12.5: Synapses

Synapse

A nerve signal reaches the end of an axon and has to connect with another entity (muscle or another nerve); releases a neurotransmitter to pass along message/signal at this synapse/connection point

Presynaptic Neuron

first neuron in signal path; releases the neurotransmitter; synapse with a dendrite, soma or the axon of a postsynaptic neuron and form an axodendritic, axosomatic, or axoaxonic synapse respectively; synaptic knob contains synaptic vesicles containing neurotransmitter

Postsynaptic Neuron

Second neuron in signal path; responds to neurotransmitter; contains proteins, receptors and ligand, gated channel

4 Major Chemical Categories of Neurotransmitters

Acetylcholine, amino acids, monoamines, and neuropeptides

Acetylcholine

Acetic acid (acetate) and choline; in a class by itself

Amino Acid Neurotransmitters

Includes glycine, glutamate, aspartate, and y(gamma)-aminobutyric acid (GABA)

Monoamines

Biogenic amines; synthesized from amino acids by the removal of the –COOH group, but retain – NHx (or amino group); includes catecholamines: epinephrine, norepinephrine, and dopamine; also includes histamine, ATP, and serotonin

Neuropeptides

chains of 2 to 40 amino acids; act at lower concentration and have longer lasting effects than other neurotransmitters; stored in secretory granules (larger than secretory vesicles); could be called neuro modulators; produced by neurons or digestive tract; includes cholecystokinin and substance P

Neurotransmitters/Neuropeptides

Organic compounds; peptides are larger than transmitters

Excitatory Cholinergic Synapse

Uses Ach as neurotransmitter; action potential depolarizes the axon terminal which opens Ca+ gated channels, Ca+ comes into synaptic knob, stimulates synaptic vesicles to release Ach(in presynapse neuron), Ach released into synapse cleft and binds to receptors on postsynaptic neuron, gates open and allows Na+ into postsynaptic neuron and K+ out of postsynaptic neuron, causing depolarization of postsynapse neuron

Inhibitory GABA-ergic Synapse

Uses y-aminobutyric acid (GABA) as neurotransmitter; Action potential triggers release of GABA into synaptic cleft; are chloride gated channels; Cl- current hyperpolarizes the post synaptic membrane and when it is inhibited, its less likely to fire action potential

Excitatory Adrenergic Synapse

Uses the monoamine neurotransmitter norepinephrine (NE) (also called noradrenaline); neurotransmitter NE travels across synapse cleft, binds with the receptor associated with G protein (second messenger scenario/MOA), activates G protein and second messenger system such as cyclic AMP (cAMP); slower to respond than other synapses but has multiple/larger effects

Synaptic Delay

An interval, about 0.5 m sec; the time from the arrival of a signal at the axon terminal of a presynaptic cell (neuron) to the beginning of an action potential in the post synaptic cell/neuron

Cessation of the Signal

Presynaptic cell stops releasing neurotransmitter (via stopping the nerve signals) or get rid of the neurotransmitters present

Neurotransmitter Degradation

Enzyme in synaptic cleft breaks down neurotransmitter; ex: (AchE) acetylcholinesterase breaks down acetylcholine into actate and choline

Reuptake

Neurotransmitter (or its breakdown products) reabsorb into axon terminal; ex: choline from ACh is recycled to make new ACh

Diffusion

Neurotransmitter (or breakdown products) diffuses too far from synapse to nearby ECF; astrocytes absorb them and return them to neurons (presynapse)

Neuromodulators

Hormones, neuropeptides, and other messengers/chemicals secreted by neurons that modify synaptic transmission/have long term effects on groups of neurons; ex: (NO) nitric oxide; stimulates a neuron to raise or lower the number of receptors in post synaptic membrane (adjusting sensitivity); alter the rate of neurotransmitter synthesis, release, reuptake, or breakdown