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Chapter six begins with an overview of muscle types including skeletal, smooth, and cardiac muscle. It discusses their differences along with their similarities in microscopic appearance and in the particular way each one functions. The anatomy of a muscle is showed both from the endomysium that wraps one muscle fiber, all the way to the epimysium that wraps an entire muscle. Then, functions of the muscle are discussed, including movement, maintenance of posture, joint stabilization, and heat generation.
As we learned in class, the muscular system is responsible for all the muscles that move the body including things within it. There are three main types of muscles such as the skeletal which are attached to the skeleton, smooth, which is found in the digestive tract and in blood vessels, and cardiac which is only found in the heart. Cardiac muscle causes contractions called systole. Systole pumps the blood out of the heart and around the body while supplying the oxygen and other vital substances to cells. Skeletal muscle is attached to tough connective tissues called tendons, which attaches to the bones. The contractions of the skeletal muscles cause the tendons to pull on the bones, which results in the movement of limbs. Cardiac and skeletal muscles share only one similarity between their structures. Both of their structures are striated and they are formed by actin and myosin myofilaments. They both are packed together closely and tightly into organized patterns so that actin is allowed to slide over myosin during contraction. All movement of the skeletal muscles occurs with your own control, therefore, it’s voluntary. On the other hand, smooth muscle contracts without being required to think about it, therefore the muscle action is called involuntary. Smooth muscle transports food through the digestive tract as well as blood throughout the whole body. Cardiac muscle also moves involuntarily. This results is our heart constantly beating while blood flows to it without being required to think about it. In addition cardiac muscle can be considered one of the strongest muscle tissue due to our hearts beating continuously throughout our entire lives.
Muscle tissue is made out of cells that have abilities to both shorten and or contract in order to allow for movement. The tissue is cellular and is greatly supplied with blood vessels. Due to the cells characteristics being long and slender, they are commonly referred to as muscles fibers. These muscle fibers are arranged in bundles surrounded by connective tissue. Proteins called actin and myosin are the main contractile proteins found in the muscle tissue.
When looking at muscle fibers they appear to be cylindrical, multinucleated, striated, and under voluntary control. Smooth muscle cells have a spindle shape, lack striations, are uninucleated, and are involuntary muscles. Cardiac muscle has branching fibers, striations, intercalated disks, along with having one nucleus per cell. Also, it is not under voluntary control. 
The next section of the chapter explains the microscopic anatomy of the skeletal muscle, as well as  the mechanism of muscle contraction. There are three types of different muscles in the body such as striated, unstriated, and cardiac. As stated before, striated muscles attach to bones and help move them. These muscles are known as skeletal muscles. As discussed in class, muscle contractions must go through three different phases. These include the the latent phase, contraction phase, and the relaxation phase. The latent phase can happen between the beginning of an action potential along with the start of a muscle contraction. The contraction period starts at the end of the latent phase and ends as the same time as muscle tension ends. The relaxation period occurs at the end of the contraction period until the muscle becomes free of tension. However, In order for a muscle contraction to happen, the stimulus must reach its threshold. Muscle contraction can be looked as a series of different processes of transmission of action potentials from one neuron to another. A chemical called acetylcholine is the neurotransmitter which is released from the presynaptic neuron. As the postsynaptic cells on the muscle cell membrane receive the acetylcholine, the channels for the cations sodium and potassium are opened. These cations create a net depolarization of the cell membrane and this electrical signal travels along the muscle fibers. Through the movement of calcium ions, the muscle action potential is put into muscle contraction with the interaction of two types of proteins, actin and myosin. 
Skeletal muscle tissue fatigues much more quickly compared to cardiac or smooth muscles. Due to cardiac muscle being responsible in delivering blood, fluids, oxygen, nutrients, and other substances very vital to our being, it must act fast. Cardiac muscles are able to hold  large amounts of blood supply and are specialized in design to avoid becoming fatigued for this exact reason. Since the lack of oxygen can immediately lead to death within minutes, cardiac muscles must be able to fulfill their duty in transporting oxygen very quickly. Also, cardiac muscle mitochondria assist with energy production capacity. It is said that skeletal muscle mass becomes greater when there is a heavy amount of force extorted on it. These muscles are much more bigger than smooth and cardiac muscles because of this. In addition, both cardiac and smooth muscles function on a subconscious, or involuntary, basis. It would obviously be impossible to make the decision to operate our heart along with maintaining a constant rhythm. Causing our heart to beat to fast would result in high blood pressure. Too slow of a heart beat could result in low blood pressure and acquiring low energy. Skeletal muscles only require voluntary contraction. However, sometimes by hitting the knee, it can cause a reflex action. Though, usually, it must be a voluntary action for us, or else we would have no control over the movement and placement of our body to do everyday tasks such as walking, standing, running, and even eating.

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