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muscle tissue

skeletal muscle

is striated muscle and is voluntary (it can be controlled consciously). It can only be stimulated by the nervous system.

cardiac muscle

is striated muscle. It is involuntary (no conscious control). It can be stimulated or inhibited by either the nervous or the endocrine systems.

smooth muscle

is nonstriated and also involuntary. It also can be stimulated or inhibited by either the nervous or endocrine function.

three functions of muscle tissue

motion, stabilize body positions and regulate organ volume, thermogenesis

characteristics of muscle tissue

excitability, contractility, extensibility, elasticity


refers to a sheet or broad band of fibrous connective tissue beneath the skin or around muscles and organs of the body. There are two types.

superficial fascia

(subcutaneous layer or hypodermis) lies immediately deep to the skin. It is composed of adipose and areolar tissues.

Four functions of Superficial fascia

store fat, insulation, mechanical protection, pathway for nerves and blood vessels

deep fascia

formed of dense irregular connective tissue. It lines the body wall and extremities and holds muscles together, separating them into functional groups. allows free movement of muscles, carries nerves, blood vessels, and lymph vessels, and fills the spaces between muscles.


is the outermost layer of deep fascia, wrapping the entire muscle.


is formed by invaginations of the epimysium, dividing the muscle into bundles of cells called fascicles (fasciculi).


is formed by invaginations of the perimysium that penetrate the fascicle and wrap each muscle cell, completely insulating it from the others.


All three layers may extend beyond the muscle as a cord of dense connective tissue that attaches the muscle to the peristeum of a bone.

tendon sheath

Some tendons, particularly those in high stress areas like the ankle and wrist, are wrapped in a layer of synovial membrane


When the three connective tissue components extend from the muscle as a flat sheet, rather that a round cord,


Within a typical skeletal muscle there are thousands of individual, very long, cylindrical cells called muscle fibers, bundled together as fascicles. They lie in parallel to one another.


the cell membrane of a muscle cell. surrounds the sarcoplasm.


lie in rows throughout the muscle fiber, located close to the muscle proteins that use ATP for the contraction-relaxation sequence.

Nuclei in skeletal muscle

cells are multinucleated, due to fusion of precursor cells during embryogenesis; the nuclei are located along the periphery of the cell, out of the way of the contractile elements within the sarcoplasm.


lie parallel to each other and extend lengthwise throughout the myofiber. give skeletal muscles their striations due to their areas of light and dark. The contractile element of skeletal muscle. Are made up of myofilaments.


functional (contracting) unit of skeletal muscle.

Z line

a dense material found at each end of a sarcomere, separating it from the next sacromere in line.

thin myofilaments

extending from each z line towards the middle of the sacromere

thick myofilaments

suspended within the sarcoplasm, between the thin myofilaments, and not attached to the z line


the main component of a thin myofilament, each molecule of which looks like a kidney bean.

myosin binding site

upon each actin filament that is twisted to form a helical strand.

tropomyosin troponin complex

In relaxed muscle, covers the myosin-binding sites on the actin molecules. This blocks the myosin-binding sites and prevents the attachment of the thick myofilaments, thus preventing contraction of the sarcomere.


Each thick myofilament is composed of about 200 molecules of a protein. is shaped like two golf clubs twisted together. The tail of the molecule extends to the center of each sarcomere. The projecting "head," called a cross bridge, extends out towards the thin myofilaments.


elastin myofilament, anchor the thick myofilaments in position and to play a role in recovery of the resting sarcomere length when a muscle cell is stretched or contracted. accounts for most of the extensibility and elasticity of the myofibrils

sarcoplasmic reticulum

A fluid filled system of tubules that encircles each myofibril. In relaxed muscle stores calcium ions by sequestering them from the sarcoplasm.

transverse tubules

are tunnel like enfolding of the sarcolemma. They penetrate the myofiber at right angles to the sarcoplasmic reticulum and the myofilaments. Are open to the outside of the muscle fiber and therefore filled with extracellular fluid.

terminal cisternae

dilated end sacs of the sarcoplasmic reticulum connected on both sides of the transverse tubule.

Muscle triad

a transverse tubule together with its two terminal cisternae.

motor neuron

originating in the brain or spinal cord, delivers the nervous stimulus that ultimately causes a muscle tissue to contract.

motor unit

One motor neuron plus ALL of the skeletal muscle cells it stimulates. On average, a single motor neuron makes contact and thus stimulates about 200 individual skeletal muscle cells. All the cells contract and relax together, as a unit.

precise movement

muscles that have as few as 2-3 muscle cells per motor neuron.

gross movement

muscle that have as many as 2,000 muscle cells per motor neuron.

total strength

the number of motor units being used at any given time.


the process of increasing the number of active motor units within a given skeletal muscle.


Excitable cells (muscle and nerve) make contact and communicate with one another at specialized regions

synaptic cleft

at each synapses a small gap, separates the two excitable cells.


The first cell, the motor neuron, communicates with the second cell, the skeletal muscle cell, across the synaptic cleft via a chemical messenger

neuromuscular junction

The type of synapse formed between the motor neuron and the skeletal muscle cell

axon terminals

At the synapse, the motor neuron branches into clusters of bulb shaped, each cluster forming a synapse with a group of muscle cells (motor unit)

motor end plate

The region of the muscle cell membrane that participates in the synapse with the axon terminal


the neurotransmitter used exclusively by motor neurons for skeletal muscles

synaptic vesicles

Within each axon terminal are many membrane-enclosed vesicles containing thousands of neurotransmitter molecules

sliding filament theory

it was proposed that skeletal muscle shortens during contraction because the thin and thick myofilaments slide past one another.


ACh is rapidly broken down in the synaptic cleft by the enzyme


calcium ions are pumped into the sarcoplasmic reticulum, where they are bound to the molecule, thus removing them from the sarcoplasm

muscle tone

Sustained contractions of a few motor units in a given skeletal muscle produce a firmness, This maintains optimum length of the muscle and makes it ideally ready for action.

asynchronous firing

different units are activated at different times

phosphagen system

energy system which utilizes stored ATP and creatinine phosphate, provides enough energy for skeletal muscles to contract maximally for about 15 seconds. It is used for maximal short bursts of activity.

aerobic system

For continued activity and with sufficient oxygen, mitochondria completely catabolize glucose to carbon dioxide, water, and a net 36 ATPs. When the oxygen supply is outstripped by the activity, glucose is incompletely catabolized to lactic acid. This leads to fatigue and oxygen debt.

glycogen lactic acid system

When stored ATP and creatinine phosphate are depleted, the cell catabolizes glucose to create new ATP. A series of reactions known as glycolysis creates 2 ATPs for each glucose and does so anaerobically (without oxygen). This provides energy for an additional 30-40 seconds.

oxygen debt

the difference between the resting state of oxygen consumption and the elevated rate following an exercise.

slow oxidative fibers

abundant mitochondria, myoglobin (stores oxygen), and blood capillaries well adapted to aerobic respiration, fibers do not fatigue easily (high endurance)

fast glycolytic fibers

abundant enzymes for the phosphagen and glycolytic lactic acid pathways, extensive sarcoplasmic reticulum for rapid calcium movement, poor in mitochondria, myoglobin, and blood capillaries, twitch times as short as 7.5 sec, atigue easily (low endurance)


detect decreases body temperature in the hypothalamus of the brain and the skin.

involuntary thermogenesis

returns body temperature to within normal homeostatic range, and the process is turned off by negative feedback.

all or none principle

states that individual muscle fibers contract fully and completely for the existing conditions, or do not contract at all.

threshold stimulus

one that initiates the formation of an electrical impulse in the sarcolemma.

twitch contraction

brief contraction of all muscle fibers in a motor unit in response to a single threshold stimulus from a motor neuron.

latent period

is the time from when the stimulus is applied to the beginning of the actual shortening of the sarcomere. During this time, membrane polarization is changing, calcium is diffusing out of the sarcoplasmic reticulum, and the cross bridge of myosin are attaching to actin.

contraction period

is the time during which the myofilaments are sliding, the sarcomere is shortening, and the muscle fiber is generating force.

relaxation period

is the time during which acetylcholinesterase is destroying acetylcholine at the synapse, calcium ions are being sequestered within the sarcoplasmic reticulum, the cross bridges are releasing from actin, and the sarcomere is sliding back to its resting strength.

refractory period

When a muscle fiber receives a threshold stimulus, it temporarily loses its excitability and cannot be stimulated again.

wave summation

If a second stimulus is applied to an excited muscle cell after the refractory period, but before the cell has finished resting from the first stimulus, the second contraction will be greater than the first. This phenomenon, in which stimuli arrive at different times and cause larger contractions

incomplete tetany

during which the muscle is contracting maximally but relaxing only partially.

complete tetany

If frequency of stimulation is increased to 80-100 per second, no relaxation period occurs at all between contractions.

treppe contraction

When a skeletal muscle has been fully rested and is then stimulated repeatedly by identical stimuli too far apart to cause wave summation, each of the first few contractions is a little stronger than the last.

isotonic contractions

occur when you move a constant load through the range of motions possible at a joint. In this type of contraction, the tension within the muscle stays the same during the contraction, while the length of the fibers shortens.

isometric contractions

occur when the muscle does not or cannot shorten, but the tension within it greatly increases.

intercalated discs

Adjacent cell membranes contact one another at thickened areas, it is through these structures that adjoining cells communicate

visceral smooth muscle

It is found wrapped in sheets that form part of the walls of hollow organs. The cells are tightly bound to each other to form a continuous network (functional syncytium), so that stimulation of one cell results in stimulation of the entire network.

Multiunit smooth muscle

consists of individual smooth muscle cells, each with its own motor neuron, so that each can contract independent of the others. Is found in the walls of blood vessels, the arrector pili of hair follicles, and in the iris of the eye.

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