A neuron consists of three main parts: the cell body (soma), dendrites, and an axon. The cell body contains the nucleus and metabolic machinery, and it integrates incoming signals. Dendrites are short, branched extensions that receive signals from other neurons or receptors and transmit them toward the cell body. The axon is a long fiber that carries electrical impulses away from the cell body to other neurons or effectors; many axons are covered by a myelin sheath produced by glial cells, which insulates the axon and increases conduction speed. The axon ends in terminal branches with synaptic knobs that participate in neurotransmitter release at synapses, enabling communication between cells.

The central nervous system (CNS) comprises the brain and spinal cord. The brain processes sensory information, makes decisions, and initiates responses; major parts include the cerebrum (higher cognitive functions, voluntary actions), cerebellum (balance and coordination), and brainstem (vital involuntary functions like breathing and heartbeat). The spinal cord acts as a conduit for signals between the brain and peripheral nerves and also mediates many reflex actions. Coordination occurs by integrating sensory inputs, processing them in CNS centres, and sending motor outputs through peripheral nerves to muscles and glands—allowing rapid, organized responses and maintaining homeostasis.

Myelin sheath is a fatty insulating layer wrapped around many axons, produced by Schwann cells in the peripheral nervous system and oligodendrocytes in the CNS. It increases the speed of nerve impulse conduction by enabling saltatory conduction, where the action potential jumps between nodes of Ranvier. Damage to the myelin sheath (demyelination) slows or blocks signal transmission, causing neurological symptoms such as muscle weakness, impaired coordination, numbness, or vision problems. Diseases like multiple sclerosis involve immune-mediated myelin damage and illustrate how crucial myelination is for normal nerve function.

Sensory (afferent) neurons carry information from sensory receptors toward the CNS—for example, pain receptors in the skin send signals to the spinal cord. Motor (efferent) neurons transmit impulses from the CNS to effectors like muscles or glands—for example, a motor neuron stimulates a skeletal muscle to contract. Relay (inter) neurons are located entirely within the CNS and connect sensory neurons to motor neurons or other interneurons, enabling complex processing and reflex circuits—for example, interneurons in the spinal cord mediate reflex arcs such as withdrawal from a hot object. Together, these neuron types form pathways for perception, processing, and response.

The human brain consists of the cerebrum, cerebellum, and brainstem (midbrain, pons, medulla). The cerebrum is the largest part with two hemispheres and is responsible for higher functions such as thinking, memory, voluntary movement, and sensory perception; the cerebral cortex processes complex information. The cerebellum lies beneath the cerebrum and coordinates movements, maintains posture, and ensures balance. The brainstem connects the brain to the spinal cord and controls involuntary functions like breathing, heartbeat, digestion, and reflexes. Subcortical structures (e.g., hypothalamus) regulate hormones, temperature, hunger, and sleep. Together, these parts allow cognitive processing, coordination, and regulation of vital activities.

The spinal cord mediates many reflex actions by forming simple neural circuits called reflex arcs that bypass conscious brain processing for speed. A typical reflex arc involves: (1) Receptor: senses a stimulus (e.g., pain), (2) Sensory neuron: transmits signal to the spinal cord, (3) Relay (inter) neuron in the spinal cord: processes the signal and connects sensory to motor neurons, (4) Motor neuron: carries impulse to effector, (5) Effector: muscle or gland that performs the response (e.g., muscle contraction to withdraw hand). Because the spinal cord handles the processing, reflexes are rapid and protective. Although the brain can receive information about the event, it is not required for the immediate reflex action.

The cerebellum processes input from sensory systems and motor pathways to fine-tune voluntary movements and maintain posture and balance. It receives information about the position of limbs, the state of muscles, and intended movements from the cerebral cortex and sensory organs (like the inner ear). By comparing intended movement with actual movement, the cerebellum adjusts muscle activity to ensure smooth, coordinated actions and corrects errors in timing and force. Damage to the cerebellum results in ataxia—uncoordinated movement, tremors, and difficulty maintaining balance—showing its role in precise motor control.

The brainstem, particularly the medulla oblongata, controls essential involuntary functions that sustain life, such as heartbeat regulation, breathing rhythm, blood pressure, and reflexes like coughing, sneezing, and swallowing. It houses centres that coordinate autonomic nervous system responses and serves as a critical pathway for nerve tracts traveling between the brain and spinal cord. Because of these roles, injury to the brainstem can be fatal or cause severe dysfunctions in vital processes, which highlights its indispensable role in maintaining homeostasis and basic life-support functions.

A nerve impulse or action potential is generated when a stimulus causes the neuron's membrane potential to reach a threshold, triggering voltage-gated sodium channels to open and sodium ions to rush in, causing rapid depolarization. Shortly after, potassium channels open and potassium ions flow out, repolarizing the membrane and restoring the resting potential via the sodium-potassium pump, which maintains ion gradients. The local depolarization spreads to adjacent patches of membrane, sequentially opening ion channels along the axon and propagating the action potential. In myelinated axons, the impulse jumps between nodes of Ranvier (saltatory conduction), increasing speed. Refractory periods ensure one-way propagation and limit firing frequency.

At a chemical synapse, an arriving action potential at the axon terminal triggers voltage-gated calcium channels to open, allowing calcium ions to enter. This influx causes synaptic vesicles loaded with neurotransmitters to fuse with the presynaptic membrane and release their contents into the synaptic cleft. Neurotransmitters diffuse across the gap and bind to specific receptors on the postsynaptic membrane, causing ion channels to open or close and thereby producing an excitatory or inhibitory postsynaptic potential. Enzymes or reuptake mechanisms then remove neurotransmitters to terminate the signal. Chemical synapses allow modulation and integration of signals but are slightly slower than electrical synapses.

Excitatory neurotransmitters increase the likelihood of the postsynaptic neuron firing an action potential by causing depolarization; an example is glutamate. Inhibitory neurotransmitters decrease the chance of firing by causing hyperpolarization; an example is GABA (gamma-aminobutyric acid). The balance between excitatory and inhibitory signals determines neuronal activity and network behaviour; disruptions in this balance can lead to conditions like epilepsy (too much excitation) or sedation (excess inhibition).

Drugs and toxins can alter synaptic transmission by mimicking neurotransmitters, blocking receptors, inhibiting neurotransmitter release, or preventing reuptake/degradation. For example, nicotine mimics acetylcholine and activates nicotinic receptors, while curare blocks acetylcholine receptors at neuromuscular junctions causing muscle paralysis. Another example: selective serotonin reuptake inhibitors (SSRIs) block the reuptake of serotonin, increasing its levels in synapses to alleviate depression. These actions demonstrate how chemical interference at synapses can profoundly affect physiology and behaviour.

The nervous system uses electrical impulses and fast synaptic transmission to deliver messages via neurons to specific target cells, enabling rapid responses (milliseconds to seconds) and precise control of muscles and glands. Neurons form direct synaptic connections allowing targeted signaling. In contrast, the endocrine system releases hormones into the bloodstream, which travel to many parts of the body and act more slowly (seconds to hours or days) but have longer-lasting effects and regulate processes like growth, metabolism, and reproduction. The nervous system is ideal for immediate, localized control, while the endocrine system manages slower, sustained regulation.

The refractory period is a brief time after an action potential during which a neuron cannot fire another action potential (absolute refractory) or requires a stronger-than-normal stimulus (relative refractory). It results from inactivation of sodium channels and the time needed to re-establish ion gradients. This period enforces unidirectional propagation of impulses along the axon, limits the maximum frequency of firing, and allows the neuron to reset before the next signal, ensuring distinct, separate impulses and preventing continuous or uncontrolled firing.

The withdrawal reflex (flexor reflex) is an automatic response that pulls a body part away from a harmful stimulus, such as touching something hot. Receptors detect the stimulus and send signals via sensory neurons to the spinal cord. Interneurons in the spinal cord transmit signals to motor neurons that activate flexor muscles to withdraw the limb and may simultaneously inhibit extensor muscles. This reflex is rapid because it is processed locally in the spinal cord without delay from the brain, protecting the body from injury and allowing immediate action to minimize tissue damage.

Clinically, reflexes like the knee-jerk (patellar) reflex or ankle reflex are tested using a reflex hammer to elicit a response. The presence, absence, or exaggeration of reflexes can indicate the integrity of specific neural pathways, helping localize neurological lesions. For example, a diminished knee reflex may suggest peripheral nerve damage, while exaggerated reflexes can indicate upper motor neuron lesions. Reflex testing is a quick, non-invasive diagnostic tool to assess nervous system function and detect disorders.

Reflex actions are innate, automatic responses to specific stimuli (e.g., blinking when an object approaches the eye) and do not require learning. Learned behaviours are acquired through experience, practice, or conditioning and often involve higher brain centres; examples include riding a bicycle, solving math problems, or language acquisition. Learned behaviours can be modified, improved, or forgotten, whereas reflexes are generally fixed and rapid. The nervous system integrates both automatic reflex pathways and higher cognitive circuits to enable flexible and adaptive behaviour.

Monosynaptic reflexes involve a single synapse between a sensory and a motor neuron, making them very fast; the patellar reflex is an example. Polysynaptic reflexes involve one or more interneurons between sensory and motor neurons, allowing for more complex processing such as reciprocal inhibition of antagonist muscles; the withdrawal reflex is polysynaptic and may recruit multiple muscles. Polysynaptic arcs can integrate multiple inputs and produce coordinated responses but are slightly slower due to additional synaptic delays.

The nervous and endocrine systems interact closely—nervous signals can stimulate endocrine glands, and hormones can influence neuronal activity. For example, the hypothalamus in the brain receives sensory information and neural inputs and signals the pituitary gland to release hormones that regulate other endocrine glands (e.g., thyroid stimulating hormone affects the thyroid). In stress, the nervous system activates the adrenal medulla to release adrenaline for immediate response, while the hypothalamic-pituitary-adrenal (HPA) axis releases cortisol for longer-term stress adaptation. Together they coordinate rapid and sustained responses to maintain internal balance.

The pituitary gland secretes several hormones that regulate growth, metabolism, reproduction, and other endocrine glands. It releases growth hormone (GH) which stimulates growth and protein synthesis, thyroid stimulating hormone (TSH) which controls thyroid activity, adrenocorticotropic hormone (ACTH) which stimulates adrenal cortex, and gonadotropins (FSH and LH) which regulate the gonads. Because its hormones control multiple other endocrine glands and influence numerous physiological processes, the pituitary is termed the 'master gland', although its activity is itself regulated by the hypothalamus and feedback from target glands.

Insulin (secreted by pancreatic beta cells) lowers blood glucose by promoting glucose uptake in muscle and adipose tissue, stimulating glycogen synthesis in the liver, and increasing protein and fat synthesis. Glucagon (from alpha cells) raises blood glucose by promoting glycogen breakdown and gluconeogenesis in the liver. The interplay between insulin and glucagon maintains blood glucose within a narrow range: after a meal insulin predominates to store excess glucose, while during fasting glucagon acts to mobilize stored glucose. Disruption in this balance causes metabolic disorders like diabetes mellitus.

Thyroid hormones (mainly thyroxine/T4) regulate basal metabolic rate, growth, and development. They influence protein synthesis, energy metabolism, and nervous system maturation. Excess thyroxine (hyperthyroidism) can lead to weight loss, increased heart rate, heat intolerance, and nervousness; prolonged excess may cause goitre if due to overactive glands. Deficiency (hypothyroidism) can cause fatigue, weight gain, cold intolerance, slowed growth in children, and developmental delays if severe (cretinism). Proper thyroid function is crucial for metabolic homeostasis and normal development.

Feedback mechanisms maintain hormone levels within desired ranges. In negative feedback, the output of a pathway reduces the activity of the pathway itself. For example, when thyroid hormone levels rise, they inhibit the release of TSH from the pituitary and TRH from the hypothalamus, which reduces stimulation of the thyroid gland and thus lowers hormone production. This feedback loop keeps thyroid hormones at appropriate concentrations. Positive feedback (less common) amplifies responses, such as oxytocin release during childbirth enhancing uterine contractions until delivery.

Follicle-stimulating hormone (FSH) and luteinizing hormone (LH), secreted by the anterior pituitary, regulate gonadal function. In females, FSH stimulates ovarian follicle growth and estrogen production; a mid-cycle surge in LH triggers ovulation and formation of the corpus luteum, which secretes progesterone to prepare the uterus for implantation. In males, FSH promotes spermatogenesis in the testes while LH stimulates testosterone production by Leydig cells, essential for sperm maturation and male secondary sexual characteristics. The coordinated action of FSH and LH ensures proper gamete production and reproductive cycles.

Diabetes mellitus is a metabolic disorder characterized by chronically elevated blood glucose due to insufficient insulin production (Type 1) or insulin resistance with relative insulin deficiency (Type 2). Type 1 is autoimmune destruction of pancreatic beta cells leading to absolute insulin deficiency; it often presents in childhood and requires insulin therapy. Type 2 is linked to obesity and lifestyle, with cells responding poorly to insulin; management includes diet, exercise, and medication. Common symptoms include excessive thirst, frequent urination, unexplained weight loss, fatigue, and blurred vision. Long-term complications affect eyes, kidneys, nerves, and blood vessels.

Auxin is a plant growth hormone that promotes cell elongation. In phototropism, light coming from one side causes auxin to redistribute toward the shaded side of the shoot. Higher auxin concentration on the shaded side stimulates greater cell elongation there compared to the lighted side, causing the shoot to bend towards the light. This differential growth enables plants to orient leaves and stems to capture more light for photosynthesis. The molecular mechanism involves auxin-regulated gene expression that modulates cell wall extensibility and growth rates.

Gibberellins are growth-promoting hormones that stimulate stem elongation, seed germination, and flowering in some plants; they break seed dormancy and promote mobilization of food reserves during germination. Ethylene is a gaseous hormone that regulates fruit ripening, leaf and flower senescence, and abscission; it can also influence root growth and responses to stress. While gibberellins generally promote growth processes, ethylene often modulates maturation and ageing processes; both are essential for normal plant development and agricultural practices (e.g., using ethylene for synchronized ripening).

Nastic movements are non-directional responses to stimuli where the direction of movement is independent of stimulus direction. They are caused by changes in turgor pressure or growth in specific cells—for example, the rapid folding of Mimosa pudica leaves on touch (seismonasty) or the opening and closing of flowers in response to light (photonasty). Tropisms, by contrast, are directional growth responses toward or away from a stimulus, such as phototropism (toward light) or geotropism (in response to gravity). Nastic movements are reversible and often rapid, while tropisms usually involve growth and are directional towards the stimulus source.

Abscisic acid (ABA) helps plants cope with drought by inducing stomatal closure to reduce transpiration and water loss. Under water stress, ABA levels rise and signal guard cells to lose turgor, closing stomatal pores; this conserves water but reduces gas exchange and photosynthesis. ABA also promotes dormancy, slows growth, and triggers expression of stress-responsive genes that protect cells. By altering physiological and developmental processes, ABA helps plants survive temporary adverse conditions until water availability improves.

Comparison: The nervous system uses electrical impulses along neurons and chemical synapses to transmit messages rapidly and specifically to target cells, producing fast and short-lived responses suited for immediate actions (e.g., muscle contraction). The endocrine system releases hormones into the bloodstream for widespread distribution, producing slower but longer-lasting effects that regulate growth, metabolism, and reproduction. Nervous control is point-to-point and precise; endocrine control is broadcast-like and systemic. Nervous responses can be voluntary or reflexive; endocrine effects are generally involuntary and modulate physiological states. Both systems interact (e.g., hypothalamus-pituitary axis) to maintain homeostasis. Exam-writing tips: Begin with a clear definition, use a table or bullets to compare features, include specific examples (e.g., insulin vs nerve impulse), and conclude with a short sentence summarizing the importance of coordination between the two systems. Use labelled diagrams where relevant to score higher marks.