Synaptic Transmission

1. Synaptic Transmission Chapter 4 Pages 95-122

2. Chemical Synapses • Most synapses in the brain are chemical. • Electronically coupled gap junction synapses occur in special circumstances (embryos, cardiac muscle) where close coordination needed. • Chemical synapses release a chemical into the synaptic cleft which transfers information (a neural signal) from one neuron to another.

4. How a Synapse Works • The presynaptic neuron experiences an action potential. • Presence of the action potential at the terminal button permits calcium (Ca++) to enter the neuron via voltage-gated calcium channels. • Calcium triggers the release of neurotransmitter from synaptic vesicles. • Neurotransmitter crosses the synaptic cleft and opens ion channels in the post-synaptic neuron.

5. Ion Channels • Found in all cells throughout the body. • Open and close in response to signals. • Selectively permeable to specific ions • High rate of flow (conductance) • Resting channels – usually open • Gated channels – open and close • Refractory period – temporarily cannot be opened

6. Control of Gating • Binding of neurotransmitters, hormones, or second messengers from within the cell. • Voltage-gated – responds to a change in the membrane potential. • Stretch or pressure gated – mechanical forces. • Phosphorylation – energy comes from a phosphate that binds with the channel. • Dephosphorylation – removal of the phosphate.

7. Kinds of Receptors • All neurotransmitters bind and act at more than one kind of receptor. • Two main kinds of receptors: • Ion channel receptors • G-protein-coupled receptors

8. Effects of Drugs • Exogenous ligands – drugs that come from outside the body. • Endogenous ligands – naturally occurring • Agonist – binds with and opens a channel. • Endogenous or exogenous (e.g., drug) • Receptors are often named for their agonists • Antagonist – binds with and closes a channel. • Reversible (curare) or irreversible (snake venom)

9. Three Kinds of Neurotransmitters • Amino acids – released from synaptic vesicles • Monoamines – released from synaptic vesicles • Peptides – larger molecules released from secretory granules. • There may be more substances that act as neurotransmitters

10. Amino Acids • Glutamate (Glu) – excitatory, widespread. • GABA – inhibitory, widespread. • Glycine (Gly) – inhibitory at some locations. • The poison Strychnine blocks GABA receptors interfering with inhibition so excitations overwhelm the brain.

11. Monoamines • Cholinergic – Acetylcholine (ACh), used by muscles • Catecholaminergic – regulate thinking, mood, synethesized from tyrosine: • Dopamine (DA) • Norepinephrine (Noradrenaline) (NE) • Epinephrine (Adrenaline) – widespread • Serotonin (5-HT) – broken down by MAO • LSD binds at 5-HT receptors.

12. Peptides • Formed by chains of amino acids. • Oxytocin – released during childbirth to regulate maternal behavior. • Vasopressin – regulates proper blood volume and salt concentration in body. • Opioids (endorphins) – naturally occurring painkillers • Morphine binds with opioid receptors.