NEET MDS Lessons
Physiology
Neurophysiology
Transmission of an action potential. This occurs in two ways:
1) across the synapse - synaptic transmission. This is a chemical process, the result of a chemical neurotransmitter.
2) along the axon - membrane transmission. This is the propagation of the action potential itself along the membrane of the axon.
Synaptic transmission - What you learned about the neuromuscular junction is mostly applicable here as well. The major differences in our current discussion are:
1) Transmission across the synapse does not necessarily result in an action potential. Instead, small local potentials are produced which must add together in summation to produce an action potential.
2) Although ACh is a common neurotransmitter, there are many others and the exact effect at the synapse depends on the neurotransmitter involved.
3) Neurotransmitters can be excitatory or inhibitory. The result might be to turn off the next neuron rather than to produce an action potential
The basic steps of synaptic transmission are the same as described at the neuromuscular junction
1) Impulse arrives at the axon terminus causing opening of Ca2+ channels and allows Ca2+ to enter the axon. The calcium ions are in the extracellular fluid, pumped there much like sodium is pumped. Calcium is just an intermediate in both neuromuscular and synaptic transmission.
2) Ca2+ causes vesicles containing neurotransmitter to release the chemical into the synapse by exocytosis across the pre-synaptic membrane.
3) The neurotransmitter binds to the post-synaptic receptors. These receptors are linked to chemically gated ion channels and these channels may open or close as a result of binding to the receptors to cause a graded potential which can be either depolarization, or hyperpolarization depending on the transmitter. Depolarization results from opening of Na+ gates and is called an EPSP. Hyperpolarization could result from opening of K+ gates and is called IPSP.
4) Graded potentials spread and overlap and can summate to produce a threshold depolarization and an action potential when they stimulate voltage gated ion channels in the neuron's trigger region.
5) The neurotransmitter is broken down or removed from the synapse in order for the receptors to receive the next stimulus. As we learned there are enzymes for some neurotransmitters such as the Ach-E which breaks down acetylcholine. Monoamine oxidase (MAO) is an enzyme which breaks down the catecholamines (epinephrine, nor-epinephrine, dopamine) and nor-epinephrine (which is an important autonomic neurotransmitter) is removed by the axon as well in a process known as reuptake. Other transmitters may just diffuse away.
Graded Potentials - these are small, local depolarizations or hyperpolarizations which can spread and summate to produce a threshold depolarization. Small because they are less than that needed for threshold in the case of the depolarizing variety. Local means they only spread a few mm on the membrane and decline in intensity with increased distance from the point of the stimulus. The depolarizations are called EPSPs, excitatory post-synaptic potentials, because they tend to lead to an action potential which excites or turns the post-synaptic neuron on. Hyperpolarizations are called IPSPs, inhibitory post-synaptic potentials, because they tend to inhibit an action potential and thus turn the neuron off.
Summation - the EPSPs and IPSPs will add together to produce a net depolarization (or hyperpolarization).
Temporal summation- this is analogous to the frequency (wave, tetany) summation discussed for muscle. Many EPSPs occurring in a short period of time (e.g. with high frequency) can summate to produce threshold depolarization. This occurs when high intensity stimulus results in a high frequency of EPSPs.
Spatial summation - this is analogous to quantal summation in a muscle. It means that there are many stimuli occurring simultaneously. Their depolarizations spread and overlap and can build on one another to sum and produce threshold depolarization.
Inhibition - When the brain causes an IPSP in advance of a reflex pathway being stimulated, it reduces the likelihood of the reflex occurring by increasing the depolarization required. The pathway can still work, but only with more than the usual number or degree of stimulation. We inhibit reflexes when allowing ourselves to be given an injection or blood test for instance.
Facilitation - When the brain causes an EPSP in advance of a reflex pathway being stimulated, it makes the reflex more likely to occur, requiring less additional stimulation. When we anticipate a stimulus we often facilitate the reflex.
Learned Reflexes - Many athletic and other routine activities involve learned reflexes. These are reflex pathways facilitated by the brain. We learn the pathways by performing them over and over again and they become facilitated. This is how we can perfect our athletic performance, but only if we learn and practice them correctly. It is difficult to "unlearn" improper techniques once they are established reflexes. Like "riding a bike" they may stay with you for your entire life!
Post-tetanic potentiation - This occurs when we perform a rote task or other repetitive action. At first we may be clumsy at it, but after continuous use (post-tetanic) we become more efficient at it (potentiation). These actions may eventually become learned reflexes
The Action Potential
The trigger region of a neuron is the region where the voltage gated channels begin. When summation results in threshold depolarization in the trigger region of a neuron, an action potential is produced. There are both sodium and potassium channels. Each sodium channel has an activation gate and an inactivation gate, while potassium channels have only one gate.
A) At the resting state the sodium activation gates are closed, sodium inactivation gates are open, and potassium gates are closed. Resting membrane potential is at around -70 mv inside the cell.
B) Depolarizing phase: The action potential begins with the activation gates of the sodium channels opening, allowing Na+ ions to enter the cell and causing a sudden depolarization which leads to the spike of the action potential. Excess Na+ ions enter the cell causing reversal of potential becoming briefly more positive on the inside of the cell membrane.
C) Repolarizing phase: The sodium inactivation gates close and potassium gates open. This causes Na+ ions to stop entering the cell and K+ ions to leave the cell, causing repolarization. Until the membrane is repolarized it cannot be stimulated, called the absolute refractory period.
D) Excess potassium leaves the cell causing a brief hyperpolarization. Sodium activation gates close and potassium gates begin closing. The sodium-potassium pump begins to re-establish the resting membrane potential. During hyperpolarization the membrane can be stimulated but only with a greater than normal depolarization, the relative refractory period.
Action potentials are self-propagated, and once started the action potential progresses along the axon membrane. It is all-or-none, that is there are not different degrees of action potentials. You either have one or you don't.
Asthma = Reversible Bronchioconstruction 4%-5% of population
Extrinsic / Atopic = Allergic, inherited (familia), chromosome 11
IgE, Chemical Mediators of inflammation
a. Intrinsic = Negative for Allergy, Normal IgE, Negative Allergic Tests
Nucleotide Imbalance cAMP/cGMP: cAMP = Inhibits mediator release, cGMP = Facilitates mediator release
b. Intolerance to Asprin (Triad Asthma)
c. Nasal Polyps & Asthma
d. Treatment cause, Symptoms in Acute Asthma
1. Bronchial dilators
2. steroids edema from Inflamation
3. Bronchiohygene to prevent Secondary Infection, (Remove Excess Mucus)
4. Education
The defecation reflex:
As a result of the mass movements, pressure is exerted on the rectum and on the internal anal sphincter, which is smooth muscle, resulting in its involuntary relaxation. Afferent impulses are sent to the brain indicating the need to defecate. The external sphincter is voluntary muscle and is controlled by the voluntary nervous system. This sphincter is relaxed along with contraction of the rectal and abdominal muscles in the defecation reflex
Urine is a waste byproduct formed from excess water and metabolic waste molecules during the process of renal system filtration. The primary function of the renal system is to regulate blood volume and plasma osmolarity, and waste removal via urine is essentially a convenient way that the body performs many functions using one process. Urine formation occurs during three processes:
Filtration
Reabsorption
Secretion
Filtration
During filtration, blood enters the afferent arteriole and flows into the glomerulus where filterable blood components, such as water and nitrogenous waste, will move towards the inside of the glomerulus, and nonfilterable components, such as cells and serum albumins, will exit via the efferent arteriole. These filterable components accumulate in the glomerulus to form the glomerular filtrate.
Normally, about 20% of the total blood pumped by the heart each minute will enter the kidneys to undergo filtration; this is called the filtration fraction. The remaining 80% of the blood flows through the rest of the body to facilitate tissue perfusion and gas exchange.
Reabsorption
The next step is reabsorption, during which molecules and ions will be reabsorbed into the circulatory system. The fluid passes through the components of the nephron (the proximal/distal convoluted tubules, loop of Henle, the collecting duct) as water and ions are removed as the fluid osmolarity (ion concentration) changes. In the collecting duct, secretion will occur before the fluid leaves the ureter in the form of urine.
Secretion
During secretion some substances±such as hydrogen ions, creatinine, and drugs—will be removed from the blood through the peritubular capillary network into the collecting duct. The end product of all these processes is urine, which is essentially a collection of substances that has not been reabsorbed during glomerular filtration or tubular reabsorbtion.
Respiration occurs in three steps :
1- Mechanical ventilation : inhaling and exhaling of air between lungs and atmosphere.
2- Gas exchange : between pulmonary alveoli and pulmonary capillaries.
3- Transport of gases from the lung to the peripheral tissues , and from the peripheral tissues back to blood .
These steps are well regulated by neural and chemical regulation.
Respiratory tract is subdivided into upper and lower respiratory tract. The upper respiratory tract involves , nose , oropharynx and nasopharynx , while the lower respiratory tract involves larynx , trachea , bronchi ,and lungs .
Nose fulfills three important functions which are :
1. warming of inhaled air .
b. filtration of air .
c. humidification of air .
Pharynx is a muscular tube , which forms a passageway for air and food .During swallowing the epiglottis closes the larynx and the bolus of food falls in the esophagus .
Larynx is a respiratory organ that connects pharynx with trachea . It is composed of many cartilages and muscles and
vocal cords . Its role in respiration is limited to being a conductive passageway for air .
Trachea is a tube composed of C shaped cartilage rings from anterior side, and of muscle (trachealis muscle ) from its posterior side.The rings prevent trachea from collapsing during the inspiration.
From the trachea the bronchi are branched into right and left bronchus ( primary bronchi) , which enter the lung .Then they repeatedly branch into secondary and tertiary bronchi and then into terminal and respiratory broncholes.There are about 23 branching levels from the right and left bronchi to the respiratory bronchioles , the first upper 17 branching are considered as a part of the conductive zones , while the lower 6 are considered to be respiratory zone.
The cartilaginous component decreases gradually from the trachea to the bronchioles . Bronchioles are totally composed of smooth muscles ( no cartilage) . With each branching the diameter of bronchi get smaller , the smallest diameter of respiratory passageways is that of respiratory bronchiole.
Lungs are evolved by pleura . Pleura is composed of two layers : visceral and parietal .
Between the two layers of pleura , there is a pleural cavity , filled with a fluid that decrease the friction between the visceral and parietal pleura.
Respiratory muscles : There are two group of respiratory muscles:
1. Inspiratory muscles : diaphragm and external intercostal muscle ( contract during quiet breathing ) , and accessory inspiratory muscles : scaleni , sternocleidomastoid , internal pectoral muscle , and others( contract during forceful inspiration).
2. Expiratory muscles : internal intercostal muscles , and abdominal muscles ( contract during forceful expiration)
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
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.