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FUNCTIONS OF THE MUSCULAR SYSTEM 1. Movement. Muscles are responsible for the movements that occur in the body (except for

cilia, flagella, and gravity). This includes movement of the body, respiration, speech, facial expression, and the movement of materials within hollow organs, such as food in the digestive tract and blood through the heart and blood vessels.

2. Posture. Muscles are necessary for the maintenance of posture. 3. Protection. Muscles provide a protective function, e.g., the abdominal muscles protect the

abdominal organs. 4. Heat production. Muscles comprise over 40% of the mass of the body and they are

metabolically active tissues. As a by-product of metabolism heat is produced. This heat is necessary for maintaining normal body temperature.

GENERAL FUNCTIONAL CHARACTERISTICS OF MUSCLE Properties of Muscle 1. Contractility. The ability to shorten with a force results from activities inside muscles.

Muscles only lengthen passively when an a force outside of the muscle pulls on the muscle. For example, gravity, or pull produced by another muscle.

2. Excitability. Muscle contracts (or relaxes) in response to nervous stimulation or hormonal

stimulation. Thus the activities of muscle can be controlled. 3. Extensibility. Muscle can be stretched. After contracting (shortening) muscle can be

stretched back to its original length. It can then contract again. 4. Elasticity. If stretched beyond its resting (noncontracted) length, muscle can recoil and

return to its original length. Types of Muscle Tissue TABLE 4.14 (p. 128) 1. Skeletal muscle.

A. Skeletal muscles attach to bones and are responsible for body movements. B. Skeletal muscle cells are long and cylindrical, multinucleated, and have striations or

bands. As we shall see, the striations are involved with contraction of the muscle cell. Skeletal muscle cells develop from myoblasts, which are multinucleated cells formed by the fusion of single nucleated cells during development.

C. Skeletal muscles are under voluntary nervous control and they do not normally contract

spontaneously.

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2. Cardiac muscle. A. Cardiac muscle is found in the heart and is responsible for the movement of blood

through the circulatory system. B. Cardiac muscle cells are short (about 10x shorter than skeletal muscle), cylindrical and

branched; have a single centrally located nucleus; and have striations. The cells are interconnected by specialized gap junctions (electrical synapses) called intercalated disks, which function to ensure that all of the muscle cells in the heart contract at about the same time.

C. Cardiac muscle is under involuntary control and does contract spontaneously. If the heart

is provided with the appropriate gases and nutrients it can beat even after being removed from the body. Example: when a heart is taken from a donor, all nerve supply to the heart is cut. When the donated heart is placed in a recipient, no new nerve supply is established.

3. Smooth muscle.

A. Smooth muscle has a variety of functions such as controlling movement of materials through hollow organs (e.g., digestive tract, urinary tract, blood vessels, gland ducts), regulates the size of the pupil of the eye, controls the shape of the lens of the eye, and causes hair to stand on end (arrector pili).

B. Smooth muscle cells are small (about 7x shorter than cardiac muscle), spindle-shaped

cells with a single, centrally located nucleus. They are not striated C. Smooth muscle is under involuntary control and is capable of spontaneous contractions.

SKELETAL MUSCLE STRUCTURE Connective Tissue FIGURE 9.2 1. Whole muscles are made-up of muscle bundles or fasciculi. The muscle bundles are made

up of muscle fibers (muscle cells). The muscle fibers can extend the length of a small muscle. Usually several muscle fibers join together to reach from one end of the muscle to the other end.

2. Surrounding connective tissue holds the muscle fibers together and attaches them to bone by

tendons. The connective tissue also acts as bridgeway for blood vessels and nerve cells to reach the muscle fiber. A. The cell membrane or sarcolemma of a muscle fiber is covered by a delicate layer of

reticular fibers called the external lamina. The endomysium (G. within muscle) covers the external lamina and sarcolemma.

B. A bundle of muscle fibers (fasciculus) is surrounded by the perimysium (G. around

muscle). C. Groups of muscle bundles makeup the entire muscle, which is bounded by the

epimysium (G. upon muscle).

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D. Fascia is a sheet of connective tissue found under the skin, around individual muscles or groups of muscles. Around a single, muscle, fascia is the same thing as the epimysium.

Nerve and Blood Vessels 1. Motor neurons are nerve cells that supply muscles. The cell bodies of motor neurons are in

the brain or spinal cord and their axons extend to muscles through nerves. 2. Motor neuron axons and blood vessels reach muscle fibers by passing through the connective

tissue layers of the whole muscle. Muscle Fibers FIGURE 9.3 1. Muscle fibers are multinucleated with cytoplasm, called sarcoplasm, which consists of

mitochondria and other cell organelles. 2. Most of the inside of the muscle fiber is filled with myofibrils, which are threadlike

structures that extend from one end of the muscle fiber to the other end. 3. Myofibrils are made-up of two kinds of protein fibers called actin myofilaments and

myosin myofilaments.

4. The myofilaments are arranged into units called sarcomeres, which are joined end to end.

Actin and Myosin Myofilaments FIGURE 9.4 1. Actin myofilament.

A. G-actin (globular actin) units join together to form a strand of F-actin (fibrous actin). Analogy: like a string of pearls.

B. Each actin myofilament has two strands of F-actin. Analogy: like two strings of pearls

twisted around each other. C. In addition, each actin myofilament has tropomyosin and troponin molecules.

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D. Functional relationships. 1) Each G-actin molecule has an active site where it can bind to myosin during

contraction. 2) Tropomyosin molecules cover the myosin active sites on G-actin. 3) Troponin has three subunits: one subunit binds to G-actin, one subunit binds to

tropomyosin and one subunit binds to calcium ions (Ca2+). As we shall see, the interactions of these molecules are involved in contraction.

2. Myosin myofilament.

A. A myosin myofilament is made up of many myosin molecules. Each myosin molecule consists of a rod portion and a globular head.

B. The myosin myofilaments are arranged so that the myosin heads are at either end of the

myofilament and the rods are in the center of the myofilament. The myosin heads can combine to actin.

C. The myosin head.

1) The myosin head has a protein that can combine with the active site on G-actin. The combination of actin and myosin is called a cross-bridge.

2) The myosin head contains ATPase, an enzyme that breaks down ATP to ADP with a

release of energy. The energy is used to drive the contraction process. 3) The myosin head is connected to the rod portion by a hingelike area that moves

during contraction. Sarcomeres 1. The myofilaments within a sarcomere are highly ordered. FIGURE 9.5

A. Z disk - sarcomeres extend from one Z disk to the next Z disk. The Z disks provide an attachment site for the actin myofilaments.

B. A band - the region formed by myosin myofilaments. It extends from end to end of the

same myosin myofilament. C. I band - the region between A bands. It includes the Z disk. It extends from end to end

of two different (adjacent) myosin myofilaments. D. H zone - the region in-between the ends of the actin myofilaments. E. M line - holds the myosin myofilaments in position. F. Other proteins also help to hold actin and myosin in place. Titin, one of the largest know

proteins, extends from Z disk to M line and holds myosin in place (see figure 9.3). Titin also has a springlike part [within the I bands] that allows muscle to stretch (extensibility) and recoil (elasticity).

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2. The arrangement of myofilaments produces a striated (banded) appearance in the myofibril, and therefore in the muscle fiber.

Sliding Filament Model FIGURE 9.6 1. Movement of the cross-bridges (more later) causes the sarcomere to shorten (contract).

A. As the cross-bridges move, the actin myofilaments slide past the myosin myofilaments. This is called the sliding filament model.

B. Note that the actin and myosin myofilaments do not change length. Therefore the A

band stays the same size during contraction. C. Because actin is attached to the Z disk, as actin slides past myosin, the Z disks are pulled

closer together. This causes the I band and the H zone to decrease in size, and the sarcomeres shorten.

2. As the sarcomeres shorten, the myofibrils shorten. 3. As the myofibrils shorten, the muscle fibers shorten. 4. As the muscle fibers shorten, the muscle bundles shorten. 5. As the muscle bundles shorten, the entire muscle shortens. PHYSIOLOGY OF SKELETAL MUSCLE FIBERS The nervous system controls the contraction of skeletal muscles by sending nerve impulses, called action potentials, along the axons of motor neurons. Membrane Potentials 1. Before an action potential can be generated, the plasma membrane must be polarized, which

means there is a charge difference across the membrane. 2. The charge difference across the plasma membrane of an unstimulated cell is called the

resting membrane potential. FIGURE 9.7

A. The resting membrane potential mainly results from the movement of potassium ions (K+) from inside the cell to the outside of the plasma membrane. That is, a slight accumulation of K+ on the outside of the plasma membrane makes it positively charged compared to the inside. Chapter 11 explains the development of the resting membrane potential.

B. The resting membrane potential is measured in units called millivolts. The resting

membrane potential is a negative number, indicating the inner surface of the plasma membrane is negative compared to the outside.

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Ion Channels 1. An action potential is the reversal of the resting membrane potential: the inside of the

plasma membrane momentarily becomes positive compared to the outside. 2. The movement of ions across the plasma membrane is responsible for the action potential.

The ions move through ion channels. 3. Ligand-gated ion channels. FIGURE 3.8 (p. 63)

A. A ligand is a molecule that binds to a receptor. A receptor is a protein or glycoprotein that has a receptor site to which a ligand can bind.

B. Ligand-gated ion channels open when a ligand binds to the receptor. C. A ligand released from the axon of a nerve cells is called a neurotransmitter. The

axons of motor neurons release a neurotransmitter called acetylcholine that binds to the receptors on ligand-gated Na+ channels in the plasma membrane of skeletal muscle cells. As a result the Na+ channels open and Na+ move into the skeletal muscle cells.

4. Voltage-gated ion channels open and close in response to small voltage changes across the

plasma membrane. Stimulation of the plasma membrane can produce these small voltage changes.

5. Ligand-gated and voltage-gated ion channels are specific for the ions that pass through them. For example, there are Na+, K+, and Ca2+ channels.

6. When an ion channel opens, ions move down their concentration gradient.

A. There is a higher concentration of Na+ and Ca2+ outside cells than inside. B. There is a higher concentration of K+ inside than outside cells.

Action Potentials 1. Stimulation of a plasma membrane can result in depolarization, which means the inside of

the plasma membrane becomes less negative (more positive). Note that this is a small change in the voltage.

FIGURE 9.8

A. If the membrane potential reaches a value called threshold, the depolarization phase of the action potential occurs. In the depolarization phase, the charge difference across the plasma membrane reverses.

B. Repolarization is the return of the membrane potential to its resting levels.

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2. Depolarization and repolarization result from the opening and closing of ion channels and the movement of ions.

FIGURE 9.9

A. In depolarization, voltage-gated Na+ channels open. Na+ move into the cell making the inside of the plasma membrane less negative and eventually positive.

B. Repolarization.

1) Na+ channels close, and Na+ movement into the cell stops. 2) K+ channels open, and K+ moves out of the cell. The movement of K+ out of the cell

makes the inside of the plasma membrane more negative and the outside more positive.

3) Repolarization ends when the K+ channels close.

3. Action potentials occur according to the all-or-none principle. A. If a stimulus is strong enough to cause a depolarization to reach threshold, all the voltage-

gated Na+ channels open and the depolarization phase occurs. An even stronger stimulus produces the same result, because all the Na+ channels open. This is the all part.

B. If a stimulus is not strong enough to cause a depolarization to reach threshold, then no

action potential is produced because all of the Na+ voltage-gated channels do not open. This is the none part.

C. Analogy: Depress the shutter on a camera until it clicks (=threshold), and the flash bulb

goes off. Whether you click the shutter gently or very forcefully, the flash bulb goes off exactly the same. If you depress the shutter but it does not click, then the flash bulb does not go off.

4. Action potentials propagate, or spread across the plasma membrane. FIGURE 9.10

A. The production of an action potential at one place on the plasma membrane is a change in voltage. The change in voltage stimulates voltage-gated Na+ channels in an adjacent area to open and produce an action potential.

B. The propagation of the action potential is like toppling a row of dominos. Each domino

falls (an action potential is produce at that spot on the plasma membrane), but no one domino actually travels the length of the row of dominos.

5. Action potential frequency is the number of action potentials produced per unit of time.

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6. The big picture. A. The resting membrane potentials is a charge difference across the plasma membrane. It

sets the stage. B. An action potential is a reversal of the resting membrane charge that acts as a signal to

the cell. In response to the signal, the cells does something. For example, a muscle cell contracts.

C. The nervous system controls the contraction of skeletal muscles by producing action

potentials that propagate through nerves toward skeletal muscle fibers. D. As the frequency of action potentials in skeletal muscles increases, the force of

contraction increases. Neuromuscular Junction 1. The neuromuscular junction or synapse is where branches of the motor neuron axon and

the sarcolemma of a muscle fiber come together. FIGURE 9.11

A. Presynaptic terminal - the end of an axon branch. B. Postsynaptic membrane - the part of the sarcolemma that forms the synapse. C. Synaptic cleft - the space in-between the presynaptic terminal and the postsynaptic

membrane.

2. Transfer of an action potential from the motor neuron to the muscle fiber. FIGURE 9.12

A. Action potentials reach the presynaptic terminal, causing voltage-gated Ca2+ channels to open. Ca2+ diffuse into the neuron and stimulate the release of acetylcholine from synaptic vesicles by exocytosis.

B Acetylcholine is an example of a neurotransmitter, a substance that is released from a

neuron and functions to stimulate (or inhibit) the production of an action potential in the postsynaptic membrane.

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C. Acetylcholine diffuses across the synaptic cleft. D. Acetylcholine binds to receptors in the postsynaptic membrane, causing ligand-gated Na+

channels to open. 1) As a result of ligand-gated Na+ channels opening, membrane permeability to Na+

increases and the membrane depolarizes. 2) Depolarization causes voltage-gated Na+ channels to open, which causes further

depolarization. 3) When the membrane depolarizes to threshold, voltage-gated Na+ channels open and

an action potential is produced in the sarcolemma. The action potential is a signal that results in muscle contraction (more later).

E. Acetylcholinesterase breaks acetylcholine into acetic acid and choline. Choline is

actively reabsorbed by the neuron and used to produce new acetylcholine. Acetic acid can be taken up by a variety of cells that use it for a source of energy.

Why is it necessary to break down acetylcholine?

A Neuromuscular Junction Problem 1. Spastic paralysis is an inability to move because muscles are contracted and will not relax.

It is caused by too many action potentials being produced in skeletal muscle. 2. Flaccid paralysis is an inability to move because muscles do not contract. It is caused by

too few action potentials being produced in skeletal muscle. ☞ Complete the following table:

Action Type of Paralysis Organophosphate Inhibits acetylcholinesterase Curare Prevents acetylcholine from binding to acetylcholine receptors

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In myasthenia gravis the immune system destroys acetylcholine receptors in the neuromuscular junction. What symptoms would you expect to observe? Would you administer an organophosphatelike drug or a curarelike drug to reverse the symptoms? Explain.

Excitation-Contraction Coupling 1. Excitation (production of an action potential) in a skeletal muscle fiber results in

contraction (shortening) of the muscle fiber. 2. For excitation-contraction coupling to occur, the action potential produced in the sarcolemma

(plasma membrane of a skeletal muscle fiber) at the neuromuscular junction has to result in events that cause contraction. The first step in this process is the propagation of action potentials to the inside of muscle fibers by T tubules.

FIGURE 9.13

A. The sarcolemma (muscle fiber membrane) has many tubelike invaginations called transverse or T tubules. The T tubules wrap around the myofibrils.

B. The smooth endoplasmic reticulum of skeletal muscle is called the sarcoplasmic

reticulum. The membrane of the sarcoplasmic reticulum is specialized to move Ca2+ into the sarcoplasmic reticulum by active transport. Thus, there is a higher concentration of Ca2+ inside the sarcoplasmic reticulum than outside.

C. On either side of the T tubule the sarcoplasmic reticulum enlarges to form a terminal

cisterna. A triad is two terminal cisternae and a T tubule.

3. Action potentials at the neuromuscular junction cause the release of Ca2+ from the sarcoplasmic reticulum.

FIGURE 9.14

Draw a neuromuscular junction on this figure.

A. An action potential is produced in the postsynaptic membrane of the neuromuscular junction.

B. The action potential propagates across the sarcolemma and down into the T tubules.

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C. At the triad, the action potential causes gated Ca2+ channels in the sarcoplasmic reticulum to open.

D. Ca2+ diffuse from the sarcoplasmic reticulum into myofibrils to actin myofilaments.

4. Ca2+ initiated contraction. A. In the resting muscle, tropomyosin is between the active myosin site of G-actin and the

head of the myosin molecule. B. When Ca2+ bind to troponin, the troponin-tropomyosin complex moves, exposing myosin

active sites on G-actin. C. The myosin head binds to actin to form a cross-bridge. D. Movement of the cross-bridges causes the actin to slide past the myosin, resulting in

contraction.

Energy Requirements for Muscle Contraction FIGURE 9.15 1. Exposure of active sites. After Ca2+ bind to troponin, tropomyosin moves, exposing active

sites to which myosin can bind. A. Note that the myosin head has an ADP and a phosphate attached to it. That is, an ATP

associated with the myosin head has already been broken down to ADP and a phosphate. The energy released has been stored in the myosin head.

B. Analogy: The myosin head is like a mouse trap. Energy has been put into the system

and it is “cocked” or armed. 2. Cross-bridge formation. Myosin binds to actin, forming a cross-bridge. The phosphate is

released when the cross-bridge forms.

3. The power stroke. The power stroke is cross-bridge movement resulting from the movement of the myosin head at its hingelike area. A. Actin slides past myosin as the myosin head moves. B. The stored energy in the myosin head, derived from the break down of ATP, is

responsible for the movement of the myosin head. C. ADP is released from the myosin head.

4. Cross-bridge release. Another ATP molecule binds to the myosin head, resulting in the

release of the myosin head from actin. 5. The recovery stroke. The recovery stroke is the return of the myosin head to its original

position. A. The ATPase in the myosin head breaks down ATP to ADP, releasing energy that is

stored by the myosin head. B. The myosin head, with ADP and phosphate attached, returns to its original position.

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6. There are two possible sequences of events following cross-bridge release. A. Continued contraction.

1) The neuron continues to stimulate the muscle fiber, producing action potentials in the muscle fiber which cause the release of Ca2+ from the sarcoplasmic reticulum.

2) Ca2+ bind to troponin, the troponin-tropomyosin complex moves, and additional actin

is exposed to myosin. 3) Cross-bridges form and there is further contraction. During a muscle contraction

there are many cross-bridge movements. It is like pulling a rope hand-over-hand in a tug-of-war game.

B. Relaxation.

1) The neuron stops stimulating the muscle fiber, there is no further action potential production in the muscle fiber, and the sarcoplasmic reticulum stops releasing Ca2+. Instead, the sarcoplasmic reticulum, by active transport, takes up and concentrates Ca2+ in the sarcoplasmic reticulum.

2) As Ca2+ levels around the actin myofibril decreases, Ca2+ unbind from troponin, and

the troponin-tropomyosin complex moves in-between actin and myosin. 3) Actin and myosin do not form cross-bridges and actin slides back to its original

position, i.e., the muscle relaxes. Given that cross-bridge movement causes the actin to slide past the myosin during

contraction, what causes the actin to slide in the opposite direction during relaxation?

Key Points 1. Ca2+ start and stop the contraction process within the muscle fiber.

A. When Ca2+ levels around the actin myofilaments increase, the muscle contracts. B. When Ca2+ levels around the actin myofilaments decrease the muscle relaxes.

2. ATP is required in two ways during this process. A. Energy from the break down of ATP is stored in the myosin molecule. ATP provides the

energy for the power stroke and is thus necessary for contraction. B. ATP is required for myosin to separate from actin. ATP is required for the recovery

stroke and is thus necessary for muscle relaxation.

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Suppose that a muscle has an adequate supply of ATP molecules and that the muscle is continuously and rapidly stimulated. How does the muscle respond? Explain.

Suppose that all of the ATP molecules in a skeletal muscle have bound to the heads of the

myosin molecules. If a stimulus is applied to the muscle, what happens? Explain.

PHYSIOLOGY OF SKELETAL MUSCLE Muscle Twitch FIGURE 9.16 1. A muscle twitch is a contraction of a muscle in response to a stimulus that causes an action

potential in one or more muscle fibers. A muscle twitch takes about 1 sec.

A. Lag, or latent, phase. Stimulation does not immediately result in contraction, so there is no change in tension. 1) The nerve supplying the muscle is stimulated but no contraction occurs. 2) When the nerve is stimulated, action potentials are produced in the neurons of the

nerve (see figure 9.9). Gated Na+ channels open and Na+ move into cells causing depolarization. Gated Na+ channels close, gated K+ channels open, and K+ moves out of the cell causing repolarization.

3) The action potentials are propagated along the axons of the nerve to neuromuscular

junctions (see figures 9.10 and 9.11).

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4) The action potential reaches the neuromuscular junction (see figure 9.12). The action potential causes voltage-gated Ca2+ channels to open, Ca2+ diffuse into the presynaptic terminal causing the release of acetylcholine. The acetylcholine diffuses across the synaptic cleft, binds to ligand-gated Na+ channels on the muscle fiber membrane. The ligand-gated Na+ channels open, Na+ diffuse into the cell causing depolarization, which causes voltage-gated Na+ channels to open, which causes further depolarization. When depolarization reaches threshold, an action potential is produced.

5) The action potential propagates along the muscle fiber membrane and down T-tubules

to the sarcoplasmic reticulum (see figure 9.13). 6) The action potential causes gated Ca2+ channels in the sarcoplasmic reticulum to open

(see figure 9.14). Calcium ions diffuse to actin myofilaments and bind to troponin. Troponin moves, causing tropomyosin to move, which exposes active sites on G actin. Myosin heads join G actin to form cross-bridges.

B. Contraction phase. The muscle contracts (shortens or increases tension).

1) When a cross bridge forms, phosphate is released from the myosin head, which changes shape. ADP is released from the myosin head. Cross-bridge movement (power stroke) slides actin past myosin (see figure 9.15).

2) The movement of actin past myosin (sliding filament model) causes sarcomeres to

shorten (see figure 9.6). 3) ATP binds to the myosin head causing cross-bridge release (see figure 9.15). ATP is

broken down, energy is stored in the myosin head, which returns to its original position (recovery stroke).

3) Many cycles of power/recovery strokes occur. Sarcomeres shorten, causing

myofibrils to shorten, causing muscle fibers to shorten, causing muscle bundles to shorten, causing the muscle to shorten (see figure 9.3).

C. Relaxation phase. The muscle relaxes (increases in length or decreases tension).

1) Ca2+ are taken up by active transport by the sarcoplasmic reticulum. 2) Troponin changes shape, causing tropomyosin to move in-between actin and myosin. 3) Passive elongation of the muscle occurs.

Stimulus Strength and Muscle Contraction 1. Stimuli of increasing strength produce different responses in individual muscle fibers, motor

units, and whole muscles. 2. Individual muscle fibers.

A. The all-or-none law of skeletal muscle contraction. If an isolated skeletal muscle fiber is directly stimulated, the fiber either does not contract, or it contracts with the same force (for a given condition) even though stimulus strength increases.

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B. The all-or-none law of skeletal muscle contraction can be explained on the basis of the all-or-none law of action potentials. 1) If there is no action potential in the skeletal muscle fiber, there is no muscle fiber

contraction. 2) If there is an action potential in the skeletal muscle fiber it will be of the same

magnitude (for a given condition) even though stimulus strength increases. The response (force of contraction) of the muscle fiber will be the same, because the action potential causes the release of the same amount of Ca2+ from the sarcoplasmic reticulum.

C. In the human body, muscle fibers are stimulated by neurons. If threshold is reached in the sarcolemma of the muscle fiber, then an action potential is produced and the muscle fiber contracts (excitation-contraction coupling). Thus, muscle fibers are either off or on. However, how rapidly the muscle fibers are stimulated can change (see below).

3. Motor units. FIGURE 9.17

A. A motor unit is a motor neuron and all the skeletal muscle fibers it supplies.

B. A subthreshold stimulus applied to the motor neuron of a motor unit results in no action potential in the motor neuron. Therefore, there is no stimulation of the muscle fibers and no contraction occurs.

Tension

Subthreshold stimulus

Threshold stimulus

Above threshold stimulus

Threshold

Membrane potential of muscle fiber

9-16

C. A threshold stimulus applied to the motor neuron produces an action potential, which propagates to all the neuromuscular junctions of the motor neuron, resulting in action potentials in all of the muscle fibers in the motor unit. As a result, all of the muscle fibers contract, according to the all-or-none law of skeletal muscle contraction.

D. In the human body, motor neurons are stimulated by other neurons in the brain or spinal

cord. Thus, the motor neuron is always off or on. However, how rapidly the motor neurons are stimulated can change (see below).

4. Whole muscles. FIGURE 9.18

A. A whole muscle either does not contract, or it contracts in a graded fashion in response to stimuli of increasing strength, i.e., a weak stimulus produces a weak contraction and a stronger stimulus produces a stronger contraction.

B. Multiple motor unit summation is an increase in the number of motor units recruited

as stimulus strength increases. In an experiment on an isolated muscle, multiple motor unit summation is achieved by increasing the strength of the stimulus (voltage) applied to the nerve supplying the muscle. As voltage increases, more and more motor units are stimulated. 1) Subthreshold stimulus - no motor units stimulated. 2) Threshold stimulus - one motor unit is stimulated. 3) Submaximal stimulus - more and more motor units are recruited as stimulus strength

increases. 4) Maximal stimulus - all the motor units are recruited and the force of contraction of

the muscle is maximal. 5) Supramaximal stimulus - a stronger stimulus than a maximal stimulus, but the force

of contraction is still the same because there are no more motor units to recruit.

C. In the human body, recruitment is achieved by the brain selecting which motor units are activated.

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Stimulus Frequency and Muscle Contraction FIGURE 9.19 1. In response to threshold or stronger stimuli of increasing frequency, individual muscle fibers,

motor units, and whole muscles contract in a graded fashion, i.e., a low frequency of stimulation produces a weak contraction and a higher frequency of stimulation produces a stronger contraction. A. The first action potential in a skeletal muscle fiber stimulates contraction (excitation-

contraction coupling). However, the action potential is completed during the lag period. The action potential sets into motion the activities that later result in contraction.

B. Additional action potentials can cause additional excitation-contraction coupling

activities that are added on to the effects of previous action potentials.

C. Increased levels of Ca2+ around the myofilaments results when the frequency of stimulation is fast enough that Ca2+ are released from the sarcoplasmic reticulum faster than they are taken up by the sarcoplasmic reticulum.

How does the increased levels of Ca2+ result in increased force of contraction?

2. Multiple wave summation is an increase in the force of contraction resulting from an

increase in the frequency of stimulation. A. Incomplete tetanus. There is partial relaxation between contractions. B. Complete tetanus. There is no relaxation between contractions.

3. Multiple wave summation occurs in motor units because they contain skeletal muscle fibers. Multiple wave summation occurs in whole muscles because they contain motor units.

4. In an experiment on an isolated muscle, multiple wave summation is achieved by increasing

the frequency of stimulation. In a human it is achieved by the brain varying the frequency of stimulation of motor units.

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Summary of the Effects of Stimulus Strength and Frequency on Muscle Contraction Muscle

Fiber Motor Unit Whole Muscle Explanation

Response to stimuli of increasing strength

All-or-none

All-or-none

Graded

The all-or-none contractions of muscle fibers and motor units result from action potentials, which are all-or-none. Graded contractions of whole muscles result from recruitment of motor units (multiple motor unit summation).

Response to stimuli of increasing frequency

Graded

Graded

Graded

A second stimulus can cause an already contracted muscle fiber to further contract (multiple wave summation). As a result, the force of contraction of muscle fibers, motor units, and whole muscles increases.

Treppe FIGURE 9.20 1. When a rested muscle is stimulated with a maximal stimulus at a frequency that allows

complete relaxation between stimuli, the second contraction is greater than the first, and the third contraction is greater than the third. After a few contractions, all of the contractions are the same.

2. The increased force of contraction in this situation is called treppe (meaning staircase). 3. Treppe results from a buildup of Ca2+ around the myofibrils as the first few stimuli cause the

release of Ca2+ from the sarcoplasmic reticulum. How might warm-up exercises improve athletic performance?

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Types of Muscle Contraction 1. Isotonic (same tension). The amount of tension generated by the muscle stays the same but

the length of the muscle changes A. In an experimentally isolated muscle, isotonic contractions can be generated. For

example, a weight can be hung from one end of a muscle while it contracts and raises the weight.

B. In the human body, muscle tension can vary somewhat through the range of motion, even

though the same weight is being moved. Nonetheless, it is still referred to as isotonic contraction.

C. There are two types of isotonic contractions.

1) In concentric contractions the length of the muscle shortens. For example, flexing the forearm while holding a weight in the hand.

2) In eccentric contractions the length of the muscle increases while the muscle is

contracting. For example, lowering (extending) the forearm while holding a weight in the hand. Note that eccentric contraction is NOT causing the muscle to get longer. Rather, it is resisting the increase in length. Eccentric contractions are known to produce more damage to muscles, and produce more muscle soreness, than concentric contractions.

Which is most likely to produce muscle soreness: running uphill or running

downhill?

2. Isometric (same length). The length of the muscle does not change but the tension the

muscle generates increases. 3. Most muscle movements are actually a combination of isometric and isotonic contractions. 4. Muscle tone is the constant tension produced by muscles for long periods of time. It results

from a small number of motor units contracting out of phase with each other. Even "relaxed" muscles have muscle tone. Muscle tone results from stimulation of the muscles by the nervous system.

Is the contraction of muscle fibers that produce muscle tone more like isotonic or isometric

contractions? Explain.

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5. A muscle twitch is a contraction of a whole muscle in response to a stimulus that causes an action potential in one or more muscle fibers. A. The muscle fibers contract and then relax. B. A muscle twitch is rare in a normal organism.

6. Most muscle contractions are smooth and steady. A. Through multiple motor unit summation the nervous system can gradually recruit

increasing number of motor units, thus controlling the force and speed of contraction. B. Through multiple wave summation the nervous system can increase the force of

contraction of the muscle fibers in a motor unit until they are in incomplete or complete tetanus. Therefore muscle fibers are not simply contracting once and relaxing as in a muscle twitch.

C. Asynchronous activation of motor units by the nervous system produces an average,

steady force of contraction. 1) As one group of motor units relaxes, another group contracts and maintains the force

of contraction that would have been lost because of relaxation of the first group. 2) Periodic relaxation of motor units helps to prevent muscle fatigue, which allows

muscles to contract for longer periods of time.

Length vs. Tension FIGURE 9.21 1. The length of a muscle before it contracts affects the amount of tension the muscle can

generated. 2. The relationship between initial length and tension can be explained on the basis of the

number of cross-bridges that can be formed during the contraction. A. If the muscle length is too short when the muscle is stimulated, there is reduced force of

contraction because there is too much overlap of the actin and myosin myofilaments, which reduces the ability of the muscle to further shorten. In addition, the elastic elements of the muscle are not as stretched, and some of the force generated by contracting simply shortens the elastic elements.

B. If the muscle is at an optimal length when stimulated, there is optimal overlap of actin

and myosin myofilaments, resulting in maximal force of contraction. Maximal force of contraction is generated when a muscle is stretched to a length approximately 20% greater than its resting length.

C. If the muscle length is too long when the muscle is stimulated, there is too little overlap

of actin and myosin myofilaments and the force of contraction is reduced.

3. Before lifting heavy weights, we often adjust our body position to achieve optimal length of our muscles.

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4. Physical shortening of muscle. Muscle fibers adjust over a period of time so that they can produce the optimal force of contraction. A. If a broken bone heals in a shortened state, the muscle will shorten. B. If a bone is broken and placed in a cast in which the muscle is in a shortened position, the

muscle fibers will adjust and shorten. After removal of the cast the muscle must be stretched back to its original length.

Fatigue 1. Fatigue is the decreased ability of a muscle to do work, i.e., contract. 2. There are three components of fatigue.

A. Psychological fatigue is the most common type of fatigue and involves the central nervous system, i.e., there is a perception that further muscular work is not possible when, in fact, the muscle is capable of further work.

B. Muscular fatigue is the second most common type of fatigue and is due to depletion of

ATP within the muscle. C. Synaptic fatigue is the least common type of fatigue and is due to a depletion of

acetylcholine in the synapse, i.e., the rate of acetylcholine release is faster than the rate of acetylcholine synthesis.

Physiological Contracture and Rigor Mortis 1. Physiological contracture occurs when a muscle is incapable of either contracting or

relaxing. It occurs under conditions of extreme muscular fatigue. Tension Start Muscular Physiological Fatigue contracture

Explain why a lack of ATP in a fatigued muscle could produce physiological contracture.

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2. Rigor mortis is the development of rigid muscles several hours after death. Immediately after death muscles become flaccid. Explain.

Several hours after death the muscles become rigid. Explain (hint: what is the function of

the sarcoplasmic reticulum and what happens to this function when death occurs).

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Energy Sources 1. ATP is the immediate energy source of muscular contraction. It can be produced by aerobic

(with oxygen) or anaerobic (without oxygen) respiration.

Glucose 2 ADP Glycolysis

Anaerobic 2 ATP respiration (2 ATP)

Pyruvic acid Lactic Acetyl acid coenzyme A Aerobic respiration (36 ATP) Citric CO2 acid cycle NADH Electron- O2 transport 34 ATP chain H2O 2. Aerobic respiration is much more efficient than anaerobic respiration and produces more

ATP. 3. Anaerobic respiration can produce a small amount of ATP rapidly.

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4. Aerobic respiration can utilize a greater variety of molecules than can anaerobic respiration. A. Aerobic respiration in skeletal muscle utilizes primarily lipids and carbohydrates. Very

small amounts of proteins can also be used. B. Anaerobic respiration uses primarily carbohydrates.

Lipids Carbohydrates Proteins Fatty Glycerol Sugar Amino acids acids Pyruvic acid Acetyl coenzyme A Citric acid cycle

5. Creatine phosphate is an energy storage molecule that can be used to produce ATP for muscle contraction.

Generated by aerobic respiration ATP ADP Creatine Creatine phosphate (energy storage molecule) ATP ADP Used for muscle contraction

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Metabolism at Rest 1. Fatty acids (mostly) and glucose are used in aerobic respiration to convert ADP to ATP. 2. Energy storage.

A. Excess fatty acids are stored as lipids in adipose tissue. B. Excess glucose is stored as glycogen in skeletal muscle and in the liver. C. Creatine is converted to creatine phosphate.

Energy storage Fatty acids Adipose tissue Aerobic respiration Glucose Glycogen (in skeletal muscle and the liver) ATP Creatine Creatine phosphate (in skeletal muscle) Maintains muscle tone and other cell activities

Metabolism During Exercise 1. ATP already present in the cell as a result of aerobic respiration is the first energy molecule

used when exercise begins. 2. Creatine phosphate is converted to creatine and ATP is produced. The ATP is then used as

an energy source for contraction. 3. Anaerobic respiration from the break down of glucose quickly provides additional ATP.

Glycogen can be broken down to provide glucose for this process. Anaerobic respiration can be maintained for only a few minutes and sustains muscle activity until aerobic respiration can provide ATP.

4. Aerobic respiration provides ATP once circulatory and respiratory changes have increased

oxygen delivery to the muscles.

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Many ATP Slowly produced Sustained process Adipose Fatty Aerobic CO2 tissue acids respiration 4. Glycogen Glucose 3. 2. Creatine Creatine Anaerobic Lactic phosphate respiration acid 1. Already present Few ATP ATP is used Rapidly produced Short term process

5. Graph of energy sources after exercise begins.

6. Aerobic respiration occurs all the time and is necessary to maintain homeostasis at rest and during sustained exercise. Anaerobic respiration adjusts for short term energy demands and is especially useful during short term intense activities. You don’t switch from aerobic to anaerobic respiration. Anaerobic respiration occurs in addition to the aerobic respiration.

7. Anaerobic respiration is limited by the depletion of creatine phosphate and glucose and by a

build-up of lactic acid. Lactic acid can diffuse out of skeletal muscle into the blood. This allows anaerobic respiration to proceed for a longer period of time.

Percent ofenergy supply

100

50

Time (minutes)Start exercise1 2 3 4

1 and 2. ATP andcreatine phosphate 4. Aerobic

3. Anaerobic

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Oxygen Debt 1. During the first few minutes of muscular activity, the circulatory system has not adjusted to

the increased demand for oxygen by the muscles. Therefore the muscle cells respire anaerobically as well as aerobically and there is a build-up of lactic acid within skeletal muscles and within the blood.

2. After exercise the lactic acid is converted into glucose. This conversion takes place mainly

in the liver, but also occurs in skeletal muscle. Skeletal muscle (anaerobic) Blood Lactic acid glucose (blood) Energy Glucose Liver (used by (aerobic) the liver) ATP ADP (aerobic respiration requires oxygen) Glycogen (stored in the liver)

3. The conversion of lactic acid to glucose requires ATP that is produced by aerobic respiration. The oxygen required to produce the ATP that is used to convert the lactic acid to glucose is called the oxygen debt.

Oxygenconsumption

Startactivity

Stopactivity

Oxygendebt

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PRACTICE PROBLEM A track athlete is in a 1-mile race. Describe her breathing and metabolism

1. halfway through the race.

2. as she sprints toward the finish line.

3. as she collapses and lays on the ground after she crosses the finish line.

Heat Production (p. 303) 1. The rate of metabolism in skeletal muscle differs before, during, and after exercise.

A. At rest, the heat released as a by-product of the chemical reactions that produce ATP through aerobic metabolism is responsible for body temperature.

What is the ATP used for in a resting muscle?

.

B. During exercise the amount of heat production increases as the amount of ATP produced

increases. Some of this heat is stored by the body and causes an increase in body temperature.

C. Following exercise the stored heat plus additional heat generated as ATP are produced to

pay back the oxygen debt are responsible for an elevated body temperature.

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2. When body temperatures fall below normal the nervous system stimulates skeletal muscles to rapidly contract (shivering). The heat produced as a by-product of the contraction process helps to elevate body temperature.

Slow and Fast Fibers Two basic kinds of skeletal muscle fibers can be recognized. 1. Slow-twitch, high oxidative, or type I muscle fibers contract slowly and are resistant to

fatigue. 2. Fast-twitch, low oxidative, or type II muscle fibers contract rapidly and fatigue rapidly.

Characteristic Slow-twitch Fast-twitch fatigue-resistant fatigable Rate of ATP Slow Fast breakdown Major method of Aerobic Anaerobic ATP generation Number of Many Few mitochondria Number of Many Few capillaries Myoglobin content High Low Glycogen content Low High The rate of ATP breakdown depends on the type of ATPase enzymes in the cell. Aerobic respiration requires oxygen and anaerobic respiration does not require oxygen. Mitochondria are the site of ATP production. Capillaries deliver oxygen to muscle fibers. Myoglobin is an oxygen-binding pigment that stores oxygen within muscle fibers. Glycogen can be broken down to glucose that is used as an energy source in anaerobic metabolism.

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Distribution of Fast-Twitch and Slow-Twitch Muscle Fibers 1. Slow-twitch fibers appear red in color because of the large number of capillaries and the high

myoglobin content. 2. Fast-twitch fibers appear white in color because of few capillaries and low myoglobin

content. Explain the significance of white breast meat in a chicken and dark breast meat in a duck.

3. Humans have both slow-twitch and fast-twitch fibers in the same muscle, although the

proportion varies. How would you expect the proportion of fiber types in the arm and the back to differ?

Explain.

4. The distribution of fiber types is genetically determined and one type of muscle fiber does

not normally change into the other type. How would you expect the thigh muscles of a world-class sprinter and a marathon runner to

differ? Explain.

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Effects of Exercise 1. When muscles hypertrophy (get larger) they get stronger.

A. It is generally believed that hypertrophy results from an increase in the size of muscle fibers and does not result from an increase in the number of muscle fibers.

B. There is some evidence that a small number of new muscle fibers can contribute to the

hypertrophy. If so, the increase in number does not contribute significantly to the hypertrophy. The production of new muscle fibers is more of clinical interest. Can a way be found to produce new muscle fibers and thereby offset the effects of diseases and age?

What organelle would you expect to increase in number in a muscle fiber that has

hypertrophied?

2. Lack of exercise results in atrophy (muscles get smaller). This involves a decrease in the

size of muscle fibers. In old age, atrophy also involves a decrease in the number of muscle fibers.

3. Training can cause hypertrophy and increase the fatigue resistance of both types of muscles.

A. Endurance type exercises (e.g., long distance running) that require aerobic respiration affect primarily slow-twitch fibers. 1) There is an increased resistance to fatigue. 2) There is a small increase in muscle size. What kinds of changes would you expect to see in an endurance trained muscle?

Why isn't there a big increase in muscle size?

B. High intensity exercises (e.g., weight lifting) that require anaerobic respiration affect

primarily fast-twitch fibers. 1) There is a large increase in size and strength. 2) There is a small increase in endurance.

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4. Although one type of muscle fiber does not switch and become the other type, it is possible to increase the size and strength of slow-twitch fibers and it is possible to increase the endurance of fast-twitch fibers. There are two types of fast twitch fibers: type IIx fibers, which are classic type II fibers, and fatigue-resistant fast-twitch fibers, or type IIa fibers, which are fast-twitch fibers that have adapted to resist fatigue.

Characteristic Slow-twitch Fast-twitch Fast -twitch fatigue-resistant fatigue-resistant fatigable Rate of ATP Slow Fast Fast breakdown Major method of Aerobic Aerobic Anaerobic ATP generation Number of Many Many Few mitochondria Number of Many Many Few capillaries Myoglobin content High High Low Glycogen content Low Intermediate High

Other factors involved in increasing muscle performance. Would an increase in the ability of

the nervous system to recruit and stimulate motor units primarily increase muscle strength or endurance? Explain.

Would an increase respiratory and cardiovascular abilities primarily increase muscle strength

or endurance? Explain.

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SMOOTH MUSCLE FIGURES 9.22 and 9.23 1. Smooth muscle fibers are spindle-shaped cells that are small compared to skeletal muscle

cells (1000 times shorter). 2. Internal organization.

A. Single nucleus. B. Shallow invaginations of the plasma membrane called caveolae may be analogous to the

T tubule system of skeletal muscle. C. The sarcoplasmic reticulum is poorly developed. It may store and release Ca2+. D. Smooth muscle has actin and myosin myofilaments, but they are not organized into

sarcomeres. Therefore smooth muscle is not striated. 1) Actin myofilaments are attached to dense bodies in the cytoplasm and to dense areas

in the sarcolemma. The dense bodies and areas function like the Z disks in skeletal muscle as anchor points for the actin myofilaments. In addition, the dense bodies and areas are connected by noncontractile intermediate filaments, which form an intracellular cytoskeleton.

2) When actin slides over myosin, the dense bodies and dense areas move closer

together, causing the smooth muscle cell to shorten.

Contraction of Smooth Muscle FIGURE 9.24 The contraction process is the same as in skeletal muscle (sliding filament model). Ca2+ initiate contraction, but in a different way than in skeletal muscle. 1. Depolarization of the smooth muscle plasma membrane, or a hormone binding to a receptor

in the plasma membrane, results in an increase in membrane permeability to Ca2+. 2. Ca2+ diffuse into the smooth muscle cell and bind to the protein calmodulin. Most of the

Ca2+ involved in smooth muscle contraction comes from outside the cell. Some of the Ca2+ are released from sarcoplasmic reticulum.

3. The calcium-calmodulin complex activates an enzyme called myosin kinase. 4. Myosin kinase promotes the breakdown of ATP to ADP. The phosphate from ATP is

attached to the myosin head and the energy from ATP is stored by the myosin head. 5. Myosin comes into contact with actin and cross-bridge movement occurs. 6. The enzyme myosin phosphatase removes phosphate from myosin, and myosin detaches

from actin.

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7. Steps 4 through 6 can be repeated or the muscle can relax. Smooth muscle typically contracts slower and relaxes slower that skeletal muscle. It takes time for the Ca2+ to diffuse into and out of the cell. The break down of ATP also occurs more slowly.

Types of Smooth Muscle 1. Visceral smooth muscle.

A. Characteristics. 1) Visceral smooth muscle is interconnected by gap junctions so that the muscle cells

function as an unit. 2) Visceral smooth muscle is often autorhythmic and contracts at regular intervals

without external stimulation. However some visceral smooth muscle contracts only when stimulated, e.g. the urinary bladder.

3) Responds to sudden stretch by contracting.

B. Examples are found in the digestive, reproductive, and urinary tracts; in the urinary bladder, gallbladder, uterus, small arteries, and veins.

2. Multiunit smooth muscle.

A. Characteristics. 1) Multiunit smooth muscle is organized and behaves much like skeletal muscle. The

muscle cells are isolated from each other (by connective tissue) and are organized into "motor units" that can contract independently of each other.

2) Multiunit smooth muscle contracts when stimulated by the nervous system. It does

not contract spontaneously. 3) Multiunit smooth muscle does not contract in response to sudden stretch.

B. Examples are the ciliary muscles of the eye, iris, arrector pili, large arteries, and large airways to the lungs (trachea, bronchi).

Electrical Properties of Smooth Muscle FIGURE 9.25 1. Slow waves of depolarization and repolarization.

A. Slow waves are slow depolarization and repolarization of the plasma membrane. They are slow because the ion channels open and close slowly.

B. Sometimes one or more “classical” action potentials are superimposed on the slow wave. C. The slow wave and action potentials can stimulate contraction of the smooth muscle

cells. Slow waves are propagated a short distance, whereas the action potentials are propagated longer distances.

2. Smooth muscle is not all-or-none. A series of action potentials cause one slow contraction

and relaxation. (In skeletal muscle, the series of action potentials cause a series of contractions).

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3. Some smooth muscle cells generate action potentials spontaneously and are called pacemaker cells because their activity stimulates other smooth cells to have action potentials and contract.

Functional Properties of Smooth Muscle 1. Autorhythmic contractions, which result from the activity of pacemaker cells. 2. Sudden stretch of smooth muscle generates a contraction (slow stretch does not stimulate

contraction). If a hollow organ (such as the intestine) is suddenly stretched, a contraction occurs. This allows the wall of the organ to resist the stretch and/or move whatever is causing the stretch.

3. Smooth muscle can maintain a steady, constant tension called smooth muscle tone.

Furthermore, smooth muscle can greatly change its length without marked changes in tension. Therefore hollow organs can change size without great changes in pressure on their contents. For example, the stomach and intestines maintain a constant pressure on their contents. Smooth muscle tone is important because it allows prolonged or even indefinite smooth muscle function.

4. The ability to shorten remains fairly constant despite large changes in smooth muscle length. A. When hollow organs fill up they are greatly stretched. The smooth muscles must be able

to contract and cause the organ to empty. B. The cells of the urinary bladder can be stretched to two and one half times their resting

length. The cells of the uterus can be eight times their original length by the end of pregnancy.

Regulation of Smooth Muscle Smooth muscle is regulated by the autonomic nervous system and is under involuntary control. Some hormones, such as epinephrine, are also important in regulating smooth muscle.

Time

Increaselength

Decreaselength

Tension

Tension

Muscle length

Skeletalmuscle

Smoothmuscle

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Types of Muscle Contraction 1. Muscle twitch. See above. Very rare in a normal organism. 2. Tetanus. See above. Complete tetanus can occur in disease conditions, such as tetanus

(lockjaw). 3. Isotonic contraction (= same tension). The muscle shortens but tension stays the same.

Lifting a weight is an example. 4. Isometric contraction (= same length). The muscle stays the same length but tension

increase. Trying to push a car that doesn't move is an example. Most activities are a combination of isotonic and isometric contractions. Explain how this is

true for walking. 5. Tonic contraction. Motor units are alternately active and inactive producing a constant

tension in the muscle. A. Tonic contractions are important in maintaining posture. A person that faints loses

muscle tone and collapses. B. Without tonic stimulation muscle fibers atrophy. Bedridden patients or patients in casts

may experience atrophy because of the lack of stimulation of the muscle. C. Too little tone produces flaccid muscles. Too much tone produces rigid or spastic

muscles.

6. Fibrillation. Following complete denervation of muscle fibers, the muscle fibers can develop spontaneous contractions (1 to 10 times per second), producing a "flutter" of the muscle. Eventually the muscle fibers atrophy and fibrillation stops (after several weeks).

7. Convulsions are strong involuntary tetanic contractions of groups of muscles. Results from

the disordered stimulation of the motor units. Fever, drugs, poisons, and hysteria are possible causes.

8. Cramp. A painful, spastic contraction of a muscle due to an involuntary tetanic contraction. 9. Rigor mortis. Following death the muscles can become rigid. Due to depletion of ATP, the

actin and myosin stay bound together. The rigidity persists until the actin and myosin are destroyed (15 to 25 hours after death).