• 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
4. Hormones only act on cells (target cells) that possess receptors
sensitive to the hormone – highly specific action.
• Have ducts
• Secrete chemicals directly onto a surface
• Examples: sweat glands, mucous cells
• Chemicals that affect only nearby cells
• Example: prostaglandins secreted with semen stimulate
muscular contractions within female reproductive organs
• 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)
• 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
• 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
Specificity of action…………. receptors on postsynaptic cell
Speed of onset……………...... <second
Duration of action……………. usually very brief
receptors on target tissues
seconds to hours
may be brief or last for days
Chemistry of Hormones
• Chemically, hormones are either:
1. Steroid or steroid-like hormones
2. Biogenic Amines
3. Peptide hormones
• Steroid hormones are derived from cholesterol
• They are composed of hydrophobic lipids (insoluble in water)
• Androgens (weak sex hormones)
• Corticoids (hormones secreted from the adrenal cortex)
• Amines are synthesized from amino acids
• Thyroid hormones (these are also hydrophobic, or water insoluble)
• Composed of long chains of amino acids (polypeptides)
• 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.
A steroid hormone crosses the cell membrane
Hormone combines with a protein receptor in the nucleus
The hormone-receptor complex activates transcription of a specific DNA region
The mRNA leave the nucleus into the cytoplasm
The mRNA is translated into a protein.
A non-steroid hormone reaches the target cell,
The hormone binds to a membrane receptor
Binding to the receptor activates an enzyme in the cell membrane (adenlyate cyclase)
Adenlyate cyclase converts ATP into cyclic adinosine monophosphate (cAMP)
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
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 13.10 Hormone secretion is under negative feedback.
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.
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.
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.
Hormones are continually secreted in the urine, and
broken down by enzymes, primarily in the liver.
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
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