As we have seen in the foregoing discussion on hormonal and neural coordination of functions in the body, both the nervous and endocrine systems are integrative. That is, each coordinates the functioning of organs and systems in separate parts of the body. In the nervous system the medium of communication is a chain of neurons. At the end of the chain a chemical such as acetyl choline, norepinephrine or serotonin is released to produce the characteristic response in the effector. In endocrine coordination, the medium of communication is a chain of blood vessels and a hormone secreted into it at one place, reaches the distant effector to produce the response. Except for a large majority of skeletal muscles, most effector organs and systems are regulated both by hormonal as well as nervous influences.
There are certain essential differences between the hormonal and nervous coordinating systems. The nervous system is involved in high-speed and short duration responses. The endocrine system on the other hand, is involved in slower, longer-lasting responses. However, an understanding of these two systems of coordination reveals that the mechanism by which both these act on the effector to elicit a response is more or less similar. Both depend on the delivery of an active response eliciting chemical to the effector organ. They differ primarily in the way in which they deliver this chemical. In a sense, a nerve cell can be considered as an endocrine cell which, in the interest of speed eliminated the vascular systems by extending itself directly to the effector organ and its secretes the active chemical directly in contact with the effector.
Previously it was thought that these two coordinating systems were two separate systems acting against or opposing each other. However, later studies showed that these two systems were, in fact, integrated into a unified system to maintain homeostasis and the responses of the animal to external and internal stimuli largely depend on a coordinated or integrated function of both the systems.
Certain nerve cells (sympathetic nerves) and some endocrine glands (adrenal medulla) secrete the same substances (norepinephrine). Another example of the close relationship between the nervous and endocrine systems is that certain neurons in the hypothalamus secrete hormones (neurosecretion) besides conducting nerve impulses. These neurons called neurosecretory neurons directly pour their hormones into the blood stream, as their axons terminate near blood vessels. The hypothalamic hormones that stimulate the anterior pituitary gland to produce its hormones, are all neurosecretory products. The hormones of the posterior pituitary, ADH and oxytosin are also products of hypothalamic neurosecretory cells. Thus endocrine glands can mimic the nervous system by producing neurotransmitters (norepinephrine from adrenal medulla) and the nervous system can mimic the endocrine system (neurosecretion).
The nature of neuroendocrine integration became clearer when information about a phenomenon called neuroendocrine reflex was known. These are certain endocrine-physiological events which follow a change in the external environment. Several examples of neuroendocrine reflexes are known. When the environmental temperature is lowered, a rat maintains its body temperature by releasing more thyroxine and oxidizing more food for heat. When birds are subjected to summer lighting schedules (long daylight) in winter, their gonads grow to spring or summer in size. In certain animals such as the rabbit and the ferret, ovulation occurs only in response to copulation (reflex ovulation). All three of these examples involve a successive series of events involving both neural as well as hormonal regulatory mechanisms. The reflex ovulation in the rabbit is diagrammatically illustrated in the figure. Copulation in the rabbit stimulates afferent neurons to the central nervous system. The nervous impulse rises in the spinal cord and eventually reaches the hypothalamus. From here a neurosecretory neuron extends into the median eminence, just above the pituitary. A neurohormone is carried by blood vessels from the median eminence into the anterior pituitary. The anterior pituitary, in turn releases LH (luteinising hormone) which goes via blood to the ovary and stimulates ovulation.
Ovulation in Rabbit as an Example of a Neuro-endocrine ReflexThree levels of integration in the neuroendocirne system are usually recognized. In the first order, neurosecretory products (neuro-hormones) act upon nonendocrine target tissues or organs (Example: hormones from posterior pituitary or pars nervosa, ADH and oxytocin).
These hormones affect target organs such as the uterus, kidneys and mammary glands. In a second order process, a neurohormone from the hypothalamus acts on an endocrine gland (pituitary) which then secretes a hormone that acts on a final set of targets. Third order neuroendocrine controls are more complex; these include hypothalamic neurohormones, anterior pituitary trophic hormones and target glands (thyroid, adrenals and gonads) and their respective targets.
Three Orders of Neuroendocrine Integration. The Broken Lines Indicate Negative Feedback