Synaptic+Transmission

=**Synaptic Transmission**=


 * 1. Be able to recognize the structural components of chemical & electrical synapses and identify their function.**

Electrical synapses allow the direct transfer of ions and larger molecules between adjacent neurons. They are made up of gap junctions composed of clusters of transmembrane ion hemichannels called connexons that form a relatively non-selective pore. Each connexon protrudes and contacts a connexon of the adjacent cell, providing a fluid-filled, low-resistance intercellular pathway directly connecting the cytoplasms of the coupled cells.

At electrical synapses, ions and other small molecules can spread directly, reliably and very rapidly between the adjacent cells. Action potentials and sub-threshold deoplarizations/hyperpolarizations are transmitted with virtually no synaptic delay. Most electrical synapses transmit bidirectionally though some display reactivation such that transmission becomes essentially unidirectional.

Chemical synapses provide the major form of cell-to-cell communication in the nervous system. Transmission is indirect in that it involves transduction of a chemical signal to a voltage response and polarized such that information flows unidirectionally. At the neuromuscular junction, one should be able to identify presynaptically, the presynaptic axon terminal and membrane, Schwann cell, synaptic cleft, synaptic vesicles, mitochondria, active zone, and basal lamina, and postsynaptically, the muscle fiber, junctional folds, and densely-packed receptors. At the neuronal synapse, one should be able to identify presynaptically, the presynaptic terminal and membrane, synaptic boutons, synaptic cleft, synaptic vesicles,mitochondria, and active zone, and postsynaptically, dendritic cell membrane and postsynaptic thickening containing densely-packed receptors. Excitatory synapses typically have round vesicles while inhibitory ones have flattened vesicles.


 * 2. Describe the sequence of events underlying transmission of a typical fast chemical synapse.**

(1) An action potential depolarizes the presynaptic terminal, activating Ca2+ channels and causing an increase in terminal Ca2+ concentration. (2) The resulting elevated Ca2+ triggers a complex set ofmolecular interactions which causes the synaptic vesicles to fuse with the presynaptic membrane and release/secrete neurotransmitter into the synaptic cleft where it diffuses to the postsynaptic membrane. (3) Neurotransmitter molecules bind to the transmembrane receptors, present at high density on the postsynaptic membrane. Binding causes the receptor to activate and its ion channel to open. Depending on the kind of receptor at the synapse, this will either allow cations (for Ach and glutamate receptors) or anions (for GABA and glycine receptors) to enter the postsynaptic cell. (4) Influx of cations will depolarize (excite) the cell whereas influx of anions will hyperpolarize (inhibit) the cell. Depolarization then leads to an appropriate response: a postsynaptic potential and action potential. (5) The synaptic delay between action potentials in pre- and postsynaptic neurons are between 0.3-3.0 ms. (6) Emptied vesicles on the presynaptic terminal are retrieved into an endosomal compartment and refilled with newly synthesized neurotransmitter. Excess neurotransmitter in the synaptic cleft is removed either by enzymatic breakdown or by reuptake mechanisms to transport the neurotransmitter back to the terminal.


 * 3. Explain the quantal nature and Ca2+ dependence of transmitter release from presynaptic nerve terminals.**

Early experiments found that action potentials produced by stimulating a motor nerve axon evokes a large, short-latency endplate potential (EPP) in the post synaptic muscle fiber. Under physiological conditions, the EPP is usually above threshold and leads to AP in the muscle fiber and muscle contraction.

The size of the muscle EPP and synaptic transmission was //critically dependent on concentration of extracellular Ca2+//. That is, removal of extracellular Ca2+ causes a reduction of EPP in a //stepwise manner with fluctuations in multiples of unitary size equal to miniature EPP’s or MEPPs// which occur spontaneously. This meant that the release of neurotransmitter from the nerve terminal is Ca2+ dependent because iontophoretic application of Ca2+ to nerve terminals bathed in low Ca2+ restores the EPP to normal amplitude and because blockage of presynaptic Ca2+ channels in terminals bathed in normal Ca2+ reduces EPP amplitude.

Additionally, neurotransmitter is released in multimolecular packets (quanta). A quantum defines the smallest unit which the neurotransmitter is normally secreted, and is now known to represent the contents of one synaptic vesicle.


 * 4. Be able to explain the basic molecular mechanisms thought to be involved in neurotransmitter release at a fast chemical synapse.**

The quantum hypothesis says that neurotransmitter is released in quantal packets (vesicles) by a process that depends on Ca2+ entry into the presynaptic termina. Prescisly how Ca2+ entry triggers vesicular fusion is unclear.

SNAREs are a class of proteins present on the outer vesicle and inner plasma membranes that are implicated in docking the vesicle with the presynaptic terminal. Synaptobrevin is on vesicle membranes while syntaxin and SNAP-25 are on presynapticplasma membranes. SNARE proteins form complexes that span the two membranes to bring them close together.

SNAREs don’t bind Ca2+. However, Synaptotagmin is a vesicular protein that does bind Ca2+. It is likely that SNARE proteins promote docking while entry of Ca2+ causes changes in synaptotagmin (perhaps to insert into the plasma membrane) that mediate the final steps of vesicular fusion.


 * 5. Summarize the activation and gating of postsynaptic nicotinic acetylcholine receptor channels.**

EPP arises from ionotrophic nicotinic ACh receptor channels. Each endplate ACh receptor channel consists of 5 transmembrane subunits either alpha2-beta-gamma-delta or alpha2-beta-epsilon-delta; ACh molecules bind to an external N-terminal domain on each of the 2 alpha subunits. These five subunits are arranged around a central pore which can be opened to permit the transit of ions.

Under normal conditions, ACh is released into the synaptic cleft and reversibly binds to most of the ACh receptors. With one ACh molecule bound to each of the ACh alpha subunits, the channel subunits undergo a rapid, reversible conformational change exposing its pore and going to an activated, “open” ion-conducting configuration. The channels open for a period of time (0.5-5.0 msec) and close again; they may reopen and close again as ACh molecules dissociate and are cleaved by the acetylcholinesterase enzymes in the synaptic cleft, allowing the ACh receptor to bind additional ACh. The reversible transition between close and open states is called gating and is a common feature of ionotrophic receptors such as ACh, GABA, glycine, and glutamate receptors. The process of ACh binding followed by gating to an open state is called activation.

Receptors such as ACh receptors are designed to respond to neurotransmitter molecules released in brief pulses (milliseconds). When ACh is available for prolonged periods, the ACh receptors become desensitized and are unlikely to open though they retain high affinity for the agonist.

The open ACh receptors are selective for monovalent cations and are significantly permeable to Na+ and K+. The normal resting potential of the postsynaptic terminal is -60mV, emaing that mostly Na+ ions will rush into the open ACh receptor channels. This causes an EPP and the resulting depolarization is usually above threshold. This causes an action potential and subsequent muscle contraction.


 * 6. Understand the different conductance mechanisms underlying excitatory versus inhibitory fast synaptic transmission.**

An endplate potential is termed an excitatory postsynaptic potential (EPSP) because it increases the likelihood that the postsynaptic cell will fire an action potential. If an input decreases the likelihood that the postsynaptic cell will fire, it is termed an inhibitory postsynaptic potential (IPSP). Whether an input is excitatory or inhibitory depends on the nature of the postsysnaptic cell’s receptor, specifically its ion selectivity and the resulting value of the reversal potential //E//rev, the potential when the flow of Na+ in will equal the K+ flow out.

Under physiological conditions, when a channel opens, the potential will approach but not go beyond the reversal potential //E//rev. If the channel’s reversal potential is more positive than the action potential threshold, it will generate an EPSP. If the reversal potential is more negative than the action potential threshold, it will generate an IPSP.

__Examples__ Glutamate receptor channels are selectively permeable to Na+ and K+ and has a reversal potential of //E//rev ``=`` 0mV. When the channels open they will bring the cell towards 0 mV, surpassing the threshold for generating an action potential and producing an EPSP.

GABA receptor channels are permeable to Cl- and has an //E//Cl- ``=`` -50 or -70 mV depending on the neuron type. When the channel opens, they will bring the cell towards -50 or -70 mV, well below the threshold for generating an action potential and producing an IPSP. Depolarizations can still generate IPSP if the reversal potential //E//rev is below action potential threshold, because now the depolarization is “clamped” at the //E//rev and cannot reach beyond it to get to threshold.


 * 7. Be able to distinguish fast from slow chemical transmission and describe the relevant mechanisms.**

Transmission via ionotropic receptors is fast with gating occurring on the order of milliseconds. Synaptic transmission can also occur at very slow time scales involving the modulation of one or more classes of ion channel.

Examples are sympathetic ganglia whose B cells receive direct synaptic input from cholinergenic nerve terminals and indirect neuropeptide (in this case, LHRH or leutinizing hormone releasing hormone) input. LHRH is released from a different cell type by a distinct set of peptidergic inputs and diffuses to B cells.

__Fast Chemical Transmission__ A single shock stimulation of preganglionic axons evokes ACh release which results in a fast EPSP mediated by ionotrophic nicotinic ACh receptors and lasts for several milliseconds.

__Slow Chemical Transmission__ Trains of stimuli cause slow EPSP in the B cell lasting up to 1-2 minutes. This depolarizing response is mediated by muscarinic ACh receptors on the B cells which act via a GTP-binding protein (G-protein) to close muscarinic K+ channels that are normally open at rest. The muscarinic ACh receptors are metabotrophic receptors that do not contain an intrinsic ion channel but has different effects on other classes of K+ channels.

Trains of stimuli delivered to distant LHRH inputs cause a late slow EPSP in the B cell lasting up to 10 min. These responses are mediated by peptide receptors on B cells which recognize and bind LHRH. Peptide receptors also act via G-protein to inhibit muscarinic K+ channel function.