Chapter 11 Physiology of the Muscular System

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Chapter 11

Physiology of the Muscular
System

Introduction

Muscular system is responsible for
moving the framework of the body

In addition to movement, muscle
tissue performs various other
functions

General Functions

Movement of the body as a whole or
of its parts

Heat production

Posture



Function of Skeletal Muscle
Tissue

Characteristics of skeletal muscle
cells


Excitability (irritability)

ability to be
stimulated


Contractility

ability to contract, or
shorten, and produce body movement


Extensibility

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


Sarcolemma

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


Myofibrils

numerous fine fibers packed
close together in sarcoplasm


Sarcomere

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



Triad

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


Myofilaments

Each myofibril contains thousands of thick
and thin myofilaments


Four different kinds of protein molecules make
up myofilaments

Myosin


Makes up almost all the thick filament


Myosin

heads


are chemically attracted to actin
molecules


Myosin

heads


are known as cross bridges when
attached to actin

Actin

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

Tropomyosin

protein that blocks the active sites on
actin molecules

Troponin

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
-
7)

Neuromuscular junction is a synapse where
neurotransmitter molecules transmit signals

Excitation and contraction


Acetylcholine

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
them


Each head releases, binds to next active site, and

pulls again


Entire myofibril shortens

The mechanism of contraction



Relaxation

Immediately after Ca
++

ions are released,
sarcoplasmic reticulum begins actively
pumping them back into sacs (Figure 11
-
3)

Ca
++

ions are removed from troponin
molecules, shutting down contraction

Energy sources for muscle
contraction


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
2

in the sarcoplasm is
bound to myoglobin


Red fibers

muscle fibers with high levels of
myoglobin


White fibers

muscle fibers with little myoglobin

Aerobic respiration occurs when adequate
O
2

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
Ca
++

Contraction phase

Ca
++

binds to troponin
and sliding of filaments occurs

Relaxation phase

sliding of filaments
ceases

Treppe

the staircase
phenomenon



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

Tetanus

smooth, sustained contractions



Multiple wave summation

multiple
twitch waves are added together to
sustain muscle tension for

a longer time


Incomplete tetanus

very short periods
of relaxation occur between peaks of
tension


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
spastic


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
contractions



Isotonic contraction

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


Concentric

muscle shortens as it contracts


Eccentric

muscle lengthens while contracting

Isotonic

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

Isometric

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
muscle


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

Syncytium

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
tetanus

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

Cardiac muscle is self
-
stimulating



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


Ca
++

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


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

Single
-
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

Multiunit


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


Each fiber responds only to nervous input