The force of contraction in a skeletal muscle is normally graded by ________.
a. The amount of calcium released from the sarcoplasmic reticulum (SR) by an action potential.
b. Activation of fast, glycolytic motor units instead of slow, oxidative motor units.
c. Summation of action potentials to prolong motor nerve depolarization.
d. Motor unit recruitment.
e. Alteration in myosin ATPase activity.
2. Gap junctions ________.
a. Are absent in cardiac muscles.
b. Are present but of little functional importance in cardiac muscle.
c. Are present and provide the pathway for rapid spread of excitation from one cardiac muscle cell to another.
d. Are absent in visceral smooth muscles.
e. Connect the sarcotubular (SR) system to individual skeletal muscle cells.
3. Acetylcholine receptors are ________.
a. Skeletal muscle contractile proteins.
b. Non-specific, small ion channels.
c. Ligand-binding hormone receptors (membrane proteins).
d. Calcium binding.
4. The action of curare, which can produce paralysis of skeletal muscles, is to ________.
a. Block the release of ACh from the nerve terminal.
b. Cause an explosive release of ACh at the neuromuscular junction.
c. Bind reversibly to the ACh receptors.
d. Bind irreversibly to the ACh receptors.
e. Inhibit acetylcholine esterase.
5. Acetylcholinesterase, the enzyme which hydrolyzes ACh, is ________.
a. Released from the presynaptic nerve terminal.
b. Blocked by antibodies in Myasthenia gravis.
c. Located in the junctional folds of the muscle cell membrane.
d. Required for membrane depolarization.
e. Dependent on increased intracellular calcium concentrations.
6. Endplate potentials, which are depolarizing due to the simultaneous movement of both sodium and potassium, are recorded at the ________.
a. Presynaptic nerve terminal.
b. Motor axon.
c. T-tubule.
d. Skeletal muscle.
e. Sarcoplasmic reticulum.