Skeletal Muscles

Anatomy and innervation of skeletal muscle tissue

• Connective tissue components:– Fascia (“bandage”) –sheet or band of fibrous

C.T. under the skin or around organs– Superficial fascia (subcutaneous fascia):

• Areolar C.T. and adipose tissue• Stores water and fat• Reduces heat loss (insulates)• Protects against trauma• Framework for nerves and blood vessels

• Deep fascia:– Dense irregular C.T. – holds muscles together

and separates them into groups– 3 layers:

• Epimysium – surrounds the whole muscle• Perimysium – separates muscle into

bundles of muscle fibers – fascicles• Endomysium – covers individual fibers

• These three layers come together to form cords of dense, regular connective tissue called tendons. Tendons attach muscle to the periosteum of bones.

• When the connective tissues form a broad, flat layer, the tendon is called an aponeurosis.

Microscopic Anatomy

• Muscle cells are called muscle fibers or myofibers

• Plasma membrane – sarcolemma

• Cytoplasm – sarcoplasm

• Myoblasts fuse to form one myofiber – several nuclei

• Myofibrils run lengthwise

Myofibrils are made of filaments

• Thin filaments – primarily actin

• Thick filament – myosin

• Elastic filaments

• Sarcomeres are the basic, functional units of striated muscle fibers.

Thick filaments

• Made of about 200 molecules of myosin

• Each myosin molecule has two “heads”

• Each head has an actin binding site and an ATP binding site

• The ATP site splits ATP and transfers energy to myosin head; which remains charged (“cocked”) until contraction.

Thin Filaments

• Actin molecules form a helix– Each actin molecule has a myosin binding site

• Other proteins:

• Tropomyosin – long, filamentous protein, it wraps around the actin and covers the myosin binding sites

• Troponin – a smaller molecule bound to tropomyosin, it has calcium binding sites.

Sarcoplasmic reticulum• Specialized smooth E. R.

• Tubes fuse to form cisternae

• In a relaxed muscle, S.R. stores Ca++ (Ca+

+ active transport pumps)

• When stimulated, Ca++ leaves through Ca+

+ release channels.

Transverse tubules (T-tubules)

• Infoldings of sarcolemma that penetrate into muscle fiber at right angles to filaments. They are filled with extracellular fluid.

• T-tubules and the cisternae on either side form a triad.

Blood and nerve supply

• Muscle contraction uses a lot of ATP

• To generate ATP, muscles need oxygen

• Each muscle fiber is in close contact with one or more capillaries

• Motor neurons – originate in brain and spinal cord; cause muscle contraction

Motor unit

• A Motor Unit is made of one motor neuron and all the muscle fibers it innervates.

• These cells all contract together.

• A single motor unit can have 2 – 2,000 muscle fibers.

• Precise movements are controlled by small motor units, and large movements by large motor units.

Neuromuscular Junction (NMJ)

• Nerves communicate with muscles and other organs at structures called synapses.

• Synaptic cleft – gap between neuron and sarcolemma

• Axon releases a chemical called a neurotransmitter – Acetylcholine (Ach)

• Axon branches into axon terminals.

• At the end of each axon terminal is a swelling called the synaptic end bulb.

• The region across the synaptic cleft from the synaptic end bulb is called the motor end plate.

• The sarcolemma of the motor end plate is folded and contains many receptors for ACh .

• When a nerve impulse reaches the synaptic end bulbs, it causes synaptic vesicles to fuse with the membrane and release ACh by exocytosis.

• Acetylcholine diffuses across the synaptic cleft, and binds with receptors on the motor end plate.

• This binding causes the receptor to change shape, and opens Na+ channels in the membrane.

• When enough Na+ channels are opened, an electrical current is generated and is carried along the sarcolemma. This is called a muscle impulse or muscle action potential. This electrical activity can be recorded in an electromyogram.

Sliding Filament Mechanism

• When a nerve impulse reaches an axon terminal, the synaptic vesicles release

acetylcholine (ACh)

• ACh crosses the synaptic cleft and binds with

receptors on the motor end plate.

• This binding opens channels that allow

sodium to rush in, beginning a muscle action potential in the sarcolemma.

• The action potential or impulse travels down the sarcolemma and into the T-tubules, causing the sarcoplasmic reticulum to release Ca++ into the sarcoplasm.

• The Ca++ binds to the troponin, which changes shape, pulling the tropomyosin away from the myosin binding sites on the actin.

• The activated myosin attaches to the actin, forming actin/myosin crossbridges.

• The myosin head moves toward the center of the sarcomere, pulling the actin filaments past the myosin. This is called a power stroke.

• When the myosin heads turn, they release ADP, and ATP binds to the heads.

• When ATP binds, it causes the myosin to release the actin.

• ATP is split, and the myosin heads again bind to the actin, but further down the filament.

• The myosin again pulls the actin.

• This action is repeated many times.

• The Z lines (discs) get closer together as the actin and myosin filaments slide past each other, and the muscle fiber shortens.

Relaxation

• ACh is broken down by an enzyme called acetylcholinesterase.

• Action potentials are no longer generated, so the Ca++ release channels in the S.R. close.

• Ca++ active transport pumps take Ca++ out of the sarcoplasm and into the S.R. where it binds to a protein called calsequestrin.

• As the Ca++ levels in the sarcoplasm fall, troponin releases tropomyosin, which falls back and covers the myosin binding sites on the actin.

• The thin filaments slip back into their relaxed positions.

Rigor mortis• After death, muscle cells begin autolysis, and

Ca++ leaks out of the S.R.

• This causes muscles to begin to contract.

• Since the body is dead, no more ATP is produced.

• Without the ATP to recharge the myosin heads, they remain linked to the actin, and neither relax nor contract any further.

• After about 24 - 72 hours it disappears as the tissues begin to disintegrate.

Origin and Insertions

• Origin – the attachment of a muscle to the less movable part (torso, etc.)

• Insertion – the attachment of a muscle to the more movable part

Interactions of muscles

• Prime mover – the muscle primarily responsible for a movement

• Synergist – stabilizes or assists prime mover

• Antagonist – opposes action of prime mover and must relax for prime mover to contract completely