Muscle Cell Functions

There are three types of muscles in your body: smooth, cardiac and skeletal muscles. Each type has distinctive characteristics and functions.

Smooth muscle fiber is found lining hollow organs throughout the body and generally contracts and relaxes involuntarily — unlike skeletal muscle. Its specific functions vary depending on its location in the body. Smooth muscle cells have a spindle-shaped appearance under a microscope, unlike skeletal muscle which appears striated.

In many cases, smooth muscles help push or propel fluids or food. For example, in the gastrointestinal system, smooth muscle contractions are responsible for moving food through the stomach and intestine. In men, they propel sperm out of the penis, and in women, they are responsible for contractions during pregnancy and labor.

In the cardiovascular system, smooth muscle lines your blood vessels and can change blood pressure and flow by contracting and relaxing to change the resistance of the vessels. It also impacts your breathing as it can change the size of the bronchiole, or the branches in your airway, in your lungs.

Other functions of smooth muscle include:

• Regulation of urine flow in the renal system
• Raising the hairs on your skin
• Dilating and constricting pupil size
• Altering the shape of the lens in your eye

Disorders affecting the smooth muscle of the body are uncommon. One such condition, multisystemic smooth muscle dysfunction syndrome, is extremely rare and may be caused by a gene mutation. This disorder causes impairment of the smooth muscles throughout the body. Some symptoms may include:

• Weakened bladder
• Hypoperistalsis, or weakened contractions of the smooth muscles in the stomach and intestines necessary for digesting food
• Lung disease
• Abnormalities in blood vessels
• Aneurysms
• Dilated pupils
• Abnormalities of white matter in the brain

If the condition is present in a newborn, one symptom may be patent ductus arteriosus. The fetus has a blood vessel that bypasses the lungs as there is no air to breathe in the uterus. When the baby is born, this vessel should close as the baby is now breathing and doesn't need the blood to bypass the lungs. Patent ductus arteriosus is the failure of this vessel to close.

The heart is made up of cardiac muscle. It is under involuntary control, unlike skeletal muscle, and is moderated by the endocrine and nervous systems. Cardiac muscle cells normally generate energy with aerobic metabolism and have the ability to store oxygen in the myoglobin of the cells. The heart primarily consists of myocardial contractile cells that allow it to beat.

Approximately 1 percent of the heart muscle consists of muscle fibers called myocardial conducting cells. These fibers are smaller than the contractile cells and allow the heart to display a unique characteristic among muscle cells in that it generates its own rhythm of contraction.

The purpose of cardiac muscle is to cause the heart to beat and pump blood throughout the body. When the muscle fibers relax, it allows blood to enter the chambers of the heart. The blood is then pumped out of the heart when the cardiac muscle contracts. From there, it circulates throughout the body.

Cardiomyopathy is a disorder that affects the cardiac muscle and causes the heart to beat less efficiently. It may also cause it to lose rhythm and beat irregularly. This condition may be caused by a viral or bacterial infection, alcohol, drug use, high blood pressure, congenital heart disease, chemotherapy or a muscular condition, such as muscular dystrophy.

This disorder, unlike many other heart conditions, often occurs in younger patients and over time, causes the shape of the heart to change. There are several types of cardiomyopathy that affect the heart muscle differently. These include:

• Hypertrophic cardiomyopathy, a thickening of the cardiac muscle in the left ventricle
• Dilated cardiomyopathy, an enlarged and stretched heart cavity that prevents normal contraction and relaxation of cardiac muscle
  
• Restrictive cardiomyopathy, the stiffening of cardiac muscle
• Arrhythmogenic right ventricular dysplasia — occurs when the cardiac muscle in the right ventricle is replaced with scar tissue or fat

The heart is also susceptible to other, often preventable conditions. One of the most common is coronary artery disease, states the Centers for Disease Control and Prevention.

This disorder is caused by a buildup of plaque in the arteries and limits the blood flow to the heart, reducing the amount of oxygen and nutrients received by the cardiac muscle. Eventually, it can weaken the heart muscle and cause an irregular heartbeat. The most common symptom of coronary artery disease is angina, or chest pain.

Risk factors for coronary artery disease include smoking, being overweight, poor diet, low physical activity levels and high blood pressure or elevated cholesterol. This condition may be managed by making lifestyle changes, including increased exercise and a healthier diet and medications to treat high blood pressure, elevated cholesterol and arrhythmia. In severe cases, surgery may be required to restore blood flow.

Read more: 10 Simple Ways to Keep Your Heart Healthy and Strong

Another common condition is a heart attack or myocardial infarction. Heart attacks are commonly caused by coronary artery disease when blood flow does not reach a portion of the heart. Although rare, they may also be caused by a sudden spasm of the coronary artery, which cuts off the blood flow. Damage to the heart muscle may occur if treatment is not provided and blood flow remains cut off.

Skeletal muscles connect to your skeleton and are responsible for the movement and stabilization of your body. Their features include:

• Voluntary control
• A long, cylindrical shape
• Striated appearance under a microscope
• Wrapped in connective tissue to maintain structure during contraction

When you think of skeletal muscle, the movement of your body is probably the main thing that comes to your mind. These muscle cells are the ones you target when you hit the gym and they give you control over your body and limbs. In addition to creating movement, they also have the ability to stop movement and hold your body or body part still.

Skeletal muscles provide stabilization for your body. They help you maintain your posture and keep you balanced. These muscles also stabilize your joints and keep them properly aligned.

These voluntary muscle fibers play an important role in protecting your internal organs by providing support to hold them in place in your abdomen and shielding them from external impact. Skeletal muscle fibers also give you voluntary control of urination, bowel movements and swallowing.

Finally, they help regulate your body temperature. When the muscles contract, they produce heat. This is what causes your temperature to rise when you exercise. It is also the reason you shiver when it is cold. Your body is contracting muscles to produce heat and warm your body.

When you work out too hard or push a muscle past its limitations, you may suffer a strain or tear in the muscle. Strains are commonly caused by pulling or stretching a muscle or tendon beyond its range of motion.

Some individuals may have chronic strains as a result of overuse or repetitive movements. In severe cases, the muscle may rupture or tear. Symptoms include pain, inflammation, muscle weakness and cramping.

Since muscles support the joints, weak or overly tight muscles may make you more prone to injuries involving the joint such as sprains, tendonitis and dislocation. Sprains are the overstretching or rupturing of a tendon that connects the bones in a joint, notes the University of Rochester Medical Center. Pain, swelling and bruising of the joint are all common symptoms.

Treat sprains and strains with rest, ice, compression and elevation, or RICE. Avoid moving or putting weight on the affected joint or muscle and apply ice for 15 to 20 minutes per session to reduce swelling. Provide compression by wrapping the injured area in a bandage and then elevate the injured body part so that it is above your heart.

Skeletal muscle is extremely important for your body to function, but it is also prone to diseases and disorders. Some conditions are present at birth or develop during childhood.

For example, muscular dystrophy is a genetic condition that causes the muscles to weaken over time. There are many types of muscular dystrophy, and the symptoms and speed at which they progress may vary. Some symptoms include:

• Decreased strength
• Muscle wasting
• Breathing problems
• Cataracts and other vision problems
• Limited joint movement and joint deformity

The muscle may also be affected by neuromuscular disorders that affect the nerves that send signals to your muscles. When these nerves die, it causes your muscles to atrophy, states Cedars-Sinai. These conditions are commonly genetic or autoimmune diseases. Some common symptoms of neuromuscular disorders include:

• Muscle weakness, pain and cramping
• Loss of muscle
• Loss of balance
• Numbness and tingling
• Vision problems such as double vision or droopy eyelids
• Difficulty breathing
• Difficulty swallowing

Read more: How Are the Muscular & Nervous Systems Connected?

A doctor will evaluate your symptoms and perform specific tests, such as a spinal tap, muscle biopsy or blood test, to diagnose the neuromuscular disorder. Some disorders include amyotrophic lateral sclerosis or ALS, multiple sclerosis, myopathy and muscular dystrophy.