Muscular System

Function

The muscular system is responsible for the movement of the human body. Attached to the bones of the skeletal system are about 700 named muscles that make up roughly half of a person's body weight. Each of these muscles is a discrete organ constructed of skeletal muscle tissue, blood vessels, tendons, and nerves.

How the Skeletal Muscles Contract

Skeletal muscle is also called striated muscle, because when it is viewed under polarized light or stained with an indicator, you can see alternating stripes of light and dark. Skeletal muscle has a complex structure that is essential to how it contracts. We will tease apart a skeletal muscle, starting with the largest structures and working our way to the smaller ones. The basic action of any muscle is contraction. For example, when you think about moving your arm using your biceps muscle, your brain sends a signal down a nerve cell telling your biceps muscle to contract. The amount of force that the muscle creates varies -- the muscle can contract a little or a lot depending on the signal that the nerve sends. All that any muscle can do is create contraction force. A muscle is a bundle of many cells called fibers. You can think of muscle fibers as long cylinders, and compared to other cells in your body, muscle fibers are quite big. They are from about 1 to 40 microns long and 10 to 100 microns in diameter. For comparison, a strand of hair is about 100 microns in diameter, and a typical cell in your body is about 10 microns in diameter. A muscle fiber contains many myofibrils, which are cylinders of muscle proteins. These proteins allow a muscle cell to contract. Myofibrils contain two types of filaments that run along the long axis of the fiber, and these filaments are arranged in hexagonal patterns. There are thick and thin filaments. Each thick filament is surrounded by six thin filaments. Thick and thin filaments are attached to another structure called the Z-disk or Z-line, which runs perpendicular to the long axis of the fiber (the myofibril that runs from one Z-line to another is called a sarcomere). Running vertically down the Z-line is a small tube called the transverse or T-tubule, which is actually part of the cell membrane that extends deep inside the fiber. Inside the fiber, stretching along the long axis between T-tubules, is a membrane system called the sarcoplasmic reticulum, which stores and releases the calcium ions that trigger muscle contraction.

Inflammatory Myopathies

The inflammatory myopathies are a group of diseases that involve chronic muscle inflammation, accompanied by muscle weakness.


Symptoms:


General symptoms of chronic inflammatory myopathy include slow but progressive muscle weakness that starts in the proximal muscles—those muscles closest to the trunk of the body. Inflammation damages the muscle fibers, causing weakness, and may affect the arteries and blood vessels that run through the muscle. Other symptoms include fatigue after walking or standing, tripping or falling, and difficulty swallowing or breathing. Some individuals may have slight muscle pain or muscles that are tender to touch.


Prevalence:


The overall prevalence of AIM was 25.0 per 100,000 persons (95% credible interval [95% CrI] 13.4-49.0) in the First Nations population and 33.8 (95% CrI 28.9-39.6) in the non-First Nations population. For both groups, prevalence was increased in women relative to men, rural women relative to urban women, and in those age >45 years.


Treatment:


The chronic inflammatory myopathies cannot be cured in most adults but many of the symptoms can be treated. Options include medication, physical therapy, exercise, heat therapy (including microwave and ultrasound), orthotics and assistive devices, and rest. Inflammatory myopathies that are caused by medicines, a virus or other infectious agents, or exposure to a toxic substance usually abate when the harmful substance is removed or the infection is treated. If left untreated, inflammatory myopathy can cause permanent disability.

Dermatomyositis

An uncommon inflammatory disease marked by muscle weakness and a distinctive skin rash.


Symptoms:


  • Skin changes. A violet-colored or dusky red rash develops, most commonly on your face and eyelids and on areas around your nails, knuckles, elbows, knees, chest and back. The rash, which can be patchy with bluish-purple discolorations, is often the first sign of dermatomyositis.
  • Muscle weakness. Progressive muscle weakness involves the muscles closest to the trunk, such as those in your hips, thighs, shoulders, upper arms and neck. The weakness affects both the left and right sides of your body, and tends to gradually worsen.


Prevalence:


Dermatomyositis affects adults and children alike. In adults, dermatomyositis usually occurs from the late 40s to early 60s. In children, the disease most often appears between 5 and 15 years of age. Dermatomyositis affects more females than males.


Treatments:


There's no cure for dermatomyositis, but periods of remission — when symptoms improve spontaneously — may occur. Treatment can clear the skin rash and help you regain muscle strength and function.