2. Muscular System
• A body tissue that functions for
contraction or shortening.
• Muscles are responsible for
essentially all body movements.
• The muscle is also a dominant
tissue in the heart and in the walls
of other hollow organs of the body.
• In all its forms, it makes up nearly
half of the body’s mass.
4. Muscle Functions
1. Producing movement
2. Maintaining posture
4. Generating heat
6. Constriction of organs and vessels
7. Contraction of the heart
5. Muscle Types
1. Skeletal muscles – voluntary
muscles attached to bones.
They are made up of single, very
long and cylindrical cells with
very obvious striations.
Locomotion, facial expressions,
posture, respiratory functions,
speech, and other body
movements are due to skeletal
muscle contraction. The nervous
system controls the voluntary
aspects of skeletal muscle.
6. Muscle Types
2. Cardiac muscles – involuntary
muscles found only in the walls
of the heart. They are made up
of branching chains of cells with
striations. Its contractions provide
the major force for moving blood
through the circulatory system.
7. Muscle Types
3. Smooth muscles – involuntary
muscles widely distributed in the
body such as in the walls of
hollow visceral organs, stomach,
intestines, uterus, blood vessels,
ducts of glands, and respiratory
passages. They are made up of
cells with no striations. Smooth
muscle contraction propels urine
through the urinary tract, mixes
food in the stomach and small
intestine, and regulates the flow
of blood through blood vessels.
9. Muscle Properties
• Contractility – is the ability of
muscle to shorten forcefully, or
• Excitability – is the capacity of the
muscle to respond to an electrical
• Extensibility – a muscle can be
stretched beyond its normal
resting length and still be able to
• Elasticity – is the ability of muscle
to spring back to its original resting
length after it has been stretched.
10. Muscle Structure
• Muscle fiber or myocyte – individual muscle cell (with a length range
from 1 to 40 mm) that contains multiple nuclei and other organelles.*
• Fascicles – bundle of parallel skeletal muscle fibers.
• Muscle fascia – layer of connective tissue that surrounds individual
muscles and groups of muscles. These outer layer keep the muscles
separate from surrounding tissues and organs.
• Epimysium – forms a connective tissue sheath that surrounds each
skeletal muscle. Its protein fibers gradually merge with the muscular
• Perimysium – connective tissue sheath that surrounds fascicles,
subdividing each whole muscle into numerous bundles of muscle fibers.
• Endomysium – delicate layer of connective tissue that separates the
individual muscle fibers within each fascicle. It serves as passageways for
nerve fibers and blood vessels that supply each separate muscle fiber.
13. Electrical Components
• Sarcolemma – is the plasma membrane of muscle fibers.*
• Transverse tubules or T tubules – are tube-like inward folds of the
sarcolemma. At regular intervals along the muscled fiber, the
sarcolemma forms T tubules by projecting and extending into the
interior of the muscle fiber. T tubules carry electrical impulses into the
center of the muscle fiber so that every contractile unit of the muscle
fiber contracts in unison.
• Sarcoplasmic reticulum – highly specialized smooth endoplasmic
reticulum in skeletal muscle fibers that stores high levels of calcium.**
• Terminal cisternae – enlarged portions of the sarcoplasmic
reticulum. T tubules lie next to it.
• Triad – formed by two terminal cisternae and their associated T
15. Mechanical Components
• Myofibril – bundle of parallel protein filaments running the length
of the muscle fiber. Hundreds to thousands of cylindrical myofibrils
occupy most of the muscle cell’s volume.*
• Sarcomere – structural and functional unit of the myofibril. It is the
smallest portion of a muscle that can contract. The myofibril is
divided into hundreds or thousands of sarcomeres.**
• Myofilaments – individual thick and thin filaments that make up a
myofibril. The interaction of these myofilaments are the basis of
16. Mechanical Components
• Actin – a thin filament of two entwined strands of proteins that
makes up the sarcomere.
• Tropomyosin – a long fibrous protein that lies in the groove along
the fibrous actin strand. In a relaxed muscle, tropomyosin is
covering the active sites of the actin where thick myosin filament
binds. A muscle cannot contract until it moves to uncover the
• Troponin – is attached to tropomyosin and lies within the groove
between actin filaments in the muscle tissue.*
17. Mechanical Components
• Myosin – a thick filament of protein that also makes up the
• Myosin heads – bind and pull on the actin filaments, causing actin
to slide in between each myosin set, therefore shortening the entire
contractile unit. When this process is duplicated across many
sarcomere units, the entire muscle or group of muscles is enabled
to contract, consequently causing a desired movement.
21. Neuromuscular Junction
• Motor unit – made up of a motor neuron synapsed to the muscle
fibers. The motor neurons carry action potentials or electrical signals
which stimulate muscles to contract.
• Neuromuscular junction or synapse – is the point of contact of motor
neuron axon branches with the muscle fiber.
• Acetylcholine – the neurotransmitter that stimulates the skeletal
muscle fibers to contract.
23. Muscle Stimulation
1. Action potentials from the CNS are conveyed along the motor
neuron’s axon, stimulating the release of acetylcholine in the
2. Acetylcholine then binds to receptor proteins on the motor end-
plate causing an electrical wave to race along the sarcolemma.
3. The electrical signal causes the muscle cell’s sarcoplasmic
reticulum to release calcium ions into the cytosol. T-tubules permit
rapid transmission of the action potential into the cell, and also play
an important role in regulating cellular calcium concentration.
24. Muscle Contraction
4. Calcium ions now in the sarcoplasm bind to troponin causing the
tropomyosin to move. This exposes the active sites of actin, allowing
it to bind to myosin forming a cross-bridge.
5. Sliding filament model: the cross bridge bends, pulling on actin and
causing to slide past myosin. This motion shortens the sarcomere
without changing the lengths of the filaments. Actin and myosin
must touch for these filaments to slide past each other. A muscle cell
contracts when the actin filaments slide with the myosin filaments.
6. The myosin head then binds a molecule of ATP, breaking it down to
ADP and a phosphate group. This releases energy prompting the
myosin head back to its original position and releases the actin. The
myosin head is now ready to contact another actin.
25. Muscle Relaxation
7. Muscle relaxation occurs when acetylcholine is no longer released
at the neuromuscular junction.
8. The cessation of action potentials along the sarcolemma stops
calcium release from the sarcoplasmic reticulum and calcium
returns into the sarcoplasmic reticulum via active transport
9. As calcium concentration decreases in the sarcoplasm, calcium
diffuses away from actin’s troponin causing tropomyosin to block
again the active sites of actin. This leads to muscle relaxation as the
cross-bridge cannot re-form anymore.*
29. Types of Muscle Contractions
1. Isometric contractions – type of contractions where a muscle does
not shorten. This type of contraction increases the tension in the
muscle, but the length of the muscle stays the same.*
2. Isotonic contractions – type of contractions where the muscle
shortens. This type of contraction increases the tension in the muscle
and the length of the muscle decreases.**
• Concentric contractions – isotonic contractions in which tension in
the muscle is great enough to overcome the opposing resistance,
and the muscle shortens.*
• Eccentric contractions – isotonic contractions in which tension is
maintained in a muscle, but the opposing resistance is great
enough to cause the muscle to increase in length.**
31. Antagonistic Muscle Pairs
Muscle contraction causes a joint, such as the elbow or knee, to flex.
Extension of the joint and lengthening of the muscle result from the
contraction of muscles that produce the opposite movement.
1. Agonist muscles – prime mover muscles that causes a particular
2. Antagonist muscles – muscles that oppose or reverse a movement.
3. Synergist muscles – help agonists by producing the same movement
or by reducing undesirable movements.
4. Fixator muscles – are specialized synergists. They hold a bone still or
stabilize the origin of the agonist muscles so all the tension can be
used to move the insertion bone.
33. Muscle Activity
• With a few exceptions, all muscles cross at least one joint and the
bulk of the muscle lies near to the joint crossed.*
• All muscles have at least 2 attachments:
• Origin or fixed end – muscle is attached to the immovable or less
movable bone in its proximal end. Some muscles have more than
• Insertion or mobile end – muscle is attached to the movable bone
in its distal end. The attached bone is being pulled toward the
other bone of the joint.*
• Muscles can only pull and can never push. The specific body
movement a muscle contraction causes is called the muscle’s action.
During contraction, the muscle insertion moves toward the origin.
35. Muscle Shapes
The shape and size of any given muscle greatly influence the degree to
which it can contract and the amount of force it can generate.
The shape of the muscle determines the type of movement it has and is
determined by the arrangement of fascicles.
Fascicle arrangements vary, producing muscles with different structures
and functional properties.*
1. Circular muscles – have their fascicles arranged in a circle around an
opening and act as sphincters to close the opening.
2. Convergent muscles – have fascicles that join at one common tendon
from a wide area, which creates muscles that are triangular in shape.
36. Muscle Shapes
3. Parallel muscles – similar with convergent muscles, have fascicles
that are organized parallel to the long axis of the muscle, but they
terminate on a flat tendon that spans the width of the entire muscle.
4. Fusiform muscles – fascicles run the length of the entire muscle and
taper at each end to terminate at tendons. These muscles have
expanded midsection than the ends.
5. Pennate muscles – have fascicles that emerge like the barbs on a
feather from a common tendon that runs the length of the entire
• Unipennate muscles – fascicles are on one side of the tendon
• Bipennate muscles – fascicles arranged on two sides of the tendon
• Multipennate muscles – fascicles arranged at many places around
the central tendon