The School of Biomedical Sciences Wiki - User contributions [en]

Synaptonemal Complex

2014-10-21T15:19:32Z

130176464:

The synaptonemal complex (SC) is an evoluntionarily-conserved protein assembly which forms between [[Homologous chromosomes|homolgous chromosomes]]. The synaptonemal complex begins to form during the [[Meiosis prophase 1|zygotene phase]] of [[Meiosis prophase 1|Prophase I]] in the first division in [[Meiosis|Meiosis]] and is complete in the [[Meiosis prophase 1|pachytene phase]]. Acting like a 'zipper' it holds the homologous chromosomes together, aligning them perfectly. After complete synapsis, crossing over occurs and in the [[Meiosis prophase 1|diplotene phase]], where the [[Meiosis prophase 1|chiasma]] is visible, the synaptonemal complex 'unzips' and disappears <ref>Hartl DL and Jones EW (2009) Genetics: Analysis of Genes and Genomes, Seventh Edition, USA, Jones and Bartlett Publishers</ref>.

The SC was discovered in 1956 and studies by electron microscopy have shown that in all sexually reproducing organisms in which it is found the complex adopts the same tripartite structure. It is made up of two lateral elements of approximately 50nm each that coat the chromosome axes, and in almost all organisms a central element of 20-40nm wdie. The elements continue along the entire chromosome axis<ref>Syrjänen JL, Pellegrini L, Davies OR. A molecular model for the role of SYCP3 in meiotic chromosome organisation. eLife 2014, 3, e02963</ref>

=== References ===

<references /> <references />

Polymerase Chain Reaction (PCR)

2014-10-21T15:05:35Z

130176464:

Polymerase Chain Reaction<ref>Hartl D. L., Ruvolo M. (2012), Genetics: Analysis of genes and genomes, Eight Edition, Jones and Bartlett learning (Chapter 2 DNA Structure and Genetic Variation)</ref> (PCR) is a technique used for the [[Amplification|amplification]] and identification of [[DNA|DNA]] or [[RNA|RNA]] of known sequence to give exponential products or copies. Also see [[MRNA|mRNA]] (including [[Transcriptase|transcriptase]]). It allows scientists to produce many millions of copies of a certain DNA sequence in a couple of hours. This technique was developed by an American biochemist [[Kary Mullis|Kary Mullis]] in 1984 <ref>Berg, J.M., Tymoczko, J.L. and Stryer, L. (2012). Biochemistry, 7 th Edition, New York , W.H.Freeman and Co Ltd. pg 151</ref> for which he was awarded the [http://www.nobelprize.org/nobel_prizes/chemistry/laureates/1993/ Nobel Prize in Chemistry in 1993] . 

[[PCR|PCR has]] three main stages:

#'''Strand Seperation''' : Heat ds[[DNA|DNA]] to 95°C  for 15s to melt and seperate the strands, denaturing the hydrogen bonds which hold them together.
#'''Hybridisation of Primer''' : Cool to 50 - 65°C to allow [[Primers|primers]] to anneal to the DNA strands. The temperature required at this stage is primer dependant.
#'''DNA Synthesis''' : Heat to 72°C to allow [[Elongation|elongation]]. This extention is usually performed by free nucleotides with taq polymerase. 

Typically these steps are repeated in a cycle about 20 to 30 times generating a large amount of identical DNA copies. Therefore, PCR is often used just before doing an [[Electrophoresis|electrophoresis]].

The most important part of the PCR reaction is the initial design of the [[Primers|primers]]. The [[Primers|primers]] are normally between 18 to 20 [[Base pairs|base pairs]] in length and must be completely complimentary to the ends of the [[DNA|DNA]] region of interest. The primers are always found at the 5' ends of the final DNA fragments<ref>Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P (2008) Molecular biology of the cell, 5th edition, New York: Garland Science. pg 544</ref>. 18 to 20 base pairs for a primer are ideal because a 18-2 base sequence is quite unique and is therefore unlikely to be present in any other section other than the target DNA. This ensures that the primers bind only to the flanking sequences associated with the target DNA sequence. For shorter genomes a smaller primer can be used. Along with this included in the PCR reaction must be both the [[Forward primers|forward]] and [[Reverse primer|reverse]] [[Primer|primers]], in addition to [[Taq polymerase|Taq Polymerase]] (which requires MgCl2  for its effective activity) and the DNA template. Alongside these substances the following must also be added; [[Magnesium Chloride|Magnesium Chloride]], free [[Nucleotides|nucleotides]] ([[DATP|dATP]], [[DCTP|dCTP]], [[DTTP|dTTP]] and [[DGTP|dGTP]]) and a [[Tris-HCl|Tris-HCl]] (pH 8.0) [[Buffer|buffer]]. 

PCR is carried out in a [[Thermal cycler|thermal cycler]], (a machine that is capable of varying temperature) when this is unavailable water baths can be used instead, and the [[Enzyme|enzyme]] '[[Taq Polymerase|Taq Polymerase]]' (isolated from ''[[Thermus aquaticus|Thermus aquaticus]]'') is used as it is [[Thermostable|thermostable]], which forms the core of PCR.Originally, [[DNA Polymerase|DNA polymerase was]] added to the PCR reaction but it was denatured by the high temperatures, so had to be added at the end of every cycle. However, because [[Taq polymerase|Taq polymerase]] is thermostable, it isn't denatured so only needs to be added at the beginning of the reaction. [[Pfu|Pfu]] (''[[Pyrococcus furiosus|Pyrococcus furiosus]]'') [[DNA Polymerase|DNA Polymerase]] can also be used as it has better thermostability than [[Taq polymerase|Taq polymerase]] and it possesses 3' to 5' [[Proof reading|proof reading]] activity. [[Pfu|Pfu is]] from an organism of the archaea that lives in submarine vents, the enzmye survives 100°C.

PCR technique has many importance amongst which is it's use in identification of the orientation of cloned inserts, also, it is used in other fields such as forensic science- at crime scenes, science in general- to diagnosize diseases. 

PCR is a very useful tool for scientists for several reasons. It can provide valuable diagnostic information in medicine as bacteria and viruses can easily be detected by the use of specific primers. For example, PCR can reveal the presence of a small amount of DNA from the human immunodeficiency virus ([[HIV/AIDS|HIV]]) in persons who have not yet mounted an immune response to the pathogen. In these patients, assays designed to detect antibodies against the virus would yield a false negative test result. This is useful in the early diagnosis of HIV as well as helping to reduce the likelihood of further transmission.

PCR shows high sensitivity and specificity to even small pieces of DNA, the results of PCR can be made available in a short amount of time (a few hours usually) and it is a relatively cheap process at £1-2 per reaction <ref>Berg, J.M., Tymoczko, J.L. and Stryer, L. (2012). Biochemistry, 7 th Edition, New York , W.H.Freeman and Co Ltd. pg 152</ref>.

Polymerase Chain Reaction first step is known as 'Strand Seperation. This is where you take the sequence of DNA you want to copy and heat it to 95oC. This causes the two strands of DNA to seperate as the [[Hydrogen bond|Hyrdrogen Bonds]] between the bases break. The strands are heated for roughly 15 seconds. The next stage is reffered to as 'Hybridization of Primers' this is where primers are added to the DNA as it is cooled to 54oC to allow the hydrogen bonds to form between the [[Primers|primers]] and the DNA strands. The primers both bind the the 3' end of the template and complementary strands. The Primers are typically 20-30 nucleotides long. The third stages is called 'DNA Synthesis'. The solution containg the DNA and primers is heated to 72oC and the enzyme [[Taq Polymerase|Taq polymerase]] is added. This specific enzyme is found at bacteria that live in hot springs, therefore it is able to function at this high temperature. The Taq polymerase lengthens the DNA in the 5'-to-3' direction and takes place on both strands <ref>Berg. J. M., Stryer. L. and Tymoczko. J.L. (2007) Biochemistry, 6th edition, New York : W. H. Freeman : Palgrave</ref>. 

=== References ===

<references />

Smooth muscle

2013-11-18T16:17:18Z

130176464:

Smooth muscle (also known as visceral muscle due to the locations in which they are present <ref>Rodney R., (2002) Human Physiology, 6th Edition, Pacific Grove, California; London: Brooks/Cole</ref>) is one of the three main types of muscle tissue that exist in the human body <ref>Barrett K. E., Barman S. M., Botiano S., Brooks H. L. (2010) Ganong’s Review of Medical Physiology, 23rd edition, New York: McGraw Hill</ref> . Smooth muscle is under involuntary control and is innervated by the autonomic nervous system . It can also be stimulated without the use of nerves, this is termed Pharmomechanical Coupling. In this type of stimulation an agent (usually hormone) is used to cause contraction rather than an action potential. When certain hormones bind to Ca2+ mobilising receptors on the sarcolemma inositol 1,4,5-triphosphate (InsP3) is produced. This secondary messenger opens InsP3-gated Ca2+ channels allowing an influx of calcium ions. In turn this influx causes ryanodine receptors (RYR) to open allowing an even greater influx of calcium ions.<ref>Koeppen B. M., Stanton B. A. (2008) Berne and Levy Physiology, 6th edition, Philadelphia: Mosby Elsevier</ref> Smooth muscle lines the walls of hollow internal organs such as the bladder and intestine <ref>Silverthorn D. U., Johnson B. R., Ober W. C., Garrison C. W., Silverthorn A. C. (2010) Human Physiology, 5th edition, San Francisco: Pearson</ref>, organs of this type are known as viscera. Smooth muscle cells compose [[Myosin|myosin]] myofilaments dispersed throughout the muscle cell cytoplasm and filaments of [[Actin|actin]] held together in contracile bundles. [[Intermediate filaments|Intermediate filaments]] exist between contractile bundles connecting them, and they are anchored by dense plaque-like bodies <ref>. [Becker W.M, Kleinsmith L.J, Hardin J, Bertoni G.P, 2009, The World of the Cell, 7th edition, Pearson]</ref>. The contractile filament bundles of [[Actin|actin]] and [[Myosin|myosin]] are loosely arranged in a diagonal fashion, in different directions around the perimeter of the smooth muscle cell. This arrangement of fibres causes the muscle cell to become globular upon contraction <ref>Silverthorn.D. U (2009) Human Physiology: An Integrated Approach, 5th Edition, Cambridge, UK: Pearson</ref>. Smooth muscle cells are fusiforn in shape meaning that they are wide in the middle with tapered ends, they also have only a single nucleus. Smooth muscle cells do not contain the sarcomeres found in skeletal and cardiac muscle and therefore appear unstriated under a microscope <ref>Fundamentals of Anatomy and Physiology 5th edition by F H Martini (Chapter 10, Smooth muscle tissue)</ref>. Smooth muscle cells are unstriated because there is no regular arrangement of actin and myosin filaments.They contain only a few [[Sarcoplasmic Reticulum|sarcoplasmic reticulua]], instead using extracellular [[Calcium|calcium]] as the source of calcium ions which initiate contraction <ref>Tortora G. and Derrickson B., Principles of Anatomy and Physiology (13th Edition, International Student Edition), 2011, pg 356</ref>.

There are two types of smooth muscle cell, multi- unit and single unit smooth muscle. Single-unit smooth muscle cells are connected by gap junctions that electrically connect cells to one another, so contract as a single unit. These can be found in the intestinal tract, the skin, and the walls of small arteries, veins and hollow organs<ref>Tortora G.and Derrickson B., Principles Of Anatomy and Physiology (13th Edition, International Student Edition), 2011, pg 354</ref>. Multi-unit cells lack gap junctions, so are not linked electrically. They must be stimulated independently, which allows fine control of contractions by selective activation of individual muscle cells. Multi-unit cells can be found in the eye <ref>Silverthorn D., Johnson B., Ober W., Garrison C., Silverthorn A. (2010) Human Physiology: An Integrated Approach, 5th edition, San Francisco: Pearson Education</ref>. Smooth muscle cell contracts in different direction because there is no regular arrangement of its contractile proteins and this is important in the movement of the intestine.

In smooth muscle contraction  Ca2+ ions enter the muscle fibre and bind to [[Calmodulin|calmodulin]] (the secondary messenger in this process <ref>Walsh.MP, ( 2008): PubMed : Calmodulin and the regulation of smooth muscle contraction. Avaliable at: http://www.ncbi.nlm.nih.gov/pubmed/7816054 Accessed: 29/11/2011</ref>.  This calcium - calmodulin complex removes the caldesmon from the [[Actin|actin sites]] where [[Myosin|myosin]] will attach. An enzyme called a myosin light chain kinase (MLCK) is activated. MLCK is responsible for phosphorylating myosin filaments so that it can form cross-bridges with actin filaments. Relaxation occurs when the myosin is dephosphorylated by myosin phosphatase (MP) removing it from actin <ref>Bruce. A, Johnson. A., Lewis. J., Raff. M., Roberts. K., Walter. P., (2008): Molecular Biology of the Cell (5th edition) pg 1029 &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp; 1028</ref>. This process is relatively slow (maximum contraction is often nearly a second long) and uses very little ATP. This means that smooth muscle doesn’t fatigue during sustained periods of activity <ref>Biology Online(2005): Muscle Available at: http://www.biology-online.org/9/10_muscle.htm Accessed: 26/11/2011</ref>.

=== References ===

<references />

Smooth muscle

2013-11-18T16:16:36Z

130176464:

Smooth muscle (also known as visceral muscle due to the locations in which they are present <ref>Rodney R., (2002) Human Physiology, 6th Edition, Pacific Grove, California; London: Brooks/Cole</ref>) is one of the three main types of muscle tissue that exist in the human body <ref>Barrett K. E., Barman S. M., Botiano S., Brooks H. L. (2010) Ganong’s Review of Medical Physiology, 23rd edition, New York: McGraw Hill</ref> . Smooth muscle is under involuntary control <ref>Koeppen B. M., Stanton B. A. (2008) Berne and Levy Physiology, 6th edition, Philadelphia: Mosby Elsevier</ref>  and is innervated by the autonomic nervous system <ref>The Human Body in Health and Disease 5th edition by Thibodeau, Patton (2010), page 267</ref>. It can also be stimulated without the use of nerves, this is termed Pharmomechanical Coupling. In this type of stimulation an agent (usually hormone) is used to cause contraction rather than an action potential. When certain hormones bind to Ca2+ mobilising receptors on the sarcolemma inositol 1,4,5-triphosphate (InsP3) is produced. This secondary messenger opens InsP3-gated Ca2+ channels allowing an influx of calcium ions. In turn this influx causes ryanodine receptors (RYR) to open allowing an even greater influx of calcium ions. Smooth muscle lines the walls of hollow internal organs such as the bladder and intestine <ref>Silverthorn D. U., Johnson B. R., Ober W. C., Garrison C. W., Silverthorn A. C. (2010) Human Physiology, 5th edition, San Francisco: Pearson</ref>, organs of this type are known as viscera. Smooth muscle cells compose [[Myosin|myosin]] myofilaments dispersed throughout the muscle cell cytoplasm and filaments of [[Actin|actin]] held together in contracile bundles. [[Intermediate filaments|Intermediate filaments]] exist between contractile bundles connecting them, and they are anchored by dense plaque-like bodies <ref>. [Becker W.M, Kleinsmith L.J, Hardin J, Bertoni G.P, 2009, The World of the Cell, 7th edition, Pearson]</ref>. The contractile filament bundles of [[Actin|actin]] and [[Myosin|myosin]] are loosely arranged in a diagonal fashion, in different directions around the perimeter of the smooth muscle cell. This arrangement of fibres causes the muscle cell to become globular upon contraction <ref>Silverthorn.D. U (2009) Human Physiology: An Integrated Approach, 5th Edition, Cambridge, UK: Pearson</ref>. Smooth muscle cells are fusiforn in shape meaning that they are wide in the middle with tapered ends, they also have only a single nucleus. Smooth muscle cells do not contain the sarcomeres found in skeletal and cardiac muscle and therefore appear unstriated under a microscope <ref>Fundamentals of Anatomy and Physiology 5th edition by F H Martini (Chapter 10, Smooth muscle tissue)</ref>. Smooth muscle cells are unstriated because there is no regular arrangement of actin and myosin filaments.They contain only a few [[Sarcoplasmic Reticulum|sarcoplasmic reticulua]], instead using extracellular [[Calcium|calcium]] as the source of calcium ions which initiate contraction <ref>Tortora G. and Derrickson B., Principles of Anatomy and Physiology (13th Edition, International Student Edition), 2011, pg 356</ref>.

There are two types of smooth muscle cell, multi- unit and single unit smooth muscle. Single-unit smooth muscle cells are connected by gap junctions that electrically connect cells to one another, so contract as a single unit. These can be found in the intestinal tract, the skin, and the walls of small arteries, veins and hollow organs<ref>Tortora G.and Derrickson B., Principles Of Anatomy and Physiology (13th Edition, International Student Edition), 2011, pg 354</ref>. Multi-unit cells lack gap junctions, so are not linked electrically. They must be stimulated independently, which allows fine control of contractions by selective activation of individual muscle cells. Multi-unit cells can be found in the eye <ref>Silverthorn D., Johnson B., Ober W., Garrison C., Silverthorn A. (2010) Human Physiology: An Integrated Approach, 5th edition, San Francisco: Pearson Education</ref>. Smooth muscle cell contracts in different direction because there is no regular arrangement of its contractile proteins and this is important in the movement of the intestine.

In smooth muscle contraction  Ca2+ ions enter the muscle fibre and bind to [[Calmodulin|calmodulin]] (the secondary messenger in this process <ref>Walsh.MP, ( 2008): PubMed : Calmodulin and the regulation of smooth muscle contraction. Avaliable at: http://www.ncbi.nlm.nih.gov/pubmed/7816054 Accessed: 29/11/2011</ref>.  This calcium - calmodulin complex removes the caldesmon from the [[Actin|actin sites]] where [[Myosin|myosin]] will attach. An enzyme called a myosin light chain kinase (MLCK) is activated. MLCK is responsible for phosphorylating myosin filaments so that it can form cross-bridges with actin filaments. Relaxation occurs when the myosin is dephosphorylated by myosin phosphatase (MP) removing it from actin <ref>Bruce. A, Johnson. A., Lewis. J., Raff. M., Roberts. K., Walter. P., (2008): Molecular Biology of the Cell (5th edition) pg 1029 &amp;amp;amp;amp;amp;amp;amp;amp;amp; 1028</ref>. This process is relatively slow (maximum contraction is often nearly a second long) and uses very little ATP. This means that smooth muscle doesn’t fatigue during sustained periods of activity <ref>Biology Online(2005): Muscle Available at: http://www.biology-online.org/9/10_muscle.htm Accessed: 26/11/2011</ref>.

=== References ===

<references />

Smooth muscle

2013-11-18T16:15:59Z

130176464:

Smooth muscle (also known as visceral muscle due to the locations in which they are present <ref>Rodney R., (2002) Human Physiology, 6th Edition, Pacific Grove, California; London: Brooks/Cole</ref>) is one of the three main types of muscle tissue that exist in the human body <ref>Barrett K. E., Barman S. M., Botiano S., Brooks H. L. (2010) Ganong’s Review of Medical Physiology, 23rd edition, New York: McGraw Hill</ref> . Smooth muscle is under involuntary control <ref>Koeppen B. M., Stanton B. A. (2008) Berne and Levy Physiology, 6th edition, Philadelphia: Mosby Elsevier</ref>  and is innervated by the autonomic nervous system <ref>The Human Body in Health and Disease 5th edition by Thibodeau, Patton (2010), page 267</ref>. It can also be stimulated without the use of nerves, this is termed Pharmomechanical Coupling. In this type of stimulation an agent (usually hormone) is used to cause contraction rather than an action potential. When certain hormones bind to Ca2+ mobilising receptors on the sarcolemma inositol 1,4,5-triphosphate (InsP3) is produced. This secondary messenger opens InsP3-gated Ca2+ channels allowing an influx of calcium ions. In turn this influx causes ryanodine receptors (RYR) to open allowing an even greater influx of calcium ions.<ref>3</ref> Smooth muscle lines the walls of hollow internal organs such as the bladder and intestine <ref>Silverthorn D. U., Johnson B. R., Ober W. C., Garrison C. W., Silverthorn A. C. (2010) Human Physiology, 5th edition, San Francisco: Pearson</ref>, organs of this type are known as viscera. Smooth muscle cells compose [[Myosin|myosin]] myofilaments dispersed throughout the muscle cell cytoplasm and filaments of [[Actin|actin]] held together in contracile bundles. [[Intermediate filaments|Intermediate filaments]] exist between contractile bundles connecting them, and they are anchored by dense plaque-like bodies <ref>. [Becker W.M, Kleinsmith L.J, Hardin J, Bertoni G.P, 2009, The World of the Cell, 7th edition, Pearson]</ref>. The contractile filament bundles of [[Actin|actin]] and [[Myosin|myosin]] are loosely arranged in a diagonal fashion, in different directions around the perimeter of the smooth muscle cell. This arrangement of fibres causes the muscle cell to become globular upon contraction <ref>Silverthorn.D. U (2009) Human Physiology: An Integrated Approach, 5th Edition, Cambridge, UK: Pearson</ref>. Smooth muscle cells are fusiforn in shape meaning that they are wide in the middle with tapered ends, they also have only a single nucleus. Smooth muscle cells do not contain the sarcomeres found in skeletal and cardiac muscle and therefore appear unstriated under a microscope <ref>Fundamentals of Anatomy and Physiology 5th edition by F H Martini (Chapter 10, Smooth muscle tissue)</ref>. Smooth muscle cells are unstriated because there is no regular arrangement of actin and myosin filaments.They contain only a few [[Sarcoplasmic Reticulum|sarcoplasmic reticulua]], instead using extracellular [[Calcium|calcium]] as the source of calcium ions which initiate contraction <ref>Tortora G. and Derrickson B., Principles of Anatomy and Physiology (13th Edition, International Student Edition), 2011, pg 356</ref>.

There are two types of smooth muscle cell, multi- unit and single unit smooth muscle. Single-unit smooth muscle cells are connected by gap junctions that electrically connect cells to one another, so contract as a single unit. These can be found in the intestinal tract, the skin, and the walls of small arteries, veins and hollow organs<ref>Tortora G.and Derrickson B., Principles Of Anatomy and Physiology (13th Edition, International Student Edition), 2011, pg 354</ref>. Multi-unit cells lack gap junctions, so are not linked electrically. They must be stimulated independently, which allows fine control of contractions by selective activation of individual muscle cells. Multi-unit cells can be found in the eye <ref>Silverthorn D., Johnson B., Ober W., Garrison C., Silverthorn A. (2010) Human Physiology: An Integrated Approach, 5th edition, San Francisco: Pearson Education</ref>. Smooth muscle cell contracts in different direction because there is no regular arrangement of its contractile proteins and this is important in the movement of the intestine.

In smooth muscle contraction  Ca2+ ions enter the muscle fibre and bind to [[Calmodulin|calmodulin]] (the secondary messenger in this process <ref>Walsh.MP, ( 2008): PubMed : Calmodulin and the regulation of smooth muscle contraction. Avaliable at: http://www.ncbi.nlm.nih.gov/pubmed/7816054 Accessed: 29/11/2011</ref>.  This calcium - calmodulin complex removes the caldesmon from the [[Actin|actin sites]] where [[Myosin|myosin]] will attach. An enzyme called a myosin light chain kinase (MLCK) is activated. MLCK is responsible for phosphorylating myosin filaments so that it can form cross-bridges with actin filaments. Relaxation occurs when the myosin is dephosphorylated by myosin phosphatase (MP) removing it from actin <ref>Bruce. A, Johnson. A., Lewis. J., Raff. M., Roberts. K., Walter. P., (2008): Molecular Biology of the Cell (5th edition) pg 1029 &amp;amp;amp;amp;amp;amp;amp;amp; 1028</ref>. This process is relatively slow (maximum contraction is often nearly a second long) and uses very little ATP. This means that smooth muscle doesn’t fatigue during sustained periods of activity <ref>Biology Online(2005): Muscle Available at: http://www.biology-online.org/9/10_muscle.htm Accessed: 26/11/2011</ref>.

=== References ===

<references />

Smooth muscle

2013-11-18T16:03:16Z

130176464:

Smooth muscle (also known as visceral muscle due to the locations in which they are present <ref>Rodney R., (2002) Human Physiology, 6th Edition, Pacific Grove, California; London: Brooks/Cole</ref>) is one of the three main types of muscle tissue that exist in the human body <ref>Barrett K. E., Barman S. M., Botiano S., Brooks H. L. (2010) Ganong’s Review of Medical Physiology, 23rd edition, New York: McGraw Hill</ref> . Smooth muscle is under involuntary control <ref>Koeppen B. M., Stanton B. A. (2008) Berne and Levy Physiology, 6th edition, Philadelphia: Mosby Elsevier</ref>  and is innervated by the autonomic nervous system <ref>The Human Body in Health and Disease 5th edition by Thibodeau, Patton (2010), page 267</ref>. Smooth muscle lines the walls of hollow internal organs such as the bladder and intestine <ref>Silverthorn D. U., Johnson B. R., Ober W. C., Garrison C. W., Silverthorn A. C. (2010) Human Physiology, 5th edition, San Francisco: Pearson</ref>, organs of this type are known as viscera. Smooth muscle cells compose [[Myosin|myosin]] myofilaments dispersed throughout the muscle cell cytoplasm and filaments of [[Actin|actin]] held together in contracile bundles. [[Intermediate filaments|Intermediate filaments]] exist between contractile bundles connecting them, and they are anchored by dense plaque-like bodies <ref>. [Becker W.M, Kleinsmith L.J, Hardin J, Bertoni G.P, 2009, The World of the Cell, 7th edition, Pearson]</ref>. The contractile filament bundles of [[Actin|actin]] and [[Myosin|myosin]] are loosely arranged in a diagonal fashion, in different directions around the perimeter of the smooth muscle cell. This arrangement of fibres causes the muscle cell to become globular upon contraction <ref>Silverthorn.D. U (2009) Human Physiology: An Integrated Approach, 5th Edition, Cambridge, UK: Pearson</ref>. Smooth muscle cells are fusiforn in shape meaning that they are wide in the middle with tapered ends, they also have only a single nucleus. Smooth muscle cells do not contain the sarcomeres found in skeletal and cardiac muscle and therefore appear unstriated under a microscope <ref>Fundamentals of Anatomy and Physiology 5th edition by F H Martini (Chapter 10, Smooth muscle tissue)</ref>. Smooth muscle cells are unstriated because there is no regular arrangement of actin and myosin filaments.They contain only a few [[Sarcoplasmic Reticulum|sarcoplasmic reticulua]], instead using extracellular [[Calcium|calcium]] as the source of calcium ions which initiate contraction <ref>Tortora G. and Derrickson B., Principles of Anatomy and Physiology (13th Edition, International Student Edition), 2011, pg 356</ref>.

There are two types of smooth muscle cell, multi- unit and single unit smooth muscle. Single-unit smooth muscle cells are connected by gap junctions that electrically connect cells to one another, so contract as a single unit. These can be found in the intestinal tract, the skin, and the walls of small arteries, veins and hollow organs<ref>Tortora G.and Derrickson B., Principles Of Anatomy and Physiology (13th Edition, International Student Edition), 2011, pg 354</ref>. Multi-unit cells lack gap junctions, so are not linked electrically. They must be stimulated independently, which allows fine control of contractions by selective activation of individual muscle cells. Multi-unit cells can be found in the eye <ref>Silverthorn D., Johnson B., Ober W., Garrison C., Silverthorn A. (2010) Human Physiology: An Integrated Approach, 5th edition, San Francisco: Pearson Education</ref>. Smooth muscle cell contracts in different direction because there is no regular arrangement of its contractile proteins and this is important in the movement of the intestine.

In smooth muscle contraction  Ca2+ ions enter the muscle fibre and bind to [[Calmodulin|calmodulin]] (the secondary messenger in this process <ref>Walsh.MP, ( 2008): PubMed : Calmodulin and the regulation of smooth muscle contraction. Avaliable at: http://www.ncbi.nlm.nih.gov/pubmed/7816054 Accessed: 29/11/2011</ref>.  This calcium - calmodulin complex removes the caldesmon from the [[Actin|actin sites]] where [[Myosin|myosin]] will attach. An enzyme called a myosin light chain kinase (MLCK) is activated. MLCK is responsible for phosphorylating myosin filaments so that it can form cross-bridges with actin filaments. Relaxation occurs when the myosin is dephosphorylated by myosin phosphatase (MP) removing it from actin <ref>Bruce. A, Johnson. A., Lewis. J., Raff. M., Roberts. K., Walter. P., (2008): Molecular Biology of the Cell (5th edition) pg 1029 &amp;amp;amp;amp;amp;amp;amp; 1028</ref>. This process is relatively slow (maximum contraction is often nearly a second long) and uses very little ATP. This means that smooth muscle doesn’t fatigue during sustained periods of activity <ref>Biology Online(2005): Muscle Available at: http://www.biology-online.org/9/10_muscle.htm Accessed: 26/11/2011</ref>.

=== References ===

<references />