Muscle is the tissue of function in the muscular system. They have three basic tasks; movement, because muscles move the skeleton as well as the organs, posture, because we would collapse without muscle there to hold us up, and heat production, because when you exercise you produce heat. There are also four main characteristics that the muscle has. They are contractility, which means it can contract forcefully, excitability, which means it can be stimulated by nerves and hormones, extensibility, which means it can be stretched out under normal conditions, and elasticity, which refers to when muscles are extended beyond normal length they can recoil back. 
3 types of muscle:
-Skeletal Muscle: This muscle is striated, which means that you can see a distinct striped pattern. Skeletal muscle is also voluntary, which means you can control it. Sometimes the nervous system automatically controls these muscles, but we can exercise control over it. Skeletal muscle is multinucleated, which means it has many nuclei. 
-Smooth Muscle: Smooth muscle does not have any noticeable strips, which means it is not striated. It has only one nucleus. It is an involuntary muscle and is located in the intestines and controls blood vessel movement. 
-Cardiac Muscle: This muscle is unique because it is found only in the heart. Cardiac muscle only has one nucleus like smooth muscle, but is striated like skeletal muscle. It does not need the nervous system for it to contract, it contracts on its own. 
Skeletal Muscle: First lets start with the fascicles; these are bundles of muscle cells. Another name for a muscle cell is a fiber because they are very long and thin. The endomysium (collagen layer) is what wraps each individual cell, then the perimysium (collagen layer) wraps the fascicle. The epimysium (collagen layer) is the outer most layer that wraps all of the fascicles together, it is also called the fascia. Then there are tendons that connect the muscle to the bone; these are made up of the same collagen fibers that make up the endomysium, which keep the tendon from extending out past the muscle belly. This then hardens and forms the dense regular connective tissue. 
Smooth muscle: The fibers that make up smooth muscle are a lot smaller than skeletal muscle fibers; therefore, you can not see any striation. There are two variations of smooth muscle; that is single-unite and multi-unite smooth muscle. The single-unite muscle fibers are gathered into sheets or bands and they run parallel but are densely and irregularly packed together. There are plasma membranes that form gap junctions between two fibers and electrical signals flow rapidly over these low resistance bridges. Then there is the multi-unite muscle and they don't have gap junctions but are just mingled together.
Cardiac muscle:This muscle is striated and is found in the heart walls, and it collectively constitutes the myocardium. Cardiac muscle's structure resembles skeletal muscles structure. One aspect in which it is different is that the T-tubules in cardiac muscle are wider than those in skeletal muscle.
Muscle contraction is explained through the sliding-filament model wherein the head of the myosin myofilament connecting to the active sites of the actin myofilament then pulling. This causes the Z-disks to come together and the H-zone and I-band to shorten which ends up in the muscle contracting. It is important to know that the length of the actin myofilaments, myosin myofilament, and the A-band do not shorten. 
Now lets look at how the head of the myosin myofilament connects to the actin myofilament. There are two proteins called tropomyosin which is a strand that lays across the actin myofilaments and the troponin which lays on top of the tropomyosin. When the sarcoplasmic reticulum releases calcium ions into the sarcoplasm the calcium ions connect to the troponin causing the tropomyosin to uncover the active site of actin myofilaments. Now the actin myofilaments attract the head of the myosin myofilament and then the connect. 
There has to be an energy source though for the head to move and that is what you get when you break down adenosine triphosphate (ATP). After you breakdown ATP there is ADP and phosphate and these connect to the myosin myofilament head. The head uses the energy to move forward which is called the power stroke, when this happens the ADP and phosphate fall of and are collected so they can be remade into ATP using cellular respiration. Now for the head to be able to let go of the active actin myofilament site there has to be ATP that comes and connects to it, this is called the return stroke. The muscle is now at its relaxed position and is ready to contract at any moment. 
Terms and Principles
First lets define some parts of the muscle. There is the origin and it is the point at which a muscle's tendon attaches to the more stationary bone. Then there is the insertion and this is the point at which a muscle's tendon attaches to the more movable bone. Another term is the belly and it is the largest part of the muscle where the muscle cells are contained. 
Now lets talk about how muscles never work individually but always work in groups. The best way to describe this is to give you and example, so lets consider the way the biceps and triceps work together. You will need to know that the muscle cant push it can only pull. So think about this if you only had a bicep then you would be able to bend your elbow but how do you get your arm back straight, that is when you need the tricep to pull it back. Muscles work together like this all over the body. 
Muscles work as levers, since muscles cant push they have to pull on a lever to be able to move the body. Levers consists of a rigid bar called the fulcrum, the effort or the force, and the resistance. There are three types of levers and they are; first class lever, were the fulcrum is between the effort and the resistance like in the back of the neck, second class lever, were the resistance is between the effort and fulcrum like the ankle, and the third class lever, were the effort is between the fulcrum and resistance. 
1. The largest muscle in the human body is the gluteus maximus. 
2. The smallest muscle in the human body is stapedius which is located in the ear. 
3. There are 640 muscles in the human body. 
4. You need 72 muscles to speak. 
5. Your jawbones can exhibit 200 pounds on your molars. 
6. The tongue is the only muscle attached at one end and it is the strongest. 
7. Your eyelid's muscles are the fastest in the human body and can blink the eyelid 5 times in a second and blink 15,000 times a day. 
8. It takes between 5 and 53 muscles to smile. 
- Wile, Jay L., and Shannon, Marilyn M. The Human Body: Fearfully and Wonderfully Made!. Cincinnati: Apologia Educational Ministries, Inc., 2001. page-number. Print.
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- Pilgram, Gray, Cardiac Muscle Structure, http://www.buzzle.com/articles/cardiac-muscle-structure.html, buzzle.com, 2010-11
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