Muscle

Functions:

Producing Movement

Maintaining Posture

Stabilizing Joints

Generating Heat

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 Tissue

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 Somatic NS

Skeletal muscle is responsible for overall body mobility.

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 voluntary (involuntary).

• 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 brief periods.

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 channels.

• 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

Pharynx

Larynx

Upper part of esophagus (skeletal muscle but involuntary in nature)

Ciliary body of eye (smooth muscle)

Involuntary muscles All smooth muscles

Skeletal muscles of Pharynx

Larynx

Upper part of esophagus

Cardiac muscle Control of contraction is myogenic, within the muscles

Autonomic NS only modifies it

According to Development

Mesodermal muscles Derived from mesodermal tissue.

Almost ALL skeletal, smooth and cardiac muscles.

Ectodermal muscles Derived from embryonic ectoderm.

Examples: Smooth muscles of Iris

Arrector pili muscle of skin

Myo Epithelial cells associated with sweat glands.

According to Phylogenetic HistorySomatic muscles

CharacterizationUniversally striated

Develop from myotomes

Controlled by Somatic NS

SubtypesSomatic 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.

Visceral musclesCharacterization

Concerned with primitive gut

Derived from mesenchyme surrounding the endodermal gut and not myotomes

controlled or modified by autonomic NS

SubtypesMuscles of pharyngeal arch apparatus

Associated with structures developing from pharyngeal arch apparatus

Striated in appearance

Visceral smooth muscles

Associated with endodermal gut tube

Non-striated

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 mitochondria.

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 mitochondria.

Few blood capillaries.

Large amount of Creatine phosphate.

Split ATP very quickly.

Fatigue easily.

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 muscle

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

Palmaris Longus

Strap or ribbon like muscles

Sartorius

Infrahyoid

Rectus Abdominis

Convergent – origin of the muscle is broad; fascicles converge toward the tendon of insertion

Pectoralis major

Pennate: Unipennate

fascicles insert into one side of the tendon

Flexor pollicis longus muscle

Bipennate fascicles insert into the tendon from both sides

Dorsal intossei

Multipennate fascicles insert into one large tendon from all sides

Subscapularis muscle

Circular – fascicles are arranged in concentric rings Orbicularis oculi

Orbicularis oris

Fan shaped muscles Temporalis muscles

Gluteus muscles

Triangular muscles Subclavius muscles

Adductor magnus muscles

Cylindrical muscles Tibialis anterior muscles

Cruciate muscles Adductor magnus muscles

According to action/role during movements

Prime movers 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.

Antagonists Muscles that oppose, or reverse, a particular movement are called

antagonists.

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

Fixators Increase the intra-articular compression and stabalize the joint

Provides the immovable base for prime movers to act upon.

Synergists 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 mover contracts

Parts of Skeletal Muscles

Origin (Proximal attachment) Proximal attachment of muscle

Doesn’t move while contraction

Can be any of the following structures… Bones

Fasciae

Tendions

Articular capsules

Intermuscular Septa

Belly Highly specialised, excitable, contractile

Highly vascular

Resistant to infection

Tendon Rounded fibrous end of a skeletal muscle

Aponeurosis Fibrous and membranous part of muscle by which it is attached to

distal parts.

Insertion Distal attachment

Movable during contraction