The Muscular System

The Muscular System

*The sole function of muscle tissue is to contract or shorten.

As it contracts it -causes movement-maintains posture-stabilizes joints-and generates heat.

Muscle Types Skeletal muscle (voluntary)

muscle attached to the skeleton cells are long, striated, and multinucleate Connective tissue coverings (endomysium, perimysium,

and epimysium) enclose and protect the muscle fibers and increase the strength of skeletal muscles.

Smooth muscle (involuntary) cells are uninucleate, spindle-shaped, and arranged in

opposing layers in the walls of hollow organs. Cardiac muscle (involuntary)

cells are striated, branching and fit closely together arranged in spiral bundles in the heart.

Skeletal Muscle

Connective Tissue Wrappings*endomysium: forms a sheath around a fiber

*perimysium: coarse membrane that surrounds several sheathed fibers

*fascicle: bundle of fibers covered in perimysium

*epimysium: tough overcoat that surrounds several fascicles that make a muscle

Attachment

Skeletal muscles attach to bone using tendons or aponeuroses. Tendons: stong, cord-like structures▪ Can cross bony projections▪ Used to stabilize joints

Aponeuroses: sheet-like structures that attach muscle indirectly

Microscopic Anatomy

•sarcolema- plasma membrane that serves as the “muscle husk”•myofibrils- long ribbon-like organelles which nearly fill the cytoplasm.

•Alternating light (I) and dark (A) bands give the striped appearance.•Midsection of the light band is the Z disc•Midsection of the dark band is the H zone

Microscopic Anatomy, Cont.

•sarcomere- unit of a myofibrile that stretches from one Z to the next Z.•Myofilaments- threadlike protein structures within the sarcomere

•Myosin- thick fibers that split ATP to generate power.

•Heads or cross bridges link filaments during contraction.

•Actin- thin fibers that are anchored to the Z disc

Microscopic Physiology

During contraction: Myosin heads pull

on actin filaments. Actin filaments slide

toward the center of the sarcomere.

Light zones disappear.

Skeletal Muscle Activity

Special functional properties: Irritability

Ability to receive and respond to stimuli

Contractility Ability to forcibly shorten

Nerves and muscles: Motor unit: one

neuron and all muscle it stimulates.

Neuromuscular junction: where nerve fiber ends (axon terminals) at skeletal muscle.

Synaptic cleft: gap between axon terminals and sarcolema.

How it works: Nerve impulse to axon Chemical release of

neurotransmitter. ACh crosses synaptic

cleft and attaches to receptors.

Muscle becomes permeable to Na+.

Inward rush of Na generates electrical impulse, Action Potential.

AP travels over muscle causing contraction.

Energy for Muscle Contractions Direct phosphorolation of ADP by

creatine phosphate. ATP transfer of phosphate group

from CP to ADP ATP Lasts about 20 seconds. (makes 1 ATP)

Aerobic Respiration C6H12O6 CO2 + H2O + energy (ATP) Yields 36 ATP. Requires oxygen.

Energy for Muscle Contractions, Cont. Anaerobic Glycolysis and lactic acid

formation. Glucose pyruvic acid + 2 ATP Without oxygen, pyruvic acid converts to

lactic acid. Fast process. Good for 30-60 seconds of

energy. Lactic acid accumulation causes muscle

fatigue and soreness.

Skeletal Muscle MovementsTypes and Names

Attachment of skeletal muscles Origin

Attachment to the immovable or less movable bone

Insertion attachment to the movable bone

**When a muscle contracts, the insertion moves toward the origin.

Example:

During contraction of the biceps, the insertion moves toward the origin.

Flexion Movement that

decreases the angle of a joint.

Bringing two bones closer together

Extension Increases the

distance or angle between two bones.

If extension is >180 degrees, it is called hyperextension.

Rotation Movement of a

bone around its longitudinal axis.▪ Lateral: away

from midline▪ Medial: toward

midline

Abduction away from

central axis of the body

Adduction closer to central

axis of the body

Circumduction Common in ball and

socket joints Proximal end of

limb is stationary while distal end moves in a circle.

Special Movements

SPECIAL MOVEMENTS OF THE FOOT.

UP AND DOWN MOVEMENTS AT THE ANKLE.

Special Movements, cont.MOVEMENTS OF THE RADIUS AROUND THE ULNA.

MOVEMENT OF THE THUMB.

Types of Skeletal Muscles

Vocabulary Prime mover

Muscle that has the major responsibility for causing a movement.

Antagonist Muscles that oppose or reverse a movement

Synergyst Help prime movers

Fixator Special synergists Stabilize the origin of the prime mover

Developmental AspectsHomeostatic Imbalance

Development

In the fetus, muscles are laid down in segments and then segments are invaded by nerves.

Occurs very early in the pregnancy.

Homeostatic Imbalance Muscular Dystrophy

Congenital muscle-destroying disease Duchenne: usually diagnosed between age 2-6▪ Normal child begins to fall, lose coordination.▪ Wheel chair by age 12▪ Generally do not live beyond teens

Myastenia gravis Happens in adults Shortage of acetylcholine receptors at

neuromuscular junctions results in generalized muscle weakness.

Homeostatic Imbalance, cont. Nerve Damage

Destruction of nerve supply to muscle causes the muscle to lose tone and become paralyzed.

Over time, the muscle with become soft and atrophy.