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Chapter 5 Skeletal and Muscular Systems 91
The cranial bones fuse together with immovable
joints called sutures. The more prominent sutures are
as follows:
Coronal suture—joins the frontal bones and two
r
parietal bones
Squamous suture—joins the temporal bone to the
r
parietal bones
Lambdoid suture—joins the occipital bone with the
r
parietal bones
Sagittal suture—joins the two parietal bones on the
r
top of the head
Skull Development
During fetal development, bone formation of the skull
is incomplete, which allows the skull to change shape
to accommodate the birth canal. The infant has areas of
membranes or “soft spots” called
grows, the cranial bones will continue to grow and eventually close at about 18 months of age. The delay in this
process also allows the brain to grow within the cranium.
fontanelles. As the infant
Frontal
Parietal
Temporal
Facial Bones
Fourteen bones compose the facial portion of the skull
and include the following:
Mandible—lower jaw bone
r
Maxillae (pl)—two bones of the upper jaw bone
r
fused at the midline
Zygomatic bones—two bones that form the promi-
r
nences of the cheeks (most pronounced area is called
the zygomatic arch)
Nasal bones—two bones that form the bridge of the
r
nose
Lacrimal bones—two bones that form the anterior
r
medial wall of the orbital cavity
Vomer—forms the inferior part of the nasal septum
r
Palatine bones—pair of bones that form the posterior
r
part of the hard palate (roof of the mouth)
Inferior nasal conchae (pl)—two bones that form the
r
side wall of the nasal cavity
Figure 5-3 is a diagram of the cranial and jaw bones
of the skull.
There are additional bones in the skull that are not
included with the cranium or facial bones. The
cles
are three tiny bones in each middle ear. The incus
(shaped like an anvil), malleus (shaped like a hammer),
and stapes (shaped like a stirrup).
Another tiny bone, the hyoid bone, is located just
below the mandible and is a U-shaped bone that attaches
the tongue and other muscles.
ossi-
Occipital
Maxilla
Palatine
Mandible
Figure 5-3 Cranial and jaw bones of the skull.
(Reprinted from Cohen BJ. Memmler’s The Human Body
in Health and Disease. 13th ed. Philadelphia, PA: Wolters
Kluwer; 2014.)
Vertebral Column
The spine is the bony support and protective cage for the
spinal cord. Figure 5-4 is a diagram of the structure of the
vertebral column. The vertebrae are irregularly shaped
bones that are smaller at the top of the column and gradually become larger at the lower end of the spine.
The vertebral column is divided into ve sections:
Cervical—7 vertebrae, C1–C7 in the neck region; C1
r
is the atlas and C2 is the axis
Thoracic—12 vertebrae, T1–T12 in the thorax (chest)
r
region
Lumbar—5 vertebrae, L1–L5 in the lower back
r
Sacrum—1 vertebrae (5 vertebrae in a child that fuse
r
together)
Coccyx—1 vertebrae, tailbone (4–5 vertebrae in a
r
child that fuse together)
Foramen
magnum
Bones of the Trunk
The axial skeleton also includes the bones in the trunk
of the body, the central portion. The vertebral column
(spine) and thorax (chest bones) are found in the trunk.
Each vertebra, other than the axis and atlas, has a body
shaped like a drum with a disk of cartilage separating each
one. In the center of each vertebra is a hole that provides the
opening for the spinal cord to pass through. The spinous
92 Section II Anatomy and Physiology
Atlas
(1st cervical)
Cervical
vertebrae
Thoracic
ve
rtebrae
Lumbar
ve
rtebrae
Sacral
ve
rtebrae
Coccygeal
ve
rtebrae
Axis
(2nd cer
Transverse
process
Intervertebral
disk
Body of
vertebra
Spinous
process
Intervertebral
foramen (for
spinal nerve)
Sacrum
Coccyx
vical)
process is the posterior projection, and a transverse process
lies on each side of the vertebra. These processes provide a
place of attachment of the muscles of the back. There are
four curves in the vertebral column that provide the exibility necessary for balance and movement.
Thoracic Bones
The thorax is formed by the ribs, which attach to the spinal
vertebrae in the back and to the sternum in the front of the
body. These bones form a cage to protect the vital internal
organs of the thorax, primarily the heart and lungs. The
upper most portion of the sternum is the manubrium. The
collarbone (clavicle) connects on the right and left sides of
the manubrium. The largest middle portion of the sternum
is the body, and the lowest part of the sternum is the tip
called the xiphoid process. It is made of cartilage, not bone.
The 12 pairs of ribs are classied as:
True ribs—rst seven pairs that attach directly to the
r
sternum
False ribs—ve remaining pairs of ribs; attach to car-
r
tilage of the rib above
Floating ribs—last two pairs of false ribs; no anterior
r
attachment to the sternum
Figure 5-4 Vertebral column. (Reprinted from Cohen BJ.
Memmler’s The Human Body in Health and Disease.
13th ed. Philadelphia, PA: Wolters Kluwer; 2014.)
T1
1
2
3
9
8
10
4
5
6
7
12
11
T11
T12
L1
L2
Floating ribs (11 and 12)
True ribs
(1-7)
False ribs
(8-12)
Each rib is separated by an intercostal space, which contains muscles, blood vessels, and nerves. Figure 5-5 shows
the thoracic bones demonstrating the different types of ribs.
Clavicular notch
Manubrium
Sternal angle
Stern
um
Body
Xiphoid
process
Intercostal space
Costal cartilage
Figure 5-5 Thoracic bones. (Reprinted from Cohen BJ. Memmler’s The Human Body in
Health and Disease. 13th ed. Philadelphia, PA: Wolters Kluwer; 2014.)
Chapter 5 Skeletal and Muscular Systems 93
The Appendicular Skeleton
The term appendicular means pertaining to the appendages. This skeleton has an upper and lower division.
The arms and shoulders are in the upper division, and
the legs and hips are in the lower division. There are
126 bones in the appendicular skeleton.
The Upper Division
The upper division includes the shoulder girdle, arms,
wrists, hands, and ngers. There are two major bones in
the shoulder girdle: the clavicle, commonly called the collar bone, and the scapula, called the shoulder blade. The
scapula has a projection, the acromion process, that connects with the clavicle. Inferior to the acromion process is
a socket, the glenoid cavity, which is the place where the
head of the humerus joins forming a ball and socket joint.
This joint allows free movement of the arm. Figure5-6 is
a view of the anterior and posterior shoulder.
The upper arm bone, the humerus, meets the forearm bones, radius and ulna, to form the elbow joint.
When standing in anatomical position, the radius is
located on the thumb side and the ulna on the little nger side. Below the forearm are the carpals (wrist bones),
the metacarpals (bones of the hand), and the phalanges
(ngers).
There are 14 phalanges, three in each nger and two
in the thumb. The phalanges connect at the joints called
the knuckles. In charting patient information, the physician will refer to the joints of the ngers as the MP
joint, metacarpophalangeal joint, the PIP joint, proximal interphalangeal joint, and the DIP joint, distal interphalangeal joint. See Figure 5-7 is a detailed diagram of
the hand.
The Lower Division
The lower division includes the pelvic girdle (hip), legs,
ankles, feet, and toes. The primary bone of the pelvic
girdle is the hip bone. This bone begins as three separate
Phalanges:
Distal phalanx
Middle phalanx
Proximal phalanx
Metacarpal
bones
Carpal
bones:
Hamate
Pisiform
Triquetral
Lunate
Ulna
Figure 5-7 Diagram of the hand. (Reprinted from
Cohen BJ. Memmler’s The Human Body in Health and
Disease. 13th ed. Philadelphia, PA: Wolters Kluwer; 2014.)
Carpal
bones:
Trapezoid
Trapezium
Capitate
Scaphoid
Radius
bones, which then fuse together to become one continuous bone. The three bones are the ilium, ischium, and
pubis. In the anterior part of the pelvis, the hip bones
come together as a joint called the pubic symphysis.
During pregnancy and childbirth, this joint becomes
exible allowing for the passage of the baby during
delivery.
The fused pelvic bones form a socket that allows the
head of the femur to t and form the hip joint. This
socket is called the acetabulum. See Figure 5-8 for a diagram of the pelvic bones.
The lower extremity, the leg, has three major bones.
The femur, the thigh bone, is the longest and strongest
bone in the body. It extends from the hip to the knee.
The lower leg, below the knee, consists of the tibia, the
shin bone, and the bula. The tibia is the larger, longer
Coracoid
process
Clavicle
Acromion
Coracoid
process
Humerus
Scapula
A
Anterior view
Glenoid
B
Acromion
cavity
Infraspinous
fossa
Posterior view
Supraspinous
fossa
Spine
Vertebral
border
Figure 5-6 Anterior and posterior
shoulder. (Reprinted from Cohen BJ.
Memmler’s The Human Body in Health and
Disease. 13th ed. Philadelphia, PA: Wolters
Kluwer; 2014.)
94 Section II Anatomy and Physiology
Ilium
Sacrum
Ischial
spine
Pubic
symph
ysis
Figure 5-8 Diagram of pelvic bones. (Reprinted from
Cohen BJ. Memmler’s The Human Body in Health and
Disease. 13th ed. Philadelphia, PA: Wolters Kluwer; 2014.)
bone. The bula is slender and lies laterally to the tibia.
The patella, kneecap, is a sesamoid bone that is held in
place by a tendon.
The ankle is formed from the distal end of the tibia
and the tarsal bones (ankle bones). The largest tarsal
bone is the calcaneus (heel bone). The metatarsal bones
form the foot and the phalanges, the toes. There are
three phalanges in each toe except the great toe, which
has two phalanges.
Pubis Ischium
Pubic arch
Iliac crest
Anterior
superior
iliac spine
ulum
Acetab
(socket for
femur)
Obturator
foramen
Study Skill
Repeat, repeat, and repeat again. Repetition is the
best way to approach learning a group of items like
the bones of the skeleton. One suggestion is to use
the diagrams in the textbook as a learning tool.
Simply place a piece of paper over the skeletal diagram and cut out part of the paper only to reveal
the bones and the lines leading to them. Make sure
none of the terms can be seen. Now, on the paper,
write the name of each bone next to the line leading
to that bone. Spelling counts, so try your best, but
always correct any misspelled words. This is a very
inexpensive way to learn the skeleton. It also provides a great visual if you are that kind of learner.
C
O
G
BONE DISORDERS
Infections
r
Structural
r
Fractures
r
Metabolic Disorders
Metabolic problems result from a breakdown of bone
tissue due to impaired function of the osteoblasts, cells
that build bone tissue, or osteoclasts, cells that breakdown bone tissue, or from a lack of specic minerals
or proteins that are required for proper bone matrix.
Osteoporosis is a condition associated with fragile,
porous bones due to a loss of bone mass. This condition will put the person at risk of fractures especially the
weight-bearing bones such as the hips and spine. Both
men and women can develop osteoporosis merely from
the aging process; however, postmenopausal women are
at a higher risk for development of osteoporosis when
the estrogen levels drop. Although hormone replacement is a treatment for this condition, HRT (hormone
replacement therapy) increases the risk of developing
breast cancer. Bone growth is also stimulated by weightbearing exercises such as walking and weight lifting.
Bone density testing is important to determine the risk
of developing osteoporosis and to monitor the progression of the disorder.
Another metabolic disorder is Paget disease or
itis deformans
. This condition causes an increase in
bone mass due to overactivity of the osteoblasts in
response to excessive osteoclast activity. Basically, bone
mass increases due to increased breakdown of bone tissue. It might sound good to have a lot of bone mass, but
the quality of the bone is weak and deformed. The bones
fracture easily.
Osteomalacia, softening of bones, is due to a lack
of calcium in the bones. This condition may stem from
a vitamin D deciency or disease of the liver or kidneys. When osteomalacia occurs in children, it is called
rickets. Rickets could be related to poor diet or inad-
equate sunlight, which is required to synthesize the
vitamin D.
A more rare metabolic disorder is osteogenesis
imperfecta
, a genetic disorder resulting in defective collagen production. The bones become brittle and break
easily. Some children suffer multiple fractures from simple causes, and some fractures are even evident prior to
birth.
oste-
There are various types of bone disorders categorized by
the origin of the disorder. X-rays and laboratory testing
are used to diagnose these disorders. The main categories of disorder include the following:
Metabolic
r
Tumors
r
Tumors
A neoplasm (new growth) is also known as a tumor.
Tumors may be benign (noncancerous) or malignant
(cancerous). Osteosarcoma and chondrosarcoma
are malignant tumors that develop within the bone
Chapter 5 Skeletal and Muscular Systems 95
(oste/o) or cartilage (chondr/o). Most osteosarcomas
develop in a young person’s bones, especially around
the knee.
Bone Infections
Bone infections are caused from bacteria invading the
bone through injury where the skin is broken or through
the bloodstream. Osteomyelitis is caused by pyogenic
bacteria (pus-forming). The infection may remain localized in one area of the bone or spread throughout the
bone infecting the marrow and periosteum. Antibiotics
are used to treat bone infections and usually have a good
outcome for the patient.
Although you may not think tuberculosis is associated with bone infections, TB is able to spread to the
bones, usually the bones of the extremities. Pott disease is tuberculosis of the spine. The vertebrae become
infected, are weakened, and collapse causing deformity
and pressure on the spinal cord.
Structural Diseases
The structural diseases of the bones include abnormal
spinal curves. Figure 5-9 shows the three abnormal spinal curves.
These include the following:
Kyphosis—known as hunchback; an exaggeration of
r
the thoracic spinal curve
Lordosis—known as swayback; an excessive lumbar
r
curve
Scoliosis—most common type of abnormal curve; a
r
lateral curvature of the spine
Another type of structural disorder occurs when the
maxillary bones (upper jaw) do not fuse during development of the fetus. This malformation results in a condition called cleft palate, an opening in the roof of the
mouth. Surgery is used to correct the defect.
Fractures
What is the difference between a broken bone and a
fractured bone? Other than the name, there is no difference. The words are interchangeable. A fracture
(break) is usually caused from external force placed on
the bone. Figure 5-10 shows the various types of bone
fractures.
There are different types of fractures including
Open fracture—a fractured bone that protrudes
r
through the skin creating a visible external wound of
the skin
Closed fracture—a fractured bone without injury to
r
the skin; no external wound
Greenstick fracture—a partial break in the bone simi-
r
lar to bending a green stick, most common in children
Impacted fracture—the bone breaks and the broken
r
ends become jammed into each other
Comminuted fracture—the bone breaks into many
r
pieces, typically seen in a crushing injury
Spiral fracture—the bone breaks as it twists, like
r
wringing a mop
Transverse fracture—the bone breaks completely
r
across the bone
Oblique fracture—the bone breaks at an angle across
r
the bone
Scoliosis Kyphosis Lordosis
Figure 5-9 Abnormal spinal curves. (Reprinted from
Cohen BJ. Memmler’s The Human Body in Health and
Disease. 13th ed. Philadelphia, PA: Wolters Kluwer; 2014.)
Case Question
A
F
F
How would you explain this to a 7-year-old? If
Hunter’s mother questioned you about the term
greenstick, how would you explain this to her?
There can be dangerous complications involved with
a fractured bone. First aid to prevent further injury is
a key element in the treatment of an individual with a
fractured bone. The fracture needs to be protected by
splinting the area “as is.” Do not try to align the bone.
Leave that to the experts. Since a main function of bones
is to provide protection to underlying structures, a fractured rib may puncture vital organs such as the heart or
lungs. A fracture of the spine may result in injury to the
spinal cord.
Hunter, the boy from our case study,
asks what the difference is between
a broken bone and a fractured bone.
96 Section II Anatomy and Physiology
Closed Open Greenstick Comminuted
Oblique
fracture
Figure 5-10 Bone fractures. (Reprinted from Kronenberger J, Ledbetter J. Lippincott Williams & Wilkins’
Comprehensive Medical Assisting. 5th ed. Philadelphia, PA: Wolters Kluwer; 2016.)
C
O
G
JOINTS
Impacted
fracture
A joint is formed by the junction of two or more bones.
Also called an articulation, they are initially classied
according to the degree of movement allowed by the
joint. There are three classications:
Synarthrosis—immovable joints; also called brous
r
joints, they come together and do not allow any
movement; an example is the sutures of the skull
Amphiarthrosis—slightly movable joints; also referred
r
to as cartilaginous joints; an example is the joint created where the pubic bones join at the symphysis pubis
Spiral
fracture
Transverse
fracture
Diarthrosis—freely movable joints; most common
r
joint in the body; joint cavity has a synovial lining
and the cavity is lled with synovial uid, which acts
as a cushion for the joint; an example is the knee joint
The synovial joints need supporting structures to provide stability to these freely moving joints.
connect bone to bone around synovial joints. They are
bands of very dense connective tissue that reinforce the
joints. The joints are also are cushioned by a layer of
articular cartilage located at the ends of the bones. Close
to some of the joints are small sacs lled with synovial
uid. These sacs, called bursae (pl), help to reduce the
stress around the joints allowing for ease of movement.
Ligaments
Chapter 5 Skeletal and Muscular Systems 97
Synovial joints are further classied by the type of
movement they provide for the body. The movement is
based on the body in anatomical position. Figure 5-11
shows the various types of joint movement.
The types of movement are as follows:
Flexion—decreases the angle of the joint; bending
r
the elbow decreases the angle between the upper and
lower arm
Dorsiexion—moving the foot upward toward the body
r
decreasing the angle between the foot and lower leg
Plantar exion—opposite of dorsiexion; bend-
ing the foot downward as in standing on the tiptoes; increases the angle between the foot and
lower leg
Extension—increases the angle of the joint; straight-
r
ening of the arm from a exed position
Hyperextension—extending a body part beyond
the anatomical position; hyperextending the knee
when kicking a ball or hyperextending the elbow
when throwing a ball
Abduction—moving away from the midline of the
r
body; example is motion when moving the arm
straight out to the side
Adduction—moving toward the midline of the body;
r
example is motion when moving the arm toward the
body, opposite of abduction
Inversion—turning the sole of the foot inward so the
r
bottom of the foot faces the other foot
Eversion—turning the sole of the foot outward so the
r
bottom of the foot faces away from the body
Circumduction—moving in a circular pattern as
r
when moving the outstretched arm in a circle
Rotation—turning a bone on its own axis; turning the
r
head from side to side to indicate no; the axis and
atlas bones allow this movement
Supination—turning the palm of the hand up; for-
r
ward when in anatomical position
Pronation—turning the palm of the hand down;
r
backward when in anatomical position
The various types of movement require different types
of synovial joints. These types include the following:
Gliding joint—bones slide over each other with little
r
change in the joint angle; example is the joints formed
between the wrist and ankle joints
Hinge joint—bones t together due to the curves they
r
have; one curves outward, the other inward; example
is the elbow and nger joints
Pivot joint—a bone with a rounded part rests on and
r
ts into another bone with a groove; example is atlas
and axis of the spine allowing rotation of the head
Condyloid joint—the oval part of a bone ts into the
r
concave part of another bone; example is the knuckles
where the metacarpals join the proximal phalanges
Saddle joint—resembles a saddle with one bone sit-
r
ting on another bone like a person sitting on a saddle;
Flexion/extension
Circumduction
Rotation
Pronation/supination
Dorsiflexion/plantar flexion
Abduction/adduction
Inversion/eversion
Figure 5-11
movement. (Reprinted from Cohen BJ.
Memmler’s The Human Body in Health
and Disease. 13th ed. Philadelphia,
PA: Wolters Kluwer; 2014.)
Types of joint
98 Section II Anatomy and Physiology
example is the thumb where the wrist bone and metacarpal of the thumb come together
Ball and socket joint—ball-shaped, rounded end of
r
the bone ts into a cup-like depression of another
bone; example is hip joint or shoulder joint
Joint Disorders
There are various conditions affecting the joints that
may be caused by injury or inammation. These disorders include the following:
Dislocation—the structures of the joint become
r
deranged. The shoulder is the most common joint to
become dislocated.
Sprain—excessive movement of a joint resulting in a
r
rupture or tearing of the ligaments surrounding the
joint. The ankle is the most common joint to become
sprained. Typically caused from inward turning of the
foot.
Bursitis—inammation of a bursa. One of the com-
r
mon locations for bursitis is in front of the knee. Also
known as “housemaid’s knee” because of the effect
kneeling has on this area.
Arthritis—inammation of a joint
r
Osteoarthritis (OA), also known as degenerative
joint disease (DJD), is a form of arthritis caused
from normal wear and tear on joints
Rheumatoid arthritis (RA), an autoimmune disease
and form of arthritis that causes swelling and crippling of the hands and feet; deformation results
from the crippling; Figure 5-12 demonstrates the
disguring effects of rheumatoid arthritis
Gout, a type of arthritis caused by a metabolic disturbance; overproduction of uric acid, which accumulates in the joints; most affected joint is the big toe
Disorders of the Spine
The vertebrae of the spine are held together by joints,
which are surrounded by ligaments, and muscles, and
have an intervertebral disk between them. The spine
is subject to numerous disorders that frequently cause
back pain, more likely low back pain.
Herniated disk—the disk between a vertebra pro-
r
trudes or “slips” out of the normal space and pushes
on surrounding nerves; sciatica results from disk pressure on the sciatic nerve
Strained ligaments of the spine—most commonly
r
located in the lumbosacral joint of the lower back
Taking Care of the Joints
There are joint disorders that may be diagnosed and
treated by primary care physicians; however, a referral to
an orthopedic specialist may be required for further evaluation and possible invasive assessment of the problem.
Orthopedic physicians also specialize in surgical procedures to correct musculoskeletal problems. Physical therapy and medication may be indicated to treat some joint
disorders. Procedures that assist in the examination and
treatment of the joints include the following:
Arthroscopy—a procedure using an arthroscope
r
(endoscope), a lighted instrument, to examine the
inside of a joint; during arthroscopy, ligament
replacement and cartilage repair can be made. See
Figure 5-13, which is an image of an endoscope used
to examine the knee joint.
Arthrocentesis—draining accumulated uid from a
r
joint cavity; typically caused by injury to the joint
Arthroplasty—surgical joint replacement; knee and
r
hip joints are most common joints that are replaced
Figure 5-12 Effects of rheumatoid arthritis. (Reprinted
from Kronenberger J, Ledbetter J. Lippincott Williams
& Wilkins’ Comprehensive Medical Assisting. 5th ed.
Philadelphia, PA: Wolters Kluwer; 2016.)
Patient Education
Throughout our lifetime, the aging process causes
changes to the musculoskeletal system. As our
body’s metabolism slows, we start to lose calcium
necessary to keep the bones strong. As the bones
become weaker and more fragile, they are subject
to injury including fractures. Muscle tissue is also
lost over time, which may affect our balance and
reexes. These changes can all lead to increased risk
of falls and injury. As we age, we also become more
sedentary and decrease exercise. One way to help
maintain the musculoskeletal system is to continue
to exercise even for a few minutes a day, and it does
not have to be strenuous exercise. Simple walking
is great to help maintain the bones and muscles. A
well balanced diet is also necessary to provide the
nutrients for proper bone and muscle repair.
Chapter 5 Skeletal and Muscular Systems 99
Endoscope
Patella
Tibia
Femur
Figure 5-13 Endoscope exam of knee joint. (Reprinted
from Cohen BJ. Memmler’s The Human Body in Health
and Disease. 13th ed. Philadelphia, PA: Wolters Kluwer;
2014.)
C
O
G
The muscular system has three types of muscle tissue:
smooth, cardiac, and skeletal muscle. Since you have
been introduced to the skeletal system in the rst part
of this chapter, this section focuses primarily on skeletal muscle. First let’s discuss the smooth and cardiac
muscle.
THE MUSCULAR SYSTEM
Smooth Muscle
This muscle is found in the walls of the hollow body
organs such as the blood vessels, digestive system, and the
pathways of the respiratory system. It is an involuntary
muscle, which means that we do not control the action
of this muscle by our conscious thoughts. It is made up
of elongated spindle-shaped cells with no striations. This
muscle functions automatically using peristalsis, a wavelike motion, to move the substances through the system.
Smooth muscle found in the blood vessels can regulate
the size or diameter of the vessel by dilating or constricting. An example of the action of smooth muscle is found
in the muscular wall of the uterus, which contracts during childbirth. In this case, the uterine muscle contracts
from hormonal stimulation.
Cardiac Muscle
Found only in the heart, cardiac muscle is also involuntary and appears striated. Each heartbeat results from
the involuntary contraction and relaxation of cardiac
muscle. The normal heart beats 60 to 80 times per minute. Cardiac muscle is regulated by nerve stimulation
and messages received from the central nervous system
(brain and spinal cord).
Skeletal Muscle
This muscle is the only voluntary muscle in the body
and relies on the nervous system to provide stimulation
to the muscle. We consciously control this muscle. It
appears striated having long and cylindrical cells. These
are referred to as muscle bers. Each muscle ber can
contract as a single unit when it is stimulated. This muscle tissue can contract and relax very rapidly allowing
body movement.
It is skeletal muscle because these muscles attach to
the bones, allowing movement of the joints. Some voluntary, skeletal muscles do not provide joint movement
such as those found in the abdominal wall. We can control the function of these muscles, but primarily, they
provide support to maintain the body in sitting and
standing positions. The facial muscles are another good
example of voluntary muscles that allow facial expression but no joint movement.
There are more than 600 skeletal muscles accounting for the largest portion of the body’s muscle mass,
approximately 40% of our total body weight. The
three primary functions of the skeletal muscles are as
follows:
Skeletal movement—the muscles are attached to the
r
bones and shorten or lengthen to cause the bones to
move at the joints.
Support body posture—some of the muscles remain
r
in a steady partial contracted state called muscle tone.
This allows the body to maintain posture.
Produce heat—the muscles generate most of the
r
body’s heat necessary to keep it at the normal temperature of 98.6°F. When we are cold, the muscles
increase contractions causing us to shiver that produces heat to warm us.
Muscle Tissue
Skeletal muscles are made of bers bound together into
bundles. These bundles are then bound together to form
a muscle. Each of these bundles and the muscle has its
own covering. The coverings are made up of connective
tissue and include the following:
Endomysium—the innermost layer that covers each
r
muscle ber
Perimysium—covers the bundles of muscle bers
r
called fascicles
Epimysium—the outermost layer that covers the
r
entire muscle
These tissues merge to form the
connective tissues that attach the muscles to bones.
Figure 5-14 is an image of a muscle and tendon attached
to the bone. It also demonstrates the intricate parts of
the muscle structure.
tendons, the dense
100 Section II Anatomy and Physiology
Muscle
Epimysium
Tendon
Bone
Figure 5-14 Muscle and tendon attached to bone.
(Reprinted from Cohen BJ. Memmler’s The Human Body
in Health and Disease. 13th ed. Philadelphia, PA: Wolters
Kluwer; 2014.)
fascicle
Muscle fiber
(cell)
Endom
Perimysium
Blood
vessels
Body of muscle
ysium
Muscle Function
Most of us take for granted that our muscles will just
do what we want them to do without giving a second
thought. There are many processes and components
necessary to make them work properly. Let’s take a
look at the basics of muscle function. Skeletal muscles
rely on nerve impulses to function and cause them to
contract. Sensory nerve impulses carry messages from
the muscles to the brain and spinal cord where the
messages are interpreted. Then, motor nerve impulses
send messages from the brain and spinal cord back
to the muscles signaling them to contract and cause
movement.
In order for a muscle to shorten and become thicker,
the muscle tissue must have contractibility. The muscle
bers contain many threads or laments that are composed of two types of protein that provide the ability of
the muscles to contract. Under a microscope, the protein
actin appears as a light lament and myosin appears
darker. The difference in the appearance of these proteins is what gives skeletal muscle the appearance of
striation.
Muscles also require energy for action. This energy
comes from adenosine triphosphate (ATP). Calcium
is also necessary for muscle contraction. It creates the
bridge between actin and myosin so the muscle can
effectively contract.
ATP can be produced through metabolism using oxygen (aerobic) or without oxygen (anaerobic). Aerobic
exercise increases oxygen use and improves muscle
endurance. Exercise such as riding a bike, swimming,
and running requires more endurance. Athletes improve
their ability to generate ATP and get rid of waste products through endurance training. Since anaerobic exercise does not require oxygen to produce ATP, this type
of muscle function uses glucose in the form of glycogen to make ATP. This form of exercise provides the
small amount of energy for short-term vigorous exercise
versus the endurance type of exercise. During anaerobic exercise, lactic acid, a chemical, can accumulate in
the muscles and cause muscle fatigue. When this substance accumulates, the muscle contractibility decreases.
Muscle cramps may occur from the increased muscle
fatigue.
Exercising the Muscles
To ensure that the muscles are well conditioned and
toned, we need to exercise them, so they will adapt
to the increased workload when needed. The type of
exercise affects the change in the muscle. For example,
weight lifting increases the size or bulk of a muscle.
This is referred to as
hypertrophy. In contrast, endur-
ance training exercises increase the muscle’s ability to
withstand lengthy exercise without increasing the size
of the muscle. Male sex hormones have a positive effect
on the building of muscles. This accounts for number of
athletes who have used steroid preparations to increase
the size of their muscles.
One of the main muscles requiring exercise is the
cardiac muscle found in the heart. Routine exercise
strengthens the heart muscle so it can continue to efciently pump throughout our lifetime. You have probably heard of this type of exercise as “cardio.” Because
they routinely exercise, well-conditioned athletes typically have a slower resting heartbeat.
Muscle Contractions
Even when you are sitting or lying down, your muscles
are contracted. This is a condition we refer to as muscle
tone. The muscles are always ready for action when the
nerves stimulate them. When we don’t use our muscles,
they become weak and abby. The body depends on two
types of contractions:
Isotonic contraction—the muscle tension remains
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the same, but it changes in length; the word literally
means equal or same tension
Isometric contraction—the muscle tension increases,
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but the size remains the same; the word means equal
or same measure
When you are walking, you are performing both
types of contractions. Isotonic contractions move
you and isometric contractions keep your body in
position.