Layman D. Biology Demystified: A Self-Teaching Guide

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10. The sharp crystals covering bone collagen ﬁbers are largely composed

of:

(a) Calcium phosphate

(b) Magnesium sulﬁde

(d) Melanin

The Giraﬀe ORDER TABLE for Chapter 13

(Key Text Facts About Biological Order Within An Organism)

1. ____________________________________________________________

2. ____________________________________________________________

3. ____________________________________________________________

4. ____________________________________________________________

5. ____________________________________________________________

The Dead Giraﬀe DISORDER TABLE for Chapter 13

(Key Text Facts About Biological Disorder Within An Organism)

1. ____________________________________________________________

2. ____________________________________________________________

3. ____________________________________________________________

CHAPTER 13 Skins and Skeletons 237

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The Spider Web ORDER TABLE for Chapter 13

(Key Text Facts About Biological Order Beyond the Individual Organism)

1. ____________________________________________________________

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CHAPTER

The Neuromuscular

(Nerve–Muscle)

Connection

Chapter 13 discussed the structure and function of the skin and skeleton.

Chapter 14 considers the neuromuscular (nur-oh-MUS-kyoo-lar) or ‘‘nerve’’

(neur) and ‘‘muscle’’ (muscul) connection. Quite fascinating is that the word

muscular (MUS-kyoo-lar) actually translates to mean ‘‘pertaining to a little

mouse (muscul)’’!

Literally speaking, then, the muscular system of humans is an organ system

composed of over 600 individual ‘‘little mice’’ – the skeletal muscle organs.In

reality, the skeletal muscle organs are a large group of individual skeletal

muscles that get their name from the fact that they are attached to the bones

of the skeleton (Figure 14.1). [Study suggestion: Flex or bend your forearm.

Now, extend or straighten it. Observe how your biceps (BUY-seps) brachii

(BRAY-kee-eye) muscle bulges, then lengthens, deep to the skin in your

upper arm. Try to duplicate the imagination of the early anatomists, by

1, Order

speculating why they called muscles like the biceps brachii the ‘‘little mice.’’

Check your thinking with the diagram in Figure 14.1, below.]

Bone–Muscle Lever Systems

Just as little mice run up-and-down, the skeletal muscles contract and

shorten, then relax and lengthen, again. Each of the skeletal muscles is

attached to a bone of the skeleton by two or more tendons (TEN-duns). A

tendon is literally a ‘‘stretcher.’’ It is actually a strap of dense ﬁbrous con-

nective tissue that anchors a skeletal muscle to a bone. When the muscle

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Fig. 14.1 An overview of the human muscular system: A collection of ‘‘little mice.’’

contracts, it exerts a pulling force, P. This pulling force stretches the tendon.

The tendon then pulls upon the bone.

The human body has a number of bone–muscle lever (LEE-ver) systems.A

lever is a rigid bar that is acted upon by some force. In a bone–muscle lever

system (Figure 14.2), the bone serves as a passive lever or rigid bar that is

pulled upon by a contracting skeletal muscle. The pulling force of the muscle

is applied to the bone through the attached tendons.

A fulcrum (FULL-crumb), abbreviated as F, is a place of ‘‘support’’ upon

which a lever turns or balances when it moves. Within a muscle–bone lever

system, a movable joint generally serves as the fulcrum. For instance, the

biceps brachii contracts and provides a pulling force (P) upon its tendons.

The tendons raise the bones of the forearm (the levers) at the elbow joint,

which serves as the fulcrum (F). A weight (W) or resistance (R) is then lifted,

as the forearm is bent or ﬂexed.

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Fig. 14.2 A bone–muscle lever system.

Some Characteristics Used for Naming Muscles

Back in Figure 14.1, you were given anterior (front) and posterior (rear)

views of several major muscles of the human body, along with their names.

Since this book is entitled Biology Demystiﬁed, we want to help you take

some of the ‘‘mystery’’ out of these muscle names! To do this, we will exam-

ine some of the general characteristics or criteria frequently employed to

name major skeletal muscles.

Characteristic #1: Body location. Many muscles are either totally or par-

tially named for their body location. Consider, for example, the tibialis (tih-

bee-AL-is) anterior muscle. From Figure 14.1, you can see that this muscle is

named for its location ‘‘in front of ’’ (anterior) to the tibia (TIB-ee-ah) or

‘‘shin bone.’’ The name tibia also means ‘‘pipe’’ or ‘‘ﬂute,’’ apparently from

its shape. The tibia (and its associated skeletal muscles) is found not just

within humans but also in amphibians, reptiles, birds, and other mammals.

Characteristic #2: Muscle shape. Just as the tibia bone (and hence the

tibialis anterior muscle) is partially named for its resemblance to an ancient

Roman ﬂute, various skeletal muscles are likewise named for their shapes.

Look at the deltoid (DELL-toyd) muscle (Figure 14.1). This muscle ‘‘resem-

bles’’ (-oid) a ‘‘triangle’’ (delt) in the upper shoulder area.

Characteristic #3: Points of attachment. Remember that each skeletal mus-

cle is attached to a bone by one or more tendons at either end. The origin of a

muscle is the least movable tendon or attachment of the muscle. Picture it like

a heavy anchor. The origin of the biceps brachii muscle, for instance, is at the

bones of the shoulder. The insertion of a muscle, on the other hand, is the

more movable tendon or attachment. The insertion of the biceps brachii is

made upon the bones in the lower arm.

Within a particular bone–muscle lever system, then, when the muscle con-

tracts and exerts a pulling force, only the insertion end moves. The origin end

remains stationary, because it is ﬁrmly anchored. During contraction, the

insertion end of a muscle moves towards the origin end. In the case of the

biceps brachii, when it contracts its lower tendons ﬂex (FLEKS) or ‘‘bend’’

the arm at the elbow joint. [Study suggestion: Put down this book for a

moment and ﬂex your own forearm. Try to imagine the lower tendons of

your biceps brachii pulling upon their insertion on your forearm bones to

raise your lower arm.]

Some muscles are even named for their points of attachment (origin and

insertion). A good example is provided by the sternocleidomastoid (ster-noh-

kleye-doh-MASS-toyd) muscle. Figure 14.3 reveals that this muscle attaches

to the sternum (STER-num) or main bone of the ‘‘chest’’ (stern). Its tendons

also hook onto the clavicle (KLAV-uh-kul) or collar bone, which is named

for its resemblance to a ‘‘little key’’ (clavicul) used in ancient times. Finally,

the sternocleidomastoid tendons attach to a third place – the mastoid

(MASS-toyd) process of the skull – which literally ‘‘resembles’’ (-oid) a ‘‘little

breast’’ (mast).

When the sternocleidomastoid contracts, it nods the head, drawing the

chin down upon the chest. [Study suggestion: From the preceding informa-

tion, which attachment do you think represents the insertion end of the

sternocleidomastoid? – What attachments are the origin end?]

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Characteristic #4: Relative size. Some muscles are named for their relative

size: that is, how big they are compared to their neighbors. Consider two

muscles in the gluteal (GLOO-tee-al) or ‘‘rump’’ region – the gluteus (GLOO-

tee-us) maximus (MACKS-ih-mus) and the gluteus minimus (MIN-ih-mus).

[Study suggestion: Which do you think is the larger muscle, the one with a

more ‘‘minimum’’ or more ‘‘maximum’’ size?]

Characteristic #5: Number of muscle ‘‘heads.’’ A muscle head is a major

division of a muscle, with one or more tendons attached. In Latin, ceps

(SEPS) means ‘‘head,’’ while bi- denotes ‘‘two.’’ Biceps (BUY-seps), there-

fore, means ‘‘two-headed.’’

Brachii (BRAY-kee-eye) stands for ‘‘arm.’’ Hence, the name of the biceps

brachii muscle translates exactly to mean a ‘‘two-headed’’ muscle in the

upper ‘‘arm.’’ The biceps brachii has two major heads or divisions to it, and

is hooked onto bones by two tendons above the muscle, and two below it.

Characteristic #6: Direction of muscle ﬁbers. Several muscles are given

Latin names describing the direction of their individual muscle ﬁbers –

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Fig. 14.3 The sternocleidomastoid muscle and its attachments.

long, slender, ﬁber-shaped muscle cells. Consider the rectus (WRECK-tus)

abdominis (ahb-DAHM-ih-nus) muscle. (Review Figure 14.1.) It is located in

the front of the abdomen (trunk midsection), and its ﬁbers are oriented in a

‘‘straight’’ (rect), vertical direction.

Characteristic #7: Association with real or mythological characters. Certain

muscles are associated with real people, or with people who only existed in

myth. A very intriguing example is the sartorius (sar-TOR-ee-us). This muscle

oddly means ‘‘presence of ’’ (-us) a ‘‘tailor’’ (sartori)! The naming connection

is to the way tailors used to sit in ancient times – cross-legged upon the

ground (Figure 14.4). The sartorius is actually located along the inner aspect

of each thigh. Thus, when it contracts, it ﬂexes (bends) the lower leg, as was

done by ancient tailors who used to sit on their bent legs!

Characteristic #8: Major body actions. Some muscles are named for their

major body actions. Consider as an example the masseter (mah-SEE-ter)

muscle in the lower jaw. Its name comes from Latin for ‘‘chewer.’’ You

use the masseter to lift your lower jaw as you chew meat or gum, of course!

The General Anatomy of a Skeletal Muscle

We have been considering the speciﬁc names and appearances of various

skeletal muscles. But there is also a general anatomy that practically all

skeletal muscles share.

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Fig. 14.4 The sartorius: A tale about tailors!

Figure 14.5 includes examples of fascia (FASH-ee-uh). A fascia is a thin

‘‘band’’ (fasci) of ﬁbrous connective tissue surrounding or penetrating a

muscle. Consider, for instance, the epimysium (ep-uh-MIS-ee-um), which is

a sheet or band of fascia that is ‘‘present’’ (-um) ‘‘upon’’ (epi-) the entire

‘‘muscle’’ (mys). [Study suggestion: Look at a raw piece of chicken. Peel the

skin back slightly from the ﬂesh, and you will see a milky looking membrane

lying upon the meat. What is the name of this membrane?]

Cutting the muscle in half, we can view the perimysium (pair-uh-MIZH-ee-

um). The perimysium penetrates deeply into the muscle organ and subdivides

it into fascicles (FAS-uh-kuls) – ‘‘little bundles’’ of muscle ﬁbers that are

surrounded by sheets of fascia. The perimysium, therefore, is the fascia pre-

sent ‘‘around’’ (peri-) each bundle or fascicle of muscle ﬁbers.

Finally, the endomysium (en-doh-MIZH-ee-um) is the fascia present

‘‘within’’ (endo-) each bundle or fascicle, and between its individual muscle

ﬁbers. You will note from Figure 14.5 that the skeletal muscle ﬁbers, them-

selves, are striated (STRY-ay-tid) or ‘‘furrowed,’’ that is cross-striped with

blackish lines.

STRUCTURES WITHIN THE MUSCLE FIBERS

An important question for us to ask is, ‘‘Okay, but so far we haven’t learned

how the internal anatomy of a skeletal muscle explains how it contracts

(shortens) and provides the pulling force for body movements.’’

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Fig. 14.5 The internal anatomy of a skeletal muscle.

Ultimately, the answer to this question will require us to examine the inner

anatomy of an individual skeletal muscle ﬁber, which is actually a long, ﬁber-

shaped cell.

As Figure 14.6 reveals, the striated (cross-striped) muscle cell or ﬁber

contains numerous myoﬁbrils (my-uh-FEYE-brils). The word myoﬁbril

literally means ‘‘little ﬁber’’ (ﬁbril) of a ‘‘muscle’’ (my). Each of these myoﬁ-

brils is actually a slender, ﬁber-shaped, cell organelle. The myoﬁbrils have a

dark-and-light banding pattern. The dark bands are called the A bands, while

the light bands are called the I bands. The striations (stry-AY-shuns) or cross-

stripes of each muscle ﬁber, then, in reality just represent the dark A bands of

their myoﬁbrils, stacked one upon the other to make a stripe.

Within the middle of each light I band is a dark, zig-zagging Z-line. These

dark lines mark oﬀ a series of sarcomeres (SAR-koh-meers). A sarcomere is a

short ‘‘segment’’ (-mere) of ‘‘ﬂesh’’ (sarc): that is, a region of myoﬁbril

between two Z-lines. Hence, each myoﬁbril organelle within a muscle ﬁber

basically consists of a series of sarcomeres, attached end-to-end.

Myofilaments and the Sliding Filament Theory

Whether one is considering the contraction of muscles in humans, ﬁsh,

amphibians, reptiles, birds, or practically any other vertebrate animals, the

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Fig. 14.6 A look within a muscle ﬁber (cell).

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