NEET MDS Lessons
Physiology
Excitability ( Bathmotropism ) : Excitability means the ability of cardiac muscle to respond to signals. Here we are talking about contractile muscle cells that are excited by the excitatory conductive system and generate an action potential.
Cardiac action potential is similar to action potential in nerve and skeletal muscle tissue , with one difference , which is the presence of plateau phase . Plateau phase is unique for cardiac muscle cells .
The resting membrane potential for cardiac muscle is about -80 mV.
When the cardiac muscle is stimulated an action potential is generated . The action potential in cardiac muscle is composed of four phases , which are :
1. Depolarization phase (Phase 0 ) :
A result of opening of sodium channels , which increase the permeability to sodium , which will lead to a rapid sodium influx into the cardiac muscle cell.
2. Repolarization : Repolarization in cardiac muscle is slow and triphasic :
a. Phase 1 (early partial repolarization ) : A small fast repolarization , results from potassium eflux and chloride influx.
b. Phase 2 ( Plateau ) : After the early partial depolarization , the membrane remains depolarized , exhibiting a plateau , which is a unique phase for the cardiac muscle cell. Plateau is due to opening of slow calcium-sodium channels , delay closure of sodium channels , and to decreased potassium eflux.
c. Phase 3 ( Rapid repolarization) : opening of potassium channels and rapid eflux of potassium.
d. Phase 4 ( Returning to resting level) in other words : The phase of complete repolarization. This due to the work of sodium-potassium pump.
Absolute refractory period:
Coincides wit phase 0,phase1 , and phase 2 . During this period , excitability of the heart is totally abolished . This prevents tetanization of the cardiac muscle and enables the heart to contract and relax to be filled by blood ..
Relative refractory period :
Coincides with the rapid repolarization and allows the excitability to be gradually recovered .
Excitation contraction relationship : Contraction of cardiac muscle starts after depolarization and continues about 1.5 time as long as the duration of the action potential and reaches its maximum at the end of the plateau. Relaxation of the muscle starts with the early partial repolarization.
Factors , affecting excitability of cardiac muscle:
I. Positive bathmotropic effect :
1. Sympathetic stimulation : It increase the heart , and thus reduces the duration of the action potentia; . This will shorten the duration of the absolute refractory period , and thus increase the excitability .
2. Drugs : Catecholamines and xanthines derivatives .
3. Mild hypoxia and mild ischemia
4. Mild hyperkalemia as it decreases the K+ efflux and opens excess Na+ channels .
5. Hypocalcemia
II. Negative bathmotropic effect :
1. Parasympathetic stimulation: The negative bathmotropic effect is limited to the atrial muscle excitability , because there is no parasympathetic innervation for the ventricles. Parasympathetic stimulation decreases the heart rate , and thus increases the duration of cardiac action potential and thus increases the duration of the absolute refractory period.
2. moderate to severe hypoxia
3. hyponatremia , hypercalcemia , and severe hyperkalemia.
Clinical Physiology : Extrasystole is a pathological situation , due to abnormal impulses , arising from ectopic focus .It is expressed as an abnormal systole that occur during the early diastole .
Extrasystole is due to a rising of excitability above the normal , which usually occurs after the end of the relative refractory period ( read about staircase phenomenon of Treppe)
Structure and function of skeletal muscle.
Skeletal muscles have a belly which contains the cells and which attaches by means of tendons or aponeuroses to a bone or other tissue. An aponeurosis is a broad, flat, tendinous attachment, usually along the edge of a muscle. A muscle attaches to an origin and an insertion. The origin is the more fixed attachment, the insertion is the more movable attachment. A muscle acts to shorten, pulling the insertion toward the origin. A muscle can only pull, it cannot push.
Muscles usually come in pairs of antagonistic muscles. The muscle performing the prime movement is the agonist, the opposite acting muscle is the antagonist. When the movement reverses, the names reverse. For example, in flexing the elbow the biceps brachii is the agonist, the triceps brachii is the antagonist. When the movement changes to extension of the elbow, the triceps becomes the agonist and the biceps the antagonist. An antagonist is never totally relaxed. Its function is to provide control and damping of movement by maintaining tone against the agonist. This is called eccentric movement.
Muscles can also act as synergists, working together to perform a movement. This movement can be different from that performed when the muscles work independently. For example, the sternocleidomastoid muscles each rotate the head in a different direction. But as synergists they flex the neck.
Fixators act to keep a part from moving. For example fixators act as postural muscles to keep the spine erect and the leg and vertebral column extended when standing. Fixators such as the rhomboids and levator scapulae keep the scapula from moving during actions such as lifting with the arms.
Abnormalities of Salt, Water or pH
- Examples:
- Hyperkalemia: caused by kidney disease & medical malpractice
- High K+ in blood- can stop the heart in contraction (systole)
- Dehydration: walking in desert- can lose 1-2 liters/hour through sweat
- Blood becomes too viscous to circulate well -> loss of temperature regulation -> hyperthermia, death
- Acidosis: many causes including diabetes mellitus and respiratory problems; can cause coma, death
- Hyperkalemia: caused by kidney disease & medical malpractice
Neurons :
Types of neurons based on structure:
a multipolar neuron because it has many poles or processes, the dendrites and the axon. Multipolar neurons are found as motor neurons and interneurons. There are also bipolar neurons with two processes, a dendrite and an axon, and unipolar neurons, which have only one process, classified as an axon.. Unipolar neurons are found as most of the body's sensory neurons. Their dendrites are the exposed branches connected to receptors, the axon carries the action potential in to the central nervous system.
Types of neurons based on function:
- motor neurons - these carry a message to a muscle, gland, or other effector. They are said to be efferent, i.e. they carry the message away from the central nervous system.
- sensory neurons - these carry a message in to the CNS. They are afferent, i.e. going toward the brain or spinal cord.
- interneuron (ie. association neuron, connecting neuron) - these neurons connect one neuron with another. For example in many reflexes interneurons connect the sensory neurons with the motor neurons.
Hormones are carried by the blood throughout the entire body, yet they affect only certain cells. The specific cells that respond to a given hormone have receptor sites for that hormone.
This is sort of a lock and key mechanism. If the key fits the lock, then the door will open. If a hormone fits the receptor site, then there will be an effect. If a hormone and a receptor site do not match, then there is no reaction. All of the cells that have receptor sites for a given hormone make up the target tissue for that hormone. In some cases, the target tissue is localized in a single gland or organ. In other cases, the target tissue is diffuse and scattered throughout the body so that many areas are affected.
Hormones bring about their characteristic effects on target cells by modifying cellular activity. Cells in a target tissue have receptor sites for specific hormones. Receptor sites may be located on the surface of the cell membrane or in the interior of the cell.
In general those protein hormones are unable to diffuse through the cell membrane and react with receptor sites on the surface of the cell. The hormone receptor reaction on the cell membrane activates an enzyme within the membrane, called adenyl cyclase, which diffuses into the cytoplasm. Within the cell, adenyl cyclase catalyzes or starts the process of removal of phosphates from ATP to produce cyclic adenosine monophosphate or c AMP. This c AMP activates enzymes within the cytoplasm that alter or change the cellular activity. The protein hormone, which reacts at the cell membrane, is called the first messenger. c Amp that brings about the action attributed to the hormone is called the second messenger. This type of action is relatively rapid because the precursors are already present and they just needed to be activated in some way.
Serum Lipids
|
LIPID |
Typical values (mg/dl) |
Desirable (mg/dl) |
|
Cholesterol (total) |
170–210 |
<200 |
|
LDL cholesterol |
60–140 |
<100 |
|
HDL cholesterol |
35–85 |
>40 |
|
Triglycerides |
40–160 |
<160 |
- Total cholesterol is the sum of
- HDL cholesterol
- LDL cholesterol and
- 20% of the triglyceride value
- Note that
- high LDL values are bad, but
- high HDL values are good.
- Using the various values, one can calculate a
cardiac risk ratio = total cholesterol divided by HDL cholesterol - A cardiac risk ratio greater than 7 is considered a warning.
1. Automatic control (sensory) of respiration is in - brainstem (midbrain)
2. Behavioral/voluntary control is in - the cortex
3. Alveolar ventilation -the amount of atmospheric air that actually reaches the alveolar per breath and that can participate in the exchange of gasses between alveoli and blood
4. Only way to increase gas exchange in alveolar capillaries - perfusion-limited gas exchange
5. Pulmonary ventiliation not effected by - concentration of bicarbonate ions
6. Central chemoreceptors - medulla - CO2, O2 and H+ concentrations
7. Peripheral chemoreceptors - carotid and aortic bodies- PO2, PCO2 and pH
8. Major stimulus for respiratory centers - arterial PCO2
9. Rhythmic breathing depends on
1. continuous (tonic) inspiratory drive from DRG (dorsal respiratory group)
2. intermittent (phasic) expiratory input from cerebrum, thalamus, cranial nerves and ascending spinal cord sensory tracts
10. Primary site for gas exchange - type I epithelial cells for alveoli