Skeletal Muscles

Chapter 21 Skeletal Muscles

Ivan Damjanov, MD, PhD

1 Describe the basic structure of 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.

2 What is a neuromuscular motor unit?

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.

3 Explain how muscle fibers are classified

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.

Whether a muscle fiber will be of one or another type depends on the innervation the cells receive. Reinnervation can thus change one muscle fiber type into another.

4 How can one recognize these two types of muscle fibers?

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.

5 How common are muscle diseases?

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.

8 Is muscle biopsy the definitive approach to diagnosing muscle diseases?

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:

Myopathic pattern: The muscle fibers show signs of injury or necrosis usually associated with an inflammatory response and repair in the form of fibrosis and limited attempts at regeneration. These findings indicate that the primary site of injury is the muscle.

Neuropathic pattern: The muscle fibers show signs of atrophy caused by denervation. The muscle changes are obviously secondary to a nerve injury or diseases of the nervous system.

Normal muscle histology: The most important disease in which signs of muscle weakness are not associated with any microscopically visible changes in the muscle biopsy is myasthenia gravis. Some of the muscle diseases presenting on normal light microscopic findings will show ultrastructural changes visible by electron microscopy (e.g., mitochondrial myopathy).

9 Name the clinical differences between primary myopathies and neurogenic muscle disease

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.

TABLE 21-1 Differences Between Primary Myopathies and Neurogenic Muscle Atrophy

Muscle Disease
Signs/Symptoms	Myopathy	Neurogenic Muscle Atrophy
Muscles involved	Proximal extremity	Distal extremity
Bilateral	Usually	Yes and no
Muscle appearance	Tender or painful	Not sensitive
Muscle appearance	Swollen or normal	Atrophic
Cramps/fasciculations	No	May be present No loss of sensation (“glove and stocking” pattern)
Onset	Gradual	Often sudden
Systemic disease	Common	Usually not present
Electromyography	Yes	Yes
Creatine phosphokinase	Elevated	Normal
Antinuclear antibody	Often elevated	No

NEUROGENIC MUSCLE ATROPHY

10 What could cause neurogenic muscle atrophy?

Denervation of muscle fibers could result from many injuries of the upper (cortical) or lower (spinal) motor neuron or their axons. Salient examples are given in Table 21-2.

TABLE 21-2 Causes of Neurogenic Muscle Atrophy

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
Axonal branches	Diabetes

* It can involve both the lower and the upper motor neurons.

11 Define 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:

Polyneuropathy: It is characterized by random involvement of numerous nerves, as typically seen in Guillain–Barré syndrome and other systemic diseases (e.g., diabetes).

Mononeuropathy: It presents with single nerve involvement, as in carpal tunnel syndrome, in which the ulnar or radial nerves are compressed by the connective tissue in the carpal tunnel.

Mononeuritis multiplex: It presents with involvement of several major nerves unrelated to each other, as in polyarteritis nodosa or leprosy.

12 What are the causes of peripheral neuropathy?

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.

To remember that diabetes is the most common cause of peripheral neuropathy and to make other possible causes of this ailment easier to remember, use the mnemonic diabetic:

Immune disorders (Guillain–Barré syndrome, polyarteritis nodosa, and amyloidosis)

B vitamin deficiencies (e.g., beri-beri [B1] and pellagra [B6 niacin])

External pressure (e.g., carpal tunnel, Paget disease, osteophytes, and tumor)

Therapeutic (toxins such as lead and hydrocarbon solvents [from glue sniffing]; drugs [such as nitrofurantoin, cisplatinum, and isoniazid])

Infections (e.g., leprosy, diphtheria, and acquired immune deficiency disease)

Congenital (e.g., Charcot–Marie–Tooth disease, Dejerine–Sottas disease, Refsum disease, etc.)

13 Discuss the two typical microscopic patterns of neurogenic muscle atrophy

Fascicular atrophy: Loss of motor neurons in the cortex or spinal cord leads to atrophy of entire fascicles of skeletal muscle. Most commonly, this type of atrophy is seen in children and adolescents affected by spinal muscular atrophy, an autosomal recessive disease caused by mutations of a gene on chromosome 5. In adults, it may be caused by cortical or spinal cord lesions resulting from cardiovascular accident or traffic injuries. Amyotrophic lateral sclerosis and poliomyelitis cause similar changes.

Single muscle fiber atrophy: Atrophic muscle fibers, which appear angulated and compressed by surrounding normal muscle fibers, are distributed at random. They may be either type I or type II fibers. Atrophy of this type develops due to a loss of single axonal branches and is most often a complication of diabetes.

These two types of muscle fiber atrophy are illustrated in Fig. 21-1.

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Jul 23, 2016 | Posted by admin in PATHOLOGY & LABORATORY MEDICINE | Comments Off

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