What Are Neurons?

Neurons are the electrically excitable cells of the nervous system that receive, integrate, and transmit information through a combination of chemical and electrical signals. Each neuron consists of a soma (cell body) containing the nucleus, dendrites that receive incoming signals from other cells, and an axon that conducts output signals to downstream neurons or effector organs. The human central nervous system contains roughly 86 billion neurons, organized into circuits that underlie every aspect of sensation, movement, cognition, and autonomic regulation.

Neurons are studied at the intersection of cell biology, biophysics, and electrical engineering. Their signaling properties are described by differential equations governing ion channel conductances and membrane voltage, making them amenable to circuit-theoretic models that have directly inspired neuromorphic hardware.

Membrane Potentials and Action Potentials

The resting membrane potential of a typical neuron is approximately -70 mV, sustained by the selective permeability of the membrane to potassium ions and by the sodium-potassium ATPase pump, which expels three sodium ions for every two potassium ions it imports. When synaptic inputs bring the membrane potential above a threshold of roughly -55 mV, voltage-gated sodium channels open rapidly, producing an inward current that drives the membrane toward +40 mV. This depolarization inactivates sodium channels and opens voltage-gated potassium channels, which repolarize the membrane and generate a brief undershoot before the resting potential is restored. The resulting stereotyped waveform, the action potential, propagates along the axon as a self-regenerating wave and carries information through its timing and firing rate rather than its amplitude. NCBI Bookshelf's Neuroanatomy, Neuron Action Potential entry describes the biophysics of action potential initiation and propagation in detail. Myelination by Schwann cells in the peripheral nervous system and oligodendrocytes in the CNS increases conduction velocity by allowing saltatory conduction between nodes of Ranvier.

Synapses and Neurotransmission

Neurons communicate primarily through chemical synapses, where an action potential arriving at the presynaptic terminal triggers calcium-dependent fusion of neurotransmitter-filled vesicles with the membrane and release of transmitter into the synaptic cleft. Ionotropic receptors on the postsynaptic membrane bind the transmitter and open ion channels directly, producing fast excitatory (glutamate at AMPA and NMDA receptors) or inhibitory (GABA at GABAA receptors) postsynaptic potentials within milliseconds. Metabotropic receptors activate intracellular signaling cascades that modulate channel gating over seconds to minutes, providing a basis for neuromodulatory control of circuit excitability. Synaptic strength is not fixed: long-term potentiation (LTP) and long-term depression (LTD) are activity-dependent changes in synaptic efficacy that represent the cellular substrate of learning and memory. A PMC article on synaptic signaling in learning and memory reviews how LTP induction at NMDA receptors drives the insertion of additional AMPA receptors, persistently strengthening the synapse.

Neural Circuits

Individual neurons are functionally meaningful only within the circuits they form. Neural circuits are assemblies of neurons connected by specific synaptic pathways that process defined classes of information. Feedforward circuits transform inputs into outputs without recurrence, as in the spinal cord reflex arc that connects sensory afferents, interneurons, and motor neurons. Recurrent circuits, in which output neurons project back to their own inputs, are prevalent in cortical columns and hippocampus and support dynamics including oscillations, persistent activity, and attractor states associated with working memory and decision-making. Neural Circuits in the NCBI Neuroscience Bookshelf describes the organizational principles of major circuit motifs across brain regions. The identification of specific circuit elements involved in disease states has informed neuromodulation targets, such as the subthalamic nucleus circuit in Parkinson's disease.

Applications

Neurons have applications in a range of fields, including:

• Brain-machine interfaces that record single-unit and multi-unit spiking activity to decode motor intent
• Computational neuroscience models that simulate circuit dynamics to study cognition and disease
• Organoid and in-vitro neural circuit platforms for drug screening and toxicology testing
• Neuromorphic chip design inspired by the biophysics of action potential generation and synaptic plasticity
• Optogenetic tools that target specific neuron types for circuit-level causal manipulation