skeletal muscle tissue

A contractile protein that makes up the thick filament. A myosin molecule consists of a tail and two myosin heads, which bind to myosin-binding sites on actin molecules of a thin filament during muscle contraction.

A contractile protein that is the main component of the thin filament. On each actin molecule is a myosin-binding site where a myosin head of a thick filament binds during muscle contraction.

A regulatory protein that is a component of the thin filament. When a skeletal muscle fiber is relaxed, tropomyosin covers the myosin-binding sites on actin molecules, thereby preventing myosin from binding to actin.

regulatory protein that is a component of the thin filament. When calcium ions (Ca2+) bind to troponin, it undergoes a change in shape; this conformational change moves tropomyosin away from myosin-binding sites on actin molecules, and muscle contraction subsequently begins as myosin binds to actin.

A structural protein that connects a Z disc to the M line of the sarcomere, thereby helping to stabilize the position of the thick filament. Because it can stretch and then spring back unharmed, titin accounts for much of the elasticity and extensibility of myofibrils.

structural protein of the Z discs that attaches to actin molecules of thin filaments and to titin molecules.

tructural protein that forms the M line of the sarcomere; it binds to titin molecules and connects adjacent thick filaments to one another.

structural protein that wraps around the entire length of each thin filament; it helps anchor the thin filaments to the Z discs and regulates the length of the thin filaments during development.

structural protein that links the thin filaments of the sarcomere to integral membrane proteins in the sarcolemma, which are attached in turn to proteins in the connective tissue matrix that surrounds muscle fibers. It is thought that dystrophin helps reinforce the sarcolemma and that it helps transmit tension generated by sarcomeres to tendons.

the repeating sequence of events that causes the filaments to slide

• ATP hydrolysis
• Attachment of myosin to actin to form crossbridges
• Power stroke
• Detachment of myosin from actin

The myosin head includes an ATP-binding site and an ATPase, an enzyme that hydrolyzes ATP into ADP (adenosine diphosphate) and a phosphate group. This hydrolysis reaction reorients and energizes the myosin head. Notice that the products of ATP hydrolysis—ADP and a phosphate group—are still attached to the myosin head.

The energized myosin head attaches to the myosin-binding site on actin and releases the previously hydrolyzed phosphate group. When the myosin heads attach to actin during contraction, they are referred to ascrossbridges.

After the crossbridges form, the power stroke occurs. During the power stroke, the site on the crossbridge where ADP is still bound opens. As a result, the crossbridge rotates and releases the ADP. The crossbridge generates force as it rotates toward the center of the sarcomere, sliding the thin filament past the thick filament toward the M line.

At the end of the power stroke, the crossbridge remains firmly attached to actin until it binds another molecule of ATP. As ATP binds to the ATP-binding site on the myosin head, the myosin head detaches from actin

contraction cycle

calsequestrin

skeletal muscle, which indicates how the forcefulness of muscle contraction depends on the length of the sarcomeres within a muscle before contraction begins

he synapse between a somatic motor neuron and a skeletal muscle fiber

synapse

small gap, separates the two cells

Suspended in the cytosol within each synaptic end bulb are hundreds of membrane-enclosed sacs

Inside each synaptic vesicle are thousands of molecules of the neurotransmitter released at the NMJ.