The brain’s ability to process thoughts, direct movements, and interpret sensations stems from the intricate communication network of billions of specialized cells called neurons. These neurons are the fundamental units of the nervous system, constantly transmitting information throughout the brain and body. This complex exchange of signals, an electrochemical event, enables all functions, from simple reflexes to complex cognitive processes.
The Neuron’s Structure
A neuron’s structure facilitates its role in transmitting information. At its center is the cell body, or soma, which contains the nucleus and genetic material. The soma also provides the energy necessary for the neuron’s work.
Extending from the cell body are tree-like structures called dendrites. These dendrites receive signals from other neurons. They are covered with synapses, specialized junctions where communication occurs between neurons. Signals received by the dendrites are then transmitted electrically towards the cell body.
Originating from the cell body is a long projection called the axon, which transmits signals away from the cell body. Many axons are covered by a fatty insulating layer, the myelin sheath, which speeds up electrical signal transmission. At the end of the axon are branching structures called axon terminals, or terminal buttons, where the neuron sends signals to other neurons, muscles, or glands.
Electrical Signals Within a Neuron
Neuronal communication begins with an electrical signal, an action potential, propagating along a neuron’s axon. This rapid change in voltage across the cell membrane is an all-or-nothing event; it either fires completely or not at all. The intensity of a stimulus is encoded by the frequency of these action potentials, not their amplitude.
Before an action potential, a neuron maintains a resting potential, where the inside of the cell is negatively charged relative to the outside, around -70 millivolts. This state is maintained by ion channels, proteins that control the flow of charged ions across the cell membrane. There is a higher concentration of positively charged sodium ions outside the neuron and potassium ions inside.
When a neuron receives a strong signal, the membrane potential at the axon hillock, where the axon connects to the cell body, begins to depolarize. Depolarization means the inside of the cell becomes less negative, or more positive. This occurs when voltage-gated sodium channels open, allowing a rapid influx of positively charged sodium ions into the cell.
As the positive charge inside the neuron increases and reaches a threshold potential, an action potential is triggered. This surge of sodium ions creates a positive spike, which propagates down the axon. Following depolarization, repolarization occurs, returning the membrane potential to its negative resting state. This is achieved by closing sodium channels and opening voltage-gated potassium channels, allowing potassium ions to flow out of the cell.
Chemical Signals Between Neurons
Once an action potential reaches the axon terminal, communication shifts from electrical to chemical at the synapse. The synapse is the synaptic cleft, a tiny gap between the axon terminal of the transmitting neuron (presynaptic neuron) and the dendrite of the receiving neuron (postsynaptic neuron). This chemical transmission ensures unidirectional communication.
The action potential’s arrival at the presynaptic axon terminal depolarizes its membrane, causing voltage-gated calcium channels to open. Calcium ion influx into the terminal triggers the fusion of synaptic vesicles with the presynaptic membrane. These synaptic vesicles store chemical messengers called neurotransmitters.
Upon fusion, neurotransmitters are released into the synaptic cleft through exocytosis. These neurotransmitters diffuse across the cleft and bind to specific receptor proteins on the postsynaptic membrane. This binding can either excite or inhibit the postsynaptic neuron, influencing whether it generates its own action potential.
Excitatory neurotransmitters, such as glutamate, make the postsynaptic neuron more likely to fire by causing depolarization through positive ion influx. Conversely, inhibitory neurotransmitters, like gamma-aminobutyric acid (GABA), decrease the likelihood of firing by making the membrane potential more negative, often by allowing negatively charged chloride ions to enter or positively charged potassium ions to leave the cell.
After binding, neurotransmitters must be rapidly cleared from the synaptic cleft to allow new signals. This termination occurs through several mechanisms: reuptake, enzymatic degradation, and diffusion. In reuptake, neurotransmitters are reabsorbed by the presynaptic neuron or nearby glial cells. Enzymatic degradation involves specific enzymes breaking down neurotransmitters into inactive components, like acetylcholinesterase breaking down acetylcholine. Alternatively, neurotransmitters can simply diffuse away from the synapse.