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Locomotor Systems
Introduction
• Anyone who has tried to catch a fly or swat a wasp will attest to the impressive accuracy and speed with which insects can maneuver.
• The investment made in the muscles that power this
movement can be substantial:
flight muscles alone can comprise as much as 65% of the total body mass of some insects.
• Compared to the modest five-fold increase in respiratory rates from rest to maximum work in
vertebrate muscles
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Introduction
• In insect flight muscles these rates can increase by 50–100 times and undergo rates of adenosine triphosphate (ATP) turnover that are triple that of hummingbirds
( نانطلا رئاطلا)• It remains essentially aerobic as a result of their high concentrations of mitochondria and adequate supply of oxygen.
The muscular apparatus
The muscular apparatus is minuscule compared to that of vertebrates.
Their efficient movement on land and in the air is a major factor in the domination of
terrestrial ecosystems by insects.
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BASIC STRUCTURE OF INSECT MUSCLES
• Muscle cells are among the most complex of all animal cells.
• like neurons, are capable of generating electrical signals.
• They also have the capability to generate force and movement, and responsible for animal behavior.
BASIC STRUCTURE OF INSECT MUSCLES
• Muscles must respond quickly and precisely to nervous signals to enable the organism to engage in complicated motions in which the timing and order of different muscle contractions is critical.
• Only a relatively small number of insect muscles have been studied, and none is known as well as the frog muscle, which has served as the basis for much of what is known about vertebrate muscle structure .
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BASIC STRUCTURE OF INSECT MUSCLES
• The giant water bug Lethocerus, the locusts
Schistocerca and Locusta, and of course, Drosophila, have been models for many of the generalizations made about the biochemistry and physiology of insect flight muscle.
• The muscles of the few insect species that have been examined show strong similarities to the structure of vertebrate muscles:
• many of the genes expressed in insect muscle and the resulting proteins have vertebrate homologues.
BASIC STRUCTURE OF INSECT MUSCLES
• There are two major types of insect muscle.
• Visceral muscles: surround the viscera but do not attach to the body wall.
• Skeletal muscles : are anchored to the
exoskeleton at either end and move parts of the skeleton relative to each other.
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Skeletal muscles
• Integumental invaginations: called
tonofibrillae are areas where the muscles attach, providing the epidermal cells with
rigidity and allowing tension to be transmitted to the cuticle.
• The protein is also involved in tube
formation during tracheal morphogenesis.
• These proteins are resistant to the molting fluid that digests the endocuticle, so they remain intact during the molting process to allow the muscles to continue functioning during ecdysis.
BASIC STRUCTURE OF INSECT MUSCLES
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NEURAL EXCITATION AND MODULATION OF MUSCLE CONTRACTION
• A typical skeletal muscle consists of many elongated muscle fibers, each of which is a single multinucleate cell
,generated by the fusion of individual myoblasts during development.
• Insect visceral muscles, in contrast, tend to have muscle fibers that contain only a single nucleus.
NEURAL EXCITATION AND MODULATION OF MUSCLE CONTRACTION
• These fibers extend the full length of the muscle.
• Many of the usual terms used to describe cell structures are different when they are applied to muscle cells.
• For example, each fiber is surrounded by an electrically excitable
sarcolemma, an outer plasma membrane that encloses the
sarcoplasm or cytoplasm of the cell.
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The structure of insect muscle.
• (Top) A leg contains several muscles each consisting of many muscle fibers.
• Each muscle fiber is a cell surrounded by an electrically excitable cell membrane.
•
• (Middle) Within the cytoplasm of muscle fibers are longitudinal arrays of myofibrils that extend its length.
• (Bottom) The banding pattern visible in the myofibrils results from the degree of overlap of actin and myosin myofilaments.
• The Z-line is the actin endplate Muscle
Muscle fibers
Muscle fiber (cell)
Myofibri
Sarcomere
Actin thin
filament
Myosin thick filamen
Z-line
The structure of insect muscle
• Within the sarcoplasm lie many myofibrils
• The rod-like elements that comprise the fiber’s contractile machinery
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The structure of insect muscle
• The myofibrils consist of repeating units of overlapping thick and thin myofilaments of the proteins myosin and actin
A sarcomere,
• The H zone within the A band represents that portion of the myosin that does not overlap.
• The I bands are areas of actin alone
• The Z-lines define the ends of the sarcomere.
Sarcomere M line
A band H zone
I band I band
Contraction
Z line Z line
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A sarcomere,
• In contraction : the action and myosin myofilaments slide across each other, shortening the sarcomere
• A sarcomere, the unit of muscle contractions .
Sarcomere M line
A band H zone
I band I band
Contraction
A sarcomere,
• The morphological regions of the sarcomere result from overlapping filaments that create a banding
pattern.
• The dark A band is a
consequence of the overlap of actin and myosin.
Sarcomere M line
A band H zone
I band I band
Contraction
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ACTIN, MYOSIN AND MUSCLE ACTIVATION
Sarcomere M line
A band H zone
I band I band
Contraction
A muscle is only able to contract .
It can lengthen only when stretched by other,
antagonistic muscles.
The shortening of the sarcomere results from the interactions between actin and myosin in the muscle fibers .
Some of the accessory proteins associated with the sarcomere.
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Cross-section within the A-bands of an insect flight muscle (top) and a visceral muscle (bottom)showing the distribution of actin and myosin filaments.
The mixture of short and long sarcomeres of the mandibular closer muscle of the ant.
The longer muscle fibers with short sarcomeres are fast and the short fibers with longer sarcomeres are slow.
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The steps in the cycle of muscle contraction
• (A) The muscle fiber at rest.
Adenosine diphosphate (ADP) is bound to the myosin heads, and the troponin-tropomyosin complex has no bound calcium.
• (B) The muscle fiber is activated when calcium is released from the sarcoplasmic reticulum and binds to the tropomyosin, causing them to expose actin-binding sites.
• This allows the myosin heads to form cross bridges between the thick and thin filaments.
Thin filament ADP
Thick filament
Troponin Actin Tropomyosin
Binding site Ca2+
ADP Ca2+
Force
ADP ATP
ADP Pi
A
B
C D
E
The steps in the cycle of muscle contraction
• (C) The attachment causes a conformational change in the myosin and they exert forces that shorten the sarcomere by
producing a greater overlap of the filaments.
• (D) When the power stroke is completed, ATP binds to the myosin head and causes it to detach from the actin.
• (E) The energy contributed by the ATP reconforms the myosin head for attachment to another
binding site.
Thin filament ADP
Thick filament
Troponin Actin Tropomyosin
Binding site Ca2+
ADP Ca2+
Force
ADP ATP
ADP Pi
A
B
C D
E
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TYPES OF SKELETAL MUSCLES
• Skeletal muscles are further divided into synchronous and asynchronous muscles, based on their mechanisms of regulation by the nervous system.
TYPES OF SKELETAL MUSCLES
• Synchronous Muscles Most of the skeletal muscles in insects are called synchronous
• because : they contract in synchrony with nervous signals from the motor neurons that innervate them.
• Each nervous signal is followed by a single contraction of the muscle, and in the case of wing muscles
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TYPES OF SKELETAL MUSCLES
• The contraction of most synchronous muscles is generally limited to a maximum of about 50%, but
• there are special supercontracting and superextending muscles associated with structures that undergo an unusual degree of extension.
TYPES OF SKELETAL MUSCLES
For example, in some insects the intersegmental muscles that allow the elongation of the abdominal segments containing the ovipositor can stretch to over 10 times their normal length, and shorten by as much as 90%
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Intersegmental muscles allow for (A) the supercontraction
(B) the superextension of the body segments.
TYPES OF SKELETAL MUSCLES
Asynchronous Muscles
• The insects in several more derived orders, including dipterans, coleopterans, hymenopterans, and some hemipterans, have evolved smaller wings that enabled them to occupy niches unavailable to insects with large wings.
• In order for these smaller wings to achieve the necessary aerodynamic forces to support flight, they must be able to beat much more rapidly.
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Tracheal Supply to Muscles
• The contraction of muscles requires energy and an adequate supply of oxygen, and the respiratory system evolved with the potential to respond to maximum
metabolic activity.
• The flight muscles of insects have the highest known rates of oxygen consumption for any animal locomotor tissues.
Tracheal Supply to Muscles
• The oxygen consumption of bumble bees in flight is between 60 and 70
mL/g/h, compared to only 40–50 mL/g/h for
hummingbirds flying at the same speed.
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Tracheal Supply to Muscles
• Compared to the resting state, flight can raise metabolic rates in some insects as much as 50–
100-fold.
• In spite of the enormous energy requirements of insect flight muscles, their respiration is always
aerobic.
مغرلا ىلع
NEURAL EXCITATION AND MODULATION OF MUSCLE CONTRACTION
• Vertebrate muscle may have hundreds of axons, but the individual muscle fibers bear only single nervous innervations and are activated in an all-or- none fashion.
• One neuron may branch to innervate several muscle fibers, which are activated together as a muscle unit.
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NEURAL EXCITATION AND MODULATION OF MUSCLE CONTRACTION
• The strength of a
contraction depends on the number of total muscle fibers within the muscle that are
recruited at one time, with fewer muscle fibers activated when smaller contractions are required.
NEURAL EXCITATION AND MODULATION OF MUSCLE CONTRACTION
• In contrast, the muscles of the smaller insects often consist of only one or two fibers .
• In these insects, for graded contractions to occur there must be some other mechanism than the recruitment of
additional muscle fibers.
جردتم
فيظوت
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NEURAL EXCITATION AND MODULATION OF MUSCLE CONTRACTION
• The motor neurons that innervate insect skeletal muscles run along the muscle fibers and repeatedly synapse at intervals .
• One muscle fiber may receive multiple
innervations from several motor neurons, and these may consist of a
combination of neurons designated fast,
intermediate, and slow.
NEURAL EXCITATION AND MODULATION OF MUSCLE CONTRACTION
• These designations refer to the speed of muscle
contraction that the neurons produce, rather than their own speed of signal transmission.
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NEURAL EXCITATION AND MODULATION OF MUSCLE CONTRACTION
• The fast neurons innervate all muscle fibers and cause rapid depolarization and maximum muscle
contraction .
• The slow neurons innervate some skeletal muscles and cause small depolarizations and slight muscle contractions.
NEURAL EXCITATION AND MODULATION OF MUSCLE CONTRACTION
• Changes in membrane potential after the
stimulation of fast and slow neurons.
• In fast neurons (top),
nervous stimulation (arrow) causes a sharp rise in
membrane potential, causing a rapid muscle contraction.
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NEURAL EXCITATION AND MODULATION OF MUSCLE CONTRACTION
• In slow neurons, each nervous impulse causes a small depolarization, but their effects can be
summated.
• This allows a muscle that consists of only a few
muscle fibers to engage in a graded contraction.
NEURAL EXCITATION AND MODULATION OF MUSCLE CONTRACTION
• Repeated firing of the slow neurons causes a
summation of
depolarization effects and allows a muscle made of only a few fibers to engage in graded contractions .
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NEURAL EXCITATION AND MODULATION OF MUSCLE CONTRACTION
• The jumping muscle of the locust hindleg is innervated by both fast and slow
neurons, with about 30% of the muscle fibers supplied by slow neurons .
• These slow neurons are used for ordinary
movements, while the fast neurons are used for
leaping. Populations of fast and slow neurons in an insect leg.
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