Neurons are the fundamental units of the nervous system, forming the intricate communication network within the brain and throughout the entire body. These specialized cells transmit information, enabling every thought, sensation, and movement. This constant flow of information underpins all bodily functions, from basic involuntary actions to complex cognitive processes. Their interconnectedness ensures seamless communication, allowing the body to respond to its environment and maintain internal balance.
The Neuron’s Basic Blueprint
A neuron possesses distinct anatomical components, each playing a specific role in processing and transmitting signals. Dendrites are branch-like extensions that receive incoming signals from other neurons. They branch extensively to maximize their surface area for receiving input.
The cell body, or soma, houses the neuron’s nucleus and integrates electrical signals collected by the dendrites. Extending from the cell body is the axon, a long, slender projection that transmits the electrical signal away toward other neurons or target cells.
Many axons are insulated by a fatty layer called the myelin sheath, which enhances signal transmission speed. The myelin sheath is formed by specialized glial cells that wrap around the axon. At the end of the axon, axon terminals are specialized structures where the neuron sends signals to other cells.
Building an Electrical Charge
A neuron’s ability to transmit messages begins with its electrical state, maintained by a charge difference across its membrane, known as the resting membrane potential. This potential ranges from -60 to -70 millivolts, with the inside of the neuron more negatively charged than the outside. This is primarily established by the unequal distribution of sodium (Na+) and potassium (K+) ions across the neuronal membrane.
Specialized proteins embedded in the membrane, called ion pumps, maintain this ion imbalance. The sodium-potassium pump, for instance, expels three sodium ions out of the cell for every two potassium ions it brings in. This action contributes to the negative charge inside the neuron. The membrane also has potassium leak channels, allowing some potassium to flow out, further contributing to the negative resting potential.
A neuron remains in this resting state until it receives a stimulus strong enough to reach a specific threshold, around -55 millivolts. When the electrical potential across the membrane reaches this threshold, it triggers a rapid change in permeability. This leads to depolarization, where voltage-gated sodium channels open, allowing a rapid influx of positively charged sodium ions into the neuron. This influx makes the inside of the neuron temporarily less negative, initiating the electrical signal.
The Electrical Signal’s Journey
Once the threshold is reached and depolarization occurs, an electrical signal known as an action potential is generated. This action potential is an “all-or-none” event, meaning it either fires completely with consistent strength or does not fire at all. It is a rapid, brief reversal of the membrane potential. This electrical pulse propagates along the axon like a wave, where each segment of the axon triggers the next.
As sodium ions rush into one section of the axon, they cause the adjacent section to depolarize to its threshold, opening new voltage-gated sodium channels there. This sequential opening of channels ensures the action potential moves unidirectionally down the axon.
Following the depolarization phase, repolarization occurs, where voltage-gated potassium channels open, allowing potassium ions to flow out of the cell. This outflow restores the negative resting membrane potential.
A brief refractory period follows repolarization, during which the neuron is temporarily unable to fire another action potential. This period ensures that the signal travels in one direction and prevents the neuron from being overstimulated. The myelin sheath, which insulates the axon, significantly increases action potential propagation speed. In myelinated axons, the electrical signal jumps from one unmyelinated gap, called a Node of Ranvier, to the next. This process, known as saltatory conduction, allows for faster signal transmission compared to unmyelinated axons.
Bridging the Gap Between Neurons
When an electrical message reaches the end of an axon, it must bridge a small gap to the next neuron or target cell. This specialized junction is called a synapse. A synapse consists of three main parts: the presynaptic terminal (the end of the transmitting neuron’s axon), the synaptic cleft (a tiny space between neurons), and the postsynaptic membrane (part of the receiving neuron’s dendrite or cell body).
Upon arrival of an action potential at the presynaptic terminal, it triggers the release of chemical messengers called neurotransmitters. These neurotransmitters are stored in small sacs called synaptic vesicles within the presynaptic terminal. The electrical signal causes these vesicles to fuse with the presynaptic membrane, releasing their contents into the synaptic cleft.
Once released, neurotransmitters diffuse across the synaptic cleft and bind to specific receptor proteins on the postsynaptic membrane. This binding changes the electrical properties of the postsynaptic neuron, either exciting it to generate its own action potential or inhibiting it.
Neurotransmitters are quickly removed from the synaptic cleft either through reuptake (reabsorption by the presynaptic neuron) or by enzymatic degradation (breakdown by specific enzymes). This rapid clearing prepares the synapse for the next signal.