Chapter 45: Hormones and the Endocrine System

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In animals, hormones are secreted into the extracellular fluid, circulate in the blood, and communicate regulatory messages throughout the body

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Hormones bind to specific receptors. A response will only occur in specific target cells

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Function of the endocrine system: Chemical signaling by hormones.

Endocrine system and nervous system are the basic systems for communication and regulation in the animal body

Function of nervous system: network of specialized cells that transmit signals along dedicated pathways.

Signaling by neurons can regulate the release of hormones, the 2 systems overlap

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Intercellular Communication

-Classified in 2 ways

- Type of secreting cell

- Route taken by the signal

1) Endocrine signaling

- Hormones secreted into extracellular fluid by endocrine cells reach target cells via the bloodstream

- Maintain homeostasis (hormones regulate responses to environmental stimuli, regulates growth and development, and triggers physical/behavioral changes in maturity and reproduction)

2) Paracrine and Autocrine signaling

-Local regulators are molecules that act over short distances and reach their target by diffusion only. Act within seconds

In Paracrine, target cells are near the secreting cell

In Autocrine, the secreting cells are the target cells

Prostaglandins - local regulators that mediate signaling. Promote inflammation and sensation of pain from injury in immune system

3) Synaptic Signaling

-Neurons form synapses with target cells and secrete molecules known as neurotransmitters that diffuse a short distance and bind to receptors on target cell

- At most synapses, neurons secrete molecules (neurotransmitters) that diffuse short distance and bind to receptors

- Neurotransmitters essential for sensation/memory/cognition/movement

Neuroendocrine Signaling

- Specialized neurons (neurosecretory cells) secrete neurohormones, which diffuse from nerve cell endings into the bloodstream

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Signaling by Pheromones

- Pheromones are chemicals that are released into the external environment (ants)

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Local Regulators

- Prostaglandins - modified fatty acids

- Nitric oxide (NO, gas) - functions as a local regulator and a neurotransmitter. When O2 levels fall, NO is released which activates an enzyme that relaxes the cells, increasing blood flow (vasodilation)

- In males, vasodilation enables sexual function by increasing blood flow to penis. Viagra sustains an erection by prolonging activity in NO pathway

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Classes of Hormones

-3 classes: Polypeptides, steroids, and amines

- Insulin is a polypeptide that contains 2 chains in its active form

-(Cortisol) steroid hormones are lipids that contain 4 fused carbon rings

-Epinephrine and thyroxine are amine hormones, which are synthesized from a single amino acid, either tyrosine or tryptophan

-Hormones vary in their solubility

- Polypeptides + amine hormones = water soluble

-steroid hormones and hydrophobic hormones = lipid soluble

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Cellular Response Pathways

- Water soluble hormones are secreted by exocytosis and travel freely in bloodstream. Because they are insoluble in lipids, they cannot diffuse through plasma membranes of target cells, so they must bind to cell-surface receptors. Triggers changes in cytoplasmic molecules and sometimes alters gene transcription

- Lipid-soluble hormones diffuse through membranes of endocrine cells. Once outside, they bind to transport proteins that keep them soluble in the aqueous blood. After leaving blood they diffuse into target cells and bind to receptors in nucleus or cytoplasm. Triggers changes in gene transcription

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PATHWAY FOR WATER SOLUBLE HORMONES

- Binding of w-s molecule to receptor protein triggers events at plasma membrane that result in cellular response

- Some cell-surface receptors cause proteins in cytoplasm to move into nucleus and alter transcription of specific genes

- Signal transduction - the series of changes in proteins that converts the extracellular chemical signal to a specific intracellular response

- Example: running to catch bus - adrenal glands secrete epinephrine (adrenaline). Once the epinephrine reaches the liver, it binds to a G Protein Coupled Receptor in the plasma membrane of target cells. The binding triggers cascade in events in liver ( synthesis of cAMP as a second messenger, activation of protein kinase A by cAMP leads to activation of enzyme required for glycogen breakdown and inactivation of an enzyme needed for glycogen synthesis) End result = liver release glucose into bloodstream, providing fuel to chase the bus

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PATHWAY FOR LIPID SOLUBLE HORMONES

- Intracellular receptors for l-s hormones perform entire task of transducing the signal within a target cell (hormone activates receptor, which then directly triggers cell's response)

- Usually, the response for l-s hormones is change in gene expression

- Most steroid hormone receptors are in cytosol prior to binding. Once bound, a hormone receptor complex forms and moves into nucleus. There, the receptor alters transcription of particular genes by interacting with specific DNA binding protein/response element in the DNA

- L-s hormones that are not steroids have receptors in nucleus. Once bound, the receptor binds to specific site in the cell's DNA and stimulates the transcription of specific genes

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- The multiple effects of hormones enhance the rapid reactions of the body

- A hormone can have multiple responses if its target cells differ in their receptor type or in the molecules that produce the response

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Endocrine glands secrete hormones into surrounding fluids

Exocrine glands carry substances onto body surfaces or into body cavities

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Pineal Gland

-Produces melatonin which is active in the regulation of biological rhythms

Hypothalamus

- Receives info and Initiates endocrine signaling

-Signals travel to pituitary gland

Pituitary gland

Anterior

- FSH and LH stimulate gonads

- TSH stimulates the thyroid

-ACTH stimulates the adrenal cortex

-Prolactin stimulates mammary gland cells

- GH stimulates growth

Posterior

- Oxytocin stimulates contraction of smooth muscle cells in uterus and mammary glands

- Vasopressin promotes retention of water by kidneys (antidiuretic)

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Thyroid gland

- T3 and T4 stimulate and maintain metabolic processes

- Calcitonin lowers blood calcium level

Parathyroid

- PTH raises blood calcium level

Adrenal Glands

-Epinephrine and norepinephrine raise blood glucose level, increase metabolic activities (adrenaline)

-Glucocorticoids raise blood glucose level

-Mineralocorticoids promote reabsorption of NA and excretion of K in kidneys

Pancreas

-Insulin lowers blood glucose

Glucagon raises blood glucose

Ovaries

-Estrogens stimulate uterine lining growth, promote development of female sex characteristics

Progestins promote uterine lining growth

Testes

-Androgens support sperm formation, promote development of male sex characteristics

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Simple Hormone Pathways

-2 types of organization - simple endocrine pathway, and simple neuroendocrine pathway

-Simple endocrine pathway - endocrine cells respond directly to an internal or environmental stimulus by secreting a particular hormone. Hormone travels in blood and reaches target cells, interacts with its receptors

-Neuroendocrine pathway - stimulus is received by a sensory neuron, which stimulates a neurosecretory cell, which then secretes a neurohormone which diffuses into blood and travels to target cells

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Feedback Regulation

-Negative - response reduces the initial stimulus

-By decreasing hormone signaling, negative feedback regulation prevents excessive pathway activity

-Positive - reinforces a stimulus, leading to a greater response

- Positive feedback amplifies both stimulus and response, negative feedback helps restore a pre existing state

Hormone pathways often involve negative feedback