The Posterior Lobe

The posterior lobe of the pituitary releases two hormones, both synthesized in the hypothalamus, into the circulation.

• Antidiuretic Hormone (ADH).
  ADH is a peptide of 9 amino acids. It is also known as arginine vasopressin. ADH acts on the collecting ducts of the kidney to facilitate the reabsorption of water into the blood.
  • A deficiency of ADH
    • leads to excessive loss of urine, a condition known as diabetes  nsipidus.
• Oxytocin
  Oxytocin is a peptide of 9 amino acids. Its principal actions are:
  • stimulating contractions of the uterus at the time of birth
  • stimulating release of milk when the baby begins to suckle

Each hormone in the body is unique.  Each one is different in it's chemical composition, structure, and action.  With respect to their chemical structure, hormones may be classified into three groups: amines, proteins, and steroids.

 Amines- these simple hormones are  structural variation of the amino acid tyrosine.  This group includes thyroxine from the thyroid gland and epinephrine and norepinephrine from the adrenal medulla.

Proteins- these hormones are chains of amino acids.  Insulin from the pancreas, growth hormone from the anterior pituitary gland, and calcitonin from the thyroid gland are all proteins.  Short chains of amino acids are called peptides.  Antidiuretic hormone and oxytocin, synthesized by the hypothalamus, are peptide hormones.

Steroids- cholesterol is the precursor for the steroid hormones, which include cortisol and aldosterone from the adrenal cortex, estrogen and progesterone from the ovaries, and testosterone from the testes.

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.

• Sensory:
  • Somatic (skin & muscle) Senses:
    Postcentral gyrus (parietal lobe). This area senses touch, pressure, pain, hot, cold, & muscle position. The arrangement is upside-down (head below, feet above) and is switched from left to right (sensations from the right side of the body are received on the left side of the cortex). Some areas (face, hands) have many more sensory and motor nerves than others. A drawing of the body parts represented in the postcentral gyrus, scaled to show area, is called a homunculus .
  • Vision:
    Occipital lobe, mostly medial, in calcarine sulcus. Sensations from the left visual field go to the right cortex and vice versa. Like other sensations they are upside down. The visual cortex is very complicated because the eye must take into account shape, color and intensity.
  • Taste:
    Postcentral gyrus, close to lateral sulcus. The taste area is near the area for tongue somatic senses.
  • Smell:
     The olfactory cortex is not as well known as some of the other areas. Nerves for smell go to the olfactory bulb of the frontal cortex, then to other frontal cortex centers- some nerve fibers go directly to these centers, but others come from the thalamus like most other sensory nerves
  • Hearing:
    Temporal lobe, near junction of the central and lateral sulci. Mostly within the lateral sulcus. There is the usual crossover and different tones go to different parts of the cortex. For complex patterns of sounds like speech and music other areas of the cortex become involved.
• Motor:
  • Primary Motor ( Muscle Control):
    Precentral gyrus (frontal lobe). Arranged like a piano keyboard: stimulation in this area will cause individual muscles to contract. Like the sensory cortex, the arrangement is in the form of an upside-down homunculus. The fibers are crossed- stimulation of the right cortex will cause contraction of a muscle on the left side of the body.
  • Premotor (Patterns of Muscle Contraction):
    Frontal lobe in front of precentral gyrus. This area helps set up learned patterns of muscle contraction (think of walking or running which involve many muscles contracting in just the right order).
  • Speech-Muscle Control:
    Broca's area, frontal lobe, usually in left hemisphere only. This area helps control the patterns of muscle contraction necessary for speech. Disorders in speaking are called aphasias.
• Perception:
  • Speech- Comprehension:
    Wernicke's area, posterior end of temporal lobe, usually left hemisphere only. Thinking about words also involves areas in the frontal lobe.
  • Speech- Sound/Vision Association:
    Angular gyrus, , makes connections between sounds and shapes of words

The pancreas

The pancreas consists of clusters if endocrine cells (the islets of Langerhans) and exocrine cells whose secretions drain into the duodenum.

Pancreatic fluid contains:

• sodium bicarbonate (NaHCO₃). This neutralizes the acidity of the fluid arriving from the stomach raising its pH to about 8.
• pancreatic amylase. This enzyme hydrolyzes starch into a mixture of maltose and glucose.
• pancreatic lipase. The enzyme hydrolyzes ingested fats into a mixture of fatty acids and monoglycerides. Its action is enhanced by the detergent effect of bile.
• 4 zymogens— proteins that are precursors to active proteases. These are immediately converted into the active proteolytic enzymes:
  • trypsin. Trypsin cleaves peptide bonds on the C-terminal side of arginines and lysines.
  • chymotrypsin. Chymotrypsin cuts on the C-terminal side of tyrosine, phenylalanine, and tryptophan residues (the same bonds as pepsin, whose action ceases when the NaHCO₃ raises the pH of the intestinal contents).
  • elastase. Elastase cuts peptide bonds next to small, uncharged side chains such as those of alanine and serine.
  • carboxypeptidase. This enzyme removes, one by one, the amino acids at the C-terminal of peptides.
• nucleases. These hydrolyze ingested nucleic acids (RNA and DNA) into their component nucleotides.

The secretion of pancreatic fluid is controlled by two hormones:

• secretin, which mainly affects the release of sodium bicarbonate, and
• cholecystokinin (CCK), which stimulates the release of the digestive enzymes.

Clinical Physiology 

Heart Failure : Heart failure is inability of the heart to pump the enough amount of blood needed to sustain the needs of organism .
It is usually called congestive heart failure ( CHF) .

To understand the pathophysiology  of the heart failure ,  lets compare it with the physiology of the cardiac output :
Cardiac output =Heart rate X stroke volume

Stroke volume is determined by three determinants : Preload ( venous return ) , contractility , and afterload    (peripheral resistance ) . Any disorder of these factors will reduce the ability of the heart to pump blood .

Preload : Any factor that decrease the venous return , either by decreasing the intravenous pressure or increasing the intraatrial pressure will lead to heart failure .

Contractility : Reducing the power of contraction such as in  myocarditis , cardiomyopathy , preicardial tamponade ..etc , will lead to heart failure .

Afterload : Any factor that may increase the peripheral resistance such as hypertension , valvular diseases of the heart may cause heart failure.

Pathophysiology : When the heart needs to contract more to meet the increased demand , compensatory mechanisms start to develope to enhance the power of contractility  . One of these mechanism is increasing heart rate , which will worsen the situation because this will increase the demands of the myocardial cells themselves . The other one is hypertrophy of the cardiac muscle which may compensate the failure temporarily but then the hypertrophy will be an additional load as the fibers became stiff  .

The stroke volume will be reduced , the intraventricular pressure will increase and consequently the intraatrial pressure and then the venous pressure . This will lead to decrease reabsorption of water from the interstitium ( see microcirculation) and then leads to developing of edema ( Pulmonary edema if the failure is left , and systemic edema if the failure is right) .