Muscle System
Muscle has four major functional characteristics: contractility,
excitability, extensibility, and elasticity. Contractility refers to the
capacity of muscle to contract or shorten forcefully. Excitability means
that muscle responds to stimulation by nerves and hormones, making it
possible for the nervous system and, in some muscle types, the endocrine
system, to regulate muscle activity. Extensibility means that muscles can
be stretched to their normal resting length and beyond to a limited
degree. Elasticity means that if muscles are stretched, they recoil to
their original resting length.
Striated or skeletal muscle is attached to bone and makes movement
possible. A muscle’s structural pattern is a series of increasingly smaller
parallel units. The muscle is composed of fascicles. Each fascicle consists
of several fibers. Each fiber is an elongated cell with many nuclei. Within
each fiber are myofibrils composed of thick and thin filaments made of
protein. The regular arrangement of these filaments gives striated
muscle its striped appearance. The basic functional unit of a muscle is
the sarcomere, a section of a myofibril. One is shown here, bordered by
the crooked blue lines. A sarcomere is composed of thick filaments of
myosin, in red, and thin filaments of actin, in blue. Muscle contraction is
the result of these thick and thin filaments sliding past one another
A skeletal muscle fiber (cell) contains numerous myofibrils, each
consisting of units called sarcomeres. A sarcomere extends from one Z
disc to the next Z disc. Thus, one myofibril is a series of sarcomeres.
The characteristic striations of a sarcomere result from the spatial
arrangement of actin and myosin filaments.
Parts of a Muscle (Part 1)
Muscle is composed of muscle fasciculi, which can be seen by the unaided eye. The fasciculi are
Parts of a Muscle (Part 2)
Each muscle fiber (cell) contains myofibrils, in which the banding patterns, called striations
Parts of a Muscle (Part 3)
The striations seen in myofibrils result from the
structure of units called sarcomeres.
Parts of a Muscle (Part 4)
Each sarcomere is a highly organized structure
extending from one Z disc to the next Z disc and
consists mainly of actin and myosin myofilaments.
The actin and myosin myofilaments are formed from
thousands of actin and myosin molecules.
T Tubules and Sarcoplasmic Reticulum
The sarcolemma has along its surface many tubelike invaginations
called transverse, or T, tubules, which are regularly arranged and
project into the muscle fiber and wrap around sarcomeres in the
region where actin myofilaments and myosin myofilaments
overlap. The lumen of each T tubule is filled with extracellular
fluid. The walls of the T tubule are continuous with the exterior
of the muscle fiber. Suspended in the sarcoplasm between the T
tubules is a highly specialized, smooth endoplasmic reticulum
called the sarcoplasmic reticulum. Near the T tubules, the
sarcoplasmic reticulum is enlarged to form terminal cisternae.
Together, a T tubule and the two adjacent terminal cisternae are
called a triad. The sarcoplasmic reticulum membrane actively
transports calcium ions from the sarcoplasm into its lumen, where
calcium ions are stored in very high concentrations compared
with the sarcoplasm.
Neuromuscular Junction
Each skeletal muscle fiber is connected by an axon from a
nerve cell called a motor neuron. This motor neuron extends
outward from the brain or spinal cord, to the muscle fiber. The
muscle fiber contracts only when a motor neuron stimulates it.
The functional connection between the motor neuron and muscle
fiber is called a neuromuscular junction. Here, the muscle fiber
membrane is specialized to form a motor end plate. In this region of
the muscle fiber, nuclei and mitochondria are abundant, and the cell
membrane (sarcolemma) is extensively folded.
The end of the motor neuron branches and projects to the motor
end plate. The cytoplasm at the terminal ends of these motor
neuron fibers is rich in mitochondria and contains many tiny vesicles (synaptic vesicles) that store chemicals called neurotransmitters
When a nerve impulse traveling from the brain or
spinal cord reaches the end of a motor neuron fiber, some of
the vesicles release a neurotransmitter into the gap (synaptic
cleft) between the neuron and the motor end plate of the
muscle fiber. Only when the neurotransmitters bind to the
muscle membrane will the muscle depolarize and lead to
stimulation of muscle contraction.
Motor Unit
A motor unit consists of one motor neuron and all the muscle fibers with which it innervates.
When a nerve impulse arrives at the end of an axon, it stimulates all the muscle fibers supplied
To activate skeletal muscle, the central nervous system initiates an action potential that travels
down the spinal cord to the motor neurons. As the nerve fiber branches, the action potential
travels down each branch. Each nerve fiber branches many times and stimulates several skeletal
muscle fibers. The union of the axon and muscle fiber is called the neuromuscular junction.
Zooming into the microscopic level, each branch of the neuron has a terminal that invaginates
the muscle fiber, while remaining outside the muscle fiber plasma membrane. The action
potential arrives at the axon terminal. In the terminal the action potential causes the release
of acetylcholine from the synaptic vesicles into space between the axon terminal and the muscle
fiber called the synaptic cleft. In the synaptic cleft, the acetylcholine binds with a receptor
site on the fiber membrane, which opens a chemically-gated ion channel. Sodium then rushes
through the ion channel into the muscle fiber, causing an action potential to form on the fiber
membrane. The action potential spreads along the muscle fiber. As more nerve branches
activate additional fibers, the action potential spreads over the entire muscle. Upon activation,
the muscle contracts.
Sliding Filament Theory
The oxygen required to support aerobic
respiration is carried in the blood and stored in
myoglobin. In the absence of sufficient oxygen,
pyruvic acid is converted to lactic acid. The
maximum number of ATPs generated per glucose
molecule varies with cell type and is 36 (2 + 34) in
skeletal muscle.
Skeletal muscles, like the biceps brachii, attach
to bone via connective tissue called tendons.
Muscles are composed of bundles of muscle
fibers. Each bundle is separated by connective
tissues known as perimysium. Each fasciculus is
made up of muscle fibers, which are separated by
connective tissue called endomysium. Skeletal
muscle fibers or cells are multinucleated and
striated in appearance. Muscle cells are composed
of subunits called myofibrils. Each myofibril is
made up of several myofilaments.
The two types of myofilaments (shown in red) are composed primarily of the protein myosin and a thin myofilament (shown in blue) composed
mainly of the protein actin. The repeating arrangement of thick and thin myofilaments serves as the fundamental subunit of striated muscle
contraction. These subunits are called sarcomeres. A sarcomere contraction is represented by the shortening of the distance between the Z
lines. The sarcomere shortens because the thin filaments slide past the thick filaments. In 3D each thick myofilament is surrounded by six thin
myofilaments arranged in a hexagonal pattern. The 3D arrangement of sliding myofilaments is the microscopic basis of muscle contraction.
At the start of contraction, stored calcium ions are released into the cytoplasm. Calcium ions expose binding sites on the actin molecules.
Globular heads on the myosin can bind to these sites, forming cross bridges. Repeated binding and release moves the filaments relative to one
another and as this occurs simultaneously in many sarcomeres, the entire muscle shortens or contracts. This process requires energy in the
form of ATP.
Smooth Muscle
Smooth muscle is distributed widely throughout the body and is
more variable in function than the skeletal muscle and cardiac
muscle. Smooth muscle cells are smaller than skeletal muscle cells.
They are spindle-shaped, with a single nucleus located in the middle
of the cell. Compared with skeletal muscle, there are fewer actin
and myosin myofilaments. Although the myofilaments approximate a
longitudinal, or spiral, orientation within the smooth muscle cell,
they are not organized into sarcomeres. Consequently, smooth
muscle does not have a striated appearance.
Smooth muscle can be either single-unit or multiunit. Single unit smooth muscle is more common than multiunit smooth muscle. The cells of a
single-unit smooth muscle are electrically coupled to each other by gap junctions, so they directly stimulate each other and a large number of
cells contract as a single unit. It normally occurs in sheets and includes smooth muscle of the digestive, reproductive, and urinary
tracts.Multiunit smooth muscle occurs as sheets such as in the walls of blood vessels, in small bundles such as in the arrector pili muscles and
the iris of the eye, or as single cells, as in the capsule of the spleen. Each motor unit of multiunit smooth muscle contracts independently of the
others
Cardiac Muscle
Cardiac muscle is found only in the heart. Cardiac muscle tissue is striated like skeletal muscle, but each cell usually contains one nucleus located
near the center unlike skeletal muscle which is multinucleated. Cardiac cells branch like a y, and at each end they form junctions with another
myocyte at a visibly dark line called an intercalated disc. The
intercalated discs have gap junctions that allow action
potentials to pass from cell to cell. Cardiac muscle cells are
autorhythmic, and one part of the heart normally acts as the
pacemaker. The action potentials of cardiac muscle are
similar to those in nerve and skeletal muscle but have a much
longer duration and refractory period