Learning Objectives
You use various muscles in your body throughout the day. Muscles are what allow you to move. Even very basic things that you do often require you to use your muscles – getting out of bed, eating, walking, and playing all use muscles! Certain parts of your arms and legs helped you do certain things, like a throw or catch a ball, kick a ball, run, etc. Can you think of some other things you do that use those same muscles? How about jumping, skipping, doing cartwheels or summersaults, playing tag or leap-frog, helping set the table or wash the dishes, brushing your teeth, and putting away your toys?
Flex your arm like the boy in the picture below. Feel the muscle of this arm. That's your biceps - a muscle on the front part of the upper arm.
Try doing the stretching exercise like shown below. Can you feel the stretch in your thigh? Those are your "quadriceps" in action. The quadriceps muscles consist of four large muscles at the front of the thigh
The muscular system is the organ system that produces movement. It is composed of specialized cells called muscle fibers.
Muscle is contractile tissue and is derived from the mesodermal layer of embryonic germ cells. It produces force and causes motion, either locomotion or movement within internal organs.
Much of muscle contraction occurs without conscious thought and is necessary for survival, as the contraction of the heart or peristalsis, which pushes food through the digestive system. Voluntary muscle contraction is used to move the body and can be finely controlled, such as movements of the finger or gross movements like those of the biceps and triceps.
There are over 650 muscles in our body. The muscles work with the skeletal system to produce movement, posture, and balance. The skeletal system together with the muscular system forms the musculoskeletal system.
Muscles are connected to bones by connective tissue called tendons. Tendons help form a connection between soft contracting muscle cells to hard bone cells.
Muscle makes up around half of the total human body weight. Muscle tissue is also around 15% denser than fat tissue.
Our longest muscle is Sartorius. It runs from the hip to the knee and helps us bend the knee and twist our leg.
The strongest muscle is in our jaw and is used for chewing.
The smallest muscle is in our ear and is called the 'stapedius'. It is attached to the smallest bone in the body, the stapes.
Let's look at some of the major muscles in the human body.
The below illustration shows the location of the major muscles in the human body.
There are three kinds of muscles:
Let's learn about each of these types of muscles in greater detail.
Skeletal muscles are composed of muscle fibers, or myocytes, which are in turn made up of myofibrils, which are made up of sarcomeres. The sarcomeres are the basic building blocks of the striated muscle tissue. Once stimulated by an action potential, the skeletal muscles conduct a coordinated contraction by way of shortening each sarcomere. In the sarcomere, myosin and actin fibers overlap in a contractile motion towards one another. Myosin filaments have heads that are club-shaped projecting towards the actin filaments.
Larger structures found along the myosin filament known as myosin heads are used for the provision of attachment points on the binding sites for the actin filaments. The myosin heads swivel toward the center of the sarcomere, detach and reattach to the nearest active site of the actin filament. This is referred to as the ratchet type drive system.
This process uses a large amount of ATP (adenosine triphosphate), which is the energy source of the cell. There are about 639 skeletal muscles in the body of a human.
The autonomic nervous system directly controls the smooth muscles. These muscles are involuntary; this means that they cannot be moved by conscious thought. Heartbeat and lungs (capable of being willingly controlled) are involuntary muscles but they are not smooth muscles.
Heart muscles are different from skeletal muscles since the muscle fibers are laterally connected to one another. In addition, their movement is involuntary just as with smooth muscles. The sinus node controls the heart muscles. The sinus node, on the other hand, is influenced by the autonomic nervous system.
At rest, the majority of ATP is aerobically produced by the body in the mitochondria without the production of lactic acid or other fatiguing byproducts. During exercise, the production of ATP differs depending on an individual’s fitness and the intensity and duration of the exercise. At low activity levels, where exercise proceeds for a long period, energy is produced aerobically through a combination of oxygen with carbohydrates and fats that are stored in the body. During a higher intensity activity, with duration decreasing with increasing intensity, the production of ATP can change to anaerobic ways, like the use of creatine phosphate or anaerobic glycolysis. Aerobic production of ATP is biochemically slower and can only be used for low-intensity, long-duration exercise, but it does not produce fatiguing waste products.
Muscle action can be classified as being either voluntary or involuntary.
The skeletal muscles move the limbs (arms and legs). They move the jaw up and down so that food can be chewed. Skeletal muscles are the only voluntary muscles. This means they are the only muscle that you can choose to move.
Skeletal muscles are further divided into two types – slow-twitch and fast-twitch.
Slow twitch (Type I) muscle contains proteins that give it a rich red color. This muscle carries more oxygen efficiently and using fats, proteins, or carbohydrates as energy slow-twitch muscle fibers contract over a long period of time. These work well for aerobic sports such as long-distance running and cycling.
Fast twitch (Type II) muscle is whiter in color as it has less myoglobin (an oxygen-carrying protein). Fast-twitch fibers contract quickly and powerfully, however they fatigue rapidly. This is useful for anaerobic exercise such as sprinting and for strength sports like weightlifting.
Smooth muscle is under involuntary control and is found in the walls of blood vessels and of structures such as the urinary bladder, the intestines, and the stomach.
Cardiac muscle makes up the mass of the heart and is responsible for the rhythmic contractions of that vital pumping organ; it too is under involuntary control.
Muscles are composed of two major protein filaments: a thick filament composed of the protein myosin and a thin filament composed of the protein actin. Muscle contraction occurs when these filaments slide over one another in a series of repetitive events.
When a muscle receives neurological signals, they open holes in its cell membrane. These holes are proteins that are called calcium channels. Then calcium ions rush into the cell. This calcium sticks to the specialized proteins actin and myosin. This triggers these proteins to contract the muscle. When the muscle contracts, this pulls the bones it's connected to closer together.
Muscles called flexors force your joints to bend. Muscles called extensors cause your limbs to straighten. A bicep is a flexor and the triceps are extensors. You may have also heard of ligaments. They are batches of connective tissue that bind bones to each other. Muscles, tendons, and ligaments can be found working together in almost all of your joints.
Exercise makes muscles get bigger and stronger. It improves both muscle strength and muscle endurance. Muscular strength is the ability of a muscle to use force during a contraction. Muscular endurance is the ability of a muscle to continue to contract over a long time without getting tired. If a person does not exercise, muscle atrophy occurs. This means their muscles get smaller and weaker.
Exercises are grouped into three types depending on the effect they have on the body:
There are three big groups of muscle diseases
1. Neuromuscular diseases – These are problems with how the nerves tell the muscles to move. Strokes, cerebral palsy, and Parkinson’s disease are neuromuscular diseases.
2. Motor endplate disease – These are problems with the place where the nerve tells the muscle to move. Tetanus and myasthenia gravis are motor endplate diseases.
3. Myopathies – These are problems with the structure of the muscle. Muscular dystrophy, cancers like Ewing’s sarcoma, and cardiomyopathy are myopathies.
