User:Douce1211/Excitatory Synapses

In neuroscience, an excitatory synapse is the junction between a presynaptic cell and a postsynaptic cell that increases the likelihood of the generation of an action potential, an EPSP. This event can be caused through intercellular signaling either electrically by depolarizing the postsynaptic cell or chemically by employing neurotransmitters to induce a cellular response.

Diversity of synapses
A synapse is the junction of two cells for the purpose of transmitting a signal between them. A synapse can be between two neurons, a neuron and a muscle cell, or a neuron and a gland. At a synapse between two neurons, an axon usually connects to a dendrite at a synaptic cleft. However, the axon can connect directly to the soma, the neuron cell body. The specific features of this junction depend on the precise nature of the signal passing through- whether it is electrical or chemical. At a neuromuscular junction, the axon of a motoneuron terminates at a motor end plate. An excitatory signal left uninterrupted at a neuromuscular junction leads to the contraction of an effector. At a neuroglandular junction, an axon connects with a gland cell, and an excitatory electrical or chemical signal from the neuron stimulates the secretion of hormones, enzymes or other chemicals produced in the specific gland.

Excitatory electrical signaling
At an electrical synapse, two cells are connected directly by a gap junction through an intercytoplasmic channel. The gap junction pores allow for the passive exchange of ions between interconnected cells. The signal at an electrical synapse can be bidirectional; "presynaptic cell" is a dynamic term relative to which cell fires first. To generate an excitatory response, a presynaptic cell generates an action potential in the spike-initiating region. The resulting electrical signal travels through the gap junction to the postsynaptic cell and induces a potential difference in the postsynaptic cell. The transmembrane potential difference opens voltage dependent sodium channels resulting in an inward sodium current. At rest, a steady leak of potassium cations generates a negative resting potential. The influx of sodium cations causes a rapid and virtually instantaneous depolarization in the membrane potential of the postsynaptic cell. If the subsequent depolarization exceeds the threshold voltage, when the inward sodium current exceeds the outward potassium current, then an action potential is generated.

An excitatory signal at an electrical synapse does not always trigger an action potential. A depolarization makes the incitement of an action potential more likely but not a certain event. The strength of a presynaptic action potential across a gap junction can be weakened by the length of the conducting nerve, the electrical resistance of the nerve itself or the structure of the postsynaptic membrane. Ionophores, the ion channels responsible for the transmembrane current, can select for various ions relative to charge and luminal diameter, the hydrated radius of the ion in an aqueous environment. Therefore, following the depolarization of the membrane, the threshold voltage is not always reached, and an action potential would not occur. .

Most synapses are found in the central nervous system to rapidly conduct information in the brain between short distances. Some neurons are distributed to the peripheral nervous system to link the brain with the rest of the body. Large axons connecting the CNS and the PNS conduct information over huge distances and require a mechanism to preserve the strength of an electric signal. Myelin is used to insulate the nerve to carry the electric signal as far as possible and to compensate for the internal resistance of a nerve. Aggregates of Schwann cells insulate the axon with myelin to form a lipid sheath that prevents the electric signal from dissipating. However, this sheath is often not sufficient to conduct electrical impulses across the entire distance. At a junction between Schwann cell clusters along the axon, supplementary excitatory electrical synapses are present. They feature an abundance of sodium channels, so that the action potential can be regenerated at various intervals along the nerve.

Excitatory chemical signaling
Chemical cell-to-cell signaling occurs though the secretion of neurotransmitters. The signal can only travel unidirectionally from a presynaptic cell to a postsynaptic cell and is significantly slower compared to electric signals. Chemical signals can either have an excitatory or inhibitory effect on the postsynaptic cell. An action potential occurring in a presynaptic cell opens voltage-gated calcium channels, resulting in an influx of calcium ions. The inward calcium current triggers the formation and the release of vesicles, which contain a specific multimolecular quantity of neurotransmitters, through exocytosis into the synaptic cleft. Neurotransmitters bind to transmembrane receptors on the surface of the postsynaptic cell. In an excitatory response, membrane permeability to sodium or other inward flowing cations increases due to a series of reactions by intermediates leading to a conformational change in the sodium channel. Sometimes, in the event of a slow EPSP, permeability to chloride ions decreases, which results in a relatively slow depolarization through disruption of the Gibbs-Donnan equilibrium. Then, neurotransmitters are rapidly removed from the postsynaptic cell and possibly reabsorbed by the presynaptic cell or degraded within the postsynaptic cell.

Very few neurotransmitters are exclusively excitatory or inhibitory. Instead, a specific neurotransmitter can induce excitement or inhibition in addition to any combination of both responses. Due to the diversity of transmembrane receptors, a single neurotransmitter can have a variety of effects depending on the individual postsynaptic cells. In general, glutamate and aspartane are the primary excitatory neurotransmitters in the cell. They are ionized amino acids that act through mostly ionotropic channels but can also act through a metabolotropic receptor. Acetylcholine is used in the central nervous system but is the principal neurotransmitter used in mammalian motoneurons in muscles. At the synapse between the motor neuron and the muscle cell, acetylcholine binds to the receptor, which opens ion channels to inward sodium current. This leads to depolarization, perhaps an action potential, and then, a muscular response in the form of movement.

A single excitatory response may not be sufficient for the generation of an action potential. Neural facilitation allows multiple excitatory signals to sum spatially and temporally so that the cumulative depolarization of multiple signals can overcome the threshold and result in an action potential. In neural integration, a cellular response to a neurotransmitter being recognized by a receptor can experience a time delay relative to the production of a potential. The duration and time generation of the resultant potential largely depends on the structure of the specific ionotropic or metabolotropic receptor. This allows for multiple excitatory or inhibitory responses or any combination of both with a unique time delay between potentials.