We are now one-half way through Biology 100.
The segment of the class deals with the structure and function of organ systems. We will focus our attention on organ systems of animals and especially of humans -- hopefully, much of the material covered in the next 6 lectures will be familiar.
In the first lecture of the semester, I stated that living organisms are able to detect and respond to environmental stimuli in order to maintain a stable internal environment (homeostasis).
In animals, two systems are primarily involved in this process: the nervous system and the endocrine system.
Nervous System: found only in multicellular animals -- rapid acting with messages being sent through body in milliseconds -- the nervous system is the basis of animal behavior -- as before, we will consider primarily the human nervous system.
Neuron (nerve cell) -- basic functional unit of the nervous system.
Three major types of neurons:
1. motor neuron -- transmit impulses from CNS to effectors (muscles, organs, glands, etc.)
2. sensory neuron -- carry impulse from sensory receptor to CNS.
3. interneuron -- found in CNS -- used for transporting messages within CNS -- used in thinking, memory.
Neuron structure-- consist of
1. dendrite(s) -- cytoplasmic projections that receive stimuli and transmit them to the cell body.
2. cell body -- contains nucleus.
3. axon (nerve fiber) -- conducts nerve impulses away from cell body -- insulated by myelin sheaths inside Schwann cells.
Figure 29.3. Neuron anatomy.
Human nervous system -- divided into 2 major components:
1. central nervous system -- brain and spinal cord -- protected by cranium and vertebral column -- 97% of all the body's neurons are in the CNS.
2. peripheral nervous system -- everything else -- 2 sub-components:
A. somatic nervous system -- nerves to skeletal muscles, skin, joints -- both motor and sensory neurons.
B. autonomic nervous system -- organs (e.g. heart, stomach, etc.) and glands -- motor neurons only.
A nerve is a bundle of neurons -- can be all motor, all sensory, or both types of neurons.
Nerve impulse -- an electrical charge or impulse -- details of how these are generated can be found in your book (pg. 475) -- involves the changing of the polarity of nerve cell membranes -- nerve must be stimulated above some threshold (action potential) -- impulse generation is an "all-or-none" process -- a 1 millisecond refractory period between stimulations.
Transmission of nerve impulses -- how do impulses get from one neuron to another or from a neuron to an effector? space between one neuron and another or between a neuron and an effector is called a synapse -- note: nerve cells are not in physical contact with one another or with effectors-- impulse generated anew at each synapse -- strength of impulse doesn't decrease.
Synapse:
1. presynaptic membrane -- membrane of axon -- end of axon also contains synaptic vesicles -- contain chemicals called neurotransmitters -- e.g. acetylcholine, norepinephrine. dopamine.
2. synaptic cleft -- space between neurons or between a neurons and effectors.
3. postsynaptic membrane -- membrane of dendrite -- contains receptor sites for neurotransmitters.
Figure 29.9. Synapse structure and function.
When a nerve impulse reaches the end of the axon, synaptic vesicles release their neurotransmitters into the synaptic cleft -- diffuse across and attach to receptor sites -- this causes a change in the polarity of the postsynaptic neuron -- neurotransmitters are either reabsorbed by presynaptic cell or broken down by enzymes produced by postsynaptic neuron (e.g. choline esterase deactivates acetylcholine).
Many drugs and poisons operate by impacting on synaptic transmission of the nervous system:
1. some stimulate or block release of neurotransmitters -- LSD inhibits release serotonin (neurotransmitter) in sensory neurons -- botulism toxin binds to neurons that synapse with muscle cells and blocks the release of acetylcholine resulting in muscular paralysis -- death often occurs due to respiratory or cardiac failure.
2. some mimic or simulate effect of neurotransmitter -- e.g. amphetamines, caffeine and nicotine are chemically similar to several neurotransmitters and mimic their effect.
Several nervous disorders are related to deficiencies of neurotransmitters --e.g. Alzheimer's disease is linked to a lack of acetylcholine -- the more severe the case, the less acetylcholine produced. Parkinson's disease is associated with a deficiency of a neurotransmitter called dopamine.
Now, let's leave the nervous system and go on to another organ system whose function is primarily homeostatic: the Endocrine System.
Endocrine system: consists of specialized glands (endocrine glands) that when stimulated secrete powerful chemicals called hormones into the blood stream -- hormones have specific "target cells" located somewhere else in the body.
We need to distinguish between endocrine and exocrine glands.
Exocrine glands: secrete products through ducts to outside -- e.g. sweat gland, mammary gland, tear glands.
Hormones and Hormone Action:
Hormones are "chemical messengers" -- stimulate cells to perform particular reactions or to synthesize particular proteins-- hormones are either steroid (lipid) or polypeptide (proteins) molecules -- mechanism of action different for steroid and polypeptide hormones.
Steroid hormones -- synthesized from cholesterol -- are small and can pass through target cell membrane -- combine with receptor molecule in cytoplasm to form a hormone-receptor complex -- moves into nucleus where the complex either promotes (gene activation) or retards (gene inhibition) transcription of a particular gene.
Figure 31.3.
Peptide hormones -- large and cannot enter target cells -- bind with receptor (protein) in cell membrane -- hormone-receptor complex activates an enzyme that produces cAMP ("second messenger") -- cAMP activates enzymes in the cell needed for the desired reactions -- your book shows an example of how the hormone glucagon works to convert glycogen into glucose.
Figure 31.4.
Let's now look at some of the endocrine glands, their hormones and their function.
Pituitary gland -- located at base of brain -- divided into anterior and posterior portions.
1. posterior secretes ADH and oxytocin.
A. anti-diuretic hormone (ADH) -- promotes reabsorption of water by kidney collecting duct cells.
B. Oxytocin -- causes contraction of uterus and ejection of milk from mammary glands.
2. anterior -- the "master gland" -- secretes 6 other hormones which stimulate other endocrine glands.
A. thyroid stimulating hormone (TSH) does just that -- stimulates thryoid to secrete its hormones (e.g. thyroxin).
B. GH (growth hormone) -- e.g. pituitary dwarf/giant -- stimulates growth of bone and tissues.
C. Prolactin -- stimulates mammary glands to produce milk.
Thyroid gland -- located in throat -- produces hormone thyroxin that regulates metabolic rate -- hyperactivity (too much thyroxin) vs. lethargy (too little thyroxin).
Adrenal glands -- lie atop kidneys -- secrete
1. epinenephrine and norepinephrine (adrenalin and noradrenalin) -- hormones cause reactions associated with stress -- e.g. increased breathing, heartbead, blood pressure, alertness -- increased muscular strength results from increased blood sugar -- "flight or fight" hormone.
2. glucocorticoids -- e.g. cortisol, cortisone -- raise amino acid level in blood by removing them from muscle -- a.a. converted to glucose in the liver -- way of generating "emergency glucose".
3. mineralocorticoids -- e.g. aldosterone -- regulate levels of Na and K in blood by reabsorption in kidney.
Pancreas -- secretes pancreatic juices into small intestine -- tissues called the islets of Langerhans produces hormones:
1. insulin -- converts glucose in blood into polysaccharide glycogen -- glycogen stored in liver and muscle cells.
2. glucagon -- breaks down glycogen into glucose.
Amounts of isulin and glucagon secreted go up and down according to blood glucose level.
Testes -- besides producing sperm (seminiferous tubules), interstitial cells produce the male sex hormone testosterone -- hormone responsible for secondary male characteristics such as deep voice, body hair, increased muscle definition.
Ovary -- produces ova and female sex hormones estrogen and progesterone -- provide secondary female characteristics and are important to menstrual and ovarian cycles.
Next time: Circulatory System.