Part 2: The Four Steps

Chemical synapse

Presynaptic neuron on one side does not immediately touch postsynaptic neuron, there is blank space junction where NTs are released & stimulated by AP

Electrical synapse

Fused presynaptic and postsynaptic membrane that allows an Ap to pass directly from one neuron to the next

For EX: this happens in crayfish

What are the four steps in order to a Neurotransmission

1. Synthesis

2. Release

3. Receptor Action

4. Inactivation

these can happen simultaneously just must happen in order

Explain the first step in Neurotransmission, synthesis

NTs derived from food or DNA instructions

NTs are transported from cell nucleus to the terminal button…others, are made from building blocks imported into the terminal and packaged into vesicles

• an individual neuron can make one to several different NTs

• Vesicles can hold one or both NTs depending on size

Explain the second step in Neurotransmission: Release

An AP sets this process into motion opening calcium channels that cover the axon terminal membrane.

Fluid surrounding the cell is filled with Ca2+ which will fill the cell when channels are open, allowing vesicle release

Explain endocytosis in the release stage of Neurotransmission

Vesicles can be refilled, defuse from membrane

Regulated by rate of neuron firing, might not undergo exocytosis

Explain exocytosis in stage 2 of Neurotransmission: release

The transmitter, in response to an AP, is released across the membrane by exocytosis

Describe the third step in Neurotransmission: Receptor Action

NT receptors are proteins located in cell membranes.

The transmitter binds to the receptor to activate it, leading to an excitatory or inhibitory effect

Describe transmitter-activated receptors

• autoreceptors (presynaptic)

• Post synaptic

They have binding sites for a specific NT

Autoreceptors: indicate to the cell if it needs to release more or less NT the next time it fires

Postsynaptic: NTs excite/inhibit cells membrane, increasing or decreasing the chance of an AP

What determines if the AP effect is excitatory or inhibitory

Receptor subtypes

Describe ionotropic receptors

Allow NTs to open a channel directly

• contains binding sites, opening or closing ion channel

• Direct, simple effects

Metabotropic

Have more indirect effects on cell membrane

Also called G-protein coupled receptor, act more slowly but last longer

What is the function of a G-protein

They are molecular switches on cell allowing messages in/out of the cell

Describe the fourth step in Neurotransmission: Inactivation and the four ways this happens

NT is cleared from synaptic clef

Can be through diffusion, degradation by enzymes, reuptake, or glial cells

Describe inactivation when done by degradation of enzymes

NTs are broken down into their component parts so they can’t fit/bind to receptor sites

• typically paired with reuptake or glial cells

What is the four criteria to be considered a NT

• chemical made or stored inside the neuron

• chemical produces a response in target cell

• if the chemical was placed on a receptor not by an AP there would be the same effect

• there must be a means for inactivation

Over 100 chemicals can act as NTs