Muscle tissue

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myology

the scientific study of muscles

sphincter

ringlike band of smooth muscle that prevents outflow of the contents of a hollow organ

thermogenesis

heat production due to muscle contraction that helps maintain normal body temperature

excitability

the ability of muscle fibers to receive and respond to stimuli

contractility

the ability of cells or parts of cells to actively generate force to undergo shortening for movements

extensibility

the ability of muscle tissue to stretch when it is pulled

elasticity

the ability of tissue to return to its original shape after contraction or extension

muscle fibers

another name for muscles cells, due to their elongated shapes

fascia

a dense sheet or broad band of dense irregular connective tissue that lines the body wall and limbs and supports and surrounds muscles and other body organs; holds muscles with similar functions together

epimysium

the outermost layer of connective tissue that encircles the entire muscle

perimysium

connective tissue layer that surrounds groups of 10 to 100 or more muscle fibers, separating them into bundles

fascicle

a small bundle or cluster of muscle fibers

endomysium

a connective tissue layer that separates individual muscle fibers from one another

tendon

a cord of dense regular connective tissue composed of parallel bundles of collagen fibers that attach a muscle to a periosteum of a bone

aponeurosis

a tendon that resembles a broad, flat layer

hypertrophy

growth resulting in an enlargement of existing muscle fibers

hyperplasia

growth due to an increase in the number of muscle fibers

fibrosis

the replacement of muscle tissue by scar tissue

sarcolemma

the plasma membrane of a muscle cell

transverse (T) tubules

tiny, cylindrical invaginations of the sarcolemma of striated muscle fibers that conduct muscle action potentials toward the center of each muscle fiber

sarcoplasm

the cytoplasm of a muscle fiber

myoglobin

red-colored, iron-containing muscle protein that binds oxygen molecules that diffuse into the muscle fiber sarcoplasm from interstitial fluid

myofibrils

threadlike structures that extend longitudinally through a muscle fiber consisting mainly of thick filaments and thin filaments

sarcoplasmic reticulum (SR)

fluid-filled system of membranous sacs that encircle each myofibril; stores and releases calcium ions

muscular atrophy

a wasting away of muscles because of progressive loss of myofibrils due to disuse or denervation

filaments

the thin and thick proteins within myofibrils that are directly involved in the contractile process

sarcomere

the basic functional contractile unit of a myofibril

Z discs

narrow, plate-shaped regions of dense protein that separate one sarcomere from the next

A band

the darker middle part of the sarcomere which extends the entire length of the thick filaments

M line

supporting proteins that hold the thick filaments together in the middle of the sarcomere

myosin

the contractile protein that makes up the thick filaments of muscle fibers

actin

a contractile protein that is part thin filaments of muscle fibers

tropomyosin

one of two regulatory proteins that is part of the thin filament; covers the myosin-binding sites on actin

troponin

one of two regulatory proteins that is part of the thin filament; binds to calciumm ions and undergoes a shape change that moves tropomyosin away from the myosin-binding sites on actin

sliding filament model

the model that describes how skeletal muscle shortens during contraction due to the thin filaments sliding past the thick filaments

crossbridges

form when the myosin heads attach to actin during contraction

somatic motor neurons

the nerve cells that stimulate skeletal muscle fibers to contract

neuromuscular junction (NMJ)

the synapse between a somatic motor neuron and a skeletal muscle fiber

synapse

a region where communication occurs between two neurons, or between a neuron and a target cell

synaptic cleft

a small gap at the synapse that separates the two cells

neurotransmitter

chemicals that allow the first cell at a synapse to communicate with the second cell

synaptic vesicles

hundreds of membrane-enclosed sacs that are suspended in the cytosol within each synaptic end bulb

acetylcholine (ACh)

the neurotransmitter stored in synaptic vesicles that is released at the neuromuscular junction

motor end plate

the region of the muscle fiber sarcolemma opposite the synaptic end bulbs

creatine phosphate

an energy-rich molecule found only in muscle fibers that is synthesized when there is excess ATP

anaerobic cellular respiration

a series of ATP-producing reactions that do not require oxygen

aerobic cellular respiration

a series of oxygen-requiring reactions that produce ATP in mitochondria

muscle fatigue

the inability of a muscle to maintain force of contraction after prolonged activity

oxygen debt (recovery oxygen uptake)

refers to the added oxygen, over and above the resting oxygen consumption, that is taken into the body after exercise

motor unit

a somatic motor neuron plus all the skeletal muscle fibers it stimulates

myogram

a visual record of a muscle contraction

latent period

the brief delay that occurs between application of the stimulus and the beginning of contraction; the muscle action potential sweeps over the sarcolemma and calcium ions are released from the sarcoplasmic reticulum

contraction period

during this time, calcium ions bind to troponin, myosin-binding sites on actin are exposed, and crossbridges form

relaxation period

during this time, calcium ions are actively transported back into the sarcoplasmic reticulum, myosin-binding sites are covered by tropomyosin, myosin heads detach from actin, and tension in the muscle fiber decreases

refractory period

the time when a muscle fiber receives enough stimulation to contract that it temporarily loses its excitability and cannot respond for a time

muscle tone

a small amount of tautness or tension in the muscle due to weak, involuntary contractions of its motor units

isotonic contraction

the tension (force of contraction) developed by the muscle remains almost constant while the muscle changes its length; used for body movements and for moving objects

isometric contraction

occurs when the tension generated is not enough to exceed the resistance of the object to be moved, and the muscle does not change its length; holding a book steady using an outstretched arm

slow oxidative (SO) fibers

muscle fibers that are the smallest in diameter, the least powerful, appearing dark red due to large amounts of myoglobin and many blood capillaries; fatigue-resistant fibers that are adapted for maintaining posture and for aerobic, endurance-type activities like running a marathon

fast oxidative-glycolytic (FOG) fibers

muscle fibers that are intermediate in diameter, appearing dark red due to large amounts of myoglobin and many blood capillaries; used in walking and sprinting

fast glycolytic (FG) fibers

muscle fibers that are the largest in diameter and contain the most myofibrils; able to generate the most powerful contractions; low myoglobin content and few blood capillaries; adapted for intense anaerobic movements of short duration such as weight lifting

cardiac muscle fibers

the contractile cells of the heart

smooth muscle tone

a state of continued partial contraction that can sustain long-term tone in the GI tract and in the walls of blood vessels to maintain blood pressure

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