1. The muscle protein, which covers the active sites on the actin filament at rest, is:
Correct answer: (c) Tropomyosin
• Tropomyosin is a protein that winds around the chains of the actin filament and covers the myosin-binding sites to prevent actin from binding to myosin [See diagram below].
• Tropomyosin binds to troponin to form a troponin-tropomyosin complex. The troponin-tropomyosin complex prevents the myosin “heads” from binding to the active sites on the actin microfilaments.
• Troponin also has a binding site for Ca2+ ions.
2. During skeletal muscle contraction, there is shortening of:
a. Actin filaments
Correct answer: (d) Sarcomere
Sarcomere shortens during skeletal muscle contraction
3. The visceral smooth muscle:
a. Is found in the iris of eye
b. Has contact type of neuromuscular junctions
c. Control is mainly through nervous stimuli
d. Is composed of individual muscle fibers
e. Shows contraction when it is stretched
Correct answer: (e) Shows contraction when stretched
• Smooth muscle found in iris of the eye is MULTI-UNIT SMOOTH MUSCLE and not visceral. VISCERAL (single-unit) SMOOTH MUSCLE is found in the walls of hollow organs in the body, such as digestive tract, urinary tract, respiratory tract, blood vessels
• Unlike skeletal muscle, there is no specialized connection between the nerve fiber and the smooth muscle cell. There is nothing equivalent to the motor end-plate in smooth muscle, therefore receptors for the neurotransmitters are located throughout the smooth muscle membrane.
• Control of visceral smooth muscle is mainly through factors such as, stretch, hormones and paracrine. Stretching the muscle can trigger its contraction (the stretch-relaxation response).
• Visceral (single-unit) smooth muscle is named because all the smooth muscle cells in the organ behave as one unit. Histologically, this is accomplished by having a lot of gap junctions interconnecting the smooth muscle cells. Gap junctions allow for electrical connections between cells.
• Visceral smooth muscles contract when stretched
4. During the neuromuscular transmission, end plate potential is produced. It:
a. Obeys all or none law
b. Has a refractory period
c. Is of low amplitude in myasthenia gravis
d. Is due to Ca2+ influx
e. Is a localized hyperpolarization
Correct answer: (c) It is of low amplitude in myasthenia gravis
• End plate potential, being a graded potential, does not obey all or none law; does not have a refractory period; is due to Na+ influx; and is a localized depolarization.
• The amplitude of end plate potential is diminished in myasthenia gravis. This disease results from destruction of skeletal muscle acetylcholine receptors. Consequently, the chemical transmitter, acetylcholine (ACh) is unable to bind to the receptors (AChR) on the postsynaptic membrane to transmit the nerve impulse to muscle fibers to produce a muscle contraction.
5. In skeletal muscle neuromuscular junctions
a. The motor end plate is the motor nerve terminal.
b. Spontaneous (miniature) potentials may be recorded in the motor nerve terminal.
c. Motor nerve terminals have vesicles containing acetylcholine.
d. There is a high concentration of acetylcholinesterase.
e. Transmission is facilitated by botulinum toxin.
A. False – It is the modified muscle membrane adjacent to the nerve terminal.
B. False – They may be recorded at the motor end plate.
C. True – This neurotransmitter, released by exocytosis, excites the end plate membrane.
D. True – This makes acetylcholine’s action transient.
E. False – Botulinum toxin blocks transmission by an action on the motor nerve terminals.
6. A skeletal muscle fiber
a. Membrane is negatively charged on the inside with respect to the outside at rest.
b. Contains intracellular stores of calcium ions
c. Is normally innervated by more than one motor neuron.
d. Becomes more excitable as its resting membrane potential falls.
e. Becomes less excitable as the extracellular ionized calcium levels fall.
A. True – This ‘resting membrane potential’ is about 90 mV.
B. True – These are released on excitation.
C. False – A single neuron supplies a group of muscle fibers.
D. True – It becomes more excitable as its membrane potential approaches the firing threshold (about 70 mV).
E. False – Decreasing extracellular Ca2 increases excitability and may lead to spontaneous contractions (tetany), possibly by increasing sodium permeability.
7. In skeletal muscle
A. Contraction occurs when its pacemaker cells depolarize sufficiently to reach the threshold for firing.
B. Calcium is taken up by the sarco-tubular system when it contracts.
C. Actin and myosin filaments shorten when it contracts.
D. The sarcomeres shorten during contraction.
E. Contraction strength is related to initial length of the muscle fibers.
A. False – Skeletal muscle has no pacemaker cells and shows no spontaneous activity.
B. False – Calcium is released from intracellular stores when it contracts.
C. False – The filaments do not shorten but slide together over one another.
D. True – There is greater overlap of the actin and myosin fibrils.
E. True – Moderate stretch increases contraction strength as in the heart.
8. Visceral smooth muscle differs from skeletal muscle in that
A. It contracts when stretched.
B. It is not paralyzed when its motor nerve supply is cut.
C. Its cells have unstable resting membrane potentials.
D. It contains no actin or myosin.
E. Excitation depends more on influx of extracellular calcium than release of calcium from endoplasmic reticulum.
A. True – The intrinsic ‘myogenic’ response in smooth muscle opposes stretch; skeletal muscle requires a nerve reflex arc for this type of response.
B. True – It continues to contract due to local pacemakers.
C. True – They show spontaneous depolarization between contractions.
D. False – In smooth muscle, actin and myosin filaments occur but are less obvious on microscopy.
E. True – Smooth muscle has a less well-developed sarcoplasmic reticulum.
9. Histological and physiological study of skeletal muscle shows that the
A. Distance between two Z lines remains constant during contraction.
B. Width of the anisotropic A band is constant during contraction.
C. Tension developed is maximal when actin and myosin molecules just fail to overlap.
D. Stimulus needed to cause contraction is minimal when applied at the Z line.
E. The T system of transverse tubules opens into the terminal cisterns of the sarcoplasmic reticulum.
A. False – This is sarcomere length and it shortens with contraction.
B. True – Its width is the length of the myosin molecule.
C. False – It is maximal when action and myosin overlap maximally and when adjoining actin molecules just fail to overlap.
D. True – This is where the transverse tubules penetrate the muscle fiber.
E. False – They are adjacent but are not directly connected.
10. Muscle tone is reduced by
A. Curare-like drugs
B. Lower motor neuron lesions.
C. Upper motor neuron lesions
D. Cerebellar lesions
E. Gamma efferent impulses to muscle spindles
A. True – These paralyze muscle by blocking transmission at neuromuscular junctions.
B. True – Lower motor neurons also paralyze skeletal muscle.
C. False – Loss of supraspinal influences results in spasticity of the affected muscles.
D. True – The cerebellum helps to maintain normal muscle tone.
E. False – These increase spindle sensitivity to stretch and hence muscle tone.