Nerve and Muscle Physiology Practice Questions

Q: Gamma neurons innervate which type of muscle fibers?

Intrafusal muscle fibers. **Explanation:** The correct answer is **B. Intrafusal muscle fibers.** **1. Why it is correct:** The muscle spindle is a sensory receptor located within skeletal muscles that detects changes in muscle length. It consists of specialized fibers called **intrafusal fibers**. These fibers are innervated by **Gamma ($\gamma$) motor neurons**, which originate in the ventral horn of the spinal cord. When gamma neurons fire, they cause the poles (ends) of the intrafusal fibers to contract, which stretches the central sensory portion, thereby maintaining the sensitivity of the muscle spindle even when the muscle itself is shortened. **2. Why the other options are incorrect:** * **Option A:** **Extrafusal muscle fibers** are the standard contractile fibers responsible for muscle bulk and force generation. These are innervated by **Alpha ($\alpha$) motor neurons**, not gamma neurons. * **Option C:** The **primary motor innervation** to skeletal muscle is provided by Alpha motor neurons, which form the "final common pathway" for motor control. * **Option D:** Inhibition of antagonistic muscles is achieved via **Ia inhibitory interneurons** during the reciprocal inhibition reflex, not directly by gamma neurons. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Alpha-Gamma Co-activation:** During voluntary movement, both alpha and gamma neurons are activated simultaneously. This ensures the muscle spindle remains "taut" and sensitive to further changes in length during contraction. * **Gamma Loop:** This refers to the pathway: Gamma motor neuron $\rightarrow$ Intrafusal fiber contraction $\rightarrow$ Ia afferent stimulation $\rightarrow$ Alpha motor neuron activation $\rightarrow$ Extrafusal fiber contraction. * **Clinical Correlation:** Overactivity of gamma motor neurons is a primary contributor to **spasticity** and increased muscle tone in Upper Motor Neuron (UMN) lesions.

Q: Regulation of smooth muscle tone is persistently influenced by:

Latch bridge mechanism. **Explanation:** The **Latch bridge mechanism** is the physiological basis for the sustained, energy-efficient contraction (tone) characteristic of smooth muscle. Unlike skeletal muscle, smooth muscle can maintain high tension for long periods with very low ATP consumption. This occurs when **myosin light chain phosphatase (MLCP)** dephosphorylates the myosin head while it is still attached to actin. This "latches" the cross-bridge in place, significantly slowing the detachment rate. This allows the muscle to maintain **persistent tone** without requiring continuous high-frequency stimulation or massive ATP hydrolysis. **Analysis of Incorrect Options:** * **A. Calcium release from SR:** While calcium is essential for initiating contraction (via binding to Calmodulin), it is not the mechanism for *persistent* tone. In fact, smooth muscle relies heavily on extracellular calcium entry through voltage-gated channels rather than just SR release. * **B. Beta 1 receptor:** These receptors are primarily located in the **heart** (increasing heart rate and contractility). Smooth muscle tone in the bronchioles and blood vessels is more typically regulated by **Beta 2** (relaxation) or **Alpha 1** (contraction) receptors. * **C. Troponin:** This is a **high-yield negative fact**. Smooth muscle **lacks troponin**. Instead, it uses **Calmodulin** and **Caldesmon/Calponin** to regulate the interaction between actin and myosin. **NEET-PG High-Yield Pearls:** * **Calmodulin** is the functional analog of Troponin C in smooth muscle. * **Myosin Light Chain Kinase (MLCK)**: Phosphorylates myosin to *initiate* contraction. * **Myosin Light Chain Phosphatase (MLCP)**: Dephosphorylates myosin to *initiate* the latch state or relaxation. * Smooth muscle has the **slowest** cycling of cross-bridges but the **greatest** force of contraction per unit area compared to skeletal muscle.

Q: What does a muscle spindle primarily detect?

Length. **Explanation:** The **muscle spindle** is a specialized sensory receptor located within the belly of skeletal muscles. Its primary physiological role is to detect changes in **muscle length** and the **rate of change in length**. 1. **Why "Length" is correct:** Muscle spindles are arranged in **parallel** with extrafusal muscle fibers. When a muscle is stretched, the spindle is also stretched, triggering sensory impulses via **Type Ia (primary)** and **Type II (secondary)** afferent fibers. This mechanism is the basis for the **stretch reflex (myotatic reflex)**, which helps maintain muscle tone and posture. 2. **Why other options are incorrect:** * **Tension (Option A):** This is the primary function of the **Golgi Tendon Organ (GTO)**. GTOs are arranged in **series** with muscle fibers and protect the muscle from damage during excessive contraction. * **Proprioception (Option C):** While muscle spindles contribute significantly to proprioception (the sense of body position), "Proprioception" is a broad category, not a specific physical parameter. "Length" is the specific stimulus detected. * **Pressure (Option D):** Pressure is primarily detected by cutaneous and deep tissue receptors like **Pacinian corpuscles**. **High-Yield Clinical Pearls for NEET-PG:** * **Innervation:** Muscle spindles receive motor innervation from **Gamma ($\gamma$) motor neurons**, which maintain spindle sensitivity during muscle contraction (Alpha-Gamma co-activation). * **Reflex Arc:** The muscle spindle is the receptor for the **monosynaptic** deep tendon reflex (e.g., Knee jerk). * **Nuclear Bag vs. Chain:** Nuclear bag fibers detect dynamic changes (velocity), while nuclear chain fibers detect static changes (length).

Q: Which equation is used to measure the resting membrane potential in case of multiple ions?

Goldman equation. The resting membrane potential (RMP) is a fundamental concept in neurophysiology, representing the electrical potential difference across a cell membrane at rest. [1] ### **Explanation of the Correct Answer** **Option A (Goldman Equation):** The Goldman-Hodgkin-Katz (GHK) equation is the correct choice because it accounts for **multiple ions** (primarily $Na^+$, $K^+$, and $Cl^-$) simultaneously. Unlike simpler models, it considers two critical factors for each ion: 1. The **concentration gradient** across the membrane. [1] 2. The **relative permeability** ($P$) of the membrane to that specific ion. [1] Since the RMP of a neuron (typically -70 mV) is a collective result of all permeant ions, the Goldman equation provides the most accurate physiological measurement. ### **Explanation of Incorrect Options** * **Option B (Nernst Equation):** This equation calculates the **equilibrium potential for a single ion** only. [1] It determines the theoretical voltage at which the electrical and chemical gradients for one specific ion are perfectly balanced (e.g., -94 mV for $K^+$ or +61 mV for $Na^+$). It does not account for the membrane's simultaneous permeability to other ions. * **Options C & D:** These are incorrect as the two equations serve distinct purposes—one for individual ion equilibrium and the other for the total membrane potential. ### **NEET-PG High-Yield Pearls** * **The "K+ Rule":** The RMP is closest to the equilibrium potential of Potassium (-94 mV) because the resting membrane is 20–100 times more permeable to $K^+$ than to $Na^+$. * **The Na+-K+ Pump:** While the Goldman equation calculates the potential based on diffusion, the $Na^+$-$K^+$ ATPase pump is "electrogenic" and directly contributes about -4 mV to the RMP. * **Clinical Correlation:** Hypokalemia makes the RMP more negative (hyperpolarization), making neurons less excitable, which clinically manifests as muscle weakness or paralysis.

Q: What is true about summation?

Subthreshold stimuli are used.. **Explanation:** Summation is the process by which individual graded potentials (post-synaptic potentials) are added together to reach the threshold required to trigger an action potential. **1. Why Option C is Correct:** Summation specifically involves **subthreshold stimuli**. A single subthreshold stimulus is insufficient to reach the firing level (threshold) of a neuron. However, if multiple subthreshold stimuli are applied in rapid succession or at different locations simultaneously, their cumulative effect can depolarize the membrane to the threshold, resulting in an action potential. If a stimulus were already suprathreshold, it would trigger an action potential on its own, making summation unnecessary. **2. Why Other Options are Incorrect:** * **Option A:** This describes **Spatial Summation**, not Temporal. Temporal summation occurs when a single presynaptic terminal fires **repeatedly in rapid succession** (one after another). * **Option B:** This describes **Temporal Summation**. Spatial summation occurs when **multiple different presynaptic terminals** fire simultaneously (together) at different locations on the same postsynaptic neuron. **NEET-PG High-Yield Pearls:** * **EPSP vs. IPSP:** Summation can be excitatory (EPSP) or inhibitory (IPSP). The net change in membrane potential depends on the algebraic sum of all inputs. * **Location:** Summation typically occurs at the **axon hillock**, which has the highest density of voltage-gated Na+ channels and the lowest threshold for firing. * **Refractory Period:** Temporal summation is possible because the postsynaptic potential lasts longer than the refractory period of the presynaptic action potential.

Question 1

Which of the following statements is NOT true regarding cardiac and skeletal muscle fibers?

Accepted Answer

Both cardiac and skeletal muscles exhibit graded contraction.

Answer

Excitation-contraction coupling in both depends upon increased intracellular Calcium.

Answer

Excitation-contraction coupling is not dependent on extracellular Ca2+ in skeletal muscle.

Answer

The cardiac action potential has a plateau and a longer refractory period.

Question 2

Gamma neurons innervate which type of muscle fibers?

Accepted Answer

Intrafusal muscle fibers

Answer

Extrafusal muscle fibers

Answer

Primary motor innervations to skeletal muscles

Answer

Inhibit antagonistic muscles

Question 3

Clasp knife rigidity is also known as:

Accepted Answer

Inverse stretch reflex

Answer

Withdrawal reflex

Answer

Lengthening reaction

Answer

Crossed extensor reflex

Question 4

What characterizes the inverse stretch reflex?

Accepted Answer

It is a disynaptic reflex activated on change in muscle tension.

Answer

It occurs when muscle length changes.

Answer

It is a monosynaptic reflex initiated by activation of the Golgi tendon organ.

Answer

It uses type II afferent fibers from the Golgi tendon organ.

Question 5

Regulation of smooth muscle tone is persistently influenced by:

Accepted Answer

Latch bridge mechanism

Answer

Calcium release from sarcoplasmic reticulum

Answer

Beta 1 receptor

Answer

Troponin

Question 6

What does a muscle spindle primarily detect?

Accepted Answer

Length

Answer

Tension

Answer

Proprioception

Answer

Pressure

Question 7

Which equation is used to measure the resting membrane potential in case of multiple ions?

Accepted Answer

Goldman equation

Answer

Nernst equation

Answer

Both Goldman and Nernst equations

Answer

Neither Goldman nor Nernst equation

Question 8

Which of the following statements is NOT true regarding action potential recording?

Accepted Answer

Compound action potential of a mixed nerve is recorded by cathode ray oscilloscope.

Answer

Biphasic action potential is recorded by a surface electrode.

Answer

Compound action potential is an algebraic summation of all the monophasic recordings from a mixed nerve.

Answer

A typical action potential is recorded by a microelectrode placed inside the nerve fiber.

Question 9

What is true about summation?

Accepted Answer

Subthreshold stimuli are used.

Answer

Temporal summation is the application of two stimuli together.

Answer

Spatial summation is the application of two stimuli one after another.

Answer

All are true.

Question 10

What property of the postsynaptic neuron would optimize the effectiveness of two closely spaced axodendritic synapses?

Accepted Answer

A high membrane resistance

Answer

A high dendritic cytoplasmic resistance

Answer

A small cross-sectional area

Answer

A small space constant

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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