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Mitochondria

2014-10-23T17:46:37Z

130127226:

[[Image:Mitochondrian.PNG|right|211x187px|A cross-section of a mitochondrion under an electron microscope]]Mitochondria (singular- Mitochondrion) are membrane bound [[Organelles|organelles]], that carry out [[Oxidative phosphorylation|oxidative phosphorylation]], to produce [[ATP|ATP]]. What is more, mitochondria produce the majority of [[ATP|ATP]] used by [[Eukaryotic|eukaryotic]] [[Organism|organisms]] and are often referred to as the power houses of the cell. Furthermore, due to the fact that mitochondria are the site [[ATP synthesis|ATP synthesis]], there is often a linear relationship between the number of mitochondria in a [[Cell|cell]] and the cells [[ATP|ATP]] requirements e.g. a [[Muscle|muscle]] cell uses vast amounts of [[ATP|ATP]] and thus often contains many mitochondria to adhere to this requirement and maintain function. A further point that must be brought to attention is that mitochondria contain their own [[DNA|DNA]] (mostly circular), referred to as [[MDNA|mDNA]]. The size of mitochondrial [[DNA|DNA]] and its percentage of total cellular DNA varies between species. In mammalian cells only about 1% of the total cellular DNA is composed of mitochondrial DNA whereas in other organisms (for example in the egg cells of amphibians) there is a much higher percentage of mitochondrial DNA. Human [[MDNA|mitochondrial DNA]] consists of 16,569 [[Base|base]] pairs coding for 13 [[Proteins|prote]][[Proteins|ins]]. In humans, mitochondrial DNA is inherited from the mother because an egg cell has many more mitochondria than a sperm cell. Mitochondria are semiautonomous [[Organelles|organelles]], depending on the host cell for their existence <ref>Berg J.M, Tymoczko J.L., Stryer L (2001) Biochemistry, 5th edition, New York: WH Freeman. p492</ref><ref>Molecular Biology of THE CELL, Fifth Edition, Alberts, Johnson, Lewis, Raff, Roberts, Watter (2008), Chapter 1 Cells and Genomes, Figure 1-33 A mitiochondrion, Page 28</ref>. 

=== Structure ===

Mitochondria:

*Range from 0.5-1μm in diameter (similar to bacteria). 
*Contain an inner (folds in) and outer [[Cell membranes|membrane]].
*Contain cristae (singular crista) - internal compartments formed by the inner membrane folds.
*Contain a matrix- large overall internal compartment. 

In sexual reproduction only the female [[Gamete|gamete]] ([[Ovum|ovum]]) has mitochondria when the gametes eventually fertilise, this is because the male gamete (sperm) draws upon all of its mitohondria for locomotion, to aid its travel to the ovum (egg). Furthermore, mitochondria in relation to the structure of the [[Sperm|sperm]], is wrapped tightly around the [[Flagellum|flagellum]] in the sperm and is fixed in this position, to enable the mitochondira to comply with the sperm's unusually high [[ATP|ATP]] consumption <ref>Bruce Alberts (et al)-2007: pg815</ref>.

Mitochondrion is the site of the [[Krebs cycle|Krebs cycle]] and the [[Electron transport chain|electron transport chain]] in [[Eukaryotic|eukaryotic]] organisms. It has a variable diameter from 0.5 to 1 micrometre thus can be easily seen under a light microscope. Using time-lapse microcinematogprahy, it has been established that mitochondria can alter their shape continuously, and are also able to fuse and separate with other mitochondria.<ref>Alberts, Johnson, Lewis, Raff, Roberts, Walter (2008) Molecular Biology of the cell, Fifth edition, p815</ref> It is surrounded by two [[Phospholipid membrane|phospholipid membranes]]: the outer and inner membrane. The inner membrane is folded inwards to form cristae and it is the location where electron transport chain occur.  On the other hand, the outer membrane is the envelope that holds all the [[Organelles|organelles]] and it is relatively permeable to small molecules. The internal mitochondrial compartment is called the matrix where link reaction and Krebs cycle occur. As a result of [[Oxidative phosphorylation|oxidative phosphorylation]], [[ATP|ATP]] is synthesised in the latter by the activity of ATP synthase on the cristae. The advantage that this has over [[Glycolysis|glycolysis]] is that it reaps 15 times more ATP for all energy-requiring reactions of the cell, both inside and outside the mitochondria. Moreover, it has its own [[DNA|DNA]] which is circular and also contains 70s [[Ribosomes|ribosomes]]. In addition, it is also beneficial since as a result of cell damage, it is the one who signals the process of [[Apoptosis|apoptosis]] (cell death) by releasing mitochondrial protein into the [[Cytoplasm|cytoplasm]] <ref>Berg J.M, Tymoczko J.L, Stryer (2012) Biochemistry, seventh Edition, New York: WH Freema.. pg 543</ref><ref>Alberts, Johnson, Lewis, Raff, Roberts, Walter (2008) Molecular Biology of the cell, Fifth edition, pg 815</ref>. 

=== References ===

<references />

Skeletal muscle

2013-11-29T15:34:45Z

130127226:

Skeletal muscle (also known as striated muscle) which acts under voluntary contraction, is attached to the [[Bone|bone]] and functions to carry out movement and help in maintaining body posture. It is made up of [[Actin|actin]] and [[Myosin|myosin]] [[Protein|proteins]] which make up the [[Sacromere|sacromere]], as well as the regulatory proteins [[Troponin|troponin]] and [[Tropomyosin|tropomyosin]]. Contraction of a skeletal muscle is stimulated by release of [[Calcium ions|calcium ions]] from the [[Sarcoplasmic reticulum|sarcoplasmic reticulum]] which bind to troponin and cause a conformational change. This change causes tropomyosin to be released which causes the myosin-binding site on the actin molecule to become visible and so myosin can bind to the actin, causing a cross-bridge to form and the muscle to contract.

The myofibrils within the skeletal muscle create a alternating banding pattern of light and dark striations due to the thickness of the myofibrils changing as the muscle contracts.

Skeletal muscle is the main muscle type in our body and makes up approximately 40% of our total body weight <ref>Silvertorn, 2010, Human phisiology, 5th edition pearson international.</ref> .

=== Structure ===

[[Image:Lrg-1348-skeletal muscle.jpg|left|Multinucleated skeletal muscle cells]] A skeletal muscle consists of muscle fibres. Each individual muscle fibre is surrounded by the endomysium, groups of muscle fibres are bound by the perimysium to form bundles called muscle fascicules and these fascicules are themselves bound together by a connective tissue called the epimysium. <ref>Gillian Pocock and Christopher D. Richards, Human Physiology, 2006, U.S. by Oxford University press inc.</ref>One muscle fibre is about 100 µm in diameter, is multinucleate and contains many [[Mitochondria|mitochondria]]. The multinucleate feature is established in myogenesis where hundreds or thousands of uninucleated myoblasts fuse together to form muscle fibres of up to several centimeters long<ref>Rossi, S.G. Vasquez, A.E. Rotundo ,R.L.. (2000). Local Control of Acetylcholinesterase Gene Expression in Multinucleated Skeletal Muscle Fibers: Individual Nuclei Respond to Signals from the Overlying Plasma Membrane. The Journal of Neuroscience. 20 (3), p919-928.</ref>.The number of muscle fibres remains constant in a man from birth - muscle building is achieved only by increasing the size  of the muscle cells (each muscle cell is one muscle fibre). In the embryo, the membranes between newly differentiated muscle cells, called [[Myoblast|myoblasts]], break down, thus forming muscle fibres with many nuclei. These nuclei are pinned randomly agaist the muscle fibre. Each muscle fibre contains lots of [[Myofibril|myofibrils]] which are lined up against nerve fibres and cause contraction of the muscle . These are approximately 1 µm in diameter. 

The [[Myofibril|myofibril]] is organised in repeating units called [[Sarcomere|sarcomeres]]. These contain thick and thin filaments; which are attached to Z discs and M lines, respectively. These thick and thin filaments, when viewed under a microscope, appear "striped" or striated. This appearance under the light microscope is the reason that skeletal muscle may also be described as striated muscle. The thick and thin filaments are made up of two different contractile [[Proteins|proteins]] called [[Actin filaments|actin]] and [[Myosin|myosin]]. The actin filaments are the thin and flexible filaments while the myosin filaments are thick filaments. Thick filaments consist of the protein [[Myosin|myosin]] II which forms a globular head and fibrous tail. The thin filaments are formed from G-actin monomers which polymerise to form F-actin <ref>Freeman S. (2007), Biological Science, 3rd edition. San Francisco, Benjamin-Cummings Pub Co</ref>.  Each G-actin has a myosin-heading binding site which is blocked during muscle relaxation by the protein [[Tropomyosin|tropomyosin.]] Tropomyosin winds around the F-actin in association with troponin. [[Troponin|Troponin]] consists of 3 subunits; I, T and C. The I and T subunits bind to the tropomyosin blocking the myosin-head binding sites by holding the tropomyosin in position. The C subunit binds to calcium ions after their release from the [[Sacroplasmic reticulum|sarcoplasmic reticulum]] during muscle stimulation. Muscle contraction occurs when the thin filaments slide along the thick filament by hydrolysing [[ATP|ATP]] <ref>Berg J., Tymoczko J and Stryer L. (2001) Biochemistry, 5th edition, New York: WH Freeman.</ref> by what is known as the [[Ratchet Mechanism|Ratchet Mechanism]], or [[The Sliding Filament Theory|Sliding Filament Theory]]. The myofibrils also contain the elastic proteins [[Titin|Titin]] and Nebulin which help the actin fibres return to their resting position in relaxation and keep the contractile proteins aligned. 

=== Contraction ===

Contraction in a muscle cell is propagated by an [[Action potential|action potential travelling]] along a motor neurone and arriving at a [[Synapse]]; it is mediated by sodium ions. The voltage gradient causes voltage-gated calcium [[Ion channels|ion channels]] in the [[Presynaptic|presynaptic neurone]] to open, triggering [[Vesicles|vesicles]] containing [[Neurotransmitter|neurotransmitters]], specifically [[Acetylcholine|acetylcholine]], travel towards the [[Sarcolemma|sarcolemma]]; fusing with the [[Membrane|membrane]] and releasing [[Acetylcholine|acetylcholine]] into the [[Synaptic cleft|synaptic cleft]] <ref>Bowness E, Braid K, Brazier J, Burrows C, Craig K, Gillham R, Towle J. (2009), A2-level Biology The Revision Guide Exam Board AQA, page 57-60, Newcastle-upon-Tyne: CGP books.</ref>. They diffuse across the cleft where they bind to specific [[Receptor|receptors]] called [[Nicotinic cholinergic receptors|nicotinic cholinergic receptors]] on the [[Sarcolemma|sarcolemma]], where the [[Depolarisation|depolarisation]] travels along the membrane and deep into the cell via [[T-tubules|T-tubules]] <ref>Bowness E, Braid K, Brazier J, Burrows C, Craig K, Gillham R, Towle J. (2009), A2-level Biology The Revision Guide Exam Board AQA, page 57-60, Newcastle-upon-Tyne: CGP books.</ref>. The sarcoplasmic reticulum is englarged where it comes into contact with the termincal cisternae and each [[T-tubules|T-tubule]] is in close contact with the [[Cisternae|cisternae]] of two regions of [[Sarcoplasmic reticulum|sarcoplasmic reticulum]].<ref>Gillian Pocock and Christopher D. Richards, Human Physiology, 2006, U.S. by Oxford University Press Inc.</ref> Therefore it allows the terminal cisternae of the [[Sarcoplasmic reticulum|sarcoplasmic reticulum]] to become depolarised, releasing [[Calcium|calcium]] [[Ions|ions]].The calcuim ions bind to [[Troponin|troponin]] on the actin filaments, the complex the moves tropomyosin therefore un blocking the myosin binding site, muscle contraction can then take place by the [[The Sliding Filament Theory|sliding filament theory]] <ref>Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P. (2008), Molecular Biology of The Cell, page 1028-1029, 5th edition, New York:Garland Science.</ref>.  

[[Image:Sliding filament theory.jpg|right|Sliding filament contraction of skeletal muscle]]

Skeletal muscles are able to undergo [[Hypertrophy|muscle hypertrophy during]] increased physical exercise, e.g. in athletes. As well as this, muscles are also able to undergo [[Atrophy|atrophy]] when the muscles are underused, such as in someone who is immobilized by paralysis or limb injury <ref name="null">Stevens A. et al. (2005), Human Histology, Third Edition, Philadelphia, Elsevier Limited</ref>.

When an action potential travels through the sarcolemma and down to T tubule, Ca2+ is released from the terminal cisternae, the section of the sarcoplasmic reticulum closest to T tubules. This release of calcium ions increases the intracellular calcium ion concentration which leads to interaction between actin and myosin which in turn causes contraction of the cell <ref>Koeppen and Stanton, 2008, Berne and Levy Physiology, 6th edtion</ref>.   



=== Types ===

''''''There are three main types of skeletal muscle fibres; Type I, Type IIA and Type IIB. Our muscles have motor units of each of these types but some are found more in particular areas of our body than others. 

Type I are also known as slow oxidative fibres. Their contraction time is slow and they are highly resistant to fatigue. They can generate ATP by [[Aerobic respiration|aerobic respiration]] and fat is their main energy source. They are red in colour due to their high [[Myoglobin|myoglobin]] levels. Type I muscles are mainly found in bodies of marathon runners <ref>Muscle Home Page. 2011. Muscle Home Page. [ONLINE] Available at: http://www.bmb.leeds.ac.uk/illingworth/muscle/. [Accessed 30 November 2011].</ref>. In an average person's body, these muscle fibers are found in postural muscles (those that maintain posture), such as the neck <ref>MACKENZIE, B. (1999) Muscle Types [WWW] Available from: http://www.brianmac.co.uk/muscle.htm [Accessed 30/11/2011]</ref>. 

Type IIA muscle fibres are also known as fast oxidative fibres. Their contraction time is fast and they are quite resistant to fatigue. [[ATP|ATP]] is generated by aerobic respiration and both fats and glucose are used as their energy source. These fibres are also red due to the high concentrations of myoglobin. Such muscle fibres are not very prominent in humans, but do usually exist in bodies of athletes who do sports requiring a lot of stamina such as long distance running <ref>Muscle Home Page. 2011. Muscle Home Page. [ONLINE] Available at: http://www.bmb.leeds.ac.uk/illingworth/muscle/. [Accessed 30 November 2011].</ref>. 

Type IIB muscle fibres are also known as fast glycolytic fibres. They contract very fast and are highly irresistant to fatigue (so can only be used briefly). They obtain ATP from [[Anaerobic|anaerobic respiration]] in which glucose is converted to lactic acid. Since they have minimal myoglobin, they are white in colour. Such types of muscle fibers are most useful for sprinters, as it requires a short burst of energy <ref>Muscle Home Page. 2011. Muscle Home Page. [ONLINE] Available at: http://www.bmb.leeds.ac.uk/illingworth/muscle/. [Accessed 30 November 2011].</ref>. They are usually found in our arms <ref>MACKENZIE, B. (1999) Muscle Types [WWW] Available from: http://www.brianmac.co.uk/muscle.htm [Accessed 30/11/2011]</ref>. 

=== Development ===

After being determined from [[Somites|somites]] in a vertebrate embryo, [[Myoblast]]s go through [[Proliferation]] and a series of changes stopping cell division, and switching on muscle specific genes related for terminal differentiation. Myoblasts then fuse together to form multinucleate muscle fibres. After [[Differentiation]], a skeletal muscle cell will never divide again; the adult number of multincucleate skeletal muscle cells is gained before birth, instead muslce fibres increase in size by cell elongation. To regulate its growth, muslce fibres secrete the [[Growth factor]] [[Myostatin]] <ref>Alberts, B. 2008. Molecular biology of the cell. New York [etc.]: Garland Science. Pages 1464-1465</ref>. 

=== References ===

<references />

Skeletal muscle

2013-11-29T15:33:35Z

130127226:

Skeletal muscle (also known as striated muscle) which acts under voluntary contraction, is attached to the [[Bone|bone]] and functions to carry out movement and help in maintaining body posture. It is made up of [[Actin|actin]] and [[Myosin|myosin]] [[Protein|proteins]] which make up the [[Sacromere|sacromere]], as well as the regulatory proteins [[Troponin|troponin]] and [[Tropomyosin|tropomyosin]]. Contraction of a skeletal muscle is stimulated by release of [[Calcium ions|calcium ions]] from the [[Sarcoplasmic reticulum|sarcoplasmic reticulum]] which bind to troponin and cause a conformational change. This change causes tropomyosin to be released which causes the myosin-binding site on the actin molecule to become visible and so myosin can bind to the actin, causing a cross-bridge to form and the muscle to contract.

The myofibrils within the skeletal muscle create a alternating banding pattern of light and dark striations due to the thickness of the myofibrils changing as the muscle contracts.

Skeletal muscle is the main muscle type in our body and makes up approximately 40% of our total body weight <ref>Silvertorn, 2010, Human phisiology, 5th edition pearson international.</ref> .

=== Structure ===

[[Image:Lrg-1348-skeletal muscle.jpg|left|Multinucleated skeletal muscle cells]] A skeletal muscle consists of muscle fibres. Each individual muscle fibre is surrounded by the endomysium, and groups of muscle fibres are bound the perimysium to form bundles called muscle fascicules. These fascicules are then bound together by a connective tissue called the epimysium. <ref>Gillian Pocock and Christopher D. Richards, Human Physiology, 2006, U.S. by Oxford University press inc.</ref>One muscle fibre is about 100 µm in diameter, is multinucleate and contains many [[Mitochondria|mitochondria]]. The multinucleate feature is established in myogenesis where hundreds or thousands of uninucleated myoblasts fuse together to form muscle fibres of up to several centimeters long<ref>Rossi, S.G. Vasquez, A.E. Rotundo ,R.L.. (2000). Local Control of Acetylcholinesterase Gene Expression in Multinucleated Skeletal Muscle Fibers: Individual Nuclei Respond to Signals from the Overlying Plasma Membrane. The Journal of Neuroscience. 20 (3), p919-928.</ref>.The number of muscle fibres remains constant in a man from birth - muscle building is achieved only by increasing the size  of the muscle cells (each muscle cell is one muscle fibre). In the embryo, the membranes between newly differentiated muscle cells, called [[Myoblast|myoblasts]], break down, thus forming muscle fibres with many nuclei. These nuclei are pinned randomly agaist the muscle fibre. Each muscle fibre contains lots of [[Myofibril|myofibrils]] which are lined up against nerve fibres and cause contraction of the muscle . These are approximately 1 µm in diameter. 

The [[Myofibril|myofibril]] is organised in repeating units called [[Sarcomere|sarcomeres]]. These contain thick and thin filaments; which are attached to Z discs and M lines, respectively. These thick and thin filaments, when viewed under a microscope, appear "striped" or striated. This appearance under the light microscope is the reason that skeletal muscle may also be described as striated muscle. The thick and thin filaments are made up of two different contractile [[Proteins|proteins]] called [[Actin filaments|actin]] and [[Myosin|myosin]]. The actin filaments are the thin and flexible filaments while the myosin filaments are thick filaments. Thick filaments consist of the protein [[Myosin|myosin]] II which forms a globular head and fibrous tail. The thin filaments are formed from G-actin monomers which polymerise to form F-actin <ref>Freeman S. (2007), Biological Science, 3rd edition. San Francisco, Benjamin-Cummings Pub Co</ref>.  Each G-actin has a myosin-heading binding site which is blocked during muscle relaxation by the protein [[Tropomyosin|tropomyosin.]] Tropomyosin winds around the F-actin in association with troponin. [[Troponin|Troponin]] consists of 3 subunits; I, T and C. The I and T subunits bind to the tropomyosin blocking the myosin-head binding sites by holding the tropomyosin in position. The C subunit binds to calcium ions after their release from the [[Sacroplasmic reticulum|sarcoplasmic reticulum]] during muscle stimulation. Muscle contraction occurs when the thin filaments slide along the thick filament by hydrolysing [[ATP|ATP]] <ref>Berg J., Tymoczko J and Stryer L. (2001) Biochemistry, 5th edition, New York: WH Freeman.</ref> by what is known as the [[Ratchet Mechanism|Ratchet Mechanism]], or [[The Sliding Filament Theory|Sliding Filament Theory]]. The myofibrils also contain the elastic proteins [[Titin|Titin]] and Nebulin which help the actin fibres return to their resting position in relaxation and keep the contractile proteins aligned. 

=== Contraction ===

Contraction in a muscle cell is propagated by an [[Action potential|action potential travelling]] along a motor neurone and arriving at a [[Synapse]]; it is mediated by sodium ions. The voltage gradient causes voltage-gated calcium [[Ion channels|ion channels]] in the [[Presynaptic|presynaptic neurone]] to open, triggering [[Vesicles|vesicles]] containing [[Neurotransmitter|neurotransmitters]], specifically [[Acetylcholine|acetylcholine]], travel towards the [[Sarcolemma|sarcolemma]]; fusing with the [[Membrane|membrane]] and releasing [[Acetylcholine|acetylcholine]] into the [[Synaptic cleft|synaptic cleft]] <ref>Bowness E, Braid K, Brazier J, Burrows C, Craig K, Gillham R, Towle J. (2009), A2-level Biology The Revision Guide Exam Board AQA, page 57-60, Newcastle-upon-Tyne: CGP books.</ref>. They diffuse across the cleft where they bind to specific [[Receptor|receptors]] called [[Nicotinic cholinergic receptors|nicotinic cholinergic receptors]] on the [[Sarcolemma|sarcolemma]], where the [[Depolarisation|depolarisation]] travels along the membrane and deep into the cell via [[T-tubules|T-tubules]] <ref>Bowness E, Braid K, Brazier J, Burrows C, Craig K, Gillham R, Towle J. (2009), A2-level Biology The Revision Guide Exam Board AQA, page 57-60, Newcastle-upon-Tyne: CGP books.</ref>. The sarcoplasmic reticulum is englarged where it comes into contact with the termincal cisternae and each [[T-tubules|T-tubule]] is in close contact with the [[Cisternae|cisternae]] of two regions of [[Sarcoplasmic reticulum|sarcoplasmic reticulum]].<ref>Gillian Pocock and Christopher D. Richards, Human Physiology, 2006, U.S. by Oxford University Press Inc.</ref> Therefore it allows the terminal cisternae of the [[Sarcoplasmic reticulum|sarcoplasmic reticulum]] to become depolarised, releasing [[Calcium|calcium]] [[Ions|ions]].The calcuim ions bind to [[Troponin|troponin]] on the actin filaments, the complex the moves tropomyosin therefore un blocking the myosin binding site, muscle contraction can then take place by the [[The Sliding Filament Theory|sliding filament theory]] <ref>Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P. (2008), Molecular Biology of The Cell, page 1028-1029, 5th edition, New York:Garland Science.</ref>.  

[[Image:Sliding filament theory.jpg|right|Sliding filament contraction of skeletal muscle]]

Skeletal muscles are able to undergo [[Hypertrophy|muscle hypertrophy during]] increased physical exercise, e.g. in athletes. As well as this, muscles are also able to undergo [[Atrophy|atrophy]] when the muscles are underused, such as in someone who is immobilized by paralysis or limb injury <ref name="null">Stevens A. et al. (2005), Human Histology, Third Edition, Philadelphia, Elsevier Limited</ref>.

When an action potential travels through the sarcolemma and down to T tubule, Ca2+ is released from the terminal cisternae, the section of the sarcoplasmic reticulum closest to T tubules. This release of calcium ions increases the intracellular calcium ion concentration which leads to interaction between actin and myosin which in turn causes contraction of the cell <ref>Koeppen and Stanton, 2008, Berne and Levy Physiology, 6th edtion</ref>.   



=== Types ===

''''''There are three main types of skeletal muscle fibres; Type I, Type IIA and Type IIB. Our muscles have motor units of each of these types but some are found more in particular areas of our body than others. 

Type I are also known as slow oxidative fibres. Their contraction time is slow and they are highly resistant to fatigue. They can generate ATP by [[Aerobic respiration|aerobic respiration]] and fat is their main energy source. They are red in colour due to their high [[Myoglobin|myoglobin]] levels. Type I muscles are mainly found in bodies of marathon runners <ref>Muscle Home Page. 2011. Muscle Home Page. [ONLINE] Available at: http://www.bmb.leeds.ac.uk/illingworth/muscle/. [Accessed 30 November 2011].</ref>. In an average person's body, these muscle fibers are found in postural muscles (those that maintain posture), such as the neck <ref>MACKENZIE, B. (1999) Muscle Types [WWW] Available from: http://www.brianmac.co.uk/muscle.htm [Accessed 30/11/2011]</ref>. 

Type IIA muscle fibres are also known as fast oxidative fibres. Their contraction time is fast and they are quite resistant to fatigue. [[ATP|ATP]] is generated by aerobic respiration and both fats and glucose are used as their energy source. These fibres are also red due to the high concentrations of myoglobin. Such muscle fibres are not very prominent in humans, but do usually exist in bodies of athletes who do sports requiring a lot of stamina such as long distance running <ref>Muscle Home Page. 2011. Muscle Home Page. [ONLINE] Available at: http://www.bmb.leeds.ac.uk/illingworth/muscle/. [Accessed 30 November 2011].</ref>. 

Type IIB muscle fibres are also known as fast glycolytic fibres. They contract very fast and are highly irresistant to fatigue (so can only be used briefly). They obtain ATP from [[Anaerobic|anaerobic respiration]] in which glucose is converted to lactic acid. Since they have minimal myoglobin, they are white in colour. Such types of muscle fibers are most useful for sprinters, as it requires a short burst of energy <ref>Muscle Home Page. 2011. Muscle Home Page. [ONLINE] Available at: http://www.bmb.leeds.ac.uk/illingworth/muscle/. [Accessed 30 November 2011].</ref>. They are usually found in our arms <ref>MACKENZIE, B. (1999) Muscle Types [WWW] Available from: http://www.brianmac.co.uk/muscle.htm [Accessed 30/11/2011]</ref>. 

=== Development ===

After being determined from [[Somites|somites]] in a vertebrate embryo, [[Myoblast]]s go through [[Proliferation]] and a series of changes stopping cell division, and switching on muscle specific genes related for terminal differentiation. Myoblasts then fuse together to form multinucleate muscle fibres. After [[Differentiation]], a skeletal muscle cell will never divide again; the adult number of multincucleate skeletal muscle cells is gained before birth, instead muslce fibres increase in size by cell elongation. To regulate its growth, muslce fibres secrete the [[Growth factor]] [[Myostatin]] <ref>Alberts, B. 2008. Molecular biology of the cell. New York [etc.]: Garland Science. Pages 1464-1465</ref>. 

=== References ===

<references />

Skeletal muscle

2013-11-29T15:33:04Z

130127226:

Skeletal muscle (also known as striated muscle) which acts under voluntary contraction, is attached to the [[Bone|bone]] and functions to carry out movement and help in maintaining body posture. It is made up of [[Actin|actin]] and [[Myosin|myosin]] [[Protein|proteins]] which make up the [[Sacromere|sacromere]], as well as the regulatory proteins [[Troponin|troponin]] and [[Tropomyosin|tropomyosin]]. Contraction of a skeletal muscle is stimulated by release of [[Calcium ions|calcium ions]] from the [[Sarcoplasmic reticulum|sarcoplasmic reticulum]] which bind to troponin and cause a conformational change. This change causes tropomyosin to be released which causes the myosin-binding site on the actin molecule to become visible and so myosin can bind to the actin, causing a cross-bridge to form and the muscle to contract.

The myofibrils within the skeletal muscle create a alternating banding pattern of light and dark striations due to the thickness of the myofibrils changing as the muscle contracts.

Skeletal muscle is the main muscle type in our body and makes up approximately 40% of our total body weight <ref>Silvertorn, 2010, Human phisiology, 5th edition pearson international.</ref> .

=== Structure ===

[[Image:Lrg-1348-skeletal muscle.jpg|left|Multinucleated skeletal muscle cells]] A skeletal muscle consists of muscle fibres. Each individual muscle fibre is surrounded by the endomysium, and groups of muscle fibres are bound the perimysium to form bundles called muscle fascicules. These fascicules are then bound together by a connective tissue called the epimysium. <ref>Gillian Pocock and Christopher D. Richards, Human Physiology, 2006, U.S. by Oxford University press inc.</ref>One muscle fibre is about 100 µm in diameter, is multinucleate and contains many [[Mitochondria|mitochondria]]. The multinucleate feature is established in myogenesis where hundreds or thousands of uninucleated myoblasts fuse together to form muscle fibres of up to several centimeters long<ref>Rossi, S.G. Vasquez, A.E. Rotundo ,R.L.. (2000). Local Control of Acetylcholinesterase Gene Expression in Multinucleated Skeletal Muscle Fibers: Individual Nuclei Respond to Signals from the Overlying Plasma Membrane. The Journal of Neuroscience. 20 (3), p919-928.</ref>.The number of muscle fibres remains constant in a man from birth - muscle building is achieved only by increasing the size  of the muscle cells (each muscle cell is one muscle fibre). In the embryo, the membranes between newly differentiated muscle cells, called [[Myoblast|myoblasts]], break down, thus forming muscle fibres with many nuclei. These nuclei are pinned randomly agaist the muscle fibre. Each muscle fibre contains lots of [[Myofibril|myofibrils]] which are lined up against nerve fibres and cause contraction of the muscle . These are approximately 1 µm in diameter. 

The [[Myofibril|myofibril]] is organised in repeating units called [[Sarcomere|sarcomeres]]. These contain thick and thin filaments; which are attached to Z discs and M lines, respectively. These thick and thin filaments, when viewed under a microscope, appear "striped" or striated. This appearance under the light microscope is the reason that skeletal muscle may also be described as striated muscle. The thick and thin filaments are made up of two different contractile [[Proteins|proteins]] called [[Actin filaments|actin]] and [[Myosin|myosin]]. The actin filaments are the thin and flexible filaments while the myosin filaments are thick filaments. Thick filaments consist of the protein [[Myosin|myosin]] II which forms a globular head and fibrous tail. The thin filaments are formed from G-actin monomers which polymerise to form F-actin <ref>Freeman S. (2007), Biological Science, 3rd edition. San Francisco, Benjamin-Cummings Pub Co</ref>.  Each G-actin has a myosin-heading binding site which is blocked during muscle relaxation by the protein [[Tropomyosin|tropomyosin.]] Tropomyosin winds around the F-actin in association with troponin. [[Troponin|Troponin]] consists of 3 subunits; I, T and C. The I and T subunits bind to the tropomyosin blocking the myosin-head binding sites by holding the tropomyosin in position. The C subunit binds to calcium ions after their release from the [[Sacroplasmic reticulum|sarcoplasmic reticulum]] during muscle stimulation. Muscle contraction occurs when the thin filaments slide along the thick filament by hydrolysing [[ATP|ATP]] <ref>Berg J., Tymoczko J and Stryer L. (2001) Biochemistry, 5th edition, New York: WH Freeman.</ref> by what is known as the [[Ratchet Mechanism|Ratchet Mechanism]], or [[The Sliding Filament Theory|Sliding Filament Theory]]. The myofibrils also contain the elastic proteins [[Titin|Titin]] and Nebulin which help the actin fibres return to their resting position in relaxation and keep the contractile proteins aligned. 

=== Contraction ===

Contraction in a muscle cell is propagated by an [[Action potential|action potential travelling]] along a motor neurone and arriving at a [[Synapse]]; it is mediated by sodium ions. The voltage gradient causes voltage-gated calcium [[Ion channels|ion channels]] in the [[Presynaptic|presynaptic neurone]] to open, triggering [[Vesicles|vesicles]] containing [[Neurotransmitter|neurotransmitters]], specifically [[Acetylcholine|acetylcholine]], travel towards the [[Sarcolemma|sarcolemma]]; fusing with the [[Membrane|membrane]] and releasing [[Acetylcholine|acetylcholine]] into the [[Synaptic cleft|synaptic cleft]] <ref>Bowness E, Braid K, Brazier J, Burrows C, Craig K, Gillham R, Towle J. (2009), A2-level Biology The Revision Guide Exam Board AQA, page 57-60, Newcastle-upon-Tyne: CGP books.</ref>. They diffuse across the cleft where they bind to specific [[Receptor|receptors]] called [[Nicotinic cholinergic receptors|nicotinic cholinergic receptors]] on the [[Sarcolemma|sarcolemma]], where the [[Depolarisation|depolarisation]] travels along the membrane and deep into the cell via [[T-tubules|T-tubules]] <ref>Bowness E, Braid K, Brazier J, Burrows C, Craig K, Gillham R, Towle J. (2009), A2-level Biology The Revision Guide Exam Board AQA, page 57-60, Newcastle-upon-Tyne: CGP books.</ref>. The sarcoplasmic reticulum is englarged where it comes into contact with the termincal cisternae and each [[T-tubules|T-tubule]] is in close contact with the [[Cisternae|cisternae]] of two regions of [[Sarcoplasmic_reticulum|sarcoplasmic reticulum]].<references /><ref>Gillian Pocock and Christopher D. Richards, Human Physiology, 2006, U.S. by Oxford University Press Inc.</ref> Therefore it allows the terminal cisternae of the [[Sarcoplasmic reticulum|sarcoplasmic reticulum]] to become depolarised, releasing [[Calcium|calcium]] [[Ions|ions]].The calcuim ions bind to [[Troponin|troponin]] on the actin filaments, the complex the moves tropomyosin therefore un blocking the myosin binding site, muscle contraction can then take place by the [[The Sliding Filament Theory|sliding filament theory]] <ref>Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P. (2008), Molecular Biology of The Cell, page 1028-1029, 5th edition, New York:Garland Science.</ref>.  

[[Image:Sliding filament theory.jpg|right|Sliding filament contraction of skeletal muscle]]

Skeletal muscles are able to undergo [[Hypertrophy|muscle hypertrophy during]] increased physical exercise, e.g. in athletes. As well as this, muscles are also able to undergo [[Atrophy|atrophy]] when the muscles are underused, such as in someone who is immobilized by paralysis or limb injury <ref name="null">Stevens A. et al. (2005), Human Histology, Third Edition, Philadelphia, Elsevier Limited</ref>.

When an action potential travels through the sarcolemma and down to T tubule, Ca2+ is released from the terminal cisternae, the section of the sarcoplasmic reticulum closest to T tubules. This release of calcium ions increases the intracellular calcium ion concentration which leads to interaction between actin and myosin which in turn causes contraction of the cell <ref>Koeppen and Stanton, 2008, Berne and Levy Physiology, 6th edtion</ref>.   



=== Types ===

''''''There are three main types of skeletal muscle fibres; Type I, Type IIA and Type IIB. Our muscles have motor units of each of these types but some are found more in particular areas of our body than others. 

Type I are also known as slow oxidative fibres. Their contraction time is slow and they are highly resistant to fatigue. They can generate ATP by [[Aerobic respiration|aerobic respiration]] and fat is their main energy source. They are red in colour due to their high [[Myoglobin|myoglobin]] levels. Type I muscles are mainly found in bodies of marathon runners <ref>Muscle Home Page. 2011. Muscle Home Page. [ONLINE] Available at: http://www.bmb.leeds.ac.uk/illingworth/muscle/. [Accessed 30 November 2011].</ref>. In an average person's body, these muscle fibers are found in postural muscles (those that maintain posture), such as the neck <ref>MACKENZIE, B. (1999) Muscle Types [WWW] Available from: http://www.brianmac.co.uk/muscle.htm [Accessed 30/11/2011]</ref>. 

Type IIA muscle fibres are also known as fast oxidative fibres. Their contraction time is fast and they are quite resistant to fatigue. [[ATP|ATP]] is generated by aerobic respiration and both fats and glucose are used as their energy source. These fibres are also red due to the high concentrations of myoglobin. Such muscle fibres are not very prominent in humans, but do usually exist in bodies of athletes who do sports requiring a lot of stamina such as long distance running <ref>Muscle Home Page. 2011. Muscle Home Page. [ONLINE] Available at: http://www.bmb.leeds.ac.uk/illingworth/muscle/. [Accessed 30 November 2011].</ref>. 

Type IIB muscle fibres are also known as fast glycolytic fibres. They contract very fast and are highly irresistant to fatigue (so can only be used briefly). They obtain ATP from [[Anaerobic|anaerobic respiration]] in which glucose is converted to lactic acid. Since they have minimal myoglobin, they are white in colour. Such types of muscle fibers are most useful for sprinters, as it requires a short burst of energy <ref>Muscle Home Page. 2011. Muscle Home Page. [ONLINE] Available at: http://www.bmb.leeds.ac.uk/illingworth/muscle/. [Accessed 30 November 2011].</ref>. They are usually found in our arms <ref>MACKENZIE, B. (1999) Muscle Types [WWW] Available from: http://www.brianmac.co.uk/muscle.htm [Accessed 30/11/2011]</ref>. 

=== Development ===

After being determined from [[Somites|somites]] in a vertebrate embryo, [[Myoblast]]s go through [[Proliferation]] and a series of changes stopping cell division, and switching on muscle specific genes related for terminal differentiation. Myoblasts then fuse together to form multinucleate muscle fibres. After [[Differentiation]], a skeletal muscle cell will never divide again; the adult number of multincucleate skeletal muscle cells is gained before birth, instead muslce fibres increase in size by cell elongation. To regulate its growth, muslce fibres secrete the [[Growth factor]] [[Myostatin]] <ref>Alberts, B. 2008. Molecular biology of the cell. New York [etc.]: Garland Science. Pages 1464-1465</ref>. 

=== References ===

<references />

Skeletal muscle

2013-11-29T15:22:37Z

130127226:

Skeletal muscle (also known as striated muscle) which acts under voluntary contraction, is attached to the [[Bone|bone]] and functions to carry out movement and help in maintaining body posture. It is made up of [[Actin|actin]] and [[Myosin|myosin]] [[Protein|proteins]] which make up the [[Sacromere|sacromere]], as well as the regulatory proteins [[Troponin|troponin]] and [[Tropomyosin|tropomyosin]]. Contraction of a skeletal muscle is stimulated by release of [[Calcium ions|calcium ions]] from the [[Sarcoplasmic reticulum|sarcoplasmic reticulum]] which bind to troponin and cause a conformational change. This change causes tropomyosin to be released which causes the myosin-binding site on the actin molecule to become visible and so myosin can bind to the actin, causing a cross-bridge to form and the muscle to contract.

The myofibrils within the skeletal muscle create a alternating banding pattern of light and dark striations due to the thickness of the myofibrils changing as the muscle contracts.

Skeletal muscle is the main muscle type in our body and makes up approximately 40% of our total body weight <ref>Silvertorn, 2010, Human phisiology, 5th edition pearson international.</ref> .

=== Structure ===

[[Image:Lrg-1348-skeletal muscle.jpg|left|Multinucleated skeletal muscle cells]] A skeletal muscle consists of muscle fibres. Each individual muscle fibre is surrounded by the endomysium, and groups of muscle fibres are bound the perimysium to form bundles called muscle fascicules. These fascicules are then bound together by a connective tissue called the epimysium. <ref>Gillian Pocock and Christopher D. Richards, Human Physiology, 2006, U.S. by Oxford University press inc.</ref>One muscle fibre is about 100 µm in diameter, is multinucleate and contains many [[Mitochondria|mitochondria]]. The multinucleate feature is established in myogenesis where hundreds or thousands of uninucleated myoblasts fuse together to form muscle fibres of up to several centimeters long<ref>Rossi, S.G. Vasquez, A.E. Rotundo ,R.L.. (2000). Local Control of Acetylcholinesterase Gene Expression in Multinucleated Skeletal Muscle Fibers: Individual Nuclei Respond to Signals from the Overlying Plasma Membrane. The Journal of Neuroscience. 20 (3), p919-928.</ref>.The number of muscle fibres remains constant in a man from birth - muscle building is achieved only by increasing the size  of the muscle cells (each muscle cell is one muscle fibre). In the embryo, the membranes between newly differentiated muscle cells, called [[Myoblast|myoblasts]], break down, thus forming muscle fibres with many nuclei. These nuclei are pinned randomly agaist the muscle fibre. Each muscle fibre contains lots of [[Myofibril|myofibrils]] which are lined up against nerve fibres and cause contraction of the muscle . These are approximately 1 µm in diameter. 

The [[Myofibril|myofibril]] is organised in repeating units called [[Sarcomere|sarcomeres]]. These contain thick and thin filaments; which are attached to Z discs and M lines, respectively. These thick and thin filaments, when viewed under a microscope, appear "striped" or striated. This appearance under the light microscope is the reason that skeletal muscle may also be described as striated muscle. The thick and thin filaments are made up of two different contractile [[Proteins|proteins]] called [[Actin filaments|actin]] and [[Myosin|myosin]]. The actin filaments are the thin and flexible filaments while the myosin filaments are thick filaments. Thick filaments consist of the protein [[Myosin|myosin]] II which forms a globular head and fibrous tail. The thin filaments are formed from G-actin monomers which polymerise to form F-actin <ref>Freeman S. (2007), Biological Science, 3rd edition. San Francisco, Benjamin-Cummings Pub Co</ref>.  Each G-actin has a myosin-heading binding site which is blocked during muscle relaxation by the protein [[Tropomyosin|tropomyosin.]] Tropomyosin winds around the F-actin in association with troponin. [[Troponin|Troponin]] consists of 3 subunits; I, T and C. The I and T subunits bind to the tropomyosin blocking the myosin-head binding sites by holding the tropomyosin in position. The C subunit binds to calcium ions after their release from the [[Sacroplasmic reticulum|sarcoplasmic reticulum]] during muscle stimulation. Muscle contraction occurs when the thin filaments slide along the thick filament by hydrolysing [[ATP|ATP]] <ref>Berg J., Tymoczko J and Stryer L. (2001) Biochemistry, 5th edition, New York: WH Freeman.</ref> by what is known as the [[Ratchet Mechanism|Ratchet Mechanism]], or [[The Sliding Filament Theory|Sliding Filament Theory]]. The myofibrils also contain the elastic proteins [[Titin|Titin]] and Nebulin which help the actin fibres return to their resting position in relaxation and keep the contractile proteins aligned. 

=== Contraction ===

Contraction in a muscle cell is propagated by an [[Action potential|action potential travelling]] along a motor neurone and arriving at a [[Synapse]]; it is mediated by sodium ions. The voltage gradient causes voltage-gated calcium [[Ion channels|ion channels]] in the [[Presynaptic|presynaptic neurone]] to open, triggering [[Vesicles|vesicles]] containing [[Neurotransmitter|neurotransmitters]], specifically [[Acetylcholine|acetylcholine]], travel towards the [[Sarcolemma|sarcolemma]]; fusing with the [[Membrane|membrane]] and releasing [[Acetylcholine|acetylcholine]] into the [[Synaptic cleft|synaptic cleft]] <ref>Bowness E, Braid K, Brazier J, Burrows C, Craig K, Gillham R, Towle J. (2009), A2-level Biology The Revision Guide Exam Board AQA, page 57-60, Newcastle-upon-Tyne: CGP books.</ref>. They diffuse across the cleft where they bind to specific [[Receptor|receptors]] called [[Nicotinic cholinergic receptors|nicotinic cholinergic receptors]] on the [[Sarcolemma|sarcolemma]], where the [[Depolarisation|depolarisation]] travels along the membrane and deep into the cell via [[T-tubules|T-tubules]] <ref>Bowness E, Braid K, Brazier J, Burrows C, Craig K, Gillham R, Towle J. (2009), A2-level Biology The Revision Guide Exam Board AQA, page 57-60, Newcastle-upon-Tyne: CGP books.</ref>. Therefore it allows the terminal cisternae of the [[Sarcoplasmic reticulum|sarcoplasmic reticulum]] to become depolarised, releasing [[Calcium|calcium]] [[Ions|ions]].The calcuim ions bind to [[Troponin|troponin]] on the actin filaments, the complex the moves tropomyosin therefore un blocking the myosin binding site, muscle contraction can then take place by the [[The Sliding Filament Theory|sliding filament theory]] <ref>Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P. (2008), Molecular Biology of The Cell, page 1028-1029, 5th edition, New York:Garland Science.</ref>.  

[[Image:Sliding filament theory.jpg|right|Sliding filament contraction of skeletal muscle]]

Skeletal muscles are able to undergo [[Hypertrophy|muscle hypertrophy during]] increased physical exercise, e.g. in athletes. As well as this, muscles are also able to undergo [[Atrophy|atrophy]] when the muscles are underused, such as in someone who is immobilized by paralysis or limb injury <ref name="null">Stevens A. et al. (2005), Human Histology, Third Edition, Philadelphia, Elsevier Limited</ref>.

When an action potential travels through the sarcolemma and down to T tubule, Ca2+ is released from the terminal cisternae, the section of the sarcoplasmic reticulum closest to T tubules. This release of calcium ions increases the intracellular calcium ion concentration which leads to interaction between actin and myosin which in turn causes contraction of the cell <ref>Koeppen and Stanton, 2008, Berne and Levy Physiology, 6th edtion</ref>.   



=== Types ===

''''''There are three main types of skeletal muscle fibres; Type I, Type IIA and Type IIB. Our muscles have motor units of each of these types but some are found more in particular areas of our body than others. 

Type I are also known as slow oxidative fibres. Their contraction time is slow and they are highly resistant to fatigue. They can generate ATP by [[Aerobic respiration|aerobic respiration]] and fat is their main energy source. They are red in colour due to their high [[Myoglobin|myoglobin]] levels. Type I muscles are mainly found in bodies of marathon runners <ref>Muscle Home Page. 2011. Muscle Home Page. [ONLINE] Available at: http://www.bmb.leeds.ac.uk/illingworth/muscle/. [Accessed 30 November 2011].</ref>. In an average person's body, these muscle fibers are found in postural muscles (those that maintain posture), such as the neck <ref>MACKENZIE, B. (1999) Muscle Types [WWW] Available from: http://www.brianmac.co.uk/muscle.htm [Accessed 30/11/2011]</ref>. 

Type IIA muscle fibres are also known as fast oxidative fibres. Their contraction time is fast and they are quite resistant to fatigue. [[ATP|ATP]] is generated by aerobic respiration and both fats and glucose are used as their energy source. These fibres are also red due to the high concentrations of myoglobin. Such muscle fibres are not very prominent in humans, but do usually exist in bodies of athletes who do sports requiring a lot of stamina such as long distance running <ref>Muscle Home Page. 2011. Muscle Home Page. [ONLINE] Available at: http://www.bmb.leeds.ac.uk/illingworth/muscle/. [Accessed 30 November 2011].</ref>. 

Type IIB muscle fibres are also known as fast glycolytic fibres. They contract very fast and are highly irresistant to fatigue (so can only be used briefly). They obtain ATP from [[Anaerobic|anaerobic respiration]] in which glucose is converted to lactic acid. Since they have minimal myoglobin, they are white in colour. Such types of muscle fibers are most useful for sprinters, as it requires a short burst of energy <ref>Muscle Home Page. 2011. Muscle Home Page. [ONLINE] Available at: http://www.bmb.leeds.ac.uk/illingworth/muscle/. [Accessed 30 November 2011].</ref>. They are usually found in our arms <ref>MACKENZIE, B. (1999) Muscle Types [WWW] Available from: http://www.brianmac.co.uk/muscle.htm [Accessed 30/11/2011]</ref>. 

=== Development ===

After being determined from [[Somites|somites]] in a vertebrate embryo, [[Myoblast]]s go through [[Proliferation]] and a series of changes stopping cell division, and switching on muscle specific genes related for terminal differentiation. Myoblasts then fuse together to form multinucleate muscle fibres. After [[Differentiation]], a skeletal muscle cell will never divide again; the adult number of multincucleate skeletal muscle cells is gained before birth, instead muslce fibres increase in size by cell elongation. To regulate its growth, muslce fibres secrete the [[Growth factor]] [[Myostatin]] <ref>Alberts, B. 2008. Molecular biology of the cell. New York [etc.]: Garland Science. Pages 1464-1465</ref>. 

=== References ===

<references />