Pain sensation Pain receptors are also called as nociceptors They are located at the ends of small 'C unmyelinated or myelinated A delta fibers a) Definition Pain sensation is unpleasant but protective sensation aroused by noxious stimuli that damage or can damage body tissues b) Physiology (properties & reaction) Purpose or importance: Protective Stimulus: noxious (chemicals like- Ach, bradykinin, serotonin, hydrogen ions, potassium ions, prostaglandins or mechanical or thermal) Receptors: free nerve endings (polymodal receptors) Adaptation : non or slow adapting receptors Nerve fibers: fast pain is carried by A-delta nerve fibers while slow pain by 'C' type. Neurotransmitters: glutamic acid (at spinal cord) for fast pain, substance P (at spinal cord) for slow pain and Lewis P factor for muscle pain, Pathway: lateral spinothalamic (specific neo spinothalamic for fast pain and diffuse and non specific paleo spinothalamic for slow pain) Reaction : pain is a
Thymus is a specialized primary lymphoid organ of the immune system. It is situated behind the sternum in upper and anterior part of chest.
It consists of a capsule, outer cortex and inner medulla.
T cell maturation
Functions of Thymus
The thymus facilitates the maturation of T cells which are responsible for cell mediated immunity. T cells begin as hematopoietic precursors from the bone-marrow, and then migrate to the thymus for maturation, where they are referred to as thymocytes.
In the thymus they undergo a process of maturation, which involves ensuring the cells react against antigens (positive selection), but that they do not react against antigens found on body tissue (negative selection). Once mature, T cells emigrate from the thymus to lymphnodes to provide vital functions in the immune system
Positive selection
This process occurs in the cortex of Thymus. The survival and nature of the T cell in thymus depends on its interaction with surrounding thymic epithelial cells. Here, the T cell receptor interacts with the MHC molecules on the surface of epithelial cells.
A T cell with that doesn't react, or reacts weakly will die by apoptosis. A T cell that does react will survive and proliferate,
Negative selection
This process occurs in the medulla of Thymus. T cells that attack the body's self antigens are eliminated in the thymus by the process known as 'negative selection'.
Epithelial cells in the medulla and dendritic cells in the thymus express major proteins from elsewhere in the body. Thymocytes that react strongly to self antigens do not survive, and die by apoptosis
Clinical
Immunodeficiency
As the thymus is where T cells develop congenital problems with the development of the thymus can lead to immunodeficiency, whether because of a problem with the development of the thymus gland, or a problem specific to thymocyte development,
Immunodeficiency can be profound.
Autoimmune diseases
Thymus removes T cells that are reactive to self antigens and that's why potentially dangerous for body's own cells by Negative selection. Any abnormality in this mechanism may lead to release of mature T cell which are reactive to self antigens and that's why they cause Autoimmune diseases e.g. Myasthenia Gravis.
Thymic hyperplasia or Thymoma could be commonly associated with autoimmune diseases. Surgical removal of Thymus (Thymectomy) may prove useful in such conditions.
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