NEET MDS Lessons
Physiology
The Nerve Impulse
When a nerve is stimulated the resting potential changes. Examples of such stimuli are pressure, electricity, chemicals, etc. Different neurons are sensitive to different stimuli(although most can register pain). The stimulus causes sodium ion channels to open. The rapid change in polarity that moves along the nerve fiber is called the "action potential." In order for an action potential to occur, it must reach threshold. If threshold does not occur, then no action potential can occur. This moving change in polarity has several stages:
Depolarization
The upswing is caused when positively charged sodium ions (Na+) suddenly rush through open sodium gates into a nerve cell. The membrane potential of the stimulated cell undergoes a localized change from -55 millivolts to 0 in a limited area. As additional sodium rushes in, the membrane potential actually reverses its polarity so that the outside of the membrane is negative relative to the inside. During this change of polarity the membrane actually develops a positive value for a moment(+30 millivolts). The change in voltage stimulates the opening of additional sodium channels (called a voltage-gated ion channel). This is an example of a positive feedback loop.
Repolarization
The downswing is caused by the closing of sodium ion channels and the opening of potassium ion channels. Release of positively charged potassium ions (K+) from the nerve cell when potassium gates open. Again, these are opened in response to the positive voltage--they are voltage gated. This expulsion acts to restore the localized negative membrane potential of the cell (about -65 or -70 mV is typical for nerves).
Hyperpolarization
When the potassium ions are below resting potential (-90 mV). Since the cell is hyper polarized, it goes to a refractory phrase.
Refractory phase
The refractory period is a short period of time after the depolarization stage. Shortly after the sodium gates open, they close and go into an inactive conformation. The sodium gates cannot be opened again until the membrane is repolarized to its normal resting potential. The sodium-potassium pump returns sodium ions to the outside and potassium ions to the inside. During the refractory phase this particular area of the nerve cell membrane cannot be depolarized. This refractory area explains why action potentials can only move forward from the point of stimulation.
Factors that affect sensitivity and speed
Sensitivity
Increased permeability of the sodium channel occurs when there is a deficit of calcium ions. When there is a deficit of calcium ions (Ca+2) in the interstitial fluid, the sodium channels are activated (opened) by very little increase of the membrane potential above the normal resting level. The nerve fiber can therefore fire off action potentials spontaneously, resulting in tetany. This could be caused by the lack of hormone from parathyroid glands. It could also be caused by hyperventilation, which leads to a higher pH, which causes calcium to bind and become unavailable.
Speed of Conduction
This area of depolarization/repolarization/recovery moves along a nerve fiber like a very fast wave. In myelinated fibers, conduction is hundreds of times faster because the action potential only occurs at the nodes of Ranvier (pictured below in 'types of neurons') by jumping from node to node. This is called "saltatory" conduction. Damage to the myelin sheath by the disease can cause severe impairment of nerve cell function. Some poisons and drugs interfere with nerve impulses by blocking sodium channels in nerves. See discussion on drug at the end of this outline.
Exchange of gases takes place in Lungs
- A person with an average ventilation rate of 7.5 L/min will breathe in and out 10,800 liters of gas each day
- From this gas the person will take in about 420 liters of oxygen (19 moles/day) and will give out about 340 liters of carbon dioxide (15 moles/day)
- The ratio of CO2 expired/O2 inspired is called the respiratory quotient (RQ)
- RQ = CO2 out/O2 in = 340/420 = 0.81
- In cellular respiration of glucose CO2 out = O2 in; RQ = 1
- The overall RQ is less than 1 because our diet is a mixture of carbohydrates and fat; the RQ for metabolizing fat is only 0.7
- All of the exchange of gas takes place in the lungs
- The lungs also give off large amounts of heat and water vapor
Exchange of gases:
- External respiration:
- exchange of O2 & CO2 between external environment & the cells of the body
- efficient because alveoli and capillaries have very thin walls & are very abundant (your lungs have about 300 million alveoli with a total surface area of about 75 square meters)
- Internal respiration - intracellular use of O2 to make ATP
- occurs by simple diffusion along partial pressure gradients
Serum Proteins
Proteins make up 6–8% of the blood. They are about equally divided between serum albumin and a great variety of serum globulins.
After blood is withdrawn from a vein and allowed to clot, the clot slowly shrinks. As it does so, a clear fluid called serum is squeezed out. Thus:
Serum is blood plasma without fibrinogen and other clotting factors.
The serum proteins can be separated by electrophoresis.
- The most prominent of these and the one that moves closest to the positive electrode is serum albumin.
- Serum albumin
- is made in the liver
- binds many small molecules for transport through the blood
- helps maintain the osmotic pressure of the blood
- The other proteins are the various serum globulins.
- alpha globulins (e.g., the proteins that transport thyroxine and retinol [vitamin A])
- beta globulins (e.g., the iron-transporting protein transferrin)
- gamma globulins.
- Gamma globulins are the least negatively-charged serum proteins. (They are so weakly charged, in fact, that some are swept in the flow of buffer back toward the negative electrode.)
- Most antibodies are gamma globulins.
- Therefore gamma globulins become more abundant following infections or immunizations.
Cystic Fibrosis
→ Thick mucus coagulates in ducts, produces obstruction, Too thick for cilia to move
→ Major Systems Affected: Respiratory System, G. I. Tract,Reproductive Tract
→ Inherited, autosomal recessive gene, most common fatal genetic disorder
→ Major characteristic, Altered electrolyte composition (Saliva & sweat Na+, K+, Cl-)
→ Family history of Cystic Fibrosis
→ Respiratory Infections & G.I.Tract malabsorption
→ Predisposes lung to Secondary infection (Staphylococcus, Pseudomonas)
→ Damages Respiratory Bronchioles and Alveolar ducts, Produces Fibrosis of Lungs, Large cystic dilations)
An anti-diruetic is a substance that decreases urine volume, and ADH is the primary example of it within the body. ADH is a hormone secreted from the posterior pituitary gland in response to increased plasma osmolarity (i.e., increased ion concentration in the blood), which is generally due to an increased concentration of ions relative to the volume of plasma, or decreased plasma volume.
The increased plasma osmolarity is sensed by osmoreceptors in the hypothalamus, which will stimulate the posterior pituitary gland to release ADH. ADH will then act on the nephrons of the kidneys to cause a decrease in plasma osmolarity and an increase in urine osmolarity.
ADH increases the permeability to water of the distal convoluted tubule and collecting duct, which are normally impermeable to water. This effect causes increased water reabsorption and retention and decreases the volume of urine produced relative to its ion content.
After ADH acts on the nephron to decrease plasma osmolarity (and leads to increased blood volume) and increase urine osmolarity, the osmoreceptors in the hypothalamus will inactivate, and ADH secretion will end. Due to this response, ADH secretion is considered to be a form of negative feedback.
Bronchitis = Irreversible Bronchioconstriction
. Causes - Infection, Air polution, cigarette smoke
a. Primary Defect = Enlargement & Over Activity of Mucous Glands, Secretions very viscous
b. Hypertrophy & hyperplasia, Narrows & Blocks bronchi, Lumen of airway, significantly narrow
c. Impaired Clearance by mucocillary elevator
d. Microorganism retension in lower airways,Prone to Infectious Bronchitis, Pneumonia
e. Permanent Inflamatory Changes IN epithelium, Narrows walls, Symptoms, Excessive sputum, coughing
f. CAN CAUSE EMPHYSEMA