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AN7.1-8 | Introduction to the nervous system — Practice Quiz
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The sympathetic division of the autonomic nervous system originates from which spinal cord levels?
Correct! Sympathetic = Thoracolumbar outflow (T1–L2). Parasympathetic = Craniosacral (CN III, VII, IX, X + S2–S4).
Mnemonic: Sympathetic = "Thoracolumbar" (T1–L2). Parasympathetic = "Craniosacral". Both use ACh at pre-ganglionic synapse; sympathetic post-ganglionic uses noradrenaline (except sweat glands = ACh).
Incorrect. Sympathetic = T1–L2 (thoracolumbar). Parasympathetic = craniosacral (CN III/VII/IX/X + S2–S4).
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Which glial cell is responsible for myelination within the central nervous system?
Correct! Oligodendrocytes myelinate CNS axons (one cell can myelinate multiple axons). Schwann cells myelinate PNS axons (one cell per axon segment).
Memory aid: "Oligo = CNS; Schwann = PNS". In MS, oligodendrocyte myelin is destroyed → plaques in CNS. In GBS, Schwann cell myelin is attacked → PNS demyelination.
Incorrect. CNS myelin = oligodendrocytes. PNS myelin = Schwann cells. MS destroys oligodendrocyte-derived myelin.
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The primary sensory neurons of the dorsal root ganglion are morphologically classified as:
Correct! DRG neurons are pseudounipolar — they develop from bipolar neurons whose two processes fuse into one T-shaped process. Both the peripheral and central branches are structurally axons.
DRG neuron: One process leaves soma → splits into peripheral branch (to skin/muscle) and central branch (enters dorsal horn). No synapse in DRG — the cell body is bypassed. This is why sensory signals can bypass the cell body even when it is damaged.
Incorrect. DRG = pseudounipolar (T-shaped single process). True bipolar = retina, cochlea, vestibule. Multipolar = motor neurons and most interneurons.
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The dorsal (posterior) root of a spinal nerve contains:
Correct! The dorsal root carries sensory information (central processes of DRG pseudounipolar neurons) into the spinal cord. The DRG houses the cell bodies.
Bell-Magendie Law: Dorsal root = sensory (afferent); Ventral root = motor (efferent). Clinical: Dorsal root section (posterior rhizotomy) relieves pain without motor loss. Ventral root lesion = LMN motor paralysis.
Incorrect. Dorsal root = sensory (DRG central processes). Ventral root = motor (anterior horn axons + T1–L2 preganglionic sympathetics).
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A "motor unit" in the context of muscle innervation refers to:
Correct! A motor unit = 1 alpha LMN + all muscle fibres it innervates. Small motor units (few fibres) allow fine control (e.g., extraocular muscles, hand intrinsics). Large motor units allow power (e.g., gastrocnemius).
Motor unit ratio examples: Extraocular muscles = 1:3–1:10 (fine control). Hand intrinsics = 1:100. Gastrocnemius = 1:2000. Smaller ratio = finer movement. This is why hand function is lost precisely in nerve injuries.
Incorrect. Motor unit = 1 LMN + ALL its muscle fibres. The ratio determines precision vs power.
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A 60-year-old man presents with sudden onset right hemiplegia and brisk deep tendon reflexes on the right side. A Babinski sign is positive on the right. This presentation is most consistent with:
Correct! Brisk DTRs + positive Babinski = UMN lesion. Right-sided hemiplegia = left hemisphere/left corticospinal tract lesion (fibres cross in medulla). Left-sided UMN lesion → right-sided signs.
UMN signs: spasticity, hyperreflexia, Babinski+, clasp-knife rigidity, extensor plantar. LMN signs: flaccidity, areflexia, Babinski−, fasciculations, rapid muscle wasting. Corticospinal tract decussates in the medulla (pyramidal decussation) — lesion above decussation → contralateral signs.
Incorrect. Brisk reflexes + Babinski+ = UMN. Contralateral: the corticospinal tract crosses in the medulla, so a left-sided lesion causes right-sided weakness.
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Which type of synapse is most commonly responsible for presynaptic inhibition in the spinal cord?
Correct! Axoaxonic synapses (axon → axon terminal of another neuron) mediate presynaptic inhibition — they reduce neurotransmitter release from the terminal before it can act on the postsynaptic cell.
Presynaptic inhibition (axoaxonic) is clinically important: GABA-B receptor-mediated presynaptic inhibition is the mechanism of baclofen (used for spasticity in India). Benzodiazepines and barbiturates enhance GABA-A postsynaptic inhibition.
Incorrect. Presynaptic inhibition = axoaxonic synapse. Axodendritic = most excitatory. Axosomatic = common inhibitory. Dendrodendritic = retina.
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Which of the following correctly distinguishes a sympathetic ganglion from a dorsal root ganglion?
Correct! Sympathetic ganglia contain synapses between preganglionic and postganglionic neurons. DRGs house sensory cell bodies only — the impulse bypasses the cell body without any synapse inside the ganglion.
DRG = no synapse (impulse passes through the T-shaped process bypassing the soma). Sympathetic ganglion = relay station with synaptic transmission. Both have satellite cells. This distinction is tested in pharmacology: ganglion blockers (hexamethonium) act on sympathetic ganglionic ACh receptors, not DRGs.
Incorrect. Key distinction: synapse present in sympathetic ganglia (pre→post relay), absent in DRG. Sympathetic = multipolar motor; DRG = pseudounipolar sensory.
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After a peripheral nerve injury, the affected motor neuron cell body shows dissolution of Nissl bodies (chromatolysis). Nissl bodies are composed of:
Correct! Nissl bodies = rough ER + polyribosomes, the site of protein synthesis. Chromatolysis (Nissl body dissolution) occurs in the cell body after axon injury, indicating the cell is switching from normal function to axon repair/regeneration mode.
Chromatolysis is the histological sign of neuronal reaction to injury: cell body swells, nucleus moves peripherally, Nissl bodies dissolve. Absent from: axon hillock and axon (these never have Nissl bodies even normally). Chromatolysis ≠ cell death — the neuron is trying to regenerate.
Incorrect. Nissl bodies = rough ER + polyribosomes. Chromatolysis after axon injury means the cell redirects metabolic activity toward axon repair.
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A patient with hypertension is prescribed prazosin (alpha-1 blocker). At which synapse in the sympathetic pathway does this drug primarily act?
Correct! Prazosin blocks alpha-1 adrenoceptors at the postganglionic sympathetic neuroeffector junction on vascular smooth muscle → vasodilation → BP reduction. Noradrenaline is the postganglionic sympathetic neurotransmitter (acting on alpha-1 receptors of blood vessels).
ANS drug targets: (1) Ganglionic blockers = nicotinic ACh receptors in sympathetic/parasympathetic ganglia. (2) Alpha/beta blockers = noradrenaline receptors at postganglionic sympathetic neuroeffector junctions. (3) Muscarinic blockers (atropine) = ACh at parasympathetic neuroeffector junctions. Understanding the ANS anatomy maps directly to pharmacology.
Incorrect. Prazosin = alpha-1 blocker = postganglionic sympathetic → smooth muscle junction. Not ganglionic (that uses nicotinic ACh). Not NMJ (that uses nicotinic ACh).
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