Neurosecretory Cells and Neurosecretion
We have before said that the neurosecretory cells are an important component of the non- chordate endocrine system. Of course, they are as well present in chordates. But unlike chordates, among non-chordates there are fewer epithelial endocrine glands, and thus they have to depend much more heavily on neurosecretory cells for chemical coordination. Berta Scharrer and Ernst Scharrer are considered progenitors of the concept of neurosecretion. Put in a easy way, it is the concept of neurones taking to secretary activity and producing hormones. We can simply locate and identify these NSCs in histological sections of the brain or ganglia, because they consist of plenty of stainable material or colloids, unlike ordinary neurones, which do not consist of any stainable colloids. We believe that these colloids are carrier substances for hormones. Very frequently we can prove the presence of such hormonal principles in such cells by experimental means. Not like the neurosecretory neurones, the ordinary neurones do not contain or secrete any hormones. Even though they release neurotransmitters at synapse and neuromuscular junctions, these are not released into blood and are short lived, not like the hormones secreted by NSCs. In the live brain of certain insects, in dissections, we can easily see the NSCs as tiny bluish white specks by the brain sheath, using a binocular dissection microscope. 'The neurosecretory material containing the hormone is produced mostly in the cell body, and is transported through the axons to the finger-like axon terminals. If we follow these axons, we see that these axonal endings frequently form swellings in association with blood spaces at a distance from the neuronal cell bodies. The secretory material is stored at the swellings and hormones are released from them in blood stream. Such types of organs seen in association with blood spaces and containing the swollen nerve endings of NSCs are called neurohemal organs. You will see that there are neurohemal organs in some non-chordates. Sinus gland in crustaceans and corpus cardiacum in insects are instances of neurohemal organs. Vertebrates also have their own neurohemal organs - the neural lobe of pituitary is one such neurohemal organ.