Physiology of Synapse:- Excitatory synapses and Inhibitory synapses

Synapse is simply a junction between two neurons. There are two types of synapses in central nervous system.

·         Excitatory synapses

·         Inhibitory synapses

Excitatory synapses

Pre synaptic knob secretes as excitatory substance. (Ex: Acetyl choline, Noradrinalin, Gutamic acid)

Transmission:

Nerve impulse à pre synaptic knob releases acetyl choline (Ach)àAch diffuse across synaptic cleftàAch combine with receptor sites of post synaptic membraneàpermeability of post synaptic membrane changesà permeability to Na⁺ increases and Na⁺ enter the post synaptic knobà Excitatory post synaptic potential (EPSP) is obtainedàthreshold potential is reachedà action potential is formedà distribution of action potential

A combined EPSP depolarize the membrane to a threshold value to generate an action potential is known as summation. Summation can occur in two ways as temporal summation which is obtained by repeat discharge of single pre synaptic knob and spatial summation which is obtained by numerous pre synaptic knobs.

Inhibitory synapses

Pre synaptic knob secretes an inhibitory substance. (Ex: alpha butric acid and glycine)

This inhibitory synapses function in two ways as post synaptic inhibition and pre synaptic inhibition.

Post synaptic inhibition

Nerve impulseà pre synaptic knob release an inhibitory substance (GABA)àGABA diffuse across synaptic cleftà GABA binds with receptor sites on post synaptic membraneà permeability of post synaptic membrane changesà permeability to K⁺ and Cl^- ions increaseà K⁺ leaves and Cl^- ions enteràInhibitory post synaptic potential (IPSP) is obtainedà membrane hyperpolarizedà no action potential initiated

Pre synaptic inhibition

Nerve impulseà pre synaptic knob release inhibitory substancesàhyperpolarize the pre synaptic  excitatory membraneà prevent excitatory substances releasingà no excitation occurs on post synaptic membrane     

Sequence of events take place during action potential in brief

>In resting state, membrane is permeable to K⁺ and relatively impermeable to Na⁺.

>When a stimulus depolarize the membrane into the threshold value the voltage gated Na⁺ channels open, vastly increasing the membrane’s permeability to Na⁺.

>Na⁺ enters the cell across the membrane under the influence of both concentration gradient and the electric gradient.

>Na⁺ makes the membrane more depolarized and that makes more voltage gated sodium channels to open.

>As the membrane potential approaches equilibrium potential of Na⁺ the driving force of Na⁺ is reduced. Therefore less number of Na⁺ reach into the cell.

>After a short time the voltage gated Na⁺ channels are inactivated and stop the taken in of the Na⁺.  Here the membrane potential rise to +45mV.

>As this occurs, the voltage gated K⁺ channels open and greatly increases the permeability to K⁺ ions. Therefore K⁺ leaves the cell under the influence of concentration gradient and electric gradient.

>As K⁺ leaves, the positive charge inside the cell is reduced. After awhile the equilibrium potential of K⁺ is obtained and the membrane is hyperpolarized. Now the membrane potential is -75mV. Hyperpolarization is due to the delay of closing the K⁺ channels compared to Na⁺ channels.

>The voltage gated K⁺ channels close and by then the Na⁺ channels recovered by inactivation.

Controlling metabolism at cellular level: Compartmentation

Metabolism is composed of reaction sequences termed as metabolic pathways. The changes at each step in a metabolic pathway are regulated in order to maintain the stability and survival of the organism. Mainly there are four ways of controlling metabolism. Such one important way of controlling metabolism at cellular level is compartmentation.

When considering a eukaryotic cell; it’s composed of many different compartments known as sub cellular organelles in contrast to a prokaryotic cell. Each organelle contributes a specific metabolic function with different enzymatic activities under different conditions. These metabolic pathways needed to be interconnected to maintain the metabolism. Therefore compartmentation allows the distribution of metabolites and enzymes between the membranes bounded organelles. It’s the method that enables the cell to obtain different metabolic activities at the same time for more energy efficiency. For an example, glycolysis occurs in the cytoplasm and provides the mitochondria substrates such as pyruvates from carbohydrate oxidation. TCA cycle occurs within the mitochondrion matrix while oxidative phosphorylation occurs in mitochondria inner membrane. However, altogether all these three processes are included in one metabolism called ‘cell respiration’. Enzymes of each process are found where the processes take place.

Fat metabolism is another one good example for compartmentation. Fatty acids synthesis takes place in cytoplasm. Cytoplasm contains the required enzymes for the anabolism of fatty acid synthesis. But these fatty acids are degregaded in mitochondria supplying Acetyl Co A for TCA cycle. Mitochondria also contain the enzymes require for the catabolism of fat.

RNA polymerase allows RNA synthesis by using a template of DNA strand. It catalyzes the synthesis of RNA. RNA synthesis takes place in nucleus. DNA transcription also occurs in nucleus. Therefore both the RNA polymerase and DNA templates are available, so that the RNA synthesis is favorable. When considering protein synthesis, all the organelles used for it are close to one another. The nucleolus makes ribosome which synthesizes proteins. Endoplasmic reticulum is near the nucleus to receive Ribosomes and synthesized proteins. The Golgi apparatus is also near the endoplasmic reticulum for storing, packaging and distribution. Therefore all the activities within a cell are cooperated with one another.

Hence, compartmentation helps metabolic controlling by maintaining substrates, regulators and enzymes in separate locations with only favoured access between them.

Endocrine Glands

Found on: www.hormone.org

The following list of glands make up the endocrine system.

• Pituitary Gland
• Hypothalmus
• Thymus
• Pineal Gland
• Testes
• Ovaries
• Thyroid
• Adrenal Glands
• Parathyroid
• Pancreas

Pituitary Gland

The pituitary gland is sometimes called the "master gland" because of its great influence on the other body organs. Its function is complex and important for overall well-being.

The pituitary gland is divided into two parts, front (anterior) and back (posterior).

The anterior pituitary produces several types of hormones:

• 1. Prolactin or PRL - PRL stimulates milk production from a woman's breasts after childbirth and can affect sex hormone levels from the ovaries in women and the testes in men.

• 1. Growth hormone or GH - GH stimulates growth in childhood and is important for maintaining a healthy body composition. In adults it is also important for maintaining muscle mass and bone mass. It can affect fat distribution in the body. (For more information go to the Growth section on this site)

• 1. Adrenocorticotropin or ACTH - ACTH stimulates production of cortisol by the adrenal glands. Cortisol, a so-called "stress hormone," is vital to survival. It helps maintain blood pressure and blood glucose levels.

• 1. Thyroid-stimulating hormone or TSH - TSH stimulates the thyroid gland to make thyroid hormones, which, in turn, control (regulate) the body's metabolism, energy, growth and development, and nervous system activity.

• 1. Luteinizing hormone or LH - LH regulates testosterone in men and estrogen in women.

• 1. Follicle-stimulating hormone or FSH - FSH promotes sperm production in men and stimulates the ovaries to release eggs (ovulate) in women. LH and FSH work together to allow normal function of the ovaries or testes.

The posterior pituitary produces two hormones:

1. Oxytocin - Oxytocin causes milk letdown in nursing mothers and contractions during childbirth.
2. Antidiuretic hormone or ADH - ADH, also called vasopressin, is stored in the back part of the pituitary gland and regulates water balance. If this hormone is not secreted properly, this can lead to problems of sodium (salt) and water balance, and could also affect the kidneys so that they do not work as well.

In response to over- or underproduction of pituitary hormones, the target glands affected by these hormones can produce too many or too few hormones of their own, leading to hormone imbalance. For example, too much growth hormone can cause gigantism, or excessive growth (referred to as acromegaly in adults), while too little GH may cause dwarfism, or very short stature.

Hypothalamus

The hypothalamus is part of the brain that lies just above the pituitary gland. It releases hormones that start and stop the release of pituitary hormones. The hypothalamus controls hormone production in the pituitary gland through several "releasing" hormones. Some of these are growth hormone-releasing hormone, or (controls GH release); thyrotropin-releasing hormone, or TRH (controls TSH release); and corticoptropin-releasing hormone, or CRH (controls ACTH release). Gonadotropin-releasing hormone (GnRH) tells the pituitary gland to make luteinizing hormone (LH) and follicle-stimulating hormone (FSH), which are important for normal puberty.

Thymus

The thymus is a gland needed early in life for normal immune function. It is very large just after a child is born and weighs its greatest when a child reaches puberty. Then its tissue is replaced by fat. The thymus gland secretes hormones called humoral factors. These hormones help to develop the lymphoid system, which is a system throughout the body that help it to reach a mature immune response in cells to protect them from invading bodies, like bacteria.

Pineal Gland

Scientists are still learning how the pineal gland works. They have found one hormone so far that is produced by this gland: melatonin. Melatonin may stop the action of (inhibit) the hormones that produce gonadotropin, which causes the ovaries and testes to develop and function. It may also help to control sleep patterns.

Testes

Males have twin reproductive glands, called testes, that produce the hormone testosterone. Testosterone helps a boy develop and then maintain his sexual traits. During puberty, testosterone helps to bring about the physical changes that turn a boy into an adult male, such as growth of the penis and testes, growth of facial and pubic hair, deepening of the voice, increase in muscle mass and strength, and increase in height. Throughout adult life, testosterone helps maintain sex drive, sperm production, male hair patterns, muscle mass, and bone mass. 

Testicular cancer, which is the most common form of cancer for males between ages 15 and 35, may need to be treated by surgical removal of one or both testicles. The resulting decrease or absence of testosterone may cause decreased sexual drive, impotence, altered body image, and other symptoms.

Ovaries

The two most important hormones of a woman's twin reproductive glands, the ovaries, are estrogen and progesterone. These hormones are responsible for developing and maintaining female sexual traits, as well as maintaining a pregnancy. Along with the pituitary gonadotropins (luteinizing hormone or LH and follicle-stimulating hormone or FSH), they also control the menstrual cycle. The ovaries also produce inhibin, a protein that curbs (inhibits) the release of follicle-stimulating hormone from the anterior pituitary and helps control egg development.

The most common change in the ovarian hormones is caused by the start of menopause, part of the normal aging process. It also can occur when ovaries are removed surgically. Loss of ovarian function means loss of estrogen, which can lead to symptoms of menopause including hot flashes, thinning vaginal tissue, lack of menstrual periods, mood changes and bone loss, or osteoporosis.

A condition called polycystic ovary syndrome (PCOS) is caused by overproduction of male hormones in females. PCOS can affect menstrual cycles, fertility, and hormone levels, as well as cause acne, facial hair growth, and male pattern balding.

Thyroid

The thyroid is a small gland inside the neck, located in front of your breathing airway (trachea) and below your Adam's apple. The thyroid hormones control your metabolism, which is the body's ability to break down food and store it as energy and the ability to break down food into waste products with a release of energy in the process. The thyroid produces two hormones, T3 (called tri-iodothyronine) and T4 (called thyroxine).

Thyroid disorders result from an underactive or overactive thyroid producing, respectively, too little or too much thyroid hormone. Symptoms of hypothyroidism (too little hormone) include decreased energy, slow heart rate, dry skin, constipation, and feeling cold all the time. In children, hypothyroidism most commonly leads to slowed growth. Infants born with hypothyroidism can have delayed development and mental retardation if not treated. In adults, this disorder often causes weight gain. An enlarged thyroid, or goiter, may develop.

Hyperthyroidism (too much hormone) may impact normal thyroid size and result in exophthalmic goiter, or Grave's disease. Symptoms of this thyroid disease include anxiety, fast heart rate, diarrhea, and weight loss. An enlarged thyroid gland (goiter) and swelling behind the eyes that causes the eyes to push forward, or bulge out, are common.

Adrenal Glands

Each adrenal gland is actually two endocrine organs. The outer portion is called the adrenal cortex. The inner portion is called the adrenal medulla. The hormones of the adrenal cortex are essential for life. The types of  hormones secreted by the adrenal medulla are not.

The adrenal cortex produces glucocorticoids (such as cortisol) that help the body control blood sugar, increase the burning of protein and fat, and respond to stressors like fever, major illness, and injury. The mineralcorticoids (such as aldosterone) control blood volume and help to regulate blood pressure by acting on the kidneys to help them hold onto enough sodium and water. The adrenal cortex also produces some sex hormones, which are important for some secondary sex characteristics in both men and women.

Two important disorders caused by problems with the adrenal cortex are Cushing's syndrome and Addison's disease. Cushing's syndrome is the result of too much cortisol, and Addison's disease occurs when there is too little cortisol.

The adrenal medulla produces epinephrine (adrenaline), which is secreted by nerve endings and increases the heart rate, opens airways to improve oxygen intake, and increases blood flow to muscles, usually when a person is scared, excited, or under stress.

Norepinephrine also is made by the adrenal medulla, but this hormone is more related to maintaining normal activities as opposed to emergency reactions. Too much norepinephrine can cause high blood pressure.

Parathyroid

Located behind the thyroid gland are four tiny parathyroid glands. These make hormones that help control calcium and phosphorous levels in the body. The parathyroid glands are necessary for proper bone development. In response to too little calcium in the diet, the parathyroid glands make parathyroid hormone, or PTH, that takes calcium from bones so that it will be available in the blood for nerve conduction and muscle contraction.

If the parathyroids are removed during a thyroid operation, low blood calcium will result in symptoms such as irregular heartbeat, muscle spasms, tingling in the hands and feet, and possibly difficulty breathing. A tumor or chronic illness can cause too much secretion of PTH and lead to bone pain, kidney stones, increased urination, muscle weakness, and fatigue.

Pancreas

The pancreas is a large gland behind your stomach that helps the body to maintain healthy blood sugar (glucose) levels. The pancreas secretes insulin, a hormone that helps glucose move from the blood into the cells where it is used for energy. The pancreas also secretes glucagon when the blood sugar is low. Glucagon tells the liver to release glucose, stored in the liver as glycogen, into the bloodstream. 

Diabetes, an imbalance of blood sugar levels, is the major disorder of the pancreas. There are two types of diabetes. Type I, and Type II diabetes. Type I diabetes occurs when the pancreas does not produce enough insulin. Type II diabetes occurs when the body is resistant to the insulin in the blood). Without enough insulin to keep glucose moving through the metabolic process, the blood glucose level rises too high.

In Type I diabetes, a patient must take insulin shots. In Type II diabetes, a patient may may not necessarily need insulin and can sometimes control blood sugar levels with exercise, diet and other medications.

A condition called hyperinsulinism (HI) is caused by too much insulin and leads to hypoglycemia (low blood sugar). The inherited form, called congenital HI, causes severe hypoglycemia in infancy. Sometimes it can be treated with medication but often requires surgical removal of part or all of the pancreas. An insulin-secreting tumor of the pancreas, or insulinoma, is a less common cause of hypoglycemia. Symptoms of low blood sugar include anxiety, sweating, increased heart rate, weakness, hunger, and light-headedness. Low blood sugar stimulates release of epinephrine, glucagon and growth hormone, which help to return the blood sugar to normal.