CHEMICAL SIGNALS IN ANIMALS CHAPTER 45 CHEMICAL SIGNALS IN ANIMALS

I. AN INTRODUCTION TO REGULATORY SYSTEMS ANIMALS HAVE TWO REGULATORY SYSTEMS THAT COORDINATE INTERNAL BODY FUNCTIONS: THE NERVOUS SYSTEM AND THE ENDOCRINE SYSTEM. CHEMICAL MESSENGERS CALLED HORMONES REGULATE THE ACTIVITY OF TARGET ORGANS AT DISTANT SITES.

NEUROSECRETORY CELLS OF THE NERVOUS SYSTEM SECRETE MANY HORMONES; A.    THE ENDOCRINE SYSTEM AND THE NERVOUS SYSTEM ARE STRUCTURALLY, CHEMICALLY, AND FUNCTIONALLY RELATED THE ENDOCRINE AND NERVOUS SYSTEMS OFTEN FUNCTION INSEPARABLY IN MAINTAINING HOMEOSTASIS, DEVELOPMENT, AND REPRODUCTION. NEUROSECRETORY CELLS OF THE NERVOUS SYSTEM SECRETE MANY HORMONES; SEVERAL HORMONES, SUCH AS NOREPINEPHRINE, ARE USED AS SIGNALS BY BOTH THE ENDOCRINE AND THE NERVOUS SYSTEM; AND MANY BODY FUNCTIONS ARE REGULATED BY BOTH SYSTEMS, OFTEN BY FEEDBACK MECHANISMS.

III. THE VERTEBRATE ENDOCRINE SYSTEM (SEE FIG. 45.6)

A.    THE HYPOTHALAMUS AND PITUITARY INTEGRATE MANY FUNCTIONS OF THE VERTEBRATE ENDOCRINE SYSTEM NEUROSECRETORY CELLS OF THE HYPOTHALAMUS INTEGRATE ENDOCRINE AND NEURAL FUNCTION BY INFLUENCING THE PITUITARY GLAND. THE POSTERIOR PITUITARY IS AN EXTENSION OF THE BRAIN THAT STORES AND RELEASES TWO HORMONES (OXYTOCIN AND ANTIDIURETIC HORMONE, ADH) PRODUCED BY NEUROSECRETORY CELLS IN THE HYPOTHALAMUS. OXYTOXIN INDUCES UTERINE CONTRACTIONS AND MILK EJECTION, AND ADH ENHANCERS WATER REABSORPTION IN THE KIDNEYS.

UNDER THE DIRECTION OF RELEASING AND INHIBITING HORMONES CONVEYED BY SPECIAL PORTAL VESSELS FROM THE HYPOTHALAMUS, THE ANTERIOR PITUITARY PRODUCES AN ARRAY OF HORMONES, INCLUDING: THYROID-STIMULATING HORMONE (TSH) FOLLICLE STIMULATING HORMONE (FSH) LUTEINIZING HORMONE (LH) GROWTH HORMONE (GH) PROLACTIN (PRL) ADRENOCORTICOTROPIC HORMONE (ACTH) MELANOCYTE-STIMULATING HORMONE (MSH) THE ENDORPHINS.

THE CHEMICALLY RELATED TROPIC HORMONES TSH AND THE GONADOTROPINS (FSH AND LH) STIMULATE THE THYROID GLAND AND THE GONADS, RESPECTIVELY TO PRODUCE THEIR HORMONES. GH PROMOTES GROWTH DIRECTLY AND STIMULATES THE PRODUCTION OF GROWTH FACTORS. PROLACTIN, NAMED FOR ITS STIMULATION OF LACTATION IN MAMMALS, HAS DIVERSE EFFECTS IN DIFFERENT VERTEBRATES. ACTH STIMULATES THE ADRENAL CORTEX. MSH INFLUENCES SKIN PIGMENTATION IN SOME VERTEBRATES AND FAT METABOLISM IN MAMMALS. ENDORPHINS, THE BRAINS’S NATURAL OPIATES, INHIBIT THE PERCEPTION OF PAIN.

B. THE PINEAL GLAND IS INVOLVED IN BIORHYTHMS THE PINEAL GLAND SECRETES MELATONIN, WHICH INFLUENCES SKIN PIGMENTATION, BIOLOGICAL RYTHYMS, AND REPRODUCTION IN VARIOUS VERTEBRATES. C. THYROID HORMONES FUNCTION IN DEVELOPMENT, BIOENERGETICS, AND HOMEOSTASIS D. PARATHYROID HORMONE AND CALCITONIN BALANCE BLOOD CALCIUM E. ENDOCRINE TISSUES OF THE PANCREAS SECRETE INSULIN AND GLUCAGON, ANTAGONISTIC HORMONES THAT REGULATE BLOOD GLUCOSE

THE ENDOCRINE PORTION OF THE PANCREAS CONSISTS OF ISLET CELLS THAT SECRETE INSULIN AND GLUCAGON. HIGH BLOOD GLUCOSE LEVELS STIMULATE THE RELEASE OF INSULIN, WHICH INCREASES THE CELLULAR UPTAKE OF GLUCOSE, PROMOTES THE FORMATION AND STORAGE OF GLYCOGEN IN THE LIVER, AND STIMULATES PROTEIN SYNTHESIS AND FAT STORAGE. LOW BLOOD GLUCOSE LEVELS TRIGGER GLUCAGON RELEASE, WHICH INCREASES BLOOD GLUCOSE IN THE LIVER AND INCREASING THE BREAKDOWN OF FAT AND PROTEIN. TYPE I DIABETES IS AN AUTOIMMUNE DISORDER RESULTING IN A LACK OF INSULIN. TYPE II DIABETES IS USUALLY CAUSED BY THE LOSS OF RESPONSIVENESS OF TARGET CELLS TO INSULIN.

THE ADRENAL MEDULLA AND ADRENAL CORTEX HELP THE BODY MANAGE STRESS G. GONADAL STEROIDS REGULATE GROWTH, DEVELOPMENT, REPRODUCTIVE CYCLES, AND SEXUAL BEHAVIOR. THE GONADS- TESTES AND OVARIES- PRODUCE VARYING PROPORTIONS OF THREE KINDS OF STEROID HORMONES: ANDROGENS, ESTROGENS, AND PROGESTINS.