Connective Tissue Terminology _________________- each individual skeletal muscle fiber is surrounded by this delicate connective tissue layer. –_________________- groups of skeletal muscle fibers _________________- connective tissue that binds together fascicles. _________________- fibrous connective tissue that surrounds groups of fascicles.

Microscopic Anatomy of Skeletal Muscle Muscle cells –Are very large in size –Have a threadlike or fiberlike shape. –Usually are multi-nucleated –Made up of smaller myofibrils composed of _______________ (thin) and _____________ (thick) filaments Network of sarcoplasmic reticulum (similar to ER) –Stores ____________ for muscle contraction

A band- large dark band made up of myosin filaments I Band- large light band made up of actin filamaments Z line- dark band in center of I band, disk that is viewed as a line and is attachment site for actin filaments. Sarcomere- area from one z line to the next z line. Basic contracting unit of skeletal muscle. When all sarcomeres contract, leads to overall muscle fiber shortening.

Neuromuscular Junction Skeletal muscle is under voluntary control If nerve supply is interrupted for long period of time, muscle will ______________ (shrink down) –Neuromuscular junctions- sites where the ends of motor nerve fibers connect to muscle fibers. The nerve is not actually touching the muscle, there is a space called the synaptic space or cleft between the nerve & the muscle

Neuromuscular Junction _______________________- sacs at end of a nerve fiber; contain neurotransmitter (e.g., acetylcholine) –_______________- neurotransmitter chemical that travels across synapse to activate muscle fiber Attaches to receptor on sarcolemma –__________________- enzyme in the synaptic space that removes acetylcholine If muscle is to contract again, another impulse must be sent

Initiation of Muscle Contraction and Relaxation –Nerve impulse comes down motor nerve fiber, reaches neuromuscular junction and acetylcholine is released into synaptic space. –Acetylcholine binds to receptors on surface of sarcolemma (cell membrane) of the muscle fiber. –This starts impulse that travels along sarcolemma and through the T tubules to the interior of the cell.

cont’d…. –Once impulse reaches sarcoplasmic reticulum it causes release of stored calcium ions (Ca ++ ) into the sarcoplasm (cytoplasm). –As Calcium diffuses into myofibrils, initiates contraction process which is powered by ATP. –As contraction occurs, Calcium is pumped back out of myofibrils which shuts down contraction process. Both relaxation and contraction requires energy

Mechanics of Muscle Contraction When a muscle fiber is relaxed, actin and myosin overlap a little. When stimulated _______ ________ (levers on the myosin filaments) ratchet back and forth and pull the actin filaments on both sides toward center of the myosin filaments. Sliding of filaments shortens sarcomere, thereby causing contraction.

Motor Unit One nerve fiber and all muscle fibers it innervates – Muscles that make small, delicate movements have only a few muscle fibers per nerve fiber in each motor unit – Large, powerful muscles may have a hundred or more muscle fibers per motor unit

Characteristics of Muscle Contraction _____-____-_____________ principle –An individual muscle fiber either contracts completely when it receives an impulse or not at all. –Movements can vary in strength due to number of muscle fibers stimulated. small movements only require a few fibers to contract, whereas larger, powerful movements require more fibers Nervous system sends out impulse based on muscle _______ - or idea of how many fibers need to be stimulated for that particular activity. –ex: shooting a basketball vs. knitting a sweater

Phases of twitch contraction (contraction of a single muscle) 1. _______________ phase –Brief pause between nerve stimulus and beginning of actual contraction (lasts about 0.01 seconds) 2. ________________ phase –Actual contraction is taking place (lasts about 0.04 seconds) 3. ________________ phase –When cells go back to a relaxed state (lasts about 0.05 seconds) Some fibers are relaxing while others contract, so smooth muscle contractions can occur