The Muscular System

A

I. Introduction

 

- Muscles convert chemical energy into mechanical energy.

- Muscles make up the bulk of the body and account for about one-

third of its weight.

Their ability to contract not only enables the body to move, but

also provides the force that pushes substances, such as blood and

food, through the body.

- Without the muscular system, none of the other organ systems

would be able to function.

 

II. Types of Muscle Tissue

 

1.    Skeletal Muscle Cells are striated (striped) with multiple nuclei;

are arranged as many, long, cylindrical cells bundled together;

with several bundles enclosed in tough connective tissue sheath

to form a muscle; they are responsible for voluntary movement,

and are generally connected to bones via tendons

Skeletal muscle fibers are multinucleated, with the cell's nuclei located just beneath the plasma membrane. The cell comprises a series of striped or striated, thread-like myofibrils. Within each myofibril there are protein filaments that are anchored by dark Z lines. The fiber is one long continuous thread-like structure. The smallest cross section of skeletal muscle is called a sarcomere which is the functional unit within the cell. It extends from one Z line to the next attached Z line. The individual sarcomere has alternating thick myosin and thin actin protein filaments. Myosin forms the center or middle of each sarcomere. The exact center of the sarcomere is designated the M line. Thinner actin filaments form a zig zag pattern along the anchor points or Z line.

Upon stimulation by an action potential, skeletal muscles perform a coordinated contraction by shortening each sarcomere. The best proposed model for understanding contraction is the sliding filament model of muscle contraction. Actin and myosin fibers overlap in a contractile motion towards each other. Myosin filaments have club-shaped heads that project toward the actin filaments.

Larger structures along the myosin filament called myosin heads are used to provide attachment points on binding sites for the actin filaments. The myosin heads move in a coordinated style, they swivel toward the center of the sarcomere, detach and then reattach to the nearest active site of the actin filament. This is called a rachet type drive system. This process consumes large amounts of adenosine triphosphate (ATP).

Energy for this comes from ATP, the energy source of the cell. ATP binds to the cross bridges between myosin heads and actin filaments. The release of energy powers the swiveling of the myosin head. Muscles store little ATP and so must continuously recycle the discharged adenosine diphosphate molecule (ADP) into ATP rapidly. Muscle tissue also contains a stored supply of a fast acting recharge chemical, creatine phosphate which can assist initially producing the rapid regeneration of ADP into ATP.

Calcium ions are required for each cycle of the sarcomere. Calcium is released from the sarcoplasmic reticulum into the sarcomere when a muscle is stimulated to contract. This calcium uncovers the actin binding sites. When the muscle no longer needs to contract, the calcium ions are pumped from the sarcomere and back into storage in the sarcoplasmic reticulum.

2.    Smooth Muscle Cells are not striated, have a single nucleus; the cells are tapered at both ends, are held together by tight junctions; and are located in the walls of "hollow organs" i.e., blood vessels, stomach, bladder, and other internal organs; they are responsible for involuntary movements i.e., peristalsis, and

although they are wrapped by connective tissue, they are not

connected by tendons. Although considered involuntary muscles,

they do respond to psychological states such as stress and

excitement

3.    Cardiac Muscle Cells are striated with multiple nuclei per cell

like skeletal muscles; however, the cells are branched and

connected end-to-end by junctions (fused cell membranes) that

allow electrical current flow; they are located only in the heart

wall. Although also considered involuntary muscles, they do

respond to psychological states such as stress and excitement,

and significant evidence suggests that some degree of voluntary

control can be gained over cardiac muscles, through practices

such as meditation and biofeedback

Control of muscle contraction

Neuromuscular junctions are the focal point where a motor neuron attaches to a muscle. Acetylcholine, (a neurotransmitter used in skeletal muscle contraction) is released from the axon terminal of the nerve cell when an action potential reaches the microscopic junction, called a synapse. A group of chemical messengers cross the synapse and stimulate the formation of electrical changes, which are produced in the muscle cell when the acetylcholine binds to receptors on its surface. Calcium is released from its storage area in the cell's sarcoplasmic reticulum. An impulse from a nerve cell causes calcium release and brings about a single, short muscle contraction called a muscle twitch. If there is a problem at the neuromuscular junction, a very prolonged contraction may occur, tetanus. Also, a loss of function at the junction can produce paralysis.

Skeletal muscles are organized into hundreds of motor units, each of which involves a motor neuron, attached by a series of thin finger-like structures called axon terminals. These attach to and control discrete bundles of muscle fibers. A coordinated and fine tuned response to a specific circumstance will involve controlling the precise number of motor units used. While individual muscle units contract as a unit, the entire muscle can contract on a predetermined basis due to the structure of the motor unit. Motor unit coordination, balance, and control frequently come under the direction of the cerebellum of the brain. This allows for complex muscular coordination with little conscious effort, such as when one drives a car without thinking about the process.

III. Functional Organization of Skeletal Muscle

Types of Skeletal Muscle Cells

Not all skeletal muscle cells are alike. Some are for speed and strength, and some are fore endurance. How fast they can use ATP and how they replace it determines their category.

Slow twitch fibers

The ATPase's which burn ATP are relatively slow acting enzymes in slow twitch fibers. Because of this and the fact that they use the more efficient aerobic pathway to replace used ATP, they may be able to replace ATP as quickly as it is used. Thus they are very fatigue-resistant. These cells have rich blood cell and loads of mitochondria to ensure a good supply of blood glucose and oxygen. They also store lots of oxygen using a molecule called myoglobin, a molecular relative of hemoglobin. Because the blood usually supplies enough glucose, they have relatively low levels of stored glycogen.

Fast twitch fibers

Fast twitch fibers have ATPase's which burn ATP rapidly. In fact, they burn it so rapidly that they must use the fast, but inefficient anaerobic pathway to replace ATP. In order to stave off fatigue as long as possible, these cells are loaded with glycogen, but are low in myoglobin and mitochondria since they are not using the aerobic pathway as their main source of replacement ATP.

Intermediate fibers (AKA fast twitch fatigue-resistant fibers)

Fast twitch fibers also have ATPase's which burn ATP rapidly. However, they are set up to attempt to use the efficient (even if slower) aerobic pathway to replace ATP. These cells are loaded with myoglobin and mitochondria, and have a rich blood supply to try to keep up with the need for ATP. They also have an intermediate supply of glycogen. Using the aerobic pathway as their main source of replacement ATP does allow the cell to fatigue significantly slower than a fast twitch fiber. But because the slower ATP production of the aerobic pathway falls farther and farther behind in replacing rapidly used ATP, the cell does eventually fatigue well before a slow twitch fiber would.

Most muscles are made of a mixture of these different fibers so that they can perform a variety of functions. However it is also true that many muscles in the body have one of these types of cells as their dominant variety. For examples, the postural muscles along the vertebral column have a high proportion of slow twitch fibers for the endurance required to stand or sit upright for many hours. Arm muscles have a larger proportion of fast twitch fibers. Leg muscles often represent the most mixed muscle as they have all three types. This is sensible considering that leg muscles are called upon to stand for long periods, to walk or run, and to jump. The basis for true athletic talent may come from the way these fibers are distributed, as, for example, a world class sprinter may have a larger than normal percentage of fast twitch fibers in her legs.

1) Single Muscle Fibers

 

1. A muscle fiber is a single, multinucleated muscle cell.

2. A muscle is made up of hundreds or even thousands of muscle

fibers, depending on the muscles size.

3.    Although muscle fiber makes up most of the muscle tissue, a

large amount of connective tissue, blood vessels, and nerves are

also present.

4.    Connective tissue covers and supports each muscle fiber and

reinforces the muscle as a whole.

5.    The health of muscle depends on a sufficient nerve and blood

supply. Each skeletal muscle has a nerve ending that controls its

activity (innervation), and an individual system to supply and

drain blood (vascularization).

 

2) Bundles of Muscles

 

6. Muscle Fibers consist of bundles of threadlike structures called

myofibrils.

7.    Each myofibril is made up of two types of protein filaments-

Thick ones and Thin ones.

8. The thick filaments are made up of a protein called myosin.

9. The thin filaments are made of a protein called actin.

10. Myosin and Actin filaments are arranged to form overlapping

patterns, which are responsible for the light and dark bands

(striations) that can be seen in skeletal muscle.

11. Thin actin filaments are anchored at their midpoints to a

structure called the z-line.

12. The region from one z-line to the next is called a sarcomere,

which is the functional unit of muscle contractions.

 

IV. How Muscles and Bones Interact

 

1.     Skeleton muscles generate force and produce movement only by

contracting or pulling on body parts.

2. Individual muscles can only pull; they cannot push.

3. Skeleton muscles are joined to bone by tough connective tissue

called tendons.

4.    Tendons attach muscle to bone; the origin is the more stationary

bone, the insertion is the more movable bone.

5.    Tendons are attached in such a way that they pull on the bones

and make them work like levers. The movements of the muscles

and joints enable the bones to act as levers.

6. Most skeletal muscles work in pairs.

7. When one muscle or set of muscles contracts, the other relaxes.

8. The muscles of the upper arm are a good example of this dual

action, a flexor, is a muscle that bends a joint, while an extensor

is a muscle that straightens a joint.

a.     when the biceps muscle (on the front of the upper arm,

flexor) contracts, it bends or flexes the elbow joint.

b.     when the triceps muscle (on the back of the upper arm,

extensor) contracts, it opens, or extends, the elbow joint.

c. a controlled movement requires contraction by both

muscles.

E. Names of Skeletal Muscles

Individual muscles are named on the basis of several criteria, each of which focuses on a particular structural or functional characteristic. Paying close attention to these cues can greatly simplify your task of learning muscle names and actions:

The rectus femoris is a straight muscle of the thigh. The external oblique is a muscle slanted across the abdomen.

The gluteus maximus is the largest muscle of the gluteus muscle group.

The temporalis and frontalis muscles overlie the temporal and frontal bones of the skull.

The biceps, triceps, and quadriceps have two, three, and four origins respectively.

The sternocleidomastoid muscle has its origin on the sternum (sterno) and clavicle (cleido) and inserts on the mastoid process of the temporal bone.

The deltoid muscle is roughly triangular. (Deltoid means "triangular".)

 

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