A General Introduction To The Muscular System

The muscular system is responsible for movement in collaboration with the nervous system to form impulses for motion. Muscles also contribute to internal functions of the human body which include motion in the intestines and circulatory system. Exercise of this organ system is critical to prevent wasting from age or the possibility of disease. Muscles are placed under the body’s skin sorted in overlapping layers on the skeletal system. All muscle is not the same and be placed into three different types. This includes smooth, skeletal, and cardiac muscle. The human body can contain up to a range of 640 to 650 voluntary muscles. Like other organ systems of the human body, it functions almost like a machine. Various parts and structures perform a role to produce an output.The human body is capable of complex movements due to various organ systems . The muscular system also serves an essential role in athletic performance.

         All muscle is not the same. There are three major muscle tissue types responsible for various functions. The muscles that are the most recognizable are the skeletal muscles. These are connected to bone and produce movement. These muscles can sometimes be referred to as voluntary muscles. Under the microscope they appear striated. Skeletal muscle contains multiple cell nuclei located around the periphery of the cell. Not all skeletal muscle is under voluntary control. The chest wall of the body is automatic to allow for breathing.

Skeletal muscle
Skeletal Muscle ( microscopic image)

Smooth muscle often referred to as non-voluntary is responsible for actions of the nervous system. An example of this would be the dilating and constriction of arteries or movement in the stomach. These actions require automatic response to assist other organ systems. The smooth muscle cells have spindle like shapes. Narrow at a point to both ends attaching to one nucleus in the cells center.

Smooth_muscle_tissue
Smooth Muscle (microscopic image)

Cardiac muscle is the most essential. The responsibility of this tissue is to transmit electric messages fast and in the most efficient manner. Heart muscle is designed to be branched out. The heart needs this type of muscle tissue to ensure it beats correctly. Cardiac muscle can have two or more nuclei at the center of each cell. This demonstrates that organ systems do not work in isolation, but in a coordinated effort.

Cardiac muscle

Muscle cells are the smallest units of the muscular system. Together they form part of the human body mass. Muscle can account for close to 50% of total body mass. This depends on the physical fitness level of the individual and the amount of fat that is also stored in the body. There is a difference in muscle mass between men and women. Men’s muscle fibers are slightly larger in size. Women have higher body fat levels which can range on average 11%, which is higher than men’s range. At the cellular level, there is no difference between male and female muscle.

      Muscles can have a superficial layer and others are deep with in the body. The names of muscles do correspond to their shape. The trapezius obviously takes its name from trapezoid. The muscle that rotates the shoulder blade has that distinct shape. The human body contains many muscles from the face down to the lower body.

Muscle anatomy

Muscular anatomy 2

Each muscle moves are particular part of the body. The muscles of the arm include the deltoid, biceps brachii, triceps, flexor carpi radialis, and barchiordalis. The biceps brachii flexes the forearm at the elbow and can pull the palm upward. The deltoid is capable of moving the away from the body to the front, side. or rear. The triceps have a medial head which covers the forearm at the elbow and can straighten it. There is also a long head of the triceps that allows for straightening of the the forearm. The brachialis brings the forearm to the shoulder. The arm when flexed at the elbow happens due to movement by the brachiordalis. When the hand is flexed at the wrist it is done by the flexor carpi  radialis.

The trunk of the body aids in the movement of the neck and other limbs branching out from the human body. The pectoralis major moves the shoulder blade. The internal intercostal pulls adjacent ribs together while the external intercostal elevates them. The scalenus has to maintain proper assistance in breathing and  the flexing of the neck. The rectus admoninius  flexes the spinal cord, while directing the pelvis forward. The linea  alba separates divides the abdominal muscles .The serratus anterior has to pull shoulder blades away from the spine. Despite its long name the sternocledomastoid only serves the function of tilting and moving parts of the neck.

There are also muscles of the back. It should be noted that while muscles are named in accordance with shape, sometimes names come from the bones they are connected with. The illocostalis runs from the one back muscle to the ribs reaching ilium. The tres major and minor contribute to lifting the arm and stabilizing the shoulder. The infraspinatus allows for rotation of the arm the stabilization of the shoulder joint. The supraspinatus can raise the arm when needed. The rhomboideus major and minor are designed to retract the shoulder blade and return it to a rest position. For the spine to be straightened it require the use of the erector spinae. This muscle is made of three other muscles, which include the spinalis, longissimus, and  illocostalis. The latissimus dorsi rotates arms and pulls shoulders back. It has the largest surface area of any muscle in the human body.  the obliques assist in breathing and abdominal wall stabilization. Internal obliques must focus on pressure inside the abdomen. Obliques both internal and external have the ability to flex and rotate the trunk.

Continuing further there are muscles of the lower body and legs. When humans became bipedal during the course of evolution, this was a major milestone. Humankind is the only primate species that can walk upright. The hamstring consists of three muscles which include the biceps femoris ( rotates the leg ), semitendinosus ( extends thigh, hip and flexes the knee), and semimembranosus (extends thigh, rotates leg and flexes the knee). The hips get stabilization from the quadratus femoris. The muscle also allows for rotation. The gluteus maximus has to straighten the hip by pulling the thighs back in the event of running, walking or jumping. The muscle is not alone in the locomotion process; the soleus has to flex the foot when walking happens. The gastrocnemius which is a calf muscle must contract to flex the ankle and pull the heel up. This is done when standing on a tip-toe position. There is a muscle that also gives the leg thrust when walking called the flexor hallucis longus. The toes and foot itself need control and that function falls on the flexor digitorum longus. The knee also has more assistance with movement with the popliteus. The adductor brevis  rotates thighs in toward the body, while the adductor longus draws into the body. Gracilis also flexes and rotates that leg as needed. There are also muscles in your face which aid in expressions you make. Whether it is a smile or grimace muscle once again work together.

The muscles of the face can be either attached to bone or connective tissues known as aponeuroses. The presence of this fibrous and sheet structured tissue means that facial muscles are connected to one another. The muscles of the head and neck interact to produce facial expressions, which can vary depending on emotional state. The facial muscles are controlled by a nerve known as cranial VII. If damage is inflicted on this nerve facial mobility would be lost, making it difficult to communicate. The structures such as the occipitofrontalis raise the eye brows and zygomaticus major pulls the corner of the mouth up and out. The sternohyoid depresses the larynx, while the platysma  lowers the mandible and covers the mouth. The muscles of the face and neck are critical for speech and even a function as simple as chewing. Facial muscles can fill the role as sphincters ( such as the orbicularis oculi  that allows your eyes to close). The function of sphincter is to open and close a particular orifice. Besides basic functions of eating, talking, or drinking the head must be balanced on the body. The average adult head weights about 5kg ( 11 lbs). Muscles of the neck , back and shoulders must act as a stabilizing force. They are active in a constant fashion tensing and contracting based on particular movements.

The rectus capitis posterior( minor and major) ,  oblique (inferior and superior) , levator scapulae, semispinalis captis and splenius captis are needed for balance of the head on the body. When moving the face and the neck it is hard to image such labor has to be put in to make a simple action possible. Chewing requires the force of the masseter muscle. Without it the mouth could not close the mandible when eating. This is only a general delineation of the structure of the muscular system. While the estimate of skeletal muscles is placed at 650, more could be classified as such. This depends on if a muscle is classified individually or part of a much larger skeletal muscle.

         Skeletal muscle tends to be the majority of the muscular system, when compared to cardiac and smooth muscle. Skeletal muscles have extra classification based on their unique shape. The two bellied parallel muscle contains two separate muscles separated by an intersection of sinew. Three headed parallel muscle is designed to attached to three different places. The quadrate muscle forma a flat four sided shape, which acts more like protective covering.  The straight muscles run parallel converging at a tendon point. Orbiculars are muscles that function like  sphincters and triangular muscles form a fan shape. The reason for this is to allow for the maximum force of contraction. These muscle also have a common site of attachment. Two headed parallel muscle splits at a point nearest to the body. Flat muscles serve mostly as a covering. The abdominal wall would be considered a flat muscle. Fusiform muscles have fibers that are positioned parallel to each other in the middle then meet to form a tendon at one or two ends. These muscles are responsible for moving the fingers, specifically the flexor pollicus longus. The last type of skeletal muscle is the pennate muscles. They contain strong muscle fibers, but fatigue quickly. They have a feather like appearance.

      The physiology of the muscular system is just as impressive as its numerous structures. The skeletal muscle is connected to tendons, which means they can only pull when contracting. During this process they shorten. Muscle in a controlled setting can achieve a threshold stimuli and can respond to the next stimulus without relaxing completely. Tetanus happens when full sustained contraction is achieved. The tetanic contraction happens when muscles are actively used. Even when there is no active motion muscles still exhibit tone. Muscle tone refers to fibers still contracting while at rest. if this did not happen during rest the body could collapse around the neck, trunk, and legs.

Muscle tone is the key to maintaining good posture. The muscle fibers are packed into bundles. The muscle fiber cell has various components. There is a plasma membrane known as the sarcolemma, which forms a transverse system. The T tubules move down into the cell making contact with the sarcoplasmic reticulum. There are storage sites present for calcium. The sarcoplasmic reticulum is home to thousands of myofibrilis which are contractile muscle fiber tissue. The myofibril has sarcomeres which are contractile units. The myofibrils are cylinder shaped and can be as long as the muscle fiber. On the tissue itself are striations, which are formed by the sarcomeres. When in a state of rest dark lines known as Z lines. What should be understood is that the muscle fiber functions on different protein filaments. Actin filaments slide past myosin, which induces contraction. Myosin pulls the actin through cross bridges including split ATP. The process can be described as the sliding filament model.

1022_Muscle_Fibers_small

P49xp
Adenosine Triphosphate  (ATP)

The sarcomeres shortens causing the actin filaments to slide past the myosin filaments. The I band will shorten and the H zone disappears. This is how muscle contractions behaves. Like two people doing a short of tug of war, this shows how the filaments move. Besides ATP, myoglobin stores oxygen and phosphocreatine helps with energy needs. Phosphocreatine does not directly get involved in muscle contraction, but has the ability to anaerobically regenerate ATP.  This helps supply enough energy for muscular contraction. Once the phosphocreatine is depleted the mitochondria can produce enough ATP for muscular contraction to proceed. Another method for supplying energy is fermentation. This does not require oxygen, but can happen during strenuous exercise. ATP can only be supplied for a short period. Lactate then amasses and there will be fatigue. Lactate may not cause muscles to ache, but rather act as a protective measure to prevent harm to the muscle.

This explains why at some point when exercising intensely muscles will fatigue. Breathing patterns even change during long term intense exercise. The oxygen debt has to be restored. This means cells must return to their original energy state. The intake of oxygen must complete the metabolic process for lactate. The lactate must be transported to the liver, where it will be broken down into carbon dioxide and water ( 20% ) . There will be ATP gained from respiration, in which 80% of the lactate will be converted into glucose. People who consistently train ( athletes for example ) the number of mitochondria increases which decreases reliance on fermentation. As a result ATP is produced more efficiently and there is less of both lactate and oxygen debt.

        While there is very little difference in structure and function of the muscular system between the sexes, there is one aspect that effects athletic performance. Men have more type II muscle fibers. Fast twitch muscle fibers generate more power, but fatigue at a faster rate. They are not oxygen dependent. Women have more type I muscle fibers which are more resistant to fatigue and can contract for longer periods. The slow twitch fibers get their energy for contraction  from the break down fat from blood, muscle cells, and adipose tissue. This explains why women have more of these muscle fibers, because estrogen means women will have higher body fat percentages. Fat breakdown needs only oxygen and some glycogen . Fast twitch requires phosphocreatine and glycogen reserves in the muscle itself. Glucose remains stored in the blood and glycogen in the liver. Sex is not the only factor in muscle fiber composition. It can be based on either genetics or physical fitness training. Marathon runners would have more type I muscle fibers, compared to the type II weightlifters. The difference in physical strength is not just due to the size of the muscle fiber, but endocrinology. Men produce higher levels of testosterone which allows for a greater amount of muscular hypertrophy. This explains why a man and a woman who do the exact same training regimen, it is more likely the man will have more absolute strength.

It is not just the difference in size of the muscles or endocrinology. The difference in absolute strength comes also from the nervous system. Men’s nervous system signals faster when muscle contraction is occurring. The common misconception is that women would not benefit from weight lifting because they have smaller muscles. Women’s muscles still can experience hypertrophy. The difference appears to be mostly in quantity, rather than quality of the muscular structure. Then one must considered starting point. If men have lower body fat levels and more muscle mass to begin with their total strength would be higher. Women would gain more relative to their size, seeing as their natural strength levels are lower. People with mesomorphic body types will have a natural advantage when engaging in a train regimen. The female body is capable of developing strength. The muscular system is almost like a multilayered puppet with tendons and muscles working in collaboration. This organ system is pivotal to athletic performance and overall health.

 References

Mader, Sylvia. Biology. New York : McGraw-Hill, 2007.

Parker, Steve. The Human Body Book. New York: DK Publishing, 2013.

Brewer, Sarah. The Human Body: A Visual Guide to Anatomy. London: Quercus, 2012.

 

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A General Introduction To The Muscular System

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