How Neurons Work: The Brain's Signaling Cells

Neuron Structure and Types

Every neuron shares a basic architecture consisting of three main parts: the cell body (soma), dendrites, and an axon. The cell body contains the nucleus and the molecular machinery needed to produce proteins, generate energy, and maintain the cell. Dendrites are tree-like extensions that branch outward from the cell body, receiving incoming signals from other neurons through specialized contact points called synapses. A single neuron may have thousands of dendritic branches, each studded with tiny protrusions called dendritic spines where most excitatory synapses form.

The axon is a long, thin fiber that extends from the cell body and carries outgoing signals to target cells. Axons range in length from less than a millimeter in local interneurons to over a meter in motor neurons that connect the spinal cord to the muscles of the feet. Many axons are wrapped in myelin, a fatty insulating layer produced by specialized glial cells called oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system. Myelin dramatically increases signal transmission speed by enabling saltatory conduction, in which the electrical impulse jumps between gaps in the myelin sheath called nodes of Ranvier.

Neurons come in many specialized types, classified by their shape, function, and the neurotransmitters they release. Sensory neurons carry information from the body's sense organs to the brain. Motor neurons transmit commands from the brain and spinal cord to muscles and glands. Interneurons, which make up the vast majority of neurons in the brain, connect other neurons and form the complex circuits responsible for processing information. Within these broad categories, researchers have identified hundreds of distinct neuron types based on their gene expression profiles, morphology, and electrophysiological properties.

The Action Potential

Neurons communicate through electrical signals called action potentials. At rest, a neuron maintains a voltage difference of about negative 70 millivolts across its cell membrane, with the inside of the cell being more negative than the outside. This resting potential is maintained by ion pumps and channels that control the flow of sodium, potassium, chloride, and calcium ions across the membrane.

When excitatory signals from other neurons depolarize the membrane to a threshold value of approximately negative 55 millivolts, voltage-gated sodium channels open rapidly, allowing sodium ions to rush into the cell. This influx of positive ions causes a rapid spike in voltage to about positive 40 millivolts. Within a millisecond, the sodium channels close and voltage-gated potassium channels open, allowing potassium ions to flow out of the cell and restoring the negative resting potential. This entire sequence, the action potential, lasts roughly one to two milliseconds.

Action potentials follow an all-or-nothing principle: once the threshold is reached, the neuron fires a full-strength signal regardless of how much the threshold was exceeded. The strength of a stimulus is encoded not by the amplitude of individual action potentials but by their frequency. A strong stimulus produces rapid firing, sometimes exceeding 100 action potentials per second, while a weak stimulus produces slower, sparser firing. After each action potential, the neuron enters a brief refractory period during which it cannot fire again, setting an upper limit on firing rate.

Synaptic Transmission

When an action potential reaches the axon terminal, it triggers the opening of voltage-gated calcium channels. The influx of calcium ions causes small membrane-bound vesicles containing neurotransmitter molecules to fuse with the cell membrane and release their contents into the synaptic cleft, a gap of about 20 nanometers separating the sending and receiving neurons. This process, called exocytosis, occurs within a few hundred microseconds of the action potential's arrival.

Released neurotransmitter molecules diffuse across the synaptic cleft and bind to receptor proteins on the postsynaptic neuron. These receptors come in two main types: ionotropic receptors, which are ion channels that open directly upon neurotransmitter binding, producing fast synaptic responses lasting milliseconds, and metabotropic receptors, which activate intracellular signaling cascades that produce slower but longer-lasting effects on cell function, gene expression, and synaptic plasticity.

After neurotransmitter molecules have activated their receptors, they must be rapidly cleared from the synapse to prevent continuous receptor activation. This occurs through three mechanisms: enzymatic breakdown in the synaptic cleft, reuptake into the presynaptic terminal through transporter proteins, and diffusion away from the synapse. Many psychoactive drugs work by interfering with these clearance mechanisms, altering the duration and intensity of neurotransmitter signaling.

Neuronal Plasticity

Neurons are not fixed, static cells but continuously modify their structure and function in response to activity and experience. At the molecular level, synaptic plasticity involves changes in the number and sensitivity of neurotransmitter receptors, alterations in neurotransmitter release probability, and modifications to the intracellular signaling pathways that link receptor activation to cellular responses. These changes can strengthen or weaken specific synaptic connections, forming the cellular basis of learning and memory.

Structural plasticity extends to physical changes in neuronal morphology. Dendritic spines can grow, shrink, or disappear over hours to days in response to patterns of neural activity. New synapses can form between previously unconnected neurons, while unused synapses can be eliminated through a process called synaptic pruning. Even the axon can undergo remodeling, with new branches sprouting to reach additional target cells. These structural changes are particularly pronounced during development but continue throughout adult life.

Glial Cells: The Other Half of the Brain

While neurons receive most of the attention, the brain also contains a roughly equal number of glial cells that perform essential support functions. Astrocytes, the most abundant glial type, form tight associations with synapses, regulating the chemical environment by absorbing excess neurotransmitters and potassium ions. They also supply neurons with metabolic fuel, help form the blood-brain barrier, and release their own signaling molecules called gliotransmitters that modulate synaptic transmission. Recent research suggests that astrocytes participate actively in information processing, not merely supporting it.

Oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system produce the myelin sheath that insulates axons and dramatically increases signal transmission speed. The loss or damage of myelin, as occurs in multiple sclerosis, disrupts neural communication and can produce a wide range of neurological symptoms. Microglia serve as the brain's resident immune cells, constantly surveying the neural environment for signs of damage, infection, or debris. During development, microglia also play a crucial role in synaptic pruning, selectively eliminating weak or unnecessary synapses to refine neural circuits.

How Neurons Develop and Survive

During embryonic development, neurons are generated from neural stem cells in a process called neurogenesis. Newly born neurons migrate to their final positions in the brain, extend axons to find their target cells, and form initial synaptic connections. This process is guided by molecular signals including growth factors, cell adhesion molecules, and chemical gradients that direct axonal growth cones toward their destinations. Remarkably, developing axons can navigate over distances of centimeters or more, guided by a combination of attractive and repulsive cues, to reach their precise targets.

Not all neurons that are generated survive to adulthood. During development, roughly half of all newly born neurons undergo programmed cell death, or apoptosis. Neurons that successfully form connections with appropriate target cells receive survival signals, particularly neurotrophic factors like nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF), while those that fail to connect die. This competitive process ensures that only neurons integrated into functional circuits survive, optimizing the efficiency of neural networks.

In the adult brain, neurogenesis was long thought to be impossible, but research beginning in the 1990s demonstrated that new neurons are continuously generated in at least two brain regions: the hippocampus, where they contribute to memory formation, and the olfactory bulb, where they participate in odor processing. Adult neurogenesis is influenced by physical exercise, environmental enrichment, learning, and stress, suggesting that the brain retains some capacity for cellular renewal throughout life.