CH. 10 Muscle Tissue and Organization

Question 1

List and explain the five unique properties of muscle tissue.

Answer 1

Excitability is the ability of a cell to respond to a stimulus (e.g.,
chemical, stretch). When a muscle cell is stimulated, a local
electrical change occurs that then sweeps across the entire plasma
membrane. Skeletal muscle cells are specifically stimulated by
neurotransmitter released from neurons (see section 10.3b).
■ Conductivity involves an electrical change that travels along
the plasma membrane during a muscle or nerve impulse. Both
muscle cells and neurons exhibit conductivity.
■ Contractility is exhibited when contractile proteins within
skeletal muscle cells slide past one another and the muscle cell
shortens. Contractility is what enables muscle cells to cause
body movement and to perform the other functions of muscles.
■ Elasticity is the ability of a muscle to return to its original
length following either shortening or lengthening of the muscle.
■ Extensibility is the lengthening of a muscle cell. For example,
when you flex your elbow joint, you are shortening the biceps
brachii on the anterior side of your arm, while the triceps
brachii on the posterior side is lengthening with the motion.
The reverse is true when you straighten your elbow joint.

Question 2

What are the 5 functions of skeletal muscle tissue?

Answer 2

■ Body movement. Bones of the skeleton move when muscles
contract and pull on the tendons that attach the muscles to the
bones. The integrated function of the muscles, bones, and joints
can produce both the large movements involved in running
and localized movements such as underlining a word in a
book. Other types of body movements include those associated
with producing facial expressions, speaking, breathing, and
swallowing.
■ Maintenance of posture. Contraction of specific skeletal
muscles stabilizes joints and helps maintain the body’s posture,
such as holding the head and trunk erect. These postural
muscles contract continuously when you are awake to keep you
from collapsing.
■ Protection and support. Some skeletal muscles are arranged
in layers along the walls of the abdominal cavity and the
floor of the pelvic cavity. These layers of muscle protect the
internal organs and support their normal position within the
abdominopelvic cavity.
■ Regulating elimination of materials. Circular muscle bands,
called sphincters (sfingk′ter; sphincter = a band), contract and
relax to regulate passage of material. These skeletal muscle
sphincters at the orifices (or′i-fis; orificium = opening) of the
gastrointestinal and urinary tracts allow you to voluntarily
control the expulsion of feces and urine, respectively.
■ Heat production. Energy is required for muscle tissue
contraction, and heat is always produced by this energy use.
Thus, muscles are like small furnaces that continuously
generate heat and function to help maintain your normal
body temperature. When you exercise, heat is produced
by your working muscles and you feel warmer. When
you shiver, your muscles are contracting and relaxing to
produce heat.

Question 3

List and explain specific characteristics of skeletal muscle tissue.

Question 4

What is the level of organization in skeletal muscle cells?

Answer 4

If you look at a cross section (figure 10.1), each skeletal muscle
is composed of fascicles (fas′i-kĕl; fascis = bundle), which are
bundles of muscle fibers. Muscle fibers, in turn, contain cylindrical
structures called myofibrils, which are composed of myofilaments.

Question 5

What is the level of organization of layers of CT within muscle from innermost to outermost?

Answer 5

The endomysium (en′dō-mis′ē-ŭm, -miz′ē-ŭm; endon = within,
mys = muscle) is the innermost connective tissue layer. It is a
delicate, areolar connective tissue layer that surrounds and electrically
insulates each muscle fiber. It contains reticular fibers to help
bind together neighboring muscle fibers and support capillaries near
these fibers.
The perimysium (per′i-mis′ē-ŭm, -miz′ē-ŭm; peri = around)
surrounds the fascicles. The dense irregular connective tissue sheath
of the perimysium contains extensive arrays of blood vessels and
nerves (called neurovascular bundles) that branch to supply each
individual fascicle.
The epimysium (ep′i-mis′ē-ŭm; epi = upon) is a layer of dense
irregular connective tissue that surrounds the whole skeletal muscle.
Deep fascia (fash′ē-ă; band or filler), (also called visceral or
muscular fascia), is an additional expansive sheet of dense irregular
connective tissue that lies external to the epimysium. It separates
individual muscles, binds together muscles with similar functions,
forms sheaths to help distribute nerves, blood vessels, and lymphatic
vessels, and fills spaces between muscles. The deep fascia is internal
to a layer called the superficial fascia (also called the subcutaneous
layer). The superficial fascia is composed of areolar and adipose connective
tissue that separates muscle from skin.

Question 6

Explain muscle attachment and how this relates to insertion and origin points of muscles.

Answer 6

Most skeletal muscles extend between bones and cross at
least one mobile joint. Upon contraction, one of the bones moves
while the other bone usually remains fixed. Often the less mobile
attachment is called the origin and the more mobile attachment is
called the insertion.

Question 7

Why are skeletal muscles classified as voluntary muscles?

Answer 7

they are controlled by the somatic (voluntary) nervous system and
we can consciously move our skeletal muscles

Question 8

Give a general breakdown of the microscopic anatomy of skeletal muscle

Answer 8

Question 9

Differentiate between thick filaments and thin filaments.

Answer 9

Thick filaments have a diameter of about 11 nanometers and
are assembled from bundles of 200 to 500 myosin protein molecules.
Each myosin molecule in a thick filament consists of two strands; each
strand has a free, globular head and an attached, elongated tail. The
tails of the two strands are intertwined. The myosin molecules are
oriented so that their long tails extend toward the center of the thick
filament and their heads extend toward the edges of the thick filament
and project outward toward the surrounding thin filaments. During a
contraction, myosin heads form crossbridges by binding thick filaments
to actin in the thin filaments.

In contrast, thin filaments are only about 5 to 6 nanometers in
diameter (about half the diameter of thick filaments). They are primarily
composed of two strands of the protein actin twisted around
each other to form a helical shape. In each helical strand of actin,
many (300 to 400) small, spherical molecules are connected to form
a long filament resembling a string of beads. Each spherical molecule
is called G (globular) actin, and each filament composed of a strand of G-actin molecules is called F (filamentous) actin. Two regulatory
proteins, tropomyosin (trō′pō-mī ′ō-sin) and troponin (trō ′pō-nin),
are part of the thin filaments. The tropomyosin molecule is a short,
thin, twisted filament that covers small sections of the actin strands.
The troponin attaches to actin and tropomyosin. It also provides a
binding site for calcium ions.

Question 10

Explain the structure of a sarcomere.

Answer 10

The thick filaments and thin filaments overlap within a sarcomere,
forming the following regions:
■ I bands extend from both directions of a Z disc and are
bisected by the Z disc. These end regions contain only
thin filaments; this region appears light when viewed with
a microscope. At maximal muscle shortening, the thin
filaments are pulled parallel along the thick filaments,
causing the I band to disappear.
■ The A band is the central region of a sarcomere that contains
the entire thick filament. Thin filaments partially overlap the
thick filament on each end of an A band. The A band appears
dark when viewed with a microscope. The A band does not
change in length during muscle contraction.
■ The H zone (also called the H band) is the most central portion
of the A band in a resting sarcomere. This region does not have
thin filament overlap; only thick filaments are present. During
maximal muscle shortening, this zone disappears when the thin
filaments are pulled past thick filaments.
■ The M line is a thin transverse protein meshwork structure in
the center of the H zone. It serves as an attachment site for the
thick filaments and keeps the thick filaments aligned during
contraction and relaxation events.

Question 11

What occurs in the sliding filament theory during contaction of muscles?

Answer 11

The following changes occur within a
sarcomere
during a contraction:
■ The width of the A band remains constant.
■ The H zone disappears.
■ The I bands narrow or shorten in length.
■ The Z discs in one sarcomere move closer together.
■ The sarcomere narrows or shortens in length.

Question 12

What are the components of a neuromuscular junction?

Answer 12

■ The synaptic (si-nap′tik; syn = with or together, hapto = to
clasp) knob of the neuron is an expanded tip of an axon. When
it nears the sarcolemma of a muscle fiber, it expands further to
cover a relatively large surface area of the sarcolemma. A nerve
impulse travels through the axon to the synaptic knob.
■ The synaptic knob cytoplasm houses numerous synaptic
vesicles (small membrane sacs) filled with molecules of the
neurotransmitter acetylcholine (a-sē′til-kō′lēn) (ACh).
■ The synaptic cleft is a narrow space separating the synaptic
knob and the motor end plate.
■ The motor end plate is a specialized region of the
sarcolemma. It has folds and indentations to increase the
membrane surface area covered by the synaptic knob.

• ACh receptors in the motor end plate act like doors that

normally are closed. ACh is the only “key” to open these
receptor doors.
■ The enzyme acetylcholinesterase (a-sĕ′til-kō′lin-es′ter-ās)
(AChE), which resides in the synaptic cleft, rapidly breaks
down molecules of ACh that are released into the synaptic
cleft. Thus, AChE is needed so that ACh will not continuously
stimulate the muscle.

Question 13

What are the events of excitation-contraction coupling?

(hint: 5 steps)

Answer 13

1. A nerve impulse causes ACh release at a neuromuscular
   junction. ACh binds receptors on the motor end plate, initiating
   a muscle impulse.
2. The muscle impulse spreads quickly along the sarcolemma
   and into the muscle fiber along T-tubule membranes, causing
   calcium ions to be released into the sarcoplasm.
3. Calcium ions bind to troponin, causing tropomyosin to move
   and expose active sites on actin. Myosin heads attach to the
   actin and form crossbridges.
4. Myosin heads go through cyclic “attach–pivot–detach–return”
   events as the thin filaments are pulled past the thick filaments.
   ATP is required to detach the myosin heads and complete the
   sequence of cyclic events. The sarcomere shortens, and the
   muscle contracts. The cyclic events continue as long as calcium
   ions remain bound to the troponin.
5. Calcium ions are moved back into the sarcoplasmic reticulum
   by ATP-driven ion pumps to reduce calcium concentration
   in the sarcoplasm, leading to relaxation. Termination of the
   muscle impulse results in the passive sliding of myofilaments
   back to their original state.

Question 14

What are the two types of muscle contraction?

Answer 14

During an isometric (ī′sō-met′rik; iso = same, metron = measure)
contraction, the length of the muscle does not change because
the tension produced by this contracting muscle never exceeds the resistance
(load).

In an isotonic (ī′sō-ton′ik; tonos = tension) contraction, the
tension produced equals or is greater than the resistance, and then
the muscle fibers shorten, resulting in movement.

Question 15

What are the two types of isotonic contractions?

Answer 15

Concentric contractions actively shorten a muscle

Eccentric
contractions actively lengthen a muscle

Question 16

List and describe the 3 types of fibers found in muscles.

Answer 16

■ Slow oxidative (SO) fibers, also called type I, typically have
half the diameter of other skeletal muscle fibers and contain slow
myosin ATPase. These cells produce contractions that are slower
and less powerful. However, they can contract over long periods
of time without fatigue because ATP is supplied primarily
through aerobic cellular respiration. These fibers appear dark red
because of the presence of large amounts of myoglobin.
■ Fast oxidative (FO) fibers, also called intermediate fibers, or
type IIa, are the least numerous of the skeletal muscle fiber types.
They are intermediate in size and contain fast myosin ATPase.
They produce a fast, powerful contraction with ATP provided
primarily through aerobic respiration. However, the vascular
supply to fast oxidative fibers is less extensive than the network of
capillaries serving SO fibers—thus, the delivery rate of nutrients
and oxygen is lower. These fibers also contain myoglobin, but
less than the amount found in SO fibers. Consequently, these
fibers can be distinguished from SO fibers on a microscopic
image because they appear a lighter red than SO fibers.
■ Fast glycolytic (FG) fibers, also called fast anaerobic fibers,
or type IIb, are the most prevalent skeletal muscle fiber type.
They are largest in diameter, contain fast myosin ATPase, and
provide both power and speed. However, they can contract for
only short bursts because ATP is provided primarily through
anaerobic cellular respiration. These fibers appear white
because of the relative lack of myoglobin.

Question 17

What are the four different patterns of fascicle arrangement?

Answer 17

• Circular Muscle: fibers are concentrically arranged around an opening or recess
• Parallel Muscle: fascicles are parallel to the long axis of the muscle; body of muscle increases in diameter with contraction
• Convergent Muscles: triangular muscle with common attachment site; direction of pull of muscle can be changed
• Pennate Muscles: muscle body has one or more tendons and fascicles at oblique angle to tendon; multiple variations

Question 18

What are the 3 types of pennate muscles?

Answer 18

■ In a unipennate muscle, all of the muscle fibers are on the
same side of the tendon. The extensor digitorum, a long muscle
that extends the fingers, is a unipennate muscle.
■ A bipennate muscle, the most common type, has muscle fibers
on both sides of the tendon. The palmar and dorsal interosseous
muscles that attach to the metacarpals are composed of
bipennate muscle that helps adduct and abduct the digits.
■ A multipennate muscle has branches of the tendon within the
muscle. The triangular deltoid that covers the superior surface
of the shoulder joint is a multipennate muscle.

Question 19

Explain the 3 classes of levers in the human body. Which is the most prominent?

Answer 19

First-Class Levers
A first-class lever has a fulcrum in the middle, between the effort
and the resistance. An example of a first-class lever is a pair of scissors.
The effort is applied to the handle of the scissors, while the
resistance is at the cutting end of the scissors. The fulcrum (pivot for
movement) is along the middle of the scissors, between the handle
and the cutting ends. In the body, an example of a first-class lever is
the atlanto-occipital joint of the neck, where the muscles on the posterior
side of the neck pull inferiorly on the nuchal lines of the skull
and oppose the tendency of the head to tip anteriorly.

Second-Class Levers
The resistance in a second-class lever is between the fulcrum and the applied
effort. A common example of this type of lever is lifting the handles
of a wheelbarrow, allowing it to pivot on its wheel at the opposite end
and lift a load in the middle. The load weight is the resistance, and the
upward lift on the handle is the effort. A small force can balance a larger
weight in this type of lever, because the force is always farther from the
fulcrum than the resistance. In the body, second-class levers are rare, but
one example occurs when the foot is depressed (plantar flexion) so that
a person can stand on tiptoe. The contraction of the calf muscle causes a
pull superiorly by the calcaneal tendon attached to the calcaneus.

Third-Class Levers
In a third-class lever, an effort is applied between the resistance and
the fulcrum, as when picking up a small object with a pair of forceps.
Third-class levers are the most common levers in the body. One example
is found at the elbow, where the fulcrum is the joint between the
humerus and ulna, the effort is applied by the biceps brachii muscle,
and the resistance is provided by any weight in the hand or by the
weight of the forearm itself. In addition, the mandible acts as a thirdclass
lever when you bite with your incisors on a piece of food. The
temporomandibular joint is the fulcrum, and the temporalis muscle
exerts the effort, while the resistance is the item of food being bitten.

Question 20

What the 3 types of primary actions in muscle groups?

Answer 20

An agonist (ag′on-ist; agon = a contest), also called a prime
mover, is a muscle that contracts to produce a particular movement,
such as extending the forearm. The triceps brachii of the posterior
arm is an agonist that causes forearm extension.

An antagonist (an-tag′ŏ-nist; anti = against) is a muscle whose
actions oppose those of the agonist. If the agonist produces extension, the antagonist produces flexion. The contraction of the agonist stretches the antagonist, and vice versa. As this movement occurs, the
stretched muscle usually does not relax completely. Instead, the tension
within the muscle being stretched is adjusted to control the speed
of the movement and ensure that it is smooth. For example, when the
triceps brachii acts as an agonist to extend the forearm, the biceps
brachii on the anterior side of the humerus acts as an antagonist to
stabilize the movement and produce the opposing action, which is
flexion of the forearm.

A synergist (sin′ĕr-jist; ergon = work) is a muscle that assists the
agonist in performing its action. The contraction of a synergist usually
either contributes to tension exerted close to the more mobile attachment
of the muscle or stabilizes the less mobile attachment of the muscle.
Usually, synergists are most useful at the start of a movement when
the agonist is lengthened and cannot exert much power. Examples of
synergistic muscles are the biceps brachii and the brachialis muscles of
the arm. Both muscles work synergistically (together) to flex the elbow
joint. Synergists may also assist an agonist by preventing movement
at a joint and thereby stabilizing the origin of the agonist. In this case,
these synergistic muscles are called fixators.

Question 21

What are the 6 criteria in which muscles are named?

Answer 21

1. Muscle action
2. Specific body region
3. Muscle attachments
4. Orientation of muscle fibers
5. Muscle shape and size
6. Muscle heads/proximal tendons

Question 22

What characteristic is only specific to cardiac muscle and what are its features?

Answer 22

Intercalated discs; composed of desmosomes and gap junctions and join individual cells to adjacent muscle cells