Nerve and Muscle Physiology Practice Questions

Q: What is the fixed length of a myosin filament?

1.6 micrometers. ***1.6 micrometers*** - Myosin filaments, also known as **thick filaments**, are integral components of muscle contraction and have a characteristic fixed length. This length is precisely **1.6 micrometers** in mammalian skeletal muscle. - This consistent length is crucial for the **sliding filament model** of muscle contraction, ensuring proper overlap with actin filaments and efficient force generation. *0.16 nm* - This value is significantly too small; **nanometers (nm)** are typically used for atomic or molecular distances, not for entire protein filaments like myosin. - A myosin filament is composed of hundreds of myosin molecules, making its overall length much larger than a fraction of a nanometer. *16 nm* - While nanometers are used for molecular structures, 16 nm is still too small for a myosin filament. The entire filament is roughly **100 times larger** than this value. - This dimension might be more appropriate for the diameter of a single myosin molecule's head region, but not the entire filament's length. *1.6 mm* - This value is significantly too large; **millimeters (mm)** are visible to the naked eye and represent macroscopic objects. - Muscle filaments are microscopic structures, and a length of 1.6 mm would imply they are many times longer than an entire muscle cell.

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: Which of the following is NOT a location where multi-unit smooth muscle is present?

Gut. ***Gut*** - The gut primarily contains **unitary (single-unit) smooth muscle**, characterized by cells connected by **gap junctions** that allow for synchronized contractions (e.g., peristalsis). - This type of smooth muscle exhibits **spontaneous rhythmic contractions** due to pacemaker cells, and its activity is modulated by neural and hormonal inputs rather than requiring individual innervation of each cell. - Multi-unit smooth muscle is **NOT present** in the gut. *Blood vessels* - Many larger blood vessels (e.g., large arteries) contain **multi-unit smooth muscle**, which allows for **fine, graded control** over vascular tone and blood flow. - Each muscle cell is typically **innervated individually**, enabling precise regulation of contraction strength. *Iris* - The iris contains **multi-unit smooth muscle** (e.g., sphincter pupillae and dilator pupillae muscles) which control pupil size. - These muscles require **individual innervation** to allow for very fine and precise movements in response to light intensity changes. *Ciliary muscle* - The ciliary muscle of the eye contains **multi-unit smooth muscle**, which controls the shape of the lens for accommodation (focusing). - These muscle fibers are **individually innervated** to allow precise control of lens curvature for near and far vision.

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

What is the fixed length of a myosin filament?

Accepted Answer

1.6 micrometers

Answer

0.16 nm

Answer

16 nm

Answer

1.6 mm

Question 2

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 3

Which of the following is NOT a location where multi-unit smooth muscle is present?

Accepted Answer

Gut

Answer

Blood vessels

Answer

Iris

Answer

Ciliary muscle

Question 4

The plateau phase of this graph is due to:

Accepted Answer

The calcium channels remaining open longer than the sodium channels

Answer

Increased membrane permeability to potassium ions

Answer

A decrease in calcium diffusion across the membrane

Answer

The movement of fewer sodium ions across the cell membrane

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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