The body’s function, from coordinating movement to generating thought, depends on a rapid and complex internal communication system. This system, known as neural communication, operates as a sophisticated electrochemical signaling network that transmits information across the body. It allows sensory input to be processed and coordinated responses to be generated, providing the fundamental mechanism for all nervous system activity.
The Neuron and Its Supporting Cells
The foundation of neural communication rests on two primary cell types: the neuron and the glial cell. Neurons are the functional signaling units, characterized by a unique structure that facilitates sending and receiving information. The neuron’s central part, the cell body (soma), houses the nucleus and cellular machinery.
Branching outward from the cell body are dendrites, which receive signals from thousands of other neurons. The neuron’s transmitting extension is the axon, a single, long projection that conducts the signal away from the cell body toward target cells. This structure allows a single neuron to transmit its signal over distances that can be up to a meter long in humans.
Glial cells provide necessary support for the neurons. Astrocytes help maintain the chemical environment and provide nutritional support. Microglia act as the immune cells of the nervous system, scavenging pathogens and cellular debris. Oligodendrocytes and Schwann cells produce myelin, a fatty insulating layer essential for signal speed.
Electrical Signaling Within the Neuron
The signal traveling down the axon is an electrical impulse called the action potential. This impulse is generated when the neuron’s internal electrical charge changes from its resting potential, typically around -70 millivolts (mV). At rest, the cell maintains this negative charge by actively pumping three sodium ions (Na+) out for every two potassium ions (K+) pumped in.
A signal arrives and causes the voltage inside the neuron to become less negative, known as depolarization. If this depolarization reaches a threshold, typically around -55 mV, it triggers the action potential, which is an “all-or-nothing” event. Voltage-gated sodium channels open, allowing a rapid influx of sodium ions into the cell. This rush of positive charge causes the membrane potential to peak sharply, reaching about +30 mV.
Following this peak, the sodium channels inactivate, and voltage-gated potassium channels open. The outflow of potassium ions rapidly returns the cell’s internal charge to a negative value, a phase called repolarization. The potassium channels may remain open briefly, causing a temporary hyperpolarization before the cell returns to its stable resting state, ready for the next impulse.
Chemical Transfer Between Neurons
Once the electrical signal reaches the end of the axon, it must cross the synaptic cleft to communicate with the next cell. This junction, the synapse, relies on chemical messengers called neurotransmitters to bridge the gap. The end of the signaling neuron is the presynaptic terminal, which contains vesicles filled with these neurotransmitters.
When the action potential arrives, it triggers the opening of voltage-gated calcium channels. The influx of calcium ions causes the vesicles to fuse with the cell membrane. This fusion releases the neurotransmitters into the synaptic cleft, a process known as exocytosis.
The released neurotransmitter molecules diffuse across the cleft and bind to specific receptor proteins on the postsynaptic neuron. This binding causes ion channels on the postsynaptic neuron to open, changing the receiving cell’s internal electrical charge. Excitatory messengers like glutamate depolarize the cell, making it more likely to fire an action potential. Inhibitory messengers such as GABA hyperpolarize the cell, making it less likely to generate a signal.
Processing Multiple Signals and Increasing Transmission Speed
Neurons integrate thousands of signals arriving simultaneously from multiple presynaptic neurons. Signal integration, or summation, determines whether a neuron will reach the threshold necessary to fire an action potential. Spatial summation occurs when the postsynaptic neuron adds up incoming signals from several different synapses across its surface at the same moment.
Temporal summation happens when a single presynaptic neuron fires repeatedly in quick succession, allowing successive signals to add together. The neuron algebraically combines all incoming excitatory and inhibitory signals. Only if the total depolarization exceeds the threshold is a new action potential generated. This integration allows the nervous system to make decisions based on a continuous stream of information.
The speed of signal transmission is enhanced by the myelin sheath, the fatty insulation wrapped around many axons. This wrapping prevents electrical current from leaking out, forcing the action potential to “jump” between small, unmyelinated gaps called the Nodes of Ranvier. This mechanism, known as saltatory conduction, greatly increases the speed of impulse propagation. Saltatory conduction allows signals to travel up to 120 meters per second, enabling fast reflexes and complex cognitive functions.