Allow cells to communicate with each other to regulate body activities
controlled release of chemicals from a cell
Classes of Chemical Messengers
- 1. Autocrine
- 2. Paracrine
- 3. Neurotransmitter
- 4. Endocrine
Autrocrine chemical messengers
-Stimulates the cell that originally secreted it-secreted by cells in a local area
- Example: white blood cells during an infection
- -Eicosanoids (prostaglandins, thromboxanes, prostacyclins, leukotrienes)
Paracrine Chemical Messengers
-act locally on nearby cells-secreted by one cell type into the extracellular fluid and affect surrounding cells
- Example: Histamine stimulates vasodilation in nearby blood vessels during allergic reactions
- -Somatostatin, histamine, eicosanoids
- -secreted by neurons that activate an adjacent cell, with it is another neuron, a muscle cell, or a glandular cell.
- -secreted into a synaptic cleft (not bloodstream) by presynaptic nerve terminals
- -travels short distances
- -influences postsynaptic cells
Examples: acetylcholine, epinephrine
Endocrine chemical messengers
- -secreted into the bloodstream by certain cells and glands
- -travels some distance to target tissues
- -results in coordinated regulation of cell function
Examples: thyroid hormones, growth hormones, insulin, epinephrine, estrogen, progesterone, testosterone, prostaglandins
-composed of endocrine glands and specialized endocrine cells located throughout the body
- endo = within
- krino = secrete
secreted by endocrine glands into bloodstream (circulatory system) to specific sites called target tissues (effectors) where they produce a coordinated response of the target tissues
- -produced in small quantities-secreted into intercellular space
- -transported some distance in circulatory system
- -acts on target tissues elsewhere in body
- -regulates activities of body structures
Greek: set into motion
have ducts that carry their secretions to the outside of the body (i.e. saliva, sweat, breast milk, and digestive enzymes) or into a hollow organ (i.e. stomach or intestines.)
Comparison of Nervous and Endocrine systems - SIMILARITIES
- 1. associated with the brain (hypothalamus: neural functions and hormone production)
- 2. May use same chemical messenger as neurotransmitter (i.e. epinephrine into synaptic cleft) and hormone (epinephrine into bloodstream)
- 3. Two systems work together to regulate critical body processes
- -Some neurons secrete hormones. Nervous system secretes neuroendocrine peptides (neurohormones), into circulatory system
- -some parts of endocrine system innervated directly by nervous system
Comparison of Nervous and Endocrine systems - differences
- 1. Mode of Transport:
- -Endocrine system secretes hormones which are transported into the bloodstream.
- -Nervous system secretes neurotransmitters, which are released directly onto their target cells
- 2. Speed of response
- -nervous system generally responds faster than the endocrine system (i.e. acetylcholine is delivered to target cells in milliseconds).
- -endocrine system responds more slowly than nervous system (hormones delivered to target cells in seconds).
- 3. Duration of response
- - nervous: milliseconds/seconds
- - endocrine: minutes/days/weeks
- hormones secreted by most endocrine glands, which consist mainly of fluctuations in the concentration of hormones in the bloodstream over a period of time ranging from minutes to hours.
- -the concentration of the hormone determines the strength of the signal and the magnitude of the response.
- --for most:
- a small concentration of a hormone=week signal & small response.
- larger concentration=stronger signal & greater response
- all-or-nothing action potentials carried along axons, which vary in frequency not amplitude (size). All action potentials are the same size in a given tissue.
- Low frequency of action potentials = weak stimulus
- High frequency of action potentials = strong stimulus
Characteristics of hormones
- 1. Stable
- 2. Communication
- 3. Distribution
Stability of hormones
- -some hormones are more stable than others.
- -half-life: amount of time it takes for 50% of the circulating hormone to be removed from the ciculation and exreted.
- -some have short half-lives, while others have long half lives.
Communication of hormones
interact with target tissue in a specific manner in order to activate a coordinated set of events...Hormones must be able to regulate specific cellular pathways once they arrive at their targets.
Distribution of hormones
transported by blood to many locations and have potential to activate any cell in the body, including those far away from where they are produced. However, only cells with their specific receptors can respond.
Hormones requiring transport assistance bind to blood proteins called binding proteins and are called bound proteins.
- When hormones dissociate from their binding proteins at their target tissues.
- (some hormones are always free hormones because they do not have specific binding proteins)
Lipid soluble hormones
- -includes steroids (testosterone, aldosterone), thyroid hormone (thyroxine - only LS amino acid derivative), fatty acid derivatives (prostoglandins & certain eicosanoids)
transportation of lipid soluble hormones
low solubility = travel in bloodstream bound to binding proteins. without binding proteins, the LSH would diffuse out of capillaries and be degraded by enzymes and removed from the body.
when L-SH ar removed from the circulation and certain enzymes in the liver attach water-soluble molecules to the hormones. Once the hormones are conjugated the kidneys and liver excrete them into urine & bile.
Water soluble hormones
- -polar molecules
- -include protein hormones (TSH, growth) , peptides (Insulin & thyrotropin-releasing), and most amino acid derivatives (epinephrine)
Transportation of Water-soluble hormones
-most dissolve into blood w/out binding protein
Patterns of Hormone Secretion
- 1. Chronic
- 2. Acute
- 3. Episodic (Cylic)
Note: lipid-soluble hormones tend to exhibit chronic & episodic patterns. Water-soluble hormones tend to exhibit acute (irregular) secretion patterns because of their short half-lives
Chronic hormone secretion
- -hormone regulation
- -maintenance of relatively constant concentration of hormone over a long period of time. (i.e. thyroid hormones)
Acute hormone secretion
- -Hormones reapidly increase in the blood for a short time in response to a specific stimulus (i.e. insulin secretion following a meal)
- -smaller stimulus does not activate as much hormone secretion as a larger stimulus
- Chronic stress causes increased levels of cortisol to be released from the adrenal cortex leading to adrenal fatigue and:
- -destruction of healthy muscle & bone
- -decreased healing and normal cell regeneration
- -impaired digestion, metabolism and mental function, and weakened immune system
Episodic (Cyclic) hormone secretion
- hormone is stimulated so that it increases & decreases in the blood at a relatively consistent time and to about the same amount
- (i.e. reproductive hormones regulating menstration)
Control of hormone secretion
most hormones controlled by negative feedback systems
most hormones are not secreted at constant rate but their secretion is regulated by 3 different methods
3 types of stimuli regulate hormone release
- -Humoral = body fluids
- -circulate in the blood
- -Blood-borne molecules can directly stimulate the release of some hormones
- -when the blood level of the particular molecule changes, the hormone is released in response to the molecule's concentration.
Control by neural stimuli
neural stimuli of endocrine glands
- 1. Action potential (AP) in a neuron innervates an endocrine cell stimulates secretion of a stimulatory neurotransmitter
- 2. endocrine cell secretes its hormone into the blood where it will travel to its target
- 3. AP in neuron stimulates secretion of an inhibitory neurotransmitter.
- 4. Endocrine cell is inhibited and does not secrete its hormone
when neuropeptides stimulate secretion from other endocrine cells
Control by hormonal stimuli
-hormone is secreted and in turn, stimulates the secretion of other hormones (i.e. tropic hormones)
- 1. neurons in the hypothalamus release stimulatory hormones called releasing hormones. Releasing hormones travel in the blood to the anterior pituitary gland.
- 2. Releasing hormones stimulate the release of tropic hormones from the anterior pituitary, which travel in the blood to their target endocrine cell.
- 3. Target endocrine cell secretes its hormone into the blood, where it travels to its target and produces a response
- 4. Hormone from the target endocrine cell also inhibits the hypothalamus and anterior pituitary from secreting releasing hormone and the tropic hormone. This is negative feedback.
- 5. In some instances, the hypothalamus can also secrete inhibiting hormones, which prevent the secretion of anterior pituitary tropic hormones
Inhibition of hormone release??
Regulation of hormone levels in the blood
-how most hormones are regulated
- 1. anterior pituitary gland secretes a tropic hormone, which travels in the blood to the target endocrine cell
- 2. hormone from the target endocrine cell travels to its target.
- 3. hormone from the target endocrine cells also has a negative feedback effect on the anterior pituitary and hypothalamus and decreases secretion of the tropic hormone.