Transferring the Message: The Synapse

The synapse is the place where one neuron meets another.
The neurons do not physically touch—there is a gap between them.
The message is transferred across this gap from one neuron to another by special chemicals called neurotransmitters.
Therefore, information in the nervous system is transferred:
1. Electrically, along the surface of neurons
2. Chemically, across the synapse between neurons.
This process is repeated again and again as information moves through the nervous system.

Synaptic terminal: Located at the end of axon; there can be hundreds or thousands.
Synaptic vesicle: Organelle which contains neurotransmitters
Presynaptic membrane: Membrane at the end of the synaptic terminal, where the neurotransmitter is released
Synaptic cleft: Gap between the presynaptic membrane and the postsynaptic membrane
Postsynaptic membrane: Membrane of the target cell that has receptors for the neurotransmitter

Figure 48.10, page 971, Campbell's Biology, 5th Edition

An action potential arrives at the end of the axon, causing an influx of calcium ions into the cell.
- This causes the synaptic vesicles to fuse with the presynaptic membrane.
- A neurotransmitter spills out into the synaptic cleft and diffuses toward the postsynaptic membrane.
- The neurotransmitter binds with receptors on the postsynaptic membrane and causes ion channels to open.
Result: either excitation or inhibition of the target cell

Integration of Multiple Synaptic Inputs

A postsynaptic neuron can receive input from hundreds or thousands of other presynaptic neurons.
The input is of two types:
- Excitatory (green)
- Inhibitory (red)
Whether an action potential is generated by all these signals is determined at the axon hillock.
- If the summation of all the input reaches a threshold, then an action potential is generated and travels down the axon.

Figure 48.11(a), page 972, Campbell's Biology, 5th Edition

Integration of the synaptic input is done through a process of summation—the signals are simply added together.
Signals received by the postsynaptic neuron can be summed in one of two ways:
- Temporally: if the signals are received close enough together in time, they can be added.
- Spatially: if the signals are received close enough together in space, they can be added.
Illustrations:
1. Two signals from the same excitatory presynaptic neuron are received, but the temporal spacing of the signal precludes them from generating an action potential.
2. Two separate signals from an excitatory presynaptic neuron are received close enough together in time that they can be (temporally) summed such that they reach the threshold and generate an action potential.
3. Two separate signals from two separate excitatory presynaptic neuron are received close enough together in space that they can be (spatially) summed such that they reach the threshold and generate an action potential.
4. One neuron (green) generates an excitatory postsynaptic potential (EPSP) in the postsynaptic neuron, and another neuron (red) generates an inhibitory postsynaptic potential (IPSP) in the postsynaptic neuron; these potentials are summed such that they cancel each other out, so the threshold is not reached and an action potential is not generated.

Figures 48.12(a), 48.12(b), and 48.12(c), page 973, Campbell's Biology, 5th Edition

	Neurotransmitters	Hormones
Binding method	Bind to a receptor on the cell surface	Bind to a receptor on the cell surface or inside the cell
Effect	The opening of an ion channel in the cell membrane	Either a chemical change in the cell or the translation of a protein
Structure	Simple: modified amino acids, amino acids, short peptides (must be cheap and easy to make, since we need so many of them)	More complicated: modified amino acids, steroids, peptides (can be up to several hundred amino acids long)
Distance travelled	Short: across the synapse	Long: often (but not always) through the circulatory system
Time to effect	Quick-acting (less than a second)	Slow-acting (minutes to hours)

Neuropharmacology: using drugs to change brain states
- Certain drugs look like the neurotransmitters.
- Therefore, the postsynaptic neuron is fooled.
A drug-induced hallucination is a message that gets transmitted without a neurotransmitter.
- Serotonin is a common neurotransmitter in the brain; the brain is full of serotonin receptors located on the dendrites and cell bodies of billions of neurons.
- LSD, mescaline, and psilocybin are drugs that all look like serotonin—they fit into the serotonin receptors and stimulate them.
  - They stimulate neurons randomly.
  - The brain tries to organize this random stimulation and give it structure.
  - The result is a hallucination.

Acetylcholine is probably the most common neurotransmitter in both invertebrates and vertebrates.
It is the only neurotransmitter used in stimulating muscle cells to contract.
Native South Americans discovered that a particular vine extract could be placed on the tips of arrows, causing the prey to die quickly.
- They called the solution curare, which means “poison” in their language.
- Curare functions by binding to the acetylcholine receptors on the muscles.
  - By blocking the receptors from being stimulated by acetylcholine, the muscles cannot contract.
  - Death is usually due to the cessation of breathing.

Unknown source; Modified piece of figure 48.11(a), page 972, Campbell's Biology, 5th Edition

Neurotransmitters like acetylcholine need to be broken down after they bind to their receptors on the postsynaptic neuron.
- This prevents the postsynaptic neuron from being overstimulated.
- The breakdown is accomplished by the enzyme acetylcholinesterase.
The products of the breakdown are reabsorbed into the presynaptic neuron and used to construct new acetylcholine/

Nerve gases like sarin and VX are acetylcholinesterase inhibitors.
By blocking the action of acetylcholinesterase:
- Acetylcholine cannot be broken down.
- Acetylcholine builds up at the synapse.
- Acetylcholine continues to act.
Symptoms:
- Muscle paralysis
- Cessation of breathing
An animation of this process is available.

Many animals use neurotoxins as a defensive or offensive weapon; they act on the synapse in some way.
- This website has more information.
Tetrodotoxin and zombies:
- Wade Davis's book: The Serpent and the Rainbow
- How to Make a Zombie