Psyc 255 : Neural communication - chemical

What happens when an AP reaches the end of the axon?

• It sends a signal to the next neuron

• This signal is sent via chemicals across a synapse

Two structural classes of chemical synapse

Type 1 synapse

• Excitatory

• Typically on dendrites

• Large active zone

• Wide cleft

• Round vesicles

• Dense material on membranes

Type 2 synapse

• Inhibitory

• Typically on cell body

• Small active zone

• Narrow cleft

• Flat vesicles

• Sparse material on membranes

Synapse locations vary

• Axodendritic synapse : the axon terminal of a neuron synapses with a dendrite or dendritic spine of another neuron

• Axomuscular synapse : an axon synapses with a muscle end plate, releasing acetylcholine

A chemical message in 5 steps

1. AP arrival

2. NT release

3. NT crosses synapse

4. Receptor action

5. Inactivation

Step 1 : The AP arrives

• When the AP arrives at the axon terminal, voltage-gated Ca2+ channels open

• Ca2+ enters axon terminal

Step 2 : NT release

The influx of Ca2+ does 2 main things

1. It triggers a calcium dependent enzyme to detach vesicles from microfilaments

• These vesicles move to the active zone at the cell membrane

2. At the same time, Ca2+ binds to proteins, forming a complex(a tool formed when the two molecules temporarily join to perform a job)

• The complex causes vesicles in the active zone to fuse with the cell membrane, releasing NTs into the synaptic cleft. This process is called exocytosis

Step 3 : NT crosses the synapse

• NTs diffuse across the synaptic cleft

• When they reach the postsynaptic membrane, NTs bind with receptors

  • NT - chemical messenger

  • Receptor - protein that responds to that chemical messenger

• Like a key finding its lock

Step 4 : Receptor action

• What happens when the NT binds with a receptor ?

  • It passes on a message by opening or closing gated channels

• The message depends on the receptor

  • Two major receptor types: ionotropic and metabotropic

  • Two major signals : excitatory and inhibitory

Receptors : Ionotropic vs Metabotropic

Ionotropic : These receptors have an ion channel (Ligand-gated ion channels)

• NT binds → ion channel opens or closes

• Fast acting, direct action

Metabotropic : Don’t have a channel themselves (G-protein coupled receptors)

• NT bind → G portein activated →. intracellular signal → ion channel might open or close

• Slower, indirect, prolonged

Receptors : Excitatory vs. Inhibitory

• Ionotropic and metabotropic receptors can be inhibitory or excitatory

  • When activated, they can change the neuron’s membrane potential, it depends on which ion channels they’re associated with

• Excitatory example

  • If the NT affects Na+ or Ca++ ion channels:

    • Sodium/calcium influx → membrane becomes less negative

• Inhibitory example

  • If the NT affects Cl- or K+ ion channels :

    • Cl- enters/K+ → membrane becomes more negative → hyperpolarisation