Nervous System — Set 5

Correct Answer: B. Sciatic nerve

• **Sciatic nerve** = the longest and thickest nerve in the human body, originating from the lumbosacral plexus (spinal roots L4–S3) in the lower back and running down through the buttock and the back of each leg to the foot. • **Width and length** — as thick as a thumb at its widest point and extending nearly the full length of the lower limb; it branches into the tibial and common peroneal nerves just above the knee. • Compression or irritation of the sciatic nerve causes sciatica — a sharp, shooting pain that radiates from the lower back down the leg. • 💡 Option A (Optic nerve) is short, running only ~5 cm from the eye to the optic chiasm; Option C (Vagus nerve) is the longest cranial nerve but does not extend into the limbs; Option D (Ulnar nerve) runs along the arm to the little finger and is far shorter than the sciatic nerve.

Correct Answer: C. Salivating at the smell of food

• **Salivating at the smell of food** = a conditioned (acquired) reflex because the association between the smell and food must be learned through repeated experience — an infant does not automatically salivate at the smell of cooked food until it has been eaten before. • **Pavlov's experiment** — Ivan Pavlov demonstrated this principle: dogs naturally salivate at food (unconditioned reflex) but, after repeatedly pairing a bell with food, began salivating at the bell alone — the conditioned stimulus — showing that reflexes can be learned. • Conditioned reflexes are stored in the cerebral cortex, making them dependent on brain function, unlike innate spinal reflexes. • 💡 Option A (Pulling hand from heat) is an innate spinal reflex requiring no learning; Option B (Sneezing due to dust) is an inborn protective reflex of the respiratory tract; Option D (Blinking when light flashes) is an unconditioned protective reflex present from birth.

Correct Answer: D. Fovea

• **Fovea (fovea centralis)** = a tiny central pit in the macula of the retina densely packed with cone photoreceptors (~150,000 per mm²) and containing no rods, providing the sharpest, most detailed colour vision in the entire visual field. • **No blood vessels, direct cone-to-ganglion wiring** — the fovea is free of blood vessels and each cone connects to its own ganglion cell, maximising spatial resolution; we automatically move our eyes so objects of interest fall on the fovea. • Reading, recognising faces, and threading a needle all rely on foveal vision; the surrounding retina provides peripheral (lower-acuity) vision. • 💡 Option A (Iris) is the coloured muscular ring that controls pupil size and has no visual receptors; Option B (Blind spot) is where the optic nerve exits the retina — it has no photoreceptors at all; Option C (Sclera) is the tough white outer coat of the eye, not involved in image formation.

Correct Answer: A. Cerebellum

• **Cerebellum** = the name literally means 'little brain' in Latin; it is located at the back and bottom of the brain and closely resembles a smaller version of the cerebrum, with its own folded cortex called the cerebellar cortex. • **Motor coordination and balance** — fine-tunes voluntary movements, maintains posture and equilibrium, and coordinates timing of muscle activity; damage results in ataxia — unsteady, uncoordinated movement. • Despite occupying only about 10% of brain volume, the cerebellum contains more than half of all the neurons in the entire brain (~69 billion of the ~100 billion total). • 💡 Option B (Cerebrum) is the 'big brain' — the large, dominant hemisphere responsible for higher thinking; Option C (Pons) is the brainstem bridge that relays signals between the cerebrum and cerebellum; Option D (Midbrain) is the upper brainstem segment involved in visual and auditory reflexes.

Correct Answer: B. -70 mV

• **−70 mV** = the resting membrane potential of a typical neuron, meaning the inside of the cell is about 70 millivolts more negative than the outside, maintained by the differential distribution of Na⁺, K⁺, Cl⁻, and large negatively charged proteins. • **Na⁺-K⁺ pump maintenance** — continuously pumps 3 Na⁺ out and 2 K⁺ in, keeping the inside negative; this polarised state stores the electrical energy needed to fire an action potential. • The resting potential is the 'charged battery' state of the neuron; stimulation to the threshold of −55 mV triggers depolarisation and an action potential. • 💡 Option A (+70 mV) is the approximate peak of an action potential (inside becomes positive), not the resting state; Option C (−55 mV) is the threshold potential at which an action potential is triggered, not the rest; Option D (0 mV) would represent no potential difference across the membrane, which never occurs in a living neuron at rest.

Correct Answer: B. Wernicke's area

• **Wernicke's area** = located in the posterior superior temporal gyrus of the dominant (usually left) hemisphere; it is responsible for comprehension of both spoken and written language. • **Wernicke's aphasia** — damage produces fluent but meaningless speech ('word salad') and severely impaired comprehension; the patient speaks easily but what they say makes no sense and they cannot understand others. • Discovered by German neurologist Carl Wernicke in 1874, one year after Broca's discovery, completing the two-node model of language processing. • 💡 Option A (Corpus callosum) is the fibre bundle connecting the two hemispheres; its damage causes disconnection syndromes, not language comprehension loss; Option C (Broca's area) controls speech production — its damage causes non-fluent aphasia where understanding is intact but speaking is laboured; Option D (Cerebellum) coordinates movement and balance and plays no direct role in language comprehension.

Correct Answer: B. Dopamine

• **Dopamine** = a catecholamine neurotransmitter released mainly by neurons in the ventral tegmental area (VTA) and substantia nigra; it transmits signals of reward, motivation, and pleasure through the mesolimbic pathway (the 'reward pathway'). • **Addiction and motor control** — nearly all addictive drugs (cocaine, amphetamines, alcohol) hijack the dopamine reward system by flooding the nucleus accumbens; dopamine deficiency in the substantia nigra causes Parkinson's disease. • Dopamine also regulates attention, working memory, and motor learning, making it essential for everyday cognitive function. • 💡 Option A (GABA) is the brain's primary inhibitory neurotransmitter — it calms neural activity, not reward; Option C (Serotonin) regulates mood, appetite, and sleep — its deficiency is linked to depression, not primarily reward; Option D (Melatonin) is a hormone produced by the pineal gland that regulates sleep, not the reward system.

Correct Answer: D. Temporal

• **Temporal lobe** = the primary auditory cortex (Heschl's gyri, Brodmann areas 41 and 42) is located on the superior surface of the temporal lobe within the lateral sulcus; it is the first cortical region to receive and process sound information from the ears. • **Tonotopic organisation** — the auditory cortex is arranged so that different frequencies (pitches) are processed in distinct areas, from high frequency at one end to low frequency at the other, like a piano keyboard laid across the cortex. • The surrounding secondary auditory cortex (including Wernicke's area) then interprets the meaning of the sounds, turning raw sound into language and music. • 💡 Option A (Frontal lobe) contains the motor cortex and Broca's area for speech production, not sound reception; Option B (Occipital lobe) is dedicated to processing visual information from the eyes; Option C (Parietal lobe) integrates touch, pain, temperature, and spatial orientation, not auditory signals.

Correct Answer: B. Enteric nervous system

• **Enteric nervous system (ENS)** = an extensive network of approximately 500 million neurons embedded in the walls of the gastrointestinal tract, from the oesophagus to the rectum; it independently regulates digestion, peristalsis, enzyme secretion, and local blood flow. • **Semi-autonomous 'second brain'** — the ENS can function even when all connections to the brain and spinal cord are severed; it communicates bidirectionally with the brain via the vagus nerve (the gut-brain axis), influencing mood and even producing ~95% of the body's serotonin. • Research links gut microbiome disruption to ENS dysfunction, connecting digestive health to anxiety and depression. • 💡 Option A (Central system) consists of the brain and spinal cord and oversees everything but is not nicknamed the gut brain; Option C (Somatic system) controls voluntary skeletal muscles and has nothing to do with gut function; Option D (Sympathetic system) is a division of the autonomic system that inhibits digestion during stress ('fight or flight') but is not the gut brain.

Correct Answer: C. Cranium

• **Cranium** = the upper part of the skull that forms a rigid, dome-shaped bony case enclosing and protecting the brain; it is composed of eight flat bones (frontal, two parietal, two temporal, occipital, sphenoid, and ethmoid) fused together at immovable joints called sutures. • **Layered protection** — inside the cranium, the brain is further cushioned by the three meninges (dura mater, arachnoid mater, pia mater) and bathed in cerebrospinal fluid (CSF) that absorbs mechanical shocks. • The cranium is the hardest and most protective bone structure in the body; neurosurgeons must drill through it (craniotomy) to access the brain. • 💡 Option A (Sternum) is the flat breastbone in the centre of the chest that protects the heart; Option B (Scapula) is the shoulder blade, a flat triangular bone of the upper back; Option D (Vertebra) are the individual bones of the spinal column that protect the spinal cord, not the brain.