SNAREs are proteins that tether. v-SNAREs are tethered to vesicles. t-SNAREs are tethered to the presynaptic membrane and attach to v-SNAREs to begin the fusion process. Synaptotagmin are attached to the vesicles and help finish the fusion process.

Proteins Involved in Exocytosis

Since exocytosis describes the release of neurotransmitters from vesicles with the help of enzymes during synaptic transmission, it is understandable that the following three things affect the rate of exocytosis: 1.) The rate of production of neurotransmitters, 2.) The rate of production of vesicles, and 3.) The activity of the neuron (i.e., the frequency of action potentials). Normally, the axon terminal contains enough neurotransmitters and vesicles to prepare itself for an action potential. These neurotransmitters and vesicles are produced in the cell body with the help of enzymes and transported to the axon terminal. 

The Rate of Exocytosis

The constitutive secretory pathway is a continuous transport process in all cells where secretory vesicles carry soluble proteins to the cell membrane and release them to the exterior without requiring external triggers.

Constitutive Secretory Pathway

The regulated secretory pathway is a specialized transport process in which cells store soluble proteins within secretory vesicles and only release them to the cell's exterior in response to specific extracellular events or signals.

Regulated Secretory Pathway

Histamine secretion by mast cells is an example of regulated secretion where these immune cells release histamine—a compound that triggers immune defense mechanisms to clear pathogens—in response to the detection of foreign objects or pathogens.

Histamine Secretion by Mast Cells

Synaptic transmission is the process by which a neuron communicates with an adjacent one. It begins when an action potential reaches the axon terminal, causing neurotransmitters to be released into the synaptic cleft. These chemicals then travel across the synapse and bind to receptors on the next neuron's dendrites. If this signal is potent enough, it will trigger a new action potential, thus propagating the nerve impulse.

Synaptic Transmission

Exocytosis is the process by which secretory vesicles move toward and fuse with the plasma membrane to release their contents outside the cell. Cells utilize exocytosis to eliminate waste products and to secrete chemical signals that other cells can receive.

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The passing of information received from an action potential through the synapse. It occurs in the following steps:
- Exocytosis of neurotransmitter-containing vesicles occurs as the incoming action potential depolarizes the presynaptic membrane, leading to a rush of Ca 2+ entering the terminal
- As a result of exocytosis, transmitters are released into the synaptic cleft and bind with receptor molecules in the postsynaptic membrane
- These bindings cause ion channels in the postsynaptic membrane to open, creating a local EPSP or IPSP in the postsynaptic neuron
- The effects of the released transmitters are halted through either degradation or reuptake
- Some transmitters bind to autoreceptors, which further regulates the release of transmitters

The endomembrane system is composed of several interconnected organelles and structures, which include the endoplasmic reticulum, the Golgi apparatus, lysosomes, and vesicles.

Components of the Endomembrane System

In eukaryotic cells, the cytoskeleton is an internal network of fibers and tubes consisting of microfilaments, intermediate filaments, and microtubules. It provides structural support, anchors organelles, and serves as a transport network to move materials within the cell.

Eukaryotic Cytoskeleton

Breedlove, S. & Watson, N. (2017). Behavioral neuroscience, eighth edition. Oxford University Press.

Behavioral Neuroscience 8th Edition

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Microbiology covers the scope and sequence requirements for a single-semester microbiology course for non-majors. The book presents the core concepts of microbiology with a focus on applications for careers in allied health. The pedagogical features of the text make the material interesting and accessible while maintaining the career-application focus and scientific rigor inherent in the subject matter. Microbiology’s art program enhances students’ understanding of concepts through clear and effective illustrations, diagrams, and photographs.

Microbiology is produced through a collaborative publishing agreement between OpenStax and the American Society for Microbiology Press. The book aligns with the curriculum guidelines of the American Society for Microbiology.

OpenStax Microbiology Textbook

After the transmitters have been released due to exocytosis, they bind to proteins called receptor molecules (or receptors). This binding is similar to how a key fits into a lock. If a transmitter, known as a ligand during this process, is not the correct shape, it will not be able to bind with a receptor. If a transmitter is the correct shape, it will be able to bind with the receptor at ligand-binding sites. Each transmitter is able to bind to a group of different receptors.

Receptor Molecules

The binding of transmitters to receptors causes the ion channels to open.

Ion Channels Open

A receptor in the presynaptic membrane that regulates how much transmitter is released. It tells the presynaptic cell the amount of transmitter that has been released into the synaptic cleft, and it can use this information to adjust the amount of transmitter it releases in the future.

Autoreceptors

The classic synapses are axo-dendritic synapses (i.e., synapses that forms between a dendrite and an axon) or axo-somatic synapses (i.e., synapses that form between the axon and the cell body or soma). However, there are many other different types of synapses. Synapses can vary by location on the neuron as well as by the mechanism of synaptic transmission (i.e., electrical or chemical transmission). 

Types of Synapses

Ectopic transmission is a form of cell-cell communication between neurons that involves the release of neurotransmitters at locations other than traditional synapses. In this form of neural communication, neurotransmitters are act on regions other than traditional synapses. 

Ectopic Transmission

Once a neural signal is delivered, excess neurotransmitters must be removed from the synaptic cleft in a process known as clearing the synapse. This clearance occurs through three primary mechanisms: the neurotransmitters may drift away, be broken down into inactive fragments, or be reabsorbed into the presynaptic neuron via reuptake. Clearing the synapse is essential because it establishes a distinct "on" and "off" state between signals and helps regulate the overall production of neurotransmitters.

Clearing the Synapse

Exocytosis

Transport vesicles are small, membranous spheres composed of a single lipid bilayer with a hollow interior that function to carry molecules within a cell.

Transport Vesicles

Lysosomes are membrane-bound organelles belonging to the endomembrane system that contain digestive enzymes. They function to break down external particles, such as food, microorganisms, and immune complexes, as well as internal debris like damaged organelles, while keeping these enzymes compartmentalized to protect the cytoplasm.

Lysosomes

Microfilaments are dynamic eukaryotic cytoskeletal cables, measuring about 6 nm in diameter, composed of two intertwined strands of actin monomers that polymerize and depolymerize in response to cellular signals.

Microfilaments

Intermediate filaments are diverse, relatively permanent cytoskeletal fibers with a diameter of $${}10\text{ nm}$$ (thicker than actin but thinner than microtubules) that are composed of multiple strands of polymerized subunits and function to maintain the position of the nucleus, form the nuclear lamina, and anchor cells together in animal tissues.

Intermediate Filaments

Microtubules are dynamic eukaryotic cytoskeletal structures composed of tubulin dimers (alpha-tubulin and beta-tubulin) that form hollow tubes with a diameter of $${}23\text{ nm}$$. They serve as internal support structures, transport tracks for organelles and vesicles, components of flagella and cilia, and form the mitotic spindle during cell division.

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