Muscle

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Skeletal Muscle

A skeletal muscle muscle consists of muscle fibres. One muscle fibre is approximatels 100 µm in diameter and consists of several nuclei and many mitochondria. Each muscle fibre contains myofibrils. These are approximately 1 µm in diameter.

The myofibril is organised in repeating units called sarcomeres. These contain thick and thin filaments; these viewed under a microscope, and for this reason they are also known as striated muscle cells. Muscle contraction occurs when the thin filaments slide along the thick filament by hydrolysing ATP [1] by what is known as the Sliding Filament Theory.

Contraction in a muscle cell is produced by an action potential travelling along a motor neurone and arriving at a Synapse. The voltage gradient causes voltage-gated calcium ion channels in the presynaptic neurone to open, triggering vesicles containing neurotransmitters, specifically acetylcholine, to travel towards the sarcolemma; fusing with the membrane and releasing acetylcholine into the synaptic cleft [2]. They diffuse across the cleft where they bind to specific receptors called nicotinic cholinergic receptors on the sarcolemma, where the depolarisation travels along the membrane and deep into the cell via T-tubules [3]. Therefore it allows the sarcoplasmic reticulum to become depolarised, releasing calcium ions and triggering muscle contraction to take place by the sliding filament theory [4].

Smooth Muscle

A smooth muscle cell's location are mainly on the walls of hollow organs such as the urinary, reproductive, intestinal and respiratory tracts of both females and males. They also contribute to other major functions such as peristalsis and vasoconstriction. Due to the smooth muscle cell having many different functions the cells are organised into two groups. These are catagorized as Multiunit smooth muscles and Single unit smooth muscles. 

Unlike skeletal muscles they are 2 to 10 µm and have only one nuclei. They contain similar components to both cardiac and skeletal muscle cells; myosin, actin and tropomyosin but they do not have troponin. The non-straited cells contain more actin than myosin in the fibre composition. Therefore, there is a larger proportion of thin filaments than thick filaments in smooth muscles than striated muscles. 

The mode of control is governed by the autonomic nervous system, meaning it is an involuntary control. Whereas, the skeletal muscles are innervated by the somatic nervous system control. The neuron can make contact with the smooth muscle cell at many points on the cell. This forms a swelling called a varicosity which contains the components for vesicular neurotransmitter release. The multiunit smooth muscle's cells each receive a nervous input and act independently to each other. The single unit muscle cells recieve a nervous input together and due to the many gap junctions electrical communication and take place. This allows the cells to act in unison[5].  

References

  1. Berg J., Tymoczko J and Stryer L. (2001) Biochemistry, 5th edition, New York: WH Freeman.
  2. 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.
  3. 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.
  4. 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.
  5. Animal Physiology, Second Edition, Richard W.Hill Michigan State University, Gordon A. Wyse University of Massachusetts Amherst, Margaret Anderson Smith College,
 
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