Sabtu, 19 Juni 2010

it just about animal phisiology

The endocrine or hormonal system (i.e. the use of body fluid-borne chemical messengers) together with the nervous system makes up the control and coordinating systems of animals. However, there are major differences in the way in which control is achieved within the two systems. Firstly, the endocrine system works by transmitting chemical rather than electrical signals, although the nervous system utilizes chemical messengers at synapses. Secondly, the endocrine system has much slower response time then the nervous system. 
   The hormone that regulating for following activities are:
·         Metabolism             : Gastrin and Corticosterone
·         Growth                   : Thyroxin
·         Reproduction          : Estrogen, Progesterone, Testosterone
·         Osmotic and ionic   : aldosterone , parathormone/calcitonin, melanocyte-stimulating hormon
The hypothalamus is part of the vertebrate brain and sits beneath the thalamus. Its main function is an interface between the nervous and endocrine system. A major role of the hypothalamus is to control the pituitary gland-the so-called master gland. Hormones are synthesized in the cell bodies of hypothalamic neurons. ADH and oxytocin are released from the axon terminals into the posterior pituitary gland. The releasing and release-inhibiting hormones are released from axon terminals into capillaries and are transported to the anterior pituitary via blood. These hormones stimulate the release of other hormones from the endocrine cells of the anterior pituitary.
The posterior pituitary (or neurohypophysis) comprises the posterior lobe of the pituitary gland and is part of the endocrine system. The posterior pituitary consists mainly of neuronal projections (axons) extending from the supraoptic and paraventricular nuclei of the hypothalamus. These axons release peptide hormones into the capillaries of the hypophyseal circulation. In addition to axons, the posterior pituitary also contains pituicytes, specialized glial cells resembling astrocytes. Hormones known classically as posterior pituitary hormones are synthesized by the hypothalamus. They are then stored and secreted by the posterior pituitary into the bloodstream. They are oxytocin and vasopressin. Hormone that not synthesized by the neurohypophyis are groeth hormone, Tyrotropin-stimulating hormone (TSH), prolactin, leutenizing hormone, follicle-stimulating hormone, melanocyte-stimulating hormone, and corticotropin (ACTH). They are released by anterior pituitary gland.
 Invertebrate neuroendocrine system:
·         Coelenterates
Coelenterates, such as Hydra, have cells which secrete substances involved with reproduction, growth and regeneration. If the head of a Hydra is removed a peptide molecule is secreted by the rest of the body. This substance is called the ‘head activator’. Its effect is to cause the reminder of the body to regenerate the mouth and tentacles which make up the head region.
·         Platyhelminthes
In a similar manner to the coelenterates, substances have been found in the flatworms which are involved in the processes of regeneration. It has also been suggested that hormones are involved in osmotic and ionic regulation as well as in reproductive processes.
·         Nematodes
Evidence for a role for neuroendocrine control in the nematodes has come from those members of the phyla which are parasitic. As with many parasitic organisms, different stages of the life cycle are quite often completed in different hosts. This means, therefore, that developmental changes in the nematode must coincide with the movement of the nematode to a new environment or new host. It has been established that nematodes have neuroendocrine secretory structures associated with their nervous systems, in the ganglia in the head region and some of the nerve cords which run the entire length of the body. It is possible that changes in the immediate environment of the worm are the triggers for activity in this neuroendocrine control system and the consequent changes in physiology which are necessary for these animals. 
There are three main groups of neuroendocrine cells in the nervous system of insects:
·         The median neurosecretory cells, which send axons down to the paired corpora cardiaca which act as storage and release sites for the neurohormones;
·         The lateral group of neurosecretory cells, which also send their axons down to the corpora cardiaca;
·         The subesophageal neurosecretory cells, which send their axons down to the corpora allata, which are classical endocrine glands.
In normal condition, hypothalamus releases the corticotropin-releasing hormone (CRH) that stimulates the anterior pituitary gland to release the cortiicotropin (adrenocorticotropic hormone, ACTH). The ACTH is stimulating the adrenal cortex to release cortisol. The secretion levels are controlled via a negative feedback mechanism. There are three ways to control the cortisol. First, if the cortisol in high levels, it could be stimulate the hypothalamus to inhibit the CRH secretion. Second, if the ACTH in high levels, it may possibly stimulate the hypothalamus to inhibit the CRH secretion. The third is if the cortisol is in high levels, it could be stimulate the anterior pituitary gland to inhibit further ACTH secretion. So that reduced levels of the cortisol and ACTH have the opposite effect.

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