Neurons are the fundamental communication units of the brain and nervous system. These specialized cells transmit information throughout the body, from simple reflexes to complex thoughts. Understanding how neurons generate and transmit signals is fundamental to comprehending the nervous system.
The Neuron’s Resting State
A neuron not actively transmitting a signal maintains an electrical difference across its cell 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 electrical difference arises from the unequal distribution of specific ions, particularly sodium (Na+) and potassium (K+), across the membrane. Sodium-potassium pumps actively maintain this imbalance, moving three sodium ions out for every two potassium ions into the cell, contributing to the negative internal charge. This polarized state allows the neuron to respond rapidly to incoming stimuli.
Receiving and Triggering a Signal
Neurons receive incoming signals through their dendrites, specialized branches extending from the cell body. These signals arrive as chemical messengers called neurotransmitters, released from other neurons. Neurotransmitters bind to specific receptors on the dendrite’s membrane, causing small, localized changes in the neuron’s electrical potential.
These changes can be either excitatory, making the inside of the neuron slightly less negative, or inhibitory, making it more negative. A single excitatory signal is insufficient to trigger a response; multiple signals must sum together, either arriving at different times (temporal summation) or from different locations (spatial summation). If the combined effect of excitatory signals reaches the threshold potential, around -55 millivolts, the neuron will initiate an electrical impulse.
The Electrical Impulse: An Action Potential
Once the threshold potential is reached, the neuron generates a rapid, all-or-nothing electrical event called an action potential. This impulse begins with a sudden depolarization phase, where voltage-gated sodium channels in the membrane open, allowing an influx of positively charged sodium ions into the cell. This influx causes the inside of the neuron to become positively charged, briefly reversing the membrane potential to +30 to +40 millivolts.
Following this depolarization, the repolarization phase occurs as voltage-gated sodium channels inactivate, and voltage-gated potassium channels open, allowing potassium ions to flow out of the cell. This outflow of positive charge restores the negative charge inside the neuron. The membrane potential briefly becomes even more negative than the resting potential, a state called hyperpolarization, before returning to its resting state, known as the refractory period, during which the neuron is less likely to fire another action potential.
Sending the Message Onward
After an action potential is generated, it propagates down the axon, a long extension of the neuron, to its terminal end. When the electrical impulse reaches the axon terminal, it triggers the release of chemical messengers called neurotransmitters into a tiny gap known as the synaptic cleft. These neurotransmitters diffuse across the cleft and bind to specific receptor proteins on the membrane of the receiving, or postsynaptic, neuron. This binding can either excite the postsynaptic neuron, making it more likely to generate its own action potential, or inhibit it, making it less likely to fire. To ensure precise communication, neurotransmitters are removed from the synaptic cleft either through reuptake into the presynaptic neuron or by enzymatic degradation, preventing continuous stimulation or inhibition.