E n d o c r i n e S y s t e m

Section 1, Chapter 13

I n t r o d u c t i o n• The endocrine system assists the nervous system with communication and control of the body

• Characteristics of endocrine glands1. They are ductless2. Endocrine glands secrete hormones3. Hormones are carried to distant target cells through the

bloodstream4. Hormones only act on cells (target cells) that possess receptors

sensitive to the hormone – highly specific action.

1. Exocrine glands• Have ducts • Secrete chemicals directly onto a surface• Examples: sweat glands, mucous cells

2. Paracrine signals• Chemicals that affect only nearby cells• Example: prostaglandins secreted with semen stimulate

muscular contractions within female reproductive organs

3. Autocrine signals• Chemicals that affects only the cells that produced it.• Example: T-cells secrete interleukins (IL), which stimulate the

proliferation of the T-cells (monoclonal population)

4. Neuroendocrine • Nervous tissue that secretes hormones• Example: hormone secretion from the hypothalamus.

O t h e r s i g n a l s

Endocrine vs. Nervous TissueThe endocrine and nervous systems communicate using chemical signals• Neurons release neurotransmitters into a synapse affecting

postsynaptic cells

• Endocrine glands release hormones into the bloodstream to specific target cell receptors

Figure 13.2 Chemical communication. (a) neurons release neurotransmitters onto synapses, affecting postsynaptic cells. (b) Glands release hormones into the bloodstream. Blood carries hormone molecules throughout the body, but only target cells respond.

Endocrine vs. Nervous Tissue

Cell…………………………………. Neuron Glandular EpitheliumSignal………………………………. neurotransmitter hormoneSpecificity of action…………. receptors on postsynaptic cell receptors on target tissuesSpeed of onset……………...... <second seconds to hoursDuration of action……………. usually very brief may be brief or last for days

Nervous System Endocrine System

Chemistry of Hormones

• Chemically, hormones are either:1. Steroid or steroid-like hormones

2. Biogenic Amines

3. Peptide hormones

Steroid Hormones

Include• Estrogen• Testosterone• Androgens (weak sex hormones)• Aldosterone• Corticoids (hormones secreted from the adrenal cortex)

Properties• Steroid hormones are derived from cholesterol• They are composed of hydrophobic lipids (insoluble in water)

Biogenic AminesProperties• Amines are synthesized from amino acids

Include• Norepinephrine• Epinephrine• Melatonin • Thyroid hormones (these are also hydrophobic, or water insoluble)

Peptide HormonesProperties• Composed of long chains of amino acids (polypeptides)

Include• Hypothalamic hormones• Pituitary hormones• Pancreatic hormones• Gastrointestinal hormones

Water Solubility & Membrane Permeability

Steroid + Thyroid Hormones• Are hydrophobic – transported in the plasma attached to proteins

• Cell membrane permeable – due to their hydrophobic properties, these hormones readily cross the phospholipid bilayer of the cell membrane.

All other Hormones• Are hydrophilic– freely dissolved in plasma

• Cell membrane impermeable – these hormones do not cross the cell membrane, and must rely on 2nd messengers to relay a signal into target cells.

• 2nd messenger – molecule that relays and amplifies a hormone signal into the cell.

Actions of steroid hormones

1. A steroid hormone crosses the cell membrane2. Hormone combines with a protein receptor in the nucleus3. The hormone-receptor complex activates transcription of a specific DNA region4. The mRNA leave the nucleus into the cytoplasm 5. The mRNA is translated into a protein.

Actions of Non-steroid

hormones

1. A non-steroid hormone reaches the target cell,2. The hormone binds to a membrane receptor3. Binding to the receptor activates an enzyme in the cell membrane (adenlyate cyclase)4. Adenlyate cyclase converts ATP into cyclic adinosine monophosphate (cAMP)5. cAMP is a second messenger that promotes a series of reactions leading to the cellular

changes associated with the hormone’s action.

Control of Hormonal SecretionsHormone secretion is generally controlled in three ways:1. Negative Feedback2. Hormone Deactivation3. Up/Down Regulation

Negative Feedback

Figure 13.10 Hormone secretion is under negative feedback.

The endocrine gland, or system controlling it senses the concentration of the hormone that gland secretes.

When the level of a specific hormone drops below needed levels, the endocrine gland is stimulated to secrete more hormone.

As the hormone concentration reaches the needed level, stimulation of that endocrine gland is reduced, and production of that hormone is reduced.

Figure 13.8 Examples of endocrine system control. (a) one way the hypothalamus controls the anterior pituitary, which in turn controls other glands (b) the nervous system controls some glands directly, and (c) some glands respond directly to changes in the internal environment.

Indicates negative feedback inhibition.

Negative Feedback

Figure 13.11 As a result of negative feedback, hormone concentration s remain relatively stable, although they may fluctuate slightly above and below average concentrations.

Hormone DeactivationHalf-life: measures the time for half of the hormone molecules to be removed from plasma

Example of half-life: a hormone with a half-life of 10 minutes, decreases in concentration by half every 10 minutes.

Time Hormone Concentration 0 minutes 100%10 minutes 50%20 minutes 25%30 minutes 12.5%

Hormones are continually secreted in the urine, and broken down by enzymes, primarily in the liver.

Up/Down RegulationUp-regulation increases the number of receptors on the target cell• Up regulation increases a cell’s sensitivity to a hormone

Down-regulation decreases the number of receptors on target cells.• Down regulation decreases a cell’s sensitivity to a hormone