Nervous System — Set 4

Correct Answer: D. Midbrain

• **Midbrain** = the short brainstem segment sandwiched directly between the thalamus (above) and the pons (below), forming the uppermost portion of the brainstem. • **Key functions** — controls eye and eyelid movements, relays auditory and visual signals, and houses the substantia nigra that produces dopamine for smooth movement. • It also contains the cerebral aqueduct, the narrow channel that connects the third and fourth ventricles for CSF flow. • 💡 Option A (Medulla) sits below the pons, not between thalamus and pons; Option B (Cerebellum) is a separate posterior structure behind the brainstem; Option C (Cerebrum) is the massive upper hemisphere, not part of the brainstem at all.

Correct Answer: D. Sodium and Potassium

• **Sodium (Na⁺) and Potassium (K⁺)** = the two ions that together establish the resting membrane potential of approximately −70 mV in a typical neuron. • **Na⁺-K⁺ ATPase pump** — actively pumps 3 Na⁺ out and 2 K⁺ in per cycle, keeping Na⁺ high outside and K⁺ high inside, creating the electrochemical gradient that drives the resting potential. • Leak channels allow K⁺ to slowly diffuse out, making the inside more negative, while the cell membrane is nearly impermeable to Na⁺ at rest. • 💡 Option A (Sodium and Calcium) is wrong because Ca²⁺ is crucial for neurotransmitter release, not resting potential; Option B (Iron and Magnesium) play no role in membrane potential; Option C (Potassium and Chlorine) — Cl⁻ plays a minor role but is not the primary pair establishing resting potential.

Correct Answer: D. Olfactory receptors

• **Olfactory receptors** = chemoreceptors located in the olfactory epithelium of the nasal cavity that detect airborne chemical molecules (odorants) and convert them into nerve signals. • **Direct pathway to brain** — uniquely, olfactory signals bypass the thalamus and travel directly to the olfactory cortex and limbic system, which is why smells powerfully trigger memories and emotions. • Humans possess about 400 types of functional olfactory receptor proteins, each detecting a range of odorant molecules. • 💡 Option A (Photoreceptors) are rods and cones in the retina for light; Option B (Thermoreceptors) detect temperature changes in the skin; Option C (Gustatory receptors) are taste buds on the tongue for the sense of taste, not smell.

Correct Answer: B. Frontal lobe

• **Frontal lobe** = the largest lobe of the cerebrum, located at the front of the skull, responsible for executive functions including problem-solving, planning, decision-making, and personality expression. • **Prefrontal cortex** — the anterior portion of the frontal lobe is the seat of personality, social behavior, and judgment; damage to it (as in the famous Phineas Gage case) dramatically alters a person's character. • The frontal lobe also contains the primary motor cortex (controls voluntary movement) and Broca's area (speech production). • 💡 Option A (Occipital lobe) processes visual information only; Option C (Temporal lobe) handles hearing, language comprehension, and memory; Option D (Parietal lobe) integrates sensory information like touch, pain, and spatial awareness.

Correct Answer: B. Myelinated axons

• **Myelinated axons** = axons wrapped in a fatty white myelin sheath produced by oligodendrocytes in the CNS; this lipid-rich coating gives white matter its characteristic pale colour. • **Myelin as insulator** — acts like electrical insulation, enabling saltatory conduction where nerve impulses jump between Nodes of Ranvier, increasing signal speed up to 100 m/s compared to ~1 m/s in unmyelinated fibres. • White matter forms the brain's internal communication highway, connecting different cortical regions and the cortex to the spinal cord. • 💡 Option A (Dendrites) are short branching extensions that receive signals and are found in grey matter; Option C (Connective tissue) supports organs but does not form white matter; Option D (Cell bodies) of neurons make up grey matter, not white matter.

Correct Answer: A. Broca's area

• **Broca's area** = located in the left inferior frontal gyrus (frontal lobe) of the dominant hemisphere; it coordinates the muscle movements needed to produce spoken language. • **Motor speech control** — damage to Broca's area causes Broca's aphasia: the patient understands language perfectly but can only speak in slow, laboured, telegraphic words. • Discovered by French surgeon Paul Broca in 1861 after observing a patient who could only say the word 'tan'. • 💡 Option B (Wernicke's area) is in the temporal lobe and handles language comprehension, not production; Option C (Thalamus) relays sensory signals but does not produce speech; Option D (Hypothalamus) regulates homeostasis, hormones, and autonomic functions, not language.

Correct Answer: C. Spinal reflex

• **Spinal reflex** = an involuntary, automatic response processed entirely within the spinal cord without any involvement of the brain, allowing a faster reaction than if signals had to travel to the cerebrum. • **Monosynaptic arc** — the knee-jerk (patellar reflex) is the simplest reflex arc: the stretch receptor in the quadriceps sends a signal directly to the motor neuron in the spinal cord, which fires back to contract the muscle — only one synapse involved. • Clinically, doctors test the knee-jerk to assess the integrity of the L3–L4 spinal segments and peripheral nerve function. • 💡 Option A (Cerebral reflex) would require brain processing, making it too slow for a true reflex; Option B (Conditional reflex) is a learned response acquired through experience, like Pavlov's salivation experiment; Option D (Visual reflex) involves the visual system, e.g., the pupillary light reflex.

Correct Answer: D. Oligodendrocytes

• **Oligodendrocytes** = glial cells of the CNS (brain and spinal cord) that wrap their flat extensions around multiple axons to form the myelin sheath, insulating and speeding up nerve impulse conduction. • **One cell, many axons** — a single oligodendrocyte can myelinate up to 50 different axon segments simultaneously, making it highly efficient; damage to these cells causes diseases like multiple sclerosis. • Oligodendrocytes are also responsible for the structural support and metabolic maintenance of the axons they cover. • 💡 Option A (Astrocytes) regulate the blood-brain barrier, provide nutrients, and maintain ion balance but do not produce myelin; Option B (Microglia) are the brain's immune cells that clear debris and pathogens; Option C (Schwann cells) produce myelin in the Peripheral Nervous System, not the CNS.

Correct Answer: A. Autonomic system

• **Autonomic nervous system** = controls involuntary body functions including pupil size; in low light, the sympathetic branch dilates the pupil (mydriasis) to allow more light onto the retina. • **Sympathetic vs. parasympathetic** — sympathetic fibres (via the long ciliary nerve) dilate the pupil; parasympathetic fibres (via cranial nerve III) constrict it in bright light — together they form the pupillary light reflex. • This automatic adjustment happens without conscious effort and is tested clinically using a penlight to check brainstem integrity. • 💡 Option B (Endocrine system) uses hormones through the bloodstream and is far too slow for rapid pupil adjustments; Option C (Somatic system) controls voluntary skeletal muscle movements only; Option D (Excretory system) handles waste removal from the body and has no role in pupil control.

Correct Answer: B. Pineal gland

• **Pineal gland** = a small pea-sized endocrine gland embedded deep in the brain that secretes melatonin in response to darkness, signalling the body to prepare for sleep and regulating the 24-hour circadian rhythm. • **Light-dark signal pathway** — light detected by the retina suppresses melatonin release via the suprachiasmatic nucleus (SCN); as darkness falls, melatonin levels rise, promoting drowsiness and lowering body temperature. • Called the 'third eye' by philosopher Descartes; in some reptiles it is literally light-sensitive and positioned beneath transparent skull skin. • 💡 Option A (Medulla) controls vital autonomic functions like heart rate and breathing, not the sleep cycle; Option C (Thalamus) acts as a sensory relay station but does not produce sleep-regulating hormones; Option D (Pons) plays a role in REM sleep stages but is not the primary regulator of the overall sleep-wake cycle.