Hormones and Reproduction

Integrative Systems of the Body

Unlike unicellular organisms, multicellular organisms have numerous specialized groups of cells and organs, the many different functions of which must be expediently integrated and coordinated (see also p. 2). In mammals, the nervous system and endocrine system are chiefly responsible for control and integration, while the immune system serves as an information system for corporal immune defense (^ p.94ff.). These systems communicate by way of electrical and/or chemical signals (^ A).

Nerve impulses and hormonal signals serve to control and regulate (^ p. 4) the metabolism and internal milieu (blood pressure, pH, water and electrolyte balance, temperature, etc.), physical growth and maturation, reproductive functions, sexual response, and responses to the social environment. The signals received by sensors (= sensory receptors) in the inner organs, musculoskeletal system, skin and the sensory organs, as well as psychological factors, skeletal muscles and other factors also play a part in regulation and control. The signals are used by many feedback mechanisms in the body (^ p. 4).

Nerve fibers are specifically adapted for rapid transmission of finely graded signals. The nervous system consists of the central nervous system (CNS; ^p.310ff.) and peripheral nervous system . The latter consists of: ! The somatic nervous system, which conducts impulses from non-visceral sensors to a center (afferent neurons) and controls the skeletal musculature (efferent neurons). ! The peripheral autonomic nervous system (^ p. 78ff.), which consists of efferent neurons and mainly functions to control the circulatory system, inner organs and sexual functions. It is supplemented by:

! Visceral afferent neurons, i.e., nerve fibers that conduct signals from inner organs to a center. They are usually located in the same nerves as autonomous fibers (e.g., in vagus nerve); and the

! Enteric nervous system, which integrates the local functions of the esophagus, stomach and gut (^ p. 234).

Hormones. Like neurotransmitters (see below) and the immune system's cytokines and chemokines (^ p. 94ff.), hormones serve as messenger substances that are mainly utilized for slower, long-term transmission of signals. Endocrine hormones are carried by the blood to target structures great distances away. Paracrine hormones (and other para-crine transmitters) only act on cells in the immediate vicinity of the cells from which they are released. Hormones that act on the cells that produced the messenger substance are referred to as autocrine hormones.

Hormones are synthesized in specialized glands, tissues and cells (e.g., neuroendocrine cells). Their target organ is either a subordinate endocrine gland (glandotropic hormone) or non-endocrine tissue (aglandotropic hormone). The target cells have high-affinity binding sites (receptors) for their specific hormone, so very low concentrations of the hormone suffice for signal transduction (10-6 to 10-12 mol/L). The receptors on the target cells pick out the substances specifically intended for them from a wide variety of different messenger substances in their environment.

Hormones work closely with the nervous system to regulate digestion, metabolism, growth, maturation, physical and mental development, maturation, reproduction, adaptation, and the internal milieu of the body (homeostasis) (^ A). Most of these actions are predominately autonomous functions subject to central control by the hypothalamus, which is controlled by higher centers of the brain (^ p. 330).

Neurotransmitters released at chemical synapses of nerve endings transmit signals to postsynaptic nerve fibers, muscles or glands (^ p. 50ff.). Some neuropeptides released by presynaptic neurons also exert their effects in neighboring synapses, resulting in a kind of "paracrine" action.

Neurons can also secrete hormones, e.g., epinephrine, oxytocin and antidiuretic hormone. Some transmitter substances of the immune system, e.g. thymosin and various cy-tokines, also have endocrine effects.

|— A. Regulation of autonomic nervous system functions (overview)

Signals from the environment

Psychological factors

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