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)
Functions of the nervous system:
1) Integration of body processes
2) Control of voluntary effectors (skeletal muscles), and mediation of voluntary reflexes.
3) Control of involuntary effectors ( smooth muscle, cardiac muscle, glands) and mediation of autonomic reflexes (heart rate, blood pressure, glandular secretion, etc.)
4) Response to stimuli
5) Responsible for conscious thought and perception, emotions, personality, the mind.
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Damage to Spinal Nerves and Spinal Cord |
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Damage |
Possible cause of damage |
Symptoms associated with innervated area |
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Peripheral nerve |
Mechanical injury |
Loss of muscle tone. Loss of reflexes. Flaccid paralysis. Denervation atrophy. Loss of sensation |
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Posterior root |
Tabes dorsalis |
Paresthesia. Intermittent sharp pains. Decreased sensitivity to pain. Loss of reflexes. Loss of sensation. Positive Romberg sign. High stepping and slapping of feet. |
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Anterior Horn |
Poliomyelitis |
Loss of muscle tone. Loss of reflexes. Flaccid paralysis. Denervation atrophy |
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Lamina X (gray matter) |
Syringomyelia |
Bilateral loss of pain and temperature sense only at afflicted cord level. Sensory dissociation. No sensory impairment below afflicted level |
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Anterior horn and lateral corticospinal tract |
Amyotrophic lateral sclerosis |
Muscle weakness. Muscle atrophy. Fasciculations of hand and arm muscles. Spastic paralysis |
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Posterior and lateral funiculi |
Subacute combined degeneration |
Loss of position sense. Loss of vibratory sense. Positive Romberg sign. Muscle weakness. Spasticity. Hyperactive tendon reflexes. Positive Babinski sign. |
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Hemisection of the spinal cord |
Mechanical injury |
Brown-Sequard syndrome |
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Below cord level on injured side |
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Flaccid paralysis. Hyperactive tendon reflexes. Loss of position sense. Loss of vibratory sense. Tactile impairment |
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Below cord level on opposite side beginning one or two segments below injury |
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Loss of pain and temperature |
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The large intestine (colon)
The large intestine receives the liquid residue after digestion and absorption are complete. This residue consists mostly of water as well as materials (e.g. cellulose) that were not digested. It nourishes a large population of bacteria (the contents of the small intestine are normally sterile). Most of these bacteria (of which one common species is E. coli) are harmless. And some are actually helpful, for example, by synthesizing vitamin K. Bacteria flourish to such an extent that as much as 50% of the dry weight of the feces may consist of bacterial cells. Reabsorption of water is the chief function of the large intestine. The large amounts of water secreted into the stomach and small intestine by the various digestive glands must be reclaimed to avoid dehydration.
Functional Divisions of the Nervous System:
1) The Voluntary Nervous System - (ie. somatic division) control of willful control of effectors (skeletal muscles) and conscious perception. Mediates voluntary reflexes.
2) The Autonomic Nervous System - control of autonomic effectors - smooth muscles, cardiac muscle, glands. Responsible for "visceral" reflexes
The Body Regulates pH in Several Ways
- Buffers are weak acid mixtures (such as bicarbonate/CO2) which minimize pH change
- Buffer is always a mixture of 2 compounds
- One compound takes up H ions if there are too many (H acceptor)
- The second compound releases H ions if there are not enough (H donor)
- The strength of a buffer is given by the buffer capacity
- Buffer capacity is proportional to the buffer concentration and to a parameter known as the pK
- Mouth bacteria produce acids which attack teeth, producing caries (cavities). People with low buffer capacities in their saliva have more caries than those with high buffer capacities.
- Buffer is always a mixture of 2 compounds
- CO2 gas (a potential acid) is eliminated by the lungs
- Other acids and bases are eliminated by the kidneys
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Partial Pressures of O2 and CO2 in the body (normal, resting conditions):
- Alveoli
- PO2 = 100 mm Hg
- PCO2 = 40 mm Hg
- Alveolar capillaries
- Entering the alveolar capillaries
- PO2 = 40 mm Hg (relatively low because this blood has just returned from the systemic circulation & has lost much of its oxygen)
- PCO2 = 45 mm Hg (relatively high because the blood returning from the systemic circulation has picked up carbon dioxide)
- Entering the alveolar capillaries
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While in the alveolar capillaries, the diffusion of gasses occurs: oxygen diffuses from the alveoli into the blood & carbon dioxide from the blood into the alveoli.
- Leaving the alveolar capillaries
- PO2 = 100 mm Hg
- PCO2 = 40 mm Hg
- Blood leaving the alveolar capillaries returns to the left atrium & is pumped by the left ventricle into the systemic circulation. This blood travels through arteries & arterioles and into the systemic, or body, capillaries. As blood travels through arteries & arterioles, no gas exchange occurs.
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- Entering the systemic capillaries
- PO2 = 100 mm Hg
- PCO2 = 40 mm Hg
- Body cells (resting conditions)
- PO2 = 40 mm Hg
- PCO2 = 45 mm Hg
- Entering the systemic capillaries
- Because of the differences in partial pressures of oxygen & carbon dioxide in the systemic capillaries & the body cells, oxygen diffuses from the blood & into the cells, while carbon dioxide diffuses from the cells into the blood.
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- Leaving the systemic capillaries
- PO2 = 40 mm Hg
- PCO2 = 45 mm Hg
- Leaving the systemic capillaries
- Blood leaving the systemic capillaries returns to the heart (right atrium) via venules & veins (and no gas exchange occurs while blood is in venules & veins). This blood is then pumped to the lungs (and the alveolar capillaries) by the right ventricle.