Ch 9 The Muscular System

Major characteristics of skeletal muscle

•Responsible locomotion, facial expressions, posture, respiratory movements, other types of body movement.

•Voluntary and controlled by the nervous system.

•Cell shape: Very long and cylindrical (1 mm–4 cm, or as much as 30 cm in length, 10 μm–100 μmin diameter)

•Nucleus: multiple nuclei, peripherally located

•Cell to cell attachments: none

•Striations: yes

Major characteristics of smooth muscle

•Walls of hollow organs, blood vessels, eye, glands, skin.

•Some functions: propel urine, mix food in digestive tract, dilating/constricting pupils, regulating blood flow.

•In some locations, autorhythmic.

•Controlled involuntarily by endocrine and autonomic nervous systems.

Cell shape: Spindle-shaped (15–200 μm in length, 5–8 μm in diameter, smaller fibers than those in skeletal muscle)

•Nucleus: single, centrally located

•Cell to cell attachments: Gap junctions join some visceral smooth muscle cells together

•Striations: no

•Caveolae: indentations in sarcolemma; may act like T tubules.

• Dense bodies instead of Z disks as in skeletal muscle that actin attached to; also have noncontractile intermediate filaments.

Major characteristics of cardiac muscle

•Heart (only in the heart): major source of movement of blood.

•Autorhythmic.

•Controlled involuntarily by endocrine and autonomic nervous systems.

Cell shape: Cylindrical and branched (100–500 μm in length, 12–20 μm in diameter)

•Nucleus: single, centrally located

•Cell to cell attachments: Intercalated disks join cells to one another, and gap junctions, these allow action potentials to pass from 1 cell to another (Action potentials of longer duration and longer refractory period)

•Striations: yes

Functions of muscular system

1. Movement

   2. Maintain posture

   3. Respiration

   4. Production of body heat

   5. Communication

   6. Heart beat

   7. Contraction of organs and vessels

Four functional properties of muscle tissue

•Contractility: ability of a muscle to shorten with force.

•Excitability: capacity of muscle to respond to a stimulus (usually from nerves).

•Extensibility: muscle can be stretched beyond it normal resting length and still be able to contract.

•Elasticity: ability of muscle to recoil to original resting length after stretched.

Describe the connective tissue components of skeletal muscle

•Motor neurons stimulate skeletal muscle contraction.

Blood supply and innervation of skeletal muscle

•An artery and 1 to 2 veins extend with a nerve through the C T layers.

•Extensive capillary beds surround muscle fibers.

Origin of muscle fibers and how muscle hypertrophy occurs

•Develop from fusion of myoblasts, resulting in large, multinucleated muscle cells.

•Number of fibers remains relatively constant after birth; muscles get larger due to hypertrophy of muscle fibers.

Components of a muscle fiber

—Electrical component structures can respond to and transmit electrical signals; they include the following:

•Sarcolemma – plasma membrane; surrounds sarcoplasm(cytoplasm) and other contents of cell.

•Transverse tubules (T tubules) – inward folds of sarcolemma; project into the interior of muscle cell.

•Sarcoplasmic reticulum (S R) – specialized smooth endoplasmic reticulum; stores calcium.

•Enlarged portions called terminal cisternae lie adjacent to T tubules.

•Two terminal cisternae and their associated T tubule form a triad.

—Mechanical component structures allow muscles to contract; due to:

•Myofibrils – bundles of protein filaments; contain the protein filaments (myofilaments) that cause contraction.

•Myofilaments

Types of myofilaments

•Actin (thin) myofilaments.

•Myosin (thick) myofilaments.

•Myofilaments arranged into orderly units called sarcomeres.

Actin (thin) myofilaments

•Two strands of fibrous (F) actin form a double helix extending the length of the myofilament; attached at either end at sarcomere.

•Composed of G actin monomers each of which has an active site.

•Actin site can bind myosin during muscle contraction.

•Tropomyosin: an elongated protein winds along the groove of the F actin double helix.

•Troponin is composed of three subunits: one that binds to actin, a second that binds to tropomyosin, and a third that binds to calcium ions. Spaced between the ends of the tropomyosin molecules in the groove between the F actin strands.

•The tropomyosin/troponin complex regulates the interaction between active sites on G actin and myosin.

Myosin (thick) myofilament

•Many elongated myosin molecules shaped like golf clubs.

•Molecule consists of myosin heavy chains wound together to form a rod portion lying parallel to the myosin myofilament and two myosin heads that extend laterally.

•Myosin heads.

1.Can bind to active sites on the actin molecules to form cross-bridges.

2.Attached to the rod portion by a hinge region that can bend and straighten during contraction.

3.Are A T Pase enzymes: activity that breaks down adenosine triphosphate (A T P), releasing energy. Part of the energy is used to bend the hinge region of the myosin molecule during contraction.

Diagram of myofilaments in a sarcomere

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How does the sliding filament model explain the contraction of muscle fibers

—Actin myofilaments sliding over myosin to shorten sarcomeres.

•Actin and myosin do not change length.

•Shortening sarcomeres responsible for skeletal muscle contraction.

—During relaxation, sarcomeres lengthen because of some external force, like contraction of antagonistic muscles.

•Muscles that produce the opposite effect.

What happens to the A band, I band and H zone during contraction

In a contracted muscle, the A bands do not narrow because the length of the myosin myofilaments does not change. The ends of the actin myofilaments are pulled to and overlap in the center of the sarcomere, shortening it and the H zone disappears.

Resting membrane potential

—Nervous system controls muscle contractions through action potentials.

—Membrane voltage difference across membranes (polarized).

•Inside cell more negative due to accumulation of large protein molecules. More K* on inside than outside. K* leaks out but not completely because negative proteins hold some back.

•Outside cell more positive and more Nat on outside than inside.

•Nat/K* pump maintains this situation.

—Must exist for action potential to occur.

Role of ion channels in the production of an action potential

—Ligand-gated: Ligands are molecules that bind to receptors. Receptor: protein or glycoprotein with a receptor site.

•Example: neurotransmitters.

•Gate is closed until neurotransmitter attaches to receptor molecule.
When Ach attaches to receptor on muscle cell, Na* gate opens.
Na moves into cell due to concentration gradient.

—Voltage-gated:

•Open and close in response to small voltage changes across plasma membrane.

—Each is specific for certain ions.

Action potential

Phases:

• Depolarization: Inside of plasma membrane becomes less negative. If change reaches threshold (when voltage-gated Na* gates open), depolarization occurs.

• Repolarization: return of resting membrane potential. Note that during repolarization, the membrane potential drops lower than its original resting potential, then rebounds. This is because Nat plus K+ together are higher, but then Na*/K+ pump restores the resting potential.

All-or-nothing principle

As it pertains to action potentials, if threshold is reached, the cell will respond completely

Describe the structure of a neuromuscular junction

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How is an action potential transmitted across the junction

When an action potential reaches the presynaptic terminal of a motor neuron, it causes voltage-gated Ca2+ channels in the presynaptic membrane to open. As a result, Ca2+ diffuse into the axon terminal.

Explain the events of excitation/contraction coupling

•Links electrical and mechanical components of contraction.

•Action potential produced on sarcolemma - propagated into T tubules - calcium channels on S R terminal cisternae open - calcium enters sarcoplasm, binds troponin - muscle contraction.

Events of cross-bridge movement

During a single contraction, each myosin myofilament goes through the cycle of cross-bridge formation, movement, release, and return to its original position many times. Many cycles of power and recovery strokes occur during each muscle contraction.

Power stroke vs the recovery stroke

•Power stroke: movement of myosin head

•Recovery stroke: returns the myosin head to its “high energy” position from its “low energy” position

Conditions needed for muscle relaxation

Three major A TP-dependent events are required for muscle relaxation.

1. After an action potential has occurred in the muscle fiber, the sodium-potassium pump must actively transport Na* out of the muscle fiber and K+ into the muscle fiber to return to and maintain resting membrane potential.

2. ATP is required to detach the myosin heads from the active sites for the recovery stroke.

3. ATP is needed for the active transport of Ca?* into the sarcoplasmic reticulum from the sarcoplasm.

What is a muscle twitch

The response of a muscle fiber to a single action potential along its motor neuron.

Phases of a muscle twitch

•Lag or latent – from the stimulus to the beginning of the contraction.

•Contraction – released and cross-bridging cycling occurs.

•Relaxation – returns to SR and muscle fiber returns to precontraction length.

Describe a motor unit

A single motor neuron and all muscle fibers innervated by it. An action potential in the neuron of a motor unit causes all the muscle fibers of that unit to contract

How do motor unit numbers affect muscle control

•Large muscles have motor units with many muscle fibers.

•Small muscles that make delicate movements contain motor units with few muscle fibers.

How do whole muscles respond in a graded fashion

—The muscle responds in a graded fashion, depending on the force generated in the individual muscle fibers.

—Increasing the number of cross-bridges allows a fiber to contract with more force. Factors that increase the number of cross-bridges are:

•Frequency of stimulation

•Muscle fiber diameter

•Muscle fiber length at the time of contraction

How can the force of contraction be increased

By a larger muscle fiber in diameter (this is because larger diameter fibers have more myofibrils and therefore can form more cross-bridges which provides more force of contraction)

Summarize what occurs in treppe

It’s a phenomenon observed in muscle tissue where consecutive stimulations result in progressively stronger contractions until a plateau is reached. This occurs when muscle fibers are stimulated frequently without allowing them to fully relax, leading to an increased efficiency in muscle contraction.

Explain multiple-motor-unit recruitment

Strength of contraction depends upon recruitment of motor units (a muscle has many motor units)

Describe wave summation in terms of incomplete tetanus and complete tetanus

•Wave summation: the phenomenon where repeated muscle contractions occur so rapidly that each new contraction builds upon the tension from the previous one, resulting in a progressively stronger contraction

•Incomplete tetanus is when muscle stimulation occurs at a frequency where some relaxation occurs between contractions

•Complete tetanus is the eventual progression if the stimulation frequency increases further, eliminating any relaxation phase

Explain the connection between the initial length of a muscle and the amount of tension produced

—Active tension: force applied to an object to be lifted when a muscle contracts.

—Active tension increases or decreases as a muscle fiber changes in length; active tension curve.

•Stretched too far – fewer cross-bridges can form.

•If not stretched at all – thick filaments touch Z disks and very little contraction can occur.

—Passive tension: tension applied to load when muscle stretches but is not stimulated.

—Total tension: sum of active and passive tension.

Distinguish between isometric and isotonic contractions

Isometric: no change in length of muscle (so no movement)

Isotonic: change in length but tension constant

How is muscle tone maintained

Constant tension by muscles for long periods of time; due to small percentage of all motor units contracting out of phase with one another

What is a slow-twitch muscle fiber

•It is type 1

•Contract more slowly, smaller in diameter, better blood supply, more mitochondria, more fatigue-resistant than fast-twitch, large amount of myoglobin.

•Postural muscles, more in lower than upper limbs. Dark meat of chicken.

What is a fast-twitch muscle fiber

•Type 2

•Respond rapidly to nervous stimulation, contain myosin that can break down A T P more rapidly than that in Type I, less blood supply, fewer and smaller mitochondria than slow-twitch.

•Lower limbs in sprinter, upper limbs of most people. White meat in chicken.

•Comes in oxidative and glycolytic (anaerobic) forms.

Describe how training can increase the size and efficiency of both types of muscle fibers

Effects of exercise: change in size of muscle fibers.

—Hypertrophy: increase in muscle size.

•Increase in myofibrils.

•Increase in nuclei  due to fusion of satellite cells.

•Increase in strength due to better coordination of muscles, increase in production of metabolic enzymes, better circulation, less restriction by fat.

—Atrophy: decrease in muscle size.

•Reverse except in severe situations where cells die.

How does muscle metabolism cause normal body temperature

•Exercise: metabolic rate and heat production increase.

•Post-exercise: metabolic rate stays high due to oxygen debt

How do muscles respond to changes from normal body temperature

•Excess heat lost because of vasodilation and sweating.

•Shivering: uncoordinated contraction of muscle fibers resulting in shaking and heat production.

Describe 4 sources of energy for ATP production in muscles

1. Conversion of two A D P to one A T P and A M P

•Adenylate kinase

2. Transfer of phosphate from creatine kinase to A D P 
to form A T P

•Creatine kinase

3. Anaerobic respiration

•Occurs in absence of oxygen and results in breakdown of glucose to yield A T P and lactic acid

4. Aerobic respiration

•Requires oxygen and breaks down glucose to produce A T P, carbon dioxide and water

•More efficient than anaerobic

Difference between oxygen deficit and excess postexercise oxygen consumption

•Oxygen deficit refers to the initial period during exercise where the body's oxygen demand exceeds the available supply, forcing it to rely on anaerobic energy systems

•Excess post-exercise oxygen consumption (EPOC) is the elevated oxygen consumption that occurs after exercise as the body restores itself to its resting state, essentially "paying back" the oxygen debt created during the deficit phase

Definition of fatigue

Decreased capacity to work and reduced efficiency of performance

Compare the mechanisms involved in the major types of muscle fatigue

—Acidosis and A T P depletion due to either an increased A T P consumption or a decreased A T P production.

—Oxidative stress, which is characterized by the buildup of excess reactive oxygen species (R O S; free radicals).

—Local inflammatory reactions

•Physiological contracture - state of fatigue where due to lack of A T P neither contraction nor relaxation can occur.

•Psychological fatigue – most common type; comes from the central nervous system rather than the muscles

Contrast physiological contracture and rigor mortis

Physiological contracture: the abnormal, permanent shortening and hardening of muscle tissue, leading to restricted joint mobility and potential deformities

Rigor mortis: development of rigid muscles several hours after death. Leaks into sarcoplasm and attaches to myosin heads and crossbridges form. Ends as tissues start to deteriorate.

Steps of smooth muscle contraction

•If the phosphate is removed from myosin while the cross-bridges are attached to actin, the cross-bridges release very slowly. This explains how smooth muscle is able to sustain tension for long periods and without extensive energy expenditure. This period of sustained tension is often called the latch state of smooth muscle contraction.

•Exhibits relatively constant tension: smooth muscle tone

How do smooth and skeletal muscle contraction differ

•Smooth muscle contraction differs from skeletal muscle contraction primarily in its involuntary control, slower contraction speed, ability to sustain contractions for longer periods, and a different mechanism for calcium regulation

•Skeletal muscle contractions are voluntary and rapid, with clear striations due to the organized arrangement of actin and myosin filaments

Compare the 2 types of smooth muscle as to their action and locations

•Single-unit (also called visceral) smooth muscle, which contracts as a single unit due to gap junctions between cells and is found in most internal organs like the digestive tract and uterus

•Multi-unit smooth muscle cells contract individually and are located in areas like the iris of the eye and arrector pili muscles of the skin, allowing for more precise control over muscle contractions

Explain how smooth muscle activities are regulated

• Ca2+ required to initiate contractions; binds to calmodulin which regulates myosin kinase. Cross-bridging occurs.

• Relaxation: caused by enzyme myosin phosphatase.