Neuron | How Do These Tiny Cells Create Our Thoughts and Actions?

A neuron is the fundamental unit of the nervous system, a specialized cell designed to transmit information to other nerve cells, muscle, or gland cells. Its structure is uniquely adapted for this purpose. The main part of the cell is the soma, or cell body. The soma contains the nucleus, which houses the cell's genetic material, and it is responsible for the metabolic work that keeps the neuron alive and functioning. Extending from the soma are branch-like structures called dendrites. The primary role of dendrites is to receive electrochemical signals from the axons of other neurons. They act as the primary receivers of information for the neuron. The third key component is the axon, a long, slender projection that carries nerve impulses away from the soma and toward other neurons. The axon can be quite long, sometimes over a meter, and it acts as the primary transmission cable of the neuron. At its end, the axon branches into several axon terminals. These terminals form specialized junctions called synapses with the dendrites or somas of other neurons. Many axons are covered by a fatty substance called the myelin sheath, which insulates the axon and significantly increases the speed at which electrical signals, known as action potentials, travel along it. This entire structure—soma, dendrites, and axon—works in concert to ensure rapid and efficient communication throughout the brain and the entire body.

Neural communication is an electrochemical process. It begins when a neuron receives sufficient stimulation, typically through its dendrites, causing a change in the electrical charge across its cell membrane. This initiates an all-or-nothing electrical impulse known as an action potential. The action potential travels down the length of the axon like a wave. When this electrical signal reaches the axon terminals, it triggers a chemical process. The terminals contain small sacs called vesicles, which are filled with chemical messengers known as neurotransmitters. The arrival of the action potential causes these vesicles to merge with the cell membrane and release their neurotransmitters into the synapse, the microscopic gap between the axon terminal of one neuron and the dendrite of another. These neurotransmitter molecules cross the synapse and bind to specific receptor sites on the postsynaptic neuron (the receiving cell). This binding opens or closes ion channels, changing the electrical potential of the receiving neuron's membrane. Depending on the type of neurotransmitter and receptor, this can either excite the next neuron, making it more likely to fire its own action potential, or inhibit it, making it less likely to fire. This intricate sequence of electrical signaling along the axon and chemical signaling across the synapse is the fundamental basis of all brain function.

Memories are not stored in individual neurons but in the connections between them. The formation of memories is primarily attributed to a process called synaptic plasticity. This term refers to the ability of synapses to strengthen or weaken over time in response to increases or decreases in their activity. A key mechanism for this is long-term potentiation (LTP), which is a persistent strengthening of a synaptic connection. When two neurons are frequently activated together, the connection between them becomes more robust. This makes communication between them more efficient, meaning the presynaptic neuron can more easily trigger an action potential in the postsynaptic neuron. This strengthening of specific neural pathways is considered the cellular basis for learning and memory.

The capacity for neuronal repair and regeneration in the central nervous system (the brain and spinal cord) is notably limited. Unlike many other cells in the body, most neurons in the adult brain do not undergo cell division. If a neuron's cell body is destroyed, it cannot be replaced. However, the brain does possess a limited ability for regeneration, a process called neurogenesis. This occurs primarily in two specific regions: the subventricular zone and the subgranular zone of the hippocampus, an area critical for memory. In the peripheral nervous system (nerves outside the brain and spinal cord), axons have a greater capacity for regeneration if the cell body remains intact, allowing for some recovery of function after injury.

Neurons and their communication pathways are central to the biology of mental health disorders, including depression. The leading hypothesis, known as the monoamine hypothesis, posits that depression is linked to a deficiency in the brain of certain neurotransmitters, particularly serotonin, norepinephrine, and dopamine. These chemical messengers are crucial for regulating mood, motivation, and emotional responses. In a healthy brain, these neurotransmitters are released into the synapse, bind to receptors on the next neuron to transmit a signal, and are then cleared from the synapse through reuptake or enzymatic degradation. In depression, it is theorized that there may be insufficient release of these neurotransmitters or that they are cleared too quickly from the synapse, disrupting the normal signaling between neurons in key mood-regulating circuits. Many antidepressant medications, such as Selective Serotonin Reuptake Inhibitors (SSRIs), work by blocking the reuptake of serotonin. This action increases the concentration of serotonin in the synapse, enhancing its effect on the postsynaptic neuron and helping to restore more balanced neural communication.