Nerve and Muscle Physiology Practice Questions

Q: Which ion movement is primarily responsible for hyperpolarization of the cell membrane?

Potassium (K+) efflux. ***Potassium (K+) efflux*** - **Potassium efflux** (K+ leaving the cell) is the **primary mechanism** responsible for hyperpolarization of the cell membrane across most cell types. - When K+ channels open, positive charges leave the cell, making the intracellular environment more negative relative to the extracellular space, thereby **hyperpolarizing** the membrane. - This mechanism is responsible for: - **Afterhyperpolarization** following action potentials - Setting the **resting membrane potential** close to the K+ equilibrium potential (-90 mV) - **Repolarization and hyperpolarization phases** of action potentials - Examples include delayed rectifier K+ channels and calcium-activated K+ channels. *Chloride (Cl-) influx* - While Cl- influx can cause hyperpolarization (especially through **GABA-A receptors** in neurons), it is a **secondary or specialized mechanism**, not the primary one. - In many mature neurons, the Cl- equilibrium potential is close to the resting potential, limiting its hyperpolarizing effect. - This mechanism is important in **inhibitory neurotransmission** but not universally across all cell types. *Sodium (Na+) influx* - **Sodium influx** through voltage-gated sodium channels is responsible for the **depolarization phase** of an action potential. - This makes the inside of the cell significantly more positive (+30 to +40 mV), which is the opposite of hyperpolarization. *None of the options* - This option is incorrect because **potassium efflux** is indeed the primary mechanism for membrane hyperpolarization.

Q: Myosin and actin filaments are kept in place by?

Titin. ***Titin*** - **Titin** is a large, elastic protein that extends from the **Z-disc to the M-line** in a sarcomere. - It plays a crucial role in maintaining the structural integrity and **passive elasticity** of muscle cells by keeping myosin and actin filaments properly aligned. *Tropomyosin* - **Tropomyosin** is a regulatory protein that wraps around the actin filament, **blocking the myosin-binding sites** in relaxed muscle. - Its primary function is to regulate muscle contraction, not to structurally anchor the filaments in place. *Troponin* - **Troponin** is a complex of three proteins (**troponin I, T, and C**) that is associated with tropomyosin on the actin filament. - It binds **calcium ions**, leading to a conformational change that moves tropomyosin away from the myosin-binding sites, allowing contraction to occur. *Actinin* - **Alpha-actinin** is a protein found in the **Z-disc** that helps to anchor the plus ends of actin filaments to the Z-disc. - While it's involved in actin filament organization, it does not directly keep both myosin and actin filaments in their precise functional arrangement along the length of the sarcomere in the same way titin does.

Q: The most distinguishing feature between skeletal and smooth muscle is the absence of ------ in smooth muscle.

Troponin. ***Troponin*** - **Smooth muscle** lacks the **troponin complex** (troponin I, C, and T) that is essential for initiating contraction in skeletal and cardiac muscle. - Instead of troponin, smooth muscle uses **calmodulin** to bind calcium, which then activates **myosin light chain kinase** to regulate contraction. *Tropomyosin* - **Tropomyosin** is present in both **skeletal** and **smooth muscle**, though it plays a different regulatory role in smooth muscle. - In smooth muscle, tropomyosin does not block myosin binding sites, but rather modulates the interaction between actin and myosin. *Myosin* - **Myosin** is a fundamental motor protein found in all types of muscle, including both **skeletal** and **smooth muscle**. - It forms thick filaments and interacts with actin to generate force and muscle contraction. *Actin* - **Actin** is a primary component of thin filaments and is universally present in all muscle types, including **skeletal** and **smooth muscle**. - It provides the framework along which myosin heads slide to produce muscle shortening.

Q: Protein connecting Z-lines to M-lines is:

Titin. ***Titin*** - **Titin** is a giant protein that extends from the **Z-disc to the M-line** in the sarcomere, acting as a molecular spring. - It maintains the **structural integrity** of the sarcomere and provides passive elasticity to muscles. *Kinin* - **Kinin** is a protein involved in **inflammation and blood pressure regulation**, not a structural component of muscle sarcomeres. - Examples include **bradykinin**, which mediates pain and vasodilation. *Desmin* - **Desmin** is an **intermediate filament protein** that forms a scaffold around the Z-discs, linking myofibrils together. - While it connects myofibrils, it does not directly span between the Z-line and M-line within a single sarcomere. *Actin* - **Actin** is a primary component of **thin filaments** in the sarcomere and participates in muscle contraction, but it does not connect the Z-line to the M-line. - Thin filaments are anchored at the **Z-disc** but only extend partway into the A-band.

Q: What is the equilibrium potential for sodium?

+60 mV. ***+60 mV*** - The **equilibrium potential for sodium** (E_Na) is approximately +60 mV. This is calculated using the **Nernst equation**, considering the higher extracellular concentration of sodium ions compared to their intracellular concentration. - At this potential, the **electrical force** driving sodium out of the cell is equal and opposite to the **chemical force** driving sodium into the cell, resulting in no net movement of sodium ions across the membrane. *-70 mV* - This value typically represents the **resting membrane potential** of a neuron, which is primarily determined by the high permeability to potassium ions. - At -70 mV, there is a strong electrochemical gradient for **sodium influx**, rather than equilibrium. *-90 mV* - This value is close to the **equilibrium potential for potassium** (E_K), due to the high intracellular concentration of potassium ions and its significant membrane permeability at rest. - At -90 mV, potassium ions are close to equilibrium, but sodium ions are far from their equilibrium. *0 mV* - This represents the potential where there is **no electrical gradient** across the membrane, but it is not the equilibrium potential for sodium. - At 0 mV, the chemical gradient would still drive sodium ions into the cell, as sodium concentration remains higher outside the cell.

Q: Which of the following statements about nerve impulses is true?

Travels in one direction at synapses.. ***Travels in one direction at synapses.*** - At **synapses**, neurotransmitters are released from the **presynaptic neuron** and bind to receptors on the **postsynaptic neuron**, ensuring unidirectional signal transmission. - This **unidirectional flow** is crucial for organized communication within the nervous system. *Travels slower than the speed of electrical signals in wires.* - While impressive, **nerve impulse conduction** involves electrochemical processes that are significantly slower than the almost instantaneous flow of electrons in wires. - The maximum speed of nerve conduction in humans is about **120 m/s**, whereas electrical signals in wires approach the speed of light. *Nerve impulses respond to increased stimulus strength.* - Nerve impulses (action potentials) operate on an **"all-or-none" principle**; once the **threshold potential** is reached, the amplitude of the action potential is maximal and does not increase with a stronger stimulus. - The nervous system encodes stimulus strength by *increasing the frequency* of action potentials, not their amplitude. *Nerve impulses can propagate bidirectionally along an axon.* - This is **FALSE**. Nerve impulses propagate **unidirectionally** along the axon due to the **refractory period**. - After an action potential passes through a segment, that segment enters an **absolute refractory period** during which it cannot fire again, preventing backward propagation. - This ensures orderly signal transmission from the axon hillock toward the axon terminals.

Q: In the context of muscle physiology, which structure is described as a threadlike component that extends along the length of a muscle fiber?

Myofibril. ***Myofibril*** - A **myofibril** is a cylindrical organelle that runs longitudinally inside a muscle fiber and contains **contractile proteins**. - Myofibrils are composed of repeating units called **sarcomeres**, which are the fundamental units of muscle contraction. *Sarcomere* - A **sarcomere** is the basic contractile unit of a muscle fiber, extending from one Z-disc to the next. - While it is a key component for muscle contraction, it is a **segment within a myofibril**, not the threadlike component that extends the entire length of the fiber. *Sarcolemma* - The **sarcolemma** is the cell membrane of a muscle fiber, responsible for transmitting nerve impulses to the muscle cell. - It encloses the muscle fiber but is not an internal, threadlike contractile component. *Myofilament* - **Myofilaments** are the individual protein filaments (actin and myosin) that make up a sarcomere within a myofibril. - They are the **smallest contractile elements**, but they are not the threadlike structure that extends along the entire muscle fiber.

Question 1

A man slept with his head over his forearm. The next morning, he complains of tingling and numbness over the forearm. If this were primarily due to hypoxia affecting nerve fibers, which of the following statements about nerve fiber sensitivity to hypoxia is correct?

Accepted Answer

B fibers are most sensitive to hypoxia, followed by A fibers, and C fibers are least sensitive.

Answer

All nerve fibers are equally sensitive to hypoxia.

Answer

C fibers are most sensitive to hypoxia, followed by B fibers, and A fibers are least sensitive.

Answer

A fibers are most sensitive to hypoxia, followed by B fibers, and C fibers are least sensitive.

Question 2

Which ion movement is primarily responsible for hyperpolarization of the cell membrane?

Accepted Answer

Potassium (K+) efflux

Answer

Chloride (Cl-) influx

Answer

Sodium (Na+) influx

Answer

None of the options

Question 3

Myosin and actin filaments are kept in place by?

Accepted Answer

Titin

Answer

Tropomyosin

Answer

Troponin

Answer

Actinin

Question 4

The most distinguishing feature between skeletal and smooth muscle is the absence of ------ in smooth muscle.

Accepted Answer

Troponin

Answer

Actin

Answer

Tropomyosin

Answer

Myosin

Question 5

Protein connecting Z-lines to M-lines is:

Accepted Answer

Titin

Answer

Kinin

Answer

Desmin

Answer

Actin

Question 6

What is the equilibrium potential for sodium?

Accepted Answer

+60 mV

Answer

-70 mV

Answer

-90 mV

Answer

0 mV

Question 7

Which of the following statements correctly describes Group B sympathetic postganglionic nerve fibers?

Accepted Answer

Myelinated fibers associated with the autonomic nervous system.

Answer

Myelinated fibers that are parasympathetic postganglionic.

Answer

Unmyelinated fibers primarily conveying pain.

Answer

Myelinated fibers that are sympathetic postganglionic.

Question 8

What is the role of tropomyosin in muscle contraction?

Accepted Answer

Covers the binding sites on actin to regulate the interaction with myosin

Answer

Helps in the fusion of actin and myosin

Answer

Slides over myosin

Answer

Causes Ca2+ release

Question 9

Which of the following statements about nerve impulses is true?

Accepted Answer

Travels in one direction at synapses.

Answer

Travels slower than the speed of electrical signals in wires.

Answer

Nerve impulses respond to increased stimulus strength.

Answer

Nerve impulses can propagate bidirectionally along an axon.

Question 10

In the context of muscle physiology, which structure is described as a threadlike component that extends along the length of a muscle fiber?

Accepted Answer

Myofibril

Answer

Sarcomere

Answer

Sarcolemma

Answer

Myofilament

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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