The nervous system relies on communication between specialized cells called neurons to process information and control the body. Neurons are the fundamental units responsible for transmitting signals that govern complex thought, memory, muscle movement, and sensation. Understanding how these cells communicate is central to comprehending all bodily functions. This process involves an electrical signal traveling within a single cell and a chemical signal bridging the gap to the next cell.
Generating the Electrical Signal
Communication begins with generating an electrical signal that travels the length of a single neuron. This process relies on maintaining a resting membrane potential, typically between -50 and -75 millivolts (mV), where the inside of the neuron is negatively charged relative to the outside. This negative charge is established by the unequal distribution of ions: sodium ions (\(\text{Na}^{+}\)) are concentrated outside the cell, and potassium ions (\(\text{K}^{+}\)) are concentrated inside. The sodium-potassium pump actively maintains these gradients, constantly moving three \(\text{Na}^{+}\) ions out for every two \(\text{K}^{+}\) ions it moves in.
The electrical signal, known as an action potential, is a rapid, temporary reversal of the resting potential, often swinging to about \(+40\) mV. This event is triggered when incoming signals cause the membrane potential to rise toward a specific threshold potential, often around \(-55\) mV. Reaching this threshold causes voltage-gated sodium channels to open suddenly, allowing a rapid influx of positively charged \(\text{Na}^{+}\) ions into the cell. This rush of positive charge creates the steep rising phase of the action potential, known as depolarization.
Almost immediately after the sodium channels open, they become inactivated, and voltage-gated potassium channels open in response to the positive voltage change. Since \(\text{K}^{+}\) ions are concentrated inside the cell, they rush out, carrying positive charge away. This outward flow causes the membrane potential to return quickly toward its negative resting state, a phase called repolarization. The potassium channels often remain open slightly longer, causing a brief period where the membrane potential dips even lower than the resting potential, a state called hyperpolarization, before ion pumps restore the cell to its original resting state.
The Synaptic Connection Point
Once the electrical signal reaches the end of the neuron, it must cross a specialized junction called the synapse to communicate with the next cell. The vast majority of these junctions use a chemical transmission method. The synapse is not a direct physical connection but a specialized site where the two cells are brought into close proximity.
The synapse consists of three main components. The presynaptic terminal, the axon terminal of the sending neuron, contains the machinery for releasing chemical messengers. Separating the neurons is the synaptic cleft, a narrow gap approximately 20 to 30 nanometers wide that the signal must bridge. The final component is the postsynaptic membrane, located on the dendrite or cell body of the receiving neuron, which is equipped with specialized receptors.
Chemical Transmission Across the Gap
The arrival of the action potential at the presynaptic terminal initiates the conversion from an electrical signal to a chemical one. This impulse causes voltage-gated calcium ion (\(\text{Ca}^{2+}\)) channels in the terminal membrane to open. Since the concentration of \(\text{Ca}^{2+}\) is higher outside the cell, these ions rush inward, raising the calcium concentration inside the terminal.
The influx of calcium acts as the trigger for signal release, binding to sensor proteins within the terminal. This binding causes synaptic vesicles—small sacs filled with chemical messengers called neurotransmitters—to fuse with the presynaptic membrane. This fusion process, known as exocytosis, releases the neurotransmitters into the synaptic cleft.
Once released, neurotransmitter molecules diffuse across the cleft and bind to specific receptor proteins on the postsynaptic membrane. This binding acts like a key fitting into a lock, causing the receptor to change shape and resulting in the opening of ion channels in the receiving neuron. To prevent continuous signaling, the neurotransmitter signal must be terminated rapidly. Termination occurs either by being broken down by enzymes in the cleft or by being transported back into the presynaptic terminal for recycling (reuptake).
Processing the Incoming Information
The binding of neurotransmitters generates a local electrical response in the receiving neuron. If the binding causes positively charged ions to flow into the cell, it results in a small depolarization, pushing the neuron closer to its firing threshold. This temporary depolarization is called an Excitatory Postsynaptic Potential (EPSP). Conversely, if the neurotransmitter causes the flow of ions that makes the cell interior more negative, it results in hyperpolarization, making the neuron less likely to fire. This is known as an Inhibitory Postsynaptic Potential (IPSP).
A single neuron receives hundreds or thousands of EPSPs and IPSPs simultaneously from many different presynaptic terminals. The cell must integrate all these inputs to decide whether to generate its own action potential and continue the signal. This integration occurs through summation, which takes place at a specialized region of the cell body.
Summation involves adding up the effects of all incoming signals, both excitatory and inhibitory. Spatial summation occurs when the effects of multiple signals arriving from different synapses at the same time are combined. Temporal summation occurs when a single presynaptic neuron fires repeatedly in quick succession, causing its EPSPs or IPSPs to pile up over time. The net result of this algebraic summation determines the outcome: if the combined excitatory signals are strong enough to overcome any inhibition and reach the necessary threshold, the receiving neuron will fire its own action potential.