Secretory cell in the context of "Bacterial secretion system"

Myelin (/ˈmaɪ.əlɪn/ MY-ə-lin) is a lipid-rich material that in most vertebrates surrounds the axons of neurons to insulate them and increase the rate at which electrical impulses (called action potentials) pass along the axon. The myelinated axon can be likened to an electrical wire (the axon) with insulating material (myelin) around it. However, unlike the plastic covering on an electrical wire, myelin does not form a single long sheath over the entire length of the axon. Myelin ensheaths part of an axon known as an internodal segment, in multiple myelin layers of a tightly regulated internodal length.

The ensheathed segments are separated at regular short unmyelinated intervals, called nodes of Ranvier. Each node of Ranvier is around one micrometre long. Nodes of Ranvier enable a much faster rate of conduction known as saltatory conduction where the action potential recharges at each node to jump over to the next node, and so on until it reaches the axon terminal. At the terminal the action potential provokes the release of neurotransmitters across the synapse, which bind to receptors on the post-synaptic cell such as another neuron, myocyte or secretory cell.

Large dense core vesicle (LDCVs) are lipid vesicles in neurons and secretory cells which may be filled with neurotransmitters, such as catecholamines or neuropeptides. LDVCs release their content through SNARE-mediated exocytosis similar to synaptic vesicles. One key difference between synaptic vesicles and LDCVs is that protein synaptophysin which is present in the membrane of synaptic vesicles is absent in LDCVs. LDCVs have an electron dense core which appears as a black circle in micrographs obtained with transmission electron microscopy.