Chapter 11 Physiology of the Muscular System

wastecypriotInternet και Εφαρμογές Web

10 Νοε 2013 (πριν από 4 χρόνια και 6 μήνες)

142 εμφανίσεις

Chapter 11

Physiology of the Muscular


Muscular system is responsible for
moving the framework of the body

In addition to movement, muscle
tissue performs various other

General Functions

Movement of the body as a whole or
of its parts

Heat production


Function of Skeletal Muscle

Characteristics of skeletal muscle

Excitability (irritability)

ability to be


ability to contract, or
shorten, and produce body movement


ability to extend, or
stretch, allowing muscles to return to
their resting length

Overview of the muscle cell

Muscle cells are called fibers because
of their threadlike shape


plasma membrane of
muscle fibers

Sarcoplasmic reticulum (SR)

Network of tubules and sacs found within
muscle fibers

Membrane of the sarcoplasmic reticulum
continually pumps calcium ions from the
sarcoplasm and stores the ions within its
sacs for later release

Overview of the muscle cell

Muscle fibers contain many
mitochondria and several nuclei


numerous fine fibers packed
close together in sarcoplasm


Segment of myofibril between two
successive Z lines

Each myofibril consists of many sarcomeres

Contractile unit of muscle fibers

Overview of the muscle cell

Striated muscle Dark stripes called A bands;
light H zone runs across midsection of each
dark A band

Light stripes called I bands; dark Z line extends
across center of each light I band

T tubules

Transverse tubules extend across sarcoplasm at right
angles to long axis of muscle fiber

Formed by inward extensions of sarcolemma

Membrane has ion pumps that continually transport
Ca++ ions inward from sarcoplasm

Allow electrical impulses traveling along sarcolemma
to move deeper into cell

Overview of the muscle cell


Triplet of tubules; a T tubule sandwiched
between two sacs of sarcoplasmic reticulum;
allows an electrical impulse traveling along a
T tubule to stimulate the membranes of
adjacent sacs of the sarcoplasmic reticulum


Each myofibril contains thousands of thick
and thin myofilaments

Four different kinds of protein molecules make
up myofilaments


Makes up almost all the thick filament



are chemically attracted to actin



are known as cross bridges when
attached to actin


globular protein that forms two fibrous strands
that twist around each other to form bulk of thin


protein that blocks the active sites on
actin molecules


protein that holds tropomyosin molecules
in place

Myofilaments (cont.)

Thin filaments attach to both Z lines (Z
disks) of a sarcomere and extend
partway toward the center

Thick myosin filaments do not attach to
the Z lines

The mechanism of contraction

Excitation and contraction

A skeletal muscle fiber remains at rest until
stimulated by a motor neuron

Neuromuscular junction

motor neurons
connect to sarcolemma at motor endplate
(Figure 11

Neuromuscular junction is a synapse where
neurotransmitter molecules transmit signals

Excitation and contraction


neurotransmitter released
into synaptic cleft that diffuses across gap,
stimulates receptors, and initiates impulse
in sarcolemma

Nerve impulse travels over sarcolemma and
inward along T tubules, which triggers
release of calcium ions

Calcium binds to troponin, causing
tropomyosin to shift and expose active sites
on actin

Excitation and contraction

Sliding filament model

When active sites on actin are exposed, myosin
heads bind to them

Myosin heads bend, pulling the thin filaments past

Each head releases, binds to next active site, and

pulls again

Entire myofibril shortens

The mechanism of contraction


Immediately after Ca

ions are released,
sarcoplasmic reticulum begins actively
pumping them back into sacs (Figure 11


ions are removed from troponin
molecules, shutting down contraction

Energy sources for muscle

Hydrolysis of ATP yields energy required for
muscular contraction

Adenosine triphosphate (ATP) binds to
myosin head and then transfers its energy
to myosin head to perform work of pulling
thin filament during contraction

Muscle fibers continually resynthesize ATP
from breakdown of creatine phosphate (CP)

Energy sources for muscle contraction

Catabolism by muscle fibers requires
glucose and oxygen

At rest, excess O

in the sarcoplasm is
bound to myoglobin

Red fibers

muscle fibers with high levels of

White fibers

muscle fibers with little myoglobin

Aerobic respiration occurs when adequate

is available

Energy sources for muscle contraction

Anaerobic respiration occurs when low levels
of O2 are available and results in formation
of lactic acid

Glucose and oxygen supplied to muscle
fibers by blood capillaries

Skeletal muscle contraction produces waste
heat that can be used to help maintain set
point body temperature

Twitch contraction

A quick jerk of a muscle that is
produced as a result of a single, brief
threshold stimulus (generally occurs
only in experimental situations)

The twitch contraction has three phases

Latent phase

nerve impulse travels to the
sarcoplasmic reticulum to trigger release of

Contraction phase


binds to troponin
and sliding of filaments occurs

Relaxation phase

sliding of filaments


the staircase

Gradual, steplike increase in the
strength of contractions seen in a series
of twitch contractions that occur 1
second apart

Eventually, the muscle responds with
less forceful contractions, and relaxation
phase becomes shorter

If relaxation phase disappears
completely, a contracture occurs


smooth, sustained contractions

Multiple wave summation

twitch waves are added together to
sustain muscle tension for

a longer time

Incomplete tetanus

very short periods
of relaxation occur between peaks of

Complete tetanus

the stimulation is
such that twitch waves fuse into a single,
sustained peak

Muscle tone

Tonic contraction

continual, partial
contraction of a muscle

At any one time, a small number of muscle
fibers within a muscle contract, producing a
tightness or muscle tone

Muscles with less tone than normal are flaccid

Muscles with more tone than normal are

Muscle tone is maintained by negative
feedback mechanisms

Graded strength principle

Skeletal muscles contract with varying degrees

of strength at different times

Factors that contribute to the phenomenon of
graded strength

Metabolic condition of individual fibers

Number of muscle fibers contracting simultaneously;
the greater the number of fibers contracting, the
stronger the contraction

Number of motor units recruited

Isotonic and isometric

Isotonic contraction

Contraction in which the tone or tension
within a muscle remains the same as the
length of the muscle changes


muscle shortens as it contracts


muscle lengthens while contracting


literally means

same tension

All of the energy of contraction is used to
pull on thin myofilaments and thereby
change the length of a fiber

s sarcomeres

Isotonic and isometric contractions

Isometric contraction

Contraction in which muscle length remains
the same while the muscle tension increases


literally means

same length

Most body movements occur as a result
of both types of contractions

Cardiac Muscle Tissue

Cardiac muscle

Found only in the heart, forming the
bulk of the wall of each chamber

Also known as striated involuntary

Contracts rhythmically and continuously
to provide the pumping action needed
to maintain a constant blood flow

Cardiac Muscle Tissue

Cardiac muscle resembles skeletal muscle but has
specialized features related to its role in continuously
pumping blood

Each cardiac muscle contains parallel myofibrils

Cardiac muscle fibers form strong, electrically
coupled junctions (intercalated disks) with other
fibers; individual cells also exhibit branching


continuous, electrically coupled mass

Cardiac muscle fibers form a continuous,
contractile band around the heart chambers that
conducts a single impulse across a virtually
continuous sarcolemma

Cardiac Muscle

Cardiac muscle

T tubules are larger and form diads with a
rather sparse sarcoplasmic reticulum

Cardiac muscle sustains each impulse longer
than in skeletal muscle; therefore, impulses
cannot come rapidly enough to produce

Cardiac muscle does not run low on ATP and
does not experience fatigue

Cardiac muscle is self

Smooth Muscle Tissue

Smooth muscle

Smooth muscle is composed of small, tapered
cells with single nuclei

No T tubules are present, and only a loosely
organized sarcoplasmic reticulum is present


comes from outside the cell and binds to
calmodulin instead of troponin to trigger a

No striations, because thick and thin
myofilaments are arranged differently than in
skeletal or cardiac muscle fibers; myofilaments
are not organized into sarcomeres

Smooth Muscle Tissue

Two types of smooth muscle tissue

unit (visceral)

Gap junctions join smooth muscle fibers into
large, continuous sheets

Most common type; forms a muscular layer in
the walls of hollow structures such as the
digestive, urinary, and reproductive tracts

Exhibits autorhythmicity, producing peristalsis


Does not act as a single unit but is composed
of many independent cell units

Each fiber responds only to nervous input