NEET MDS Lessons
Physiology
Cardiac Control: The Cardiac Center in the medulla.
Outputs:
The cardioacceleratory center sends impulses through the sympathetic nervous system in the cardiac nerves. These fibers innervate the SA node and AV node and the ventricular myocardium. Effects on the SA and AV nodes are an increase in depolarization rate by reducing the resting membrane polarization. Effect on the myocardium is to increase contractility thus increasing force and therefore volume of contraction. Sympathetic stimulation increases both rate and volume of the heart.
The cardioinhibitory center sends impulses through the parasympathetic division, the vagus nerve, to the SA and AV nodes, but only sparingly to the atrial myocardium, and not at all to ventricular myocardium. Its effect is to slow the rate of depolarization by increasing the resting potential, i.e. hyperpolarization.
The parasympathetic division controls the heart at rest, keeping its rhythm slow and regular. This is referred to as normal vagal tone. Parasympathetic effects are inhibited and the sympathetic division exerts its effects during stress, i.e. exercise, emotions, "fight or flight" response, and temperature.
Inputs to the Cardiac Center:
Baroreceptors in the aortic and carotid sinuses. The baroreceptor reflex is responsible for the moment to moment maintenance of normal blood pressure.
Higher brain (hypothalamus): stimulates the center in response to exercise, emotions, "fight or flight", temperature.
Intrinsic Controls of the Heart:
Right Heart Reflex - Pressoreceptors (stretch receptors) in the right atrium respond to stretch due to increased venous return. The reflex acts through a short neural circuit to stimulate the sympathetic nervous system resulting in increased rate and force of contraction. This regulates output to input
The Frank-Starling Law - (Starling's Law of the Heart) - Like skeletal muscle the myocardium has a length tension curve which results in an optimum level of stretch producing the maximum force of contraction. A healthy heart normally operates at a stretch less than this optimum level and when exercise causes increased venous return and increased stretch of the myocardium, the result is increased force of contraction to automatically pump the increased volume out of the heart. I.e. the heart automatically compensates its output to its input.
An important relationship in cardiac output is this one:
Blood Flow = D Pressure / Resistance to Blood Flow
Oxygen Transport in Blood: Hemoglobin
A. Association & Dissociation of Oxygen + Hemoglobin
1. oxyhemoglobin (HbO2) - oxygen molecule bound
2. deoxyhemoglobin (HHb) - oxygen unbound
H-Hb + O2 <= === => HbO2 + H+
3. binding gets more efficient as each O2 binds
4. release gets easier as each O2 is released
5. Several factors regulate AFFINITY of O2
a. Partial Pressure of O2
b. temperature
c. blood pH (acidity)
d. concentration of “diphosphoglycerate” (DPG)
B. Effects of Partial Pressure of O2
1. oxygen-hemoglobin dissociation curve
a. 104 mm (lungs) - 100% saturation (20 ml/100 ml)
b. 40 mm (tissues) - 75% saturation (15 ml/100 ml)
c. right shift - Decreased Affinity, more O2 unloaded
d. left shift- Increased Affinity, less O2 unloaded
C. Effects of Temperature
1. HIGHER Temperature --> Decreased Affinity (right)
2. LOWER Temperature --> Increased Affinity (left)
D. Effects of pH (Acidity)
1. HIGHER pH --> Increased Affinity (left)
2. LOWER pH --> Decreased Affinity (right) "Bohr Effect"
a. more Carbon Dioxide, lower pH (more H+), more O2 release
E. Effects of Diphosphoglycerate (DPG)
1. DPG - produced by anaerobic processes in RBCs
2. HIGHER DPG > Decreased Affinity (right)
3. thyroxine, testosterone, epinephrine, NE - increase RBC metabolism and DPG production, cause RIGHT shift
F. Oxygen Transport Problems
1. hypoxia - below normal delivery of Oxygen
a. anemic hypoxia - low RBC or hemoglobin
b. stagnant hypoxia - impaired/blocked blood flow
c. hypoxemic hypoxia - poor lung gas exchange
2. carbon monoxide poisoning - CO has greater Affinity than Oxygen or Carbon Dioxide
Heart sounds
Heart sounds are a result of beating heart and resultant blood flow . that could be detected by a stethoscope during auscultation . Auscultation is a part of physical examination that doctors have to practice them perfectly.
Before discussion the origin and nature of the heart sounds we have to distinguish between the heart sounds and hurt murmurs. Heart murmurs are pathological noises that results from abnormal blood flow in the heart or blood vessels.
Physiologically , blood flow has a laminar pattern , which means that blood flows in form of layers , where the central layer is the most rapid . Laminar blood flow could be turned into turbulent one .
Turbulent blood flow is a result of stenotic ( narrowed ) valves or blood vessels , insufficient valves , roughened vessels` wall or endocardium , and many diseases . The turbulent blood flow causes noisy murmurs inside or outside the heart.
Heart sounds ( especially first and second sounds ) are mainly a result of closure of the valves of the heart . While the third sound is a result of vibration of ventricular wall and the leaflets of the opened AV valves after rapid inflow of blood from the atria to ventricles .
Third heart sound is physiologic in children but pathological in adults.
The four heart sound is a result of the atrial systole and vibration of the AV valves , due to blood rush during atrial systole . It is inaudible neither in adults nor in children . It is just detectable by the phonocardiogram .
Characteristic of heart sounds :
1. First heart sound (S1 , lub ) : a soft and low pitch sound, caused by closure of AV valves.Usually has two components ( M1( mitral ) and T1 ( tricuspid ). Normally M1 preceads T1.
2. Second heart sound ( S2 , dub) : sharp and high pitch sound . caused by closure of semilunar valves. It also has two components A2 ( aortic) and P2 ( pulmonary) . A2 preceads P2.
3. Third heart sound (S3) : low pitched sound.
4. Fourth heart sound ( S4) very low pitched sound.
As we notice : the first three sounds are related to ventricular activity , while the fourth heart sound is related to atrial activity.
Closure of valves is not the direct cause for heart sounds , but sharp blocking of blood of backward returning of blood by the closing valve is the direct cause.
Conductivity :
Means ability of cardiac muscle to propagate electrical impulses through the entire heart ( from one part of the heart to another) by the excitatory -conductive system of the heart.
Excitatory conductive system of the heart involves:
1. Sinoatrial node ( SA node) : Here the initial impulses start and then conducted to the atria through the anterior inter-atrial pathway ( to the left atrium) , to the atrial muscle mass through the gap junction, and to the Atrioventricular node ( AV node ) through anterior, middle , and posterior inter-nodal pathways.
The average conductive velocity in the atria is 1m/s.
2- AV node : The electrical impulses can not be conducted directly from the atria to the ventricles , because of the fibrous skeleton , which is an electrical isolator , located between the atria and ventricles. So the only conductive way is the AV node . But there is a delay in the conduction occurs in the AV node .
This delay is due to:
- the smaller size of the nodal fiber.
- The less negative resting membrane potential
- fewer gap junctions.
There are three sites for delay:
- In the transitional fibers , that connect inter-nodal pathways with the AV node ( 0.03 ) .
- AV node itself ( 0.09 s) .
- In the penetrating portion of Bundle of Hiss ( 0.04 s) .
This delay actually allows atria to empty blood in ventricles during the cardiac cycle before the beginning of ventricular contraction , as it prevents the ventricles from the pathological high atrial rhythm.
The average velocity of conduction in the AV node is 0.02-0.05 m/s
3- Bundle of Hiss : A continuous with the AV node that passes to the ventricles through the inter-ventricular septum. It is subdivided into : Right and left bundle. The left bundle is also subdivided into two branches: anterior and posterior branches .
4- Purkinje`s fibers: large fibers with velocity of conduction 1.5-4 m/s.
the high velocity of these fibers is due to the abundant gap junctions , and to their nature as very large fibers as well.
The conduction from AV node is a one-way conduction . This prevents the re-entry of cardiac impulses from the ventricles to the atria.
Lastly: The conduction through the ventricular fibers has a velocity of 0.3-0.5 m/s.
Factors , affecting conductivity ( dromotropism) :
I. Positive dromotropic factors :
1. Sympathetic stimulation : it accelerates conduction and decrease AV delay .
2. Mild warming
3. mild hyperkalemia
4. mild ischemia
5. alkalosis
II. Negative dromotropic factors :
1. Parasympathetic stimulation
2. severe warming
3. cooling
4. Severe hyperkalemia
5. hypokalemia
6. Severe ischemia
7. acidosis
8. digitalis drugs.
1 - Passive processes - require no expenditure of energy by a cell:
- Simple diffusion = net movement of a substance from an area of high concentration to an area of low concentration. The rate of diffusion is influenced by:
- concentration gradient
- cross-sectional area through which diffusion occurs
- temperature
- molecular weight of a substance
- distance through which diffusion occurs
- Osmosis = diffusion of water across a semi permeable membrane (like a cell membrane) from an area of low solute concentration to an area of high solute concentration
- Facilitated diffusion = movement of a substance across a cell membrane from an area of high concentration to an area of low concentration. This process requires the use of 'carriers' (membrane proteins). In the example below, a ligand molecule (e.g., acetylcholine) binds to the membrane protein. This causes a conformational change or, in other words, an 'opening' in the protein through which a substance (e.g., sodium ions) can pass.
2 - Active processes - require the expenditure of energy by cells:
- Active transport = movement of a substance across a cell membrane from an area of low concentration to an area of high concentration using a carrier molecule
- Endo- & exocytosis - moving material into (endo-) or out of (exo-) cell in bulk form
Proteins:
- about 50 - 60% of the dry mass of a typical cell
- subunit is the amino acid & amino acids are linked by peptide bonds
- 2 functional categories = structural (proteins part of the structure of a cell like those in the cell membrane) & enzymes
Enzymes are catalysts. Enzymes bind temporarily to one or more of the reactants of the reaction they catalyze. In doing so, they lower the amount of activation energy needed and thus speed up the reaction
Pain, Temperature, and Crude Touch and Pressure
General somatic nociceptors, thermoreceptors, and mechanoreceptors sensitive to crude touch and pressure from the face conduct signals to the brainstem over GSA fibers of cranial nerves V, VII, IX, and X.
The afferent fibers involved are processes of monopolar neurons with cell bodies in the semilunar, geniculate, petrosal, and nodose ganglia, respectively.
The central processes of these neurons enter the spinal tract of V, where they descend through the brainstem for a short distance before terminating in the spinal nucleus of V.
Second-order neurons then cross over the opposite side of the brainstem at various levels to enter the ventral trigeminothalamic tract, where they ascend to the VPM of the thalamus.
Finally, third-order neurons project to the "face" area of the cerebral cortex in areas 3, 1, and 2 .
Discriminating Touch and Pressure
Signals are conducted from general somatic mechanoreceptors over GSA fibers of the trigeminal nerve into the principal sensory nucleus of V, located in the middle pons.
Second-order neurons then conduct the signals to the opposite side of the brainstem, where they ascend in the medial lemniscus to the VPM of the thalamus.
Thalamic neurons then project to the "face" region of areas 3, I, and 2 of the cerebral cortex.
Kinesthesia and Subconscious Proprioception
Proprioceptive input from the face is primarily conducted over GSA fibers of the trigeminal nerve.
The peripheral endings of these neurons are the general somatic mechanoreceptors sensitive to both conscious (kinesthetic) and subconscious proprioceptive input.
Their central processes extend from the mesencephalic nucleus to the principal sensory nucleus of V in the pons
The subconscious component is conducted to the cerebellum, while the conscious component travels to the cerebral cortex.
Certain second-order neurons from the principal sensory nucleus relay proprioceptive information concerning subconscious evaluation and integration into the ipsilateral cerebellum.
Other second-order neurons project to the opposite side of the pons and ascend to the VPM of the thalamus as the dorsal trigeminothalamic tract.
Thalamic projections terminate in the face area of the cerebral cortex.