Chapter 21 Skeletal Muscles
The basic histologic component of all skeletal muscles is the skeletal muscle cells, also known as muscle fibers or myofibers. Myofibers are grouped into larger units called muscle fascicles, which are enveloped together by connective tissue (called epimysium) into anatomically recognized muscles. Epimysium extends internally between fascicles, forming septa that are called perimysium. Perimysium extends into still thinner strands, called endomysium, enveloping individual myofibers (muscle cells). The connective tissue skeleton attaches the muscles to the tendons and bones. It supports the muscles and contains the blood vessels and nerves.
A motor unit comprises the lower motor neuron located in the anterior horn of the spinal cord and the muscle fibers that it innervates. The size of each motor unit varies. In lower extremities, individual axons form numerous branches that innervate several hundred muscle fibers, whereas in the eye muscles, most motor units are composed of only 20 muscle cells.
Muscle fibers are classified as type I or type II, depending on their biochemical and functional properties. Type I fibers are also known as red or slow-twitch fibers, whereas type II fibers are known as white or fast-twitch fibers.
In some animals, such as chickens, white and red fibers are grouped into individual skeletal muscles. White muscles of the breast are responsible for the fast movement of wings. The slow-twitch, red muscle fibers, used for prolonged (sustained) action, are found in the leg muscles. In human muscles, type I and type II muscles are not segregated into distinct anatomic muscles and are usually found intermixed in a checkerboard manner.
In routine hematoxylin and eosin (H&E)-stained microscopic sections, type I and type II muscle fibers cannot be distinguished from one another. However, these fibers can be identified by enzyme histochemical stains that differentiate type I and type II fibers. Type I fibers have numerous mitochondria and stain strongly with mitochondrial stains, whereas type II fibers contain fewer mitochondria and thus appear paler in these sections.
Cumulatively, skeletal muscles represent the largest organ in the body. Muscles are often traumatized or involved in various metabolic, autoimmune, and infectious diseases. In most cases, the underlying pathology of these secondary muscle diseases is not studied in greater detail and remains unknown. For example, there are no “hard data” about fibromyalgia, even though this disease is diagnosed daily by most family practitioners. Most people who have muscle aches during a bout of flu probably have a form of viral myositis, but few physicians will go to the trouble of proving it. These common diseases are not studied by pathologists, and because so little is known about them, medical students and residents can be assured that the pathology of these everyday diseases will not show up on medical school or board examinations.
Unfortunately not. Muscles have a limited capacity to respond to injury, and the changes seen in muscle biopsy specimens are often nonspecific. Accordingly, muscle biopsy findings can be meaningfully interpreted only in the context of other clinical and laboratory data.
Muscle diseases are associated with three morphologic patterns, and the changes seen in the biopsied muscle often do not provide definitive answers about the causes of the disease. These three patterns are:
The differences between primary myopathies and neurogenic muscle diseases are not always obvious. Some of the most important clinical signs and symptoms favoring one or another of these diseases are given in Table 21-1.
|Signs/Symptoms||Myopathy||Neurogenic Muscle Atrophy|
|Muscles involved||Proximal extremity||Distal extremity|
|Bilateral||Usually||Yes and no|
|Muscle appearance||Tender or painful||Not sensitive|
|Swollen or normal||Atrophic|
|Cramps/fasciculations||No||May be present No loss of sensation (“glove and stocking” pattern)|
|Systemic disease||Common||Usually not present|
|Antinuclear antibody||Often elevated||No|
|Location of Injury||Example|
|Upper (cortical) motor neuron||Apoplexy|
|Axon of the upper motor neuron||Spinal cord|
|Lower (spinal) motor neuron||Poliomyelitis, amyotrophic lateral sclerosis*|
|Axon of the lower motor neuron||Nerve transection, peripheral neuropathy|
The term peripheral neuropathy is used to describe a number of diseases of peripheral nerves that present with a loss of motor or sensory functions and are accordingly classified as primary motor, primary sensory, or sensorimotor neuropathies. According to the involvement of nerves, clinicians distinguish three forms:
In the United States, the most common cause of peripheral polyneuropathy is diabetes mellitus. Diabetes is a microangiopathy, so the lesions are caused in part by ischemia and in part by the metabolic changes that affect axons and myelin sheaths. Alcohol (ethanol) is another common cause of neuropathy. Nerve biopsy is often used to determine the cause of neuropathy and to prescribe treatment.