Download Chapter 6: The Muscular System

Chapter 6 Notes: The Muscular System
I.
Overview of Muscle Tissues
The essential function of muscle is contraction- a unique characteristic that sets it apart
from any other body tissue.

In all its forms, muscle makes up nearly half the body’s mass.
A. Muscle Types: Table 6.1, pg. 155

There are three types of muscle tissue. In some ways, muscle tissues are the same.
1. All muscle cells are elongated. So they are all called muscle fibers.
2. The ability to contract depends on two types of myofilaments.
3. Terminology – myo and mys mean muscle; sacrco means flesh.

The three types of muscles differ in cell structure, body location, and how they are
stimulated to contract.
1. Skeletal Muscle
a. Skeletal muscle fibers are packaged into the organs called skeletal muscles
that attach to the body’s skeleton.
b. Skeletal muscle fibers are cigar-shaped, multinucleate cells, and the largest of
the muscle fiber types.
c. Striated muscle because its fibers appear to be striped
d. Voluntary muscle because it is the only muscle type subject to conscious
control.
e. Skeletal muscle can contract rapidly and with great force, but it tires easily
and must rest after short periods of activity.
f. The reason skeletal muscles are not ripped apart as they exert force is that
thousands of their fibers are bundled together by connective tissue, which
provides strength and support to the muscle as a whole. FIG. 6.1, pg. 156

Each muscle fiber is enclosed in a delicate connective tissue sheath
called an endomysium.

Several sheathed muscle fibers are then wrapped by a courser
fibrous membrane called a perimysium to form a bundle of fibers
called a fascicle.

Many fascicles are bound together by an even tougher overcoat of
connective tissue called an epimysium, which covers the entire
muscle. These epimysia blend into the tendons, which attach
muscles indirectly to bones, cartilages, or connective tissue coverings
of each other.
2. Smooth Muscle
a. Smooth muscles are spindle shaped and have a single nucleus.
b. Smooth muscles are arranged in layers. One layer usually runs circularly, and
the other layer longitudinally. As the two layers alternately contract and relax,
they change the size and shape of the organ.
c. Smooth muscle has no striations.
d. Smooth muscle is involuntary, meaning we cannot control it.
e. Smooth muscle is found in the walls of hollow visceral organs such as the
stomach, urinary bladder, and respiratory passages. Smooth muscle propels
substances along a definite pathway. Ex; movement of food through the
digestive tract or emptying the bowels or bladder.
f. Smooth muscle contractions are steady and are tireless.
3. Cardiac Muscle
a. Cardiac muscle is found in only one place in the body – the heart.
b. Cardiac muscle, like skeletal muscle, is striated.

II.
c. Cardiac muscle, like smooth muscle, is involuntary.
d. The cardiac fibers are cushioned by small amounts of soft, connective tissue
and arranged in spiral or figure 8 shaped bundles.
e. Cardiac muscle fibers are branching cells joined by special junctions called
intercalated disks.
f. Cardiac muscle contracts at a fairly steady rate, but the heart can also be
stimulated by the nervous system to shift into high gear for short periods.
B. Muscle Functions

Muscles play four important roles in the body
1. Producing Movement
2. Maintaining Posture
3. Stabilizing Joints
4. Generating Heat – As ATP is used to power muscle contraction, nearly ¾ of its
energy escapes as heat. This heat is vital in maintaining normal body temperature.
Microscopic Anatomy of Skeletal Muscle: Fig. 6.3, pg. 159
A. Many oval nuclei can be seen just beneath the plasma membrane, which is called the
sarcolemma in muscle cells.
B. The nuclei are pushed aside by long ribbonlike organelles, the myofibrils, which nearly
fill the cytoplasm. The myofibrils are chains of tiny contractile units called sarcomeres,
which are aligned end to end.
C. It is the arrangement of even smaller structures (myofilaments) within the sarcomere that
produce a banding pattern.
1. Alternating light (I band) and dark (A band) bands along the length of the perfectly
aligned myofibrils give the muscle cell as a whole its striped appearance.
a. The I band has a midline interruption, a darker area, called the Z line.
b. The dark A band has a lighter central area called the H zone.
2. There are two types of myofilaments:
a. The larger thicker filaments: myosin filaments
 Made mostly of bundled molecules of the protein myosin, but they also
contain ATPase enzymes which split ATP to generate the power for
muscle contraction.
 Extend the entire length of the dark A band.
 The midparts of the thick filaments are smooth, but their ends are studded
with small projections, or myosin heads. These myosin heads are
sometimes called crossbridges because they link the thick and thin
filaments together during contraction.
b. The thin filaments: actin filaments
 Made of the contractile protein called actin, plus some regulatory proteins
that play a role in allowing or preventing myosin head binding to actin.
 Anchored to the Z line
 The light I band is an area that includes parts of two adjacent sarcomeres
and contains only the thin filaments.
 Although the thin filaments overlap the ends of the thick filaments, they
do not extend into the middle of a relaxed sarcomere, and thus the central
region (the H zone) looks a big lighter.
D. When contracting occurs the actin-containing filaments slide toward each other into the
center of the sarcomeres, these light zones disappear because the actin and myosin
filaments are completely overlapped.
E. Another very important muscle fiber organelle, the sarcoplasmic reticulum:
1. a specialized smooth endoplasmic reticulum
2. Interconnecting tubules and sacs of the SR surround each and every myofibril.
3. Major role of the system is to store calcium and to release it on demand when the
muscle fiber is stimulated to contract.
III.
Muscle Movements, Types, and Names
A. 5 Golden Rules of Skeletal Muscle Activity
1. All muscles cross at least one joint.
2. Typically, the bulk of the muscle lies proximal to the joint crossed
3. All muscles have at least two attachments; the origin (attachment at immoveable or
less moveable bone) and the insertion (attachment onto the moveable bone). When
the muscle contracts, the insertion moves toward the origin.
4. Muscles can only pull; they never push. This means that most muscles must work
in pairs. Muscles are arranged on the skeleton in such a way that whatever one
muscle can do, another can to the reverse.
5. During contraction, the muscle insertion moves toward the origin.
B. Types of Body Movements – Refer to Fig. 6.13, pgs. 169-170, or color code handout
given in class
1. Flexion:
2. Extension
3. Abduction
4. Adduction
5. Rotation
6. Circumduction
7. Pronation
8. Supination
9. Inversion
10. Eversion
11. Dorsiflexion
12. Plantar flexion
C. Types of Muscles
1. Muscles can’t push; they can only pull as they contract. Most often body
movements are the result of the activity of pairs or teams of muscles acting
together or against each other.
2. Primer mover: when several muscles are contracting at the same time, the muscle
that has the major responsibility for causing a particular movement. AND
Antagonists are muscles that oppose or reverse a movement.
o When a prime mover is active, its antagonist is stretched and relaxed.
o Example: biceps of the arm antagonized by the triceps
3. Synergists help prime movers by producing the same movement or by reducing
undesirable or unnecessary movement.
o When a muscle crosses two or more joints, its contraction will cause
movement in all the joints crossed unless synergists are there to stabilize
them.
o Example: you can make a fist without bending your wrist because
synergist muscles stabilize the wrist joints and allow the prime mover to
act n the finger joints.
D. Naming Skeletal Muscles
1. Muscles are named based on several criteria, each of which focuses on a particular
structural or functional characteristic.
a. Direction of Muscle Fibers
a. Rectus – straight, its fibers run parallel to an imaginary line
b. Oblique – slanted, its fibers run obliquely from that imaginary line
IV.
b. Relative size of the muscle
a. Maximus – largest
b. Minimus – smallest
c. Longus – long
c. Location of the muscle
a. Some muscles are named for the bone with which they are
associated.
b. Example: the temporalis and frontalis muscles overlie the
temporal and frontal bones of the skull.
d. Number of origins
a. Biceps, triceps, quadriceps forms part of a muscle name, one can
assume that the muscle has two, three, or four origins, respectively
e. Location of the muscle’s origin and insertion
a. Occasionally, muscles are named for their attachment sites.
b. Example: the sternocleidomastoid muscle has its origin on the
sternum (sterno) and the clavicle (cleido) and inserts on the
mastoid (mastoid) process of the temporal bone.
f. Shape of the muscle
a. Some have a distinctive shape for which they are named.
b. Example: deltoid muscle is triangular and deltoid means
triangular.
g. Action of the muscle
a. Extensor: extensor muscles of the wrist all extend the wrist
b. Adductor: adductor muscles of the thigh all bring about its
adduction.
Gross Anatomy of Skeletal Muscle – Fig. 6.20 and Fig. 6.21 – Be able to label major
muscles
A. Head Muscles
1. Facial Muscles
a. Frontalis – covers the frontal bone – allows you to raise eyebrows or wrinkle
forehead
b. Orbicularis Oculi – run circles around the eyes
c. Orbicularis Oris – kissing muscle
d. Zygomaticus – smiling muscle
2. Chewing Muscles
a. Masseter – closes jaw by elevating mandible
b. Temporalis- covers the temporal bone; acts as a synergist of the masseter
closing the jaw.
B. Trunk and Neck Muscles
1. Anterior Muscles
a. Platysma: pulls corners of mouth inferiorly
b. Sternocleidomastoid: flexes neck, rotates head
c. Pectoralis Major: adducts and flexes humerus
d. Intercostal Muscles: deep muscles found between the ribs
e. Rectus Abdominis: flexes vertebral column
f. External oblique: flexes and rotates vertebral column
2. Posterior Muscles
a. Trapezius: extends neck and adducts scapula
b. Latissimus dorsi: extends and adducts humerus
c. Deltoid: abducts humerus
C. Muscles of the Upper Limb
1. Muscles that Move the Arm
a. Pectoralis Major: adducts and flexes humerus
b. Latissimus Dorsi: extends and adducts humerus
c. Deltoid: abducts humerus
2. Muscles that Act on the Forearm
a. Biceps Brachii: flexes elbow and supinates forearm
b. Triceps Brachii: extends elbow
3. Muscles of the Forearm that Act on the Hand bones
D. Muscles of the Lower Limb
1. Muscles Causing Movement at the Hip Joint
a. Gluteus Maximus: extends hip
b. Gluteus Medius: abducts thigh
2. Muscles Causing Movement at the Knee Joint
a. Sartorius: flexes thigh on hip
b. Quadriceps Group
c. Hamstring Group
3. Muscles Causing Movement at the Ankle and Foot
a. Tibialis Anterior: dorsiflexes and inverts foot
b. Peroneus longus: plantar flex and evert foot
c. Gastrocnemius: plantar flexes foot and flexes knee
V.
Developmental Aspects of the Muscular System
A. Muscular dystrophy: a group of inherited diseases that affect specific muscle groups
B. Myasthenia gravis: a disease characterized by drooping of the upper eyelids, difficulty in
swallowing and talking, and generalized muscle weakness and fatigability.
VI.
Skeletal Muscle Activity
A. Stimulation and Contraction of Single Skeletal Muscle Cells
1. The Nerve Stimulus and the Action Potential
a. All skeletal muscle cells are stimulated by motor neurons. A motor neuron is
one neuron and all the skeletal muscle cells it stimulates. Fig. 6.4, pg. 160.
b. When the neuron releases a neurotransmitter, acetylcholine, the permeability
of the sarcolemma changes, allowing sodium ions to enter the muscle cell.
Fig. 6.5a
c. This produces an electrical current, the action potential, which flows across
the entire sarcolemma, resulting in release of calcium ions from the
sarcoplasmic reticulum. Fig. 6.5b
2. Mechanism of Muscle Contraction: The Sliding Filament Theory
a. Notice that in the contracted sarcomere, the light H zone in the center of the A
band has disappeared, the Z lines are closer to the thick filaments, and I bands
have nearly disappered. The A bands move closer together but do not change
in length. Fig. 6.7
b. Calcium binds to regulatory proteins on the thin filaments and exposes myosin
binding sites, allowing the myosin heads on the thick filaments to attach. Fig.
6.8 a, b
c. The attached heads pivot, sliding the thin filaments toward the center of the
sarcomere, and contraction occurs. Fig. 6.8 c
d. ATP provides the energy for the sliding process, which continues as long as
ionic calcium is present.
B. Contraction of a Skeletal Muscle as a Whole
1. Graded Responses:
 Although individual muscle cells contract completely when adequately
stimulated, a muscle (the organ) responds to stimuli to different degrees.
2. Muscle Response to Increasingly Rapid Stimulation:
 Most skeletal muscle contractions are titanic, smooth and sustained, because
rapid nerve impulses are reaching the muscle, and the muscle cannot relax
completely between contractions.
3. Muscle Response to Stronger Stimuli:
 The strength of muscle contractions reflects the relative number of muscle
cells contracting:
4. Providing Energy for Muscle Contraction:
 ATP, the immediate source of energy for muscle contraction, is stored in
small amounts in muscle fibers and is quickly used up. ATP is regenerated by
3 routes.
1. creatine phosphate with ADP
2. anaerobic glycolysis and lactic acid formation
3. aerobic respiration.
5. Muscle Fatigue and Oxygen Debt:
 If muscle activity is strenuous and prolonged, muscle fatigue occurs due to an
accumulation of lactic acid in the muscle and a decrease in its energy (ATP)
supply.
 After exercise, the oxygen debt is repaid by rapid deep breathing.
6. Types of Muscle Contractions –
 Isotonic: the muscle shortens and movement occurs
 Isometric: the muscle does not shorten, but is tension increases.
7. Muscle tone
 Muscle tone keeps muscles healthy and ready to react.
 It is a result of a staggered series of nerve impulses delivered to different cells
within the muscle.
 If the nerve supply is destroyed, the muscle loses tone, becomes paralyzed,
and atrophies.
8. Effect of Exercise on Muscles
 Inactive muscles atrophy. Muscles challenged by resistance exercise to
respond (almost) beyond their ability increase in size and strength.
 Muscles subjected to regular aerobic exercise become more efficient and
stronger and can work longer without tiring.
 Aerobic exercise also benefits other body organ systems.