Neurons are the fundamental communication units of the nervous system. This communication relies on chemical messengers called neurotransmitters. Understanding how these chemical signals lead to electrical responses within a neuron is key to comprehending brain function.
The Neuron’s Electrical Foundation
A neuron, when not actively transmitting a signal, maintains an electrical charge difference across its membrane, known as the resting membrane potential. This potential ranges from -70 to -80 millivolts, with the inside of the neuron more negatively charged than the outside. This charge difference is established and sustained by ion pumps, particularly the sodium-potassium pump. This pump actively moves three sodium ions (Na+) out of the cell for every two potassium ions (K+) it brings in, using ATP.
The unequal distribution of ions and the membrane’s selective permeability through “leak” channels contribute to this negative resting state. For instance, the neuronal membrane is more permeable to potassium ions than sodium ions at rest, allowing positive potassium ions to leak out. For a neuron to generate an electrical signal, its membrane potential must shift from this resting state to a specific voltage level called the threshold potential, usually around -50 to -55 millivolts.
How Excitatory Signals Influence a Neuron
When an excitatory neurotransmitter stimulates a neuron, it initiates events at the synapse, the specialized junction between neurons. These neurotransmitters are released from the presynaptic neuron, diffuse across the synaptic cleft, and bind to specific receptor proteins on the postsynaptic neuron’s membrane.
This binding causes ion channels on the postsynaptic membrane to open. For many excitatory neurotransmitters, such as glutamate or acetylcholine, this allows positively charged ions, primarily sodium ions (Na+), to flow into the neuron. The influx of these positive charges makes the inside of the neuron’s membrane less negative, a process known as depolarization. This localized voltage change is termed an excitatory postsynaptic potential (EPSP).
Reaching the Firing Threshold
A single, weak excitatory input often produces an EPSP too small to reach the neuron’s firing threshold. Neurons overcome this by integrating multiple inputs through summation. Two main types of summation allow sub-threshold EPSPs to combine and collectively reach the threshold.
Temporal summation occurs when a single presynaptic neuron rapidly fires multiple times in quick succession. Each successive neurotransmitter release causes an EPSP that adds to the lingering effect of the previous one, building up depolarization over time. Spatial summation involves simultaneous inputs from multiple presynaptic neurons converging on a single postsynaptic neuron. The EPSPs generated by these different inputs can combine to collectively depolarize the neuron to its threshold.
The Action Potential: The Neuron’s Response
Once the neuron’s membrane potential reaches the threshold, an electrical signal known as an action potential is generated. This event operates on an “all-or-none” principle: if the threshold is met, a full-strength action potential fires, regardless of how much the membrane potential exceeded the threshold. A stronger stimulus will not produce a larger action potential, but it might trigger more frequent action potentials.
The action potential involves a rapid sequence of depolarization and repolarization. Initially, voltage-gated sodium channels open, leading to a rapid influx of sodium ions and a sharp rise in the membrane potential to a positive value, around +30 mV. Sodium channels then inactivate, and voltage-gated potassium channels open, allowing potassium ions to flow out of the cell. This outward movement of positive charge quickly repolarizes the membrane, often briefly hyperpolarizing it before it returns to its resting state. This electrical signal then propagates along the neuron’s axon, transmitting information to subsequent neurons or target cells.