Sensory Systems — MCQs

Sensory Systems Indian Medical PG Practice Questions and MCQs

Question 331: Somatosensory area I has the largest representation for which body part?

A. Arm (Correct Answer)
B. Leg
C. Back
D. Head

Explanation: The Somatosensory Area I (Primary Somatosensory Cortex), located in the postcentral gyrus, is organized according to a **Sensory Homunculus**. The size of the cortical representation for a body part is not proportional to its physical size, but rather to the **density of sensory receptors** and the functional importance of tactile discrimination in that area. **Why Arm is Correct:** Among the given options, the **Arm** (specifically the hand and fingers) has the highest density of sensory receptors. The hand, particularly the thumb and index finger, requires precise tactile feedback for fine motor tasks. Consequently, it occupies a significantly larger area of the sensory cortex compared to the trunk or lower limbs. **Why Other Options are Incorrect:** * **Leg:** While the leg is physically large, it has a lower density of sensory units compared to the upper extremity. It is represented on the medial surface of the hemisphere. * **Back:** The back has very large receptive fields and low tactile acuity (high two-point discrimination threshold), resulting in one of the smallest cortical representations. * **Head:** While the face and lips have massive representation (often larger than the arm), the "Head" as a general category in this specific comparison—or specifically the scalp/back of head—is less represented than the highly sensitive hand/arm complex. **High-Yield NEET-PG Pearls:** 1. **Sequence of Homunculus (Medial to Lateral):** Leg → Foot → Trunk → Arm → Hand → Face → Mouth. 2. **Greatest Overall Representation:** The **Lips**, **Face**, and **Thumb/Hands** have the largest areas in the sensory homunculus. 3. **Two-Point Discrimination:** There is an inverse relationship between the two-point discrimination threshold and the size of cortical representation. 4. **Blood Supply:** The medial part (Leg/Foot) is supplied by the **Anterior Cerebral Artery (ACA)**, while the lateral part (Arm/Face) is supplied by the **Middle Cerebral Artery (MCA)**.

Question 332: Horner syndrome causes all of the following except:

A. Enophthalmos
B. Mydriasis (Correct Answer)
C. Anhidrosis
D. Narrowed palpebral fissure

Explanation: **Explanation:** Horner syndrome is a clinical condition resulting from the interruption of the **sympathetic nerve supply** to the eye. To answer this question, one must understand that the sympathetic system is responsible for "fight or flight" responses, including pupil dilation. **1. Why Mydriasis is the Correct Answer:** Sympathetic fibers innervate the **dilator pupillae** muscle. In Horner syndrome, the loss of sympathetic tone leads to the unopposed action of the parasympathetic system (via the oculomotor nerve), resulting in **miosis** (constricted pupil), not mydriasis (dilated pupil). Therefore, mydriasis is the "except" in this list. **2. Analysis of Incorrect Options:** * **Enophthalmos (A):** The loss of sympathetic supply to the **orbitalis muscle** (Müller’s muscle) causes the eye to appear slightly sunken. * **Anhidrosis (C):** Sympathetic fibers control sweat glands. A lesion (especially pre-ganglionic) leads to a loss of sweating on the affected side of the face. * **Narrowed palpebral fissure (D):** This is caused by **partial ptosis**. The superior tarsal muscle (involuntary) is sympathetically innervated; its paralysis causes the upper eyelid to droop slightly, narrowing the fissure. **High-Yield Clinical Pearls for NEET-PG:** * **The Classic Triad:** Ptosis, Miosis, and Anhidrosis. * **Apparent Enophthalmos:** It is often described as "apparent" because the narrowing of the palpebral fissure creates the illusion of a sunken eyeball. * **Cocaine Test:** In a normal eye, cocaine drops cause mydriasis. In Horner syndrome, the pupil **fails to dilate** because there is no norepinephrine in the synaptic cleft for cocaine to act upon. * **Common Causes:** Pancoast tumor (at the lung apex), carotid artery dissection, or syringomyelia.

Question 333: Stereoanesthesia is due to a lesion of which of the following?

A. Nucleus Gracilis
B. Nucleus cuneatus
C. Cerebral cortex (Correct Answer)
D. Spinothalamic tract

Explanation: **Explanation:** **Stereoanesthesia** (also known as astereognosis) is the inability to identify the shape and texture of an object by touch in the absence of primary sensory loss. This is a classic sign of a lesion in the **Cerebral Cortex**, specifically the **Somatosensory Association Cortex (Brodmann areas 5 and 7)** in the parietal lobe. ### Why the Correct Answer is Right: While the primary somatosensory cortex (Areas 3, 1, 2) perceives basic sensations like touch and pressure, the **Association Cortex** is responsible for the high-level integration and interpretation of these signals. Stereoanesthesia occurs because the brain can no longer "synthesize" individual sensory inputs into a recognizable 3D concept, even though the pathways for touch and vibration are intact. ### Why Other Options are Wrong: * **Nucleus Gracilis & Nucleus Cuneatus:** These are second-order neurons in the Dorsal Column-Medial Lemniscal (DCML) pathway located in the medulla. Lesions here cause a loss of **primary** sensations (fine touch, vibration, and conscious proprioception) from the lower and upper body, respectively. This is called "sensory loss," not anesthesia of interpretation. * **Spinothalamic Tract:** This pathway carries pain, temperature, and crude touch. A lesion here results in loss of pain and temperature sensation (analgesia/thermanesthesia) on the contralateral side, which is unrelated to object recognition. ### High-Yield Clinical Pearls for NEET-PG: * **Stereognosis:** The ability to identify an object by touch (requires intact DCML + Parietal lobe). * **Graphesthesia:** The ability to recognize numbers/letters traced on the skin. Loss of this (agraphesthesia) also indicates a **parietal lobe lesion**. * **Tactile Agnosia:** Often used interchangeably with astereognosis; it is a hallmark of cortical sensory loss. * **Rule of Thumb:** If the patient can feel the touch (primary sensation) but cannot name the object (interpretation), the lesion is always **cortical**.

Question 334: Impulses generated in the taste buds of the tongue reach the cerebral cortex via which structure?

A. Thalamus (Correct Answer)
B. Internal capsule
C. Cervical spinal nerve
D. Trigeminal nerve

Explanation: **Explanation:** The correct answer is **Thalamus**. In the sensory pathway for gustation (taste), the thalamus acts as the essential relay station before information reaches the primary gustatory cortex. **The Pathway:** 1. **First-order neurons:** Taste impulses from the tongue are carried by the Facial (CN VII), Glossopharyngeal (CN IX), and Vagus (CN X) nerves. 2. **Second-order neurons:** These fibers synapse in the **Nucleus Tractus Solitarius (NTS)** in the medulla. Axons from the NTS ascend via the central tegmental tract. 3. **Third-order neurons:** These are located in the **Ventral Posteromedial (VPM) nucleus of the Thalamus**. From here, impulses are projected to the gustatory cortex (Insula and frontal operculum). **Why other options are incorrect:** * **Internal capsule:** While taste fibers pass through the posterior limb of the internal capsule to reach the cortex, it is a white matter tract (conduit), not the relay structure where the synapse occurs. * **Cervical spinal nerve:** These carry general somatic sensation from the neck and back of the head, not special visceral afferents like taste. * **Trigeminal nerve:** This nerve carries general sensations (touch, pain, temperature) from the anterior 2/3rd of the tongue via the lingual nerve, but it does not carry taste impulses. **High-Yield NEET-PG Pearls:** * **VPM Nucleus:** Remember the mnemonic **"VPM = Very Polar Mouth"** (relays sensory/taste from the face/mouth). In contrast, the **VPL** relays sensations from the **L**imbs/body. * **Ageusia:** Loss of taste sensation. * **NTS:** Known as the "sensory nucleus" for all visceral afferents (Taste, Baroreceptors, Chemoreceptors).

Question 335: Stereognosis will be lost in a lesion of which pathway?

A. Fasciculus gracilis
B. Fasciculus cuneatus (Correct Answer)
C. Anterior spinothalamic tract
D. Lateral spinothalamic tract

Explanation: **Explanation:** **Stereognosis** is the ability to identify an object by touch and manipulation without visual input. This is a complex cortical sensation that requires intact **fine touch, pressure, and proprioception**—all of which are carried by the **Dorsal Column-Medial Lemniscal (DCML) pathway**. 1. **Why Fasciculus Cuneatus is correct:** The DCML pathway is divided into the Fasciculus Gracilis (medial) and Fasciculus Cuneatus (lateral). The **Fasciculus Cuneatus** carries sensory information from the **upper limbs and upper trunk (above T6)**. Since stereognosis is clinically tested using the hands (e.g., identifying a key or coin), a lesion in the Fasciculus Cuneatus specifically results in the loss of this ability in the upper extremities. 2. **Why other options are incorrect:** * **Fasciculus Gracilis:** While also part of the DCML, it carries sensations from the **lower limbs (below T6)**. While a lesion here would affect fine touch in the legs, stereognosis is not typically tested or functional in the lower limbs. * **Anterior Spinothalamic Tract:** This pathway carries **crude touch and pressure**. * **Lateral Spinothalamic Tract:** This pathway carries **pain and temperature** sensations. **Clinical Pearls for NEET-PG:** * **Astereognosis** is often a sign of a lesion in the **Parietal Lobe** (specifically the somatosensory association cortex) or the dorsal columns. * **Rule of Thumb:** Gracilis is "G" for Ground (legs); Cuneatus is for the arms. * **DCML vs. Spinothalamic:** DCML fibers decussate in the **medulla** (internal arcuate fibers), whereas Spinothalamic fibers decussate at the **spinal cord level** (anterior white commissure). * **Tabes Dorsalis:** A classic condition involving the destruction of dorsal columns, leading to loss of vibration, position sense, and two-point discrimination.

Question 336: Which lobe of the cerebrum contains the auditory cortex?

A. Temporal lobe (Correct Answer)
B. Parietal lobe
C. Frontal lobe
D. Occipital lobe

Explanation: **Explanation:** The **Temporal lobe** is the primary site for auditory processing. The **Primary Auditory Cortex (Brodmann areas 41 and 42)** is located on the superior temporal gyrus, specifically within the **Heschl’s gyri**. It receives sensory input from the cochlea via the medial geniculate nucleus (MGN) of the thalamus. **Analysis of Options:** * **Temporal Lobe (Correct):** Responsible for hearing, memory (hippocampus), and language comprehension (Wernicke’s area). * **Parietal Lobe:** Primarily involved in **somatosensory processing** (postcentral gyrus, areas 1, 2, 3) and spatial awareness. * **Frontal Lobe:** Responsible for **motor control** (precentral gyrus), executive functions, and motor speech (Broca’s area). * **Occipital Lobe:** Dedicated to **visual processing** (Primary visual cortex, area 17). **High-Yield Clinical Pearls for NEET-PG:** * **Tonotopic Organization:** The auditory cortex is organized by sound frequency; high frequencies are processed posteromedially, while low frequencies are processed anterolaterally. * **Wernicke’s Area:** Located in the posterior part of the superior temporal gyrus (usually in the left hemisphere). Damage leads to **Sensory/Receptive Aphasia** (fluent but meaningless speech). * **Meyer’s Loop:** Part of the visual pathway that passes through the temporal lobe; a lesion here causes **Superior Quadrantanopia** ("pie in the sky" defect). * **Mnemonic:** "The **T**emporal lobe is near the **T**ips of the ears."

Question 337: Absence of taste sensation is termed as:

A. Hypogeusia
B. Ageusia (Correct Answer)
C. Dysgeusia
D. Partial ageusia

Explanation: ### Explanation The correct answer is **B. Ageusia**. **Understanding the Concept:** Taste sensation (gustation) is mediated by taste buds located on the tongue, soft palate, and epiglottis. In medical terminology, the suffix **"-geusia"** refers to the sense of taste. The prefix **"a-"** denotes a total absence or lack thereof. Therefore, **Ageusia** is defined as the complete loss of taste functions. True ageusia is relatively rare and is often confused with anosmia (loss of smell), as flavor perception relies heavily on olfactory input. **Analysis of Incorrect Options:** * **A. Hypogeusia:** Refers to a **reduced** or diminished ability to taste. It is a quantitative decrease rather than a total absence. * **C. Dysgeusia:** Refers to a **distortion** or perversion of taste. Patients often complain of a persistent metallic, salty, or rancid taste in the mouth. * **D. Partial ageusia:** Refers to the loss of taste for **specific** modalities (e.g., unable to taste bitter but can taste sweet) or loss in a specific area of the tongue. **Clinical Pearls for NEET-PG:** * **Nerve Supply:** Remember the "2/3 and 1/3" rule. Anterior 2/3 of the tongue is supplied by the **Chorda tympani** (branch of Facial nerve, CN VII); Posterior 1/3 is supplied by the **Glossopharyngeal nerve** (CN IX). * **Zinc Deficiency:** A classic high-yield cause of hypogeusia/ageusia is **Zinc deficiency**. * **Drug-induced:** Drugs like **Captopril** (ACE inhibitor) and **Penicillamine** are frequently associated with taste disturbances (Dysgeusia). * **Pathway:** Taste fibers terminate in the **Nucleus Tractus Solitarius (NTS)** in the medulla.

Question 338: Which body part is represented superiorly and medially within the postcentral gyrus?

A. Upper limb
B. Lower limb (Correct Answer)
C. Abdomen
D. Genitalia

Explanation: ### Explanation The **postcentral gyrus** of the parietal lobe houses the **Primary Somatosensory Cortex (Brodmann areas 3, 1, and 2)**. The spatial distribution of body parts in this area follows a specific pattern known as the **Sensory Homunculus**. **1. Why the Lower Limb is Correct:** The sensory homunculus is an "upside-down" representation of the body. The **lower limb (leg and foot)** and the **perineum** are represented on the **medial surface** of the hemisphere and the most **superior (dorsal)** aspect of the postcentral gyrus. These areas are supplied by the **Anterior Cerebral Artery (ACA)**. **2. Analysis of Incorrect Options:** * **A. Upper Limb:** The hand and arm are represented on the **superolateral** aspect of the postcentral gyrus. The hand, particularly the thumb and fingers, occupies a disproportionately large area due to high receptor density. * **C. Abdomen:** The trunk and abdomen are located on the upper part of the **lateral surface**, inferior to the lower limb representation but superior to the upper limb. * **D. Genitalia:** While genitalia are also represented medially, they are located most inferiorly on the medial wall, often below the representation of the toes. In the context of "superior and medial," the lower limb is the primary landmark. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Blood Supply:** A stroke involving the **Anterior Cerebral Artery (ACA)** leads to sensory and motor deficits primarily in the **contralateral lower limb**. In contrast, a **Middle Cerebral Artery (MCA)** stroke affects the face and upper limb. * **Disproportionate Representation:** The size of the cortical area is not proportional to the size of the body part, but to the **density of sensory receptors** (e.g., the lips and hands have the largest representation). * **Sequence (Medial to Lateral):** Toes → Foot → Leg → Hip → Trunk → Upper Limb → Face → Intra-abdominal.

Question 339: Bilateral destruction of the auditory cortex in humans causes what type of hearing impairment?

A. Almost total deafness
B. Hearing defect for higher frequency sounds
C. Deficiency in the interpretation of sounds (Correct Answer)
D. Inability for the orientation of sounds

Explanation: **Explanation:** The auditory cortex (Brodmann areas 41 and 42) is located in the superior temporal gyrus. While it is the primary site for processing auditory information, its role in humans is more specialized toward **complex integration** rather than the simple detection of sound. **1. Why Option C is Correct:** Bilateral destruction of the auditory cortex does not result in total deafness because lower-level auditory processing occurs in the **medial geniculate body** and the **inferior colliculus**. A person with such a lesion can still detect the presence of sound (intensity and pitch); however, they lose the ability to interpret the meaning of sounds, distinguish complex patterns, or understand spoken language. This is often referred to as **auditory agnosia**. **2. Why the other options are incorrect:** * **Option A:** Total deafness does not occur because subcortical centers remain intact. Sound detection is preserved, though sound perception is severely disorganized. * **Option B:** Frequency discrimination is primarily a function of the **organ of Corti** (tonotopic map) and is maintained at the level of the brainstem and thalamus. * **Option D:** While the auditory cortex helps in fine-tuning sound localization, the primary centers for sound orientation (detecting time and intensity differences between ears) are the **superior olivary nuclei** in the brainstem. **Clinical Pearls for NEET-PG:** * **Unilateral Lesion:** Causes very little change in hearing acuity because auditory pathways are **bilaterally represented** (each ear sends signals to both hemispheres). * **Wernicke’s Area:** Located in the posterior part of the superior temporal gyrus; damage here leads to **sensory aphasia** (inability to understand spoken words despite hearing them). * **Primary Auditory Cortex:** Receives input from the **Medial Geniculate Body** (MGB) of the thalamus (Mnemonic: **M**edial for **M**usic/Hearing; **L**ateral for **L**ight/Vision).

Question 340: What is the function of utricle and saccule?

A. Hearing high-frequency sounds
B. Linear acceleration (Correct Answer)
C. Hearing low-frequency sounds
D. Angular acceleration

Explanation: ***Linear acceleration***- The **utricle** and **saccule** are the two **otolithic organs** of the inner ear responsible for detecting mechanical forces related to **linear motion** (acceleration, deceleration) and the pull of gravity.- The **utricle** senses horizontal linear acceleration (e.g., walking), while the **saccule** senses vertical linear acceleration (e.g., riding an elevator).*Hearing high-frequency sounds*- Sound detection (hearing) is carried out by the **cochlea** and its associated structures, primarily the hair cells in the **Organ of Corti**.- High frequencies stimulate hair cells located near the **base** of the **basilar membrane** but this is unrelated to the vestibular functions of the utricle and saccule.*Angular acceleration*- Rotational movements of the head, or **angular acceleration**, are detected solely by the three **semicircular canals**.- The canals use the movement of endolymph to displace the **cupula** within the **ampullae**, stimulating the enclosed hair cells.*Hearing low-frequency sounds*- The **cochlea** is the auditory apparatus; the utricle and saccule are part of the **vestibular system** responsible for balance and spatial orientation.- Low frequencies primarily stimulate hair cells located near the **apex** of the **basilar membrane** (apex is furthest from the oval window).

Practice by Chapter

General Sensory Physiology

Practice Questions

Somatosensation

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Pain Physiology

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Vision and Optics

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Retinal Physiology

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Visual Pathways and Processing

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Auditory System

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Vestibular System

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Taste and Smell

Practice Questions

Sensory Integration

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