Muscle cell sarcomere

Muscle cell sarcomere (Photo credit: TheJCB)

Biology 42 – Human Biology (Spring 2007)


[ Students and professors, please read. ]

Assignment Questions.

A. What is a sarcomere and what are the sequence of events that lead to contraction?

The sarcomere extends between two Z lines and is composed of thin filaments of actin (I band) and thick filaments of myosin (H band), interlocked like when you interlock your fingers.  When the muscle contracts, the sarcomeres shorten (the Z lines get closer together, and the filaments of actin and myosin interlock tighter).

When calcium is present (because it was released from the sarcoplasmic reticulum after it received impulses from the T tubules of the sarcolemma, because acetylcholine, released by the synaptic vesicles in the axon terminal of a motor neuron, has diffused across the sarcolemma and bound itself to its receptors), it combines with the troponin on the threads of tropomyosin wound about the actin filaments, exposing the binding sites for myosin heads.  The globular head of myosin (with ADP + P on its head) bends backward to attach to the now-exposed neighboring actin filament (forming a cross-bridge), then bends forward (because ADP + P release causes it to change position), dragging actin with it.  Then when ATP binds to it again, the myosin’s head detaches from the actin filament, and if calcium is still present, the whole process can start again.  If calcium is not present, there is no contraction, and ATP is built back up again (stored in creatine phosphate) for next time.  The sarcoplasmic reticulum actively transports the calcium back to the sarcoplasmic reticulum once the nerve impulses (releasing acetylcholine) cease.

B. How do muscle pairs like the bicep and tricep work? Hint: Antagonist

When the bicep muscle contracts (flexes), the tricep expands (relaxes).  Other muscle pairs work the same way – one contracts while the other expands.

C. Name the components of the appendicular and axial skeleton.

The appendicular skeleton includes the pelvic girdle, femur, patella, tibia, fibula, tarsals, metatarsals, phalanges, clavicle, scapula, humerus, radius, ulna, carpals, metacarpals and phalanges.

The axial skeleton includes the skull (cranial and facial bones), rib cage (sternum and 12 pairs of ribs), vertebral column (lecture says 26, text says 33), and disks.

D. What is a synovial joint?

Unlike fibrous and cartilaginous joints, which are mostly immobile, synovial joints are freely movable.  In a synovial joint, the two bones are separated by a cavity, held together by ligaments, and encapsulated by a synovial membrane.  Different types of synovial joints include hinge joints (knee, elbow), and ball-and-socket joints (hips, shoulders).

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