Download MUSCLE CONTRACTION

MUSCLE CONTRACTION
a) Structure of SKELETAL MUSCLE
Myofibrils are rod like elements within the myofibre which consist of smaller
units called sarcomeres. Alternating light and dark bands repeat along the
length of the myofibril. The dark bands are called A bands because they are
anisotropic (i.e. they can polarise light) and the light bands are called I bands
because they are isotropic or non polarising. The myofibril bands are nearly
perfectly aligned making continuous bands across the fibre.( hence striated
muscle). The I bands also have a midline interruption, a dark line = Z line. The
region of the myofibril between two successive Z lines is called the sarcomere.
EM of the sarcomere shows these patterns arise from the orderly arrangement
of two types of myofilament. The central thick filaments make up the A band.
The thin filaments extend across the I band and part of the way across the A
band. The Z line is a disc like protein sheet that is the point of attachment for
the array of thin filaments. In transverse section each thin filament is seen to
be surrounded by three thicks and each thick by six thins.
b) Biochemistry of muscle
Thick filaments composed primarily of myosin. Each myosin has a rod-like tail
and two globular heads (see lecture on myosin) . Each thick filament consists of
a bundle of myosin molecules with the tails forming the central portion of the
filament and the globular heads facing outwards. The heads have ATPase
activity.
Thin filaments are composed mainly of actin. (see lecture 7). Two strands of F
(fibrous) actin are wound into a helix make up a thin filament. There are
several other proteins also associated of which the most important are troponin
(globular) and tropomyosin (fibres).
c) Mechanism of contraction
Contraction mechanism involves sliding of the thin filaments past the thick ones
The myosin heads form cross bridges which link to the actin. Each cross bridge
attaches and detaches several times during contraction and so acts like an oar
which rows the thin filaments towards the middle of the sarcomere.
Cross bridge formation is set in motion by an increase in the levels of
intracellular Ca++. Below 10-8M cross bridge formation does not occur but
above this contraction occurs. In the absence of Ca++ the myosin binding sites
on the actin are blocked by the tropomyosin molecules. In the presence of
Ca++ they are bound by troponin. The troponin/Ca++complex then undergoes
a conformational change that physically moves tropomyosin into the centre of
the helical groove in the actin and away from the myosin binding sites.
click to see how contraction occurs in muscle
Sarcoplasmic reticulum = SR
SR consists of two components - T tubules and the SR proper
a) T tubules. At each Z line the sarcolemma penetrates deeply into the
cell to form a hollow tube extending deep within the muscle. This carries the
electrical depolarisation produced by the action potential on the muscle
membrane to all parts of the interior of the muscle fibre.
b) The SR forms a system of interconnecting tubules in the narrow
space between the myofibrils. At the level of the A/I junctions they form sac
like swellings called terminal cisternae. As each T tubule penetrates into the
muscle interior it associates with the terminal cisternae. The association is seen
in the EM as three profiles ie cisterna, T, cisterna = triad
The SR controls Ca++ levels in the sarcoplasm. The cisternae store Ca++ and
release it to a signal from the T tubule. Ca++ in the sarcoplasm interacts with
troponin, which leads to a conformational change in troponin /tropomyosin
complex. This exposes myosin binding sites on the actin. Myosin interacts with
the actin to produce actomyosin , therefore contraction occurs.
The rest of the SR pumps Ca++ out of the sarcoplasm into the SR. Thus
concentration goes down, tropomyosin displaces myosin cross bridges and
muscle relaxes. The Ca++ is then pumped back into storage in the terminal
cisternae.
SMOOTH MUSCLE
Much smaller cells than skeletal muscle. 2-10um in diameter and 50 200um long (skeletal muscle 50- 500um diameter and may be as much as a
meter long).
The SR of smooth muscle fibres is poorly developed and no T tubules. No
striations visible because of differences in the organisation of the myofilaments
i.e.:
1) ratio of thick to thin is 1:16 ( 1:2 in skeletal muscle)
2) troponin is absent (a different Ca regulating protein instead =
calmodulin) although tropomyosin is present
3) No sarcomeres but thick and thin filaments are collected into less
regular bundles
4) No Z lines; instead are dark staining dense bodies connected by non
contractile intermediate filaments into a cytoskeletal network which is also
anchored to the sarcolemma. The actin filaments are connected to the dense
bodies
MECHANISM OF CONTRACTION
Smooth muscle cells are electrically coupled by gap junctions. So, if an
action potential is generated in one cell, it can spread to adjacent cells. In this
way a wave of depolarisation and hence contraction can spread over a sheet of
smooth muscle.
The mechanism of contraction is essentially similar in that:
1) actin and myosin interact by a sliding filament mechanism
2) the trigger for contraction is a rise in intracellular Ca++
3) the sliding process is energised by ATP.
But it is much, much slower; a smooth muscle contraction takes some 30 x
longer to complete than a striated muscle contraction . This is because :
1) The increased intracellular Ca++ comes mainly from the extracellular space
(some from the small amounts of SR also)
2) Myosin ATPase activity in smooth muscle only 1/10th that of striated muscle.
3) Also, Ca++ regulation mechanism different and slower via calmodulin rather
than troponin/tropomyosin.
Although much slower contracting, smooth muscle can maintain the same
tension of contraction as skeletal muscle at less than 1% of the energy cost.
Vital in viscera and blood vessels where smooth muscle tone often continuous.
ATP requirements for smooth muscle contraction is so small that it can be
supplied entirely by anaerobic pathways.
Furthermore, smooth muscles are able to develop tension over a much greater
range of length than skeletal muscle because a) they have a much more
irregular highly overlapping arrangement of contractile filaments and b)
because they tend to contract in a corkscrew manner. While skeletal muscle
can develop tension only over a length change of 60% smooth muscle can do
so over 150% of its length. This allows organ cavities both to accommodate
large volumes and also to maintain tone when emptied.
REGULATION OF CONTRACTION
1) neural stimulation
Neural activation of smooth muscle is similar to that in striated muscle ie
neurotransmitter molecules bind to receptors to depolarise the membrane and
lead to an action potential (frequently different in wave form however; ie some
have prolonged plateaus)
2) neural relaxation (inhibition)
In addition smooth muscle receives innervation from nerves releasing
different transmitters. the effect of a given neurotransmitter on a given type of
smooth muscle depends on the type of receptor molecules. e.g. ACh receptors
of smooth muscles in the bronchioles cause strong contraction, thus narrowing
the airways. On the other hand noradrenaline on the same muscle fibres, from
sympathetic endings causes relaxation and dilation of the airways. On the other
hand the binding of NA to the smooth muscle cells of most blood vessels is
stimulatory and causes contraction - narrowing of the blood vessel.
3) chemical stimulation and inhibition.
Some smooth muscle layers receive no neural stimulation at all and
instead depolarise spontaneously or in response to chemical stimuli. Chemical
factors can promote smooth muscle contraction or relaxation without an action
potential by enhancing or inhibiting calcium ion entry through the membrane.
Thus, smooth muscle can respond directly to certain hormones, lack of oxygen,
excess carbon dioxide and low pH.
4)Mechanical stimulation
Stretching of smooth muscle elicits contraction; useful for example in
moving substances along the gut. But the increased tension persists for only a
few minutes and then the tension returns to normal - called stress-relaxation
response This allows a hollow organ to accommodate an increased internal
volume without promoting expulsive contractions important since stomach and
bladder must be able to store their contents temporarily
CARDIAC MUSCLE
Cardiac muscle is intermediate in properties between striated muscle and
smooth muscle. It does not need to be as flexible in its properties as smooth
muscle, yet it must contract in precise synchrony and is never able to rest.
Features in common with striated muscle:
1) Arrangement of contractile filaments.
Similar to skeletal muscle in having sarcomeres therefore striated appearance
2) Speed of contraction is slower than most skeletal muscles but is as fast as
the fastest of smooth muscle contractions
3) Same type of Ca++ regulation as striated muscle with troponin on actin thin
filaments and sarcoplasmic reticulum
4) Like skeletal muscle, cardiac muscle responds to stretch with a more
vigorous contraction, whereas smooth muscle responds with stress relaxation
response
Features in common with smooth muscle
1) Cardiac cells are connected together with gap junctions, so contraction in
one muscle cell leads to contraction in neighbouring cells
2) Cardiac muscles are myogenic; that is they show spontaneous contractile
activity even when deprived of innervation.
3) Like smooth muscle, cardiac muscle receives dual innervation from the
autonomic nervous system - sympathetic excites and parasympathetic inhibits
CONCLUSION.
Note adaptation for different requirements. Skeletal muscle designed to
produce fast movements needed to interact with the environment. Different
types of skeletal muscle developed according to whether sustained or
spasmodic movements necessary.
Smooth muscle contracts 30 times slower than striated muscle but for the jobs
smooth muscles do, speed is not the most important criterion. Much more
important to be able to generate tension over a large length range, and to do it
with high energy economy.
Heart muscle which has some of the properties of both the other tissue types is
adapted to continuous, aerobic, rhythmic contractions.