The Muscular System

• Author: Samuel D. Hodge, Jr./Jack E. Hubbard
• Profession: Skilled litigator, is chair of the department of legal studies at Temple University/Professor of Neurology at the University of Minnesota
• Pages: 203-255

The Muscular System

No muscle uses its

power in pushing[,]

but always in drawing

to itself the parts that

are joined to it.

Leonardo da Vinci

(1452–1519)

4

The spine would not be able to move without the muscles and ligaments that surround

it.1 In fact, there is a complex set of muscles known as extensors, flexors, and obliques that

work together to support the spine, help hold the body upright, and enable the trunk of

the body to move, twist, and bend in a variety of directions.2 These soft tissues, however,

are not limited to the back and neck. The body contains more than 700 muscles that allow

us to act upon our thoughts and react to the challenges and pleasures presented by the

environment as well as the events that go on about us. Muscles also help with temperature

regulation by causing shivering, an automatic response to a drop in body temperature.

Shivering allows the skeletal muscles to alternately contract and relax, generating heat

by the increased muscle metabolism. Body musculature, in many individuals, also defines

body form and shape.

This chapter will examine the microscopic structure of muscle, how muscles allow

people to move, the major muscle groups of the body, a clinical discussion of diseases of

and injuries to muscle, and the legal considerations of muscular issues.

The body has three types of muscle based upon their microscopic appearance as well as

function: skeletal, cardiac, and smooth. Figure 4-1. Skeletal muscles are under the control

of the voluntary nervous system and connect bone-to-bone, as well as to skin and other

structures in order to move those parts of the body such as the eye.3 Cardiac muscle is

only found in the heart; that perpetual muscular pump is under involuntary control of

the autonomic nervous system. Smooth muscle, also controlled by the autonomic nervous

system, surrounds vessels and organs to cause a squeezing or constricting action such as the

contractions of the uterus during labor, peristaltic activity of the intestines during digestion,

and constriction of the arteries to increase blood pressure. Since the muscular system only

pertains to skeletal (striated) muscles, this chapter is limited to that type of muscle.

Skeletal muscle accounts for about 40 percent of body mass and is able to do its job

due to its unique properties of being (1) contractile, (2) excitable, and (3) elastic. Muscles

are contractile, meaning that they are able to shorten and thus move the structures to

which they are attached. Muscles are excitable, contracting in response to electrical and

chemical stimulation, and they have significant elasticity, the ability to return to their

original resting length after contraction.

The medical specialists who treat diseases of the muscular system are neurologists;

physiatrists treat musculoskeletal disorders. However, no specific surgical specialty is

associated with the muscular system.

Microscopic Structure

A skeletal muscle cell is a marvel of biomechanical engineering. Each cell contains

approximately 2,000 myofibrils (myo = muscle) that provide for contraction and thus movement.

204 ◆ CHAPTER 4

Each myofibril contains two types of long protein molecules called myofilaments. Figure 4-2.

One type of myofilament, the thicker of the two, is called myosin and is surrounded by six of the

thinner myofilaments, called actin, in a very precise, regular pattern. The actin myofilaments

extend from a common point, called the Z line (or Z disk), with the myosin myofilaments

positioned in between the actin myofilaments. The myosin does not reach the Z line.

The segment between each Z line is called a sarcomere or the basic unit of a striated

muscle4 and is the contractile unit of a muscle cell. The alternating concentrations of actin

and myosin myofilaments in a sarcomere each give an alternating light and dark coloring

when muscles are prepared and treated for certain histological stains. These alternating

bands give a striated appearance; hence, skeletal muscle is also known as striated muscle.

Each myosin myofilament has a head made of the energy molecule ATP (adenosine

triphosphate), which cross-bridges with the thinner actin myofilaments (see figure

4-2). Each of the actin myofilaments also contains regulatory proteins, troponin and

tropomysin, which are important during the contraction process. During a muscle

contraction, very complex biochemical and neurophysiological changes occur. Calcium

ions are stored in the sarcoplasmic reticulum, a tubular system surrounding the myofibrils,

and are released. Figure 4-3. In the presence of calcium ions, the ATP head of the myosin

myofilaments is activated, causing the myosin myofilament to be pulled along the actin

myofilament. The myosin makes contact with the next segment of actin and is pulled

along again. This action is repeated over and over, with the actin and myosin sliding

along each other, thereby shortening the length of the sarcomere and causing the muscle

to contract. As long as calcium is present, the contraction is maintained. Termination of

contraction is due to reuptake of calcium ions into the sarcoplasmic reticulum. The body

stiffening that occurs with rigor mortis upon death is actually the result of permanently

contracted sarcomeres from which calcium has not been removed and taken back up into

the sarcoplasmic reticulum because of the lack of required energy-dependent systems.

A way to visualize this contractile process is to hold your hands out in front of you and

interlace the fingers of your left hand with those on your right hand. The fingers of the

left hand represent actin; those of the right, myosin. As you slide your fingers together,

mimicking a muscle contraction, notice how the thumbs come closer together. Now, if

each thumb represents tendons of the mode that are attached to bones across a joint, it is

easy to visualize how body movement occurs.

The calcium ion is critical to muscle contraction and is released from the sarcoplasmic

reticulum where it is stored. Calcium is released in response to a nerve impulse (action

potential) generated by an axon carried in a motor nerve. Each neuronal axon makes

FIGURE 4-1.

Muscle types.

Three types of muscle are found

in the body—cardiac, skeletal, and

smooth. Cardiac muscle, found

only in the heart, causes it to

beat; skeletal muscle moves body

parts under voluntary control;

smooth muscle causes constriction

of blood vessels and squeezing of

hollow organs such as the stomach

under involuntary (autonomic)

control.

THE MUSCULAR SYSTEM ◆ 205

contact with a number of muscle cells; the combination is termed a motor unit. Figure 4-4.

A motor unit is an axon and all the muscle cells that it connects with and innervates.

When an electrical impulse travels down a motor axon, all of the muscle cells within the

motor unit that it innervates contract simultaneously.

The connection between the motor axon and muscle cell is called the myoneural (also

called neuromuscular) junction. The neurotransmitter acetylcholine (ACh) is released in

packets called synaptic vesicles from the axon terminal. ACh, in turn, stimulates acetylcholine

receptors found in the highly folded postjunctional membrane of the muscle cell. The

action of acetylcholine is stopped by acetylcholinesterase, an enzyme that breaks down

the acetylcholine. Some nerve poisons work as acetylcholinesterase inhibitors, causing

sustained diffuse muscle contraction, resulting in death.

b

aFIGURE 4-2.

Microscopic view

of muscle cell.

4-2a: Each muscle cell consists of

hundreds of two types of myo-

fibrils—actin and myosin—that

are bundled together to form a

contracting unit, the sarcomere.

With muscle contraction, the two

myofibrils slide across each other

powered by the energy molecule

adensine triphosphate (ATP).

4-2b: Multiple myofibrils are

bundled together to form a muscle

fascicle, wrapped together by con-

nective tissue (endomysium). A

number of fascicles are wrapped

together by another connec-

tive tissue layer, the perimysium.

The entire muscle is wrapped

by a third connective tissue, the

epimysium. Covering the muscle is

a loose connective tissue, fascia. A

tendon is a connective tissue seg-

ment that attaches the muscle to

the periosteum covering of bone.