In this lesson we look at synaptic connections in the brain and neurotransmission.
Key Concepts to Remember from this Lesson:
- Your brain is made of tens of billions of neurons, and the "average" neuron in your brain will be connected to many thousand other neurons via chemical synapses.
- In the mammalian central nervous system, synapses are almost always axo-dendritic (axon to dendrite) or axo-somatic (axon to cell body).
- Presynaptic butons contain tiny vesicles of neurotransmitter.
- The brain uses many different substances as neurotransmitters, including glutamate, GABA, glycine, acetylcholine, serotonin (also called 5HT), dopamine, noradrenaline, enkephaline, and others. But any one neuron will normally produce and release only one type of transimitter.
- When action potentials arrive at the presynaptic buton, voltage gated Ca++ channels let Ca++ in, which causes "exocytosis" of neurotransmitter: a few (perhaps just one) of the neurotransmitter vesicles fuses with the outer membrane and sheds its content in the synaptic cleft.
- Neurotransmitters diffuse across the cleft and bind to receptor proteins on the postsynaptic cell membrane.
- Receptors can either be chemically gated ion channels which open when they bind neurotransmitter (so called ionotropic receptors), or they can interact with other proteins to work indirectly via a "2nd messanger cascade" (metabotropic receptors).
- When receptors detect neurotransmitter and open ion channels (either directly or indirectly) then postysnaptic currents can flow into or out of the postsynaptic cell. This affects the cell membrane voltage of the postsynaptic cell membrane (causes a "postsynaptic potential").
- Receptors which open channels that are permeable to Na+ or Ca++ depolarize (excite) the cell. Receptors which open Cl- or K+ channels hyperpolarize (inhibit) the cell.
- The chemical signal in the synapse must be terminated by removal of the neurotransmitter from the synaptic cleft, either by reuptake pumps or by enzymes that destroy the transmitter. Certain drugs and poisons make synaptic signals stronger by fiddling with these termination signals.
- The many hundreds to thousands synaptic inputs received by any one neuron every few miliseconds are combined ("integrated") and make the neuron more or less likely to fire action potentials itself.
- Chemical synapses also used by motor neurons to tell your muscles when and how to move. These synapses are particularly vulnerable to poisoning because they are not protected by the blood brain barrier. (Nerve gas, botox etc...)
- Synapses can be "plastic", meaning the strength of the link provided by certain types of synapses can change over time. Synaptic plasticity is thought to play a key role in phenomena such as adaptation and memory.