Muscle Tissue Lab

The Muscular System

HASPI Medical Anatomy & Physiology 09a

Background

The Muscular System

The main function of the muscular system is movement.  This includes walking, breathing, pumping the heart, and moving food through your digestive tract, just to name a few important examples.  Muscles also create heat as they contract, helping to maintain a constant body temperature.  Muscle tissue makes up nearly half of an individual’s total body weight.  Each muscle is an individual organ made up of muscle tissue, nerves, blood vessels, and connective tissue. There are three types of muscle tissue: skeletal, cardiac, and smooth.

Skeletal Muscle

Approximately 650 skeletal muscles are attached to bones by tendons.  Skeletal muscles contract voluntarily, meaning that you can control them consciously.  Skeletal muscle cells appear to be striped under the microscope, and these stripes are called striations.  Most skeletal muscles are attached to two bones over a joint.  When they contract, they pull the attachment points closer to one another.  

Tendons are made up of extremely tough collagen fibers in the form of dense regular connective tissue.  They attach to skeletal muscle fibers on one side and are intermeshed in bone on the other.  Tendons must handle a great degree of strain when a muscle contracts.  When identifying skeletal muscles and their function, the location, origin, and insertion are important.  The location is the area of the body where the muscle is found, the origin is the point where the muscle connects to a stationary bone, and the insertion is the point where the muscle connects to a moving bone.

Cardiac Muscle

Cardiac muscle is only found in the heart and continuously contracts and relaxes to push blood through the blood vessels of the body.  Cardiac muscle contracts involuntarily, meaning that a person cannot control when they contract.  The heart has its own pacemaker that initiates the consistent contraction of cardiac tissue.  Cardiac muscle tissue is striated like skeletal muscle, but also has areas called intercalated disks.  These disks are areas where cells interlock to form very tight bonds, allowing cardiac muscle tissue to withstand a great amount of pressure and force over an individual’s lifetime. 

Smooth Muscle

Smooth muscle, also known as visceral muscle, is the weakest of the muscle tissues and is found within organs.  Smooth muscles also contract involuntarily.  They can be found lining blood vessels, the gastrointestinal tract, and the bladder.  When smooth muscle contracts it moves substances, such as blood or food, through the organ.

Skeletal Muscle Structure

Muscle cells, also known as myofibers, are specialized to contract and therefore look very different from neurons or skin cells.  The cell membrane of a muscle cell is called the sarcolemma, and the cytoplasm is called sarcoplasm.  Hundreds of long filaments, called myofibrils, extend the length of the cell.  Each myofibril is made up of thick filaments, called myosin, and thin filaments, called actin, that are  responsible for the actual muscle contraction.  The arrangement of these filaments gives skeletal muscle its striated appearance.  An entire muscle is actually made up of bundles of muscle cells held together by connective tissue. 

Muscular Disorders

Normal muscle function is crucial to overall health.  A variety of abnormalities caused by disease or disorders can affect the ability of muscles to contract and perform daily functions.  For example, paralysis of the diaphragm muscle would prevent respiration and result in death.  The following table summarizes a few neuromuscular disorders.  Prevalence and mortality is based on annual numbers from 2009 in the U.S.

Diagnostic Tests for Muscular Disorders

There are many tests that can be performed to assess and treat neuromuscular disorders.  The following list summarizes a few common procedures.

• Manual Muscle Test – This is the most common test for muscular disorders and involves clinical observation to determine muscle weakness, atrophy, or paralysis.
• Electromyography – Records the electrical activity of a muscle during contraction and relaxation.
• Muscle Biopsy – A section of muscle tissue is collected to test for abnormal cellular structures.
• Nerve Conduction Study – Tests whether the nerve fibers for a particular portion of the body are functioning normally.