Muscular System

Muscles of the body

Muscle origins and insertions

The origin of a muscle is the point at which it attaches to a bone (usually) or another muscle. The structure that the origin is attached to is not moved by the contraction of the muscle^. The opposite end of the muscle is called the insertion. This definition means that there is a functional aspect to the definition of a muscle's origin and insertion. Both origin and insertion are important for understanding the physiological function of the muscle.

Muscle Functions

Agonist or prime mover

This is the muscle which is responsible for movement and lengthens during contraction

Antagonist

This is the muscle assisting the movement and lengthens during contraction.

Fixator

These are the muscles supporting the movement and providing a base for the other muscles to pull against. Fiaxtors stabilise the origin. They stabilise the origin so that the agonist can achieve maximum and effective contraction.

Synergists

These are the other muscles which prevent any unwanted movement during the action. They work together to enable the agonists to operate more effectively. Synergists add a little extra force to the same movement to help agonists. 

Origin

This is the part of the body usually a bone where the muscle attaches and does not move when the muscle contracts

Insertion

This is the part of the body where the muscle attaches and moves when the muscles contracts.

Types of muscles

Cardiac

 This type of muscle only occurs in the heart and forms the bulk of the wall of the heart. It is specifically found in the myocardium, which is the middle and the thickest layer of the heart wall. This muscle is responsible for pumping blood through the heart chambers and into the blood vessels the heart beats non stop about 100,000 times each day, it can do this because of the cardiac muscle. Its does this by contracting when it is relaxed it fills your heart with blood. Unlike other muscles the cardiac muscles never gets tired, it works constantly without pausing to rest. It consists of specialised fibres which do not tire. The cardiac muscle is also controlled but it is also an involuntary muscle which can continue to function without the generation of nerve impulses, so it works automatically. The heart muscle has a built in pacemaker known as the sino-atrial node. The sino-atrial node controls the rate of the heartbeat. The rate of which the heart beats is involuntary but it can be influenced by factors such as stress, medication, illness and exercise. These influences change the reaction of the nervous system and the hormones that are released, this results in a change of heart rate. Cardiac muscle cells rely on a blood supply to deliver oxygen and nutrients and to remove waste products like carbon dioxide. The cardiac muscle is adapted to be highly resistant to fatigue, so it has a high number of mitochondria. The cells are y shaped and are shorter and wider than skeletal muscle cells. Some of the cardiac muscle cells are auto-rhythmic. Cardiac muscle is reliant on oxygen to function. It also has its own blood supply.  

Skeletal

 Skeletal muscle tissue are named for there location, they are attached to bones by tendons. Skeletal muscles cover your skeleton which gives your body its shape. When you work out you will gain good muscle tone and shape and it is skeletal muscle you are training.The entire muscle is surrounded by a layer of connective tissue which is called the epimysium. The epimysium is a connective tissue that surrounds and holds muscles in the body. The epimysium provides a smooth surface which allows other muscles to glide on. Within the muscles are large bundles of fibres or fasiculi surrounded by the perimysium. The perimysium is a membrane that protects and supports groups of fibres within the skeletal muscles. Each individual fibre is wrapped by a thin layer of connective tissue layers which are connected to each other so that when the muscle fibres contract they are ultimately linked to the tendons, this creates movement. Skeletal muscles control movement so you control what they do and they are voluntary. Most of your body movements are controlled by skeletal muscles contracting. Skeletal muscle is also important for keeping your bones in the correct position and prevents your joints from dislocating. Skeletal muscles come in many different sizes and shapes to allow them to do many types of jobs. Some of your biggest and most powerful muscles are in your back, near your spine. These muscles help keep you upright and standing tall. They also give your body the power it needs to lift and push things. They also generate heat as a by product from muscle activity, this is heat that keeps your normal body temperature. Skeletal muscles also have the ability to stretch or contract and still return to their original shape. Two main types of striated skeletal muscle can be distinguished on the basis of their speed of contraction. Type1 and type 2.        

Smooth

This is concerned with the movements of internal organs. So it is found in the walls of organs like the stomach, the intestines, blood vessels and urinary bladder. Smooth muscles are used for many functions. Muscles in your bladder wall contract to expel urine from your body. When they're relaxed, they allow you to hold in urine until you can get to the bathroom. Then they contract so that you can push the urine out. Smooth muscles in a woman's womb help to push babies out of the body during childbirth and the muscular walls of your intestines contract to push food through your body. Smooth muscles work in your eyes as well, the muscles help the eye focus.  You can see rows of smooth muscle cells running circularly around blood vessels, especially prominent around muscular arteries Smooth muscles are arranged in layers with the fibres in each layer running in a different direction. This makes the muscle contract in all directions.  Smooth muscles are involuntary muscles and work automatically. I.e. you’re not under conscious control. Your brain and body tell smooth muscles what to do without you even having to think. It is well adapted to producing long slow contractions. Some smooth muscle tissue can undergo hypertrophy. The structure and function is basically the same in smooth muscle cells in different organs. Most smooth muscle is of the single-unit variety, that is, either the whole muscle contracts or the whole muscle relaxes. Smooth muscle containing tissue tends to demonstrate greater elasticity and function within a larger length-tension curve than striated muscle. This ability to stretch and still maintain contractility is important in organs like the intestines and urinary bladder. In the relaxed state, each cell is spindle-shaped, 20-500 micrometers in length. Smooth muscle fibres contain no striations and sarcomeres.      

The smooth muscle works like the sliding filament to contract the muscle cells.  Intermediate filamets (desmin and vimentin) help with the contraction by pulling the cell ends in (shortening the cell).

Differences and similarities of the type of muscles  

A similarity of cardiac muscle and skeletal muscles is that they are multinucleate and striated. This means they have one or more nucleus in one cell and have fibres that have combined into parallel fibres. This is different to smooth muscles because they are not striated or multinucleate. A similarity of cardiac and smooth muscle is that they both have nuclease centrally located. Cardiac muscle differs from both skeletal muscle and smooth muscle because it has cells that are branched and are joined to one another by an intercalated disk. Intercalated disks allow communication between the cells. Cardiac muscle also differs from the other two muscle types in that contraction can occur even without an initial nervous input, these cells are called pacemaker cells. Another similarity between cardiac and smooth muscles are that they both are involuntary and work automatically and a skeletal muscles is voluntary e.g. when you kick a ball you have to think about it and this will allow your skeletal muscles contract to allow movement. A difference of all the muscles types are that they are all found in different places. The skeletal muscles are found on the skeleton, the cardiac muscle is found in the heart and the smooth muscles are found in internal organs. Another difference is that they all do different jobs. The skeletal muscle makes you move and protects your skeleton, the cardiac muscle pumps blood around the body and fills your heart with blood and the smooth muscles help push out babies and push food through your body. The cardiac muscle never gets tired, it works constantly without pausing to rest.    

Muscle fibre types

Type 1 (slow twitch or slow oxidative fibres)

Type 1 fibres are also known as slow twitch or slow oxidative fibres. They contain large amounts of myoglobin which make them red in colour, they also have many mitochondria and many blood capillaries. This type of fibre split ATP at a slow rate and has a slow contraction velocity. These fibres are very resistant to fatigue and have a high capacity to generate ATP. The slow muscles are more efficient at using oxygen to generate more fuel (known as ATP) for continuous, extended muscle contractions over a long time. Slow twitch muscle fibres use oxygen for power. Type 1 fibres suit activates that need endurance. Athletes such as marathon runners have a high number of this type of fibre. Marathon runners have a high rate of type 1 fibres because they run for a long time at a steady speed without getting fatigue. Paula Radcliffe would have a high number of type 1 fibres because she is a marathon runner and has to run for a long time without getting fatigue. You can tell she has a high rate of type 1 fibres because she can run for a long time at the same speed without getting fatigue. Also slow twitch fibres are great at helping athletes bicycle for hours and long distance swimmers because they can go for a long time without getting fatigue. Type 1 fibres are generally employed at the beginning of exercise, regardless of the intensity of exercise.

Type 2 A fibres (fast twitch or fast oxidative)

These fibres are called type 2 A or fast twitch or fast oxidative fibres. They contain very large amounts of myoglobin, mitochondria and blood capillaries like type 1 fibres. They also are red in colour like type 1 fibres. These fibres split ATP at a very high rate and also have a very high capacity for generating ATP. They have a fast contraction velocity and are resistant to fatigue. They can use both aerobic and anaerobic metabolism almost equally to create energy. In this way, they are a combination of Type I and Type II muscle fibers.  400m runners would have a high number of type 2A fibres because the race is not completely aerobic or anaerobic it’s in between, so they have to run at a fast pace for a while. Michael Johnson would have a lot of type 2A fibres because he was a 400m runner and had to run intensely for 400meters.   


Type 2 B fibres (Fast twitch or fast glycolytic fibres)

These fibres are also called fast twitch or fast glycolytic fibres they contain a low content of myoglobin so they are white in colour. They also contain low mitochondria and low blood capillaries content but have large amounts of glycogen. Type 2 B fibres generate ATP anaerobically and are not able to supply skeletal fibres continuously with ATP so fatigue easily. The purpose of this type of muscle is to provide rapid movement for short periods of time. Fast twitch muscles do not use oxygen - they use glycogen. Reactions using glycogen require anaerobic enzymes to produce power. They split ATP at a fast rate and have a fast contraction velocity. 100m sprinters have a high rate of type 2B fibres because they sprint for a short time but intense. Usain Bolt will have a high rate of type 2B fibres because he is a 100m sprinter and has to sprint intensely for 100m, you can tell he has a high rate of type 2B fibre because he looks fatigue after a race.      

Differences and similarities of muscle fibre types

A similarity between Type 1 fibres and Type 2A fibres are that they both contain a large amount of myoglobin. This makes both fibres red in colour. Type 2B is different to these two fibres because it is white in colour because it has a low content of myoglobin. It is also different to type 1 and type 2A because it has a low mitochondira and blood capillaries content and the other two have a high content of mitochondria and blood capillaries. A similarity between type 2A and type 2B are that they both split ATP at a very fast rate and have a fast contraction velocity. Type 1 is different to both other fibres because it splits ATP at a slow rate and has a slow contraction velocity. Another similarity between type 1 and type 2A fibres are that they both are resistant to fatigue. This is different to Type 2B because they fatigue easily. Another similarity between type 1 and type 2A fibres are that they generate ATP at a very high capacity but type 2B is different because they generate ATP anaerobically. A difference between all muscle fibre types is that type 1 fibres are found in large numbers in the neck, type 2A fibres are infrequently found in humans and type 2B fibres are found in large numbers in the muscles of the arms. Type 2 fibres adapt to high intensity anaerobic exercise involving explosive or powerful movements, but they are also increasingly employed during low intensity endurance workout as performer fatigue increases.    

Muscle Contraction

Isotonic contraction

Isotonic contractions are those which cause the muscle to change length as it contracts and causes movement of a body part

Concentric

This is the main type of muscle contraction. In this type of muscles contraction the muscles gets shorter in length and the two ends of the muscle move closer together. This happens when the muscle contracts. This type of contraction is most common type of muscle contraction and occurs frequently in daily and sporting activities.

 An example of an isotonic contraction is when we flex the bicep muscle. The bicep muscle shortens as it contracts, the two ends of the muscle gets closer. Another example is a squat and a pull up.  Concentric contractions are common to many sports in which you need to generate a lot of power or explosive force.

Eccentric

Eccentric contractions are the opposite of concentric. So in this type of muscle contraction the muscle increases in length as it contracts. The two ends of the muscle move further apart. This type of contraction is normally evident in a

Downwards phase in a movement. An example is lowering phase of a bicep curl. Another example is when when kicking a football the Hamstrings contracts eccentrically to decelerate the motion of the lower leg. Another example is walking .

Isometric

In this type of muscle contraction the muscle stays the same length and doesn’t change shape. There is no movement of the muscle or body part that is attached. Balancing or pushing against something is an example of an isometric contraction so a ski squat is a good example. Another example is when you grip something e.g. a tennis racket. There is no movement in the joints of the hand, but the muscles are contracting to provide a force sufficient enough to keep a steady hold on the racket. Another example is during the crucifix position on the rings in gymnastics. Tension occurs in the muscle but the distance between the ends stay the same. Other examples are a wall sit, holding free weights at a static position and the plank.     

Differences and similarities of muscle contractions

A difference between the muscle contractions is the concentric contraction gets shorter in length when they contract, eccentric contractions get longer when they contract and isometric contractions stay the same when they contract. 

Muscle Structure

Each fibre itself is made up of smaller fibres called myoribrils. It is in here that the contractile process takes place in very small units called sarcomeres

Sarcolemma is the cell membrane.

Sarcoplasmic reticulum is a network of internal membranes that run throughout the sarcoplasm and are responsible for the transportation of materials within the cell.

T vesicle is a sac containing chemicals needed to start muscle contraction.

Sliding filament theory

At first the muscle is relaxed. To get the muscle to contract the actin has to be brought close together. To get the actin together the myosin has cross bridges which pull them near each other but the actin has proteins tropmyosin and troponin which stop the cross bridges from pulling them together. Actin is a blue filament and myosin is a green filament, they work together to produce these contractions, as they are arranged in filaments that slide past each other, giving sliding filament theory its name Troponin and tropomyosin are proteins that form part of the thin or actin filament. The tropomyosin is rod shaped and stiffens the actin core. Tropoin binds to the tropomyosin and helps it bind to the actin. To get rid of the troponin and tropmyosin, calcium (Ca++) which is an Ion comes along and breaks them off which allows the cross bridges to pull the actin together which makes the muscle contract. The skeleton acts as a major mineral storage site for the element and releases calcium ions into the bloodstream under controlled conditions. The ions are stored in the sarcoplasmic reticulum of muscle cells.