Chapter 10

Muscle Tissue

Lecture Outline

INTRODUCTION

• Motion results from alternating contraction (shortening) and relaxation of muscles; the skeletal system provides leverage and a supportive framework for this movement.

• The scientific study of muscles is known as myology.

OVERVIEW OF MUSCLE TISSUE

• Types of Muscle Tissue• Skeletal muscle tissue is primarily attached to

bones. It is striated and voluntary.• Cardiac muscle tissue forms the wall of the heart.

It is striated and involuntary.• Smooth (visceral) muscle tissue is located in

viscera. It is nonstraited (smooth) and involuntary.

• Table 4.4 compares the different types of muscle.

Functions of Muscle Tissue

• Producing body movements• Stabilizing body positions• Regulating organ volumes

– bands of smooth muscle called sphincters

• Movement of substances within the body– blood, lymph, urine, air, food and fluids, sperm

• Producing heat– involuntary contractions of skeletal muscle

(shivering)

Properties of Muscle Tissue

• Excitability– respond to chemicals released from nerve cells

• Conductivity– ability to propagate electrical signals over membrane

• Contractility– ability to shorten and generate force

• Extensibility– ability to be stretched without damaging the tissue

• Elasticity– ability to return to original shape after being stretched

Skeletal Muscle -- Connective Tissue

• Superficial fascia is loose connective tissue & fat underlying the skin

• Deep fascia = dense irregular connective tissue around muscle

• Connective tissue components of the muscle include– epimysium = surrounds the whole muscle – perimysium = surrounds bundles (fascicles) of 10-

100 muscle cells– endomysium = separates individual muscle cells

• All these connective tissue layers extend beyond the muscle belly to form the tendon

Connective Tissue Components

• Tendons and aponeuroses are extensions of connective tissue beyond muscle cells that attach muscle to bone or other muscle.

• A tendon is a cord of dense connective tissue that attaches a muscle to the periosteum of a bone (Figure 11.22).

• An aponeurosis is a tendon that extends as a broad, flat layer (Figure 11.4c).

Nerve and Blood Supply

• Each skeletal muscle is supplied by a nerve, artery and two veins.

• Each motor neuron supplies multiple muscle cells (neuromuscular junction)

• Each muscle cell is supplied by one motor neuron terminal branch and is in contact with one or two capillaries.– nerve fibers & capillaries are found in the

endomysium between individual cells

Sarcolemma, T Tubules, and Sarcoplasm

• Skeletal muscle consists of fibers (cells) covered by a sarcolemma (Figure 10.3b).– The fibers contain T tubules and sarcoplasm– T tubules are tiny invaginations of the sarcolemma

that quickly spread the muscle action potential to all parts of the muscle fiber.

• Sarcoplasm is the muscle cell cytoplasm and contains a large amount of glycogen for energy production and myoglobin for oxygen storage.

Transverse Tubules

• T (transverse) tubules are invaginations of the sarcolemma into the center of the cell– filled with extracellular fluid– carry muscle action potentials down into cell

• Mitochondria lie in rows throughout the cell– near the muscle proteins that use ATP during contraction

Myofibrils & Myofilaments

• Muscle fibers are filled with threads called myofibrils separated by SR (sarcoplasmic reticulum)

• The sarcoplasmic reticulum encircles each myofibril. It is similar to smooth endoplasmic reticulum in nonmuscle cells and in the relaxed muscle stores calcium ions.

• Myofilaments (thick & thin filaments) are the contractile proteins of muscle

Sarcoplasmic Reticulum (SR)

• System of tubular sacs similar to smooth ER in nonmuscle cells

• Stores Ca+2 in a relaxed muscle• Release of Ca+2 triggers muscle contraction

Filaments and the Sarcomere• Thick and thin filaments overlap each

other in a pattern that creates striations (light I bands and dark A bands)

• The I band region contains only thin filaments.

• They are arranged in compartments called sarcomeres, separated by Z discs.

• In the overlap region, six thin filaments surround each thick filament

Sarcomere

• Figure 10.5 shows the relationships of the zones, bands, and lines as seen in a transmission electron micrograph.

• Exercise can result in torn sarcolemma, damaged myofibrils, and disrupted Z discs (Clinical Application).

The Proteins of Muscle

• Myofibrils are built of 3 kinds of protein– contractile proteins

• myosin and actin

– regulatory proteins which turn contraction on & off

• troponin and tropomyosin

– structural proteins which provide proper alignment, elasticity and extensibility

• titin, myomesin, nebulin and dystrophin

The Proteins of Muscle -- Myosin

• Thick filaments are composed of myosin – each molecule resembles two golf clubs twisted together– myosin heads (cross bridges) extend toward the thin

filaments

• Held in place by the M line proteins.

The Proteins of Muscle -- Actin

• Thin filaments are made of actin, troponin, & tropomyosin

• The myosin-binding site on each actin molecule is covered by tropomyosin in relaxed muscle

• The thin filaments are held in place by Z lines. From one Z line to the next is a sarcomere.

Sliding Filament Mechanism Of

Contraction • Myosin cross bridges

pull on thin filaments• Thin filaments slide

inward• Z Discs come toward

each other• Sarcomeres

shorten.The muscle fiber shortens. The muscle shortens

• Notice :Thick & thin filaments do not change in length

How Does Contraction Begin?

1. Nerve impulse reaches an axon terminal & synaptic vesicles release acetylcholine (ACh)

2. ACh diffuses to receptors on the sarcolemma & Na+ channels open and Na+ rushes into the cell

3. A muscle action potential spreads over sarcolemma and down into the transverse tubules

4. SR releases Ca+2 into the sarcoplasm

5. Ca+2 binds to troponin & causes troponin-tropomyosin complex to move & reveal myosin binding sites on actin--the contraction cycle begins

Contraction Cycle

• Repeating sequence of events that cause the thick & thin filaments to move past each other.

• 4 steps to contraction cycle– ATP hydrolysis– attachment of myosin to actin to form crossbridges– power stroke– detachment of myosin from actin

• Cycle keeps repeating as long as there is ATP available & there is a high Ca+2 level near the filaments.

ATP and Myosin

• Myosin heads are activated by ATP

• Activated heads attach to actin & pull (power stroke)

• ADP is released. (ATP released P & ADP & energy)

• Thin filaments slide past the thick filaments

• ATP binds to myosin head & detaches it from actin

• All of these steps repeat over and over– if ATP is available &– Ca+ level near the troponin-tropomyosin complex is high

Relaxation• Acetylcholinesterase (AChE) breaks down

ACh within the synaptic cleft

• Muscle action potential ceases

• Ca+2 release channels close

• Active transport pumps Ca2+ back into storage in the sarcoplasmic reticulum

• Calcium-binding protein (calsequestrin) helps hold Ca+2 in SR (Ca+2 concentration 10,000 times higher than in cytosol)

• Tropomyosin-troponin complex recovers binding site on the actin

Events Occurring After a Nerve Signal1. Arrival of nerve impulse at nerve terminal causes

release of ACh from synaptic vesicles

2. ACh binds to receptors on muscle motor end plate opening the gated ion channels so that Na+ can rush into the muscle cell

3. Inside of muscle cell becomes more positive, triggering a muscle action potential that travels over the cell and down the T tubules

4. The release of Ca+2 from the SR is triggered and the muscle cell will shorten & generate force

5. Acetylcholinesterase breaks down the ACh attached to the receptors on the motor end plate so the muscle action potential will cease and the muscle cell will relax.

Pharmacology of the NMJ• Botulinum toxin blocks release of neurotransmitter at the NMJ

so muscle contraction can not occur– bacteria found in improperly canned food– death occurs from paralysis of the diaphragm

• Curare (plant poison from poison arrows)– causes muscle paralysis by blocking the ACh receptors – used to relax muscle during surgery

• Neostigmine (anticholinesterase agent)– blocks removal of ACh from receptors so strengthens weak muscle

contractions of myasthenia gravis– also an antidote for curare after surgery is finished

MUSCLE METABOLISM

• Creatine phosphate and ATP can power maximal muscle contraction for about 15 seconds and is used for maximal short bursts of energy (e.g., 100-meter dash) (Figure 10.13a).– Creatine phosphate is unique to muscle

fibers.

MUSCLE METABOLISM

• The partial catabolism of glucose to generate ATP occurs in anaerobic cellular respiration (Figure 10.13b). This system can provide enough energy for about 30-40 seconds of maximal muscle activity (e.g., 300-meter race).

• Muscular activity lasting more than 30 seconds depends increasingly on aerobic cellular respiration (reactions requiring oxygen). This system of ATP production involves the complete oxidation of glucose via cellular respiration (biological oxidation) (Figure 10.13c).

Muscle Fatigue

• Inability to contract after prolonged activity

• Factors that contribute to fatigue– central fatigue is feeling of tiredness and a

desire to stop (protective mechanism)– insufficient release of acetylcholine from

motor neurons – depletion of creatine phosphate– decline of Ca+2 within the sarcoplasm– insufficient oxygen or glycogen– buildup of lactic acid and ADP

The Motor Unit

• Motor unit = one somatic motor neuron & all the skeletal muscle cells (fibers) it stimulates (10 cells to 2,000 cells)– muscle fibers normally scattered throughout belly of muscle– One nerve cell supplies on average 150 muscle cells that all contract in unison.

• Total strength of a contraction depends on how many motor units are activated & how large the motor units are

Motor Unit Recruitment• Motor units in a whole muscle fire asynchronously

– some fibers are active others are relaxed – delays muscle fatigue so contraction can be sustained

• Produces smooth muscular contraction– not series of jerky movements

• Precise movements require smaller contractions– motor units must be smaller (less fibers/nerve)

• Large motor units are active when large tension is needed

Cardiac versus Skeletal Muscle

• More sarcoplasm and mitochondria

• Larger transverse tubules located at Z discs, rather than at A-l band junctions

• Less well-developed SR

• Limited intracellular Ca+2 reserves– more Ca+2 enters cell from extracellular

fluid during contraction

• Prolonged delivery of Ca+2 to sarcoplasm, produces a contraction that last 10 -15 times longer than in skeletal muscle

Physiology of Cardiac Muscle

• Autorhythmic cells– contract without stimulation

• Contracts 75 times per min & needs lots of O2 • Larger mitochondria generate ATP aerobically • Extended contraction is possible due to slow

Ca+2 delivery– Ca+2 channels to the extracellular fluid stay open

SMOOTH MUSCLE

• Smooth muscle tissue is nonstriated and involuntary and is classified into two types: visceral (single unit) smooth muscle (Figure 10.18a) and multiunit smooth muscle (Figure 10.18b).

– Visceral (single unit) smooth muscle is found in the walls of hollow viscera and small blood vessels; the fibers are arranged in a network and function as a “single unit.”

– Multiunit smooth muscle is found in large blood vessels, large airways, arrector pili muscles, and the iris of the eye. The fibers operate singly rather than as a unit.

Two Types of Smooth Muscle

• Visceral (single-unit)– in the walls of hollow

viscera & small BV– autorhythmic– gap junctions cause

fibers to contract in unison

• Multiunit – individual fibers with own

motor neuron ending– found in large arteries,

large airways, arrector pili muscles,iris & ciliary body

Physiology of Smooth Muscle• Contraction starts slowly & lasts longer

– no transverse tubules & very little SR– Ca+2 must flows in from outside

• In smooth muscle, the regulator protein that binds calcium ions in the cytosol is calmodulin (in place of the role of troponin in striated muscle); – calmodulin activates the enzyme myosin

light chain kinase, which facilitates myosin-actin binding and allows contraction to occur at a relatively slow rate.

Smooth Muscle Tone• The prolonged presence of calcium ions in

the cytosol of smooth muscle fibers provides for smooth muscle tone, a state of continued partial contraction.

• Smooth muscle fibers can stretch considerably without developing tension; this phenomenon is termed the stress-relaxation response.

• Useful for maintaining blood pressure or a steady pressure on the contents of GI tract

Atrophy and Hypertrophy

• Atrophy– wasting away of muscles– caused by disuse (disuse atrophy) or severing of the

nerve supply (denervation atrophy)– the transition to connective tissue can not be

reversed

• Hypertrophy– increase in the diameter of muscle fibers – resulting from very forceful, repetitive muscular

activity and an increase in myofibrils, SR & mitochondria

Muscle Attachmen

t Sites:Origin and Insertion

• Skeletal muscles shorten & pull on the bones they are attached to

• Origin is the bone that does not move when muscle shortens (normally proximal)

• Insertion is the movable bone (some 2 joint muscles)• Fleshy portion of the muscle in between attachment sites

= belly

HOW SKELETAL MUSCLES ARE NAMED

• The names of most of the nearly 700 skeletal muscles are based on several types of characteristics.

• These characteristics may be reflected in the name of the muscle.

• The most important characteristics include the direction in which the muscle fibers run, the size, shape, action, numbers of origins, and location of the muscle, and the sites of origin and insertion of the muscle

• Examples from Table 11.2– triceps brachii -- 3 sites of origin– quadratus femoris -- square shape– serratus anterior -- saw-toothed edge

Muscles of Abdominal Wall

• 4 pairs of sheetlike muscles– rectus abdominis = vertically oriented– external & internal obliques and transverses abdominis

• wrap around body to form anterior body wall• form rectus sheath and linea alba

• Inguinal ligament from anterior superior iliac spine to upper surface of body of pubis

• Inguinal canal = passageway from pelvis through body wall musculature opening seen as superficial inguinal ring

• Inguinal hernia = rupture or separation of abdominal wall allowing protrusion of part of the small intestine (more common in males)

Compartment Syndrome

• Skeletal muscles in the limbs are organized in units called compartments.

• In compartment syndrome, some external or internal pressure constricts the structures within a compartment, resulting in damaged blood vessels and subsequent reduction of the blood supply to the structures within the compartment.

• Without intervention, nerves suffer damage, and muscle develop scar tissue that results in permanent shortening of the muscles, a condition called contracture.