NEET MDS Lessons
Physiology
There are three types of muscle tissue, all of which share some common properties:
- Excitability or responsiveness - muscle tissue can be stimulated by electrical, physical, or chemical means.
- contractility - the response of muscle tissue to stimulation is contraction, or shortening.
- elasticity or recoil - muscles have elastic elements (later we will call these their series elastic elements) which cause them to recoil to their original size.
- stretchability or extensibility - muscles can also stretch and extend to a longer-than-resting length.
The three types of muscle: skeletal, cardiac, and visceral (smooth) muscle.
Skeletal muscle
It is found attached to the bones for movement.
cells are long multi-nucleated cylinders.
The cells may be many inches long but vary in diameter, averaging between 100 and 150 microns.
All the cells innervated by branches from the same neuron will contract at the same time and are referred to as a motor unit.
Skeletal muscle is voluntary because the neurons which innervate it come from the somatic or voluntary branch of the nervous system.
That means you have willful control over your skeletal muscles.
Skeletal muscles have distinct stripes or striations which identify them and are related to the organization of protein myofilaments inside the cell.
Cardiac muscle
This muscle found in the heart.
It is composed of much shorter cells than skeletal muscle which branch to connect to one another.
These connections are by means of gap junctions called intercalated disks which allow an electrochemical impulse to pass to all the connected cells.
This causes the cells to form a functional network called a syncytium in which the cells work as a unit. Many cardiac muscle cells are myogenic which means that the impulse arises from the muscle, not from the nervous system. This causes the heart muscle and the heart itself to beat with its own natural rhythm.
But the autonomic nervous system controls the rate of the heart and allows it to respond to stress and other demands. As such the heart is said to be involuntary.
Visceral muscle is found in the body's internal organs and blood vessels.
It is usually called smooth muscle because it has no striations and is therefore smooth in appearance. It is found as layers in the mucous membranes of the respiratory and digestive systems.
It is found as distinct bands in the walls of blood vessels and as sphincter muscles.
Single unit smooth muscle is also connected into a syncytium similar to cardiac muscle and is also partly myogenic. As such it causes continual rhythmic contractions in the stomach and intestine. There and in blood vessels smooth muscle also forms multiunit muscle which is stimulated by the autonomic nervous system. So smooth muscle is involuntary as well
Water: comprises 60 - 90% of most living organisms (and cells) important because it serves as an excellent solvent & enters into many metabolic reactions
- Intracellular (inside cells) = ~ 34 liters
- Interstitial (outside cells) = ~ 13 liters
- Blood plasma = ~3 liters
40% of blood is red blood cells (RBCs)
plasma is similar to interstitial fluid, but contains plasma proteins
serum = plasma with clotting proteins removed
intracellular fluid is very different from interstitial fluid (high K concentration instead of high Na concentration, for example)
- Capillary walls (1 cell thick) separate blood from interstitial fluid
- Cell membranes separate intracellular and interstitial fluids
- Loss of about 30% of body water is fatal
Ions = atoms or molecules with unequal numbers of electrons and protons:
- found in both intra- & extracellular fluid
- examples of important ions include sodium, potassium, calcium, and chloride
Ions (Charged Atoms or Molecules) Can Conduct Electricity
- Giving up electron leaves a + charge (cation)
- Taking on electron produces a - charge (anion)
- Ions conduct electricity
- Without ions there can be no nerves or excitability
- Na+ and K+ cations
- Ca2+ and Mg2+ cations control metabolism and trigger muscle contraction and secretion of hormones and transmitters
Na+ & K+ are the Major Cations in Biological Fluids
- High K+ in cells, high Na+ outside
- Ion gradients maintained by Na pump (1/3 of basal metabolism)
- Think of Na+ gradient as a Na+ battery- stored electrical energy
- K+ gradient forms a K+ battery
- Energy stored in Na+ and K+ batteries can be tapped when ions flow
- Na+ and K+ produce action potential of excitable cells
GENERAL VISCERAL AFFERENT (GVA) PATHWAYS
Pain and Pressure Sensation via the Spinal Cord
Visceral pain receptors are located in peritoneal surfaces, pleural membranes, the dura mater, walls of arteries, and the walls of the GI tube.
Nociceptors in the walls of the GI tube are particularly sensitive to stretch and overdistension.
General visceral nociceptors conduct signals into the spinal cord over the monopolar neurons of the posterior root ganglia. They terminate in laminae III and IV of the posterior horn as do the pain and temperature pathways of the GSA system , their peripheral processes reach the visceral receptors via the gray rami communicantes and ganglia of the sympathetic chain
Second-order neurons from the posterior horn cross in the anterior white commissure and ascend to the thalamus in the anterior and lateral spinothalamic tracts,
Projections from the VPL of the thalamus relay signals to the sensory cortex.
The localization of visceral pain is relatively poor, making it difficult to tell the exact source of the stimuli.
Blood Pressure, Blood Chemistry, and Alveolar Stretch Detection
The walls of the aorta and the carotid sinuses contain special baroreceptors (pressure receptors) which respond to changes in blood pressure. These mechanoreceptors are the peripheral endings of GVA fibers of the glossopharyngeal (IX) and vagus (X) nerves
The GVA fibers from the carotid sinus baroreceptors enter the solitary tract of the brainstem and terminate in the vasomotor center of the medulla (Fig-14). This is the CNS control center for cardiovascular activity.
Stretch receptors in the alveoli of the lungs conduct information concerning rhythmic alveolar inflation and deflation over GVA X fibers to the solitary tract and then to the respiratory center of the brainstem. This route is an important link in the Hering-Breuer reflex, which helps to regulate respiration.
Carotid body chemoreceptors, sensitive to changes in blood PO2 and, to a lesser extent, PCO2 and pH, conduct signals to both the vasomotor and respiratory centers over GVA IX nerve fibers
GVA X fibers conduct similar information from the aortic chemoreceptors to both centers
Concentration versus diluting urine
Kidney is a major route for eliminating fluid from the body to accomplish water balance. Urine excretion is the last step in urine formation. Everyday both kidneys excrete about 1.5 liters of urine.
Depending on the hydrated status of the body, kidney either excretes concentrated urine ( if the plasma is hypertonic like in dehydrated status ) or diluted urine ( if the plasma is hypotonic) .
This occurs thankful to what is known as countercurrent multiplying system, which functions thankfully to establishing large vertical osmotic gradient .
To understand this system, lets review the following facts:
1. Descending limb of loop of Henle is avidly permeable to water.
2. Ascending limb of loop of Henly is permeable to electrolytes , but impermeable to water. So fluid will not folow electrolytes by osmosis.and thus Ascending limb creates hypertonic interstitium that will attract water from descending limb.
Pumping of electrolytes
3. So: There is a countercurrent flow produced by the close proximity of the two limbs.
Juxtamedullary nephrons have long loop of Henle that dips deep in the medulla , so the counter-current system is more obvious and the medullary interstitium is always hypertonic . In addition, peritubular capillaries in the medulla are straigh ( vasa recta) in which flow is rapid and rapidly reabsorb water maintaining hypertonic medullary interstitium.
In distal tubules water is diluted. If plasma is hypertonic, this will lead to release of ADH by hypothalamus, which will cause reabsorption of water in collecting tubules and thus excrete concentrated urine.
If plasma is hypotonic ADH will be inhibited and the diluted urine in distal tubules will be excreted as diluted urine.
Urea contributes to concentrating and diluting of urine as follows:
Urea is totally filtered and then 50% of filtrated urea will be reabsorbed to the interstitium, this will increase the osmolarity of medullary interstitium ( becomes hypertonic ). Those 50% will be secreted in ascending limb of loop of Henle back to tubular fluid to maintain osmolarity of tubular fluid. 55% of urea in distal nephron will be reabsorbed in collecting ducts back to the interstitium ( under the effect of ADH too) . This urea cycle additionally maintain hypertonic interstitium.
Gonadotropin-releasing hormone (GnRH)
GnRH is a peptide of 10 amino acids. Its secretion at the onset of puberty triggers sexual development.
Primary Effects
FSH and LH Relaese
Secondary Effects
Increases estrogen and progesterone (in females)
testosterone Relaese (in males)
Growth hormone-releasing hormone (GHRH)
GHRH is a mixture of two peptides, one containing 40 amino acids, the other 44. GHRH stimulates cells in the anterior lobe of the pituitary to secrete growth hormone (GH).
Corticotropin-releasing hormone (CRH)
CRH is a peptide of 41 amino acids. Its acts on cells in the anterior lobe of the pituitary to release adrenocorticotropic hormone (ACTH) CRH is also synthesized by the placenta and seems to determine the duration of pregnancy. It may also play a role in keeping the T cells of the mother from mounting an immune attack against the fetus
Somatostatin
Somatostatin is a mixture of two peptides, one of 14 amino acids, the other of 28. Somatostatin acts on the anterior lobe of the pituitary to
- inhibit the release of growth hormone (GH)
- inhibit the release of thyroid-stimulating hormone (TSH)
Somatostatin is also secreted by cells in the pancreas and in the intestine where it inhibits the secretion of a variety of other hormones.
Antidiuretic hormone (ADH) and Oxytocin
These peptides are released from the posterior lobe of the pituitary
The Posterior Lobe
The posterior lobe of the pituitary releases two hormones, both synthesized in the hypothalamus, into the circulation.
- Antidiuretic Hormone (ADH).
ADH is a peptide of 9 amino acids. It is also known as arginine vasopressin. ADH acts on the collecting ducts of the kidney to facilitate the reabsorption of water into the blood.- A deficiency of ADH
- leads to excessive loss of urine, a condition known as diabetes nsipidus.
- A deficiency of ADH
- Oxytocin
Oxytocin is a peptide of 9 amino acids. Its principal actions are:- stimulating contractions of the uterus at the time of birth
- stimulating release of milk when the baby begins to suckle
Alveolar Ventilation: is the volume of air of new air , entering the alveoli and adjacent gas exchange areas each minute . It equals to multiplying of respiratory rate by ( tidal volume - dead space).
Va = R rate X (TV- DsV)
= 12 X ( 500-150)
= 4200 ml of air.