Synaptic Transmission Chapter 4 Pages 95-122. Chemical Synapses Most synapses in the brain are chemical. Electronically coupled gap junction synapses.
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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.
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.
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
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.
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
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)
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
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.
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.
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.