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
Next Section
Turn to “Smooth Muscle” on page 255 of your text
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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