4.3 b Video Postsynaptic Potentials and Neuronal Signaling Integration

Release and Binding Process: When neurotransmitters are released from the presynaptic axon terminal, they diffuse across the synaptic cleft to reach the postsynaptic membrane.
Location of Receptors: These receptors are situated on the membrane of the postsynaptic cell, directly across from the synaptic cleft.
Types of Postsynaptic Receptors: * Chemically Gated Ion Channels (Ligand-Gated Ion Channels): These channels open directly in response to the binding of a neurotransmitter. This allows for immediate ion flux through the postsynaptic membrane, leading to a change in flow that results in either hyperpolarization or depolarization. Examples include specific receptors for Acetylcholine ( $ACh$ ), Glutamate, and $GABA$ . * G-Protein Coupled Receptors (Metabotropic Receptors): These do not open a channel directly. Instead, binding activates G-proteins that can influence or activate/inhibit specific effector enzymes or specific ion channels. This can lead to the phosphorylation of proteins within the cell and other specified cellular effects. These receptors change the excitability of the cell. An example is the class of Muscarinic receptors found in the autonomic nervous system.

Definition: A graded potential is a change in the membrane potential that decreases with distance. Because of this decrease, they are considered localized responses.
Characteristics: * They vary in size (can be small or large). * They vary in nature (can be excitatory or inhibitory). * They are summed at the axon hillock to determine if an action potential will be elicited.

Mechanism: A localized depolarization caused by the opening of ligand-gated Sodium ( $Na^+$ ) or Calcium ( $Ca^{2+}$ ) channels on the postsynaptic membrane.
Ion Flux: This leads to an influx of these positive ions into the cell.
Effect on Membrane Potential: The depolarization spreads toward the axon hillock, moving the membrane potential closer to the threshold potential.
Requirement for Action Potential: Usually, a single $EPSP$ is insufficient; multiple $EPSPs$ are required to reach the threshold to initiate an action potential.
Examples of Excitatory Neurotransmitters: * Acetylcholine ( $ACh$ ): Specifically at the neuromuscular junction in the Peripheral Nervous System ( $PNS$ ). * Glutamate: Primary excitatory neurotransmitter in the Central Nervous System ( $CNS$ ).

Mechanism: Occurs when a neurotransmitter binds to cause hyperpolarization of the postsynaptic membrane.
Ion Flux: This is characterized by the opening of Chloride ( $Cl^-$ ) channels or Potassium ( $K^+$ ) channels. The movement of these ions makes the internal environment of the cell more negative.
Effect on Membrane Potential: The localized hyperpolarization spreads toward the axon hillock and moves the membrane potential away from the threshold potential, making it significantly harder to produce an action potential.
Examples of Inhibitory Neurotransmitters: * $GABA$ and Glycine: These are specifically found in the Central Nervous System ( $CNS$ ) and are not present in the Peripheral Nervous System ( $PNS$ ).

Neural Integration: A single postsynaptic neuron receives inputs from thousands of other neurons simultaneously through its various dendrites. The timing and location of these multiple inputs determine whether an action potential is fired.
Temporal Summation: * Relates to the firing rate of a single neuron. * Occurs when a second stimulus is received from the same synapse shortly after the first. * The second stimulus adds to the first, potentially reaching the threshold for action potential propagation.
Spatial Summation: * Relates to simultaneous input from multiple different neurons. * Action potentials propagating down different axons arrive at the postsynaptic cell at the same time, leading to neurotransmitter release at multiple sites. * The combined excitation of these different regions can reach the threshold for the postsynaptic neuron.

The Trigger Zone: The axon hillock connects to the initial segment of the axon. It serves as the site where all graded potentials (both $EPSPs$ and $IPSPs$ ) are integrated or summed.
Threshold Dynamics: The total change in membrane potential determines if the threshold is reached. For example, moving from a resting potential of $-70\,mV$ to $-60\,mV$ triggers the process.
All-or-Nothing Principle: Action potentials are all-or-nothing; once the threshold is reached, the action potential will fire.
The Balance of Signals: * If signals are dominated by $EPSPs$ , the cell reaches or gets close to the threshold. * If signals are dominated by $IPSPs$ , the threshold will not be reached due to hyperpolarization.
Facilitation: If $EPSPs$ bring the membrane potential closer to the threshold but do not reach it, the cell is "facilitated." In this state, the cell is more easily excited by subsequent $EPSPs$ because it requires less of an increase in potential to reach the firing point.

Propagation: Action potentials are the primary form of neuronal communication, allowing signaling from neuron to neuron and cell to cell across long distances, including from the brain, down the spinal cord, and into the peripheral nervous system.
Key Summary Points: * Postsynaptic receptors are activated by neurotransmitters using direct ion channels or second messenger systems. * Graded potentials decrease with distance and sum at the axon hillock. * $EPSPs$ involve $Na^+$ or $Ca^{2+}$ channels (depolarization). * $IPSPs$ involve $Cl^-$ or $K^+$ channels (hyperpolarization). * Success at the axon hillock leads to action potential initiation in the initial segment of the axon.