Page 275 - Anatomy and Physiology of Farm Animals, 8th Edition

260 / Anatomy and Physiology of Farm Animals

glucose in the body fluids. An increase in Hypothalamopituitary Axis
glucose concentration causes the β‐cells to
VetBooks.ir increase their release of insulin. One effect The hypothalamus is ventral to the thala­

of insulin is to promote the uptake of glu­
cose by skeletal muscle cells. As glucose is mus in the diencephalon and forms the
floor and part of the wall of the third ventri­
removed from the body fluids, the stimu­ cle (see Chapter 10). The pituitary gland,
lus for insulin release is removed, and this or hypophysis cerebri, is attached to its
has a negative effect on insulin release. base by the infundibulum. Cell bodies of
This negative feedback regulation of insu­ neurons whose axons form the infundibu­
lin release is a major factor in determining lum are found in the hypothalamus, and
a normal plasma concentration of glucose. their termini abut on the capillaries in the
The negative feedback regulation of neural part of the pituitary gland (neurohy-
insulin by changes in plasma glucose is a pophysis, posterior pituitary, or pars ner-
relatively simple and straightforward feed­ vosa) such that the infundibulum is really
back loop. The plasma constituent, glucose, just a stalk of nervous tissue formed by the
being regulated by the hormone, insulin, axons of these neurons. Associated with
has a direct effect on the cells releasing the the infundibulum is a unique system of
hormone. However, negative feedback arterioles and capillaries called the hypo-
loops can be quite complex and have multi­ thalamohypophysial portal system. This
ple organs in the loop. Some of the more system is a true vascular portal system in
complex loops involve the hormones regu­ that blood from a capillary network in the
lating reproduction in domestic animals hypothalamus flows through portal vessels
and the hypothalamus, the anterior pitui­ (similar to veins) to the glandular portion of
tary gland, and the gonads. These are dis­ the pituitary (adenohypophysis, anterior
cussed further in Chapters 25 and 27. pituitary, or pars distalis), where it enters
A second type of feedback regulation, a second capillary network (Fig. 13‐3).
that is seen much less frequently than neg­ During development, the pituitary gland
ative feedback, is positive feedback regu- forms by the conjoining of a ventral out­
lation. In this case, the hormone brings pouching of the diencephalon (the future
about a biologic response that produces a infundibulum and neurohypophysis) and a
further increase in the release of the hor­ diverticulum, Rathke’s pouch, which arises
mone. This type of regulation is unusual, from the nearby dorsal ectoderm of the
and it is not designed to maintain a stable pharynx. The cells of Rathke’s pouch
or homeostatic level of some activity or become the adenohypophysis.
blood constituent. One of the few exam­ The neurotransmitters released by
ples of this type of regulation is the rela­ hypothalamic neurons whose termini end
tionship between oxytocin release and in the neurohypophysis enter the blood
dilation of the uterine cervix. An increase and are carried to distant sites to function
in oxytocin release is associated with dila­ as systemic hormones (Fig. 13‐3). These
tion of the uterine cervix during parturi­ peptides are manufactured by the neuronal
tion (details in chapters on reproduction), cell bodies within the hypothalamus, trans­
and oxytocin acts on the smooth muscle of ported via axons in the infundibulum into
the uterus to increase uterine contractions. the neurohypophysis, and are released
When the cervix dilates during parturition directly into blood vessels when action
and oxytocin is released, the contractions potentials arrive at the telodendria.
of the uterus move the fetus out of the Other hypothalamic neurons release
uterus through the cervix. This further neurotransmitters that are carried from
dilates the cervix, providing a greater stim­ the hypothalamus to the adenohypo­
ulus for secretion of oxytocin. The overall physis via the hypothalamohypophysial
effect is to expel the fetus when the cervix portal system (Fig. 13‐3). These neuro­
is dilated. transmitters act on endocrine cells in the