Neuronal Signaling and Synaptic Transmission

Exam Tip: Remember that the resting potential is negative relative to the outside of the cell. If a question asks about the charge of the interior of a resting neuron, the answer is negative.

The Action Potential: Electrical Signaling

When a neuron receives a stimulus, the membrane potential can change. If the stimulus is strong enough to reach a specific "threshold" (usually around $-55\text{ mV}$), an action potential is triggered. An action potential is an "all-or-none" electrical impulse that travels down the axon.

The lifecycle of an action potential can be broken down into four distinct phases:

1. Depolarization: Upon reaching the threshold, voltage-gated $\text{Na}^+$ channels open rapidly. $\text{Na}^+$ ions rush into the cell, driven by both the concentration gradient and the negative internal charge. This causes the membrane potential to spike toward a positive value (often $+30\text{ mV}$ to $+40\text{ mV}$).
2. Repolarization: At the peak of the action potential, $\text{Na}^+$ channels close (inactivate), and voltage-gated $\text{K}^+$ channels open. $\text{K}^+$ ions rush out of the cell, removing positive charge and bringing the membrane potential back down toward a negative state.
3. Hyperpolarization: Often, the $\text{K}^+$ channels stay open slightly too long, causing the membrane potential to become even more negative than the original resting potential (e.g., $-90\text{ mV}$). This is known as the "undershoot".
4. Return to Rest: The $\text{Na}^+/\text{K}^+$ pump and leak channels work to restore the original resting membrane potential.

The Refractory Period

During and immediately after an action potential, there is a period called the refractory period. During this time, it is difficult or impossible for the neuron to fire another action potential. This ensures that the signal travels in only one direction—from the cell body toward the axon terminals—and prevents the signal from overlapping with itself.

Synaptic Transmission: Chemical Communication

An action potential travels down the axon until it reaches the axon terminal. However, neurons do not physically touch each other. Instead, they are separated by a microscopic gap called the synaptic cleft. To bridge this gap, the electrical signal must be converted into a chemical signal.

The Step-by-Step Process of Transmission

The process of synaptic transmission follows a highly regulated sequence:

1. Calcium Influx: When the action potential reaches the axon terminal, it triggers the opening of voltage-gated calcium ($\text{Ca}^{2+}$) channels. $\text{Ca}^{2+}$ ions rush into the terminal.
2. Neurotransmitter Release: The rise in intracellular $\text{Ca}^{2+}$ causes synaptic vesicles (small membrane-bound sacs filled with chemicals) to fuse with the presynaptic membrane. This releases neurotransmitters into the synaptic cleft via exocytosis.
3. Receptor Binding: The neurotransmitters diffuse across the cleft and bind to specific receptors on the postsynaptic membrane (the membrane of the receiving neuron).
4. Postsynaptic Potentials: The binding of neurotransmitters causes ion channels on the receiving neuron to open, leading to one of two results:
   • Excitatory Postsynaptic Potential (EPSP): If the binding causes $\text{Na}^+$ to enter the cell, the neuron becomes more positive (depolarized), making it more likely to fire an action potential.
   • Inhibitory Postsynaptic Potential (IPSP): If the binding causes $\text{Cl}^-$ to enter or $\text{K}^+$ to leave, the neuron becomes more negative (hyperpolarized), making it less likely to fire an action potential.

Termination of the Signal

To prevent continuous stimulation, the neurotransmitter must be removed from the cleft. This occurs through three primary mechanisms:

• Reuptake: The presynaptic neuron pumps the neurotransmitter back into itself for reuse.
• Enzymatic Degradation: Specific enzymes break down the neurotransmitter in the cleft.
• Diffusion: The neurotransmitter simply drifts away from the synaptic area.

Summary Table: Electrical vs. Chemical Signaling

Worked Example: Imagine a neuron receiving a signal. If it receives ten different excitatory signals (EPSPs) that sum together to push the membrane potential from $-70\text{ mV}$ to $-55\text{ mV}$, the neuron will fire an action potential. However, if it receives five inhibitory signals (IPSPs) that pull the potential down to $-80\text{ mV}$, it will remain silent. This "summation" is how neurons make decisions.