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Structure and Function of the Muscular System in Dogs

By: Virginia Wells

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What Is the Muscular System?

The muscular system is one of the largest systems in the dog's body. It is composed of units of tissue that have the power to contract, and hence to produce movement.

Where Are Muscles Located?

Skeletal muscles are found throughout the entire skeletal system and are attached to bones, other muscles and skin. They account for about half the weight of an animal. Parts of the walls of hollow internal organs such as the heart, stomach and intestines, and blood vessels are composed of smooth muscles.

What Is the General Structure of Muscles?

Most dogs are built for endurance, not speed, and their muscles reflect this. The muscle structure of the dog is much the same as its ancestor, the wolf. Muscles allow the animal to jump, run, stalk and engage in battle.

Muscles can be under voluntary or involuntary control. Voluntary muscles, such as those found in the arms and legs, can be controlled by thought. Involuntary muscles are those that are automatically controlled by the nervous system and cannot be moved at will. Examples of involuntary muscles are the muscles of the heart, diaphragm, and intestines.

There are three types of muscle tissue:

  • Smooth muscle. Smooth muscles are found within the internal organs such as the intestines, stomach, and bladder. They are involuntary muscles and are under the control of the autonomic nervous system.

  • Skeletal muscle. These muscles are called striated muscles and are voluntary muscles. They predominately attach to portions of the skeleton. They are involved with such things as walking, eating, tail wagging and eye movement.

  • Cardiac muscle. Cardiac muscle fibers are found in the heart and are involuntary muscles. Cardiac muscle is unique in that it is striated muscle, like skeletal muscle, but is not under voluntary control.

    Each individual muscle is composed of many cells held together by connective tissue. Skeletal muscles attach to bones by means of connective tissue tendons, which are elastic and strong. When muscles contract, they pull on the tendons, which then pull on the bones and cause the limbs to move.

    Each muscle fiber receives its own nerve impulses, which trigger various motions. Once a signal or an impulse travels down the nerve to the muscle, the muscle fiber changes chemical energy into mechanical energy, and the result is muscle contraction.

    Muscles receive a generous blood supply from neighboring blood vessels. The arteries supplying a muscle enter at rather definite places and often connect within the muscle. A rich capillary (the smallest of all blood vessels) distribution helps to deliver oxygen to muscles. High intake of oxygen by the muscles is necessary for endurance running or other types of performance. Veins also accompany the arteries. During contraction, blood circulates by being forced into the larger veins.

    What Is the Function of Muscles?

    The primary function of muscles is to bring about movement to all or part of the dog's body. Muscle is used to stabilize joints to prevent their collapse under a load. Smooth muscles maintain continence of the bladder and propel food through the bowel. Muscles also help to generate heat by shivering.

    Voluntary muscles can contract and pull, but they cannot push, so they must work in pairs that flex and extend. Extensor muscles straighten the limbs and attach to the bones, so the bones act as levers. The flexor muscles, which bend the joints, act to pick up the limb. Their partners, the extensor muscles, in turn contract to bring the limb back down. The abductor muscles move the limbs away from the midline, and the adductor muscles move the limbs toward the midline.

    What Are Common Diseases of Muscles?

    Common disease that affect muscle include:

  • Congenital Disorders. Congenital muscle disorders are rare in the dog. They are usually inherited and are seen most often in purebred dogs.

    Muscular dystrophy. Muscular dystrophy is the most prevalent type of congenital muscle disorder. It is a sex-linked disorder, so females carry the disease and males show clinical signs. It affects a wide variety of dogs, including the Alaskan malamute, golden retriever, Irish terrier, Samoyed, rottweiler and Welsh corgi. The disease in the golden retriever is similar to Duchenne-type muscular dystrophy of children.

    Muscle fiber deficiency. This is an inherited muscle disorder that causes a disturbance of gait in young Labrador retrievers.

    Familial reflex myoclonus. This disorder occurs in Labrador retrievers and is characterized by intermittent overzealous contraction of the skeletal muscles of the limbs.

    Congenital myotonia. This disease of skeletal muscle is characterized by involuntary active contraction of a muscle that persists after some sort of voluntary effort or stimulation of the muscle. It is occasionally seen in the chow chow, cocker spaniel, Labrador retriever and West Highland white terrier.

  • Inflammatory Disorders

    Polymyositis. This disorder is inflammation of several or many muscles at once, along with degenerative and regenerative changes, and is marked by muscle weakness.

    Eosinophilic myositis. This is an acute or chronic inflammation of striated muscle. It affects primarily the muscles of the head and to a lesser extent the shoulders and generally occurs in larger breed dogs.

    Myositis caused by infection. Infectious agents such as bacteria, protozoa and some parasites may cause either localized or generalized inflammation.

  • Neoplasia

    Primary cancers of muscle are rare. With the exception of rhabdomyosarcoma (a malignancy of striated muscle), most muscle tumors occur in adult or older animals.

    Secondary tumors that arise from nearby bone, cartilage and soft tissue organs may invade muscles and restrict their motion and function.

    What Types of Diagnostic Tests Are Used to Evaluate Muscles?

  • Serum muscle enzymes – These tests measure the enzymes of muscles that are released into the blood stream when the tissue is damaged or inflamed or when a lot of muscle activity has occurred.

  • Electromyography (EMG) – This test assesses the electrical properties of skeletal muscle.

  • Muscle biopsy – Biopsy is the removal and microscopic examination of muscle tissue. It is used to detect inflammation and infection within the muscle and to determine whether a tumor of muscle is malignant or benign.

  • Radiography – X-rays do not reveal much information about muscles, but they can detect abnormalities in the surrounding bony structures.

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