Unit 15: Human Nervous System — Long Questions

A neuron (nerve cell) is the structural and functional unit of the nervous system. Though neuron shapes vary, all neurons have three basic parts: (1) Cell body — an enlarged region containing the nucleus and most organelles; its cytoplasm shows Nissl's granules (groups of ribosomes and rough endoplasmic reticulum). (2) Dendrites — membrane-bounded extensions that receive information from other neurons/cells, convert it into nerve impulses, and carry it toward the cell body; motor and inter-neurons, which receive input from many sources, have highly branched dendrites. (3) Axon — a long, membrane-bounded extension that transmits impulses away from the cell body; its end, the axon terminal, contacts a muscle, gland, or another neuron. Many axons are wrapped by neuroglia (Schwann cells in the PNS, oligodendrocytes in the CNS) that form an insulating myelin sheath, speeding up impulse conduction; the sheath is interrupted every 1–2 mm by gaps called nodes of Ranvier. Neurons without such a sheath are non-myelinated.

Sensory neurons carry impulses from receptors to the CNS; inter-neurons (within the CNS) process/relay impulses; motor neurons carry impulses from the CNS to effectors. This is illustrated by the knee-jerk reflex: tapping the patellar tendon stimulates a receptor in the quadriceps muscle, which generates a nerve impulse in a sensory neuron. This impulse travels to the spinal cord, where the sensory neuron directly synapses with a motor neuron (without an inter-neuron). The motor neuron carries the impulse to the quadriceps (effector), which contracts, extending the lower leg. At the same time, the sensory neuron also activates an inter-neuron in the spinal cord; this inter-neuron releases inhibitory chemicals that suppress the motor neurons supplying the hamstrings, causing the hamstrings to relax during the leg's extension.

A resting neuron is polarized, with RMP about –70 mV (inside negative). Generation of an impulse (action potential) involves: Depolarization — Na⁺ gates open, Na⁺ rushes in, and the potential rises from –70 mV toward 0 and up to about +50 mV; this reversal lasts about one millisecond. Repolarization — Na⁺ gates close and K⁺ gates open; K⁺ diffuses out, making the inside negative again. Hyperpolarization — because K⁺ gates close with a slight delay, the potential briefly becomes more negative than the RMP; during this refractory period a new action potential cannot be generated. Return to RMP — the Na⁺–K⁺ pump restores the original ionic distribution, returning the membrane to –70 mV, ready for the next impulse. In transmission, the action potential does not literally travel; rather, each region's depolarization stimulates the next region to produce its own action potential, while the previous region repolarizes — so the impulse is propagated, regenerating in strength, along the length of the axon.

Polarization the state of a resting neuron, with the inside of the membrane negative and the outside positive; this is the resting membrane potential, about –70 mV. Depolarization: when Na⁺ gates open and Na⁺ rushes into the cell, reducing the inside's negative charge; the potential reverses from –70 mV toward 0 mV and up to about +50 mV, producing the action potential over a short region of the neuron for about a millisecond. Hyperpolarization: after Na⁺ gates close and K⁺ gates open (repolarization), the continued, slightly delayed efflux of K⁺ carries the potential to a value briefly more negative than the RMP; during this period (the refractory period) the neuron cannot generate another action potential.

In myelinated axons, the myelin sheath insulates the membrane so that action potentials can only be produced at the nodes of Ranvier (the non-myelinated gaps between sheath segments, occurring every 1–2 mm). The impulse effectively jumps from one node of Ranvier to the next, skipping over the myelinated regions; this is called saltatory conduction. It is up to 50 times faster than conduction in the fastest non-myelinated axons. Non-myelinated human neurons conduct impulses at 1–3 m/s, while myelinated neurons conduct at speeds of up to 120 m/s. The strength (amplitude) of the action potential remains constant along the axon, but its velocity depends on myelination and axon diameter.

A synapse is the junction between the axon terminal of one neuron and the dendrites of another neuron, or an effector cell (muscle or gland). The two cells are not in direct contact; they are separated by a fluid-filled synaptic cleft. The neuron whose axon carries the impulse toward the synapse is the presynaptic neuron; its axon terminal bears swollen synaptic knobs containing numerous synaptic vesicles packed with neurotransmitter chemicals. The cell on the far side of the cleft, which has receptor proteins on its membrane, is the postsynaptic cell. (The synapse between a motor neuron and a muscle fibre is also called a neuromuscular junction.)

Synaptic transmission occurs as follows: (1) When an action potential reaches the synaptic knobs, calcium gates open in the presynaptic membrane. (2) Ca²⁺ ions from the synaptic cleft diffuse rapidly into the presynaptic axon. (3) This Ca²⁺ influx causes synaptic vesicles to fuse with the presynaptic membrane, releasing neurotransmitter by exocytosis. (4) The neurotransmitter diffuses across the cleft and binds to receptor proteins on the postsynaptic membrane. (5) This binding opens Na⁺ gates on the postsynaptic membrane, generating a new action potential there. (6) The neurotransmitter is then removed from the cleft — at many synapses it is reabsorbed by the presynaptic neuron, while at others it is broken down by enzymes — preventing continuous stimulation.

Excitatory neurotransmitters generate a nerve impulse in the postsynaptic neuron by increasing membrane permeability to Na⁺. Examples: acetylcholine (motor neuron–skeletal muscle junctions and many other synapses), glutamate (in the CNS), and the biogenic amines — norepinephrine (regulates sleep, focus, alertness), epinephrine (role in sleep and alertness), dopamine (controls body movement, motivation-related pleasure, emotional stimulation), and serotonin (regulates appetite, sleep, memory/learning, temperature, mood, muscle contraction). Inhibitory neurotransmitters decrease the chance of impulse transmission by opening chloride channels instead of sodium channels, causing hyperpolarization. Examples: GABA (chief inhibitory neurotransmitter of the CNS, reduces anxiety/stress and promotes relaxation and sleep), glycine (inhibitory especially in the spinal cord, brainstem, and retina), and endorphins (reduce release of excitatory neurotransmitters, blocking pain signals).

Cerebrum the largest part, divided into left and right hemispheres joined by the corpus callosum; each hemisphere has four lobes — frontal (executive functions: self-control, planning, reasoning, abstract thought), parietal (processes sensory information, body/environment position awareness, attention, planning and executing motor movement), temporal (visual memory retention, language comprehension, emotions), and occipital (vision). The cerebral cortex (grey matter) contains motor areas (send impulses to skeletal muscles), sensory areas (receive sensory information, e.g., auditory and visual cortices), and association areas (higher mental activities). Brainstem: midbrain (relays visual/auditory information, contains the reticular formation), pons (transmits impulses between cerebral hemispheres and cerebellum), and medulla oblongata (relay and control centre for heart rate, respiration, and other homeostatic activities). Diencephalon: thalamus (relay centre between sensory information and the cerebrum) and hypothalamus (regulates body temperature, hunger/satiety, thirst, emotional states, and controls the pituitary gland). Limbic system (hypothalamus, amygdala — controls emotions like fear/anger/love, and hippocampus — consolidates short-term into long-term memory). Cerebellum: coordinates muscle actions, processes information about body position, and controls posture and balance.

The brain (enclosed in the cranium) and spinal cord (enclosed in the vertebral column) are both covered by three meninges: dura mater (outermost, thick, contains large blood vessels), arachnoid (thin, barrier between blood and CSF), and pia mater (delicate, adheres to the surface, carries nourishing blood capillaries). The brain's major parts are the cerebrum, brainstem (midbrain, pons, medulla oblongata), diencephalon (thalamus and hypothalamus) with the limbic system, and the cerebellum; it contains four fluid-filled ventricles that produce and circulate cerebrospinal fluid. The spinal cord is a cable of neurons running from the medulla oblongata down through the vertebral column. Like the brain, it is made of grey matter and white matter, but their arrangement is reversed: in the brain, grey matter (cerebral cortex) is on the outside and white matter is underneath, whereas in the spinal cord the inner H-shaped/butterfly-shaped zone is grey matter (containing cell bodies of inter- and motor neurons and non-myelinated axons) and the outer zone is white matter (myelinated axons). The spinal cord's central canal, continuous with the brain's fourth ventricle, carries CSF. The spinal cord acts as an information highway carrying messages between the body and the brain.

The autonomic nervous system consists of motor neurons (of spinal and cranial nerves) that control smooth muscles and glands; it is involuntary, regulating internal body conditions such as respiration, heartbeat, and digestion. It has two subdivisions: Sympathetic nervous system — its motor neurons originate from the thoracic and lumbar regions of the spinal cord; when activated, it prepares the body for "fight or flight" under physical/emotional stress: heart beats faster and stronger, blood glucose concentration increases, blood flow is diverted to muscles and heart, bronchioles dilate, pupils dilate, salivary/gastric secretions decrease, digestive motility decreases, and bladder relaxes. Parasympathetic nervous system — its motor neurons originate from the brain and sacral regions of the spinal cord (many travel in the vagus nerve); it antagonizes the sympathetic response once the stress has passed, restoring normal conditions: heart rate slows, blood flow to heart/skeletal muscles decreases, digestive organs increase secretion and motility, bladder contracts, and pupils constrict.

The receptors for smell (olfaction) are chemoreceptors located in the upper part of the nasal cavities. They are actually the endings of sensory neurons, present in a thin layer of tissue called the nasal epithelium, packed between supporting cells (which protect them) and basal cells (stem cells that replace old olfactory neurons). The dendrites of these sensory neurons bear tiny hair-like cilia that project into the moist mucus lining the nasal cavity. Airborne chemicals that enter the nose dissolve in this mucus and bind to specific receptor sites on the cilia. This binding generates a nerve impulse in the sensory neuron, which is transmitted via the olfactory nerves (cranial nerve I) to the olfactory bulb, where the perception of smell is produced.

Taste is sensed by about 10,000 chemoreceptors called taste buds, most of which are embedded in bumps (papillae) on the tongue, with some in the throat and on the roof of the oral cavity. A taste bud is bulb-shaped, made of receptor cells bearing hair-like structures and basal cells that produce new receptor cells; it opens into the mouth through a taste pore. There are four kinds of taste buds, responding to salty, sweet, sour, and bitter stimuli, concentrated in particular regions of the tongue (sweet at the tip, sour at the sides, bitter at the back, salty over most of the surface), with overlap between regions. When chemicals in food or drink stimulate the chemoreceptor cells, nerve impulses are generated in associated sensory neurons and travel via cranial nerves VII, IX and X to the brainstem, and then to the taste-sensing areas of the cerebral cortex, where the information is combined with smell and texture to create the overall perception of taste.

Pain receptors (nociceptors) are free nerve endings found throughout the body, especially near surfaces; they respond to extremes of temperature, intense pressure, or specific chemicals (including those released by injured cells), and detect pain — a stimulus that causes or threatens tissue damage. Touch receptors in the skin are mechanoreceptors responding to different physical stimuli: Meissner's corpuscles (capsule-enclosed endings in the upper skin layer, especially lips, fingertips, palms, and soles; detect light touch and vibration); hair follicle receptors (neuron endings around hair follicle bases; detect hair movement from mechanical/thermal stimuli); Merkel cells (oval receptors in the deepest epidermis; detect light touch in highly sensitive areas like fingertips); Pacinian corpuscles (capsule-enclosed endings deeper in the skin; detect deep pressure and vibration); and Ruffini endings (capsule-enclosed dermal endings; detect sustained pressure and stretching). Impulses from these receptors travel via sensory neurons to the spinal cord and then to the brain, where they are processed into sensations of touch.

"Use" refers to the appropriate, medically prescribed use of a drug; narcotics (opioid drugs like morphine, codeine, and oxycodone) are legitimately used for pain relief. "Abuse" refers to inappropriate, non-medical use of such psychoactive drugs. An important example is heroin — a highly addictive opioid derived from the opium poppy, taken by injection, smoking, or snorting. It produces a false sense of euphoria and relaxation but leads quickly to dependence and addiction, along with negative effects such as respiratory depression, infections, and mental health disorders. Narcotics such as morphine and heroin have an analgesic (pain-relieving) effect because they are chemically similar to the body's own endorphins and so readily bind to endorphin receptors, but this same property gives them high potential for addiction and abuse.

Drug addiction is a disorder of uncontrollable drug desire and use despite negative effects. Drug tolerance is the body becoming adaptive and less sensitive to a drug's effects with continued use, so higher doses are needed to achieve the same response; tolerance increases addiction. Caffeine (found in coffee, tea, energy drinks) blocks the sleep-promoting neurotransmitter adenosine, increasing alertness and energy. Regular use causes tolerance, so higher doses are needed for the same alertness; stopping abruptly causes withdrawal symptoms such as headaches and fatigue. Nicotine (in tobacco products) binds to nicotine receptors in the brain, triggering release of dopamine and other neurotransmitters that produce pleasurable feelings. Regular use causes tolerance, requiring higher doses for the same pleasure; abrupt cessation causes withdrawal symptoms such as irritability, anxiety, and cravings. Long-term nicotine use is also linked to cancer, heart disease, and respiratory problems.

Nervous disorders are classified into five categories: (1) Vascular disorders — conditions affecting the blood vessels of the brain/spinal cord, e.g., stroke and cerebral venous thrombosis. (2) Infectious disorders — infections of the CNS caused by viruses, bacteria, fungi, protozoa, or flatworms, e.g., meningitis. (3) Structural disorders — resulting from abnormalities or damage to the physical structure of the nervous system, e.g., brain tumour and spinal cord injuries. (4) Functional disorders — affect normal nervous system functioning without any apparent structural or organic cause, e.g., headache/migraine. (5) Degenerative disorders — progressive deterioration of nerve cells in the brain, spinal cord, or peripheral nerves, e.g., Parkinson's disease, Huntington's disease, multiple sclerosis, and Alzheimer's disease.

EEG (Electroencephalography) based on detecting the electrical nerve impulses transmitted among neurons. Metal electrodes attached to the scalp are connected to an amplifier that magnifies the signals, which are recorded as waveforms on a screen/paper. It is used to evaluate seizures, epilepsy, and brain tumours; it is painless and needs no sedation or anaesthesia. CT scan (Computed Tomography): based on the fact that different tissues absorb X-rays to different degrees. The patient lies in a doughnut-shaped gantry with a rotating X-ray tube; detectors on the opposite side record X-ray absorption through tissues, and a computer constructs detailed cross-sectional images. It is used to image soft tissues (brain, lungs, abdomen), detect bone abnormalities, and diagnose cancers, infections, and injuries. MRI (Magnetic Resonance Imaging): based on the fact that different tissues contain varying amounts of hydrogen atoms, which have magnetic properties. The patient lies in a scanner with a powerful magnet creating a strong magnetic field; radio waves cause hydrogen atoms to emit signals, which a receiver detects and a computer uses to build a detailed 3D image. It is used to image soft tissues (brain, spinal cord, internal organs) and detect abnormalities in blood vessels and tumours.