NEET MDS Lessons
Physiology
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.
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)
Red blood cell cycle:
RBCs enter the blood at a rate of about 2 million cells per second. The stimulus for erythropoiesis is the hormone erythropoietin, secreted mostly by the kidney. RBCs require Vitamin B12, folic acid, and iron. The lifespan of RBC averages 120 days. Aged and damaged red cells are disposed of in the spleen and liver by macrophages. The globin is digested and the amino acids released into the blood for protein manufacture; the heme is toxic and cannot be reused, so it is made into bilirubin and removed from the blood by the liver to be excreted in the bile. The red bile pigment bilirubin oxidizes into the green pigment biliverdin and together they give bile and feces their characteristic color. Iron is picked up by a globulin protein (apotransferrin) to be transported as transferrin and then stored, mostly in the liver, as hemosiderin or ferritin. Ferritin is short term iron storage in constant equilibrium with plasma iron carried by transferrin. Hemosiderin is long term iron storage, forming dense granules visible in liver and other cells which are difficult for the body to mobilize.
Some iron is lost from the blood due to hemorrhage, menstruation, etc. and must be replaced from the diet. On average men need to replace about 1 mg of iron per day, women need 2 mg. Apotransferrin (transferrin without the iron) is present in GI lining cells and is also released in the bile. It picks up iron from the GI tract and stimulates receptors on the lining cells which absorb it by pinocytosis. Once through the mucosal cell iron is carried in blood as transferrin to the liver and marrow. Iron leaves the transferrin molecule to bind to ferritin in these tissues. Most excess iron will not be absorbed due to saturation of ferritin, reduction of apotransferrin, and an inhibitory process in the lining tissue.
Erythropoietin Mechanism:
Myeloid (blood producing) tissue is found in the red bone marrow located in the spongy bone. As a person ages much of this marrow becomes fatty and ceases production. But it retains stem cells and can be called on to regenerate and produce blood cells later in an emergency. RBCs enter the blood at a rate of about 2 million cells per second. The stimulus for erythropoiesis is the hormone erythropoietin, secreted mostly by the kidney. This hormone triggers more of the pleuripotential stem cells (hemocytoblasts) to follow the pathway to red blood cells and to divide more rapidly.
It takes from 3 to 5 days for development of a reticulocyte from a hemocytoblast. Reticulocytes, immature rbc, move into the circulation and develop over a 1 to 2 day period into mature erythrocytes. About 1 to 2 % of rbc in the circulation are reticulocytes, and the exact percentage is a measure of the rate of erythropoiesis.
Structural Divisions of the nervous system:
1) Central Nervous System (CNS) - the brain and spinal cord.
2) Peripheral Nervous System (PNS) - the nerves, ganglia, receptors, etc
Lipids:
- about 40% of the dry mass of a typical cell
- composed largely of carbon & hydrogen
- generally insoluble in water
- involved mainly with long-term energy storage; other functions are as structural components (as in the case of phospholipids that are the major building block in cell membranes) and as "messengers" (hormones) that play roles in communications within and between cells
- Subclasses include:
- Triglycerides - consist of one glycerol molecule + 3 fatty acids (e.g., stearic acid in the diagram below). Fatty acids typically consist of chains of 16 or 18 carbons (plus lots of hydrogens).
- phospholipids - Composed of 2 fatty acids, glycerol, phosphate and polar groups , phosphate group (-PO4) substitutes for one fatty acid & these lipids are an important component of cell membranes
steroids - have 4 rings- cholesterol, some hormones, found in membranes include testosterone, estrogen, & cholesterol
Levels of Organization:
CHEMICAL LEVEL - includes all chemical substances necessary for life (see, for example, a small portion - a heme group - of a hemoglobin molecule); together form the next higher level
CELLULAR LEVEL - cells are the basic structural and functional units of the human body & there are many different types of cells (e.g., muscle, nerve, blood)
TISSUE LEVEL - a tissue is a group of cells that perform a specific function and the basic types of tissues in the human body include epithelial, muscle, nervous, and connective tissues
ORGAN LEVEL - an organ consists of 2 or more tissues that perform a particular function (e.g., heart, liver, stomach)
SYSTEM LEVEL - an association of organs that have a common function; the major systems in the human body include digestive, nervous, endocrine, circulatory, respiratory, urinary, and reproductive.
There are two types of cells that make up all living things on earth: prokaryotic and eukaryotic. Prokaryotic cells, like bacteria, have no 'nucleus', while eukaryotic cells, like those of the human body, do.
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