A small fraction of cardiac muscle fibers have myogenicity and autorhythmicity.

Myogenicity is the property of spontaneous impulse generation. The slow sodium channels are leaky and cause the polarity to spontaneously rise to threshold for action potential generation. The fastest of these cells, those in the SA node, set the pace for the heartbeat.

Autorhythmicity - the natural rhythm of spontaneous depolarization. Those with the fastest autorhythmicity act as the 1. heart's pacemaker.

Contractility - like skeletal muscle, most cardiac muscle cells respond to stimuli by contracting. The autorhythmic cells have very little contractility however. Contractility in the other cells can be varied by the effect of neurotransmitters.

Inotropic effects - factors which affect the force or energy of muscular contractions. Digoxin, epinephrine, norepinephrine, and dopamine have positive inotropic effects. Betal blockers and calcium channel blockers have negative inotropic effects 

Sequence of events in cardiac conduction: The electrical events in the cardiac cycle.

1) SA node depolarizes and the impulse spreads across the atrial myocardium and through the internodal fibers to the AV node. The atrial myocardium depolarizes resulting in atrial contraction, a physical event.

2) AV node picks up the impulse and transfers it to the AV Bundle (Bundle of His). This produces the major portion of the delay seen in the cardiac cycle. It takes approximately .03 sec from SA node depolarization to the impulse reaching the AV node, and .13 seconds for the impulse to get through the AV node and reach the Bundle of His. Also during this period the atria repolarize.

3) From the AV node the impulse travels through the bundle branches and through the Purkinje fibers to the ventricular myocardium, causing ventricular depolarization and ventricular contraction, a physical event.

4) Ventricular repolarization occurs.

The thyroid gland is a double-lobed structure located in the neck. Embedded in its rear surface are the four parathyroid glands.

The Thyroid Gland

The thyroid gland synthesizes and secretes:

• thyroxine (T₄) and
• calcitonin

T₄ and T₃

Thyroxine (T₄ ) is a derivative of the amino acid tyrosine with four atoms of iodine. In the liver, one atom of iodine is removed from T₄ converting it into triiodothyronine (T₃). T₃ is the active hormone. It has many effects on the body. Among the most prominent of these are:

• an increase in metabolic rate
• an increase in the rate and strength of the heart beat.

The thyroid cells responsible for the synthesis of T₄ take up circulating iodine from the blood. This action, as well as the synthesis of the hormones, is stimulated by the binding of TSH to transmembrane receptors at the cell surface.

Diseases of the thyroid

1. hypothyroid diseases; caused by inadequate production of T₃

• cretinism: hypothyroidism in infancy and childhood leads to stunted growth and intelligence. Can be corrected by giving thyroxine if started early enough.
• myxedema: hypothyroidism in adults leads to lowered metabolic rate and vigor. Corrected by giving thyroxine.
• goiter: enlargement of the thyroid gland. Can be caused by:
  • inadequate iodine in the diet with resulting low levels of T₄ and T₃;
  • an autoimmune attack against components of the thyroid gland (called Hashimoto's thyroiditis).

2. hyperthyroid diseases; caused by excessive secretion of thyroid hormones

Graves´ disease. Autoantibodies against the TSH receptor bind to the receptor mimicking the effect of TSH binding. Result: excessive production of thyroid hormones. Graves´ disease is an example of an autoimmune disease.

Osteoporosis. High levels of thyroid hormones suppress the production of TSH through the negative-feedback mechanism mentioned above. The resulting low level of TSH causes an increase in the numbers of bone-reabsorbing osteoclasts resulting in osteoporosis.

Calcitonin

Calcitonin is a polypeptide of 32 amino acids. The thyroid cells in which it is synthesized have receptors that bind calcium ions (Ca²⁺) circulating in the blood. These cells monitor the level of circulating Ca²⁺. A rise in its level stimulates the cells to release calcitonin.

• bone cells respond by removing Ca²⁺ from the blood and storing it in the bone
• kidney cells respond by increasing the excretion of Ca²⁺

Both types of cells have surface receptors for calcitonin.

Because it promotes the transfer of Ca²⁺ to bones, calcitonin has been examined as a possible treatment for osteoporosis

Basic Properties of Gases

A.    Dalton's Law of Partial Pressures

1.    partial pressure - the "part" of the total air pressure caused by one component of a gas 

     Gas            Percent            Partial Pressure (P)
    ALL AIR        100.0%                760 mm Hg
    Nitrogen       78.6%                   597 mm Hg    (0.79 X 760)
    Oxygen          20.9%                l59 mm Hg    (0.21 X 760)
    CO₂              0.04%                  0.3 mm Hg    (0.0004 X 760) 

2.    altitude - air pressure @ 10,000 ft = 563 mm Hg
3.    scuba diving - air pressure @ 100 ft = 3000 mm Hg

B.    Henry's Law of Gas Diffusion into Liquid

1.    Henry's Law - a certain gas will diffuse INTO or OUT OF a liquid down its concentration gradient in proportion to its partial pressure

2.    solubility - the ease with which a certain gas will "dissolve" into a liquid (like blood plasma)

HIGHest solubility in plasma            Carbon Dioxide
                                                      Oxygen
                                        
LOWest solubility in plasma             Nitrogen

C.    Hyperbaric (Above normal pressure) Conditions

1.    Creates HIGH gradient for gas entry into the body

2.    therapeutic - oxygen forced into blood during: carbon monoxide poisoning, circulatory shock, asphyxiation, gangrene, tetanus, etc.

3.    harmful - SCUBA divers may suffer the "bends" when they rise too quickly and Nitrogen gas "comes out of solution" and forms bubbles in the blood

Respiratory system plays important role in maintaining homeostasis . Other than its major function , which is supplying the cells with needed oxygen to produce energy and getting rid of carbon dioxide , it has other functions :

1 Vocalization , or sound production.
2 Participation in acid base balance .
3 Participation in fluid balance by insensible water elimination (vapors ).
4 Facilitating venous return .
5 Participation in blood pressure regulation : Lungs produce Angiotensin converting enzyme ( ACE ) .
6 Immune function : Lungs produce mucous that trap foreign particles , and have ciliae that move foreign particles away from the lung. They also produce alpha 1 antitrepsin that protect the lungs themselves from the effect of elastase and other proteolytic  enzymes

Structure of a nerve:

A peripheral nerve is arranged much like a muscle in terms of its connective tissue. It has an outer covering which forms a sheath around the nerve, called the epineurium. Often a nerve will run together with an artery and vein and their connective coverings will merge. Nerve fibers, which are axons, organize into bundles known as fascicles with each fascicle surrounded by the perineurium. Between individual nerve fibers is an inner layer of endoneurium.

 The myelin sheath in peripheral nerves consists of Schwann cells wrapped in many layers around the axon fibers. Not all fibers in a nerve will be myelinated, but most of the voluntary fibers are. The Schwann cells are portrayed as arranged along the axon like sausages on a string. Gaps between the Schwann cells are called nodes of Ranvier. These nodes permit an impulse to travel faster because it doesn't need to depolarize each area of a membrane, just the nodes. This type of conduction is called saltatory conduction and means that impulses will travel faster in myelinated fibers than in unmyelinated ones.

The myelin sheath does several things:

1) It provides insulation to help prevent short circuiting between fibers.

2) The myelin sheath provides for faster conduction.

3) The myelin sheath provides for the possibility of repair of peripheral nerve fibers. Schwann cells help to maintain the micro-environments of the axons and their tunnel (the neurilemma tunnel) permits re-connection with an effector or receptor  CNS fibers, not having the same type of myelination accumulate scar tissue after damage, which prevents regeneration.

Properties of cardiac muscle

Cardiac muscle is a striated muscle like the skeletal muscle , but it is different from the skeletal muscle in being involuntary and syncytial .

Syncytium means that cardiac muscle cells are able to excite and contract together due to the presence of gap junctions between adjacent cardiac cells.

Cardiac muscle has four properties , due to which the heart is able to fulfill its function as a pumping organ. Studying and understanding these properties is essential for students to understand the cardiac physiology as a whole.

1. Rhythmicity ( Chronotropism )
2. Excitability ( Bathmotropism ) 
3. Conductivity
4. Contractility