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Endocrine System I: Superior Glands Homeostatic Mechanisms

Endocrine System vs Nervous System

Endocrine vs Exocrine Glands

Types and Actions of Hormones

Interaction of Hormones with Target Cells

Effects of Hormones on Target

Control Mechanisms of Endocrine Glands

Endocrine Signaling as Simple/Complex Reflexes

Major Superior Endocrine Organs

• Pituitary

Anterior: GH, Pl, FSH, LH, TSH, ACTH

Posterior: Oxytocin, ADH

• Thyroid: TH synthesis and release; Calcitonin

• Parathyroids: PTH

Homeostasis Maintenance of a stable internal environment =

a dynamic state of equilibrium

Feedback Mechanisms Example of Negative feedback

Receptor and

Low blood glucose

Feedback Mechanisms Examples of Positive feedback










• Pituitary





The Endocrine System A more broad-based and long-lasting

communication system than the nervous system

Uses chemical messages (hormones) that are released into the blood

Hormones control several major processes

• Reproduction

• Growth and development

• Mobilization of body defenses

• Maintenance of much of homeostasis

• Regulation of metabolism

Comparing Two Control Systems

System Mode of Messaging

Speed of Signaling

Length of Response Time Range of Focus

Nervous System

Endocrine System

Neuron “wiring” leading

directly to and from target

Hormones travel

through blood stream

systemically

Rapid, in milliseconds

Short duration; response stops in

seconds or minutes

Long-lasting;

minutes to days

Narrow & specific, a few cells to part of one organ

Broad & general; many target

tissues and organs stimulated

collectively

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QuickTime™ and a decompressor

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Slow; minutes,

hours, or days










• Pituitary





Glandular Epithelium Gland – one or more cells that secretes a

particular product

Two major gland types

• Endocrine gland

Ductless

Secretions are hormones

• Exocrine gland

Empty through ducts to the epithelial surface

Include sweat and oil glands

Types of Hormones Peptide-based hormones

Steroid hormones

Prostaglandins and catecholamines

Insulin Growth hormone

http://en.wikipedia.org/wiki/Image:Insulin.jpg

Hormones as Chemical Messengers

Endocrine gland (source)

Target organs or glands

Hormones move through the bloodstream to target organs










• Pituitary





Hormone Interaction with Target Cells Hormones bind to receptors sticking out from the plasma membrane of target

cells or within target cells

growth factor insulin epinephrine

Hormones

Examples of receptors found in the plasma membrane of cells

Receptors

Two Mechanisms of Hormone Action

Steroid hormone action1. Diffuses through the plasma membrane of target cells2. Enters the nucleus or binds to cytoplasmic receptor3. Binds to a specific protein within the nucleus if not

already bound4. Binds to specific sites on the cell’s DNA5. Activates genes that result in synthesis of new proteins

Non-steroid hormone action

1. Hormone binds to a membrane receptor; does not enter cell

2. Sets off a reaction where a G protein with bound GTP activates adenylate cyclase enzyme.

3. Adenylate cyclase produces cyclic AMP (second messenger) by converting ATP --> cAMP

5. cAMP, in turn, activates phosphorylating activation proteins (protein kinases) that trigger additional intracellular changes (enzyme activation, secretion, ion channel changes) to promote a specific response

(A few peptide hormones activate Ca+2 release via second messengers in the PIP2 calcium signaling system).

G protein

AdenylateCyclase

cAMP










• Pituitary





Effects Caused by HormonesChanges in target cells by hormonal activation

• Changes in plasma membrane permeability or electrical state

• Synthesis of proteins, such as enzymes

• Activation or inactivation of enzymes

• Stimulation of mitosis

• Activation of transcription of certain genes

Target cell activation depends on three factors

1. Blood levels of the hormone

2. Relative number of receptors on or in the target cell

3. Affinity of binding between receptor and hormone

Target Cell Activation

Hormones influence the number of their receptors

• Up-regulation—target cells form more receptors in response to the hormone

• Down-regulation—target cells lose receptors in response to the hormone

Hormones are removed from the blood by

• Degrading enzymes

• Kidneys

• Liver

• Half-life—the time required for a hormone’s blood level to decrease by half

Control of Hormone Release Hormone levels in the

blood are maintained by negative feedback

A stimulus or low hormone levels in the blood triggers the release of more hormone

Hormone release stops once an appropriate level in the blood is reached










• Pituitary






Hormonal: Chemical stimulus (i.e. endocrine

glands are activated by other hormones

Humoral: Changing blood levels of certain ions

stimulate hormone release

Neural: Nerve impulses stimulate hormone release; most are under

control of the sympathetic nervous system

Endocrine Signaling (Reflexes)

Figure 9.3

Simple Endocrine Reflex • Involves only one hormone• Controls hormone secretion by the heart, pancreas, parathyroid gland, and digestive tract

Complex Endocrine ReflexInvolves:

• One or more intermediary steps• Two or more hormones (tropic hormones cause secretion of a second hormone in target glands)• The hypothalamus

Location of Major Endrocrine Organs

Figure 9.3

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