Download MUSCLE Three types of muscles based on morphological and

MUSCLE
Three types of muscles based on morphological and functional difference
Skeletal muscle
Cardiac muscle
Smooth muscle
Composed of bundles of very long
cylindrical multinucleated cells that
has cross-striations.
Composed of elongated or branched
individual cells that run parallel to
each other. Also have crossstriations.
Consists of collections of fusiform
cells, no cross-striations.
Contraction is quick, forceful and
usually under voluntary control.
Contraction is involuntary, vigorous
and rhythmic.
Contraction is slow and involuntary
Sarcomere
Sarcomere is a basic functional unit of myofibril. Myofibrils show a pattern of alternate light bands and dark bands
known as (I band and A band). Each sarcomere contains two types of filaments: thick and thin filaments.
Structure of sarcomere:
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In the central region of the sarcomere are the thick filaments and they appear as dark band. This band is
termed the A band. The thick filaments contain the protein myosin. The dark band is supported in centre by
M line.
The thin filaments contain the protein myosin and are attached at either end of the sarcomere at a structure
known as the Z line. The limits of a sarcomere are defined by two successive Z lines. The thin filaments
extend from the Z lines toward the center of the sarcomere where they overlap with the thick filaments.
The I band represents the region between the ends of the A bands of two adjoining sarcomeres. In this
region, only thin filaments are present.
The H zone corresponds to the space between the ends of the thin filaments, where only thick filaments are
present.
Arrangement of sarcomere:
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The thick and thin filaments are arranged hexagonally with respect to one another.
Each thick filament is surrounded by six thin filaments; each thin filament is surrounded by three thick
filaments.
The thin filaments of 2 adjacent sarcomeres show a polarity which can be observed by interacting them with
isolated myosin heads with decorate the thin filaments at specific angles. The decoration takes place in the
ration of one head per monomer.
Why would muscle exhibit constant volume contraction?
Every time the muscle contracts, sliding of filaments occur and there is a change in the angels at the zigzag regions of
the Z-line. Consequently, the muscle bulged out.
The sliding filament theory
Muscle contraction occurs as the result of the sliding of the thick and thin filaments past one another. The length of
individual filaments remains unchanged. The myosin cross bridges produce the sliding of the filaments. When bound
to the actin filaments, the movement of cross bridge causes the sliding of thin and thick filaments past each other.
Events between nerve action potential and contraction and relaxation of a muscle fiber
The action potential in the axon causes the release of the neurotransmitter known as acetylcholine from the axons.
Once released, acetylcholine binds to the receptor sites on the motor end plate. Acetylcholine increases the
permeability of motor end plate to sodium and potassium ions, causing the depolarization of the end plate. The
electrical signal spreads along the membrane down the T-tubules. Eventually, this will result in the release of calcium
ions from the terminal cisternae of the sarcoplasmic reticulum.
 At low Ca2+, TN-I binds strongly to actin, tropomyosin is in a blocking position preventing the attachment of
myosin heads to the actin monomers.
 When Ca2+ is released from the sarcoplasmic reticulum, it binds to TN-C, strengthens the linkage of TN-C to
TN-I, and weakens the TN-I actin bond.
 The TN-I moves off the actin and carries the tropomyosin from its blocking position to one nearer the groove.
 The muscle contraction cycles begins.
 The Ca2+ is re-sequestered in the sarcoplasmic reticulum, the troponin-tropomyosin moves back and the
muscle relaxes.
Muscle contraction cycle:
 A myosin head unit with a bound ATP is in the rotated position and unattached to actin.
 A series of reaction takes place during which the myosin head is rotated to the cocked position relative to
actin. This is probably a charged M-ATP* complex.
 In this state, the M-ATP* complex binds to actin in a rapid process. With actin bound, myosin is able to split
ATP and ATP leaves the complex as ADP and P.
 The myosin head rotates while attached and generates a tension stroke.
 In the rotated state, another ATP attaches in a fast reaction. The actin dissociates leaving a myosin ATP
complex.
 This cycle repeats again.
Rigor Mortis
Rigor Mortis is a phenomenon in which the muscles of the body become very stiff and rigid after death. It results
directly from the loss of ATP in the dead muscles cells because the actin-myosin complex will not dissociate of no
ATP is bind to the myosin head.
Role of ATP in muscle contraction
Muscle at rest: ATP + creatine -> ADP + phosphocreatine
Working muscle: Phosphocreatine + ADP (in the presence of creatine kinase) -> Creatine + ATP
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Myosin head conformational change
Dissociation of actin-myosin complex
Active transport of Ca1+
Maintenance of Na+ and K+ gradients across membrane
Single twitch and summation
The mechanical response of a muscle fiber to a single electrical signal is known as twitch. Muscle fibers like nerve
fibers have a refractory period after one stimulus, during which they will not respond to a second stimulus. The
refractory period in skeletal muscle is so short that muscle can respond to second stimulus while still contracting in
response to first. The result of this is the summation of contractions, which leads to a greater than normal
shortening of muscle fibre.
Tetanus
Tetanus tension is the maximum tension builds up when all the muscle fibers are contracting at their maximally rate.
Tetanus is the results of muscle contraction in response to a string of action potentials that fire to the motor end
plate. There are two types of tetanus that based on the rate at which the electrical signal is delivered to a muscle
fiber after the refractory period.
 If the muscle is stimulated at a lower rate, at this point, it will partially relax, causing a wave in contractions.
This is known as incomplete tetanus or unfused tetanus.
 However, if the muscle is stimulated at a higher rate and it cannot relax at all, causing it to be contracted all
the time. This is known as completed or fused tetanus.
Tetanus ends when muscle fatigue. When muscle is fatigue, it is unable to contract anymore. This is primarily
induced by the accumulation of lactic acid.
Control of muscle tension
The total tension that a muscle can develop depends on two factors:
 The number of muscle fibers contracting at a given time.
 The tension developed by each fiber.
The number of muscle fibers that are contracting at any given time depends upon the number of motor units being
stimulated.
 Each motor neuron innervates several muscle fibers, forming a motor unit.
 Stimulation of a motor neuron produces a contraction in all muscle fibers in the motor unit.
 One cannot simultaneously fire all the motor units in a muscle. The motor neurons fire in an asynchronous
pattern.
o Prevents fatigue of the muscle.
o Maintaining a nearly constant tension in the muscle. Both Parkinson ‘s disease and normal shivering
inhibit the subcortical centers in the brain, and movement is now dominated by the local feedback
loops from the stretch receptors, and thus the motor neurons firing tend to be synchronous and
oscillatory.
The number of muscle fibers associated with a single motor neuron varies.
 In muscles of the hand, for example, which are able to produce delicate movements, the individual motor
units is small.
 In the muscle of leg, for example, each motor unit contains hundreds of muscle fibers.
 The smaller the number of fibers in a motor unit, the more delicate the movement obtainable and the larger
is the area of the motor cortex associated with that muscle.
Antagonistic muscle
Muscles can only exert a pull but not a push. For this reason, muscles are typically arranged in antagonistic pairs.
When one piece of muscle within the pair contract, the other piece would have to stretch. Eg: Biceps and triceps.
Relaxation of muscle has done nothing with the contraction of triceps. However, contraction of triceps stretched the
relaxed biceps.
In an arm-wrestling event, we should keep our arms bended at as small an angle as possible. This is because the
length of the biceps would have been shorten, meaning that there s already a great overlap between thick and thin
filaments within sarcomeres at the start of the competition.
Isotonic and isometric contraction
Moderate exercise increases the diameter of muscle cells, thus enlarging and strengthening the gross muscle being
exercised.
 Isometric contraction: If one exercise by pushing against an immovable object or by opposing antagonistic
muscle to each other, the resulting contraction does not actually shorten the muscle. However, sarcomeres
shorten, generating force, but elastic elements stretch, along muscle length remain the same.
 Isotonic contraction: It the exercise involves movement, as in weight lifting, it is said to be isotonic, for
though a muscle does shorten during such exercise, its tension does not greatly increase. Sarcomeres
shorten more, but, since elastic elements are already stretched, the entire muscle must shorten.
Different types of muscles with different ATPase activity and differing speeds of contraction
Fast muscle low oxidation
Fast muscle high oxidation
Slow muscle high oxidation
Low mitochondria
Many mitochondria
Many mitochondria
White or pink, few oxidative
enzymes. No myoglobin.
Darker pink oxidative enzymes. Low
myoglobin.
Dark red oxidative enzymes. High
myoglobin.
Uses only glucose
Uses glucose, fat and protein
Uses glucose, fat and protein
Easily fatigue; lactic acid formed
Easily fatigue; lactic acid formed
Long lasting; little lactic acid formed
Fast ATPase
Intermediate ATPase
Slow ATPase
Muscle number and size
Although muscle cells enlarge with body growth and exercise, they do not increase in number, so that as they die
they cannot be replaced. Thus, the number of muscle cells in all muscles drastically decreases in old ages. Part of this
decrease is no doubt due to minor injuries that occurs in the course of a life time.