Sensory Systems — MCQs

Sensory Systems Indian Medical PG Practice Questions and MCQs

Question 381: Stimulation of posterior semicircular canal produces

A. Rotatory nystagmus
B. Horizontal nystagmus
C. Vertical nystagmus (Correct Answer)
D. All of the options

Explanation: ***Vertical nystagmus*** - Stimulation of the **posterior semicircular canal** primarily produces **vertical nystagmus** with a torsional (rotatory) component. - The posterior canal detects rotation in the sagittal plane and its activation triggers the **vestibulo-ocular reflex (VOR)**, causing predominantly **vertical eye movements** (typically downbeating) with associated torsion. - The posterior canal connects via the **vestibular nerve** to the **vestibular nuclei**, which project to the **oculomotor nuclei** to produce eye movements in the plane of the stimulated canal. *Rotatory nystagmus* - While the posterior canal does produce a **torsional (rotatory) component** accompanying the vertical nystagmus, **purely rotatory nystagmus** alone is not the most characteristic response. - Isolated rotatory nystagmus is more commonly associated with **anterior semicircular canal** pathology or combined canal involvement. *Horizontal nystagmus* - **Horizontal nystagmus** is specifically associated with stimulation of the **horizontal (lateral) semicircular canal**, which detects rotation in the horizontal plane. - Posterior canal stimulation does not produce horizontal eye movements as it operates in a different anatomical plane. *All of the options* - This option is incorrect because isolated stimulation of the **posterior semicircular canal** produces a specific pattern: primarily **vertical nystagmus with a torsional component**, not all types of nystagmus. - Horizontal nystagmus is distinctly absent with isolated posterior canal stimulation.

Question 382: Which of the following has cyclic GMP mediated action?

A. Thyroid hormone action
B. Steroidogenesis
C. Recruitment of glucose transporters to cell membrane
D. Photochemical reactions of visual cycle (Correct Answer)

Explanation: ***Photochemical reactions of visual cycle*** - The **photochemical reactions** in the visual cycle, particularly in **rod cells**, are mediated by **cyclic GMP (cGMP)**. - Light stimulation activates **rhodopsin**, which in turn activates **transducin**, leading to the activation of **cGMP phosphodiesterase**, which hydrolyzes cGMP, causing **hyperpolarization** of the photoreceptor cell. *Thyroid hormone action* - **Thyroid hormone (T3 and T4)** action is mediated by **intracellular receptors** that act as **transcription factors**, directly regulating gene expression. - Their mechanism typically does not involve **second messengers** like cGMP. *Steroidogenesis* - **Steroidogenesis** is the process of producing **steroid hormones**, which is primarily regulated by **intracellular signaling pathways** involving **G protein-coupled receptors** (e.g., ACTH stimulating cortisol synthesis) and ultimately gene expression, but not typically cGMP. - While some steroid hormones can influence cGMP levels, cGMP is not the primary mediator for the *process* of steroidogenesis itself. *Recruitment of glucose transporters to cell membrane* - The recruitment of **glucose transporters** (e.g., GLUT4) to the cell membrane, particularly in response to **insulin**, is mediated by **insulin receptor** signaling pathways, involving **tyrosine kinase activity** and downstream protein phosphorylation cascades. - This process primarily uses **intracellular signaling molecules** like **PIP3** and **Akt**, not cGMP.

Question 383: Pitch discrimination is best between:

A. 1000-4000 Hz (Correct Answer)
B. 20-20,000 Hz
C. 100-1000 Hz
D. 0-100 Hz

Explanation: ***1000-4000 Hz*** - Humans exhibit the **best pitch discrimination** in the **mid-to-high frequency range**, which corresponds to frequencies between approximately **1000 Hz and 4000 Hz**. - This range demonstrates the **smallest just-noticeable difference (JND)** for frequency changes, meaning the ear can detect the most subtle pitch variations. - This optimal range is crucial for **speech intelligibility**, as it encompasses the **formant frequencies** of vowels and many consonants. - The **cochlear mechanics** and **density of hair cells** are optimized for maximum frequency resolution in this range. *100-1000 Hz* - While pitch perception is functional in this range, the **discrimination ability is not as precise** as in the 1000-4000 Hz range. - This lower-mid frequency range is important for fundamental frequencies in speech and music, but **JND values are larger**. *20-20,000 Hz* - This represents the **entire human audible frequency range**, but it is too broad to describe the specific range of **optimal pitch discrimination**. - Pitch discrimination varies significantly across this spectrum, with **poorest discrimination at the extremes** (very low and very high frequencies). *0-100 Hz* - This range covers **very low frequencies**, where pitch discrimination is **poor**. - Sounds in this range are often perceived more as **vibrations** or **rhythmic pulses** rather than distinct musical pitches. - The **temporal resolution** rather than frequency resolution dominates perception at these low frequencies.

Question 384: Static equilibrium is due to -

A. Organ of Corti
B. Cristae ampullaris
C. Cupula
D. Macula (Correct Answer)

Explanation: ***Macula*** - The **macula** contains hair cells embedded in a gelatinous layer with otoliths, which respond to **linear acceleration** and **head tilt**, thus sensing static equilibrium. - It is located within the **utricle and saccule** of the inner ear, responsible for detecting the head's position relative to gravity. *End organ of corti* - The **organ of Corti** is the sensory organ for **hearing**, located within the cochlea, transducing sound vibrations into electrical signals. - It is not involved in balance or equilibrium, but rather with the perception of **sound**. *Cristae ampullaris* - The **cristae ampullaris** are sensory organs found within the **ampullae of the semicircular canals** and are primarily responsible for detecting **dynamic equilibrium** (angular acceleration). - They respond to rotational movements of the head, not static changes in head position or gravity. *Cupula* - The **cupula** is a gelatinous structure that covers the hair cells within the **cristae ampullaris**. - It moves with the endolymph during head rotation, stimulating the hair cells for **dynamic equilibrium**, not static equilibrium.

Question 385: Destruction of the right labyrinth causes nystagmus to:

A. Rotatory nystagmus
B. No nystagmus
C. Right side
D. Left side (Correct Answer)

Explanation: ***Left side*** - Destruction of the right labyrinth leads to a **reduction in tonic firing** from the right vestibular nerve. This creates an **imbalance** where the left labyrinth's signals are now relatively stronger. - The brain interprets this imbalance as if the head is turning to the left, causing **vestibulo-ocular reflex** to induce nystagmus with its **fast phase** to the left (away from the side of the lesion and towards the relatively intact labyrinth). *Rotatory nystagmus* - While nystagmus can have a rotatory component, the predominant direction of acceleration or slow phase will be either horizontal or vertical and indicates the **direction of the lesion** or excitation. - Describing nystagmus purely as "rotatory" does not specify the **direction of the fast component** in relation to the lesion. *No nystagmus* - **Unilateral destruction** of a labyrinth creates an acute imbalance in vestibular input, which **always results in nystagmus**. - Nystagmus is a key clinical sign of acute **vestibular dysfunction**, as the brain perceives an ongoing head movement that isn't occurring. *Right side* - A fast phase directed towards the right would imply either **excitation** of the right labyrinth or **destruction** of the left labyrinth. - In this case, destruction of the right labyrinth leads to nystagmus with its **fast phase away from the lesion**, meaning to the left.

Question 386: Which substance is most likely to increase in the rods of the retina when the light is turned on?

A. Cyclic guanosine monophosphate (cGMP)
B. Metarhodopsin II (Correct Answer)
C. Cyclic adenosine monophosphate (cAMP)
D. Rhodopsin

Explanation: ***Metarhodopsin II*** - When **light strikes rhodopsin**, it undergoes a conformational change, forming **metarhodopsin II**, which is the active form that initiates the phototransduction cascade. - **Metarhodopsin II** activates a **G-protein (transducin)**, leading to a decrease in cGMP and subsequent rod hyperpolarization. *Cyclic guanosine monophosphate (cGMP)* - **Light activation** of rhodopsin triggers a cascade that **decreases cGMP concentration** in the rods, leading to closing of cGMP-gated sodium channels. - In the **dark**, cGMP levels are high, keeping the sodium channels open and the rod depolarized. *Cyclic adenosine monophosphate (cAMP)* - **cAMP** is a significant second messenger in many cellular processes but is **not directly involved in the primary phototransduction pathway** in rods. - Its levels do not acutely increase in response to light in the same manner as molecules in the phototransduction cascade. *Rhodopsin* - **Rhodopsin** is the **light-sensitive pigment** located in the rod outer segment membranes. - When light is turned on, rhodopsin is **converted** into its active form, metarhodopsin II, meaning the amount of intact rhodopsin itself will decrease, not increase.

Question 387: Tuning fork of 512 Hz is used to test the hearing because it is -

A. Produces overtones
B. Better heard (Correct Answer)
C. Better felt
D. Not heard

Explanation: ***Better heard*** - A 512 Hz tuning fork produces a frequency that falls within the range of **optimal human hearing sensitivity** (around 500 Hz to 4000 Hz), making it clearly perceptible for hearing tests. - **Key clinical advantage**: 512 Hz minimizes **tactile bone vibration** compared to lower frequencies (128 Hz or 256 Hz), which are "better felt" than heard, making 512 Hz more specific for testing actual auditory perception. - This frequency allows for accurate assessment of **conductive** and **sensorineural hearing loss** using Rinne and Weber tests. - It produces fewer **overtones** compared to higher frequencies, ensuring a clear fundamental tone for testing. *Produces overtones* - While all tuning forks produce some **overtones** (harmonics), 512 Hz produces relatively fewer overtones compared to higher frequencies. - The primary reason for choosing 512 Hz is **not** its overtone production but rather its clear fundamental tone and optimal audibility. - Excessive overtones can interfere with the clarity of hearing assessment. *Better felt* - This describes **lower frequency tuning forks** (128 Hz, 256 Hz), which produce more **bone vibration** that can be felt tactilely. - 512 Hz is specifically chosen because it minimizes this tactile sensation, making it a **better auditory test** rather than a vibration test. - The goal of hearing tests is to assess **auditory perception**, not tactile sensation. *Not heard* - This statement is incorrect; a 512 Hz tuning fork is specifically chosen because its frequency is **well within the human auditory range** and is easily heard. - If a patient cannot hear this frequency, it indicates potential **hearing impairment**, which is the goal of the test to identify.

Question 388: Colour vision is with the help of:

A. Cortex
B. Optic disc
C. Rods
D. Cones (Correct Answer)

Explanation: ***Cones*** - **Cones** are photoreceptor cells in the retina responsible for **color vision** and high spatial acuity in bright light conditions. - There are three types of cones, each sensitive to different wavelengths of light (red, green, and blue). *Cortex* - The **visual cortex** in the brain processes visual information, including color, but it relies on input from the photoreceptors. - The cortex itself does not initially detect color; it interprets signals sent from the retina. *Optic disc* - The **optic disc** is the point where the optic nerve leaves the eye, and it contains no photoreceptor cells. - This area is therefore known as the "blind spot" and cannot detect light or color. *Rods* - **Rods** are photoreceptor cells primarily responsible for **scotopic (low-light) vision** and peripheral vision. - They are highly sensitive to light but do not detect color, only shades of gray.

Question 389: The parvocellular pathway, from the lateral geniculate nucleus to the visual cortex, carries signals for the detection of

A. Luminance contrast
B. Temporal frequency
C. Color vision, shape and fine details. (Correct Answer)
D. Movement, depth and flicker.

Explanation: ***Color vision, shape and fine details.*** - The **parvocellular pathway** is specialized for processing **detailed visual information**, including **color vision**, **fine spatial resolution** (shape), and **texture**. - Its neurons have **small receptive fields** and a sustained response, making them ideal for precise analysis of stationary objects and their features. *Luminance contrast* - While the parvocellular pathway contributes to luminance contrast, it is not its primary or most distinguishing feature compared to its role in color and fine detail. - Both parvocellular and **magnocellular pathways** process luminance contrast, but **magnocellular** is more involved in rapid changes. *Temporal frequency.* - **Temporal frequency** (detecting changes over time, like motion or flicker) is primarily processed by the **magnocellular pathway**. - The **parvocellular pathway** has a relatively **slow, sustained response** and is less adept at detecting rapid temporal changes. *Movement, depth and flicker.* - **Movement**, **depth perception**, and **flicker detection** are predominantly functions of the **magnocellular pathway**. - This pathway has large receptive fields and responds transiently to stimuli, making it optimized for detecting motion and rapid changes in the visual scene.

Question 390: At what intensity does the sound of a normal voice reach the ear from a distance of 1 meter?

A. 60 dB (Correct Answer)
B. 80 dB
C. 20 dB
D. 40 dB

Explanation: **60 dB** - A **normal conversational voice** at a distance of about 1 meter typically has an intensity around **60 decibels (dB)**. - This level is considered moderate and easily audible without discomfort in a quiet environment. *80 dB* - An intensity of **80 dB** is significantly louder, comparable to a **garbage disposal** or a **loud alarm clock**. - While audible, it would generally be perceived as quite loud for a normal conversational voice. *20 dB* - An intensity of **20 dB** is very quiet, equivalent to a **whisper** or the **rustling of leaves**. - It would be too low for a normal conversational voice to be heard clearly at 1 meter. *40 dB* - An intensity of **40 dB** is softer than typical conversation, similar to the sound of a **quiet office** or **refrigerator hum**. - While audible, it would likely be considered a **soft voice** rather than a normal conversational level.

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

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Sensory Integration

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