Download A.L. Wafa`a sameer 2014 Muscles physiology Muscle tissue Types of

A.L. Wafa’a sameer
2014
Muscles physiology
Muscle tissue
Types of muscle contractions :1- Isotonic contraction ( same tension ) : When a tension in a muscle is sufficient
to lift a load , the contraction is said to be isotonic . On the other hand , during
isotonic contraction , the muscle is shorten .
2- Isometric contraction ( same length ) : When the tension in a muscle is used
to support a load ( a book you are carrying , for example ) or to push against
an immovable object , the contraction is said to be isometric .
In an isometric contraction the muscle maintains the same length but develops
tension during the period of the contraction .
Simple muscle twitch :
There are three phases of twitch can be distinguished :1- The latent period : is the time between the stimulation & the beginning of the
contraction . During this period : 1) the nerve action potential is conducted to
the axon terminals , 2) A.Ch. is released and binds to the motor end plate . 3)
the action potential passes over the membrane , 4) Ca++ inhibit the action
troponin & tropomyosin . or Ca++ exposes the myosin active site .
2- The contraction period : is the time during which tension is rising or
shortening is occurring .
3- The relaxation period : is the time during which tension is declines or muscle
back to its original length .
 With regard to whole muscles the degree of contraction ( amount of tension
or amount of shortening produced ) is determined by two factors :
1- How many motor units are stimulated .
A.L. Wafa’a sameer
2014
Muscles physiology
2- How frequently the fiber in the muscle are being stimulated .
 Thus , the response of a whole muscle is the sum of the responses of the
individual fibers .
 When the frequency of stimulation increases , the force of the contraction
increases up to the maximum possible for the fiber being stimulated .
Summation of contraction :
If a second stimulus is given before complete
relaxation of the muscles response to first stimulus , the force produced by the
second contraction will be stronger than the first . similarly , the third will be
stronger than the second . this phenomenon is called summation .
Incomplete tetanus ( Clonus ) :When stimuli are applied at a fairly rapid rate ( 20-30
) stimuli / second in frog muscle , for eg. ) incomplete tetanus occurs ; the muscle
barely begins to relax when the next stimulus arrives .
Complete tetanus :
When stimuli are applied at an even more rapid rate (40-60
stimuli / sec. in frog m. ) , complete tetanus occurs ; the muscle cannot being to
relax before the next stimulus arrives .
Eg. If we maintain our muscles at maximum levels of contraction ( when doing
work ) , we are maintaining them in tetanic contraction because the m. fibers are
being stimulated at a high frequency .
 Most of the work we do involves short – term tetanic contraction of one set of
muscles followed by short – term tetanic contraction of other set of muscles .
A.L. Wafa’a sameer
2014
Muscles physiology
Treppe
When a series of maximal stimuli are delivered to a resting muscle at
intervals long enough to allow relaxation , the phenomenon of atreppe is observed .
under this condition each of the first few contractions produces more tension than
the preceding one , creating a staircase effect on graph .
Fatigue :Occur when a muscle has been repeatedly stimulated , and the tension the
muscle can produce decreases .
Smooth muscle
SM.M. is innervated by both sympathetic and parasympathetic
neurons . The mechanism of stimulation in smooth muscle differs from that in
Sk.m. in that Sm.m. has no discrete neuromuscular junctions . Swollen area on
an axon simply release their transmitter in to extracellular fluids , and it diffuses
to receptor sites on the membranes of several adjacent Sm.m. cells . Thus ,
A.L. Wafa’a sameer
2014
Muscles physiology
Sm.m. respond more slowly to nerve stimuli and less precise in its action than in
Sk.m. .
In contrast to Sk.m. ( which can be stimulated only by neural stimuli ) Sm.m
can be either excited or inhibited by various neural and hormonal stimuli , such
as acetyl choline and norepinephrine .
Sm.m. classified according to its properties as 1) multiunit 2) visceral ( singleunit ) . However , some Sm.m. has some of the properties of both types .
Visceral Sm.m. is found in sheets in the walls of hollow organs such as those
of the digestive tract , the ureters of the urinary tract , & the uterus of the female
reproductive system . In addition to responding to neural stimuli , visceral Sm.m.
can contract spontaneously and rhythmically without being stimulated by
neurotransmitters or hormones . The mechanism by which these cells
spontaneously contract is not well understood , but its appears that they can
generate an electrical activity that spreads from the membrane of one cell to that
of adjacent cells . Gap junctions that link the cytoplasm of adjacent cells permit
ions to flow from cell to cell , making a sheet cell function together .
Visceral Sm.m. responds to stretching by fist
increase its tension
&
then gradually decreasing its tension when stretching is prolonged . This property
called plasticity .
Multiunit Sm.m. contracts in more discrete smaller units than visceral m. because
electrical activity is not transferred from cell to cell as in visceral m. . However ,
multiunit m. is very sensitive to chemicals such as acetyl choline & norepinephrine
from neurons . These chemicals tend to persist & cause repeated firing of the muscle
cell rather than generating a single action potential , thus prolonging the duration of
contraction .
 Multiunit Sm.m. are found in some large bl.ves. & in the iris & the ciliary
body of the eye .
A.L. Wafa’a sameer
2014
Muscles physiology
 Cardiac muscle
Cardiac m. tissue forms the bulk of the wall of the heart . It is
striated at the microscopic level & involuntary .

The fibers of cardiac m. tissue are roughly quadrangular and usually have
only a single centrally located nucleus .

The thin sarcolemma of cardiac m. fibers is similar to that of Sk.m. , but the
sarcoplasm is more abundant & the mitochondria are larger & more numerous .

Cardiac m. fibers have the same arrangement of actin & myosin & the same
bands , zones , & lines as Sk.m. fibers .

Myofilaments in cardiac m. fibers are not arranged in discrete myofibrils as in
Sk.m. .
 The transverse tubules of mammalian cardiac m. are larger than those of Sk.m.
& are located at the Z lines rather than at the A-I junctions as in Sk.m. fibers .
 The sarcoplasmic R. of cardiac m. is less well developed than that in Sk.m. .
Cardiac m. fibers branch & interconnested with each other . Cardiac m. fibers
form two separate networks :
1- The muscular walls & partition of the upper champers of the heart ( atria )
2- The muscular walls & partition of the lower chambers of the heart
(ventricles ) .
Each fiber in a network is separated from the next fiber by an irregular transverse
thickening of the sarcolemma called an intercalated disk . These disks contain
desmosomes , ( which hold fibers together ) & gap junctions ( which aid in
conduction of m. action potentials from one m. fiber to other ) . When a single
fiber of either network is stimulated , all the fibers in the network become
stimulated as well . thus, each network contracts as a functional unit . When the
A.L. Wafa’a sameer
2014
Muscles physiology
fibers of the atria contract as a unit , blood moves in to the ventricles . Then , when
the ventricular fibers contract as a unit , bloob is pumped in to atrium .