Volume transmission

When a neurotransmitter is released at a synapse, it reaches its highest concentration inside the narrow space of the synaptic cleft, but some of it is certain to diffuse away before being reabsorbed or broken down. If it diffuses away, it has the potential to activate receptors that are located either at other synapses or on the membrane away from any synapse. The extrasynaptic activity of a neurotransmitter is known as volume transmission.^[18] It is well established that such effects occur to some degree, but their functional importance has long been a matter of controversy.^[19]

Recent work indicates that volume transmission may be the predominant mode of interaction for some special types of neurons. In the mammalian cerebral cortex, a class of neurons called neurogliaform cells can inhibit other nearby cortical neurons by releasing the neurotransmitter GABA into the extracellular space. Approximately 78% of neurogliaforms do not form classical synapses. This may be the first definitive example of neurons communicating chemically where synapses are not present.^[20]

Relationship to electrical synapses

An electrical synapse is an electrically conductive link between two abutting neurons that is formed at a narrow gap between the pre- and postsynaptic cells, known as a gap junction. At gap junctions, cells approach within about 3.5 nm of each other, rather than the 20 to 40 nm distance that separates cells at chemical synapses.^[21][22] As opposed to chemical synapses, the postsynaptic potential in electrical synapses is not caused by the opening of ion channels by chemical transmitters, but rather by direct electrical coupling between both neurons. Electrical synapses are faster than chemical synapses.^[10] Electrical synapses are found throughout the nervous system, including in the retina, the reticular nucleus of the thalamus, the neocortex, and in the hippocampus.^[23] While chemical synapses are found between both excitatory and inhibitory neurons, electrical synapses are most commonly found between smaller local inhibitory neurons. Electrical synapses can exist between two axons, two dendrites, or between an axon and a dendrite.^[24][25] In some cases electrical synapses can be found within the same terminal of a chemical synapse, as in Mauthner cells.^[26]

Effects of drugs

Main article: neuropharmacology

One of the most important features of chemical synapses is that they are the site of action for the majority of psychoactive drugs. Synapses are affected by drugs such as curare, strychnine, cocaine, morphine, alcohol, LSD, and countless others. These drugs have different effects on synaptic function, and often are restricted to synapses that use a specific neurotransmitter. For example, curare is a poison which stops acetylcholine from depolarizing the postsynaptic membrane, causing paralysis. Strychnine blocks the inhibitory effects of the neurotransmitter glycine, which causes the body to pick up and react to weaker and previously ignored stimuli, resulting in uncontrollable muscle spasms. Morphine acts on synapses that useendorphin neurotransmitters, and alcohol increases the inhibitory effects of the neurotransmitter GABA. LSD interferes with synapses that use the neurotransmitter serotonin. Cocaine blocks reuptake of dopamine and therefore increases its effects.

Nonsynaptic Plasticity – Пластичность нервной системы - способность к функциональным мозговым перестройкам в ответ на действие значимых внешних и внутренних факторов. Особенной пластичностью нервные структуры обладают в раннем онтогенезе, за счет чего возможна существенная перестройка их структуры и связей при различных повреждениях. С возрастом пластичность снижается. Для зрелого мозга свойство функциональной пластичности может проявляться и на нейронном, и на системном уровнях.

Пластичность в физиологии, способность клеток и органов животных и растений менять в известных пределах свои свойства в зависимости от условий их функционирования. Так, говорят о П. центральной нервной системы, проявляющейся, например, в её функциональных перестройках, компенсирующих потерю той или иной части вещества мозга, о П. синапсов и т.п.

Nonsynaptic plasticity is a form of neuroplasticity that involves modification of ion channel function in the axon, dendrites, and cell body that results in specific changes in the integration of EPSPs and IPSPs, thus modifying the intrinsic excitability of the neuron. It interacts with synaptic plasticity, however it is considered separate from synaptic plasticity itself. This process is visible in neurons that are actively involved in learning. Nonsynaptic plasticity affects synaptic integration, subthreshold propagation, spike generation, and other fundamental mechanisms of neurons at the neuronal level. These individual neuronal alterations can result in changes in higher brain function, especially concerning learning and memory. However, as an emerging field in neuroscience, much of the knowledge about nonsynaptic plasticity is uncertain and still requires further investigation to better define its role in brain function and behavior.