MUSCULAR SYSTEM

The muscular system is generally regarded as consisting of one type of muscle found in the body-skeletal muscle. However, there are two other types of muscle tissue found in the body, smooth muscle and cardiac muscle, that are important components of other systems. These three types of muscle can be characterized by whether they are controlled voluntarily or involuntarily, whether they appear striated (striped) or smooth, and whether they are associated with the body wall (somatic), or with organs and blood vessels (visceral). Skeletal muscle forms the majority of the muscle tissue in the body. It consists of parallel bundles of long, multinucleated fibers with transverse stripes, is capable of powerful contractions, and is innervated by somatic and branchial motor nerves. This muscle is used to move bones and other structures, and provides support and gives form to the body. Individual skeletal muscles are often named on the basis of shape (e.g., rhomboid major muscle), attachments (e.g., sternohyoid muscle), function (e.g., flexor pollicis longus muscle), position (e.g., palmar interosseous muscle), or fiber orientation (e.g., external oblique muscle). Cardiac muscle is striated muscle found only in the walls of the heart (myocardium) and in some of the large vessels close to where they join the heart. It consists of a branching network of individual cells linked electrically and mechanically to work as a unit. Its contractions are less powerful than those of skeletal muscle and it is resistant to fatigue. Cardiac muscle is innervated by visceral motor nerves. Smooth muscle (absence of stripes) consists of elongated or spindle-shaped fibers capable of slow and sustained contractions. It is found in the walls of blood vessels (tunica media), associated with hair follicles in the skin, located in the eyeball, and found in the walls of various structures associated with the gastrointestinal, respiratory, genitourinary, and urogenital systems. Smooth muscle is innervated by visceral motor nerves.

In the clinic

Muscle paralysis

Muscle paralysis is the inability to move a specific muscle or muscle group and may be associated with other neurological abnormalities, including loss of sensation. Paralysis may be due to abnormalities in the brain, the spinal cord, and the nerves supplying the muscles. Paralysis may also be caused by drugs that affect the neurotransmitters at the nerve endings and their action upon the muscle themselves. Major causes include stroke, trauma, poliomyelitis, and iatrogenic factors.

In the long term, muscle paralysis will produce secondary muscle wasting and overall atrophy of the region due to disuse.

Certain drugs used in anesthesia affect the neurotransmitters at the neuromuscular junction, in effect paralyzing the muscle. This has two results. First, it enables the operator to enter the region of the body without agonistic and antagonistic patient muscle response, making the procedure easier to perform. Second, muscle paralysis prevents the patient from breathing, which requires the anesthetist to mechanically ventilate the patient. Importantly, there are specific drugs that reverse the muscle paralysis drugs, which can be used at the end of the procedure.

In the clinic

Muscle injuries and strains

Muscle injuries and strains tend to occur in specific muscle groups and usually are related to a sudden exertion and muscle disruption. They typically occur in athletes.

Muscle tears may involve a small interstitial injury up to a complete muscle disruption (Fig. 1.27). It is important to identify which muscle groups are affected and the extent of the tear to facilitate treatment and obtain a prognosis, which will determine the length of rehabilitation necessary to return to normal activity.

 Figure 1.27 Axial inversion recovery series, which suppresses fat and soft tissue and leaves high signal intensity where fluid is seen. A muscle tear in the right adductor longus with edema in and around the muscle is shown.

In the clinic

Muscle atrophy

Muscle atrophy is a wasting disorder of muscle. It can be produced by a variety of causes, which include nerve damage to the muscle, and disuse.

Muscle atrophy is an important problem in patients who have undergone long-term rest or disuse, requiring extensive rehabilitation and muscle building exercises to maintain normal activities of daily living.

Muscle atrophy can be a considerable problem in the postsurgical patient who has undergone anterior cruciate ligament reconstruction. In patients with anterior cruciate ligament disruption there is often marked muscle wasting of the quadriceps, which occurs rapidly because of disuse. Before reconstruction of the cruciate ligament, patients will have to undergo a course of physiotherapy to increase muscle bulk. After the operation, this improved muscle bulk will lead to quicker return to normal daily living and will prevent the possibility of graft dysfunction.