Biomedical Engineering Reference
In-Depth Information
resentation of its information-processing functions in a way that is both biologically
referrable in its parameters and quantitatively accurate in its match to experimental
data, while being also founded on explicit assumptions in a manner that gives it true
predictive and explanatory power.
7.2
The hypothalamo-hypophysial system
The magnocellular neurosecretory neurons of the hypothalamus are concentrated in
the supraoptic and paraventricular nuclei; axons from these cells project to the pos-
terior pituitary gland (also known as the neural lobe, or neurohypophysis) where the
hormones that they synthesize, oxytocin and vasopressin, are released into the circu-
lation. These hormones are released in response to spikes that are generated at the
cell bodies and conducted along the axons; hormone release from these neurosecre-
tory terminals is analogous to neurotransmitter release from conventional neurons,
but unlike transmitter release, hormone release occurs in such large amounts that it
can be measured very easily. The soma and axons of the magnocellular neurons are
readily identifiable and accessible for experimental manipulation through a wide va-
riety of
in vivo
and
in vitro
experimental approaches. Thus, they are one of the few
groups of central neurons in which changes in activity pattern can be related to the
physiological stimulus and the precise neuronal response to the stimulus, the state of
the organism and the hormonal secretion, respectively (see [5, 6, 7, 8, 9] for reviews).
Through its role as the antidiuretic hormone, vasopressin is primarily concerned
with body fluid homeostasis. Vasopressin is released in response to increased plasma
osmotic pressure, and in response to reduced plasma volume, and it acts on the
kidney to promote conservation of water by concentrating the urine, and to restrict
plasma volume by vasopressor actions on blood vessels. The classical roles of oxy-
tocin are in lactation and parturition. At parturition, oxytocin stimulates uterine con-
tractions to promote parturition. During lactation, oxytocin is released in response
to suckling in a pulsatile manner, and promotes milk let-down from the mammary
gland. Oxytocin is also released in response to increased osmotic pressure, hypov-
olemia, and gastric distension, reflecting a secondary role at the kidney to stimulate
sodium excretion in response to increased sodium intake. Oxytocin and vasopressin
also have intriguing behavioral actions. These are intriguing first because oxytocin
and vasopressin released into the blood does not re-enter the brain, which is protected
by a blood-brain barrier; so these behavioral effects are mediated by central release
of vasopressin and oxytocin. The oxytocin and vasopressin cells have few axonal
endings within the brain, but they can release very large amounts of these peptides
from their dendrites. The behavioral actions seem remarkable apposite to the pe-
ripheral roles of the hormones. Oxytocin for instance promotes maternal behavior
after parturition, seen in rats as nest building and retrieval of young. These behav-
ioral actions are typical of the effects of central injection of peptides endogenous
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