An Introduction to Muscle Tissue

An Introduction to Muscle Tissue

Muscle Tissue

A primary tissue type, divided into

Skeletal muscle

Cardiac muscle

Smooth muscle

Skeletal Muscles

Are attached to the skeletal system

Allow us to move

The muscular system

Includes only skeletal muscles

Functions of Skeletal Muscles

Produce skeletal movement

Maintain body position

Support soft tissues

Guard openings

Maintain body temperature

Store nutrient reserves

Skeletal Muscle Structures

Muscle tissue (muscle cells or fibers)

Connective tissues

Nerves

Blood vessels

Organization of Connective Tissues

Muscles have three layers of connective tissues

Epimysium:

–exterior collagen layer

–connected to deep fascia

–Separates muscle from surrounding tissues

Perimysium:

–surrounds muscle fiber bundles (fascicles)

–contains blood vessel and nerve supply to fascicles

Endomysium:

–surrounds individual muscle cells (muscle fibers)

–contains capillaries and nerve fibers contacting muscle cells

–contains myosatellite cells (stem cells) that repair damage

Muscle attachments

Endomysium, perimysium, and epimysium come together:

–at ends of muscles

–to form connective tissue attachment to bone matrix

–i.e.,tendon (bundle) or aponeurosis (sheet)

Nerves

Skeletal muscles are voluntary muscles, controlled by nerves of the central nervous system (brain and spinal cord)

Blood Vessels

Muscles have extensive vascular systems that

Supply large amounts of oxygen

Supply nutrients

Carry away wastes

Skeletal Muscle Fibers

Are very long

Develop through fusion of mesodermal cells (myoblasts)

Become very large

Contain hundreds of nuclei

Internal Organization of Muscle Fibers

The sarcolemma

The cell membrane of a muscle fiber (cell)

Surrounds the sarcoplasm (cytoplasm of muscle fiber)

A change in transmembrane potential begins contractions

Transverse tubules (T tubules)

Transmit action potential through cell

Allow entire muscle fiber to contract simultaneously

Have same properties as sarcolemma

Myofibrils

Lengthwise subdivisions within muscle fiber

Made up of bundles of protein filaments (myofilaments)

Myofilaments are responsible for muscle contraction

Types of myofilaments:

–thin filaments:

»made of the protein actin

–thick filaments:

»made of the protein myosin

Sarcoplasmic reticulum (SR)

A membranous structure surrounding each myofibril

Helps transmit action potential to myofibril

Similar in structure to smooth endoplasmic reticulum

Forms chambers (terminal cisternae) attached to T tubules

Triad

Is formed by one T tubule and two terminal cisternae

Cisternae:

–concentrate Ca2+ (via ion pumps)

–release Ca2+ into sarcomeres to begin muscle contraction

Sarcomeres

The contractile units of muscle

Structural units of myofibrils

Form visible patterns within myofibrils

Muscle striations

A striped or striated pattern within myofibrils:

–alternating dark, thick filaments (A bands) and light, thin filaments (I bands)

Sarcomeres

M Lines and Z Lines:

–M line:

»the center of the A band

»at midline of sarcomere

–Z lines:

»the centers of the I bands

»at two ends of sarcomere

Zone of overlap:

–the densest, darkest area on a light micrograph

–where thick and thin filaments overlap

The H Band:

–the area around the M line

–has thick filaments but no thin filaments

Titin:

–are strands of protein

–reach from tips of thick filaments to the Z line

–stabilize the filaments

Transverse tubules encircle the sarcomere near zones of overlap

Ca2+ released by SR causes thin and thick filaments to interact

Muscle Contraction

Is caused by interactions of thick and thin filaments

Structures of protein molecules determine interactions

Four Thin Filament Proteins

F-actin (Filamentous actin)

Is two twisted rows of globular G-actin

The active sites on G-actin strands bind to myosin

Nebulin

Holds F-actin strands together

Tropomyosin

Is a double strand

Prevents actin–myosin interaction

Troponin

A globular protein

Binds tropomyosin to G-actin

Controlled by Ca2+

Initiating Contraction

Ca2+ binds to receptor on troponin molecule

Troponin–tropomyosin complex changes

Exposes active site of F-actin

Thick Filaments

Contain twisted myosin subunits

Contain titin strands that recoil after stretching

The mysosin molecule

Tail:

–binds to other myosin molecules

Head:

–made of two globular protein subunits

–reaches the nearest thin filament

Myosin Action

During contraction, myosin heads

Interact with actin filaments, forming cross-bridges

Pivot, producing motion

Skeletal Muscle Contraction

Sliding filament theory

Thin filaments of sarcomere slide toward M line, alongside thick filaments

The width of A zone stays the same

Z lines move closer together

The process of contraction

Neural stimulation of sarcolemma:

–causes excitation–contraction coupling

Cisternae of SR release Ca2+:

–which triggers interaction of thick and thin filaments

–consuming ATP and producing tension

The Neuromuscular Junction

Is the location of neural stimulation

Action potential (electrical signal)

Travels along nerve axon

Ends at synaptic terminal

Synaptic terminal:

–releases neurotransmitter (acetylcholine or ACh)

–into the synaptic cleft (gap between synaptic terminal and motor end plate)

The Neurotransmitter

Acetylcholine or ACh

Travels across the synaptic cleft

Binds to membrane receptors on sarcolemma (motor end plate)

Causes sodium–ion rush into sarcoplasm

Is quickly broken down by enzyme (acetylcholinesterase or AChE)

Action Potential

Generated by increase in sodium ions in sarcolemma

Travels along the T tubules

Leads to excitation–contractioncoupling

Excitation–contraction coupling:

–action potential reaches a triad:

»releasing Ca2+

»triggering contraction

–requires myosin heads to be in “cocked” position:

»loaded by ATP energy

The Contraction Cycle

Five Steps of the Contraction Cycle

Exposure of active sites

Formation of cross-bridges

Pivoting of myosin heads

Detachment of cross-bridges

Reactivation of myosin

Fiber Shortening

As sarcomeres shorten, muscle pulls together, producing tension

Contraction Duration

Depends on

Duration of neural stimulus

Number of free calcium ions in sarcoplasm

Availability of ATP

Relaxation

Ca2+ concentrations fall

Ca2+ detaches from troponin

Active sites are re-covered by tropomyosin

Sarcomeres remain contracted

Rigor Mortis

A fixed muscular contraction after death

Caused when

Ion pumps cease to function; ran out of ATP

Calcium builds up in the sarcoplasm

The Contraction Cycle

Skeletal muscle fibers shorten as thin filaments slide between thick filaments

Free Ca2+ in the sarcoplasm triggers contraction

SR releases Ca2+ when a motor neuron stimulates the muscle fiber

Contraction is an active process

Relaxation and return to resting length are passive

Tension Production

The all–or–none principle

As a whole, a muscle fiber is either contracted or relaxed

Tension of a Single Muscle Fiber

Depends on

The number of pivoting cross-bridges

The fiber’s resting length at the time of stimulation

The frequency of stimulation

Tension of a Single Muscle Fiber

Length–tension relationship

Number of pivoting cross-bridges depends on:

–amount of overlap between thick and thin fibers

Optimum overlap produces greatest amount of tension:

–too much or too little reduces efficiency

Normal resting sarcomere length:

–is 75% to 130% of optimal length

Frequency of stimulation

A single neural stimulation produces:

–a single contraction or twitch

–which lasts about 7–100 msec.

Sustained muscular contractions:

–require many repeated stimuli

Three Phases of Twitch

Latent period before contraction

The action potential moves through sarcolemma

Causing Ca2+ release

Contraction phase

Calcium ions bind

Tension builds to peak

Relaxation phase

Ca2+ levels fall

Active sites are covered

Tension falls to resting levels

Treppe

A stair-step increase in twitch tension

Repeated stimulations immediately after relaxation phase

Stimulus frequency <50/second

Causes a series of contractions with increasing tension

Wave summation

Increasing tension or summation of twitches

Repeated stimulations before the end of relaxation phase:

–stimulus frequency >50/second

Causes increasing tension or summation of twitches

Incomplete tetanus

Twitches reach maximum tension

If rapid stimulation continues and muscle is not allowed to relax, twitches reach maximum level of tension

Complete Tetanus

If stimulation frequency is high enough, muscle never begins to relax, and is in continuous contraction

Tension Produced by Whole Skeletal Muscles

Depends on

Internal tension produced by muscle fibers

External tension exerted by muscle fibers on elastic extracellular fibers

Total number of muscle fibers stimulated

Motor units in a skeletal muscle

Contain hundreds of muscle fibers

That contract at the same time

Controlled by a single motor neuron

Recruitment (multiple motor unit summation)

In a whole muscle or group of muscles, smooth motion and increasing tension are produced by slowly increasing the size or number of motor units stimulated

Maximum tension

Achieved when all motor units reach tetanus

Can be sustained only a very short time

Sustained tension

Less than maximum tension

Allows motor units rest in rotation

Muscle tone

The normal tension and firmness of a muscle at rest

Muscle units actively maintain body position, without motion

Increasing muscle tone increases metabolic energy used, even at rest

Two Types of Skeletal Muscle Tension

Isotonic contraction

Isometric contraction

Two Types of Skeletal Muscle Tension

Isotonic Contraction

Skeletal muscle changes length:

–resulting in motion

If muscle tension > load (resistance):

–muscle shortens (concentric contraction)

If muscle tension < load (resistance):

–muscle lengthens (eccentric contraction)

Isometric contraction

Skeletal muscle develops tension, but is prevented from changing length

Note: iso- = same, metric = measure

Resistance and Speed of Contraction

Are inversely related

The heavier the load (resistance) on a muscle

The longer it takes for shortening to begin

And the less the muscle will shorten

Muscle Relaxation

After contraction, a muscle fiber returns to resting length by

Elastic forces

Opposing muscle contractions

Gravity

Elastic Forces

The pull of elastic elements (tendons and ligaments)

Expands the sarcomeres to resting length

Opposing Muscle Contractions

Reverse the direction of the original motion

Are the work of opposing skeletal muscle pairs

Gravity

Can take the place of opposing muscle contraction to return a muscle to its resting state

ATP and Muscle Contraction

Sustained muscle contraction uses a lot of ATP energy

Muscles store enough energy to start contraction

Muscle fibers must manufacture more ATP as needed

ATP and CP Reserves

Adenosine triphosphate (ATP)

The active energy molecule

Creatine phosphate (CP)

The storage molecule for excess ATP energy in resting muscle

Energy recharges ADP to ATP

Using the enzyme creatine phosphokinase (CPK or CK)

When CP is used up, other mechanisms generate ATP

ATP Generation

Cells produce ATP in two ways

Aerobic metabolism of fatty acids in the mitochondria

Anaerobic glycolysis in the cytoplasm

Aerobic metabolism

Is the primary energy source of resting muscles

Breaks down fatty acids

Produces 34 ATP molecules per glucose molecule

Anaerobic glycolysis

Is the primary energy source for peak muscular activity

Produces two ATP molecules per molecule of glucose

Breaks down glucose from glycogen stored in skeletal muscles

Energy Use and Muscle Activity

At peak exertion

Muscles lack oxygen to support mitochondria

Muscles rely on glycolysis for ATP

Pyruvic acid builds up, is converted to lactic acid

Muscle Fatigue

When muscles can no longer perform a required activity, they are fatigued

Results of Muscle Fatigue

Depletion of metabolic reserves

Damage to sarcolemma and sarcoplasmic reticulum

Low pH (lactic acid)

Muscle exhaustion and pain

The Recovery Period

The time required after exertion for muscles to return to normal

Oxygen becomes available

Mitochondrial activity resumes

The Cori Cycle

The removal and recycling of lactic acid by the liver

Liver converts lactic acid to pyruvic acid

Glucose is released to recharge muscle glycogen reserves

Oxygen Debt

After exercise or other exertion

The body needs more oxygen than usual to normalize metabolic activities

Resulting in heavy breathing

Skeletal muscles at rest metabolize fatty acids and store glycogen

During light activity, muscles generate ATP through anaerobic breakdown of carbohydrates, lipids, or amino acids

At peak activity, energy is provided by anaerobic reactions that generate lactic acid as a byproduct

Heat Production and Loss

Active muscles produce heat

Up to 70% of muscle energy can be lost as heat, raising body temperature

Hormones and Muscle Metabolism

Growth hormone

Testosterone

Thyroid hormones

Epinephrine

Muscle Performance

Power

The maximum amount of tension produced

Endurance

The amount of time an activity can be sustained

Power and endurance depend on

The types of muscle fibers

Physical conditioning

Muscle Fiber Types

Three Types of Skeletal Muscle Fibers

Fast fibers

Slow fibers

Intermediate fibers

Fast fibers

Contract very quickly

Have large diameter, large glycogen reserves, few mitochondria

Have strong contractions, fatigue quickly

Slow fibers

Are slow to contract, slow to fatigue

Have small diameter, more mitochondria

Have high oxygen supply

Contain myoglobin (red pigment, binds oxygen)

Intermediate fibers

Are mid-sized

Have low myoglobin

Have more capillaries than fast fibers, slower to fatigue

Muscles and Fiber Types

White muscle

Mostly fast fibers

Pale (e.g., chicken breast)

Red muscle

Mostly slow fibers

Dark (e.g., chicken legs)

Most human muscles

Mixed fibers

Pink

Muscle Hypertrophy

Muscle growth from heavy training

Increases diameter of muscle fibers

Increases number of myofibrils

Increases mitochondria, glycogen reserves

Muscle Atrophy

Lack of muscle activity

Reduces muscle size, tone, and power

Physical Conditioning

Improves both power and endurance

Anaerobic activities (e.g., 50-meter dash, weightlifting):

–use fast fibers

–fatigue quickly with strenuous activity

Improved by:

–frequent, brief, intensive workouts

–hypertrophy

Improves both power and endurance

Aerobic activities (prolonged activity):

–supported by mitochondria

–require oxygen and nutrients

Improved by:

–repetitive training (neural responses)

–cardiovascular training

What you don’t use, you lose

Muscle tone indicates base activity in motor units of skeletal muscles

Muscles become flaccid when inactive for days or weeks

Muscle fibers break down proteins, become smaller and weaker

With prolonged inactivity, fibrous tissue may replace muscle fibers

Cardiac Muscle Tissue

Structure of Cardiac Tissue

Cardiac muscle is striated, found only in the heart

Seven Characteristics of Cardiocytes

Unlike skeletal muscle, cardiac muscle cells (cardiocytes)

Are small

Have a single nucleus

Have short, wide T tubules

Have no triads

Have SR with no terminal cisternae

Are aerobic (high in myoglobin, mitochondria)

Have intercalated discs

Intercalated Discs

Are specialized contact points between cardiocytes

Join cell membranes of adjacent cardiocytes (gap junctions, desmosomes)

Functions of intercalated discs

Maintain structure

Enhance molecular and electrical connections

Conduct action potentials

Coordination of cardiocytes

Because intercalated discs link heart cells mechanically, chemically, and electrically, the heart functions like a single, fused mass of cells

Four Functions of Cardiac Tissue

Automaticity

Contraction without neural stimulation

Controlled by pacemaker cells

Variable contraction tension

Controlled by nervous system

Extended contraction time

Ten times as long as skeletal muscle

Prevention of wave summation and tetanic contractions by cell membranes

Long refractory period

Smooth Muscle in Body Systems

Forms around other tissues

In blood vessels

Regulates blood pressure and flow

In reproductive and glandular systems

Produces movements

In digestive and urinary systems

Forms sphincters

Produces contractions

In integumentary system

Arrector pili muscles cause “goose bumps”

Structure of Smooth Muscle

Nonstriated tissue

Different internal organization of actin and myosin

Different functional characteristics

Eight Characteristics of Smooth Muscle Cells

Long, slender, and spindle shaped

Have a single, central nucleus

Have no T tubules, myofibrils, or sarcomeres

Have no tendons or aponeuroses

Have scattered myosin fibers

Myosin fibers have more heads per thick filament

Have thin filaments attached to dense bodies

Dense bodies transmit contractions from cell to cell

Smooth Muscle Tissue

Functional Characteristics of Smooth Muscle

Excitation–contraction coupling

Length–tension relationships

Control of contractions

Smooth muscle tone

Excitation–contraction coupling

Free Ca2+ in cytoplasm triggers contraction

Ca2+ binds with calmodulin:

–in the sarcoplasm

–activates myosin light–chain kinase

Enzyme breaks down ATP, initiates contraction

Length–Tension Relationships

Thick and thin filaments are scattered

Resting length not related to tension development

Functions over a wide range of lengths (plasticity)

Control of contractions

Multiunit smooth muscle cells:

–connected to motor neurons

Visceral smooth muscle cells:

–not connected to motor neurons

–rhythmic cycles of activity controlled by pacesetter cells

Smooth muscle tone

Maintains normal levels of activity

Modified by neural, hormonal, or chemical factors