Essay: The Muscular System

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  • The Muscular System
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The human body is a complex but fascinating study and it is highly important to understand normal anatomy and physiology. The oxford dictionary defines normal and states that ‘(Of a person) free from physical or mental disorders’ meaning that for the body to work as a whole all of the bodily systems must work together, if one of the systems or organs does not work normally a patient is described as having disease (Oxford dictionaries, 2016). As a practitioner this is the foundation for diagnosing a sick patient and to be able to source a treatment firstly you must understand how the body functions in a normal state and compare this to a patient who is suffering from abnormalities. This then allows a practitioner to educate their patient about their illness and referring them to the correct health sector for treatment. Without a good knowledge of normal anatomy, a patient could be wrongly diagnosed and therefore mistreated. This essay intends to discuss normal anatomy of the muscular system and its interaction with the nervous system (Barrow,2004). An auto immune condition known as multiple sclerosis where the immune system fails within the myelin protein that covers and protects the nerves that send impulses from the brain to the rest of the body are damaged. This causes loss of vision, muscle stiffness or uncontrolled movements or difficulty with balance and coordination.
There are three main functions to the muscular system and muscles enable the skeleton to move, maintain posture and control the generation of heat throughout the body. There are three types of muscle firstly smooth, an involuntary muscle made of non-striated tissue found in bodily organs such as the digestive tract used to help push food through to the rectum, and in the arteries used to aid blood flow by contracting. Cardiac muscle is also known as involuntary which is found in the heart, and is the muscle that contracts the heart to circulate blood around the body. Lastly the skeletal muscle which is located throughout the body connected to the skeleton and this aids the voluntary movement of a person when needed to perform desired actions or duties (Zimmermann, 2014). Muscles have five major properties excitable and irritable meaning they are capable of receiving and responding to stimulation from nerves, contractible meaning that after receiving stimulation they are capable of contracting and shortening, extensible meaning muscles can be stretched by force without damage, elasticity meaning after extension or contraction a muscle can return to its original state and adaptable meaning the muscular system can be changed in response to how it is used, for instance is a person was to perform extensive muscle training through exercise the muscle would then tear, throughout the muscle repair process new muscle would grow making the muscle larger (Zimmermann, 2014). There are around six hundred and forty muscles in the body and each range and vary in size depending on the location within body, for instance your gluteus maximus located in the buttocks is very thick containing hundreds of thousands of muscle cells and is used for hip extension, whereas the stapedius muscle located in the middle ear may only have a few hundred muscle cells as it is the smallest muscle in the body and is used to control the movement of the stirrup bone, and all these different muscles are sized accordingly with the skeletal system to work together effectively as a unit. (Zilkowsky, 2013). Muscles are the body’s supportive structures called tendons and ligaments. Tendons attach muscle to bone and ligaments attach muscle to muscle. Tendons are strong, flexible and made of collagen fibres, this allows the bone to move by means of muscle contraction. (Minnet, Wayne, and Rubenstein, 1989, pp.12-13). The tendon is coated in epimysium, a sheath of fibrous elastic tissue used for protection and separates one muscle from another, (Jarmey, 2008, pp. 28-29).
Deeper in the muscle is fascicles, bundles of muscle fibres, coated with perimysium, a connective tissue containing mainly collagen which is used to protect each fascicle. Each single muscle fibre is coated in a fine protective tissue called endomysium which includes blood vessels, nerves and lymphatics. Muscle fibres are the smallest unit of a fascicle, this is where muscle contraction takes place (Jenkins, 1996, pp. 101-102). Hundreds of cylindrical Myofibrils are inside each muscle fibre which are packed with protein strands and each myofibril consists of two types of protein thick and thin filaments. Within each muscle fibre the myofibrils line up with each other so that the z lines of the sarcomeres are adjoining in the next muscle fibre. The muscle fibre cannot contract unless the myofibril contracts. The sarcomere is a single contacting unit of a myofibril and has a striated nature due to the alignment of bands. The sarcomere is the unit between two Z lines, the Z lines are proteins which show where the sarcomere begins and ends (Krans, 2010). The thick filament inside the myofibrils are known as myosin and the thin filaments are known as actin, together with the troponin and the tropomyosin these build the structure of the muscle fibres (Krans, 2010). A single myosin molecule has a shape of a tail and a head, the head is what extends and forms the cross bridges. Approximately 200 myosin molecules form a single myosin thick filament and are arranged so that their tails are parallel to each other. The protein that makes the thin filaments are known as actin, which are the attachment point for the myosin heads. When contraction takes place the actin will bind with a ATP molecule assembling and dissembling itself when needed for contraction (Krans, 2010). The I band is made up of actin filaments and is located either side of the z line and extends over two sarcomeres, it is light in colour due to the thin actin filaments as there are no thick filaments present, the size of the I band depends on whether the muscle is relaxed or contracted. The A band is in the centre of the sarcomere and darker in colour as this is where thick and thin filaments overlap. The H zone is also located in the middle of a single sarcomere and is also dark in colour as it contains actin and myosin, but does not have any thin filaments. Together all of these layers and tissues work with one another to aid muscle contraction (Krans, 2010).
Nerve impulses travel from the brain or spinal cord to trigger contraction of skeletal muscles. A message travels down a motor neuron to the muscle fibre, the site where the motor neuron excites the muscle fibre is known as the neuro muscular junction. This connection is called chemical synapse and is the point where the axon terminals and the motor end plate of the muscle fibre meet. An action potential travel down the motor neuron to the axon terminal, and voltage gated calcium channels open and calcium ions diffuse into the terminal. Calcium entry causes the synaptic vesicles to release acetylcholine by a process called exocytosis. Acetylcholine diffuses across the synaptic cleft and binds to the acetylcholine receptors which contain ligand gated cation channels. The ligand gated cation channels open, and sodium ions enter the muscle fibre and potassium ions exit. Once the membrane potential reaches threshold value the action potential spreads along the sarcolemma. Neuro transmission to the muscle fibre ceases when acetylcholine is removed from synaptic cleft.
Motor neurons from the brain or spinal cord travel to muscle fibres for muscle contraction, this is known as excitation contraction coupling. Within a muscle fibre an action potential travels across the whole sarcolemma and then enters through transverse tubules which makes contact with the calcium storage called sarcoplasmic reticulum. The Sarcoplasmic reticulum forms sacs called terminal cisternae around a T tubule which is known as a triad. Membranes from a T tubule and terminal cisternae are linked together by proteins that control calcium release. As action potential travels down the T tubule it causes voltage sensitive protein to change shape. This change causes the calcium release channels to open allowing calcium ions to flood the sarcoplasm. This rapid invasion of calcium triggers contraction of the muscle fibre.
Muscles will relax or contract when receiving signals from the nervous system. The muscle contracts when the thick myosin filament and thin actin filament slide across each other. Myosin is anchored at the centre of the sarcomere called the M line, and the actin is anchored at the outside of the sarcomere called the Z line, and because of this when muscle contraction occurs the sarcomere shortens on both sides and the myosin pulls the actin along, shortening the muscle fibre by around 30 to 40 percent (Bragg, 2013). The cross bridges of the myosin filament attach to the actin binding sites and force them to move towards the centre known as the sliding filament mechanism which was discovered by Andrew F. Huxley and Rolf Niedergerke in 1954 (Krans, 2010). A contraction begins when a bound ATP molecule is hydrolysed to ADP and phosphate, this causes the myosin head to extend and attach to a binding site on actin to form what is known as a cross bridge. An action called power stroke is triggered allowing myosin to pull the actin toward the M line and shortening the sarcomere. ADP and phosphate are released during the power stroke. The myosin remains attached to actin until a new ATP molecule binds, therefore freeing the myosin to either repeat the process and bind again or to relax. This process is completely controlled by calcium, the thin actin filament is associated with proteins called troponin and tropomyosin. When a muscle is relaxed tropomyosin blocks the cross bridge binding sites on actin, when calcium levels are high enough and ATP is present, calcium ions bind with troponin and displaces tropomyosin exposing the binding sites on acting allowing the myosin heads to form a cross bridge (Krans, 2010).

To finalise, understanding normal human anatomy is paramount when working in the community health sector to be able to diagnose and treat patients who are receiving care for illness. The muscular systems intricate make up is accountable for every body movement a person makes, however it is dependent on the nervous system as billions of neurons that are constantly touching each other monitor and regulate sensory functions and your conscious mind relays an impulse to the central nervous system which then sends another impulse to your peripheral nervous system and then to the specific nerves that are located at the muscle that is needed to contract. These systems interlock and work together to make posture, movement and organ function possible. Without an understanding of normal anatomy patients who suffer with multiple sclerosis would not get the correct treatment as the practitioner would not be able to identify the symptoms and the link between the muscular and nervous system (Bragg, 2013).

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