32 Muscular System Introduction

When you think of muscles you probably have a picture in your mind of some muscular people who are pleasing to the eye. Did you know that you have over 650 muscles in your body? That sounds like a lot but a caterpillar has even more, around 4000 muscles! Without muscles we would never be able to move. The main function of the muscular system is movement. But did you know that muscle contractions generate 85% of our body heat? Muscles also protect our organs. Your abdominal muscles keep your “guts” and internal organs in place. Some muscles are circular, such as the one that encircles your mouth for kissing or puckering. Other circular muscles form sphincters which control when you defecate or urinate. One important muscle, known as your diaphragm, is necessary for breathing. If your diaphragm stopped working you would die in minutes because you would not breathe.

The Muscular System and Homeostasis

Case Study: Let’s check out the muscular system in action in the following case study.

Seymour is a nineteen year old power lifter and college student. You sit next to Seymour in psychology class and have gotten to know him this semester. On Monday, he sits down next to you and lets out a wince of pain as he takes his seat. You ask him, “What’s wrong?”

He replies, “Oh, it’s probably nothing, but my stomach has been hurting especially when I lift something heavy.”

“Don’t you work at The Dark Horse Tavern? And work out a lot? I bet that must be tough on you. Do you think you pulled a muscle?” you ask him.

Seymour answers, “I don’t know, I’ve never felt this kind of pain before, it hurt a lot at work and I haven’t been able to lift as much at the gym.”

“Why don’t we check this out on my iPhone, we still have 5 minutes before class begins” you reply.

Seymour suggests going to WebMD.com for a quick look. “Hmm, I don’t see anything specific for stomach pain while exercising,” you tell him. “What other symptoms do you have?”

“Well, it’s kind of embarrassing but I have noticed this lump in my navel especially while lifting.”

“My cousin had something like that before, I think it was called a hernia, let’s Google that. Oh – man, it sounds like something called an umbilical hernia, it says that the bulge might be your intestines!” you whisper to him. “Seymour, you better get to your doctor right away, this can’t be good.”

Seymour goes a little pale and says, “I’m going to call my mom right after class.”

The muscular system is mainly composed of skeletal muscle, such as those that cover the anterior portion of your abdomen and help compress the abdominopelvic cavity and digestive organs. As we saw with Seymour, if these muscles are weakened, they might separate and allow the underlying organs to protrude. Seymour’s constant exposure to high intra-abdominal pressure has weakened the muscle. If a piece of Seymour’s intestine is bulging through his abdominal muscles this can have serious consequences. The small intestine is a critical component of the digestive system and if it is pinched off that section could become necrotic and die.

There are two other types of muscle tissue that are necessary for life: cardiac and smooth muscle. Cardiac muscle is found in the heart and its rhythmic contraction is responsible for your heart beat, while smooth muscle is found in many organs and blood vessels. Smooth muscle is a significant part of your cardiovascular system (blood vessels), respiratory system (bronchioles), digestive system (esophagus, stomach, small and large intestines), urinary system (ureters and urinary bladder), and your reproductive system (uterus, vas deferens).

As stated earlier, skeletal muscle has many important functions in maintaining our homeostasis. Skeletal muscle contractions are critical in producing heat for our body. In fact, when you are very cold your skeletal muscles increase contractions to generate more heat to warm you – this is known as shivering. When you exercise and your skeletal muscles are more active you produce more body heat and your body temperature increases. Your body will sweat to help cool itself.

The diaphragm is a skeletal muscle that contracts to allow us to inhale, we exhale when it relaxes. Any disruption in this important muscle’s function can be fatal. The diaphragm is a critical organ of the respiratory system.

Clearly, skeletal muscle and muscle tissue is much more important than just giving us a “buff” body!

The Components of the Muscular System

The muscular system consists of skeletal muscle connected to bones via tendons. Tendons are a type of dense regular connective tissue that joins muscles to bone. The abdominal muscles are also held with broad tendon sheaths termed aponeuroses. In Seymour’s case either the muscle separated or pulled away from its aponeurosis that helped to anchor the muscle fibers together.

As mentioned, there are three types of muscle tissue. Skeletal muscle attaches to bones, is voluntary, and has a striped (striated) appearance. Cardiac muscle is found in the heart, is involuntary and is striated. Smooth, sometimes known as visceral muscle, is found in many organs and blood vessels, is not striated but is involuntary.

All muscle tissue is composed of muscle cells known as muscle fibers. These fibers are bundled and held together with connective tissue. The basic unit of the muscle cell is the sarcomere. Muscles contract because sarcomeres shorten. Calcium is essential for all types of muscle tissue to function properly. You will investigate the role of calcium in muscle contraction in this module.

Muscle contains many proteins, mainly myosin and actin. Skeletal muscle is under control of the nervous system and will not contract unless a neural command reaches the muscle and instructs it to do so. The nervous system communicates with skeletal muscle via chemical messengers known as neurotransmitters. The neurotransmitter, acetylcholine, is responsible to stimulating skeletal muscle so it contracts. The process of muscle contraction occurs at the cellular level and will be studied in this unit.

Muscle fibers or cells are not all alike. There are cells that are specialized for endurance events, these are known as slow-twitch fibers. There are also cells or fibers that are better suited for power and sprinting. These fatigue a lot faster and are known as fast-twitch fibers.

When you study the muscular system you will need to understand the microscopic anatomy and physiology so you can learn how muscle contracts. You will also study the gross anatomy of muscles, their names, locations, and functions in the human body. The muscular system is a fascinating system. Enjoy your journey as you discover it!



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