The nervous system is the body's specialised communication and control network, consisting principally of the brain, spinal cord and nerves. It receives information from inside and outside the body, processes that information, generates responses and coordinates functions ranging from voluntary movement and sensation to automatic regulation of breathing, heart rate and digestion. The nervous system is conventionally divided into the central nervous system (CNS), consisting of the brain and spinal cord, and the peripheral nervous system (PNS), consisting of nerves and associated structures extending beyond the CNS.
Neurons are the principal communication cells of the nervous system. A typical neuron contains a cell body, dendrites that receive information and an axon that carries electrical signals towards distant targets. Axon terminals communicate with other neurons or with effector cells such as muscle or gland cells. Bundles of axons form nerves in the peripheral nervous system. Different neurons have different specialised functions, including sensory neurons that detect stimuli, motor neurons that control muscles and interneurons that connect and process information within the CNS.
Communication within the nervous system involves both electrical and chemical signalling. An action potential is an electrical change that propagates along a neuron's membrane when appropriate threshold conditions are met. At the axon terminal, the electrical signal can trigger release of neurotransmitters into a synapse. These chemical messengers cross the synaptic gap and bind receptors on another cell, producing excitatory, inhibitory or modulatory effects. The next neuron can then integrate these signals and determine whether to generate its own electrical response. This combination of electrical conduction and chemical transmission allows complex networks to process information rapidly.
The nervous system includes glial cells as well as neurons. Glia are not merely passive support cells; different types provide structural support, metabolic assistance, insulation, immune functions, regulation of neurotransmitters and other essential processes. Myelin, produced by specialised glial cells, electrically insulates portions of axons and allows action potentials to propagate more efficiently. Other glial populations help maintain the chemical environment around neurons and participate in repair or immune responses. Normal nervous-system function therefore depends on interactions among many cell types rather than neurons acting alone.
The peripheral nervous system can be functionally divided into somatic and autonomic components. The somatic system is associated principally with conscious sensation and voluntary skeletal-muscle control. The autonomic nervous system regulates involuntary functions and includes sympathetic, parasympathetic and enteric components. Sympathetic activity contributes to responses such as increased heart rate during stress, whereas parasympathetic activity generally supports functions associated with rest and digestion. The enteric nervous system contains extensive neural circuitry within the gastrointestinal tract and can coordinate many digestive processes locally.
Sensory processing allows the nervous system to monitor both the external environment and the body's internal state. Receptors detect stimuli such as light, sound, pressure, temperature, chemicals and tissue damage. Sensory signals travel through peripheral pathways to the CNS, where networks interpret the information and generate appropriate responses. Motor signals then travel through descending pathways and peripheral nerves to muscles or glands. The process can be extremely rapid: sensory information can reach the CNS, be integrated and generate a motor response within milliseconds in some pathways.
The nervous system is also responsible for higher functions such as learning, memory, language, attention, emotion, decision-making and consciousness. These functions do not arise from one isolated "thinking centre"; they emerge from coordinated activity across networks of specialised and interconnected brain regions. Experience can alter neural connections, a property often described as neuroplasticity. Development, learning, injury, disease and repeated experience can all influence the organisation and efficiency of neural circuits. The nervous system therefore combines relatively stable anatomical structures with continuously changing patterns of activity and connectivity.
Nervous-system disorders can result from many mechanisms, including trauma, infection, vascular injury, inflammation, degeneration, abnormal electrical activity, tumours, inherited genetic changes and metabolic disturbances. Examples include stroke, epilepsy, Parkinson's disease, Alzheimer's disease, peripheral neuropathy and spinal-cord injury. Symptoms depend strongly on which structures or pathways are affected and can include weakness, sensory loss, abnormal movement, seizures, pain, cognitive changes or disturbances of autonomic function. Understanding the nervous system is therefore fundamental not only to neuroscience but also to diagnosing and treating neurological disease.
