Signal propagation: The movement of signals between neurons
How does information travel?
- Information is sent as packets of messages called action potentials
- Action potentials travel down a single neuron cell as an electrochemical cascade, allowing a net inward flow of positively charged ions into the axon.
- Within a cell, action potentials are triggered at the cell body, travel down the axon, and end at the axon terminal
- The axon terminal has vesicles filled with neurotransmitters ready to be released
- The space between the axon terminal of one cell and the dendrites of the next is called the synapse
The Pre-Synaptic Cell
- Chemicals known as neurotransmitters are stored in membrane-bound vesicles at the axon terminal of neurons
- Membrane proteins on the vesicle bind membrane proteins at the axon terminal to tether the vesicles in place
- A protein known as complexin acts like a brake, and stop the vesicles from fusing into the membrane and releasing their contents [remember: complexin complicates the process of vesicle fusion]
- The vesicle protein synaptotagmin can bind and release complexin in the presence of calcium
- The major excitatory neurotransmitter is glutamate
- The major inhibitory neurotransmitter is GABA (gamma-aminobutyric acid)
- Other well-known neurotransmitters include dopamine, serotonin, adrenaline, and histamine
- Glutamate and GABA are released by nearly all the inhibitory or excitatory neurons of the brain, while specific neurotransmitters are found at precise points of the brain to help aid in learning, memory, motor function, and other neurologic functions.
- Some neurotransmitters also have other functions in different parts of the body (like adrenaline and the stress response, or histamine and allergies)
- The post-synaptic terminal is full of neurotransmitter receptors
- The neurotransmitter receptors are, for the most part, ligand-gated ion channels, that open in response to being bound by the neurotransmitter they are specific for
- Whether a neurotransmitter is excitatory or inhibitory depends on the type of channel they open – excitatory neurotransmitters bind channels that let in positive ions like Na, inhibitory neurotransmitters open up Cl channels
- Some receptors set off a signal cascade when activated, and lead to the transcription of new proteins or the insertion of more receptors into the cell membrane