Nerve and Muscle Physiology Practice Questions

Q: Which of the following is NOT a characteristic of a biphasic action potential of a mixed nerve?

Two or more positive peaks. ***Two or more positive peaks*** - A **biphasic action potential** of a mixed nerve, when recorded extracellularly, typically consists of two phases: an initial **negative deflection** followed by a **positive deflection**. It does not exhibit multiple positive peaks for a single action potential. - The shape is determined by the propagation of the action potential past two recording electrodes, illustrating the **depolarization and repolarization** of the nerve. *All or none phenomenon* - This is a fundamental characteristic of **individual nerve fibers** and thus applies to the action potentials propagating within a mixed nerve. - If a stimulus reaches a threshold, a full-sized action potential is generated; otherwise, none is generated, regardless of stimulus strength. *Refractory period* - The **refractory period** is a crucial characteristic of nerve excitability, ensuring unidirectional propagation and limiting the frequency of action potentials. - This period, comprising absolute and relative phases, applies to the individual fibers within the mixed nerve. *Recorded on surface* - **Compound action potentials (CAPs)** of mixed nerves are typically recorded extracellularly (on the surface) using electrodes, often seen in nerve conduction studies. - This contrasts with intracellular recordings which measure the potential across the cell membrane directly.

Q: Tetanic contraction is due to accumulation of?

Ca 2+ . ***Ca 2+ *** - **Tetanic contraction** results from a rapid succession of muscle stimulations, leading to the sustained elevation of **intracellular calcium (Ca 2+ )** levels. - This persistent high Ca 2+ concentration in the sarcoplasm allows for continuous binding to **troponin**, maintaining the activation of cross-bridge cycling. *Na + * - **Sodium (Na + )** influx is primarily responsible for the **depolarization** of the muscle cell membrane, leading to an **action potential**. - While essential for initiating the contraction, Na + accumulation itself does not directly cause the sustained high Ca 2+ levels characteristic of tetany. *K + * - **Potassium (K + )** efflux is crucial for the **repolarization** of the muscle cell membrane after an action potential. - Accumulation of K + in the extracellular space can contribute to muscle fatigue and reduce excitability, but it does not directly lead to tetanic contraction. *Cl - * - **Chloride (Cl - )** ions play a significant role in stabilizing the resting membrane potential and contributing to muscle **repolarization**, particularly in skeletal muscle. - While important for muscle function, changes in Cl - concentration do not directly cause the sustained Ca 2+ release required for tetanic contraction.

Q: When the tension in a muscle fibre is maximum, its length is called?

Optimum length. ***Optimum length*** - This is the muscle length at which the **maximum number of cross-bridges** can form between actin and myosin filaments. - At this length, the sarcomere allows for the **greatest overlap** of thick and thin filaments without excessive stretching or compression, leading to peak tension generation. *Equilibrium length* - This term usually refers to the **resting length** of a muscle fiber when no external forces are acting upon it. - At equilibrium, the tension generated by the muscle may not necessarily be at its maximum. *Initial length* - This is a general term that refers to the **starting length** of a muscle fiber before it contracts or is stretched. - It does not specifically denote the length at which maximum tension is achieved. *None of the options* - This option is incorrect because **optimum length** accurately describes the muscle length yielding maximum tension.

Q: Burning pain is carried by which type of fibres?

C fibres. ***C fibres*** - These are **unmyelinated** and have a **slow conduction velocity**, responsible for transmitting **slow, dull, burning, or aching pain**. - They also transmit sensations of **temperature** and **itch**. *A alpha fibres* - These are **large, myelinated fibres** with the **fastest conduction velocity**, primarily involved in proprioception (sense of body position) and motor control. - They are not involved in the transmission of burning pain. *A delta fibres* - These are **small, myelinated fibres** that transmit **fast, sharp, localized pain** (the "first pain") and cold sensations. - While they transmit pain, it is characteristically sharp, not burning. *A beta fibres* - These are **large, myelinated fibres** that primarily transmit **touch and pressure sensations**. - They have a fast conduction velocity and are not involved in pain transmission.

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: 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: 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: 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.

Question 1

Which of the following is NOT a characteristic of a biphasic action potential of a mixed nerve?

Accepted Answer

Two or more positive peaks

Answer

Refractory period

Answer

All or none phenomenon

Answer

Recorded on surface

Question 2

What is the primary factor that determines the resting membrane potential in a nerve fiber?

Accepted Answer

Is primarily determined by the equilibrium potential of potassium ions.

Answer

Is equal to the resting potential of cardiac muscle fibers.

Answer

Can be accurately measured using intracellular electrodes.

Answer

Increases with elevated extracellular potassium concentration.

Question 3

Tetanic contraction is due to accumulation of?

Accepted Answer

Ca<sup>2+</sup>

Answer

Na+

Answer

K+

Answer

Cl<sup>-</sup>

Question 4

When the tension in a muscle fibre is maximum, its length is called?

Accepted Answer

Optimum length

Answer

None of the options

Answer

Initial length

Answer

Equilibrium length

Question 5

Burning pain is carried by which type of fibres?

Accepted Answer

C fibres

Answer

A alpha fibres

Answer

A delta fibres

Answer

A beta fibres

Question 6

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 7

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 8

Myosin and actin filaments are kept in place by?

Accepted Answer

Titin

Answer

Tropomyosin

Answer

Troponin

Answer

Actinin

Question 9

Protein connecting Z-lines to M-lines is:

Accepted Answer

Titin

Answer

Kinin

Answer

Desmin

Answer

Actin

Question 10

What is the equilibrium potential for sodium?

Accepted Answer

+60 mV

Answer

-70 mV

Answer

-90 mV

Answer

0 mV

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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