Nervous System — Set 3

Correct Answer: A. Hypothalamus

• **Hypothalamus** = The hypothalamus controls the pituitary gland (called the 'master gland' because it regulates all other endocrine glands) by secreting specific releasing and inhibiting hormones that travel through a special portal blood system directly into the pituitary. • **Neuro-hormonal cascade** — When the hypothalamus releases Gonadotropin-Releasing Hormone (GnRH), the pituitary responds by releasing LH and FSH, which then act on the gonads — a three-level command chain demonstrating the hypothalamus as the true 'master controller' above the master gland. • The hypothalamus–pituitary axis is the command centre for growth, reproduction, stress response, and thyroid function — disruptions here affect the entire body's hormonal balance. • 💡 Option B (Pons) is a brainstem relay centre for respiratory rhythms; Option C (Cerebellum) handles motor coordination and has no endocrine function; Option D (Thalamus) relays sensory information to the cortex.

Correct Answer: C. Electrical signals

• **Electrical signals (action potentials)** = A nerve impulse is an electrochemical event called an action potential — when a neuron is sufficiently stimulated, sodium ions rush into the axon through voltage-gated channels, reversing the membrane's charge, creating a self-propagating wave of electrical depolarisation that travels down the axon. • **Speed variation** — The speed of nerve impulses ranges from 0.5 m/s in unmyelinated pain fibres to over 120 m/s in myelinated motor fibres — the myelin sheath enables this 240-fold speed increase through saltatory conduction. • The signal itself is entirely electrical within the neuron, but it converts to chemical (neurotransmitter release) at the synapse to cross to the next neuron — a seamless electrical-to-chemical-to-electrical relay. • 💡 Option A (Sound waves) are mechanical pressure waves in air; Option B (Chemical waves) describe synaptic transmission, not conduction along the axon; Option D (Magnetic waves) play no role in nerve conduction.

Correct Answer: A. Thalamus

• **Thalamus** = The thalamus is the brain's central relay station — it receives almost all incoming sensory signals (vision, hearing, touch, taste, pain) and routes them to the appropriate areas of the cerebral cortex for conscious processing. • **Smell bypasses the thalamus** — Olfactory (smell) signals are the only sensory signals that travel directly to the limbic system (emotional brain) without passing through the thalamus first, which is why smells trigger powerful emotional memories — the evolutionary remnant of a more primitive, smell-driven brain. • The thalamus also regulates consciousness and sleep-wake cycles; damage to it can cause a permanent coma because the brain is effectively cut off from all sensory input. • 💡 Option B (Cerebrum) processes sensory information after receiving it from the thalamus; Option C (Midbrain) handles visual and auditory reflexes; Option D (Hypothalamus) manages homeostasis and hormone control.

Correct Answer: C. Cell bodies and dendrites

• **Cell bodies and dendrites** = Gray matter is composed primarily of neuronal cell bodies (somata), dendrites, and unmyelinated axons — it is where actual information processing, computation, and integration of signals occurs in the brain. • **Gray vs. white matter** — Gray matter appears darker because it lacks the white myelin sheath; white matter (containing myelinated axons) appears lighter and acts as the communication highway, while gray matter is the processing centre — like the difference between computer processors and cables. • The cerebral cortex — the thin outer layer of the cerebrum — is entirely gray matter and is responsible for the most sophisticated human cognitive functions, which is why having more cortical folds (gyrification) increases the surface area and cognitive capacity. • 💡 Option A (Fatty tissue) is found in myelin but is not the defining component of gray matter; Option B (Myelinated axons) are the defining component of white matter, not gray matter; Option D (Blood plasma) is the liquid part of blood.

Correct Answer: D. Rest and digestion

• **Rest and digestion** = The parasympathetic nervous system is the 'rest and digest' division — it is dominant when the body is at rest, promoting digestion, slowing heart rate, constricting pupils, and stimulating glandular secretions to conserve and restore energy. • **SLUDD mnemonic** — Parasympathetic activation causes Salivation, Lacrimation (tears), Urination, Digestion, and Defecation — all restorative functions that require the body to be calm and not in danger. • The parasympathetic and sympathetic systems are constantly in dynamic opposition; the balance between them determines heart rate, blood pressure, and digestive rate at any given moment. • 💡 Option A (Sudden fear) strongly activates the sympathetic system; Option B (Intense exercise) requires maximum sympathetic output to increase blood flow to muscles; Option C (Stressful events) trigger the sympathetic fight-or-flight response.

Correct Answer: B. Temporal lobe

• **Temporal lobe** = The temporal lobe is located on the sides of the brain above the ears and contains the primary auditory cortex (Heschl's gyrus) that processes sound, pitch, and volume, as well as Wernicke's area which is essential for understanding spoken and written language. • **Memory and facial recognition** — Beyond hearing, the temporal lobe houses the hippocampus (critical for forming new memories) and the amygdala (emotional processing and fear), making it a hub for both sensory and emotional experience. • Damage to Wernicke's area in the left temporal lobe causes Wernicke's aphasia — the person can speak fluently but produces nonsensical sentences and cannot understand language. • 💡 Option A (Parietal lobe) processes touch, spatial awareness, and proprioception; Option C (Frontal lobe) handles voluntary movement, personality, and executive functions; Option D (Occipital lobe) is dedicated exclusively to visual processing.

Correct Answer: D. 31 pairs

• **31 pairs** = The human spinal cord gives rise to exactly 31 pairs of spinal nerves that emerge between adjacent vertebrae — 8 cervical (neck), 12 thoracic (chest), 5 lumbar (lower back), 5 sacral (pelvis), and 1 coccygeal — each carrying sensory and motor fibres for a specific body region. • **Dermatomes** — Each spinal nerve pair serves a defined skin territory called a dermatome; doctors map these to pinpoint which spinal level is damaged when a patient reports numbness in a specific area, making the 31-pair count clinically vital for diagnosis. • Unlike cranial nerves that emerge directly from the brain, all 31 pairs exit the spinal cord through gaps between vertebrae (intervertebral foramina), explaining why herniated discs cause pain along specific nerve pathways. • 💡 Option A (33 pairs) matches the number of vertebrae, not nerve pairs; Option B (24 pairs) is incorrect; Option C (12 pairs) is the count of cranial nerves, not spinal nerves.

Correct Answer: B. Occipital lobe

• **Occipital lobe** = The occipital lobe is located at the very back of the cerebrum and contains the primary visual cortex (V1) that receives raw visual data from the eyes via the optic nerve and optic radiations, interpreting shape, colour, motion, and depth. • **Vision continues after the eyes** — Complete destruction of the occipital lobe causes total cortical blindness even if the eyes are perfectly healthy, because the raw signals have nowhere to be processed — proving vision is a brain function, not just an eye function. • The visual cortex is organised so that the upper half processes the lower visual field and vice versa; the right occipital lobe processes the left visual field of both eyes, which is why a stroke on the right side causes blindness on the left side. • 💡 Option A (Frontal lobe) manages voluntary movement, decision-making, and speech production; Option C (Temporal lobe) processes auditory signals and memory; Option D (Parietal lobe) integrates touch, spatial orientation, and body awareness.

Correct Answer: D. Voluntary muscle control

• **Voluntary muscle control** = The somatic nervous system is the division of the peripheral nervous system that carries motor commands from the CNS to skeletal (striated) muscles, enabling all conscious, purposeful movements like walking, writing, and speaking. • **Bidirectional communication** — The somatic nervous system also carries sensory information from the skin, joints, and muscles back to the CNS, giving the brain conscious awareness of touch, temperature, pain, and body position (proprioception). • Because it controls voluntary movements, the somatic system requires conscious brain involvement — cutting the somatic nerve pathway (as in spinal cord injury) causes paralysis of the specific body parts served by those nerves. • 💡 Option A (Digestion) is controlled by the autonomic nervous system's parasympathetic branch; Option B (Involuntary movements) are governed by the autonomic system; Option C (Heart rate control) is a function of the autonomic nervous system.

Correct Answer: C. A neuron fires completely or not at all

• **A neuron fires completely or not at all** = The all-or-none law states that once a neuron receives a stimulus that reaches its threshold voltage, it fires a full-strength action potential regardless of how much stronger the stimulus is — the response is binary, like a light switch with no dimmer. • **Intensity is coded by frequency, not amplitude** — Since every action potential is the same size, the nervous system signals stimulus strength by increasing the rate (frequency) of firing: a gentle touch fires 10 impulses/second while a sharp pain fires 100 impulses/second from the same nerve. • This law was first established by American physiologist Henry Bowditch (1871) studying cardiac muscle, and later confirmed for all neurons — it is a fundamental principle explaining how the nervous system encodes information. • 💡 Option A (No neurons fire without blood) is not a recognised physiological law; Option B (All neurons fire at once) would cause uncontrolled seizures; Option D (Signals travel in all directions) is incorrect — signals travel in one direction only at synapses.