synapses

• Neurons communicate by transmitting chemicals at junctions, called “synapses”, which were coined by Charles Scott Sherrington in 1906 to describe the specialized gap that existed between neurons.
• Sherrington’s discovery of synapses was a major feat of scientific reasoning.
• A century ago, Cajal hypothesized that sites of contact between nerve cells, later termed synapses, were fundamental in the processing of information by the brain.
• Sherrington gave the first functional evidence of the presence of gaps between neurons.
• Sherrington inferred the existence of a specialized gap between neurons from synaptic delay.
• Sherrington investigated how neurons communicate with each other by studying reflexes, which are automatic muscular responses to stimuli, in a process known as a reflex arc (leg flexion reflex).
• A nervous reflex consists of five elements: sensory receptors, afferent neural pathways, control centers in the CNS, efferent neural pathways, and motor endplates.
• Some of the neurotransmitter simply diffuses away from the synaptic cleft and is no longer available to bind to receptors.
• Ionotropic receptors and Metabotropic receptors are types of receptors.
• Autoreceptors, Heteroreceptors, and Postsynaptic receptors are types of receptors.
• The neurotransmitter is brought back into the presynaptic axon terminal; by-products of degradation by enzymes also may be taken back into the terminal to be used again.
• Neurotransmitter may interact with receptors on the presynaptic membrane.
• The quantum leap refers to the amount of neurotransmitter needed to send a message.
• Enzymes in the synaptic cleft break down the neurotransmitter.
• Autoreceptor is a self-receptor on the presynaptic membrane that responds to the transmitter that the neuron releases.
• Many quanta are required to be released at the same time to produce an Action Potential (AP).
• To be effective, chemical neurotransmission needs to be terminated.
• Electrical synapses are faster than all chemical transmissions, operate at a gap junction, and involve the depolarization of both cells, resulting in the two neurons acting as if they were one.
• Neurotransmission in 5 steps involves the anterograde synaptic transmission process of transmitting information across a chemical synapse from the presynaptic side to the postsynaptic neuron.
• Otto Loewi’s work contributed to the demonstration of the chemical nature of synapses.
• At the terminal, the action potential opens voltage-sensitive calcium (Ca 2+ ) channels, allowing Ca 2+ to enter the terminal and bind to the protein calmodulin, forming a complex.
• Types of synapses include electrical synapses, which involve current flow through gap junctions, and chemical synapses, which involve neurotransmitters.
• Chemical synapses involve the junction where messenger molecules (neurotransmitters) are released from one neuron to excite or inhibit the next neuron.
• The Ca2+ complex causes some vesicles to empty their contents into the synapse and others to get ready to empty their contents.
• Tripartite synapses involve astrocytes, which regulate excitability and variations of Ca 2+ concentration in the cytoplasm.
• Most synapses in the mammalian nervous system are chemical.
• Neurotransmitters are derived in two general ways: synthesized in the axon terminal (small-molecule) or synthesized in the cell body (peptide transmitters).
• Transmitter-activated receptors are protein embedded in the membrane of a cell that has a binding site for a specific neurotransmitter.
• After release, the neurotransmitter diffuses across the synaptic cleft to activate receptors on the postsynaptic membrane.
• Receptor properties are responsible for the effect on the postsynaptic cell, as the same neurotransmitter can elicit different effects.
• The hypothalamus maintains a fairly constant circulating level of hormones through a negative-feedback system.
• Hormones are chemicals secreted by a gland or other cells that are transported to other organs by the blood where they alter activity.
• Hormones are produced by endocrine glands and are important for triggering long-lasting changes in multiple parts of the body.
• Sherrington observed that synapses produce on and off responses and vary enormously in their duration of effects.
• Effectors are the relationship among a sensory neuron, intrinsic neuron, and motor neuron.
• The effect of two synapses at the same time can be more than double the effect of a single synapse.
• Sherrington’s observations include that reflexes are slower than conduction along an axon, several weak stimuli present at slightly different times or slightly different locations produce a stronger reflex than a single stimulus, and as one set of muscles becomes excited, another set relaxes.
• Sherrington observed that an interneuron in the spinal cord sent an excitatory message to the flexor muscles of one leg and an inhibitory message was sent to the other three legs, suggesting that inhibitory synapses exist.
• Sherrington also noticed that several small stimuli in a similar location produced a reflex when a single stimuli did not, leading to the idea of spatial summation.
• Sherrington observed that repeated stimuli over a short period of time produced a stronger response, leading to the idea of temporal summation.