Muscle Tissue

Chapter 10

Overview of Muscle Tissue There are three types of muscle tissue

– Skeletal muscle– Cardiac muscle– Smooth muscle

These muscle tissues differ in the structure of their cells, their body location, their function, and the means by which they are activated to contract

Overview of Muscle Tissue

All skeletal and smooth muscle cells are elongated and are referred to as muscle fibers

Muscle contraction depends on two types of myofilaments, actin and myosin

All prefixes of myo or mys and sarco reference muscle

Skeletal Muscle Tissue

Skeletal muscle tissue appears as distinct skeletal muscle that attach to the skeletal system

Skeletal muscle has obvious striations It is a voluntary muscle under conscious control

Cardiac Muscle Tissue

Cardiac muscle occur only in the heart The muscle is striated but involuntary Cardiac fibers are short, fat, branched and

interconnected Cardiac muscle cells are interlocked by

intercalated discs and function as a single unit

Smooth Muscle Tissue

It is found in the walls of hollow organs such as the stomach, urinary bladder, and intestines

It has no striations It is not subject to voluntary control

Differences in Contractions

Skeletal muscle can contract rapidly but tire easily and must be rested

Skeletal muscle contractions vary in force depending on use

Cardiac muscle contracts at a steady rate but can accelerate to cope with demand

Smooth muscle contracts in steady, sustained contractions and continues on tirelessly

Muscle Functions Muscle performs four important

functions in the body:– Producing movement– Maintaining posture– Stabilizing joints– Generating heat

Producing Movement

Movement results from skeletal muscle contraction

Skeletal muscle are responsible for all locomotion and manipulation

Allows you to interact or react with your external environment

It controls eye movement, facial expression (skeletal); circulation (cardiac), and moves gas, liquids, and solids through organs (smooth)

Maintaining Posture

Skeletal muscles are utilized constantly to maintain sitting, standing, and moving postures

Postural muscle develop to compensate for the never ending pull of gravity– Our developmental milestones as an infant

are our initial victories over gravity Curves of the spinal column are shaped

by the interplay of skeletal muscle and gravity

Stabilizing Joints

Skeletal muscle provide the dynamic stability of joints

Many joints are poorly reinforced by ligaments and connective tissue

Many joints have noncomplementary surface which do not contribute to stability

Generating Heat

Muscles generate heat as they contract The heat generated is vitally important to

maintain normal body temperature Skeletal muscle generates most of the

heat because it represents 40% of body mass

Excess heat must released to maintain body temperature

Functional Characteristics Excitability or irritability

– It has the ability to respond to a stimulus Contractility

– It has the ability to shorten forcibly Extensibility

– Muscle fibers can be stretched Elasticity

– Resume its normal length after being shortened

Skeletal Muscle

Anatomy of a Skeletal Muscle Each skeletal

muscle is a discrete organ with thousands of fibers

Muscle fibers predominate the tissue but it also contains, blood vessels, nerve fibers, and connective tissue

Connective Tissue Wrappings Each muscle fiber

is wrapped by fine sheath of areolar connnective called endomysium

Several fibers are gathered side by side into bundles called fascicles

Each fascicle is bound by collagen a fiber layer called the perimysium

Connective Tissue Wrappings Fascicles are

bound by a dense fibrous connective tissue layer called the epimysium

The epimysium surrounds the entire muscle

External to the epimysium is the deep fascia that binds muscles into functional groups

Connective Tissue Wrappings All the connective tissue

layers are continuous with one another as well as with the tendons that join muscles to bone

When muscle fibers contract they pull these connective tissue sheaths which in turn transmit the force to the bone to be moved

Connective tissues supports each cell

Nerve and Blood Supply Normal activity of skeletal muscle is totally

dependent on its nerve and blood supply Each skeletal muscle fiber is controlled by a

nerve ending (neuromuscular junction) Contracting muscle fibers use huge amounts

of energy which requires a continuous supply of oxygen and nutrients

In general, each muscle is served by an artery and one or more veins

Attachments Most muscles span joints and have at least

two attachments an origin and an insertion Origin

– Attachment of a muscle that remains relatively fixed during muscular contraction

– Generally a more proximal or axial location Insertion

– Attachment of a muscle that moves during muscular contraction

– Generally a more distal or appendicular attachment

Attachments Direct attachments

have the epimysium attaching directly to the periosteum of the bone or perichondrium of a cartilage

Indirect attachments have the epimysium attaching to a tendon or an aponeurosis

Temporalis has both muscle attachments

Contraction of Skeletal Muscle The principles of contraction of a muscle

cell can be generalized to the entire muscle The force exerted is called tension, the

resistance to the force is called the load A contracting muscle does not always

shorten (isometric or isotonic) Skeletal muscle can contract with varying

force for different periods of time which enhances its efficiency

The Motor Unit Each muscle is served by at least one

motor nerve which contains hundreds of motor neuron axons

As a nerve enters a muscle it branches into a number of axonal terminals, each of which forms a neuromuscular junction with a single nerve fiber

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

The Motor Unit When a motor

neuron transmits an electrical impulse, all the muscle fibers that it innervates respond by contracting

The average number of muscle fibers per unit is 150, but it ranges from 4 to several hundred

The Motor Unit Muscles that exert

very fine control have small motor units (eyes, fingers)

Large muscles of locomotion and weight bearing have large motor units and as a consequence have less precise control

The Motor Unit The muscle fibers

in a unit are not clustered together but rather are spread throughout the entire muscle

Stimulation of a single unit causes a weak contraction of the entire muscle

This allows control of the intensity of the contraction

Skeletal Muscle Fiber Skeletal muscle fibers

are long and cylindrical These cells are huge Diameter of 10-100 m

up to 10 times average cell size

Length is phenomenal for a cell - from several centimeters to dozens of centimeters in long muscles

Skeletal Muscle Fiber These cells actually

form by the fusion of hundreds of embryonic cells

Because of its development skeletal muscle fiber contains many nuclei

Nuclei lie at the cell periphery, just deep to the sarcolemma

Myofibrils and Sarcomeres Under the microscope

stripes called striations are visible in skeletal muscle fibers

These striations result from the internal structure of long rods called myofibrils within the sarcoplasm

Note that fibrils are to be distinguished from fibers and filaments

Myofibrils and Sarcomeres Myofibrils are

unbranched cylinders that are present in large numbers making up 80% of the sarcoplasm

Myofibrils can be conceptualized as specialized contractile cellular organelles unique to muscle fibers

Myofibrils and Sarcomeres Different myofibrils in

a fiber are separated and surrounded by narrow regions of sarcoplasm that contain rows of mitrochondria and glycosomes that supply energy for muscle contraction

Myofibrils and Sarcomeres Distinguishing

individual myofibrils is histologically difficult because the striations of adjacent myofibrils line up almost perfectly

Myofibrils and Sarcomeres A myofibril is a long

row of repeating segments called sarcomeres

The sarcomere is the basic unit of contraction in skeletal muscle

The boundaries at each end of the sarcomere are called z discs

Myofibrils and Sarcomeres Attached to each Z disc

and extending toward the center of the sarcomere are many fine myofilaments called thin (actin) filaments, which consist primarily of the protein actin, although they contain other proteins as well

Myofibrils and Sarcomeres In the center of each

sarcomere and overlapping the inner ends of the thin filaments is a cylindrical bundle of thick (myosin) filaments

Myofibrils and Sarcomeres Myosin filaments are

mostly myosin and some ATPase enzymes that split ATP to release energy required for muscle contraction

Both ends of a thick filament are studded with knobs called myosin head or cross bridges

Types of Skeletal Muscle Fiber Not all skeletal muscle fibers are alike as

they vary on the type of contractions they produce

Muscle fiber can be divided by the strength, speed, and endurance of the contraction to which they contribute

Specifically the fibers are referred to as red slow twitch, white fast twitch and intermediate fast-twitch fibers

Types of Skeletal Muscle Fiber Red slow twitch are relatively thin fibers They are named for the abundant

myoglobin (oxygen binding pigment) in their sarcoplasm

Red fibers obtain their energy from aerobic (oxygen requiring) reactions and thus have relatively large numbers of mitrochondria (the site of aerobic metabolism) and a rich blood supply from an extensive network of capillaries

Types of Skeletal Muscle Fiber Red slow twitch fibers contract slowly,

are resistant to fatigue as long as oxygen is present

Deliver prolonged contractions Used in many of the postural muscles of

the axial skeleton Because their fibers are thin, slow twitch

fibers do not generate much power

Types of Skeletal Muscle Fiber White fast twitch fibers are pale because

they contain little myoglobin The fibers are about twice the diameter

of red slow twitch fibers, they contain more myofibrils and generate more power

The fibers depend on anaerobic pathways (no oxygen used) to make ATP

Types of Skeletal Muscle Fiber They contain few mitrochondria or

capillaries but have many glycosomes containing glycogen as a fuel source

White fast twitch fibers contract rapidly and tire quickly

This fiber type is common in the muscle of the upper limbs

Used to lift heavy objects for brief periods

Types of Skeletal Muscle Fiber Intermediate fast twitch are sized

between the other two fiber types Like white fibers they contract quickly;

like slow twitch they are oxygen dependent and have a high myoglobin content and a rich supply of capillaries

Because they are intermediate fibers they depend largely on aerobic metabolism, and are less fatigue resistant

Types of Skeletal Muscle Fiber They are more powerful than red fibers,

but not as strong as white This type of fiber is abundant in the

muscles of the lower limbs Used to move the body for long periods of

time in activities like walking and jogging

Types of Skeletal Muscle Fiber Because muscles contain a mixture of the

three fiber types, each muscle can perform different tasks at different times

For example the Gastrocnemius muscle can be used for sprinting, walking and as a postural muscle

Although everyone’s muscles contain mixtures of the three fiber types, some people have relatively more of one type

These differences are genetically controlled

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Smooth Muscles Smooth muscle

lacks the courser connective tissue seen in skeletal muscle

Small amounts of endomysium is found between smooth muscle fibers

Smooth Muscles Smooth muscles are

organized into sheets of closely apposed fibers

These sheets occur in the walls of all but the smallest blood vessels and in the walls of hollow organs of the respiratory, urinary digestive and reproductive tracts

Smooth Muscles In most cases two

sheets of muscles are present with their fibers aligned at right angle to each other

These forms the longitudinal (long axis) and circular (encircling) layer

These two layers squeeze the contents of the organ

End of Chapter

Chapter 9