Neuron

Neurons—also called nerve cells—are electrically excitable cells that form the core functional units of the nervous system. They generate and transmit rapid electrical impulses, enabling communication across neural circuits in both vertebrates and invertebrates. Neurons coordinate sensory input, motor output and complex integrative processes, making them fundamental to perception, movement, cognition and homeostasis.

Evolutionary Background

Molecular evidence indicates that electrically excitable cells evolved around 700 to 800 million years ago during the Tonian period. The earliest precursors of neurons were peptidergic secretory cells, which later acquired specialised gene modules for postsynaptic scaffolding and voltage-gated ion channels. These innovations allowed cells to transmit fast electrical signals. Neurons are present in all animals except sponges and placozoans; plants and fungi do not possess nerve cells.

Types of Neurons

Neurons are commonly classified by their functional roles:

• Sensory neurons, which detect stimuli such as light, sound, pressure or chemical signals, and transmit information to the brain or spinal cord.
• Motor neurons, which carry signals from the central nervous system to muscles or glands, controlling voluntary and involuntary responses.
• Interneurons, which form connections between neurons within the same region of the brain or spinal cord and constitute the majority of neurons in complex nervous systems.

Functionally connected groups of neurons form neural circuits, which underlie processes such as reflexes, memory, emotion and higher cognitive functions.

Structure and Organisation

Although neurons share many organelles with other cell types—such as a nucleus, mitochondria and Golgi apparatus—they possess unique structural features essential for signal transmission:

• Soma (cell body): Contains the nucleus and most of the cell’s biosynthetic machinery. Somata range from about 4 to 100 micrometres in diameter.
• Dendrites: Branching extensions that receive synaptic input. Their numerous spines increase receptive surface area and contribute to synaptic strength.
• Axon: A long, cable-like projection that carries electrical impulses away from the soma. Human axons can exceed one metre in length; in larger animals like giraffes, individual axons may span several metres. The axon originates from the axon hillock, the region with the highest density of voltage-gated sodium channels and the primary site for action potential initiation.
• Axon terminals: The distal ends of axonal branches containing synaptic boutons, where neurotransmitters are released. Some boutons occur en passant along the axon shaft.

Not all neurons conform to the typical pattern: some lack dendrites, others lack axons, and the term neurite may be used when distinctions are unclear, particularly in developing cells.
Microscopically, neuron cell bodies contain Nissl substance—aggregates of rough endoplasmic reticulum and ribosomal RNA—which stain basophilically and are involved in protein synthesis.

Membrane Properties and Electrical Excitability

Neurons are electrically excitable because they maintain a voltage gradient across their plasma membrane. The membrane is a lipid bilayer embedded with ion channels and pumps that regulate the movement of sodium, potassium, chloride and calcium ions. Key features include:

• Voltage-gated ion channels, which open or close in response to changes in membrane potential.
• Chemically gated channels, which respond to neurotransmitters or other signalling molecules.
• Ion pumps, which maintain ionic gradients using metabolic energy.

If the membrane potential reaches a critical threshold, the neuron generates an action potential, a rapid all-or-none electrical spike that travels along the axon. Some myelinated axons conduct impulses at speeds up to approximately 120 m/s. When the action potential reaches the axon terminals, it triggers the release of neurotransmitters into the synaptic cleft.

Synaptic Communication

Communication between neurons typically occurs at chemical synapses, where neurotransmitters diffuse across a small gap to bind receptors on the postsynaptic cell. Depending on the receptor type, the postsynaptic response may be:

• Excitatory, producing depolarisation and increasing the likelihood of firing an action potential.
• Inhibitory, producing hyperpolarisation and reducing excitability.

Although most synapses are axon-to-dendrite, they may also form between axons, between dendrites or even between soma and axon initial segments.

Neurogenesis and Cellular Lifespan

Most neurons are generated during embryonic development and early childhood from neural stem cells. In many regions of the mammalian brain, neurogenesis declines sharply in adulthood, although some adult neural stem cells persist and contribute to limited regeneration. Fully differentiated neurons remain in the G0 phase and do not undergo further cell division.
Astrocytes—star-shaped glial cells—have demonstrated the capacity to differentiate into neurons under certain circumstances, reflecting a degree of plasticity.

Neurons in the Nervous System

Neurons work alongside glial cells, which provide structural, metabolic and protective support. The nervous system comprises:

• Central nervous system: Brain and spinal cord, containing the majority of neurons.
• Peripheral nervous system: Somatic, autonomic and enteric divisions, containing sensory and motor neurons.

Axons in the PNS bundle into nerves, while in the CNS they form nerve tracts.

Identified Neurons in Other Species

In species with simpler nervous systems, some neurons are “identified”, meaning they have consistent location, structure and function across individuals. The best-known vertebrate example is the Mauthner cell of fish, a giant brainstem neuron whose single action potential triggers a rapid escape reflex. Numerous other identified neurons exist in invertebrates, including the extensively mapped nervous system of nematodes.

Significance

Neurons are the foundational units of neural communication and adaptation. Their highly specialised architecture, rapid electrochemical signalling and capacity for complex interactions allow nervous systems to process information, regulate bodily functions and generate behaviour. Their evolution represents a critical step in the rise of animal complexity, forming the basis for sensation, movement and cognition across the animal kingdom.