Neuroanatomy — MCQs

Neuroanatomy Indian Medical PG Practice Questions and MCQs

Question 1641: The commonest cranial nerve involved in acoustic neuroma is:

A. X
B. VIII (Correct Answer)
C. IX
D. VI

Explanation: ***Correct Answer: VIII (Vestibulocochlear Nerve)*** - Acoustic neuroma, also known as **vestibular schwannoma**, arises from the **Schwann cells** of the **vestibular branch of cranial nerve VIII**. - Its symptoms, such as **hearing loss**, **tinnitus**, and **balance problems**, directly result from the compression and dysfunction of the vestibulocochlear nerve. - This is the **primary nerve involved** as the tumor originates from it. *Incorrect: X (Vagus Nerve)* - The **vagus nerve (cranial nerve X)** is involved in diverse functions like **swallowing, phonation, and parasympathetic innervation of organs**. - While a large acoustic neuroma can eventually affect adjacent cranial nerves, it is not the primary nerve involved or the origin of the tumor. *Incorrect: IX (Glossopharyngeal Nerve)* - The **glossopharyngeal nerve (cranial nerve IX)** is primarily responsible for **taste, salivation, and sensation from the pharynx**. - Involvement of this nerve typically presents with symptoms like **dysphagia** or altered taste, which are not initial or common features of an acoustic neuroma. *Incorrect: VI (Abducens Nerve)* - The **abducens nerve (cranial nerve VI)** controls the **lateral rectus muscle**, responsible for moving the eye outward. - Involvement would lead to **diplopia** or a convergent squint, which occurs only in very large acoustic neuromas that cause significant brainstem compression.

Question 1642: In a diving accident that severed the spinal cord below the sixth cervical vertebra, which of the following muscles would be affected?

A. Deltoid
B. Infraspinatus
C. Levator Scapulae
D. Latissimus Dorsi (Correct Answer)

Explanation: ***Latissimus Dorsi*** - The **latissimus dorsi muscle** is primarily innervated by the **thoracodorsal nerve**, which arises from the **C6, C7, and C8** nerve roots (with C7 and C8 being the predominant contributors) [1]. - A spinal cord injury below the sixth cervical vertebra would affect the C7 and C8 segments, thereby disrupting the nerve supply to the latissimus dorsi, leading to weakness or paralysis. - This muscle is responsible for adduction, extension, and internal rotation of the shoulder. *Deltoid* - The **deltoid muscle** is innervated by the **axillary nerve**, which arises predominantly from the **C5 and C6** nerve roots. - Since the injury is below the C6 vertebra, the upper cervical segments (C5 and C6) would remain intact above the level of injury. - Therefore, deltoid function would be preserved. *Infraspinatus* - The **infraspinatus muscle** is innervated by the **suprascapular nerve**, which arises from the **C5 and C6** nerve roots. - Similar to the deltoid, its innervation originates above the level of the spinal cord injury and would be spared. *Levator Scapulae* - The **levator scapulae muscle** receives innervation from the **C3, C4, and C5** spinal nerves, as well as contributions from the dorsal scapular nerve (predominantly C5). - All of these nerve roots originate well above the level of injury, so this muscle would not be affected.

Question 1643: Somatic efferent does not include:

A. Facial nerve (Correct Answer)
B. Abducent nerve
C. Trochlear nerve
D. Oculomotor nerve

Explanation: ***Facial nerve*** - The **facial nerve (CN VII)** is a **mixed nerve** that does **NOT** contain **general somatic efferent (GSE)** fibers - Its motor component consists of **special visceral efferent (SVE)** fibers that innervate **muscles of facial expression** derived from the **2nd pharyngeal arch**, not from somites - Also contains **general visceral efferent (GVE)** fibers (parasympathetic to lacrimal, submandibular, and sublingual glands) and sensory fibers - **This is the correct answer** because it lacks GSE fibers *Abducent nerve* - The **abducent nerve (CN VI)** is a **pure general somatic efferent (GSE) nerve** - Exclusively innervates the **lateral rectus muscle** of the eye, which is derived from **somites** - Its function is to cause **abduction** of the ipsilateral eye - This nerve DOES contain somatic efferent fibers, so it is incorrect *Trochlear nerve* - The **trochlear nerve (CN IV)** is also a **pure general somatic efferent (GSE) nerve** - Innervates the **superior oblique muscle** of the eye, derived from **somites** - Responsible for **depression**, **abduction**, and **internal rotation** of the eye - This nerve DOES contain somatic efferent fibers, so it is incorrect *Oculomotor nerve* - The **oculomotor nerve (CN III)** contains **general somatic efferent (GSE) fibers** that innervate most **extraocular muscles** (superior rectus, inferior rectus, medial rectus, inferior oblique, and levator palpebrae superioris) - Also contains **general visceral efferent (GVE)** fibers (parasympathetic to pupillary sphincter and ciliary muscles) [1] - This nerve DOES contain somatic efferent fibers, so it is incorrect

Question 1644: Which of the following cranial nerves passes through the internal auditory meatus?

A. 9th cranial nerve
B. 7th cranial nerve (Correct Answer)
C. 11th cranial nerve
D. 10th cranial nerve

Explanation: ***7th cranial nerve*** - The **facial nerve (CN VII)** passes through the internal auditory meatus along with the vestibulocochlear nerve (CN VIII). - This nerve is responsible for **facial expression**, taste from the anterior two-thirds of the tongue, and parasympathetic innervation of some glands. *9th cranial nerve* - The **glossopharyngeal nerve (CN IX)** exits the skull through the **jugular foramen**, not the internal auditory meatus. - It is involved in taste, swallowing, and sensation from the posterior tongue and pharynx. *11th cranial nerve* - The **accessory nerve (CN XI)** exits the skull through the **jugular foramen**, supplying the sternocleidomastoid and trapezius muscles. - Its spinal root ascends into the skull through the foramen magnum and then joins the cranial root before exiting. *10th cranial nerve* - The **vagus nerve (CN X)** also exits the skull through the **jugular foramen**, alongside CN IX and CN XI. - It has extensive innervation to the heart, lungs, and digestive tract.

Question 1645: Dissociated sensory loss in a case of tumor of central Spinal cord is due to lesion of-

A. Dorsal column fibres
B. Anterior Spinothalamic tract
C. Decussating fibres of lateral spinothalamic tract (Correct Answer)
D. Central spinal centre of spinal cord

Explanation: ***Decussating fibres of lateral spinothalamic tract*** - A tumor in the central spinal cord, such as a **syringomyelia**, primarily affects the decussating fibers of the **lateral spinothalamic tract**. - This typically results in a **dissociated sensory loss**, meaning loss of **pain and temperature sensation** while preserving light touch, proprioception, and vibration. *Dorsal column fibres* - Lesions here would typically cause loss of **proprioception**, **vibration**, and **fine touch**, not primarily dissociated sensory loss involving pain and temperature [1]. - These fibers ascend ipsilaterally and do not decussate in the spinal cord, so they would be less likely to be affected by a central lesion in a dissociated pattern [1]. *Anterior Spinothalamic tract* - This tract primarily mediates **crude touch** and **pressure** and is less commonly the sole cause of dissociated sensory loss as described [1]. - While it does decussate, isolated damage to this tract alone would not typically explain the classic dissociated pain and temperature loss pattern. *Central spinal center of spinal cord* - This is a broad and less specific term; the specific fibers affected within the central spinal cord, leading to dissociated sensory loss, are the **decussating fibers of the lateral spinothalamic tract**. - While a central lesion is the cause, specifying "central spinal center" doesn't precisely identify the neural pathway responsible for the characteristic sensory deficit.

Question 1646: Which of the following extraocular muscles is not supplied by oculomotor nerve?

A. Inferior oblique
B. Medial rectus
C. Inferior rectus
D. Lateral rectus (Correct Answer)

Explanation: ***Lateral rectus*** - The **lateral rectus muscle** is innervated by the **abducens nerve (CN VI)**, not the oculomotor nerve (CN III). [1] - Its primary action is **abduction** of the eye, moving it laterally away from the midline. [1] *Inferior oblique* - The **inferior oblique muscle** is innervated by the **oculomotor nerve (CN III)**. [1] - Its actions include **extorsion**, elevation, and abduction of the eye. [1] *Medial rectus* - The **medial rectus muscle** is innervated by the **oculomotor nerve (CN III)**. [1] - Its primary action is **adduction** of the eye, moving it medially towards the midline. [1] *Inferior rectus* - The **inferior rectus muscle** is innervated by the **oculomotor nerve (CN III)**. [1] - Its actions include **depression**, extorsion, and adduction of the eye. [1]

Question 1647: Which structure carries axons from the nucleus gracilis to the thalamus?

A. Medial lemniscus (Correct Answer)
B. Fasciculus gracilis
C. Fasciculus lemniscus
D. Lateral spinothalamic tract

Explanation: ***Medial lemniscus*** - The **medial lemniscus** is formed by the decussation of internal arcuate fibers, which originate from the **nucleus gracilis** and nucleus cuneatus [1]. - These fibers carry **fine touch**, **vibration**, and **proprioception** from the body to the thalamus [1]. *Fasciculus gracilis* - The **fasciculus gracilis** is part of the **dorsal column** in the spinal cord, ascending ipsilaterally [1]. - It carries sensory information from the **lower body** to the nucleus gracilis in the medulla, not directly to the thalamus [1]. *Fasciculus lemniscus* - This is an **incorrect anatomical term**; there is no recognized neurological structure called the fasciculus lemniscus. - The term "lemniscus" refers to ascending sensory tracts, but it does not combine with "fasciculus" in this manner. *Lateral spinothalamic tract* - The **lateral spinothalamic tract** carries information about **pain** and **temperature** from the body to the thalamus [1]. - It originates from the dorsal horn of the spinal cord and decussates at the spinal cord level, distinct from the dorsal column-medial lemniscus pathway [1].

Question 1648: Neurologic examination reveals bilateral medial rectus paresis on attempted lateral gaze, monocular horizontal nystagmus in the abducting eye, and unimpaired convergence. The lesion is in the:

A. Rostral midbrain, bases pedunculorum.
B. Midpontine tegmentum, dorsomedial zones, bilateral. (Correct Answer)
C. Caudal midbrain tectum.
D. Caudal pontine base.

Explanation: ***Midpontine tegmentum, dorsomedial zones, bilateral.*** - The combination of **bilateral medial rectus paresis** on attempted lateral gaze, **monocular horizontal nystagmus** in the abducting eye, and **unimpaired convergence** is the classic presentation of **bilateral internuclear ophthalmoplegia (INO)**. - Bilateral INO is caused by lesions in the **medial longitudinal fasciculus (MLF)**, which is located in the **dorsomedial tegmentum** of the midpons and is crucial for coordinating horizontal eye movements. *Rostral midbrain, bases pedunculorum.* - Lesions in the **rostral midbrain** are more likely to affect vertically oriented eye movements or cause disorders like **Parinaud's syndrome**, not bilateral INO. - The **bases pedunculorum** primarily contain motor tracts and would cause motor deficits, not isolated eye movement disorders like INO. *Caudal midbrain tectum.* - The **caudal midbrain tectum** contains the superior and inferior colliculi, which are involved in visual and auditory reflexes respectively, not direct control of conjugate horizontal gaze. - Lesions here are unlikely to cause the specific pattern of bilateral medial rectus paresis and abducting nystagmus seen in INO. *Caudal pontine base.* - The **caudal pontine base** contains the abducens nucleus and the paramedian pontine reticular formation (PPRF), lesions of which cause different oculomotor deficits such as **gaze palsies** or **abducens nerve palsies**, not bilateral INO with preserved convergence. - A lesion here would typically affect the generation of horizontal gaze, leading to an inability to look in one direction, rather than dysconjugate eye movements.

Question 1649: Gustatory pathway involves which nerve(s)?

A. Glossopharyngeal
B. Vagus
C. Facial
D. Facial and Glossopharyngeal (Correct Answer)

Explanation: ***Facial and Glossopharyngeal*** - The **facial nerve (cranial nerve VII)** innervates the taste buds on the **anterior two-thirds of the tongue** via the **chorda tympani** branch [1]. - The **glossopharyngeal nerve (cranial nerve IX)** innervates the taste buds on the **posterior one-third of the tongue** and the circumvallate papillae [1]. *Glossopharyngeal* - While the **glossopharyngeal nerve** is involved in taste sensation for the posterior tongue, it does not cover the entire taste pathway. - It specifically transmits taste from the **posterior one-third of the tongue** and **circumvallate papillae** [1]. *Vagus* - The **vagus nerve (cranial nerve X)** plays a minor role in taste sensation, primarily innervating taste buds on the **epiglottis and pharynx** [1]. - Its contribution to the overall gustatory pathway is not as significant as the facial and glossopharyngeal nerves. *Facial* - The **facial nerve** is crucial for taste sensation, specifically transmitting taste from the **anterior two-thirds of the tongue** [1]. - However, it does not innervate the posterior portion of the tongue, making it an incomplete answer for the entire gustatory pathway.

Question 1650: Spinal nerve roots are supplied by -

A. Posterior spinal artery
B. Radicular arteries (Correct Answer)
C. Anterior spinal artery
D. Vertebral artery

Explanation: ***Radicular arteries*** - The **radicular arteries** are branches of the segmental arteries (e.g., intercostal, lumbar, lateral sacral arteries) that supply the **spinal nerve roots** and their coverings. - They provide crucial blood supply to the peripheral parts of the spinal cord and the nerve roots as they exit the vertebral canal. *Posterior spinal artery* - The **posterior spinal artery** supplies the posterior one-third of the spinal cord, primarily the **posterior columns** and posterior horns. - It does not directly supply the spinal nerve roots themselves as they emerge from the cord. *Anterior spinal artery* - The **anterior spinal artery** supplies the anterior two-thirds of the spinal cord, including the **anterior horns** and lateral funiculi. - While it's a major supplier to the spinal cord, it does not directly vascularize the spinal nerve roots. *Vertebral artery* - The **vertebral arteries** form the **basilar artery** and primarily supply the brainstem, cerebellum, and posterior cerebrum. - They give off the anterior and posterior spinal arteries, but they do not directly supply the spinal nerve roots.

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