Page 286 - Anatomy and Physiology of Farm Animals, 8th Edition

Endocrinology / 271

Glucagon causes liver cells to break including humans and domestic animals,
the pineal has no photoreceptors, and its
down glycogen to release glucose, stimu­
VetBooks.ir lates adipocytes to release fatty acids, and location deep inside the braincase renders
increases the synthesis of glucose in the
it incapable of detecting photoperiods
liver from substrates other than carbohy­ directly. The epiphysis nonetheless does
drates, such as amino acids. The stimulus receive information about light and dark
for glucagon release is a decrease in blood cycles indirectly from a nucleus of the
glucose to levels associated with fasting. hypothalamus. The cells of the epiphysis,
α‐Cells detect such decreases and respond although neuronal by lineage, are secre­
by secreting glucagon in proportion to the tory, and they are supported by neuroglia
reduction in blood glucose. There are and receive axonal input. These special­
many several other important substances ized cells are called pinealocytes.
associated with appetite, digestion, and The pinealocytes manufacture seroto­
energy balance reviewed in Chapter 21. nin and an enzyme that converts this pep­
Insulin is necessary for the uptake of tide to the hormone melatonin. Synthesis
glucose by many cells, including skeletal and secretion of melatonin exhibits a
muscle, which makes up most of the profound diurnal rhythm, with releases
body mass. Without sufficient insulin, into the blood peaking during darkness.
glucose accumulates in the blood after a Melatonin is viewed by many scientists as
meal, for it cannot be transported across the internal clock that is able to synchro­
cell membranes into cells where it can nize biological rhythms related to the
be used for fuel. The metabolic conse- sleep–wake cycle and cyclic changes in
quences of insufficient insulin (or of body temperature rhythm. Reproductive
resistance to its effect) create the condi- cycles of many domestic species are closely
tion called diabetes mellitus. linked to photoperiod and may be medi­
ated in part by melatonin. Early discover-
Epiphysis (Pineal Gland) ies about the link between melatonin
and sleep and reproduction have
The epiphysis cerebri (pineal gland or spawned a wave of enthusiastic specula-
pineal body) is a midline structure on the tion about its possible use as a cure for
dorsocaudal aspect of the diencephalon. In insomnia, as an aphrodisiac, as a treat-
fish, amphibians, and some reptiles, it pos­ ment for jet lag, and as an antiaging
sesses photoreceptors, and its proximity to drug, among others. While some con-
the thin calvaria makes it literally a third trolled studies support some of these
eye, the function of which is thought to claims, the role of melatonin in some
involve setting daily and yearly biologic of these processes is not yet fully
cycles based on photoperiod. In mammals, understood.