Although not usually cited as a major muscle
function, skeletal muscles also protect more
fragile internal organs (the viscera) by enclosure.
Also, smooth muscle, in particular, forms valves to
regulate the passage of substances through
internal body openings.
Functional Characteristics of Muscle
Excitability is the ability to receive and respond to a
stimulus, that is, any change in the environment
whether inside or outside the body. In the case of
muscle, the stimulus is usually a chemical.
Contractility is the ability to shorten forcibly when
adequately stimulated. This property sets muscle
apart from all other tissue types.
Extensibility is the ability to be stretched or
extended. Muscle fibers shorten when contracting,
but they can be stretched, even beyond their resting
length, when relaxed.
Elasticity is the ability of a muscle fiber to recoil and
resume its resting length after being stretched.
Classifications of muscles
According to structure
1. Skeletal muscle
attach to and cover the bony skeleton.
Skeletal muscle fibers are the long, cylindrical and
multi-nucleated and contractile.
Under microscope, shows alternating dark and
light bands, these stripes are called striations.
A.k.a. voluntary muscle because it is the only
type subject to conscious control, i.e. controled by
Skeletal muscle is responsible for overall body
It can contract rapidly, but it tires easily and must
rest after short periods of activity.
2. Cardiac muscle tissue
• occurs only in the heart
• Cylinderical, branching and anastomosing cells
• cardiac muscle cells are also striated, but cardiac muscle is not
• Most of us have no conscious control over how fast our heart beats,
controlled by autonomic NS.
• Cardiac muscle usually contracts at a fairly steady rate set by the heart’s
pacemaker, but neural controls allow the heart to “shift into high gear” for
3. Smooth muscle tissue
• Long, spindle shaped
• is found in the walls of hollow visceral organs, such as the stomach,
urinary bladder, and respiratory passages.
• Its role is to force fluids and other substances through internal body
• It has no striations (nonstriated), and like cardiac muscle, it is not
subject to voluntary control, controlled by autonomic NS.
• Contractions of smooth muscle fibers are slow and sustained.
According to function Voluntary muscles
All skeletal muscles EXCEPT
Upper part of esophagus (skeletal muscle but involuntary in
Ciliary body of eye (smooth muscle)
All smooth muscles
Skeletal muscles of
Upper part of esophagus
Control of contraction is myogenic, within the muscles
Autonomic NS only modifies it
According to Development
Derived from mesodermal tissue.
Almost ALL skeletal, smooth and cardiac muscles.
Derived from embryonic ectoderm.
Smooth muscles of Iris
Arrector pili muscle of skin
Myo Epithelial cells associated with sweat glands.
According to Phylogenetic History
Develop from myotomes
Controlled by Somatic NS
Somatic Axial muscles
Limited to central or axial skeleton e.g.
trunk muscles, muscles of eye ball
Somatic appendicular muscles
Limited to appendicular muscles, muscles
of upper and lower limb.
Concerned with primitive gut
Derived from mesenchyme surrounding the
endodermal gut and not myotomes
controlled or modified by autonomic NS
Muscles of pharyngeal arch apparatus
Associated with structures developing from
pharyngeal arch apparatus
Striated in appearance
Visceral smooth muscles
Associated with endodermal gut tube
Classification of Skeletal
Muscles According to relative color of skeletal muscle cell
Red muscle fiber
White muscle fiber
1. Red Muscle Fiber
Large amounts of myoglobin (gives the red color appearance!)
Many blood capillaries.
Generate ATP by the aerobic system.
Split ATP at a slow rate.
Slow contraction velocity.
Resistant to fatigue.
Found in large numbers in postural muscles.
Needed for aerobic activities like long distance running.
2. White Skeletal fibers
Low myoglobin content.
Few blood capillaries.
Large amount of Creatine phosphate.
Split ATP very quickly.
Needed for sports like sprinting.
Individual muscles are a mixture of 2 types of muscle fibers
but their proportions vary depending on the action of that
According to architecture or form
(Fascicular Architecture) Muscles can be arranged either parallel or obliquely
to the line of action of the muscle.
Parallelly arranged muscles
Quadrilateral muscles (flat quadrangle shape)
Hypoglossus muscle of tongue
Thyrohyoid muscle of larynx
Gluteus maximus muscle
Fusiform or Spindle shaped muscles
Biceps brachii muscle
Flexor Carpi radialis muscle
Strap or ribbon like muscles
Convergent – origin of the muscle is broad;
fascicles converge toward the tendon of insertion
fascicles insert into one side of the tendon
Flexor pollicis longus muscle
fascicles insert into the tendon from both sides
fascicles insert into one large tendon from all sides
Circular – fascicles are arranged in concentric
According to action/role during movements
A muscle that provides the major force for producing a specific
movement is a prime mover or agonist of that movement.
The biceps brachii muscle, which fleshes out the anterior arm (and inserts
on the radius), is a prime mover of elbow flexion.
Muscles that oppose, or reverse, a particular movement are called
When a prime mover is active, the antagonist muscles are often
stretched and may be relaxed.
flexion of the forearm by the biceps brachii muscle of the arm is antagonized by
the triceps brachii
Increase the intra-articular compression and stabalize the joint
Provides the immovable base for prime movers to act upon.
Synergists help prime movers by
adding a little extra force to the same movement
reducing undesirable or unnecessary movements that might occur as the prime
Parts of Skeletal Muscles
Origin (Proximal attachment)
Proximal attachment of muscle
Doesn’t move while contraction
Can be any of the following structures…
Highly specialised, excitable, contractile
Resistant to infection
Rounded fibrous end of a skeletal muscle
Fibrous and membranous part of muscle by which it is attached to
Movable during contraction