Introduction: Muscle tissue is one of the 4 basic tissue types. Its most unique characteristics are:

1. It has the ability to contract (become more compactly oriented).

2. It has the ability to perform work.

Both are fundamental to our survival!

The basic unit of muscle tissue is the muscle cell (or muscle fiber) which has an elongated shape.

A. Muscles are found:

1. Connected to bones where they collectively permit movement.

2. As components of other organs such as the digestive and respiratory tracts and blood vessels.

B. Muscle Types: Muscles have many different shapes and functions, but all are classified into 1 of 3 types.

1. Smooth (non-banded or striated) Muscle: These muscles consist of single contractile cells. It is found lining the walls of the digestive tract and the uterus and lining blood vessels and some ducts.

2. Cardiac (semi-banded or striated) Muscle: These muscles also are composed of single cells, but the organization of the macromolecules is greater which makes the muscles appear semi-banded. Obviously, cardiac muscle is located in the heart.

3. Skeletal (bandedor striated) Muscle: This is the most abundant muscle type and is easily identifiable by its striations or bands. In skeletal muscle, each muscle "cell" is a fused set of dozens or even hundreds of cells. The muscle cells that result are usually very long, and they are called muscle fibers or myofibrils . A large number of these fibers is what constitutes the organ called a muscle.

Striated muscle is the muscle associated with the skeleton and is the most researched and understood type.

Skeletal muscle function by exerting a force in one direction. The act of contraction is an active process, while relaxation is a passive process. This means that contraction requires a stimulus and relaxation is the result of simply the cessation of that stimulus.

Since a given muscle can only exert a force in one direction, all locomotion requires that 2 sets of muscles that move the involved body parts in opposite directions. They essentially work against each other and as such are referred to as antagonists.

eg. The leg joint is flexed by flexor muscles and extended by extensor muscles. One "undoes" the work of the other, which is essential to movement.

C. Muscle CellStructure: All skeletal muscle is under voluntary control, meaning it contracts only when stimulated by neurons that deliver impulses to it. Skeletal muscle contains bundles of muscle fibers.

Each fiber has a set of 4 to 20 rod shaped filaments called myofibrils which are bathed in cytoplasm (called sarcoplasm). Since myofibrils do work by contracting (getting shorter), they require energy. That energy is supplied by the many mitochondria that are scattered throughout the sarcoplasm.

Myofibrils exist in smaller units called sarcomeres. Each myofibril is about 1-2 m wide and up to 100 m long. It is the repetitive pattern of sarcomeres within a muscle fiber that gives muscles their striated or banded appearance.

Microscopically, sarcomeres are composed of 2 types of myofilaments; thin and thick filaments. These filaments run parallel to each other.

The thick filaments are made primarily of a protein called myosin and the thin filaments are made primarily of a protein called actin.

The point where actin filaments from adjacent sarcomeres interweave is a dense black line called the Z-line. The Z-line bisects (crosses) a clear (relatively) broad strip called the I-band.

The large dense stripe in the center of the sarcomere formed by overlapping myosin filaments is called the A-band. The A-band is bisected by a central H-zone which contains myosin filaments but no actin filaments.

Repeating A and I bands account for the broad pattern in the myofibrils of striated muscle.

The thin actin filaments are anchored to the Z-line so during muscle contraction opposing actin filaments slide along myosin filaments and the 2 sets of actin filaments are drawn toward each other. This draws the Z-lines together which shortens the distance between the Z-lines without actually shortening any of the muscle filaments. This narrows the rather pale I-band while the broader A-band approaches the Z-lines. The other pale band (the H-band) at the center also disappears in a contracted muscle because the sets of filaments nearly meet.

D. Triggers for Muscle Contraction: Actin actually consists of 2 strands of the protein that are twisted into a helix. In the grooves of this helix are found molcules of the protein tropomyosin. While the muscle is relaxed, tropomyosin prevents myosin heads from binding (attaching) to actin.

The protein troponin is also found at regular intervals along the actin filament. It binds to both tropomyosin and actin molecules. Troponin also binds to calcium ions.

Muscle cells can be triggered to contract because they start (initiate) and continue (propagate) the action potential (a nerve impulse). A neurotransmitter like acetylcholine is released when nerve impulses from the nerve cell reach the muscle cell at the neuromuscular junction and enter. This junction consists of a single muscle fiber and the terminal end of a single muscle cell. Though close, there is actually no contact between the membranes of the nerve and the muscle cells themselves. They are separated by a fluid filled space called a synapse. It is in the synapse where the neurotransmitters are released.

The impulse (trigger) is initiated in the muscle cell and propagated over the entire cell surface. This then triggers events inside the muscle cell that results in contraction.

The normal concentration of Ca²⁺ in the cytoplasm of muscle fibers is verylow because it is constantly "pumped" out of the cell or into mitochondria. In muscle cells there is a hidden reservoir of calcium in a system of folds (called transverse or T tubules) in the plasma membrane. These T tubules surround the myofibrils at the z-lines. In addition to these, another calcium source in the branches of the sarcoplasmic reticulum exists in the form of hollow sac that contains the ion.

When nerve impulses spread through the muscle cell, the T tubule membrane conduct that impulse to the terminal sacs that contain the Ca²⁺. This immediately releases the calcium which enters the sarcoplasm that baths the myosin filaments. This changes the shape of the filaments. Since troponin is linked to tropomyosin, the tropomyosin also changes its position in the helix which makes it possible for the actin to bind to the myosin heads. The result is what is called a power stroke of muscle contraction.

The more calcium that is released the greater the strength of the muscle contraction.

Relaxation is defined as the absence of stimulation which removes calcium from the sarcoplasm and returns it to the T tubules and terminal sacs for reuse during the next contraction. Though relaxation is a passive act, energy is required to pump calcium back into the "storage areas". This means that ATP is required and no muscle will relax in the absence of the required ATP needed. This unfortunate position is called tetany. A form of tetany occurs after death called rigor mortis.

Below is a summary of the events that lead to muscle fiber contraction and relaxation:

E. Graded Responsesof Muscles: All muscle fibers have the functional feature of an all or none response. This means that a fiber can contract only after it has received stimulus response that is great enough. When that happens the fiber contracts with near complete contraction strength.

Though individual muscle fibers contract completely or not at all, that doesn't mean the entire organ (the muscle itself) contracts to the same level. Instead they are under our control and the result is called a graded response. Each nerve fiber to a muscle branches, making a single nerve fiber capable of supplying up to 100 muscle fibers. The muscle fibers and the motor neuron that it stimulates form a single motor unit.

The contraction of a muscle fiber is called a twitch. The number of twitches in a muscle is called summation. Summation is a condition during which nerve impulses arrive at a muscle before its previous contraction has ended. This is because the sarcoplasmic reticulum lacks the ability to move all the calcium out of the cell's cytoplasm quickly enough. This makes the strength of summated contractions always greater than that of individuals twitches. Uncontrolled and controlled summation results in tetany (a state of maximum sustained muscle contraction).

eg. Making a fist places the muscles of the lower arm and hands into a state of tetany.

F. Some Considerations for Smooth Muscle:

1. Smooth muscle cells consists of slender, elongated cells that lack striations and are uninucleiated (striated muscles have many nuclei).

2. Smooth muscles cells are arranged in sheets in the walls of the colon or as straps around certain blood vessels. The cells are linked together by collagen fibers.

3. The cytoplasm of these cells contain many actin filaments whose ends are inserted into the inner surface of the plasma membrane. A molecule that is similar to (but not) myosin is also present that has heads like those in myosin.

4. Smooth muscles cells contract much more slowly than striated muscle cells but can sustain that contraction for far longer that striated muscles.

5. There is no sarcoplasmic reticulum in smooth muscles (also called visceral muscles). The contractions are slow and sustained. Smooth is also not under voluntary control because they lack highly structured neuromuscular junctions. Smooth muscles are referred to as visceral muscles because it is found in visceral organs.

6. Smooth muscle cells can respond to hormones.

7. Smooth muscle contains no sarcomeres, but thin and thick filaments are collected into bundles that resemble myofibrils. The ratio of thick to thin filaments is very different in smooth muscle. In skeletal muscle that ratio is 1:2, but in smooth muscle the ratio is 1:16.

8. Contraction of smooth muscles in the urinary bladder, rectum, and uterus assist with expelling their contents.

9. Smooth muscle is generally distinguished in 2 ways:

a. Single-unit smooth muscle: The cells contract as a rhythmic unit and are electrically coupled to each other. The linings of the colon, urinary bladder, and other visceral organs consist of single unit smooth muscles.

b. Multiunit smooth muscle: Here the fibers work independently of one another. There are many nerve endings in multiunit smooth muscles and a motor unit forms with a number of muscle fibers. The linings of the airway to the lungs and large arteries contain multiunit smooth muscle. So too does the erector pili muscles of the hair follicles which cause "goose bumps" when you are cold or emotionally stimulated.

G. A Few Considerations for Cardiac Muscle:

1. They are found only in the heart, and have only a single nucleus per cell.

2. Cardiac muscle is semi-striated and richly supplied with mitochondria.

3. The cells are often branched to form an interlocking network of cells. The ends of cardiac muscle cells are bound firmly together by intercalated disks. These are sites that allow easy flow of electrical current which is associated with the heart beat.

4. There is a T tubule system and sarcoplasmic reticulum in the cells of cardiac muscle.

5. Cardiac muscle is not under direct voluntary control, but there are people who have the unique ability to control their heart rate. This is not fully understood.

H. The Major Skeletal Muscles (as organs) of the Human Body:

1. External Characteristics:

a. The are relatively long and narrow with both ends usually attached directly to bones.

b. Each skeletal muscle has a fixed end (called the origin) and a movable end (called the insertion). Most of the time the origin is more medial then the insertion. Some muscles have multiple origins and insertions.

eg. The biceps brachii has 2 origins and 2 insertions.

c. In some cases the muscles is attached to the bone by a tendon. Tendons are band-like connective tissues made of protein. They vary is length from less than a inch (the cervical tendon) to more than a foot (the Achilles' tendon).

d. Muscles can also be attached to bones by aponeuroses, which are sheet-like strips of thin tendons.

e. All skeletal muscles have a specific action (function). Movement of the body requires the actions of 2 or more muscles that work in opposition to each other. The muscle(s) acting most directly and powerfully during a given movement are called prime movers or agonists. The opposers or "undoers" of that movement are called antagonists.

eg. The triceps brachii is the antagonist of the biceps brachii.

f. Most body movements are the results of the complex actions of groups of flexors, extensors, rotators, and other muscles. Many of the muscles of the body are named in association with their individual function(s), and contain in their name term such as flexor, extensor, abductor, adductor, levator, depressor. retractor, protractor, rotator, and sphincter.

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