Sensory Systems Practice Questions

Q: Tapping the patellar tendon with a reflex hammer produces a brief contraction of the knee extensors. What is the cause of this muscle contraction?

Muscle spindle activation. **Explanation:** The patellar reflex (knee-jerk) is a classic example of a **monosynaptic stretch reflex**. When the patellar tendon is tapped, it causes a rapid, transient stretch of the quadriceps muscle. 1. **Why the correct answer is right:** The primary sensory receptors for detecting muscle stretch are the **muscle spindles** (intrafusal fibers). When the quadriceps is stretched, the muscle spindles are activated, sending afferent impulses via **Type Ia nerve fibers** to the spinal cord. These fibers synapse directly (monosynaptically) with **alpha-motor neurons** in the anterior horn, which then trigger the contraction of the extrafusal muscle fibers of the quadriceps. 2. **Why the incorrect options are wrong:** * **Elastic rebound:** This is a physical property of tissue, not a neural reflex. The force of contraction in a reflex is far greater than any simple elastic recoil. * **Golgi tendon organ (GTO) response:** GTOs (Type Ib fibers) respond to **muscle tension**, not stretch. Their activation typically causes *autogenic inhibition* (relaxation) to prevent tendon avulsion, which is the opposite of the observed contraction. * **Muscle spindle unloading:** This occurs during muscle **contraction** (shortening), which actually decreases the firing rate of the spindle. It is the *loading* (stretching) that triggers the reflex. **Clinical Pearls for NEET-PG:** * **Reflex Arc Components:** Receptor (Spindle) → Afferent (Ia) → Center (L3-L4 spinal cord) → Efferent (Alpha-motor neuron) → Effector (Quadriceps). * **Gamma-motor neurons:** These regulate the sensitivity of the muscle spindle. High gamma-discharge leads to hyperreflexia. * **Jendrassik Maneuver:** A reinforcement technique used to accentuate deep tendon reflexes by increasing the excitatory drive to the spinal motor pool.

Q: Pain is appreciated when the small bowel is

Distended. **Explanation:** The sensory innervation of the gastrointestinal tract differs significantly from that of the skin. Visceral pain is mediated by sensory fibers traveling primarily with the sympathetic nervous system. **Why Distension is Correct:** The hollow viscera, such as the small bowel, are insensitive to stimuli that typically cause pain on the body surface (like cutting or burning). Instead, visceral pain receptors (nociceptors) located in the muscularis and serosa are specifically sensitive to **mechanical stretch, distension, and forceful contraction**. When the small bowel is distended (e.g., due to intestinal obstruction or gas), it triggers these tension receptors, resulting in a dull, poorly localized, and often midline aching sensation. **Why Other Options are Incorrect:** * **A, B, and C (Burned, Crushed, Cut):** These are examples of somatic stimuli. The visceral peritoneum and the bowel wall lack the specific thermoreceptors and sharp-pain nociceptors required to perceive these insults. A surgeon can biopsy or cauterize the bowel in a conscious patient without causing pain, provided there is no traction on the mesentery. **High-Yield NEET-PG Pearls:** * **Mechanism:** Visceral pain is transmitted via **Type C unmyelinated fibers**. * **Localization:** Because visceral afferents enter the spinal cord at multiple levels and converge with somatic afferents (Dermatomal rule), visceral pain is **poorly localized** and often **referred**. * **The Exception:** While the bowel wall is insensitive to cutting, the **parietal peritoneum** is richly innervated by somatic nerves and is highly sensitive to mechanical and chemical irritation (leading to localized "rebound tenderness" in peritonitis). * **Ischemia:** Along with distension and chemical irritation, **ischemia** is a potent trigger for visceral pain.

Q: All are required for balancing EXCEPT:

Olfactory system. Maintaining equilibrium and posture is a complex process coordinated by the cerebellum, which integrates sensory input from three primary systems. This is often referred to as the **"Triad of Balance."** **Why Olfactory System is the Correct Answer:** The **olfactory system (Option B)** is responsible for the sense of smell. It does not provide any spatial, positional, or gravitational information to the brain. Therefore, it plays no role in maintaining physical balance or posture. **Explanation of Incorrect Options (Systems required for balance):** 1. **Visual System (Option A):** Provides information about the body's position relative to the environment and the horizon. It allows for compensatory movements when the head or body moves. 2. **Proprioception (Option C):** Involves mechanoreceptors in joints, muscles, and tendons (e.g., muscle spindles, Golgi tendon organs). These send "position sense" signals to the brain, informing it of the location of limbs in space. 3. **Vestibular System (Option D):** Located in the inner ear (semicircular canals and otolith organs), it detects angular acceleration and linear movement, acting as the body’s primary "gyroscope." **High-Yield Clinical Pearls for NEET-PG:** * **Romberg’s Test:** This test evaluates the integration of these systems. A patient is asked to stand with eyes closed. If they lose balance, it indicates a loss of **proprioception** (sensory ataxia), as they can no longer use the **visual system** to compensate. * **The Cerebellum:** It is the main integration center. While the three systems provide the *input*, the cerebellum provides the *coordination*. * **Rule of Two:** To maintain balance, at least **two** of the three systems (Visual, Vestibular, Proprioceptive) must be functional. Loss of two or more results in significant equilibrium deficits.

Q: Which of the marked areas is involved in relieving pain when the painful site is massaged?

A. ***A*** - According to **Melzack and Wall's gate control theory**, massaging activates large **Aβ fibers** that stimulate the **substantia gelatinosa** (area A), which acts as a "gate" to inhibit pain transmission. - The substantia gelatinosa contains **inhibitory interneurons** that close the gate by reducing activity in **T cells** (transmission cells), thereby blocking pain signals from reaching higher brain centers. *B* - This area represents the **dorsal horn** but not specifically the substantia gelatinosa where the gate control mechanism occurs. - Lacks the specific **inhibitory interneurons** responsible for the gate control effect that blocks pain transmission. *C* - This area represents **deeper laminae** of the dorsal horn that are involved in processing rather than gating pain signals. - Does not contain the **gate control circuitry** necessary for the inhibitory effect seen with massage and touch stimulation. *D* - This area represents the **ventral horn** which contains **motor neurons** and is not involved in sensory pain processing. - Plays no role in the **gate control mechanism** that explains why massage relieves pain through sensory inhibition.

Q: Which of the following activates vanilloid receptors in the urinary bladder?

Temperature. **Explanation:** The correct answer is **Temperature**. **1. Why Temperature is Correct:** Vanilloid receptors, specifically the **TRPV1 (Transient Receptor Potential Vanilloid 1)** subtype, are non-selective cation channels found on the sensory (afferent) nerve endings of the urinary bladder mucosa and detrusor muscle. These receptors are primarily known as **polymodal nociceptors**. In the bladder, they are activated by **noxious heat (temperature >43°C)**, protons (acidic pH), and capsaicin. They play a crucial role in sensing bladder irritation and pain, contributing to the "burning" sensation during cystitis and the signaling of bladder fullness. **2. Why the Other Options are Incorrect:** * **Vibration and Touch (B & C):** These sensations are mediated by mechanoreceptors such as Pacinian corpuscles and Meissner’s corpuscles, respectively. They are not the primary function of vanilloid receptors. * **Pressure (D):** Bladder wall tension and pressure (distension) are primarily sensed by **mechanosensitive ion channels** (like Piezo channels) and high-threshold mechanoreceptors. While TRPV1 may modulate the threshold of these responses, pressure is not the direct activator of the vanilloid receptor itself. **3. High-Yield Clinical Pearls for NEET-PG:** * **Capsaicin & Resiniferatoxin:** These are TRPV1 agonists. Intravesical instillation of these agents is sometimes used to treat **detrusor overactivity** by initially stimulating and then "desensitizing" the sensory afferents (C-fibers). * **C-Fibers vs. A-delta Fibers:** Normal bladder filling is sensed by A-delta fibers. However, in pathological states (like spinal cord injury or inflammation), the **TRPV1-expressing C-fibers** become hyper-excitable, leading to urgency and frequency. * **Location:** TRPV1 receptors are also highly expressed in the **urothelium**, which acts as a sensory organ communicating with underlying nerves via ATP release.

Q: Substance P is increased in response to pain in the periphery, mediated by which of the following?

Nerve terminals. **Explanation:** The correct answer is **Nerve terminals (Option D)**. This phenomenon is rooted in the concept of **Neurogenic Inflammation**. When peripheral nociceptors (specifically polymodal C-fibers) are stimulated by a painful stimulus, action potentials travel orthodromically toward the spinal cord. However, some impulses also travel **antidromically** (backward) along collateral branches of the same sensory nerve. Upon reaching the peripheral nerve terminals, these impulses trigger the release of neuropeptides, primarily **Substance P** and **Calcitonin Gene-Related Peptide (CGRP)**. * **Substance P** acts on local blood vessels to increase permeability, leading to **edema** (plasma extravasation). * **CGRP** causes potent **vasodilation**, resulting in the "flare" response of the Triple Response of Lewis. **Why other options are incorrect:** * **Mast cells (A):** While Substance P triggers mast cells to release histamine (furthering the inflammatory cycle), the mast cells themselves do not synthesize or release Substance P. * **Endothelium (B):** Endothelial cells are the *target* of Substance P (leading to gap junction formation and leakage), but they are not the source. * **Plasma (C):** Substance P is a neuropeptide synthesized in the dorsal root ganglion and transported to terminals; it is not a plasma-derived mediator like bradykinin or complement proteins. **High-Yield Clinical Pearls for NEET-PG:** * **Triple Response of Lewis:** Consists of Red spot (capillary dilation), Flare (arteriolar dilation via axon reflex), and Wheal (exudation/edema). * **Substance P Receptor:** It acts via the **NK-1 (Neurokinin-1) receptor**. * **Axon Reflex:** This is a unique reflex that does not involve the CNS or a synapse; it occurs entirely within the peripheral branching of a single sensory neuron.

Question 1

A lesion in the posterior column of the spinal cord will affect which of the following modalities?

Accepted Answer

Temperature sense

Answer

Vibration sense

Answer

Fine touch

Answer

Crude touch sensation

Question 2

Tapping the patellar tendon with a reflex hammer produces a brief contraction of the knee extensors. What is the cause of this muscle contraction?

Accepted Answer

Muscle spindle activation

Answer

Elastic rebound of muscle connective tissue

Answer

Golgi tendon organ response

Answer

Muscle spindle unloading

Question 3

Pain is appreciated when the small bowel is

Accepted Answer

Distended

Answer

Burned

Answer

Crushed

Answer

Cut

Question 4

All are required for balancing EXCEPT:

Accepted Answer

Olfactory system

Answer

Visual system

Answer

Proprioception

Answer

Vestibular system

Question 5

Which of the following statements regarding color vision is true?

Accepted Answer

Ganglion cells are responsible for color vision processing.

Answer

Color blindness is more common in females.

Answer

The optic nerve head is blue-blind.

Answer

The V2 area of the occipital cortex is responsible for color vision.

Question 6

Which of the marked areas is involved in relieving pain when the painful site is massaged?

Accepted Answer

A

Answer

B

Answer

C

Answer

D

Question 7

A physiology experiment is conducted in which a glass microelectrode is inserted into a Pacinian corpuscle to record receptor potentials during different levels of stimulation. Increasing stimulus strength from 10 percent of maximum to 30 percent of maximum causes a 40 percent increase in the amplitude of the receptor potential. Increasing the stimulus potential from 70 percent of maximum to 90 percent of maximum is most likely to cause what increase in the amplitude of the receptor potential (in percent)?

Accepted Answer

10

Answer

40

Answer

60

Answer

80

Question 8

A 39-year-old neurosurgeon picks up a scalpel, which activates numerous sensory receptors in her hand. An increase in which of the following best describes the basis for transduction of the sensory stimuli into nerve impulses?

Accepted Answer

Increased ion permeability

Answer

Activation of G protein

Answer

Decreased ion permeability

Answer

Decreased transmitter release

Question 9

Which of the following activates vanilloid receptors in the urinary bladder?

Accepted Answer

Temperature

Answer

Vibration

Answer

Touch

Answer

Pressure

Question 10

Substance P is increased in response to pain in the periphery, mediated by which of the following?

Accepted Answer

Nerve terminals

Answer

Mast cells

Answer

Endothelium

Answer

Plasma

Sensory Systems — MCQs

Sensory Systems — MCQs

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