The nervous system coordinates thought, movement, and sensation through the rapid communication between billions of specialized cells called neurons. These cells form complex circuits that transmit information across the body. Neuronal communication is a two-part process: a swift electrical signal travels within a single neuron, followed by a slower chemical signal used to pass the message to the next neuron. This interplay forms the foundation of all neural activity.
Generating the Electrical Impulse
Communication begins within a neuron as the Action Potential (AP), a swift, temporary change in the electrical voltage across the neuronal membrane. The neuron maintains a resting potential, typically around \(-70\) millivolts (mV). This potential is established by the unequal distribution of sodium (\(\text{Na}^+\)) and potassium (\(\text{K}^+\)) ions, actively maintained by the sodium-potassium pump.
A stimulus causes the membrane to depolarize, making the internal voltage less negative. If this depolarization reaches a threshold, often around \(-55\) mV, voltage-gated sodium channels open. This causes a rapid influx of positively charged \(\text{Na}^+\) ions, reversing the membrane potential and creating the rising phase of the action potential.
This electrical event follows the “all-or-nothing” principle: if the threshold is met, the AP fires with full, uniform strength. A stronger stimulus does not create a “bigger” action potential, but rather causes the neuron to fire more frequently. Following the peak, voltage-gated sodium channels inactivate, and potassium channels open. The outflow of positive \(\text{K}^+\) ions rapidly repolarizes the membrane, bringing the voltage back toward the resting potential.
Chemical Transmission Across the Synapse
The electrical signal must convert to a chemical one to bridge the gap between two neurons, a tiny space called the synaptic cleft. When the action potential reaches the axon terminal of the presynaptic neuron, it opens voltage-gated calcium channels. This allows calcium ions (\(\text{Ca}^{2+}\)) to rush into the terminal.
This influx of \(\text{Ca}^{2+}\) directly triggers the release of chemical messengers, known as neurotransmitters. The calcium ions bind to specialized proteins, causing synaptic vesicles (small sacs filled with neurotransmitters) to dock and fuse with the presynaptic membrane.
This fusion event, called exocytosis, empties the vesicle contents into the synaptic cleft. The neurotransmitter molecules diffuse rapidly across the cleft and bind to specific receptor proteins embedded in the postsynaptic neuron’s membrane. This binding successfully transmits the signal and converts it back into an electrical event by causing ion channels on the receiving neuron to open.
Signal Integration: Interpreting Multiple Messages
A single neuron receives thousands of chemical messages simultaneously from various presynaptic neurons. The binding of neurotransmitters generates small electrical fluctuations called postsynaptic potentials, which are the fundamental units of neural computation. These potentials are categorized as either excitatory or inhibitory.
Excitatory Postsynaptic Potentials (EPSPs) cause temporary depolarization, making the internal voltage less negative and increasing the probability of generating an action potential. Conversely, Inhibitory Postsynaptic Potentials (IPSPs) cause hyperpolarization, making the membrane potential more negative and decreasing the likelihood of the neuron firing.
The receiving neuron algebraically sums up all these incoming EPSPs and IPSPs, a process called summation. Summation occurs in two forms: spatial and temporal. Spatial summation combines potentials arriving at different locations simultaneously. Temporal summation occurs when a single presynaptic neuron sends rapid, successive signals that add up. The net result determines whether the membrane potential at the axon hillock reaches the threshold to fire a new action potential.
Factors Affecting Communication Speed
The speed at which the electrical impulse travels down the axon is important for fast reactions and coordinated functions. The primary factor enhancing this speed is the Myelin Sheath, a fatty, insulating layer wrapped around many axons. Myelin prevents ions from leaking out, forcing the electrical signal to travel passively and faster down the insulated segments.
The myelin sheath is not continuous, featuring small, uninsulated gaps called the Nodes of Ranvier. The action potential is regenerated only at these nodes, where voltage-gated sodium channels are highly concentrated. This propagation mechanism, known as saltatory conduction, makes the impulse appear to “jump” from one node to the next, dramatically increasing transmission speed. Myelinated axons can achieve speeds up to \(150\) meters per second, compared to unmyelinated axons which conduct impulses between \(0.5\) and \(10\) meters per second. Axon diameter is also a factor, as wider axons offer less internal resistance, allowing for faster passive conduction.