AP Biology: Nervous System Signal Transmission

The ability to think, feel, and move hinges on the precise electrochemical language of neurons. Understanding signal transmission—the process by which neurons communicate—is not just a cornerstone of AP Biology but the very foundation for grasping everything from reflexes to consciousness. The elegant, rapid-fire events of the action potential and the crucial chemical handoff at the synapse explain how your nervous system encodes complex information in a simple electrical code.

Neuron Structure and Resting Potential

Before a signal can be sent, the neuron must be ready to fire. A typical neuron consists of a cell body (soma), branching dendrites that receive signals, and a long axon that transmits them. The signal travels from the dendrites, through the cell body, down the axon, and finally to the axon terminals, which connect to the next cell.

Critical to this readiness is the resting membrane potential, typically around $-70\text{ mV}$ inside the cell relative to the outside. This negative charge is maintained by the sodium-potassium pump (${\text{Na}}^{+}/{\text{K}}^{+}$ ATPase), which actively transports three sodium ions (${\text{Na}}^{+}$) out for every two potassium ions (${\text{K}}^{+}$) it pumps in, and by the differential permeability of the membrane. At rest, the membrane is much more permeable to ${\text{K}}^{+}$ than to ${\text{Na}}^{+}$, allowing ${\text{K}}^{+}$ to leak out down its concentration gradient, which contributes significantly to the negative interior.

The Action Potential: A Wave of Depolarization

An action potential is a rapid, temporary reversal of the membrane potential from about $-70\text{ mV}$ to about $+30\text{ mV}$, which then propagates like a wave down the axon. It is an all-or-none event; once the threshold is reached, it fires completely. This process is governed by voltage-gated ion channels that open or close in response to changes in membrane voltage.

1. Depolarization to Threshold: A stimulus causes some ${\text{Na}}^{+}$ channels to open. If the influx of ${\text{Na}}^{+}$ is sufficient to depolarize the membrane to the threshold potential (around $-55\text{ mV}$), it triggers the massive opening of voltage-gated sodium channels.
2. Rising Phase: Voltage-gated sodium channels open fully, allowing a torrent of ${\text{Na}}^{+}$ to rush into the cell down its electrochemical gradient. This causes rapid depolarization, peaking near $+30\text{ mV}$.
3. Falling Phase (Repolarization): The voltage-gated sodium channels quickly become inactivated. Simultaneously, slower voltage-gated potassium channels open, allowing ${\text{K}}^{+}$ to rush out of the cell, repolarizing the membrane back toward its resting potential.
4. Hyperpolarization (Undershoot): The potassium channels are slow to close, so an excess of ${\text{K}}^{+}$ leaves the cell, temporarily making the interior more negative than the resting potential (e.g., $-80\text{ mV}$). This refractory period ensures the action potential travels in one direction and limits firing frequency.

The action potential propagates unidirectionally because the region behind it is in a refractory period. The influx of ${\text{Na}}^{+}$ at one point depolarizes the adjacent segment to threshold, causing the wave to move forward.

Synaptic Transmission: The Chemical Relay

When the action potential reaches the axon terminal, it must cross the synaptic cleft—the tiny gap between neurons (or between a neuron and a muscle/gland). This is achieved through synaptic transmission, a chemical process.

1. Arrival and Calcium Influx: The depolarization from the action potential opens voltage-gated calcium (${\text{Ca}}^{2+}$) channels in the terminal. ${\text{Ca}}^{2+}$ rushes into the terminal.
2. Vesicle Fusion and Neurotransmitter Release: The influx of ${\text{Ca}}^{2+}$ causes synaptic vesicles—membranous sacs filled with neurotransmitters (e.g., acetylcholine, dopamine, serotonin)—to fuse with the presynaptic membrane and release their contents into the synaptic cleft via exocytosis.
3. Receptor Binding and Postsynaptic Potential: The neurotransmitters diffuse across the cleft and bind to specific ligand-gated receptor proteins on the postsynaptic membrane. This binding opens ion channels, causing a local change in the postsynaptic cell's membrane potential. If it's an excitatory synapse (e.g., using glutamate), ${\text{Na}}^{+}$ channels open, causing a small excitatory postsynaptic potential (EPSP), which depolarizes the cell, making an action potential more likely. If it's an inhibitory synapse (e.g., using GABA), it may open ${\text{Cl}}^{-}$ channels, causing an inhibitory postsynaptic potential (IPSP), which hyperpolarizes the cell, making an action potential less likely.
4. Termination of Signal: To prevent constant stimulation, the signal must be terminated. This occurs via reuptake (neurotransmitter pumped back into the presynaptic neuron), enzymatic degradation in the cleft (e.g., acetylcholinesterase breaks down acetylcholine), or simple diffusion away.

Encoding Signal Strength and Neural Integration

Since a single action potential is all-or-none, the intensity of a stimulus (e.g., a bright light vs. a dim one) cannot be encoded by the size of the action potential. Instead, signal strength is encoded by firing frequency. A stronger stimulus causes a higher frequency of action potentials per unit time. For instance, a gentle touch might generate 10 action potentials per second, while a painful pinch might generate 100 per second.

A single neuron receives thousands of synaptic inputs, both excitatory and inhibitory. Neural integration is the process by which the postsynaptic neuron sums all these EPSPs and IPSPs, both temporally (rapid successive signals from one synapse) and spatially (signals from many different synapses), at the axon hillock. Only if the summated depolarization reaches threshold will an action potential be initiated in the postsynaptic neuron. This integration is the fundamental basis of neural computation and decision-making.

Common Pitfalls

• Confusing Depolarization with an Action Potential: Depolarization is any change making the membrane less negative. An action potential is a specific, all-or-none sequence triggered when depolarization reaches threshold. Not all depolarizations lead to an action potential.
• Misunderstanding the "All-or-None" Law: This law applies to the action potential in a single neuron once threshold is reached. It does not mean all stimuli produce the same neural response. The nervous system varies output by changing which neurons fire and how often they fire.
• Attributing the Resting Potential Only to the Sodium-Potassium Pump: While the pump is essential for maintaining concentration gradients, the immediate cause of the resting potential is primarily the diffusion of ${\text{K}}^{+}$ out of the cell. The pump works continuously to counteract the slow leakage of ${\text{Na}}^{+}$ in and ${\text{K}}^{+}$ out.
• Thinking Neurotransmitters Are Always Excitatory: A neurotransmitter's effect is determined by the receptor it binds to on the postsynaptic cell. Some neurotransmitters, like GABA, are predominantly inhibitory. Others, like acetylcholine, can be excitatory at a neuromuscular junction but inhibitory at the vagus nerve.

Summary

• Neurons maintain a resting membrane potential of about $-70\text{ mV}$ through the action of the sodium-potassium pump and differential ion permeability.
• An action potential is a rapid, all-or-none depolarization propagated along the axon by the sequential opening of voltage-gated sodium channels (causing depolarization) and voltage-gated potassium channels (causing repolarization).
• At the synapse, the electrical signal is converted to a chemical one: an action potential triggers neurotransmitter release, which binds to receptors on the postsynaptic cell, generating either an excitatory (EPSP) or inhibitory (IPSP) signal.
• Signal strength is encoded in the frequency of action potentials, not their amplitude.
• The postsynaptic neuron integrates thousands of EPSPs and IPSPs through summation; if the net depolarization at the axon hillock reaches threshold, a new action potential is initiated.