The nervous system uses electrical impulses, formally known as action potentials, as its fundamental method of communication. These impulses are brief, rapid shifts in the electrical voltage across a cell’s membrane that travel quickly along the length of a nerve cell. The entire process, from sensing the environment to coordinating muscle movement and forming thoughts, relies on the swift transmission of these electrical signals. These rapid voltage changes allow information to be relayed over long distances within the body.
The Cells That Generate Signals
The biological unit responsible for generating and propagating these electrical signals is the neuron, or nerve cell. Neurons possess a distinct anatomy designed for communication, consisting of a central cell body and two types of extensions: dendrites and an axon. Dendrites are branch-like structures that receive signals from neighboring cells, while the single, long axon transmits the signal away from the cell body toward a target (which can be another neuron, a muscle, or a gland).
The speed and efficiency of impulse transmission are enhanced by specialized support cells known as glial cells. In the peripheral nervous system, Schwann cells wrap tightly around the axon to create a myelin sheath, a fatty layer that acts as an electrical insulator. In the central nervous system, oligodendrocytes perform this same insulating function. The myelin sheath prevents the electrical signal from dissipating and dramatically increases the speed at which the impulse travels down the axon.
The myelin sheath is not continuous, featuring small, periodic gaps along the axon known as the Nodes of Ranvier. These uninsulated regions are where the electrical signal is effectively “recharged” as it jumps from one node to the next, a process called saltatory conduction. This mechanism allows nerve impulses to be conducted at high speeds.
The Step-by-Step Mechanism of Impulse Firing
The generation of an electrical impulse begins with the neuron maintaining a resting potential, which is a negative electrical charge across its membrane, typically around -70 millivolts (mV). This negative charge is established by the unequal distribution of ions, with a higher concentration of positively charged sodium ions (\(\text{Na}^{+}\)) outside the cell and a higher concentration of potassium ions (\(\text{K}^{+}\)) inside the cell. The cell maintains this resting state through the sodium-potassium pump, which actively transports three \(\text{Na}^{+}\) ions out for every two \(\text{K}^{+}\) ions it moves in.
An action potential is triggered only when the neuron receives enough input to raise its membrane potential to a specific threshold, often around -55 mV. Once this threshold is reached, a rapid sequence of events begins, driven by voltage-gated ion channels embedded in the cell membrane. The initial event is the opening of numerous voltage-gated sodium channels, causing a sudden and massive influx of positive \(\text{Na}^{+}\) ions into the cell.
This sudden rush of positive charge is called depolarization, which quickly reverses the membrane potential from negative to positive. The electrical signal is an “all-or-nothing” event, meaning that if the threshold is met, the impulse fires with a consistent magnitude. At the peak of depolarization, the voltage-gated sodium channels quickly inactivate, stopping the flow of \(\text{Na}^{+}\) into the cell.
Almost immediately, the slower-acting voltage-gated potassium channels open, allowing \(\text{K}^{+}\) ions to flow rapidly out of the cell. This outward movement of positive charge is called repolarization, and it restores the negative charge inside the cell. The potassium channels often remain open slightly too long, causing the membrane potential to briefly dip below the resting potential, a state called hyperpolarization. This hyperpolarization contributes to the refractory period, during which the neuron is resistant or unable to fire another impulse, ensuring the signal travels in only one direction.
How Impulses Communicate Across the Body
Once the electrical impulse has traveled the length of the axon, it reaches the axon terminal, where the signal must be passed to the next cell. This transfer of information occurs at a specialized junction called a synapse, which consists of a small gap, the synaptic cleft, between the presynaptic neuron and the postsynaptic cell. The arrival of the action potential at the terminal triggers the opening of voltage-gated calcium channels.
The influx of calcium ions into the axon terminal causes tiny sacs called synaptic vesicles, which are filled with chemical messengers known as neurotransmitters, to fuse with the cell membrane. Neurotransmitters are then released into the synaptic cleft, where they diffuse across the gap. These molecules bind to specific receptor proteins on the membrane of the postsynaptic cell, which may be another neuron, a muscle cell, or a gland cell.
The binding of neurotransmitters to the postsynaptic receptors causes ion channels to open or close, generating a change in the electrical potential of the receiving cell. This effect can be excitatory, pushing the postsynaptic neuron closer to its firing threshold, or inhibitory, making it less likely to generate its own action potential. The postsynaptic cell integrates all these incoming signals, and if the net effect is sufficiently excitatory to reach the threshold, it will generate a new electrical impulse.
This continuous chain of electrical impulses and chemical transmission underlies all nervous system functions, from rapid reflexes to the complex processing involved in memory and decision-making. The precise timing and location of neurotransmitter release ensure that signals are accurately conveyed, allowing the body’s various systems to be coordinated and controlled.