Section 1, Chapter 13

• The endocrine system assists the nervous system with communication

and control of the body

• Characteristics of endocrine glands

1. They are ductless

2. Endocrine glands secrete hormones

3. Hormones are carried to distant target cells through the 3. Hormones are carried to distant target cells through the

bloodstream

4. 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 organsmuscular 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.

Endocrine vs. Nervous Tissue

The 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 Epithelium

Signal………………………………. neurotransmitter hormone

Specificity of action…………. receptors on postsynaptic cell receptors on target tissues

Speed of onset……………...... <second seconds to hours

Duration 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 hormones3. Peptide hormones

Steroid Hormones

Include

• Estrogen

• Testosterone

• Androgens (weak sex hormones)

Properties

• Steroid hormones are derived from cholesterol

• They are composed of hydrophobic lipids (insoluble in water)

• Androgens (weak sex hormones)

• Aldosterone

• Corticoids (hormones secreted from the adrenal cortex)

Biogenic Amines

Properties

• Amines are synthesized from amino acids

Include

• Norepinephrine

• Epinephrine

• Melatonin • Melatonin

• Thyroid hormones (these are also hydrophobic, or water insoluble)

Peptide Hormones

Properties

• Composed of long chains of amino acids (polypeptides)

Include

• Hypothalamic hormones

• Pituitary hormones

• Pancreatic 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 membrane

2. Hormone combines with a protein receptor in the nucleus

3. The hormone-receptor complex activates transcription of a specific DNA region

4. 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 receptor

3. 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 Secretions

Hormone secretion is generally controlled in three ways:

1. Negative Feedback

2. Hormone Deactivation

3. Up/Down Regulation

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.

Figure 13.10 Hormone secretion is under negative feedback.

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)

Indicates negative

feedback inhibition.

Negative

Feedback

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.

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 Deactivation

Half-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%30 minutes 12.5%

Hormones are continually secreted in the urine, and

broken down by enzymes, primarily in the liver.

Up/Down Regulation

Up-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