Nerve and Muscle Physiology Practice Questions

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.

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: What covers the binding sites for myosin heads on actin in skeletal muscles?

Tropomyosin. ***Tropomyosin*** - **Tropomyosin** is a two-stranded alpha-helical coiled coil protein that lies in the grooves of the **actin** filaments. - In a resting muscle, it physically blocks the **myosin-binding sites** on **actin**, preventing contraction. *Troponin* - **Troponin** is a complex of three proteins (troponin C, troponin I, and troponin T) that binds to **tropomyosin**. - It plays a crucial role in pulling **tropomyosin** away from the **myosin-binding sites** on **actin** when calcium binds to troponin C, initiating muscle contraction. *Calcium* - **Calcium ions (Ca2+)** do not directly cover the binding sites but rather bind to **troponin C**. - This binding causes a conformational change in **troponin**, which in turn shifts **tropomyosin** to expose the binding sites. *None of the above* - This option is incorrect because **tropomyosin** specifically performs the function of covering the myosin-binding sites on actin. - The other components (troponin and calcium) are involved in regulating the position of tropomyosin, not directly covering the sites themselves.

Q: Latent period of muscle twitch is 10 milliseconds, contraction period is 40 milliseconds, and the relaxation time is 50 milliseconds. What would be the tetanizing frequency?

25 Hz. ***25 Hz*** - **Tetanizing frequency** is the minimum stimulation frequency required to produce tetanus (sustained muscle contraction without complete relaxation between stimuli). - For **incomplete tetanus** to occur, the next stimulus must arrive during the relaxation phase, before the muscle fully relaxes. - The critical time window is the **latent period + contraction period** = 10 ms + 40 ms = 50 ms. However, to ensure summation occurs reliably during relaxation, stimuli typically arrive at a slightly higher frequency. - **Practical tetanizing frequency** = approximately 1/(40 ms) = **25 Hz**, which ensures stimuli arrive during the latter part of contraction or early relaxation phase, producing sustained tension. - This frequency allows sufficient overlap for tetanic fusion while accounting for the physiological requirements of the muscle twitch cycle. *50 Hz* - This frequency (one stimulus every 20 ms) would produce a **complete tetanus** with no visible relaxation between stimuli. - This is higher than the minimum tetanizing frequency required for this muscle with its 100 ms total twitch duration. - While this would produce tetanus, it exceeds the minimum frequency needed. *75 Hz* - This very high frequency (one stimulus every 13.3 ms) would produce a **smooth, complete tetanus**. - This is approximately 3 times the minimum tetanizing frequency and represents excessive stimulation. - Such high frequencies are well beyond what is needed to prevent relaxation in this muscle. *100 Hz* - This extremely high frequency (one stimulus every 10 ms, equivalent to the latent period alone) would produce **maximal tetanic fusion**. - This is 4 times the minimum tetanizing frequency needed for this muscle. - While physiologically possible, this represents supramaximal stimulation frequency for tetanus production in this scenario.

Q: What mediates the response observed in Lewis's triple response?

Axon reflex. ***Axon reflex*** - The **axon reflex** is the characteristic **neural mechanism** that mediates the spreading **flare** (the distinctive third component) of Lewis's triple response. - This antidromic reflex occurs when sensory nerve endings are stimulated, causing impulses to travel along branches of the same axon and release **substance P and CGRP** (calcitonin gene-related peptide), producing vasodilation in surrounding areas. - The axon reflex is what makes Lewis's triple response a **neurovascular** phenomenon, distinguishing it from simple inflammatory responses. - While histamine initiates the cascade, the **axon reflex** is considered the key mediating mechanism for the complete triple response pattern. *Arteriolar dilation* - Arteriolar dilation is the **effect** or result seen in the flare, not the mediating mechanism itself. - This is what happens as a consequence of axon reflex activation, not what mediates the response. *Histamine release* - **Histamine** from mast cells is the initial **chemical trigger** that causes the red line (direct capillary dilation) and wheal (increased permeability). - However, histamine alone cannot explain the **spreading flare** beyond the site of injury, which requires the neurogenic mechanism of the axon reflex. - The question asks for the **mediating mechanism** of the observed response pattern, where the axon reflex is the distinguishing feature. *Local hormones* - This term is too vague and non-specific to describe the precise mechanism. - While bradykinin and other mediators may contribute, the **axon reflex** is the specific neural mechanism that characterizes Lewis's triple response.

Q: Upstroke of the action potential would lead to:

All of the options. ***All of the options*** - The **upstroke** or **depolarization phase** of an action potential is characterized by a rapid influx of **positive ions** (primarily sodium) into the cell. - This influx causes the **cell's membrane potential** to become less negative and then positive relative to the outside, representing an inward net current and making the cell interior more positive. *Net current in an inward direction* - The **upstroke** is driven primarily by the rapid opening of **voltage-gated sodium channels**, allowing **Na+ ions** to rush into the cell. - This movement of positive charge into the cell constitutes an **inward current**, making the inside of the cell more positive. *Cell interior becomes more positive* - As **positively charged sodium ions** flow into the cell during the upstroke, the accumulation of these charges within the cell leads to a change in membrane potential. - This change shifts the membrane potential from a **negative resting state** towards a **positive value**. *Cell interior becomes less negative* - The initial phase of the action potential upstroke involves the **depolarization** of the membrane, meaning the potential moves from its resting negative value (e.g., -70mV) towards zero. - Before becoming positive, the cell interior first becomes **less negative** as it approaches the threshold potential and then rapidly depolarizes.

Q: Which of the following conditions does not involve neuronal degeneration?

Temporary conduction block (Neuropraxia). ***Temporary conduction block (Neuropraxia)*** - **Neuropraxia** is the mildest form of nerve injury, involving a **temporary conduction block** without structural damage to the axon. - It causes a transient loss of function but does not involve **neuronal degeneration** or Wallerian degeneration. *Growth and differentiation in the fetal nervous system* - During fetal development, a significant amount of **neuronal apoptosis** (programmed cell death) occurs. - This process is crucial for sculpting the developing nervous system by eliminating redundant or improperly connected neurons. *Aging process of neurons* - The aging process is associated with some degree of **neuronal loss** and **degeneration**, particularly in specific brain regions. - This can lead to cognitive decline and other neurological changes common in older adults. *Crush injury to a nerve* - A **crush injury** to a nerve typically results in significant **axonal disruption** and **Wallerian degeneration**. - This involves the breakdown of the axon distal to the injury site, which is a form of neuronal degeneration.

Question 1

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 2

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 3

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

Question 4

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 5

What covers the binding sites for myosin heads on actin in skeletal muscles?

Accepted Answer

Tropomyosin

Answer

Troponin

Answer

Calcium

Answer

None of the above

Question 6

Latent period of muscle twitch is 10 milliseconds, contraction period is 40 milliseconds, and the relaxation time is 50 milliseconds. What would be the tetanizing frequency?

Accepted Answer

25 Hz

Answer

50 Hz

Answer

75 Hz

Answer

100 Hz

Question 7

What mediates the response observed in Lewis's triple response?

Accepted Answer

Axon reflex

Answer

Arteriolar dilation

Answer

Histamine release

Answer

Local hormones

Question 8

Upstroke of the action potential would lead to:

Accepted Answer

All of the options

Answer

Cell interior becomes less negative

Answer

Net current in an inward direction

Answer

Cell interior becomes more positive

Question 9

Which of the following conditions does not involve neuronal degeneration?

Accepted Answer

Temporary conduction block (Neuropraxia)

Answer

Aging process of neurons

Answer

Growth and differentiation in the fetal nervous system

Answer

Crush injury to a nerve

Question 10

Which of the following functions is primarily associated with neuroactive substances such as substance P, 5-hydroxytryptamine, vasoactive intestinal peptide, somatostatin, and prostaglandins?

Accepted Answer

Both modify excitability of nerve endings and affect vascular tone.

Answer

None of the above.

Answer

Affect vascular tone only.

Answer

Modify excitability of nerve endings only.

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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