Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

Cell body
One nerve cell
Axon
Nucleus
Cytoplasm
Dendrites
Telodendria
BIOLOGICAL BASIS FOR R ADIATION SAFETY 301
Figure 7-10. Structure of a neuron. The picture
also shows a synapse, which is the connection
between two neurons that allows the nerve
impulse to progress along the nervous system.
(Reproduced with permission from Lessing MS.
Review Text in Life Science. Intermediate Level.
New York, NY: Amsco School Publications; 1967.)
the next synapse. This action continues until a neuromuscular junction is reached,
where the electrical impulse stimulates the muscle. The nerve pulse itself consists of
a momentary depolarization of the wall of the neuron that travels along the neuron
at a rate on the order of meters per second. When the nerve cell is at rest, a potential
difference of about 80 mV is built up across the inside of the axon and the outside
because of the difference in concentration of Na + and K + ions. The depolarization
is caused by a momentary change in the permeability of the wall of the neuron that
allows free passage of Na + and K + ions, thereby leading to a decreased potential
difference. This change in permeability lasts about a millisecond and is followed
immediately by a repolarization of the nerve cell wall.
The nervous system is considered to be the least sensitive system to radiation
effects—the threshold for somatic damage is high, on the order of 10–20 Gy.
The Endocrine System
The endocrine system comprises a group of separate glands that are located in various
parts of the body and secrete specific substances called hormones into the blood.
These hormones are carried by the blood to specific sites, where they produce certain
specific effects. We call the endocrine glands a system because, in many instances,
they interact with each other, and the action of one may depend on interaction with
a hormone secreted by another gland. A hormone that stimulates another gland is
identified by a name whose prefix is the name of the gland that it stimulates and
whose suffix is “tropin.” Thus, the hormone called thyrotropin stimulates the secretion
of thyroxine by the thyroid gland. The various glands in the endocrine system and
their hormones include the following:
Pituitary. The pituitary gland, which is located in the base of the skull, above the
roof of the mouth, is called the master gland because it secretes a number of different
hormones, including:
Adrenocorticotropic hormone, which regulates the growth and hormonal secretion
of the adrenal cortex.
Gonadotrophic hormones, which stimulate the production of sex hormones in
the male and in the female.
◦ Follicle-stimulating hormone—In the female, this hormone stimulates the
production of the female hormone estrogen and the development of the
ovarian follicles.