In human anatomy, the head is at the top of the human body. It is maintained by the skull, which itself encloses the brain; the human head consists of a fleshy outer portion. The brain is enclosed within the skull; the head rests on the neck, the seven cervical vertebrae support it. The human head weighs between 5 and 11 pounds The face is the anterior part of the head, containing the eyes and mouth. On either side of the mouth, the cheeks provide a fleshy border to the oral cavity; the ears sit to either side of the head. The head receives blood supply through the external carotid arteries; these supply the area outside of the inside of the skull. The area inside the skull receives blood supply from the vertebral arteries, which travel up through the cervical vertebrae; the twelve pairs of cranial nerves provide the majority of nervous control to the head. The sensation to the face is provided by the branches of the trigeminal nerve, the fifth cranial nerve. Sensation to other portions of the head is provided by the cervical nerves.
Modern texts are in agreement about which areas of the skin are served by which nerves, but there are minor variations in some of the details. The borders designated by diagrams in the 1918 edition of Gray's Anatomy are similar but not identical to those accepted today; the cutaneous innervation of the head is as follows: Ophthalmic nerve Maxillary nerve Mandibular nerve Cervical plexus Dorsal rami of cervical nerves and others are in picture which show following in upper column The head contains sensory organs: two eyes, two ears, a nose and tongue inside of the mouth. It houses the brain. Together, these organs function as a processing center for the body by relaying sensory information to the brain. Humans can process information faster by having this central nerve cluster. For humans, the front of the head is the main distinguishing feature between different people due to its discernible features, such as eye and hair colors, shapes of the sensory organs, the wrinkles. Humans differentiate between faces because of the brain's predisposition toward facial recognition.
When observing a unfamiliar species, all faces seem nearly identical. Human infants are biologically programmed to recognize subtle differences in anthropomorphic facial features. People who have greater than average intelligence are sometimes depicted in cartoons as having bigger heads as a way of notionally indicating that they have a "larger brain". Additionally, in science fiction, an extraterrestrial having a big head is symbolic of high intelligence. Despite this depiction, advances in neurobiology have shown that the functional diversity of the brain means that a difference in overall brain size is only to moderately correlated to differences in overall intelligence between two humans; the head is a source for many metaphors and metonymies in human language, including referring to things near the human head, things physically similar to the way a head is arranged spatially to a body and things that represent some characteristics associated with the head, such as intelligence. Ancient Greeks had a method for evaluating sexual attractiveness based on the Golden ratio, part of which included measurements of the head.
Headpieces can signify status, religious/spiritual beliefs, social grouping, team affiliation, occupation, or fashion choices. In many cultures, covering the head is seen as a sign of respect; some or all of the head must be covered and veiled when entering holy places or places of prayer. For many centuries, women in Europe, the Middle East, South Asia have covered their hair as a sign of modesty; this trend has changed drastically in Europe in the 20th century, although is still observed in other parts of the world. In addition, a number of religions require men to wear specific head clothing—such as the Islamic Taqiyah, Jewish yarmulke, or the Sikh turban; the same goes for Christian nun's habit. A hat is a head covering. Hats may be worn as part of a uniform or used as a protective device, such as a hard hat, a covering for warmth, or a fashion accessory. Hats can be indicative of social status in some areas of the world. While numerous charts detailing head sizes in infants and children exist, most do not measure average head circumference past the age of 21.
Reference charts for adult head circumference generally feature homogeneous samples and fail to take height and weight into account. One study in the United States estimated the average human head circumference to be 55 centimetres in females and 57 centimetres in males. A British study by Newcastle University showed an average size of 55.2 cm for females and 57.2 cm for males with average size varying proportionally with height Macrocephaly can be an indicator of increased risk for some types of cancer in individuals who carry the genetic mutation that causes Cowden syndrome. For adults, this refers to head sizes greater than 58 centimeters in men or greater than 57 centimeters in women. Human body Head and neck anatomy 8. Human head Campbell, Bernard Grant. Human Evolution: An Introduction to Man's Adaptations, 4th edition
Immunoglobulin E is a type of antibody that has only been found in mammals. IgE is synthesised by plasma cells. Monomers of IgE consist of two heavy chains and two light chains, with the ε chain containing 4 Ig-like constant domains. IgE's main function is immunity to parasites such as helminths like Schistosoma mansoni, Trichinella spiralis, Fasciola hepatica. IgE is utilized during immune defense against certain protozoan parasites such as Plasmodium falciparum. IgE has an essential role in type I hypersensitivity, which manifests in various allergic diseases, such as allergic asthma, most types of sinusitis, allergic rhinitis, food allergies, specific types of chronic urticaria and atopic dermatitis. IgE plays a pivotal role in responses to allergens, such as: anaphylactic drugs, bee stings, antigen preparations used in desensitization immunotherapy. Although IgE is the least abundant isotype—blood serum IgE levels in a normal individual are only 0.05% of the Ig concentration, compared to 75% for the IgGs at 10 mg/ml, which are the isotypes responsible for most of the classical adaptive immune response—it is capable of triggering the most powerful inflammatory reactions.
IgE was discovered in 1966 and 1967 by two independent groups: Kimishige Ishizaka and his wife Teruko Ishizaka at the Children's Asthma Research Institute and Hospital in Denver, by S. G. O Johansson and Hans Bennich in Sweden, their joint paper was published in April 1969. IgE primes the IgE-mediated allergic response by binding to Fc receptors found on the surface of mast cells and basophils. Fc receptors are found on eosinophils, monocytes and platelets in humans. There are two types of Fcε receptors: FcεRI, the high-affinity IgE receptor FcεRII known as CD23, the low-affinity IgE receptorIgE can upregulate the expression of both types of Fcε receptors. FcεRI is expressed on mast cells and the antigen-presenting dendritic cells in both mice and humans. Binding of antigens to IgE bound by the FcεRI on mast cells causes cross-linking of the bound IgE and the aggregation of the underlying FcεRI, leading to degranulation and the secretion of several types of type 2 cytokines like IL-3 and Stem Cell Factor which both help the mast cells survive and accumulate in tissue, IL-4, IL-5 and IL-13 as well as IL-33 which in turn activate group 2-innate lymphoid cells.
Basophils, which share a common haemopoietic progenitor with mast cells, upon the cross-linking of their surface bound IgE by antigens release type 2 cytokines like interleukin-4 and interleukin-13 and other inflammatory mediators. The low-affinity receptor is always expressed on B cells. There has been an accumulating evidence in the past decade on the physiological role of IgE: this isotype has co-evolved with basophils and mast cells in the defence against parasites like helminths but may be effective in bacterial infections. Epidemiological research shows that IgE level is increased when infected by Schistosoma mansoni, Necator americanus, nematodes in humans, it is most beneficial in removal of hookworms from the lung. Although it is not yet well understood, IgE may play an important role in the immune system’s recognition of cancer, in which the stimulation of a strong cytotoxic response against cells displaying only small amounts of early cancer markers would be beneficial. If this were the case, anti-IgE treatments such as omalizumab might have some undesirable side effects.
However, a recent study, performed based on pooled analysis using comprehensive data from 67 phase I to IV clinical trials of omalizumab in various indications, concluded that a causal relationship between omalizumab therapy and malignancy is unlikely. Atopic individuals can have up to ten times the normal level of IgE in their blood. However, this may not be a requirement for symptoms to occur as has been seen in asthmatics with normal IgE levels in their blood—recent research has shown that IgE production can occur locally in the nasal mucosa. IgE that can recognise an allergen has a unique long-lived interaction with its high-affinity receptor FcεRI so that basophils and mast cells, capable of mediating inflammatory reactions, become "primed", ready to release chemicals like histamine and certain interleukins; these chemicals cause many of the symptoms we associate with allergy, such as airway constriction in asthma, local inflammation in eczema, increased mucus secretion in allergic rhinitis, increased vascular permeability, it is presumed, to allow other immune cells to gain access to tissues, but which can lead to a fatal drop in blood pressure as in anaphylaxis.
IgE is known to be elevated in various autoimmune disorders such as Lupus, Rheumatoid Arthritis & psoriasis, is theorized to be of pathogenetic importance in RA and SLE by eliciting a hypersensitivity reaction. Regulation of IgE levels through control of B cell differentiation to antibody-secreting plasma cells is thought to involve the "low-affinity" receptor FcεRII, or CD23. CD23 may allow facilitated antigen presentation, an IgE-dependent mechanism whereby B cells expressing CD23 are able to present allergen to specific T helper cells, causing the perpetuation of a Th2 response, one of the hallmarks o
Modes of mechanical ventilation
Modes of mechanical ventilation are one of the most important aspects of the usage of mechanical ventilation. The mode refers to the method of inspiratory support. In general, mode selection is based on clinician familiarity and institutional preferences, since there is a paucity of evidence indicating that the mode affects clinical outcome; the most used forms of volume-limited mechanical ventilation are intermittent mandatory ventilation and continuous mandatory ventilation. There have been substantial changes in the nomenclature of mechanical ventilation over the years, but more it has become standardized by many respirology and pulmonology groups. Writing a mode is most proper in all capital letters with a dash between the control variable and the strategy The taxonomy is a logical classification system based on 10 maxims of ventilator design A breath is one cycle of positive flow and negative flow defined in terms of the flow-time curve. Inspiratory time is defined as the period from the start of positive flow to the start of negative flow.
Expiratory time is defined as the period from the start of expiratory flow to the start of inspiratory flow. The flow-time curve is the basis for many variables related to ventilator settings. A breath is assisted. An assisted breath is one. For constant flow inflation, work is defined as inspiratory pressure multiplied by tidal volume. Therefore, an assisted breath is identified as a breath for which airway pressure rises above baseline during inspiration. An unassisted breath is one for which the ventilator provides the inspiratory flow demanded by the patient and pressure stays constant throughout the breath. A ventilator assists breathing using either pressure control or volume control based on the equation of motion for the respiratory system. Providing assistance means doing work on the patient, accomplished by controlling either pressure or volume. A simple mathematical model describing this fact is known as the equation of motion for the passive respiratory system: Pressure = + In this equation, pressure and flow are all continuous functions of time.
Pressure is a pressure difference across the system. Elastance and resistance are parameters assumed to remain constant during a breath. Volume control means that both flow are preset prior to inspiration. In other words, the right hand side of the equation of motion remains constant while pressure changes with changes in elastance and resistance. Pressure control means that inspiratory pressure is preset as either a constant value or it is proportional to the patient’s inspiratory effort. In other words, the left-hand side of the equation of motion remains constant while volume and flow change with changes in elastance and resistance. Time control means. In this case only the inspiratory and expiratory times are preset. Breaths are classified by the criteria that cycle inspiration; the start of inspiration is called the trigger event. The end of inspiration is called the cycle event. Trigger and cycle events can be initiated by the machine. Inspiration can be patient cycled by a signal representing inspiratory effort.
Inspiration may be machine triggered or machine cycled by preset ventilator thresholds. Patient triggering means starting inspiration based on a patient signal independent of a machine trigger signal. Machine triggering means starting inspiratory flow based on a signal from the ventilator, independent of a patient trigger signal. Patient cycling means ending inspiratory time based on signals representing the patient determined components of the equation of motion. Flow cycling is a form of patient cycling because the rate of flow decay to the cycle threshold is determined by patient mechanics. Machine cycling means ending inspiratory time independent of signals representing the patient determined components of the equation of motion. Breaths are classified as mandatory based on both the trigger and cycle events. A spontaneous breath is cycles the breath. A spontaneous breath may occur during a mandatory breath. A spontaneous breath may be unassisted. A mandatory breath is a breath for which the machine cycles the breath.
A mandatory breath can occur during a spontaneous breath. A mandatory breath is, by definition, assisted. There are 3 breath sequences: Continuous mandatory ventilation, Intermittent Mandatory Ventilation, Continuous Spontaneous Ventilation. A breath sequence is a particular pattern of spontaneous and/or mandatory breaths; the 3 possible breath sequences are: continuous mandatory ventilation, intermittent mandatory ventilation, continuous spontaneous ventilation. There are 5 basic ventilatory patterns: VC-CMV, VC-IMV, PC-CMV, PC-IMV, PC-CSV. Th
Propofol, marketed as Diprivan among other names, is a short-acting medication that results in a decreased level of consciousness and lack of memory for events. Its uses include the starting and maintenance of general anesthesia, sedation for mechanically ventilated adults, procedural sedation, it is used for status epilepticus if other medications have not worked. It is given by injection into a vein. Maximum effect takes about two minutes to occur and it lasts five to ten minutes. Common side effects include an irregular heart rate, low blood pressure, burning sensation at the site of injection, the stopping of breathing. Other serious side effects may include seizures, infections with improper use and propofol infusion syndrome with long-term use, it has not been well studied in this group. However, it is not recommended during cesarean section. Propofol is not a pain medication, so opioids such as morphine may be used. Whether or not they are always needed is unclear. Propofol is believed to work at least via a receptor for GABA.
Propofol was discovered in 1977 and approved for use in the United States in 1989. It is on the World Health Organization's List of Essential Medicines, the most effective and safe medicines needed in a health system, it is available as a generic medication. The wholesale price in the developing world is between 8.50 USD per vial. It has been referred to as milk of amnesia because of the milk-like appearance of the intravenous preparation. Propofol is used in veterinary medicine for anesthesia. Propofol is used for induction and maintenance of general anesthesia, having replaced sodium thiopental, it can be administered as part of an anaesthesia maintenance technique called total intravenous anesthesia using either manually-programmed infusion pumps or computer-controlled infusion pumps in a process called target controlled infusion or TCI. Propofol is used to sedate individuals who are receiving mechanical ventilation but are not undergoing surgery, such as patients in the intensive care unit.
In critically ill patients, propofol has been found to be superior to lorazepam both in effectiveness and overall cost. Propofol may be more expensive in the short term but decreases ICU stay length, thus overall lowering costs. One of the reasons Propofol is thought to be more effective is because studies have found that benzodiazepines like midazolam and lorazepam tend to accumulate in critically ill patients, prolonging sedation. Propofol is used instead of sodium thiopental for starting anesthesia because recovery from propofol is more rapid and "clear." Propofol is commonly used in the ICU as a sedation medication for intubated people. It can be run through central line. Propofol is paired with fentanyl in intubated and sedated people. Both are compatible in IV form. Propofol is used for procedural sedation, its use in these settings results in a faster recovery compared to midazolam. It can be combined with opioids or benzodiazepines; because of its fast induction and recovery time, propofol is widely used for sedation of infants and children undergoing MRI.
It is often used in combination with ketamine as the two together have lower rates of side effects. The Missouri Supreme Court decided to allow the use of propofol to execute prisoners condemned to death. However, the first execution by administration of a lethal dose of propofol was halted on 11 October 2013 by governor Jay Nixon following threats from the European Union to limit the drug's export if it were used for that purpose; the United Kingdom had banned the export of medicines or veterinary medicines containing propofol to the United States. Recreational use of the drug via self-administration has been reported, but is rare due to its potency and the level of monitoring required for safe use. Critically, a steep dose-response curve makes recreational use of propofol dangerous, deaths from self-administration continue to be reported; the short-term effects sought via recreational use include mild euphoria and disinhibition. Recreational use of the drug has been described among medical staff, such as anesthetists who have access to the drug, is more common among anesthetists on rotations with short rest periods.
Long-term use has been reported to result in addiction. Attention to the risks of off-label use of propofol increased in August 2009 due to the Los Angeles County coroner's conclusion that music icon Michael Jackson died from a mixture of propofol and the benzodiazepine drugs lorazepam and diazepam on June 25, 2009. According to a July 22, 2009 search warrant affidavit unsealed by the district court of Harris County, Jackson's personal physician, Conrad Murray, administered 25 milligrams of propofol diluted with lidocaine shortly before Jackson's death. So, as of 2016 propofol was not on a U. S Drug Enforcement Administration schedule. One of propofol's most frequent side effects is pain on injection in smaller veins; this pain arises from activation of the pain receptor, TRPA1, found on sensory nerves and can be mitigated by pretreatment with lidocaine. Less pain is experienced. Patients show great variability in their response to propofol, at times showing profound sedation with small doses.
Additional side effects include low blood pressure related to vasodilation, transient apnea following induction doses, cerebrovascular effects. Propofol has more pronounced hemodynamic effects relativ
Eosinophils, sometimes called eosinophiles or, less acidophils, are a variety of white blood cells and one of the immune system components responsible for combating multicellular parasites and certain infections in vertebrates. Along with mast cells and basophils, they control mechanisms associated with allergy and asthma, they are granulocytes that develop during hematopoiesis in the bone marrow before migrating into blood, after which they are terminally differentiated and do not multiply. These cells are eosinophilic or "acid-loving" due to their large acidophilic cytoplasmic granules, which show their affinity for acids by their affinity to coal tar dyes: Normally transparent, it is this affinity that causes them to appear brick-red after staining with eosin, a red dye, using the Romanowsky method; the staining is concentrated in small granules within the cellular cytoplasm, which contain many chemical mediators, such as eosinophil peroxidase, deoxyribonucleases, lipase and major basic protein.
These mediators are released by a process called degranulation following activation of the eosinophil, are toxic to both parasite and host tissues. In normal individuals, eosinophils make up about 1–3% of white blood cells, are about 12–17 micrometres in size with bilobed nuclei. While they are released into the bloodstream as neutrophils are, eosinophils reside in tissue, they are found in the medulla and the junction between the cortex and medulla of the thymus, and, in the lower gastrointestinal tract, uterus and lymph nodes, but not in the lungs, esophagus, or some other internal organs under normal conditions. The presence of eosinophils in these latter organs is associated with disease. For instance, patients with eosinophilic asthma have high levels of eosinophils that lead to inflammation and tissue damage, making it more difficult for patients to breathe. Eosinophils persist in the circulation for 8–12 hours, can survive in tissue for an additional 8–12 days in the absence of stimulation.
Pioneering work in the 1980s elucidated that eosinophils were unique granulocytes, having the capacity to survive for extended periods of time after their maturation as demonstrated by ex-vivo culture experiments. TH2 and ILC2 cells both express the transcription factor GATA-3 which promotes the production of TH2 cytokines, including the interleukins. IL-5 controls the development of eosinophils in the bone marrow, as they differentiate from myeloid precursor cells, their lineage fate is determined by transcription factors, including GATA and C/EBP. Eosinophils produce and store many secondary granule proteins prior to their exit from the bone marrow. After maturation, eosinophils circulate in blood and migrate to inflammatory sites in tissues, or to sites of helminth infection in response to chemokines like CCL11, CCL24, CCL5, 5-hydroxyicosatetraenoic acid and 5-oxo-eicosatetraenoic acid, certain leukotrienes like leukotriene B4 and MCP1/4. Interleukin-13, another TH2 cytokine, primes eosinophilic exit from the bone marrow by lining vessel walls with adhesion molecules such as VCAM-1 and ICAM-1.
When eosinophils are activated, they undergo cytolysis, where the breaking of the cell releases eosinophilic granules found in extracellular DNA traps. High concentrations of these DNA traps are known to cause cellular damage, as the granules they contain are responsible for the ligand-induced secretion of eosinophilic toxins which cause structural damage. There is evidence to suggest that eosinophil granule protein expression is regulated by the non-coding RNA EGOT. Following activation, eosinophils effector functions include production of the following: Cationic granule proteins and their release by degranulation Reactive oxygen species such as hypobromite and peroxide Lipid mediators like the eicosanoids from the leukotriene and prostaglandin families Enzymes, such as elastase Growth factors such as TGF beta, VEGF, PDGF Cytokines such as IL-1, IL-2, IL-4, IL-5, IL-6, IL-8, IL-13, TNF alphaThere are eosinophils that play a role in fighting viral infections, evident from the abundance of RNases they contain within their granules, in fibrin removal during inflammation.
Eosinophils, along with basophils and mast cells, are important mediators of allergic responses and asthma pathogenesis and are associated with disease severity. They fight helminth colonization and may be elevated in the presence of certain parasites. Eosinophils are involved in many other biological processes, including postpubertal mammary gland development, oestrus cycling, allograft rejection and neoplasia, they have been implicated in antigen presentation to T cells. Eosinophils are responsible for tissue damage and inflammation including asthma. High levels of interleukin-5 has been observed to up regulate the expression of adhesion molecules, which facilitate the adhesion of eosinophils to endothelial cells, thereby causing inflammation and tissue damage. An accumulation of eosinophils in the nasal mucosa is considered a major diagnostic criterion for allergic rhinitis. Following activation by an immune stimulus, eosinophils degranulate to release an array of cytotoxic granule cationic proteins that are capable of inducing tissue damage and dysfunction.
These include: major basic protein eosinophil cationic protein eosinophil peroxidase eosinophil-derived neurotoxin Major basic protein, eosinophil peroxidase, eosinophil cationic protein are toxic to many tissues. Eosinophil cationic protein and eosinophil-derived neurotoxin are ribonucleases w
A mast cell is a resident cell of connective tissue that contains many granules rich in histamine and heparin. It is a type of granulocyte derived from the myeloid stem cell, a part of the immune and neuroimmune systems. Although best known for their role in allergy and anaphylaxis, mast cells play an important protective role as well, being intimately involved in wound healing, immune tolerance, defense against pathogens, blood–brain barrier function; the mast cell is similar in both appearance and function to the basophil, another type of white blood cell. Although mast cells were once thought to be tissue resident basophils, it has been shown that the two cells develop from different hematopoietic lineages and thus cannot be the same cells. Mast cells are similar to basophil granulocytes in blood. Both are granulated cells that contain an anticoagulant, their nuclei differ in. The Fc region of immunoglobulin E becomes bound to mast cells and basophils and when IgE's paratopes bind to an antigen, it causes the cells to release histamine and other inflammatory mediators.
These similarities have led many to speculate that mast cells are basophils that have "homed in" on tissues. Furthermore, they share a common precursor in bone marrow expressing the CD34 molecule. Basophils leave the bone marrow mature, whereas the mast cell circulates in an immature form, only maturing once in a tissue site; the site an immature mast cell settles in determines its precise characteristics. The first in vitro differentiation and growth of a pure population of mouse mast cells has been carried out using conditioned medium derived from concanavalin A-stimulated splenocytes, it was discovered that T cell-derived interleukin 3 was the component present in the conditioned media, required for mast cell differentiation and growth. Mast cells in rodents are classically divided into two subtypes: connective tissue-type mast cells and mucosal mast cells; the activities of the latter are dependent on T-cells. Mast cells are present in most tissues characteristically surrounding blood vessels and nerves, are prominent near the boundaries between the outside world and the internal milieu, such as the skin, mucosa of the lungs, digestive tract, as well as the mouth and nose.
Mast cells play a key role in the inflammatory process. When activated, a mast cell can either selectively release or release "mediators", or compounds that induce inflammation, from storage granules into the local microenvironment. Mast cells can be stimulated to degranulate by allergens through cross-linking with immunoglobulin E receptors, physical injury through pattern recognition receptors for damage-associated molecular patterns, microbial pathogens through pattern recognition receptors for pathogen-associated molecular patterns, various compounds through their associated G-protein coupled receptors or ligand-gated ion channels. Complement proteins can activate membrane receptors on mast cells to exert various functions as well. Mast cells express a high-affinity receptor for the Fc region of IgE, the least-abundant member of the antibodies; this receptor is of such high affinity. As a result, mast cells are coated with IgE, produced by plasma cells. IgE antibodies, are specific to one particular antigen.
In allergic reactions, mast cells remain inactive until an allergen binds to IgE coated upon the cell. Other membrane activation events can either prime mast cells for subsequent degranulation or act in synergy with FcεRI signal transduction. In general, allergens are polysaccharides; the allergen binds to the antigen-binding sites, which are situated on the variable regions of the IgE molecules bound to the mast cell surface. It appears; the clustering of the intracellular domains of the cell-bound Fc receptors, which are associated with the cross-linked IgE molecules, causes a complex sequence of reactions inside the mast cell that lead to its activation. Although this reaction is most well understood in terms of allergy, it appears to have evolved as a defense system against parasites and bacteria. A unique, stimulus-specific set of mast cell mediators is released through degranulation following the activation of cell surface receptors on mast cells. Examples of mediators that are released into the extracellular environment during mast cell degranulation include: serine proteases, such as tryptase and chymase histamine serotonin proteoglycans heparin and some chondroitin sulfate proteoglycans adenosine triphosphate lysosomal enzymes β-hexosaminidase β-glucuronidase arylsulfatases newly formed lipid mediators: thromboxane prostaglandin D2 leukotriene C4 platelet-activating factor cytokines TNF-α basic fibroblast growth factor interleukin-4 stem cell factor chemokines, such as eosinophil chemotactic factor reactive oxygen species Histamine dilates post-capillary venules, activates the endothelium, increases blood vessel permeability.
This leads to local edema, warmth and the attraction of other inflammatory cells to the site of release. It depolarizes nerve endings. Cutaneous signs of histamine release are the "flare and wheal"-
Smooth muscle is an involuntary non-striated muscle. It is divided into two subgroups. Within single-unit cells, the whole bundle or sheet contracts as a syncytium. Smooth muscle cells are found in the walls of hollow organs, including the stomach, urinary bladder and uterus, in the walls of passageways, such as the arteries and veins of the circulatory system, the tracts of the respiratory and reproductive systems; these cells are present in the eyes and are able to change the size of the iris and alter the shape of the lens. In the skin, smooth muscle cells cause hair to stand erect in response to cold fear. Most smooth muscle is of the single-unit variety, that is, either the whole muscle contracts or the whole muscle relaxes, but there is multiunit smooth muscle in the trachea, the large elastic arteries, the iris of the eye. Single unit smooth muscle, however, is most common and lines blood vessels, the urinary tract, the digestive tract. However, the terms single- and multi-unit smooth muscle represents an oversimplification.
This is due to the fact that smooth muscles for the most part are controlled and influenced by a combination of different neural elements. In addition, it has been observed that most of the time there will be some cell to cell communication and activators/ inhibitors produced locally; this leads to a somewhat coordinated response in multiunit smooth muscle. Smooth muscle is fundamentally different from skeletal muscle and cardiac muscle in terms of structure, regulation of contraction, excitation-contraction coupling. Smooth muscle cells known as myocytes, have a fusiform shape and, like striated muscle, can tense and relax. However, smooth muscle tissue tends to demonstrate greater elasticity and function within a larger length-tension curve than striated muscle; this ability to stretch and still maintain contractility is important in organs like the intestines and urinary bladder. In the relaxed state, each cell is 20 -- 500 micrometers in length. A substantial portion of the volume of the cytoplasm of smooth muscle cells are taken up by the molecules myosin and actin, which together have the capability to contract, through a chain of tensile structures, make the entire smooth muscle tissue contract with them.
Myosin is class II in smooth muscle. Myosin II contains two heavy chains which constitute the tail domains; each of these heavy chains contains the N-terminal head domain, while the C-terminal tails take on a coiled-coil morphology, holding the two heavy chains together. Thus, myosin II has two heads. In smooth muscle, there is a single gene that codes for the heavy chains myosin II, but there are splice variants of this gene that result in four distinct isoforms. Smooth muscle may contain MHC, not involved in contraction, that can arise from multiple genes. Myosin II contains 4 light chains, resulting in 2 per head, weighing 20 and 17 kDa; these bind the heavy chains in the "neck" region between the head and tail. The MLC20 is known as the regulatory light chain and participates in muscle contraction. Two MLC20 isoforms are found in smooth muscle, they are encoded by different genes, but only one isoform participates in contraction; the MLC17 is known as the essential light chain. Its exact function is unclear, but it's believed that it contributes to the structural stability of the myosin head along with MLC20.
Two variants of MLC17 exist as a result of alternative splicing at the MLC17 gene. Different combinations of heavy and light chains allow for up to hundreds of different types of myosin structures, but it is unlikely that more than a few such combinations are used or permitted within a specific smooth muscle bed. In the uterus, a shift in myosin expression has been hypothesized to avail for changes in the directions of uterine contractions that are seen during the menstrual cycle; the thin filaments that form part of the contractile machinery are predominantly composed of α- and γ-actin. Smooth muscle α-actin is the predominant isoform within smooth muscle. There are lots of actin that does not take part in contraction, but that polymerizes just below the plasma membrane in the presence of a contractile stimulant and may thereby assist in mechanical tension. Alpha actin is expressed as distinct genetic isoforms such as smooth muscle, cardiac muscle and skeletal muscle specific isoforms of alpha actin.
The ratio of actin to myosin is between 10:1 in smooth muscle. Conversely, from a mass ratio standpoint, myosin is the dominant protein in striated skeletal muscle with the actin to myosin ratio falling in the 1:2 to 1:3 range. A typical value for healthy young adults is 1:2.2.. Tropomyosin is present in smooth muscle, spanning seven actin monomers and is laid out end to end over the entire length of the thin filaments. In striated muscle, tropomyosin serves to block actin–myosin interactions until calcium is present, but in smooth muscle, its function is unknown. Calponin molecules may exist in equal number as actin, has been proposed to be a load-bearing protein. Caldesmon has been suggested to be involved in tethering actin and tropomyosin, thereby enhance the ability of smooth muscle to maintain tension. All three of these proteins may have a role in inhibiting the ATPase activity of the m