Synaptic Activity

electrical synapses

membranes connected by gap junctions; local current from one cell always depolarizes the next cell; rare in neurons

chemical synapses

membranes separated by synaptic cleft; neurotransmitter from one cell sometimes depolarizes the next cell; mostly CNS and PNS synapses

excitatory neurotransmitters

cause depolarization

inhibitory neurotransmitters

cause hyperpolarization

cholinergic synapses

releases acetylcholine (ACh) into synaptic cleft; present at CNS, neuron-neuron synapses in PNS, and all NMJ/parasympathetic synapses

What are the events at a cholinergic synapse?

1. depolarization of synaptic knob

2. entry of Ca²⁺/exocytosis of ACh

3. binding of ACh to receptors; depolarization of postsynaptic membrane

4. breakdown of ACh by AChE

5. reabsorption of choline for ACh synthesis

synaptic delay

time between arrival of action potential and effect on postsynaptic membrane; 0.2-0.5 msec

synaptic fatigue

inactivity caused by shortage of ACh in the synaptic knob; occurs under intense simulation

the types of biogenic amines NT include:

NE, dopamine, and serotonin

the types of amino acid neurotransmitters include:

GABA, glycine, glutamate, and aspartate

the type of dissolved gases neurotransmitters include:

NO and CO

Norepinephrine (NE)

used at adrenergic synapses in the brain and sympathetic division of ANS; excitatory and depolarizing effect

Dopamine

inhibitory effects allow precise motor control; excitatory effects stimulates the reward center of the brain

Serotonin

effects in CNS regulate attention span, sleep/wake cycles and emotional states

Gamma aminobutyric acid (GABA)

inhibitory effects in CNS; may reduce anxiety

neuromodulators

adjusts the rate of NT release or response to postsynaptic cell; released from synaptic knobs alone or with NT

neuropeptides

class of neuromodulators; has indirect effects on membrane potential of pre or post synaptic cells

opioides

class of neuropeptides; inhibits the release of substance P; includes endorphins

Neurotransmitters with direct effects on membrane potential:

open or close gated ion channels (ionotropic effects) ex. ACh, amino acids, NE, serotonin

Neurotransmitters with indirect effects on membrane potential:

NT bind to a receptor resulting in synthesis of a second messenger; ex. GABA, dopamine, NE, serotonin

Example of indirect membrane potential:

1. receptor activates a G protein

2. G protein activates adenylate cyclase

3. adenylate cyclase produces cAMP

4. cAMP opens gated ion channels

Intracellular neurotransmitters effect on membrane potential:

enters the cell by diffusion the activates cytoplasmic enzymes which produce a second messenger ex. NO, CO