Synaptic Structures and Communication

Synapses are junctions between the terminal buttons of one neuron's axon and the membrane of another neuron.
Synapses commonly occur on dendrites (smooth surface or dendritic spines), soma, or other axons (Figure 2.21).
The presynaptic membrane faces the postsynaptic membrane across the synaptic cleft (approximately $20$ nm wide).
The synaptic cleft contains extracellular fluid for neurotransmitter diffusion and a filament meshwork for alignment.
Terminal buttons contain mitochondria (energy) and synaptic vesicles (neurotransmitter storage).

Neurons communicate through synaptic transmission, where neurotransmitters carry messages across synapses.
Neurotransmitters, released by the presynaptic cell, diffuse across the synaptic cleft to the postsynaptic cell.
This process induces postsynaptic potentials (depolarizations or hyperpolarizations) that affect the postsynaptic neuron's firing rate.
Neurotransmitters bind to specific binding sites on receptor molecules, acting as ligands.
Ligands can be naturally occurring (neurotransmitters) or artificial (drugs).

Neuromuscular junction: synapse between a terminal button and a muscle fiber.
Small synaptic vesicles contain neurotransmitter molecules.
- Consist of approximately $10,000$ lipid molecules and about $200$ protein molecules.
- Transport proteins fill vesicles; trafficking proteins aid release and recycling.
Located near the release zone of the presynaptic membrane.
Large, dense-core synaptic vesicles contain peptides.
Small synaptic vesicles are produced in the Golgi apparatus in the soma and transported to the terminal button via fast axoplasmic transport, or recycled in the terminal button.
Large synaptic vesicles are produced only in the soma then transported.

Action potentials cause synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitters into the synaptic cleft.
Voltage-dependent calcium channels at the release zone open when the terminal button membrane is depolarized.
Calcium ions (Ca2+) flow into the cell, triggering neurotransmitter release.
Calcium ions bind with proteins, prompting fusion pore formation between vesicle and presynaptic membranes.
Fusion process takes approximately $0.1$ msec.
Three pools of synaptic vesicles:
- Release-ready: Less than $1$ % of total, docked against the presynaptic membrane.
- Recycling pool: $10-15$ % of total.
- Reserve pool: $85-90$ % of total.
Low firing rates use release-ready vesicles; higher rates recruit recycling and reserve pools.
- Docking occurs when protein clusters attach to proteins in the presynaptic membrane.

Kiss and Run: Vesicles release neurotransmitter, reseal, and refill.
Merge and Recycle: Vesicles merge with the presynaptic membrane, form new vesicles, and refill.
Bulk Endocytosis: Large pieces of the terminal button membrane fold inward, break off, and form new vesicles.
Recycling speed (time):
- Readily releasable pool: Less than 1 second.
- Recycling pool: A few seconds.
- Reserve pool: A few minutes.

Neurotransmitters bind to postsynaptic receptors, opening neurotransmitter-dependent ion channels.
This process allows specific ions to pass through the membrane, changing the local membrane potential.
Neurotransmitter molecules do not enter the postsynaptic cell; only ions can enter through ion channels.
Neurotransmitters open ion channels directly or indirectly.

Direct Method: Neurotransmitter-dependent ion channel with its own binding site opens when a neurotransmitter attaches (ionotropic receptor).