Page 279 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

11 Hormones and Reproduction

Hormones. Like neurotransmitters (see
Integrative Systems of the Body
below) and the immune system’s cytokines
Unlike unicellular organisms, multicellular or- and chemokines (! p. 94ff.), hormones serve
ganisms have numerous specialized groups of as messenger substances that are mainly util-
cells and organs, the many different functions ized for slower, long-term transmission of sig-
of which must be expediently integrated and nals. Endocrine hormones are carried by the
coordinated (see also p. 2). In mammals, the blood to target structures great distances
nervous system and endocrine system are away. Paracrine hormones (and other para-
chiefly responsible for control and integration, crine transmitters) only act on cells in the im-
while the immune system serves as an infor- mediate vicinity of the cells from which they
mation system for corporal immune defense are released. Hormones that act on the cells
(! p. 94ff.). These systems communicate by that produced the messenger substance are re-
way of electrical and/or chemical signals (! A). ferred to as autocrine hormones.
Nerve impulses and hormonal signals serve Hormones are synthesized in specialized
to control and regulate (! p. 4) the metabo- glands, tissues and cells (e.g., neuroendocrine
lism and internal milieu (blood pressure, pH, cells). Their target organ is either a subordi-
water and electrolyte balance, temperature, nate endocrine gland (glandotropic hormone)
etc.), physical growth and maturation, repro- or non-endocrine tissue (aglandotropic hor-
ductive functions, sexual response, and re- mone). The target cells have high-affinity bind-
sponses to the social environment. The signals ing sites (receptors) for their specific hormone,
received by sensors (= sensory receptors) in so very low concentrations of the hormone
the inner organs, musculoskeletal system, skin suffice for signal transduction (10 – 6 to
and the sensory organs, as well as psychologi- 10 – 12 mol/L). The receptors on the target cells
cal factors, skeletal muscles and other factors pick out the substances specifically intended
also play a part in regulation and control. The for them from a wide variety of different mes-
signals are used by many feedback mechanisms senger substances in their environment.
in the body (! p. 4). Hormones work closely with the nervous
Nerve fibers are specifically adapted for system to regulate digestion, metabolism,
rapid transmission of finely graded signals. growth, maturation, physical and mental
The nervous system consists of the central development, maturation, reproduction, adap-
nervous system (CNS; ! p. 310ff.) and periph- tation, and the internal milieu of the body
eral nervous system . The latter consists of: (homeostasis) (! A). Most of these actions are
! The somatic nervous system, which con- predominately autonomous functions subject
ducts impulses from non-visceral sensors to a to central control by the hypothalamus, which
center (afferent neurons) and controls the is controlled by higher centers of the brain
skeletal musculature (efferent neurons). (! p. 330).
! The peripheral autonomic nervous system Neurotransmitters released at chemical
(! p. 78ff.), which consists of efferent neurons synapses of nerve endings transmit signals to
and mainly functions to control the circulatory postsynaptic nerve fibers, muscles or glands
system, inner organs and sexual functions. It is (! p. 50ff.). Some neuropeptides released by
supplemented by: presynaptic neurons also exert their effects in
! Visceral afferent neurons, i.e., nerve fibers neighboring synapses, resulting in a kind of
that conduct signals from inner organs to a “paracrine” action.
center. They are usually located in the same Neurons can also secrete hormones, e.g.,
nerves as autonomous fibers (e.g., in vagus epinephrine, oxytocin and antidiuretic hor-
nerve); and the mone. Some transmitter substances of the im-
! Enteric nervous system, which integrates mune system, e.g. thymosin and various cy-
the local functions of the esophagus, stomach tokines, also have endocrine effects.
and gut (! p. 234).
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