A muscle is a body part that can contract when stimulated by the nervous system. There are three main types of muscle found within the human body and skeletal muscle is the most predominant. The other types of muscle are smooth muscle and cardiac muscle. The majority of information relates to skeletal muscle unless stated otherwise.
Structure of Muscle
For the most part, skeletal muscles are connected to at least two bones unless they are the exceptions such as those connected to cartilage. They are connected to the bone by tendons which are elasticated tissue that can withstand large amounts of force.
The main part of the muscle is the body which makes up the majority of the muscles volume. The body is made up of fasicles, which are bundles of muscle cells, nerves, blood vessels and cartilage. The amount of muscle cells per fasicle can vary between 100's or 1000's and the cells are termed muscle fibres. They are elongated cells that often are as long as the muscle itself and they are wrapped within connective tissue. During embryonic development, a single muscle fibre is made up of several cells which fuse together to create one long muscle cell. Therefore, each muscle fibre has multiple nuclei which is gained from this fusion. The muscle fibres are surrounded by a membrane called the sarcolemma.
The cytoplasm of the muscle is called the sarcoplasm and it is in this that the mitochondria sit. Mitochondria are the main organelle responsible for providing the muscle with energy as they are the site of respiration. Also within the sarcoplasm are myofibrils which are a bundle of filaments that provide the contractile properties of the muscle. The two filaments involved are actin and myosin. Each myofibril is surrounded by the sarcoplasmic reticulum and T tubules. These two components are involved in activating the muscle contractions as the sarcoplasmic reticulum contains calcium and its release is essential for contraction.
Under the microscope, muscles appear to be striped and this is due to the order of the filaments. There are both thick and thin filaments present in a muscle and they are in a 2:1 ratio. The filaments are arranged into a sarcomere and the general structure has been transduced. A line called the Z line marks the end of each sarcomere and this anchors the filament in position. A M line runs straight down the middle of a sarcomere which also provides support. The thick filaments are referred to as the A band and the thin filaments are the I band. Where the two filaments overlap is the H zone.
The two filaments are made up of actin and myosin which are contractile proteins. The thin filaments are made up of G actin which are globular proteins linked together to form F actin. This is then arranged into a double helix and myosin binding sites can be found on the G actin. Two other proteins are found in the thin filaments - tropomyosin and troponin. The tropomyosin blocks the myosin binding site when the muscle is relaxed to prevent contraction and troponin is connected to the tropomyosin and will move it out of the way in the presence of calcium. The thick filaments are made up of myosin which have two subunits with one resembling a head and the other a tail. This conformation is essential to the movement of the muscle. The head has an actin binding site so that it can bind to the thin filaments.
The Sliding-Filament Model
In order for the muscle to contract, the filaments have to physically move which requires energy. The Sliding-Filament Model was proposed to describe how the filaments were able to move and cause the muscle to contract. The thin filaments slide past the thick filaments and so under the microscope one can see the I band shortening. The mechanism works as follows:
- Once calcium is released, the myosin binding site is displayed and the actin can bind to myosin. At this point ADP and a phosphate is attached to the myosin head.
- The actin is then pulled along the myosin through the connection of the myosin head.
- ATP then binds to the myosin head and allows the myosin head to be released from the actin.
- The contraction is completed and the ATP is hydrolysed so that the cycle can continously occur.
Skeletal Muscle Fibres
There are two types of muscle fibres found within different muscles. The first is slow-twitch fibres. These are usually found in the leg muscles and given the name, they contract slowly. The other type of fibre is the fast-twitch fibres and these are found in muscles such as the eye muscles. These can contract quicky. Therefore, they have different roles in muscle contraction and are used for different means. Slow twitch fibres will provide slow contractions and slow release and can be used for exercises such as running marathons. Fast-twitch fibres quickly contract and release, using up their energy stores rapidly and thus they are useful for short sprints.
Smooth muscle is found in internal organs and blood vessels. It is also found in structures that are not under voluntary control. It derives its name from its appearance under the microscope. Smooth muscle can have a number of different roles depending on where is is located. For instance, smooth muscle in the intestine is used for contraction to push food from one region of the GI tract to another. Another example is the control of blood movement through dilation or constriction in blood vessels. It has thick and thin filaments like skeletal muscle but they are arranged differently in smooth muscle. They are not arranged in sarcomeres and their contraction occurs along several different axes. The contraction of the filaments is also controlled with calcium.
Control of Smooth Muscle
Skeletal muscle is regulated by motor neurons whereas smooth muscle is under the control of autonomic neurons. It is the release of neurotransmitter that is responsible for contraction or relaxation of the muscle and this can be from sympathetic or parasympathetic input.
Types of Smooth Muscle
There are two main types of smooth muscle and this depends on their connection. Multi unit smooth muscle is where the cells are not connected by gap junctions and are therefore separated. They are surrounded by neurons and often found in the respiratory airways and large arteries. Single unit smooth muscle differs because it is joined by gap junctions. Therefore, electrical signals can easily be passed on to many cells. This form of smooth muscle is found in the GI tract.
The third type of muscle is called Cardiac muscle and it is similar to both skeletal and smooth muscle in some ways. It is arranged in sarcomeres and the mechanism of contraction is the same as skeletal muscle. However the muscle cells themselves resemble smooth muscle cells and are linked through gap junctions. The action potential transmitted through cardiac muscle lasts for a longer period of time than other muscles. This is important in ensuring that the heart beats at a regular rate. The cardiac muscle is said to be under myogenic control and it is regulated by the autonomic nervous system.
There are a range of different muscle disorders ranging from simple strains to genetic conditions. One genetic condition is called Muscular Dystrophy and the main symptoms are muscle weakness and loss. Over time, the patient will lose the ability to walk and breathe properly and therefore life expectancy is considerably shorter. Diseases of the nerves can also cause muscle disorders such as Multiple Sclerosis whereby the individual suffers from weakness and lack of coordination and control. Other nerve disorders include spinal muscular atrophy and Myasthenia Gravis. Although uncommon, there are also cases of cancers of the muscle which are referred to as soft tissue sarcomas.