I. Types of muscle

A. SKELETAL muscles attach to the skeleton. They function in movement and maintenance of posture.

1. They can be long (up to 30 cm).

2. They are voluntary and contract and relax rapidly (stimulated by ACETYLCHOLINE).

3. Cells are multinucleated and tissue is STRIATED (striped).

B. SMOOTH muscle is found in the walls of hollow organs and functions to move organ walls.

1. PERISTALSIS is the term for rhythmic

contractions and relaxations that occur in some hollow organs.

2. They are involuntary and contract and relax slowly. They are rhythmic (stimulated by acetylcholine, NOREPINEPHRINE, and hormones).

3. Cells have a single nucleus and tissue lacks striations.

C. CARDIAC muscle is found in the heart wall where it functions in pumping the heart.

1. They are involuntary and contract and relax as a unit. They are rhythmic.

2. Cells have a single nucleus and tissue is striated.

II. Structure of a skeletal muscle

A. FASCIA is the connective tissue that surrounds each individual muscle fiber making up a muscle.

1. TENDONS are made up of fascia and connect muscle to bone.

B. The EPIMYSIUM surrounds the muscle.

C. The PERIMYSIUM breaks tissue into small sections.

1. The sections of muscles fibers are called FASCICLES.

D. The ENDOMYSIUM encloses each fascicle.

III. Skeletal muscle fibers

A. They are made up of single fused cells.

B. Each fiber has a SARCOLEMA (cell membrane), SARCOPLASM (cytoplasm), SARCOPLASMIC RETICULLUM (ER), many mitochondria, and MYOFIBRILS.

1. Myofibrils are made up of the proteins ACTIN and MYOSIN that aid in contraction and cause striations in tissue.

a. I BANDS (light/made of actin) attach to Z LINES.

b. A BANDS are made of overlapping actin and myosin filaments.

c. The H ZONE of the A band is made of only myosin filaments.

IV. When actin and myosin slide past one another in the myofibrils, muscle fibers shorten and pull on there attachments. This is muscle contraction.

A. Contraction is caused by stimulation from a MOTOR NEURON. The AXON of the neuron and the muscle fiber join at the NEUROMUSCULAR JUNCTION.

1. The gap between the axon and muscle fiber is called the SYNAPTIC CLEFT.

2. NEUROTRANSMITTERS (chemicals that cause contraction) are released at the distal end of the axon and bind to receptors on the muscle fibers, thus stimulating them with a MUSCLE IMPULSE.

a. Muscle impulses are transmitted in all directions along the cell.

b. Acetylcholine is the neurotransmitter that motor neurons use to control skeletal muscle.

B. Excitation and the SLIDING FILAMENT THEORY

1. TROPONIN and TROPOMYOSIN are proteins in actin that prevent muscle contraction in the absence of calcium ions.

2. When a muscle fiber is relaxed, troponin and tropomyosin prevent myosin from linking up with actin.

3. In response to a muscle impulse, the sarcoplasmic

reticulum releases stored calcium ions into the muscle fiber. They bind with troponin, changing its shape and shifting the tropomyosin to expose myosin binding sites.

4. Actin and myosin can then form CROSS-BRIDGES.

5. According to the sliding filament theory, actin and myosin filaments slide past one another to shorten the sarcomeres and contract the muscle.

IV. Muscle fibers relax when ACETYLCHOLINESTERASE decomposes acetylcholine and calcium gets pumped back into the sarcoplasmic reticulum.

V. Energy sources for contraction

A. CREATINE PHOSPHATE can regenerate ATP from ADP (10 sec.).

B. Cellular respiration makes ATP (and gives off heat).

1. Blood supplies oxygen for the citric acid cycle.

2. MYOGLOBIN found in muscles stores oxygen.

C. When oxygen runs out during strenuous activity, glycolysis is used to make ATP.

1. Less ATP is released

2. Toxic PYRUVATE molecules are converted into lactic acid.

3. LACTIC ACID builds up in the muscles (before diffusing into blood) and causes muscle soreness.

VI. Muscular responses

A. The minimal stimulus strength required to cause contraction is called the THRESHOLD STIMULUS.

B. A TWITCH is the response of a single muscle fiber to the ACh released by the neuron.

1. Each twitch has both a period of contraction and relaxation.

2. The LATENT PERIOD describes the time between two muscle twitches.

3. The REFRACTORY PERIOD is the amount of time when a muscle fiber is unresponsive.

C. Even when muscles appear to be at rest, there are some sustained contractions going on. This is called MUSCLE TONE.

D. Types of contractions

1. In ISOTONIC contractions, force is equal and muscles change in length.

2. In ISOMETRIC contractions, force changes and muscles remain the same length.

E. FAST and SLOW muscle twitches

1. Slow twitch fibers always have oxygen and are resistant to fatigue.

2. Fast twitch fibers can be glycolytic (easily fatigue) or

fatigue resistant.

VII. Skeletal muscle actions

A. The ORIGIN is the immovable end and the INSERTION is the moveable end of the muscle. During contraction, the insertion pulls toward the origin.

B. The PRIME MOVER (AGONIST) is the main muscle responsible for the body movement.

C. SYNERGISTS contract and help a prime mover.

D. ANTAGONSITS resist movement of the prime mover and cause movement in the opposite direction.