Amygdala

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The amygdala is an almond-shaped, grey-matter structure located in the inferior portion of temporal lobe in each cerebral hemisphere. It is bigger in male than in female. It receives afferent fibers from the medulla oblongata and sends efferent nerve projections to the hypothalamus and thalamic reticular nucleus. It is also known as the amygdaloid body.

Function

Along with the hippocampus, the amygdala belongs to the limbic system. Thus, functionally, it is associated with long-term memory, playing an important role in storing memories linked to emotional events. Stimulation of the amygdala in humans leads to changes in mood, ranging from fear and rage to rest and relaxation . It is also associated with sexual hormones response, having receptors for both testosterone and estrogen.

Anatomical Description

The amygdala stretches medially to the inferior surface of the cortex of temporal lobe in both cerebral hemisphere. This is why it is also considered to be part of the cortex as well as a nuclear structure which has migrated into the white matter. It lies right below the putamen and anterior to the tail of the caudate nucleus. The fibers of the pyramidal tract run posteriorly and medially to the amygdala. The corticomedial half of it is considered to be part of the paleocortex, while the deeper portion is characterized as a nuclear complex.

Below, lateral view of left cerebral hemisphere, showing the position of the amygdala in relation to basal ganglia structures (putamen and caudate nucleus) and pyramidal tract.

Frontal view of the aforesaid grey nuclei in a coronal section of the forebrain (cerebrum)


Insular Cortex

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Also known as insula, the insular cortex is the part of the human cerebral cortex which is buried deep in the Sylvian fissure. It is a deep fold formed by the massive overgrowth of the temporal, frontal, and parietal lobe on the lateral aspect of cerebral hemispheres. It is located laterally to the lenticular nucleus. It can be accessed and seen by opening up the Sylvian fissure (lateral sulcus), pulling up the lower margin (operculum) of the parietal and frontal lobe with a bent spatula.

From a functional point of view, the insular cortex is related, along with the hippocampus, with long-term memory, emotion, as well as to self-awareness (consciousness), empathic feelings, and maturity. Its nerve cells receive and send axons from and to the temporal, frontal and parietal lobe. Its myelinated neuron axons are part of both the superior longitudinal and fronto-temporal fasciculus. This area of the cortex is supplied by branches of the middle cerebral artery.

Below, lateral aspect of left cerebral hemisphere and coronal section of brain showing the insular cortex.

 

Immune System

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The immune system is a network of highly specialized cells, organs, and tissues, which shield the body from pathogenic foreign microorganisms, such as bacteria and parasites, as well as from viruses and cancer cells. When the body is well nourished and physiologically balanced, it prevents infectious diseases and tumors from breaking out.

The main pillar of the immune system is made up of white blood cells, or leukocytes, which are highly-specialized cells that move in the bloodstream through arteries and lymphatic vessels, reaching every organ and tissue of the body. They constitute the immune response, which is the white blood cell reaction to the presence of foreign microorganisms, abnormal cells and cellular debris. They are composed of three types of cells: 1) granulocytes, which comprise neutrophils, eosinophils, and basophils; 2) lymphocytes, which include T and B-lymphocytes; 3) monocytes, which develops into macrophages or phagocytic cells.

Granulocytes protect the body from bacteria and parasites, while lymphocytes are active against viruses and cancer cells. Monocytes, on the other hand, engulf and destroy pathogenic microorganisms and cancer cells. All these white blood cells originate in the red bone marrow from stem cells. Then they travel in the bloodstream into the spleen and thymus where they mature into active leukocytes, that protect us from diseases.

When the immune system does not work properly, it is said that that someone is immunodeficient. This means, the the patient lacks enough mature white blood cells to protect him/her from pathogenic microorganisms and viruses. This imbalance of the immune system can be caused by 1) malnutrition, as when we do not eat enough proteins, which contain the twelve essential amino acids; 2) diabetes, which causes comorbidity; 3) viruses that weakens the immune system, such as HIV; 4) cancer and chemotherapy; 5) second and third degree burns.

Below, the three types of white blood cells that are essential part of the immune system.


White Blood Cells

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The white blood cells, or leukocytes, are unpigmented, nucleated cells which are the essence of the immune system. They are ubiquitous as they are found in the bloodstream, lymphatic vessels and nodes, spleen, and thymus. Through the arteries, arterioles, and capillaries, they reach every tissue of the body to protect us against foreign pathological microorganisms, viruses, parasite larvae, and cancer cells.

There are three types of white blood cells: granulocytes, lymphocytes, and monocytes. The granulocytes, in turn, are composed of three specialized leukocytes: neutrophils, basophils, and eosinophils. Lymphocytes consists of T-lymphocytes and B-lymphocytes (or simply T and B cells). Monocytes, on the other hand, are a mononuclear phagocytic cell (destroys pathogenic microorganism through phagocytosis). All of them originate from stem cells in the red bone marrow and mature in the spleen and thymus.

The granulocytes are so called because of the granule-like structures found in their cytosol. The neutrophils are polymorphonuclear leukocytes, which are highly specialized to attack and destroy bacteria through phagocytosis, using hydrolitic enzymes. Eosinophils are granulocytes whose cytoplasm contain rounded granules; they react before the presence of parasites and their parasitic larvae. Basophils, like eosinophils, play an important role in parasitic infections and allergies, appearing in many inflammatory reactions that cause allergic symptoms; they also produce histamine and heparine.

Lymphocytes are spherical in shape and contain an oval nucleus. B-lymphocytes produce antibodies by transforming into plasma cells when the detect the presence of an antigen (virus, protozoan, bacteria); these plasma cells mark the antigen for the cytotoxic T cells and monocytes can identify and destroy it. T-lymphocytes (cytotoxic T-cells) react when they detect a pathogen (virus, bacterium, etc) that has been marked by a B-lymphocyte by destroying it. There are other types of T-lymphocytes, such as helper T-cells and natural killer cells.

Below, schematic picture of the different types of white blood cells.


 

Phagocytes

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Phagocytes are white blood cells that perform phagocytosis, which is the engulfing, absorption, and destruction of pathogenic microorganisms, cancer cells, and protein particles. There are three types of phagocytic cells: 1) the neutrophil, which is a granulocyte that intercepts and kills bacteria; 2) mononuclear phagocytes, which include the monocyte and macrophage; 3) organ-specific phagocytes, which are found in the liver, spleen, lymph nodes, lungs, and brain.

The organ-specific phagocytes have different names, according to the organs and systems in which they are found. For example, in the brain, they are called microglia; and, in the liver, they called Kupffer cells. These are ¨fixed¨ phagocytes, meaning that these cells do not move around throughout the organ but they are rather disseminated (scattered) everywhere in the tissue of the parenchyma. A well-known mononuclear phagocyte, the macrophage, operates separately from the immune system, clearing cellular debris (remnants of dead cells, which usually dies from apoptosis).

Phagocytic cells engulf bacteria and cell particles in the same way amoeba eats. A bacterium or cell particle gets surrounded by cytoplasmic extensions, which are called pseudopods. These extensions derive from the plasma membrane and release enzymes, which kills and break down the pathogenic cell or particle.

Eosinophils

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Eosinophils are a type of white blood cell which destroys parasitic larvae. They are granulocytes, which means they are leukocytes that have granule-like bodies in their cytosol. The other two types of granulocytes in the immune system are neutrophils and basophils. Eosinophils comprise only 1%, sometimes 2%, of all leukocytes in the blood. A cubic millimeter (1 m3) of blood contains about 200 of them.

Description

Eosinophils are motile phagocytic cells, which means they are moving white blood cells which have the capacity to engulf and destroy pathogenic microorganisms that have invaded the body. They are similar in size to the neutrophils, with a diameter of 12 or 15 μm (micron). However, they have a different aspect in the light microscope. The nucleus has two or three lobes, which contain an striking array of red or orange eosinophilic granules.

An eosinophil has a typical cell membrane and cytoplasm, in which scattered mitochondria can be found. It also has one Golgi apparatus near the nucleus, numerous eosinophilic granules, which are modified lysosomes. These eosinophilic granules have prominent crystaloids. Microfilaments can also be found near the cortex.

Functions

The eosinophils kill parasitic larvae as the enter the peripheral blood or the lamina propria of the gut. When parasites invade the body, mast cells* draw eosinophils to the infection site. Specifically, mast cells attract eosinophils by secreting eosinophil chemotactil factor. Lymphokines also draw this type of white blood cell to the site of inflammation. Thus, eosinophils help regulate mast cell response to inflammation by releasing an enzymes, which degrade the histamines secreted by mast cells at the site of infection.

*Mast cells are produced in the bone marrow and function in allergic reactions and immune response.

Below, diagrammatic picture of an eosinophil.


Neutrophils

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Neutrophils are one of the three types of granulocytes of the immune system; the others being eosinophils and basophils. These are white blood cells (leukocytes) which contains glycogen granules in their cytoplasm. Neutrophils are polymorphonuclear cells, which means their nucleus is highly variable, having three, four and sometimes five lobes. They are the most common white blood cells in the normal human peripheral blood, constituting between 40% and 65% of all leukocytes in the bloodstream.

Function

The main function of neutrophils is the phagocytosis and destruction of bacteria. It means the engulfing and ingestion of this type of microbe. Bacteria are phagocytosed after opsonization (the process by which a bacterium is weakened before being destroyed).

Structure

A neutrophil is between 12 and 15 μm in diameter. The nucleus has three to five lobes, contains no nucleolus, and it is largely heterochromatin. The cytoplasm has only a few mitochondria, a Golgi apparatus, and little endoplasmatic reticulum; it also contains many glycogen granules. (glycogen is a form of sugar = carbohydrate). The neutrophils have two varieties of membrane-bound granules, which are modified lysosomes and have a bacteriological function. They also have cortical microfilaments and some cytoplasmic microtubules.

How a bacterium is destroyed by a neutrophil

The membrane-bound azurophilic granules (primary or type A) contains myeloperoxidase, which is an enzyme that produces bacterocidal molecular oxygen from hydrogen peroxide (H2 O2). This corrodes the bacterium membrane. Meanwhile, specific granules have a special protein called lactoferrin, which binds to the ferric ions of bacteria that is required for bacterial multiplication.

A light micrograph of two neutrophils among erythrocytes in a normal human peripheral blood smear.


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