Question

During endochondral ossification, the very first step in turning a hyaline model into bone is

		secondary ossification
		bone collar formation
		primary ossification
		chondrocyte hypertrophy

Action potentials on the surface of the sarcolemma originate at the

	A.	T-tubules
	B.	sarcomere
	C.	neuromuscular junction
	D.	Z-disk

The most immediate, most effective source of ATP for a muscle fiber that needs to perform at maximum output for a short period of time (less than 10 seconds) is

	A.	aerobic respiration
	B.	phosphagen system
	C.	anaerobic respiration
	D.	glycolysis

During contraction of the sarcomere, the role of Ca⁺² is to

	A.	bind to troponin
	B.	bind to tropomyosin
	C.	provide the energy for myosin to pull on actin
	D.	propagate action potentials

Answer 1

1. Option (B) is correct. During endochondral ossification, the very first step in turning a hyaline model into bone is primary ossification.

The first region of a bone to start ossifying is a primary ossification center, which normally appears in the central part of every developing bone during prenatal development.

2. Option (C) is correct. Action potentials on the surface of the sarcolemma originate at the neuromuscular junction.

Whenever an action potential reaches a neuromuscular junction, this triggers the release of acetylcholine into this synapse. If more of these sodium ions penetrate the muscle fiber to raise it from its resting potential of -95 mV to about -50 mV, they cause an action potential for the muscle that spreads throughout the fiber.

3. Option (D) is correct. The most immediate, most effective source of ATP for a muscle fiber that needs to perform at maximum output for a short period of time (less than 10 seconds) is Glycolysis.

Anaerobic glycolysis will produce small amounts of ATP for a short period in the absence of oxygen. Aerobic metabolism uses oxygen to produce much more ATP which allows a muscle to work for longer periods.

4. Option (A) is correct. During contraction of the sarcomere, the role of Ca⁺² is to bind to troponin.

The troponin changes shape, when calcium binds to troponin, removing tropomyosin from the binding sites. The sarcoplasmic reticulum stores calcium ions, which it releases when a muscle cell is stimulated. The calcium ions then trigger the process of contraction of the muscle across the bridge.