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 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 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 Nervous System Cell…………………………………. Neuron Signal………………………………. neurotransmitter Specificity of action…………. receptors on postsynaptic cell Speed of onset……………...... <second Duration of action……………. usually very brief
Endocrine System Glandular Epithelium hormone receptors on target tissues seconds to hours may be brief or last for days
Chemistry of Hormones â&#x20AC;˘ Chemically, hormones are either: 1. Steroid or steroid-like hormones 2. Biogenic Amines 3. Peptide hormones
Steroid Hormones Properties • Steroid hormones are derived from cholesterol • They are composed of hydrophobic lipids (insoluble in water) Include • Estrogen • Testosterone • 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 • 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 • 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. 2. 3. 4. 5.
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.
Actions of Non-steroid hormones
1. 2. 3. 4. 5.
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 hormoneâ&#x20AC;&#x2122;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. 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.
Negative Feedback Indicates negative feedback inhibition. 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.
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 0 minutes 10 minutes 20 minutes 30 minutes
Hormones are continually secreted in the urine, and broken down by enzymes, primarily in the liver.
Hormone Concentration 100% 50% 25% 12.5%
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