Chemical Synapses and Neurotransmission

In the discussion of chemical synapses, we start by visualizing the interaction between two neurons: the presynaptic neuron and the postsynaptic neuron. The synapse is the junction between these two neurons. The presynaptic neuron releases neurotransmitters into the synaptic cleft, where they subsequently bind to receptors on the postsynaptic neuron.

Presynaptic Neuron and Action Potential

The presynaptic neuron's axon terminal, referred to as the synaptic end bulb, is where the action potential travels down the axon until it reaches the end bulb. Upon reaching this point, voltage-gated calcium (Ca²+) channels open in response to the action potential, allowing calcium ions to rush into the synaptic end bulb. The influx of calcium is crucial as it initiates the next phase of neurotransmitter release.

Release of Neurotransmitters

Inside the synaptic end bulb are synaptic vesicles filled with neurotransmitter molecules, specifically acetylcholine (ACh) in this example. When calcium enters the neuron, it interacts with these vesicles, causing them to move toward the presynaptic membrane and eventually fuse with it through a process known as exocytosis. This results in the release of acetylcholine into the synaptic cleft.

Binding to Postsynaptic Receptors

Once acetylcholine is in the synaptic cleft, it diffuses across and binds to ligand-gated receptor channels located on the postsynaptic neuron's membrane. These receptors are specific to acetylcholine, including nicotinic acetylcholine receptors. When acetylcholine binds to these receptors, sodium (Na⁺) channels open, allowing sodium ions to flow into the postsynaptic neuron. This influx of sodium ions causes a depolarization of the membrane potential, leading to the generation of an excitatory postsynaptic potential (EPSP).

Threshold for Action Potential

If the EPSP is strong enough to change the membrane potential to reach the action potential threshold of approximately -55 millivolts, an action potential will be triggered at the axon hillock of the postsynaptic neuron.

Different Types of Receptor Channels

Another consideration in neurotransmitter action involves muscarinic receptors, which can also bind acetylcholine but operate differently compared to nicotinic receptors. Muscarinic receptors can lead to the opening of potassium channels, allowing potassium (K⁺) to exit the neuron. This potassium efflux can cause a hyperpolarizing graded potential known as an inhibitory postsynaptic potential (IPSP), making it less likely for the neuron to reach the action potential threshold and thus inhibit the generation of action potentials.

Summary of Graded Potentials

In summary, neurotransmitter action at the synapse can lead to either excitatory or inhibitory effects on the postsynaptic neuron. An influx of sodium associated with ACh results in an EPSP, leading to membrane depolarization; conversely, potassium efflux due to muscarinic receptor activation may result in an IPSP or membrane hyperpolarization. Understanding these processes is crucial for grasping how synapses facilitate neuronal communication and processing within the nervous system.