Chapter 9: Muscular system

Important markings

A number of other markings are important to note. A narrow, dark staining band found in the central region of the I band that looks like a series of the letters Z one on top of another is called the Z line. A slightly darker section in the middle of the dark A band is called the H band or H zone. This is where the myosin filaments are thickest and where there are no cross-bridges on the myosin filaments. The area between two adjacent Z lines is called a sarcomere (SAHR-koh-meer). It is here at the molecular level that the actual process of contraction occurs via chemical interactions, which is discussed later.

Release Acetylcholine

As the nerve impulse reaches the neuromuscular junction where the axon terminals of the nerve cell are in close proximity to the muscle and its numerous cells, it triggers the axon terminals to release a neurotransmitter substance called acetylcholine (ah-seh-till-KOH-leen). This chemical substance affects the muscle cell membrane. It causes the sodium ions (which were kept outside during the resting potential) to rush inside the muscle cell

Studying the Anatomy

As we study the figure, we see that the entire muscle consists of a number of skeletal muscle bundles called fasciculi. Each individual bundle of muscle cells, or fascicle, is surrounded by another layer of connective tissue called the perimysium. This is visible to the naked eye. This perimysium connects with the coarse irregular connective tissue that surrounds the whole muscle called the epimysium. These three layers of connective tissue act like cement holding all of the muscle cells and their bundles together. In addition, a layer of areolar tissue covers the whole muscle trunk on top of the epimysium and is called the fascia

Using your Muscles

As you read this introduction, skeletal muscles are moving your eyes to read the words. Muscles allowed you to first pick up this book and open it to the correct page. You walked to your desk, and you took this book off a shelf. All of these actions allowed you to function in your environment. In addition, smooth muscle is containing the blood in your arteries and veins, food is being pushed through your digestive tract and urine is being transported from your kidneys via the ureters to your bladder. Meanwhile, cardiac muscle is pumping the blood, carrying oxygen and nutrients to your body cells and carrying away waste.

Sarcotubular System

Electron microscopy has also revealed the fact that muscle fibrils (thousands of tiny units that make up a muscle cell) are surrounded by structures made up of membranes in the form of vesicles and tubules. These structures constitute what is referred to as the sarcotubular (sahr-koh-TYOO-byoo-lar) system. The sarcotubular system is made up of two components: 1. the T system or tubules 2. the sarcoplasmic reticulum (reh-TIK-you-lum).

Because their length is greater than their width, skeletal muscle cells are referred to as muscle _____.

Fibers

Types of Muscles

From the discussion of tissues in Chapter 5, you recall that there are three types of muscle tissue: skeletal or striated, smooth or visceral and cardiac. Recall that skeletal muscle is voluntary, that is, we can control its contraction. Under the microscope, skeletal muscle cells are multinucleated and striated; we can see alternating dark and light bands. Smooth muscle, on the other hand, is involuntary, uninucleated and nonstriated. It is found in places like the digestive tract. Cardiac muscle is involuntary, striated and uninucleated and is found only in the heart.

There are no cross-bridges in the _____ _____.

H zone

Abductor

If a limb is moved away from the midline

Myofibrils

If you put a single thread under a microscope, you would see that it was made up of hundreds of smaller units of fiber. Hence, just like the thread, the muscle cell or fiber is made up of thousands of smaller units called myofibrils. At the molecular level, each myofibril is made up of microscopic filaments of the proteins myosin (which is thick and looks dark under the microscope) and actin (which is thin and looks light under the microscope).

Another Source of Energy

In addition, skeletal muscle cells can take up free fatty acids from the blood and break them down as another source of energy into CO2, H2O and ATP. Of course, during any contraction, heat is produced as a waste product

Glycolysis

In glycolysis, you will recall from Chapter 4, glucose present in the blood enters cells where it is broken down through a series of chemical reactions to pyruvic acid. A small amount of energy is released from the glucose molecule with a net gain of two molecules of ATP

Four Sources of ATP

In summary, muscle cells have four sources of ATP for the energy of contraction: 1.Glucose + 2 ATP--> CO2 + H2O + 38 ATP (aerobic) 2.Glucose + 2 ATP --> 2 lactic acid + 2 ATP (anaerobic) 3.Phosphocreatine + ADP--> creatine + ATP 4.Free fatty acids--> CO2 + H2O + ATP

Krebs Citric Acid Cycle and Electron Transport

In the Krebs citric acid cycle and electron transport, if oxygen is present, the pyruvic acid is further broken down into CO2 and H2O and 36 more ATP molecules. If oxygen is not available to the muscle cell, the pyruvic acid changes to lactic acid and builds up in the muscle cell with only two ATP produced until oxygen again becomes available

Thousands of Myofilaments

Keep in mind that we discussed only one small part of a muscle cell's filaments. There are thousands of myofilaments in a single muscle cell, and a muscle like your biceps contains hundreds of thousands of muscle cells, all interacting and coordinating together at the molecular level to bring about contraction

Skeletal Muscle Cells

Mature skeletal or striated muscle cells are the longest and most slender muscle fibers, ranging in size from 1 to 50 mm in length and 40 to 50 micrometers in diameter. Because of this unique structure of the cell, that is, their length being much greater than their width, skeletal muscle cells are often referred to as skeletal muscle fibers

Skeletal muscle is striated and _____.

Multinucleated

Source of Energy (Adenosine triphosphate [ATP] molecule)

Muscle cells convert chemical energy (ATP) into mechanical energy (contraction). This source of energy is ATP molecules (review Chapter 4). Actin + myosin + ATP-›actomyosin + ADP + PO4 + energy (causing contraction). The energy given off by the breakdown of ATP is used when the actin and myosin filaments intermesh. ATP is synthesized by glycolysis, the Krebs citric acid cycle, electron transport and, in muscle cells, by the breakdown of phosphocreatine

What does potassium have to do with muscle contraction?

Muscle cells need potassium to maintain the electrical potential or difference in charge from inside to outside

Controlling Fine Movements

Muscles controlling very fine movements (like muscles that move the eye) are characterized by the presence of only a few muscle fibers in each motor unit. Another way to state this would be the ratio of nerve fibers to muscle cells is high. For example, each motor unit present in the ocular muscle contains about 10 muscle cells. However, gross movements (like lifting an object with your hand) will contain a motor unit with 200 or more muscle cells. On the average, a single motor nerve fiber innervates about 150 muscle cells

Functions

Muscles make up about 40% to 50% of the body's weight. They allow us to perform extraordinary physical feats of endurance (running, playing sports) and grace (ballet, figure skating). When they contract, they bring about movement of the body as a whole and cause our internal organs to function properly. Muscles of the diaphragm, chest and abdomen allow us to breathe.

Flexors

Muscles that bend a limb at a joint

Extensors

Muscles that straighten a limb at a joint

_____ is muscle pain

Myalgia

Rotators

The muscles rotating an involved limb

Release of the Calcium Ions

The release of the calcium ions from the sarcoplasmic reticulum inhibits the activity of the troponin and the tropomyosin, which have kept the actin and myosin myofilaments apart. The calcium ions attach to the troponin and now cause the myosin to become activated myosin. The myosin filaments have large heads that contain ATP molecules. The activated myosin releases the energy from the ATP at the actin active site when the myosin links up and forms actomyosin. The head linkage makes a cross-bridge that pulls the actin filaments inward among the myosin filaments and breaks down the ATP into adenosine diphosphate (ADP) and PO4 and the release of energy, which causes contraction

Shortening of the Contractile Elements

The shortening of the contractile elements in the muscle is brought about by the pulling of the actin filaments over the myosin filaments. The width of the A bands remains constant while the Z lines move closer together during contraction.

T System

The tubules of the T system are continuous with the cell membrane or sarcolemma of the muscle fiber and form a grid perforated by individual muscle fibrils. The sarcoplasmic reticulum forms an irregular curtain around each of the fibrils. Again refer to the figure on the right for these complex structures. This T system functions in the rapid transmission of a nerve impulse at the cell membrane to all the thousands of fibrils that make up the muscle cell. A muscle cell could be thought of as a single thread of cloth.

Resting Potential

These ions are all positively charged. Because of an uneven distribution of these ions, there is an electrical distribution around the muscle cell. The inside of the cell is negatively charged and the outside of the cell is positively charged

Resulting Contractile Process

These substances keep the actin and myosin protein filaments from interacting. However, when calcium ions are released by the sarcoplasmic reticulum, the action of these inhibitor substances is negated. It is the release of the calcium ions that brings about the contractile process at the molecular level in the myofilaments. When the action potential ceases to stimulate the release of the calcium ions from the reticulum, these ions begin to return and recombine with the sarcoplasmic reticulum.

Release of Stored Calcium ions

This action potential not only travels over the surface of the muscle cell membrane but passes down into the cell by way of the T tubules and also deep into all the cells that make up the muscle. This action potential causes the sarcoplasmic reticulum to release stored calcium ions into the fluids surrounding the myofibrils of the muscle cell. Surrounding the actin myofilaments are two inhibitor substances: troponin and tropomyosin

Electrical Potential

This rapid influx of sodium ions creates an electrical potential that travels in both directions along the muscle cell at a rate of 5 meters per second. This influx of Na1 causes the inside of the cell to go from being electrically negative to being positive. This is a signal to the muscle cell to generate its own impulse called the action potential. This is the signal to contract. Meanwhile the potassium ions that were kept inside begin to move to the outside to restore the resting potential, but they cannot change back to the resting potential situation because so many sodium ions are rushing in.

Sodium-Potassium Pump of the Cell Membrane

What causes this to happen is the sodium-potassium pump of the muscle cell membrane. As the sodium ions rushed into the cell and potassium rushed out to try to restore the original resting potential but could not do so, the sodium-potassium pump began operating to restore the ionic distribution to its normal resting potential. Contraction occurs in a few thousandths of a second and once the sodium-potassium pump restores ionic distribution, contraction ceases because the action potential is now stopped and all the calcium ions are once again bound to the reticulum. A continued series of action potentials is necessary to provide enough calcium ions to maintain a continued contraction. Now let's discuss the chemical interactions and those calcium ions

Under a Microscope

When skeletal muscle is viewed under a microscope, the cells appear to have alternating dark and light bands referred to as cross-striations. The striations are due to an overlapping of the dark and light bands of protein on the myofibrils. The dark bands are made of the thick filaments of the protein myosin. Being thick, they therefore appear dark and are called the A bands (hint to remember: the second letter in the word dark is A). The light bands are made of the thin filaments of the protein actin; being thin, they appear light and are called the I bands (hint to remember: the second letter in the word light is I).

Muscle twitch

When the contraction of a skeletal muscle is studied in the laboratory by applying an electrical charge to the muscle, the analysis of the contraction

Restoring Resting Potential

When the sodium-potassium pump has restored the resting potential of the cell and sodium ions are back outside and potassium ions are back inside the cell, the action potential ceases and calcium ions get reabsorbed by the sarcoplasmic reticulum. Now contraction ceases and the actin filaments get released from the myosin and the Z lines move further apart. This whole complex process occurs in 1/40 of a second

All-or-none law

a contraction or nervous transmission either occurs or does not occur

Tone

a property of muscle whereby a steady or constant state of partial contraction is maintained in a muscle

Motor unit

all of the muscle cells or fibers innervated by one motor neuron. It is important to remember that the terminal divisions or axon endings of a motor neuron are distributed throughout the belly of the whole muscle. Stimulation of a single motor unit causes weak but steady contractions in a broad area of the muscle rather than a strong contraction at one tiny specific point

The concept of a stimulus being strong enough to elicit a maximal contraction of an individual muscle cell, or no contraction at all, is known as (the) ______.

all-or-none law

Tropomyosin

an inhibitor substance found in muscle cells

Troponin

an inhibitor substance found in muscle cells

Sarcoplasmic reticulum

an irregular curtain around muscle fibrils

A decrease in muscle bulk due to a lack of exercise, as when a limb is in a cast for a prolonged period, is known as ____.

atrophy

Cardiac and smooth muscle is under the control of the _____ nervous system

autonomic

Electrical potential

caused by a rapid influx of sodium ions into a muscle cell

Epimysium

coarse, irregular connective tissue that surrounds the whole muscle

Isometric activity

contraction in which a muscle remains at a constant length while tension against the muscle increases

Isotonic contraction

contraction in which tone or tension remains the same as the muscle becomes shorter and thicker. When lifting a weight muscles become shorter and thicker

A condition, commonly occurring in individuals who are bedridden, in which a muscle shortens its length in the resting state is known as ____.

contracture

Spastic and painful contractions of muscles that occur because of an irritation within the muscle are known as ____.

cramps

A bands are the _____ bands and I bands are the _____ bands

dark, light

Fibrillation of cardiac muscle can result in _____.

death

Endomysium

delicate connective tissue that surrounds the sarcolemma of a muscle cell

Muscle cells possess four properties:

excitability, conductivity, contractility and elasticity. Muscle fibers can be excited by a stimulus. In our bodies this stimulus is a nerve cell. In the laboratory, we can stimulate and excite a muscle with an electrical charge. Besides the property of excitability, all protoplasm in the cell possesses the property of conductivity, which allows a response to travel throughout the cell. The type of response will depend on the type of tissue that is excited. In muscle cells, the response is a contraction. Elasticity then allows the muscle cell to return to its original shape after contraction. Muscle contraction is caused by the interactions of three factors: neuroelectrical factors, chemical interactions and energy sources.

The rapid, uncontrolled contraction of individual cells in the heart, resulting in the heart's inability to pump blood properly is known as ____.

fibrillation

Smooth muscle contraction occurs without the regular rearrangement of _____.

fibrils

Smooth muscle

found in hollow structures of the body like the intestines, blood vessels and urinary bladder. It cannot be controlled at will because it is under the control of the autonomic nervous system and also may be hormonally stimulated

Phosphocreatine

found in muscle tissue; provides a rapid source of high-energy ATP for muscle contraction. Muscle cells have two additional sources of ATP. Phosphocreatine is found only in muscle tissue and provides a rapid source of high-energy ATP for muscle contraction. When muscles are at rest, excess ATP is not needed for contraction so phosphate is transferred to creatine to build up a reserve of phosphocreatine. During strenuous exercise, the phosphocreatine takes up ADP to release ATP and creatine, thus supplying the muscle with an additional supply of ATP. The overall reaction, which goes in both directions is phosphocreatine + ADP ‹-›creatine + ATP.

An increase in the size of a muscle caused by an increase in the bulk of muscle cells through exercise is known as _____.

hypertrophy

Adductor

if the limb is brought in toward the midline

Fascicle

individual bundle of muscle cells

Fascia

layer of areolar tissue covering the whole muscle trunk

Perimysium

layer of connective tissue surrounding the fascicle of a muscle

I bands

light, thin bands of the protein actin in muscle cells

Sarcotubular system

membranes of vesicles and tubules that surround muscle fibrils

Skeletal muscle

muscle attached to bone

Cardiac muscle

muscle found only in the heart

When the contraction of a skeletal muscle is studied in the laboratory by applying an electrical charge to the muscle, the analysis of the contraction is called a ____.

muscle twitch

In movements of the ankle

muscles of dorsiflexion turn the foot upward, and muscles of plantar flexion bring the foot toward the ground

Agonists (prime movers)

muscles performing the actual movement

Synergists

muscles that assist the prime movers

Antagonists

muscles that relax while the agonist contracts

An inherited muscular disorder, occurring most often in males, in which the muscle tissue degenerates over time, resulting in complete helplessness is known as ____.

muscular dystrophy

Myofibrils have dark bands, known as A bands, composed of the protein _____.

myosin

Z line

narrow, dark-staining band found in the central region of the I band

Acetylcholine

neurotransmitter substance

Resting potential

normal electrical distribution around a muscle cell when it is not contracting

The all-or-none law states that a muscle contraction either _____ or it _____.

occurs, does not

The three types of ions involved in nerve impulses are _____ (K), _____(Na) and _____ (Cl).

potassium, sodium, chloride

Levators

raise a part of the body

Fibrillation

rapid, uncontrolled contraction of individual cells in the heart. This results in the heart's inability to pump the blood properly and can result in death

The actual process of contraction occurs in the area called the _____.

sarcomere

An irregular curtain around each of the fibrils is the _____ _____.

sarcoplasmic reticulum

Fasciculi

skeletal muscle bundles

H band or zone

slightly darker section in the middle of the dark A band; also called H zone

The muscle that is nonstriated and found in hollow structures is _____.

smooth

Sarcomere

the area between two adjacent Z lines in a muscle cell

A bands

the dark, thick bands of the protein myosin in muscle cells

Sarcolemma

the electrically polarized muscle cell membrane. In addition, each muscle cell or fiber is multinucleated and is surrounded by a special cell membrane. This cell membrane is electrically polarized and is called a sarcolemma.

Origin

the more fixed attachment of a muscle that serves as a basis for the action

Insertion

the movable attachment where the effects of contraction are seen

Myosin

thick filaments of protein in a muscle cell

Actin

thin filaments of protein in a muscle cell

A property of muscle whereby a steady or constant state of partial contraction is maintained is known as ____.

tone

Muscle fibrils are surrounded by membranes in the form of vesicles and _____.

tubules

T system or tubules

tubules; part of the sarcotubular system

In movements of the hand

turning the forearm when it is extended out so that the palm of the hand faces the ground is pronation, whereas turning the forearm so that the palm faces upward is supination

Action potential

when a muscle generates its own impulse to contract

Aponeurosis

wide and flat tendon