood vessels

The structures that supply each skeletal muscle with nerve impulses, oxygen, and nutrients.

Skeletal Muscle Fiber

Long, cylindrical cells that make up skeletal muscle tissue.

Sarcomere

The basic unit of contraction in skeletal muscle, composed of thin and thick filaments.

Myofibrils

Unbranched cylinders present in skeletal muscle fibers, made up of myofilaments and responsible for muscle contraction.

Thin Filaments

Fine myofilaments composed primarily of the contractile protein actin, found in the sarcomere.

Thick Filaments

Cylindrical bundle of myofilaments consisting largely of myosin molecules, found in the sarcomere.

Z Discs

Boundaries at the two ends of each sarcomere, where thin filaments are attached.

Troponin

A regulatory protein found on the thin filament, attaches the tropomyosin strand to the actin molecule.

Tropomyosin

A regulatory protein found on the thin filament, forms a thin strand that spirals around the actin molecule.

Actin

A contractile protein found in the thin filament.

Myosin

A contractile protein found in the thick filament, contains ATPase enzymes that split ATP to release energy for muscle contraction.

A Band

The dark band in the sarcomere, created by the full length of the thick filaments and the inner ends of the thin filaments.

I Band

The light band in the sarcomere, created by the thin filaments that do not overlap with the thick filaments.

H Zone

The central part of the sarcomere where no thin filaments reach.

M Line

The center of the sarcomere, overlapping the inner ends of the thin filaments.

Sarcoplasmic Reticulum

A specialized smooth endoplasmic reticulum in muscle fibers that stores and releases calcium ions for muscle contraction.

T Tubules

Invaginations of the sarcolemma that penetrate deep into the muscle fiber, allowing for the transmission of action potentials.

Myofibril

A thread-like structure within a muscle fiber that contains the contractile units called sarcomeres.

Triad

A structure in skeletal muscle that consists of a T tubule and two terminal cisterns of the sarcoplasmic reticulum.

Sarcolemma

The plasma membrane of a muscle fiber.

Mitochondria

Organelles responsible for producing energy (ATP) in the muscle fiber.

I band

A light band in the sarcomere that contains only thin filaments.

A band

A dark band in the sarcomere that contains both thick and thin filaments.

Z disc

A structure in the sarcomere that serves as the attachment site for thin filaments.

H zone

A region in the sarcomere that contains only thick filaments.

T tubule

A deep invagination of the sarcolemma that helps transmit nerve-generated impulses to the deepest regions of the muscle fiber.

Sarcoplasmic reticulum

An elaborate smooth endoplasmic reticulum in the muscle fiber that stores and releases calcium ions.

M line

A structure in the sarcomere that serves as the attachment site for thick filaments.

Titin

A spring-like molecule in sarcomeres that resists overstretching and helps maintain the organization of the A band.

Elastic filaments

Filaments in the sarcomere that contain titin and contribute to muscle elasticity.

Terminal cisterns

Enlarged portions of the sarcoplasmic reticulum that form cross channels over the junction between each A band and its adjacent I bands.

Calcium ions

Ions stored in the sarcoplasmic reticulum and released during muscle contraction.

T tubules

Deep invaginations of the sarcolemma that run between each pair of terminal cisterns and help transmit nerve-generated impulses to the deepest regions of the muscle fiber.

Triad

A complex consisting of a T tubule flanked by two terminal cisterns at the A-I junction.

Concentric contraction

A type of muscle contraction in which the muscle shortens and does work.

Eccentric contraction

A type of muscle contraction in which the muscle generates force as it lengthens.

Sliding filament mechanism

The mechanism of muscle contraction in which the myosin heads attach to actin in the thin filaments and pull them towards the center of the sarcomere.

Sarcomere

The basic functional unit of a muscle fiber, consisting of overlapping thick and thin filaments.

Motor unit

A motor neuron and all the muscle fibers it innervates.

Calcium ions

Ions released from the sarcoplasmic reticulum that bind to the troponin molecule on the thin filament, initiating muscle contraction.

ATP

Adenosine triphosphate, the molecule that provides energy for muscle contraction.

Optimal resting length

The length at which a muscle fiber generates the greatest pulling force when contracted.

Delayed-onset muscle soreness

Soreness that begins 8-24 hours after an activity, caused by microscopic tears in the muscle fibers.

Neuromuscular junction

The point at which a nerve ending and muscle fiber meet, where neurotransmitters are released to signal the muscle fiber to contract.

Acetylcholine

The neurotransmitter released at the neuromuscular junction that binds to receptor molecules on the sarcolemma, initiating muscle fiber contraction.

Terminal boutons

Enlargements at the end of the axonal process of a motor neuron that store neurotransmitters and form neuromuscular junctions with muscle fibers.

Motor neuron

Nerve cells that innervate muscle fibers and initiate muscle contraction through the release of neurotransmitters.

Motor unit

Consists of one motor neuron and all the muscle fibers it innervates, causing them to contract together when the motor neuron fires.

Recruitment

The addition of motor units to accomplish a movement, where more force is needed and additional motor units are stimulated.

Muscle twitch

The uncontrollable contraction of a small portion of a muscle, usually a single motor unit.

Slow oxidative fibers

Muscle fibers that obtain their energy from aerobic metabolic reactions, contract slowly, are resistant to fatigue, and have a high myoglobin content.

Fast glycolytic fibers

Muscle fibers that contain little myoglobin, generate much more power than slow oxidative fibers, depend on anaerobic pathways to make ATP, and tire quickly.

Fast oxidative fibers

Muscle fibers that contract quickly, are oxygen dependent, have a high myoglobin content, and are fatigue resistant but less so than slow oxidative fibers.

Myoglobin

An oxygen-binding pigment found in muscle fibers, particularly in slow oxidative fibers.

Rhabdomyolysis

A condition characterized by the disintegration of skeletal muscle, leading to the release of myoglobin into the bloodstream and potential kidney failure.

Epimysium

The connective tissue layer that surrounds the entire muscle.

Perimysium

The connective tissue layer that surrounds bundles of muscle fibers.

Endomysium

The connective tissue layer that surrounds individual muscle fibers.

Myofibrils

Structures within muscle fibers composed of sarcomeres, responsible for muscle contraction.

Sarcomeres

The functional units of muscle contraction, composed of actin and myosin filaments.

Muscular dystrophy

A noninfectious disorder characterized by progressive muscle weakness and degeneration.

Myofascial pain syndrome

A noninfectious disorder characterized by the presence of trigger points in muscle tissue, causing localized pain and muscle stiffness.

Fibromyalgia

A noninfectious disorder characterized by widespread musculoskeletal pain, fatigue, and tenderness in specific areas of the body.

Myofascial Pain Syndrome

A condition characterized by pain caused by tightened bands of muscle fibers that twitch when touched, often associated with overused or strained postural muscles.

Trigger Points

Sensitive areas of skin that are associated with myofascial pain syndrome and cause pain when touched.

Fibromyalgia

A chronic pain syndrome of unknown cause, characterized by severe musculoskeletal pain, fatigue, sleep abnormalities, and headache.

Muscular Dystrophy

A group of inherited muscle-destroying diseases that typically appear in childhood, characterized by muscle degeneration and enlargement with fat and connective tissue.

Duchenne Muscular Dystrophy

The most common and serious form of muscular dystrophy, inherited as a sex-linked recessive disease, affecting mostly males and leading to muscle weakness and respiratory failure.