Somatosensation — MCQs
Ablation of the somatosensory area I of the cerebral cortex leads to
Which sensory receptor is primarily responsible for mediating slow vibration sensation?
Fine touch and position sense is carried by
A woman with right-sided loss of sensations of both the upper and lower limb complains of shooting pain from her fingers to the right shoulder and a burning sensation when touching cold water. Motor functions are normal. Which of the following structures is likely to be involved?
Area numbers for the somatosensory area include:
Pacinian corpuscle is stimulated by which of the following?
A woman suffered a sunburn while enjoying a vacation on the beach. Now, while taking a shower, the lukewarm water (40° C) touching her back caused her to feel pain. What types of receptors were activated by the lukewarm water, and why did she experience pain?
Fine touch and rapidly adapting receptor is
Maximum area in homunculus signifies?
Name the interneuron marked X in colour purple involved in tetanus. (Recent NEET Pattern 2016-17)
Somatosensation Indian Medical PG Practice Questions and MCQs
Question 1: Ablation of the somatosensory area I of the cerebral cortex leads to
- A. Loss of tactile localization but not of two point discrimination
- B. Loss of tactile localization and two point discrimination (Correct Answer)
- C. Total loss of pain sensation
- D. Total loss of touch sensation
Explanation: ***Loss of tactile localization and two point discrimination*** - The **somatosensory area I (S1)** is crucial for processing higher-order somatic sensations, including the **discriminative aspects of touch**. - Its ablation leads to deficits in distinguishing distinct points of touch (**two-point discrimination**) and precisely identifying where touch occurred (**tactile localization**). *Loss of tactile localization but not of two point discrimination* - This option incorrectly suggests that **two-point discrimination** would be preserved, which is not the case as S1 is essential for both functions. - While other cortical areas contribute to sensation, S1 is paramount for fine tactile distinctions. *Total loss of pain sensation* - **Pain sensation** is processed across multiple brain regions, including the **thalamus**, **insula**, and **anterior cingulate cortex**, not solely S1. - S1 primarily processes discriminatory aspects of somatosensation, not the affective component of pain. *Total loss of touch sensation* - A total loss of touch sensation implies a complete unawareness of touch, which would require more extensive damage than just S1 ablation, potentially affecting **thalamic pathways** or other cortical regions. - S1 ablation primarily affects the *quality* and *interpretation* of touch, rather than its complete absence.
Question 2: Which sensory receptor is primarily responsible for mediating slow vibration sensation?
- A. Pacinian capsule
- B. Merkel's disc
- C. Meissner corpuscles (Correct Answer)
- D. Ruffini's endings
Explanation: ***Meissner corpuscles*** - These are **rapidly adapting mechanoreceptors** located in the dermal papillae, particularly abundant in glabrous (hairless) skin of fingertips, palms, and soles. - They are highly sensitive to **low-frequency vibration (10-50 Hz)**, which corresponds to **slow vibration sensation**. - They respond to **light touch** and **dynamic skin deformation**, making them ideal for detecting flutter and slow vibratory stimuli. *Ruffini's endings* - These are **slowly adapting mechanoreceptors** (Type II) located deep in the dermis and subcutaneous tissue. - They primarily detect **sustained pressure**, **skin stretch**, and **joint position**, contributing to proprioception. - They are NOT primarily involved in vibration sensation but rather in detecting continuous mechanical deformation. *Pacinian capsule* - These are **rapidly adapting mechanoreceptors** located deep in the dermis, subcutaneous tissue, and periosteum. - They are highly sensitive to **high-frequency vibration (200-300 Hz)**, which corresponds to **fast vibration sensation**. - They detect rapid changes in pressure and are the most sensitive mechanoreceptors to high-frequency stimuli. *Merkel's disc* - These are **slowly adapting mechanoreceptors** (Type I) found at the epidermal-dermal junction. - They are responsible for **sustained touch**, **pressure**, and **fine tactile discrimination** (texture, edges, shapes). - They do not detect vibration but rather respond to continuous indentation of the skin.
Question 3: Fine touch and position sense is carried by
- A. Lateral spinothalamic tract
- B. Anterior spinothalamic tract
- C. Spinocerebellar tract
- D. Dorsal column (Correct Answer)
Explanation: ***Dorsal column*** - The **dorsal column-medial lemniscus pathway** is responsible for transmitting **fine touch**, **vibration**, **proprioception**, and **two-point discrimination** from the body to the brain. - This pathway includes the **fasciculus gracilis** (lower body) and **fasciculus cuneatus** (upper body) in the spinal cord. *Lateral spinothalamic tract* - The **lateral spinothalamic tract** primarily transmits sensations of **pain** and **temperature**. - It decussates (crosses) at the level of entry in the spinal cord and ascends contralaterally. *Anterior spinothalamic tract* - The **anterior spinothalamic tract** carries information about **crude touch** and **pressure**. - Like the lateral spinothalamic tract, it decussates at the spinal cord level and ascends contralaterally. *Spinocerebellar tract* - The **spinocerebellar tracts** (anterior and posterior) convey unconscious **proprioceptive** information to the **cerebellum**. - This information helps in coordinating muscle movements and maintaining posture, but it does not carry conscious fine touch or position sense.
Question 4: A woman with right-sided loss of sensations of both the upper and lower limb complains of shooting pain from her fingers to the right shoulder and a burning sensation when touching cold water. Motor functions are normal. Which of the following structures is likely to be involved?
- A. Anterior spinothalamic tract
- B. Spinocerebellar tract
- C. Lateral spinothalamic tract (Correct Answer)
- D. Posterior column
Explanation: ***Lateral spinothalamic tract*** - The symptoms described, such as **loss of sensations**, **shooting pain** (neuropathic pain), and **burning sensation** with cold water (dysesthesia/allodynia), are characteristic of damage to the **spinothalamic tract**, which carries **pain and temperature** sensations. - Involvement of the **right-sided upper and lower limb** indicates a lesion affecting sensory pathways on the ipsilateral side of the body before decussation, or more commonly a contralateral lesion above the level of decussation for the specific tract. Given the symptoms affecting pain and temperature, the lateral spinothalamic tract is the primary candidate. *Anterior spinothalamic tract* - This tract primarily transmits **crude touch** and **pressure** sensations. - While loss of sensation is present, the prominent **shooting pain** and **burning sensation with cold water** are not typically associated with isolated anterior spinothalamic tract lesions. *Spinocerebellar tract* - This tract is responsible for transmitting **proprioceptive information** to the cerebellum for motor coordination. - Damage to the spinocerebellar tracts would manifest as **ataxia** and **incoordination**, not pain or loss of touch/temperature sensation, and motor functions are stated as normal in the patient. *Posterior column* - The posterior column (dorsal column-medial lemniscus pathway) transmits **fine touch**, **vibration**, and **proprioception**. - While a loss of sensation is present, the specific complaints of **shooting pain** and **burning sensation to cold water** are not characteristic of posterior column damage, which would typically present with deficits in discriminative touch, vibratory sense, and position sense.
Question 5: Area numbers for the somatosensory area include:
- A. 5 & 7
- B. 1, 2 & 3 (Correct Answer)
- C. 4 & 6
- D. 16 & 18
Explanation: ***1, 2 & 3*** - These Brodmann areas (1, 2, and 3) collectively represent the **primary somatosensory cortex**, located in the postcentral gyrus [1]. - This region is responsible for processing **tactile** and **proprioceptive information** from the body [1]. *5 & 7* - Brodmann areas 5 and 7 are part of the **posterior parietal cortex**, involved in **multimodal sensory association** and spatial awareness [1]. - While they process sensory information, they are considered **somatosensory association areas**, not the primary somatosensory cortex [1]. *4 & 6* - Brodmann area 4 is the **primary motor cortex**, responsible for initiating voluntary movements. - Brodmann area 6 is the **premotor and supplementary motor cortex**, involved in planning and coordinating movements [2]. *16 & 18* - Areas 16 and 18 are not associated with somatosensory function. - Brodmann area 18 is a **visual association area** (secondary visual cortex), involved in processing and interpreting visual information.
Question 6: Pacinian corpuscle is stimulated by which of the following?
- A. Pain
- B. Temperature
- C. Touch
- D. Pressure (Correct Answer)
Explanation: ***Pressure*** - **Pacinian corpuscles** are rapidly adapting mechanoreceptors that detect **deep pressure** and **vibrations**. - Their layered, onion-like structure allows them to be very sensitive to rapid changes in pressure. *Pain* - **Pain** is primarily detected by **nociceptors**, which are free nerve endings, not Pacinian corpuscles. - Nociceptors respond to various noxious stimuli, including mechanical, thermal, and chemical. *Temperature* - **Temperature** changes are detected by **thermoreceptors**, such as Krause end bulbs for cold and Ruffini endings for warmth, not Pacinian corpuscles. - These receptors have specific temperature ranges over which they are active. *Touch* - **Touch** sensation is broadly detected by several mechanoreceptors, including **Meissner's corpuscles** (light touch), **Merkel discs** (sustained touch), and hair follicle receptors. - While Pacinian corpuscles contribute to sensing touch through deep pressure, they are not the primary receptors for general light or sustained touch.
Question 7: A woman suffered a sunburn while enjoying a vacation on the beach. Now, while taking a shower, the lukewarm water (40° C) touching her back caused her to feel pain. What types of receptors were activated by the lukewarm water, and why did she experience pain?
- A. Thermal nociceptors & nociceptive pain
- B. Thermal nociceptors & allodynia (Correct Answer)
- C. Innocuous thermal receptors and hyperalgesia
- D. Innocuous thermal receptors and allodynia
Explanation: ***Thermal nociceptors & allodynia*** - Sunburn causes tissue damage, leading to the sensitization of **thermal nociceptors** (e.g., TRPV1 channels), making them responsive to normally innocuous thermal stimuli. - **Allodynia** is the perception of pain from a stimulus that does not ordinarily cause pain, such as lukewarm water after a sunburn, due to increased sensitivity of the pain pathways. *Thermal nociceptors & nociceptive pain* - While thermal nociceptors are involved, **nociceptive pain** typically refers to pain caused by direct tissue damage from a noxious (harmful) stimulus. Here, the lukewarm water itself is not noxious. - The pain experienced from the lukewarm water is not due to a *new* noxious stimulus but rather an exaggerated response to a *non-noxious* one. *Innocuous thermal receptors and hyperalgesia* - **Innocuous thermal receptors** (e.g., TRPM8 for cold, TRPV3/TRPV4 for warmth) normally detect non-painful temperature changes, not pain from sunburn. - **Hyperalgesia** is an increased response to a stimulus that *is* normally painful, but applied at a lower intensity. The lukewarm water is not normally painful. *Innocuous thermal receptors and allodynia* - As mentioned, **innocuous thermal receptors** are not primarily responsible for pain transmission, even in a sensitized state, as they are not nociceptors. - While **allodynia** is an accurate description of the pain type, the primary receptors activated for the pain sensation are sensitized nociceptors, not innocuous thermal receptors.
Question 8: Fine touch and rapidly adapting receptor is
- A. Ruffini's nerve ending
- B. Pacinian corpuscle
- C. Meissners corpuscle (Correct Answer)
- D. Merkel's disc
Explanation: ***Meissners corpuscles*** - They are responsible for **discriminative touch** and ability to differentiate between two points close together. - These receptors are **rapidly adapting**, meaning they respond strongly to initial stimulation but quickly cease firing if the stimulus is constant. *Ruffini's nerve ending* - These are **slowly adapting receptors** sensitive to **stretch** and sustained pressure. - They provide information about **proprioception** and the shape of objects. *Pacinian corpuscle* - Pacinian corpuscles are **rapidly adapting mechanoreceptors** sensitive to **vibration** and deep pressure. - They have a large receptive field and are less involved in fine touch discrimination. *Merkel's disc* - Merkel's discs are **slowly adapting mechanoreceptors** responsible for sensing sustained touch and **pressure**. - They contribute to the perception of **texture** and edges, having a small receptive field.
Question 9: Maximum area in homunculus signifies?
- A. Increased precision of movements (Correct Answer)
- B. More muscle fibers are present
- C. Low control and unskilled movements
- D. Larger size of the body part
Explanation: ***Increased precision of movements*** - A larger area on the homunculus represents a greater density of **sensory receptors** or motor neurons dedicated to that body part, allowing for more **fine-tuned control** and sensation. - This increased representation enables body parts like the hands and face to perform **complex and precise movements** such as writing or speaking. - The cortical area is proportional to the **functional importance** and precision required, not the physical size of the body part. *More muscle fibers are present* - While there may be more muscle fibers in some areas, the size on the homunculus primarily reflects the **neural innervation density** and the precision of control, not merely the bulk of muscle. - A large muscle group with relatively simple movements might have fewer cortical neurons dedicated to it than a smaller, more dexterous body part. *Low control and unskilled movements* - A maximum area in the homunculus signifies **high control** and highly skilled movements, as more cortical processing power is dedicated to these regions. - Areas with less cortical representation typically have **cruder, less precise movements** and sensory discrimination. *Larger size of the body part* - The homunculus does **not** represent the actual physical size of body parts. - Small body parts like fingers and lips have **disproportionately large** cortical representation due to their need for precision, while larger parts like the trunk have minimal representation.
Question 10: Name the interneuron marked X in colour purple involved in tetanus. (Recent NEET Pattern 2016-17)
- A. Renshaw cell (Correct Answer)
- B. Basket cell
- C. Purkinje cell
- D. Anterior horn cell
Explanation: ***Renshaw cell*** - The image depicts a **Renshaw cell (X)**, which is an **inhibitory interneuron** in the spinal cord, regulating motor neuron activity. - In tetanus, the toxin **tetanospasmin** inhibits the release of neurotransmitters (glycine and GABA) from Renshaw cells, leading to **uncontrolled muscle spasms**. *Basket cell* - **Basket cells** are found in the **cerebellar cortex** and hippocampus, playing a role in inhibiting Purkinje cell activity. - They are not located in the spinal cord gray matter in the position marked X. *Purkinje cell* - **Purkinje cells** are large, distinctive neurons found exclusively in the **cerebellar cortex**, crucial for motor coordination. - They are not present in the spinal cord and are not interneurons in the context of spinal reflexes. *Anterior horn cell* - **Anterior horn cells** are **motor neurons** whose cell bodies reside in the anterior horn of the spinal cord and directly innervate skeletal muscles. - They are not interneurons; rather, they are the target of regulation by interneurons like Renshaw cells.
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