Animal Science 434 - 2/3/98

Integration of Body Functions

Nervous System

The nervous system is a rapid communication network which serves to detect stimuli and coordinate responses to stimuli. The basic "circuitry" of the nervous system involves sensory nerves that detect and transmit information about a stimulus to the brain. The brain is a very complex network of nerve cells that processes the information and transmit a response to an effector organ via motor nerves. In most cases the effector organ is a muscle. Some noxious stimuli, such as a hot burner, require an immediate response if injury is to be avoided. In this case, a rapid "reflexive" response occurs in which the relatively slow brain processing step is eliminated.

The functional unit of the nervous system is the nerve cell, or neuron. Although their size, shape and biochemical characteristics vary considerably, their basic morphology is similar. A neuron consists of a cell body that contains the nucleus, a single long slender process called the axon, and several dendrites which are processes of the cell body. The dendrites are similar to the axon but are generally shorter.

The basic function of a neuron is to transmit information over distance similar to a telephone wire. Information is conducted along the neuron cell membrane as an electrochemical impulse. This electrochemical impulse is a highly localized event, starting near the dentritic terminals and traveling much as a wave moves through the water along the dendrite. The impulse moves toward the cell body and down the axon to the axon terminus. Thus, sensory information from physical or chemical stimuli are transduced into electrochemical impulses which are rapidly propagated along the neuron.

Sensory neurons rarely have a direct connection to the brain. Motor neurons that originate in the brain also do not connect directly to the target muscle. Rather, several neurons arranged in series relay messages through the nervous system. For example, the dendrites of sensory neurons may be located in the skin, but the sensory neuron's axon will extend deep into underlying tissue where the axon terminal meets the dendrites of a second neuron. The axon of the second neuron in turn meets the dendrites of yet another neuron. Eventually, one neuron in the series will enter the spinal cord and then connect with brain neurons.

The junction between two neurons is called a synapse. Here, messages are "relayed" from one neuron to the next. There is no physical connection between adjacent neurons. In fact, there is a narrow gap called the "synaptic cleft". Messages carried along the neuron in the form of electrochemical impulses are transferred to the adjacent neurons by chemical messengers called neurotransmitters. When the axon terminal becomes depolarized by an electrochemical impulse, it releases a minute amount of neurotransmitter substance into the synaptic cleft. The neurotransmitter travels across the cleft to the dendrites of the adjacent neuron. The arrival of neurotransmitter at the next neuron gives rise to another electrochemical impulse. The impulse is then propagated along that neuron to its axon where the synaptic transmission process is repeated again. It is in this fashion that neural information is transmitted throughout the body.

Anatomically, the nervous system may be divided into several parts. The central nervous system (CNS) includes the brain and spinal cord. The peripheral nervous system (PNS) includes the remainder of the nerves. In general, sensory nerves carry messages toward the brain and are called afferent nerves, whereas motor nerves carry messages away from the brain and are called efferent nerves. Strictly speaking, a nerve is actually a bundle of neurons (similar to a telephone cable that consists of many individual telephone wires). In general, the neurons within a given nerve innervate, or serve, a specific muscle, group of muscles, or a sense organ. For example, the facial nerve contains neurons which innervate the muscles of the face, allowing you to smile, smirk, whistle, squint, etc.

The nervous system may also be divided into the somatic nervous system which governs voluntary activities such as walking, talking or brushing your teeth, and the autonomic nervous system which controls involuntary activity such as digestion and breathing. This separation permits vital functions (breathing, blood circulation) to occur even during states of unconsciousness (sleeping, fainting, etc.) when voluntary activity has ceased.

Conscious thought (the ability to generate ideas) as well as the ability to coordinate responses to stimuli is also a property of the nervous system (the brain). Your ability to read and understand what is written in this handout is the product of no more than a well coordinated series of electrochemical impulses in the brain. In fact, the ability of the human mind to contemplate the biological basis for comprehension itself is a remarkable feat.

Endocrine System

Occasionally the activities of the nervous system join those of the other major communication systems in the body, the endocrine system. The endocrine system is generally involved with slower adjustments in response to stimuli than the nervous system. Responses from the endocrine system may be measured in hours, months or even years (such as the hormonally dependent events of puberty) instead of seconds as is seen with the nervous system. Endocrine communication is mediated by substances called hormones. By definition a hormone is a "substance produced in one tissue, secreted into the blood and transported by the vascular system to a distant target tissue, where it exerts its effect". In some instances, the hormone-secreting tissue is nerve tissue, in the other cases the target organ maybe nerve tissue, or the hormone-secreting tissue and target tissue may have nothing to do with the nervous system. The integration of the nervous system and endocrine system during response to a stimuli is refered to as activation of the neuroendocrine system.

The classic example of neuroendocrine integration in farm animals is the "milk-let down" reflex in cows. There is generally a 6-8 minute delay between the time of initial udder stimulation (suckling, manual stimulation) and full milk release. This is because milk letdown depends on oxytocin, a hormone from the posterior pituitary gland that is located at the base of the brain. The release of oxytocin depends in turn on the neural impulses from sensory neurons in the udder. So, when the udder is stimulated, neural signals travel along afferent nerves to the spinal cord and into the brain where they are relayed to secretory neurons of the posterior pituitary gland. These neurons contain oxytocin, which is secreted into the capillary bed surrounding the posterior pituitary. The blood then flows through the venous system to the heart where it travels to the lungs, back to the heart, then to the arterial system, and finally to the udders where it stimulates milk let down.

The endocrine system usually functions to regulate long acting body changes or changes which adapt the animal to its environment and provide homeostasis for the body. Traditionally, the endocrine system and the nervous system have been considered separate entities; as more is learned of each, close association of the two have become apparent. For example, the hypothalamus of the brain has direct influence on the pituitary gland by producing the pituitary hormones or regulating their release. The adrenal medulla produces a hormone, epinephrine (adrenalin), and the sympathetic nerves produce a similar substance, norepinephrine. A clear cut definition of a hormone which would separate the two becomes difficult.

Historically, the proof of the presence of a hormone and the way in which endocrine glands have been studied have consisted of the following steps:

1) Separation of the gland in question from the body.

2) Replacement of the gland at another site to determine if the body functions lost after gland removal are restored.

3) Injection of a gland extract to determine if the body functions lost after gland removal are restored.

4) Purification of the glandular extract and identification of the active component.

5) Synthesis of the hormonal substance.

6) Determination of how the hormone causes its effect on the body and specifically, how it controls a body function.

Proof from the first three steps is required before a substance is classified as a hormone. Thus, a hormone is an organic substance, produced by specific tissues which act in a regulatory manner on some other tissue, gland or organ (the target organ or organs).

The glands producing hormones are called endocrine glands. Endocrine glands are ductless glands which drop their secretions into the circulatory system for transport to a target organ. In contrast, the other major type of gland, exocrine glands, are glands that are connected by ducts to their target organs. The secretions of endocrine glands (hormones) can be elicited by a neural impulse, another hormone, or changes in blood chemistry. Examples of these are the release of the releasing hormones from the neurons in the hypothalamus (GnRH), the release of trophic hormones (LH, FSH) by the pituitary in response to the releasing hormones of the hypothalamus (GnRH), and the release of insulin from the pancreas in response to an increase in blood glucose. Endocrine glands are also often regulated by their target organ in a feedback manner. When this occurs, a substance produced by the target organ is transported by the blood back to the endocrine gland where it regulates the release of hormone from that endocrine gland. An example of this is how LH (secreted by the pituitary) stimulates the follicle to produce estrogen. The estrogen in turn inhibits or in some cases stimulates the release of LH from the pituitary.

Chemically, hormones fall into four classes. They are proteins, steroids, fatty acids or modified amino acids. Some of the hormones, specifically the steroids, are transported in blood bound to specific blood transport proteins. Hormones effect their target cells by binding to specific proteins known as receptors. Receptor proteins are located either on the surface or in the nucleus of the target cells. The location of the receptor depends on the chemical category of the hormone. The receptor-hormone complex then activates a series of responses in the target cell that may include stimulation of cell multiplication, promote the synthesis of a specific enzyme, or cause the release of a secretory product.

The pituitary gland is considered the master of the other endocrine glands because the hormones stored in the pituitary gland control production of hormones produced by other endocrine glands. There are two major lobes to the pituitary gland in which hormones are produced or stored. These are called the anterior pituitary and the posterior pituitary. Most of the function of the pituitary gland is controlled by a portion of the brain which is also an endocrine gland. This structure is called the hypothalamus. The hormones of the posterior pituitary are produced in the hypothalamus and transported to the pituitary gland along nerve axons. The release of these hormones from the pituitary gland is also controlled by the hypothalamus.

The hormones of the anterior pituitary gland are produced and stored in the anterior pituitary gland. Blood-transported releasing factors from the hypothalamus direct the anterior pituitary gland to release specific hormones into the circulatory system. There are nine hypothalamic hormones. They have names such as gonadotrophin releasing hormone, corticotropin releasing hormone, thyrotrophin releasing hormone, somatostatin and prolactin inhibiting hormone. The anterior pituitary hormones are called trophic hormones because they stimulate other endocrine glands or tissues to function. There are several anterior pituitary hormones of which growth hormone or somatotrophic hormone (GH or STH), follicular stimulating hormone (FSH), luteinizing hormone (LH), thyroid stimulating hormone (TSH), prolactin and adrenal corticotrophic hormone (ACTH) are the most important with regards to reproduction. The hormones LH and FSH are refered to as gonadotropins because they stimulate the gonads, the ovary or testis.

The gonads themselves are also endocrine glands and produce the steroid hormones estradiol (an estrogen), progesterone and testosterone. The gonads also produce the protein hormones inhibin and relaxin as well as the above mentioned steroid hormones.

In the female, the uterus is also an endocrine gland that produces the fatty acid hormone, prostaglandin. This hormone is important in regulation of the estrous cycle. During pregnancy, the placenta of the developing fetus produces the hormones, placental lactogen and chorionic gonadotropin (equines and primates). In addition, the placenta is also capable of producing estrogen, progesterone and relaxin.

A final endocrine gland that effects reproduction is the pineal gland. This gland is located at the top of the brain and secretes the hormone melatonin, that modulates the effects of light on reproduction. The pineal gland is responsible for seasonal regulation of reproduction.

There are other endocrine glands in the animal that can effect reproduction such as the adrenal, thyroid, pancreas and parathyroid, but these glands secret hormones that primarily regulate metabolic homeostasis of the animal. Indirectly these glands and their secretions, effect reproduction because if metabolism of an animal is disturbed, this ultimately has an effect on ability of the animal to reproduce.

The table which follows contains most of the important hormones and a summary of where they are secreted from and their major effects. To conserve space in the table the common abbreviations for the hormones have been used. A listing of the abbreviations and the full name of the hormones follows the table.

TABLE 1. THE ENDOCRINE SYSTEM

Endocrine Gland	Hormone Secreted	Major Actions of These Hormones
Pineal gland	melatonin	plays a role in modulating the effects of photoperiod on reproduction
Hypothalamus	Releasing Factors
	GnRH	Release of FSH and LH from pituitary
	TRH	Release of TSH and prolactin from pituitary
	CRH	Release of ACTH from pituitary
	GRH	Release of growth hormone
	Inhibiting Factors
	PIF	Inhibits release of prolactin from pituitary
	somatistatin	Inhibits GH release
Pituitary gland
Anterior Pituitary	Gonadotropins
	LH	In the ovary: formation of corpora lutea; secretion of progesterone; acts in conjunction with FSH to cause follicular growth, estrogen secretion and ovulation
		In the testis: stimulates the Leydig cells to produce the androgen testosterone.
	FSH	In the ovary: growth of ovarian follicles; functions with LH to cause follicular growth and estrogen secretion.
		In the testis: acts on seminiferous tubules to promote spermatogenesis.
	Prolactin	Causes proliferation of mammary gland and initiation of milk secretion;responsible for maternal behaviour after parturition; prolongs the functional life of the corpus luteum in rodents.
	Other trophic hormones
	TSH	Stimulates thyroid gland to secrete thyroxine (T4) and triiodothyronine (T3)
	ACTH	Stimulates adrenal gland to secrete cortisol.

Endocrine Gland	Hormone Secreted	Major Actions of These Hormones
Anterior Pituitary (cont.)
	Growth Hormone	Promotes growth and stimulates liver to produce somatomedin which promotes growth.
Intermediate lobe	MSH	effect on pigmentation
Posterior pituitary	oxytocin	milk let down and uterine contractions; also produced in ovary and may be involved in luteolysis
	vasopressin	salt and water balance
Gonads
Ovary	follicle
	estrogen	secondary sex characteristics; mating behavior; maintenance of female reproductive tract and duct system of the mammary gland; negative or positive feedback on gonadotropin secretion
	inhibin	inhibits secretion of FSH by anterior pituitary
Ovary	corpus luteum
	progesterone	maintenance of pregnancy; mammary development; negative feedback on gonadotropin secretion
	relaxin	dilation of cervix; expansion of pelvis at parturition
Testis	Leydig cells
	testosterone	maintenance and function of male reproductive tract and accessory glands; male mating behavior; spermatogenesis
	Sertoli cells
	estrogen	negative feedback on gonadotropin secretion from the anterior pituitary
	inhibin	inhibits release of FSH from the anterior pituitary
Uterus	prostaglandin
	(PGF2a)	initiates regression of the corpus luteum; induction of parturition

Endocrine Gland	Hormone Secreted	Major Actions of These Hormones
Placenta	progesterone
	estrogen
	relaxin	same functions as when produced by the ovary
	chorionic gonadotropins
	HCG	produced in primate placenta and has LH like activity
	PMSG or ECG	produced in the equine and has mostly FSH like activity but also contains some LH like activity
Thyroid gland	T4 and T3	effects general body growth, energy metabolism and heat production
Parathyroid gland	parathyroid hormone	anatomically associated with the thyroid gland and involved in calcium metabolism
Pancreas	insulin	decreases blood glucose by causing glucose to enter cells.
	glucagon	increases blood glucose by stimulating gluconeogenesis from the liver.
Adrenal glands
Adrenal cortex	cortisol	Induction of parturition; involved in stress; metabolism of protein,carbohydrate and fat.
Adrenal medulla	epinephrine and norepinephrine	effects on heart rate, blood pressure and blood flow through muscle, liver and the brain; effects on carbohydrate metabolism