1. Three modes of transmission of information between neurons.

• Electrical synapses are direct cytoplasmic connections between two nerve cells, through channels called gap junctions. Electrical synapses are often "rectifying" which means that they allow electrical charge to pass more easily in one direction than the other. These synapses were once thought to only exist in invertebrates, such as the crayfish. Recently, a limited number of types of electrical synapses have also been found in vertebrates. (Eckert, Fig. 6-9)
• Fast chemical synapses release a neurotransmitter substance that binds directly to ion channels on the postsynaptic membrane and changes the permeability of the postsynaptic membrane, thus causing an electrical potential change. (Eckert, Fig. 6-10a), (Eckert, Fig. 6-10b), (Eckert, Fig. 6-10c), (Eckert, Fig. 6-10d)
• For slow chemical synapses, the neurotransmitter substance binds to a receptor protein on the postsynaptic membrane. This binding causes release of an intracellular transmitter in the postsynaptic cell and that transmitter then binds to ion channels on the postsynaptic membrane. (Eckert, Fig. 6-11b)

2. Fast Chemical Synapses.

• Your text has superb diagrams and descriptions of the anatomical details of fast nerve-nerve and nerve-muscle (neuromuscular junction) synapses. You are responsible for the details in these diagrams and the associated text. (Eckert, Fig. 6-12a), (Eckert, Fig. 6-12b), (Eckert, Fig. 6-12c), (Eckert, Fig. 6-13)
• Release of neurotransmitter (acetylcholine - ACh - for neuromuscular junctions in verterbrates) causes synaptic potentials. These can be depolarizing (excitatory postsynaptic potentials or EPSPs) or inhibitory (inhibitory postsynaptic potentials or IPSPs) On muscle, the excitatory postsynaptic potentials are called EPPs for "end plate potentials". These EPPs are depolarizing potentials of 20mV maximum amplitude. (Eckert, Fig. 6-15a), (Eckert, Fig. 6-15b) To see the EPPs, we use Curare, which binds to postsynaptic ion channels, competing with ACh for binding sites, and reduces the EPP just enough to avoid reaching threshold and firing off action potentials. EPPs die off with distance, by electrotonic spread of current, but without any active regeneration as would be caused by voltage-gated channels. Link - "The horror of curare poisoning is that the victim is very much awake and aware of what is happening..."
• Skip the reading in the text on "synaptic currents".
• Postsynaptic potentials are made up of many miniature potentials, (mEPSP, mIPSP, etc.) each caused by release of a single vesicle of neurotransmitter. (Eckert, Fig. 6-24a)
• Neurotransmitter is released after a depolarization on the presynaptic nerve terminal causes Calcium to flow into the nerve terminal (voltage-gated Ca channels!).(Eckert, Fig. 6-28), (Eckert, Fig. 6-29)
• Many different types of molecules are used as synaptic transmitters. I'll tell you a couple of particularly interesting physiological effects related to Glutamate and Serotonin! (Eckert, Table 6-2, part 1), (Eckert, Table 6-2, part 2), (Eckert, Table 6-2, part 3)

ACh

Nicotine
• The nicotinic acetylcholine receptor protein (nicotinic Ach receptor) consists of five protein chains, called "subunits". Two of these are identical and when both bind ACh, the channel formed by the five subunits opens up, causing increased sodium permeability. Nicotine mimics the effect of acetylcholine on these receptor proteins.(Eckert, Fig. 6-32a), (Eckert, Fig. 6-32b), (Eckert, Fig. 6-32c) We will also discuss the electroplax organ.

3. Slow Chemical Synapses.

Muscarine
• Slow chemical synapses, such as the muscarinic acetylcholine receptor often, if not always, act through internal cell signaling involving G-proteins. Muscarine mimics the effect of acetylcholine on these receptor proteins. (Eckert, Fig. 6-37c)
• Another transmitter used in slow chemical synaptic transmission is a form of the hormone adrenaline (epinephrine), called noradrenaline. Note that sometimes we speak of cholinergic neurons (using ACh) or adrenergic neurons (using noradrenaline). (How adrenaline acts as a hormone!)

All text and images, not attributed to others, including course examinations and sample questions, are Copyright, 2008, Thomas J. Herbert and may not be used for any commercial purpose without the express written permission of Thomas J. Herbert.