chapter 11 anatomy and physiology
TRANSCRIPT
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Muscular System
myo = Muscle endo = Within fasc = little
epi = above peri = around apo = above
pena = feather syn = together ant = against
ramus = branch sarco = flesh blast =precursor
1. There are three kinds of muscular tissue studied in Myology, or the study of muscles, they are
cardiac muscle, skeletal muscle and smooth muscle. All muscle cells exhibit characteristics of
excitability, conductivity, contractility, extensibility, and elasticity.
I. Responsiveness (Excitability) - Responsiveness to external stimuli is a normal
property of all living cells, muscle cells and nervous cells are very responsive.
Stimuli could be chemical, mechanical, electrical and other stimuli which elicits a
response in the cell.
II. Conductivity -Stimulation is not only locally emulated but is conducted along a
pathway to neighboring cells.
III. Contractility - The ability to shorten or contract which creates a tug on bones,
and organs.
IV. Extensibility - Refers to the ability of muscle cells in being stretched, such as the
smooth muscles that make up the stomach.
V. Elasticity - Is the ability of muscle cells in returning to their original shape after
stretching or contracting.
2. Skeletal muscles are voluntary, motion is initiated by motor neurons attached to muscle fibers at
synapse, they are striated and are usually attached bones, they can be 100 um to 30 cm long.
3. Smooth muscles are involuntary and are usually moved through the control of the autonomic
nervous system and are never attached to bones.
4. Skeletal muscles vary in size from micrometers to about 30 cm, and usually appear as strands
and due to this factor muscle cells are also known as muscle fibers or myofibers.
5. Muscular tissue has four key functions: Producing body heat, stabilizing body positions, moving
substances within the body, and generating heat.
6. Muscle fibers are covered by a number of
connective fibrous layers including the tendons,
which arent excitable, conductive, or contract to
produce motion. They are however can be
stretched and are elastic so they resume shape
after being stretched within limits.
7. Fascia covers the entire muscle including groups of
muscle, which which is continuous with epimysium
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that narrows and become a tendon or an insert. Fascia can be surrounding Below the
epimysium bundles of muscle fiber or muscle cell are perimysium which surrounds about
10-100 of the muscle fibers. Endomysium surrounds individual muscle cells.
8. When muscles are contracted and motion occurs, the usual course of action in the muscles is
that a muscle stays stationary while the other bone moves. The site at which usually the muscle
is anchored where by it regulates motion is called its origin and the other end of the muscle orinsertion is where the muscle is attached to moving bones. In most cases insertions are distal
to origin.
9. Myofiber is the cell, Myofibril is the individual rod like arrangement of myofilaments within the
muscle cell. Within the myofibril exits the myofilaments that are actin and myosin filaments.
Components of muscle
Layers that cover muscle (> 2 )
16.Muscle cells are called myofibers, muscle fibers which are
distinct from myofibrils which are protein fibers that make up
the contractile component of a cell, myofibrils are made up
myofilaments which are the individual protein chains or
filaments that make up the myofibril.
17. Filaments is a long chain of protein organized into long parallel
lines. In a muscle can be broken into three categories (more
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below) thick filaments which are made up of myosin molecules, thin filaments which are made
up of a protein called actin. The last category of myofilaments are made up of elastic filaments
that anchors the thick filament or myosin to the z disc, contributes to the elastic recoil and helps
center the fiber for effective movement.
18. Sarcolemma - is the plasma membrane of the muscle cells that is continuous with the
endomysium. Below the sarcolemma lies the multiple nuclei of the muscle cell.19. Sarcoplasm - The cytoplasm of muscle cells, contains a larger amounts of glycosomes
(glycogen storage units), myoglobin (O2 binding protein), larger amounts of mitochondria, and the
sarcoplasmic reticulum (smooth ER) is mostly used for calcium storage and pumping during
excitation.
20. Sarcoplasmic Reticulum - In muscle cells the endoplasmic reticulum is called the
sarcoplasmic reticulum and completely covers each myofibril, it functions as a calcium storage
unit.
21. Transverse Tubule or T- tubules, the sarcolemma has deep infolds or invagination that
extend from outside the cell to the inside of the cell and wraps around myofibrils (bundle of
filaments).22. Triad - Each of the Transverse tubules are situated between two enlarged parts of the
sarcoplasmic reticulum called cisternae,so this structure with two cisternae with a transverse
tube in the middle is known as a Triad.
23. Fascicles - Perimysium surrounds fascicles which are bundles of muscle fibers numbering
anywhere from 10-100.
24. Muscle Fibers (Cells) - Long cylindrical cells covered by endomysium and sarcolemma, and
the nucleus is located near the edge of the cell immediately after the sarcolemma.
25. Myofibrils - Threadlike structures that make up the contractile portion or the functional portion ofthe cell. Composed of protein filaments and extend to the entire length of the cell.
Myofilaments
26.Myofibrils are made up of myofilaments and are the contractile organelle of the skeletal system,
they can be classified into three categories though there are more than three kinds of proteins
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that make up myofibrils. (above they are categorized into thick, thin and elastic, here they are
grouped by function) http://www.youtube.com/watch?v=Ct8AbZn_A8A
I. Contractile Proteins - Myosin and actin make up the contractile proteins in that their
combined function creates motion. Actin and myosin are contractile proteins in many
different cells in use in cellular transport, during meiosis and mitosis but they are
abundant in muscle cells and give the striated appearance to skeletal muscle. Myosin is a motor proteinthat it uses ATP to pull on structures to create
movement.
Myosin is organized into bundles of about 300 strands that combine to make up
the thick filament of the myofibril.
Projections of myosin called myosin headsextend outwards and become attached
to active sites on the thin filaments and propel themselves forward.
Actin molecules are arranged into spiralled filament (Helix) and have active siteswhere myosin heads can attach.
Actin molecules can be further broken down into F-actin which makes up the
chain of the molecule that makes the actin filament and G-actin makes up theactive sites where myosin head can attach.
II. Regulatory Proteins - Regulatory proteins help switch the muscle from active to
relaxed, they are both part of the actin helix. (thin filament)
Tropomyosin molecules are arranged into thin filaments that block the active sitesof actin in relaxed state.
Troponin molecules when combined with calcium ions change the tropomyosinso that the active sites are revealed so that myosin heads can attach to the
tropomyosin active sites.
(The cycle is described shortly...as soon as I understand it and not fall
asleep.hahah sleep, like I can fall asleep)
III. Structural Proteins - There are about 12 structural protein the sarcomere eachadding to the elasticity, extensibility, connect filaments and for the proper alignment of
molecules.
Nebulin - Limits the length of the thin filament. Titin the most plentiful after myosin and actin, because of its springy nature
connects M- line to the Z disc and can recoil after stretching aiding to elasticity of
the sarcomere and secures the sarcomere against overstretching.
- actinin makes up the z line of sarcomere and anchors titin molecules and
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actin filament that makes up the thin filament.
Dystrophin - (I am not sure whether if they are part of the regulatory protein, I
mean it doesnt sound like it.. but its on the book so here it goes) These
molecules transfer the contractile strength of the sarcomere
Myomesin - molecules that are lined up on the M-Line that are connected to thetitin molecules and keep the myosin molecules in a bundle.
Striations
27.The arrangement of myosin and actin fibers are organized in such way at a molecular level that
at a microscopic level it seems striated. The striations come from two alternating bands of dark
and light sections.
The A- Band is the middle part of the sarcomere and contains the entire lenghth of the
thick filament or myosin and thus appear darker in comparison to the I-Band I-Band where there are no thick filament or myosin and so it appears lighter, the center
of the I band is where each of the sarcomere ends. At the end of each sarcomere lies
proteins that anchor the filaments within the sarcomere.
28.Z- Disk is the center of each of the I-band where thin filaments are connected to them, they are
at the end of the each sarcomere and so each z-disk signal the end of a sarcomere.
29.A narrow part of the A-band contains thick filament but no thin filament this portion of the
sarcomere is called the H-Band, the center of which is the midline of a sarcomere and so its
called the M-Line.
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The Nerve Muscle Relationship
30.Muscles contract only in response to electrical signals, and those signals are delivered throughthe somatic nervous system.
31.Each of the muscle fiber is connected to one corresponding neuron, but one neuron can be
connected to a thousand muscle fiber as it is in the calf, or for highly specialized movement
such as it is is in the eye only 3-6 muscle cells are connected to a neuron. On average about
200 muscle fibers are connected to a neuron.
32.Neuronal connection is vital for neurons, with a severed nerve the muscle no longer function and
as a result it denervation atrophy.
33.Somatic motor neurons make up the activation pathways for skeletal muscle, they are located
in the CNS with their axons reaching the individual muscle cells. Once the neuron fires sending
electrical signal to the target cell, the signal travels to the end of the neuron where the fibersbranch off and connect anywhere from a few muscle cells to about a 1000 muscle cells at the
neuromuscular junction (NMJ).
34.At the Neuromuscular Junction the ends of neuron called the axon terminal divides into
cluster of synaptic bulbs (or synaptic knob). The synaptic knob and the depression in the
sarcolemma where communication occurs does not have direct contact. Communication is
carried out through synaptic vesicles with corresponding receptors in the muscle.
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35.The space between the synaptic bulbs and the receptors is called the synaptic cleft, and can
have as many as 50 million active sites for transmitters (30 - 40 million norm) . The depression
on the sarcolemma has junctional folds which increases the amount of surface area available for
receptors.
36.When neurons communicate with other cells or themselves neurotransmitters are used, a
specialized molecule that are released in vesicles to the receptor site of the target cell. Inneuron-muscle communication the neurotransmitter used is called Acetylcholine (ACh).
37.A motor unit- consists of one motor neuron and its associated muscle fibers. A motor unit could
be spread through the group of muscle.
Fibers in motion
38. The sliding filament model is the representation of the movement of the thick and thin filament
which does not change in size but slide past each other.
The thin filaments move towards the center, as this occurs the H-zones and the I bands
narrow and the Z lines become closer together. The combined mechanism of the molecules in the sarcomere add towards shortening
the length of the sarcomere.
39. All or nothing Muscle fibers of a motor unit moves in unison to stimuli, the force of
concentration for a motor unit remains the same for all contractions. The variations of strength
for muscle movement is achieved through the activation of only some of the motor unit in the
same muscle group. Muscle groups are spread throughout the muscle and so the activation of
just a fraction of a muscle can contract a muscle without having to use all of its contractile
strength.
40. Electrophysiology and excitability of cells
In a electrically polarized cell (cells that have different voltage outside the cell than the
outside) there are more negative charges (anions) within the cells than outside the cell,
this is mostly due to the negatively charged protein, nucleic acids and proteins.
Within the cells there are more K+ ions and outside the cell (ICG) there is an excess of
Na+.
Electric Potential refers to the the difference in voltage between two points, in musclecells this difference is about -90 millivolts referred to as the resting membrane
potential (RMP). This charge difference is maintained through the sodium potassium
pump.
When a stimulus triggers the cell to open its ion gates the sodium ions in the ICF rushes
in while the positive potassium ions within the cells partly due to the positive sodium and
partly due to the its own concentration gradient (there are less k+in ICF) rushes out of the
cell as the sodium rushes in. This change progresses in a steady stream of action as
one action triggers another is called action potential.
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter10/animation__
action_potentials_and_muscle_contraction.html
http://www.google.com/url?q=http%3A%2F%2Fhighered.mcgraw-hill.com%2Fsites%2F0072495855%2Fstudent_view0%2Fchapter10%2Fanimation__action_potentials_and_muscle_contraction.html&sa=D&sntz=1&usg=AFQjCNEbWZ_OD2Sg5K7XXgdsUu05Dnqumwhttp://www.google.com/url?q=http%3A%2F%2Fhighered.mcgraw-hill.com%2Fsites%2F0072495855%2Fstudent_view0%2Fchapter10%2Fanimation__action_potentials_and_muscle_contraction.html&sa=D&sntz=1&usg=AFQjCNEbWZ_OD2Sg5K7XXgdsUu05Dnqumw -
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41. Muscle movement andcross bridge cycling:
I. The signal, the nerve impulse initiated at the central nervous system is carried to the
target cell through axons and arrives at the synapse.
The electrical signal (action potential) causes voltage gated calcium channels to open,
this causes calcium to enter the synaptic knob. The newly released calcium channels initiates the exocytosis of the vesicles containing
acetylcholine.
The Neuromuscular Junctions are slightly apart from the muscle (60-100 nm wide), and
are covered by Schwann Cells (same cells that cover the axons).
The sarcolemma is covered with ACh receptors which bind to the ACh released from the
nerve. The ACh receptors are specially concentrated in the high infolds directly beneath
the neuromuscular junction called Junctional Folds.
The ligand gated ACh receptors channels open letting in sodium ions which causes
potassium channel to leak out. As the sodium rushes in the resting potential of the cell
which was -90 mV jumps to +75, but once the potassium leaves the cell membrane itgoes back down to a level closer to its resting potential.
II. Acetylcholine
Acetylcholinerase (AChE) is an enzyme in the muscles basal lamina that breaks down
acetylcholine after the muscle has been exited. It allows the muscle to relax after
excitation, although ACh molecules can leave the ACh receptors the procedure is
expedited by AChE.
Toxins that interfere with the procedure of AChE causes muscle paralysis, such as
spastic paralysis. Other conditions such as that brought on by the bacterium Clostridium
Tetaniblocks the production of glycine, which is inhibitory protein produced in the spine to
prevent unwanted muscle contractions.III. Gates open Once Acetylcholine binds to the Acetylcholine receptors, the receptors are
ligand gated ion channels and once they open sodium ions rush in followed by potassium
ions leaking out.
The sudden change in voltage change continues through the muscle and within the
muscle this change is almost instantaneous and throughout because of the transverse
tubules and its webbed network.
As the action potential travels through the transverse tubules in the triad it signals voltage
sensitive tubule proteins in the cisternae to open releasing stored up calcium into the
muscle cell.
IV. Calcium ions and actin
Once calcium is released into the cell, the calcium binds to the troponin, which alters the
shape of troponin which in turn changes the shape of tropomyosin.
Tropomyosin covers the binding site of actin (G actin) preventing myosin heads from
being attached to them, but the change in shape of troponin alters the tropomyosin and
moves them out of the binding site.
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V. Myosin and ATP
Myosin is in its cocked position when an ATP is hydrolyzed to form ADP and inorganic
phosphate.
The activated myosin head binds to a G protein as the tropomyosin moves away.
After the connection has been made between the myosin and actin the inorganic
phosphate leaves the myosin head strengthening the bond between the two proteins.VI. Power stroke
After the inorganic phosphate leaves the myosin and the bond is made stronger, the
myosin head moves backward propelling the actin filament toward the M line, thus
narrowing the H zone (bare zone with no actin filament) and I zone.
As the thin filaments are pulled closer to each other the space between the actin
filaments shorten which shorten the length of a sarcomere and as a result the combined
effort of many sarcomeres produce contraction.
At maximal contraction the actin filaments not only get closer to each other but can
overlap one another.
As the thin filaments are pulled toward the midline, a protein structure called dystrophin
translates the force to ECf and to the endomysium which pulls on the connective tissues
including and eventually the tendons producing motion or force.
VII. Myosin becomes detached
After the myosin has exerted its force on the actin filament, the myosin remains attached
to the actin until it binds to another ATP molecule.
Once ATP binds to the myosin head the bond between actin and myosin becomes
weaker and the two separates.
VIII. ATP Splits ATP is hydrolyzed forming ATP and inorganic phosphate and the cycle begins all over
again.
The myosin head attaches and detaches about five times per second as long the G-
actin sites remain open.
Myosin heads in their state of relaxations stays with an unhydrolyzed ATP molecule, and
in the extended (not cocked) position.
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IX. Acetylcholine stops.
Once the motor neuron stops firing, the release of Acetylcholine ceases, some of the
ACh leaves through diffusion but most of them are broken down through
Acetylcholinerase into acetic acid and choline, Choline is absorbed back into the synaptic
knob.
Active transport pumps calcium back into the cisternae from the cytosol, calcium isstored attached to proteins called calsequestrin.
As calcium leaves the cytosol they dissociate from troponin which alters the shape of
tropomyosin and the active sites for myosin binding is closed.
X. Summary
A motor neuron releases Acetylcholine which triggers an action potential, which is carried
through the T-tubules and that releases calcium ions from SR.
Calcium ions bind to troponin, tropomyosin changes shape and myosin attaches to actin.
The myosin heads power-strokes toward the center pullin actin, which produces force
that translates to muscle contractions.
When acetylcholine is no longer released from the neuromuscular junction it is absorbedback into the sarcoplasmic reticulum, and trpomyosin reverts back to covering the actin
sites.
42. The length tension relationship
Overly Contracted - When a muscle is overly contracted the length of the
sarcomere is the length of the myosin filament, and any force produced
would act against the z disk and no force would be produced.
Optimum resting length - The optimum level for contraction is when
the fibers are set in a way where all of the myosin heads can
make contact and there is room for the filaments so thatmaximum motion can take place.
Overly stretched - When a muscle is overly stretched only a
fraction of the myosin heads can generate force and so the
force produced will also be a fraction of the total possible generatable force.
43. Threshold Latent period and Twitch (The twitch and the latent period between twitches are
achieved through activation of muscle fibers rather than nerve fibers, thus the twitch here does
not reflect the usual course of action in a muscle but rather
characteristics of muscle as studied in a laboratory through
stimulation and the resulting myogram )
Muscle contractions are initiated only when sufficient
electric activity brings forth an action potential wave,
the minimum amount of stimulation necessary for
muscle movement is called the threshold.
Once the stimulus is received from the nerve there is a
delay of about 2 milliseconds before the electric
stimulus becomes muscle contraction, this period is known as the Latent Period. This
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period is also includes the time in which the muscles becomes taught and the internal
tension becomes stronger in order to overcome the elastic properties of muscle.
The muscle and connective tissue would have to contract more than the amount that it
can stretch so that it can act upon the tendons and bones to produce movement
At stimulus that produces a muscle potential equalling or higher amount of action
potential causes a Twitch, which is a quick cycle of contraction followed by relaxation. Between two twitches there exists a latent period where the muscle does not contract
because time is required for excitation and excitation-contraction coupling.
Contraction Phase is when the components of myofibers have gone through theirelasticity and sarcomere contraction takes place, which is soon followed by relaxation
phase, where the calcium is pumped back into the sarcoplasm.
44.Muscle tonerefers to the bodys tendency to maintain optimum resting length of
sarcomeres for optimum production of force. This is achieved through partial contraction
of fibers in the muscle and through connective tissues that maintain elasticity.
45. Contraction Strength of Twitches Muscle cells when stimulated at nerves rather than the muscle produce stronger force,
because they stimulate more of the muscle fibers, the process of adding more motor
units in order to produce a stronger contractile strength is called Recruitment or
multiple motor unit summation.
Twitch Sudden excitation followed by relaxation, where the muscle produces strength ofequal degree.
TreppeA phenomenon exhibited by muscle to series of stimuli at the same strength,since not all calcium is absorbed back their strength becomes increasingly stronger, but
the muscle relaxes even if for a short time between excitation.
Incomplete Tetanus is achieved when a muscle is not allowed to completely relaxbetween stimuli. (20-40 stimuli / sec), in incomplete tetanus there is so small an interval
of time between stimulus that complete relaxation of the muscle never takes place.
Complete Tetanus is achieved when no relaxation at all takes place between stimuli.(40-50 stimuli/ sec), where the muscle never goes into relaxation but soon fatigues after
continues after complete tetanus.
The neuron rarely stimulates at rates higher than 25 stimuli/ second, the above noted
characteristics are observed in laboratory.
If a muscle is exited before it is allowed to relax then the two waves of excitation
combines together forming a larger wave of excitation.
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46. Isometric and Isotonic Contractions
Isometric contraction is when a muscle develops tension but is unable to producemovement. (pushing against an object producing no movement)
Isotonic contraction is where the as a result of contraction produces movement, if themovement is achieved through shortening of the muscle then its Isotonic concentric
contraction and if the motion is when the muscle lengthen under tension its calledeccentric contraction.
Isometric, no movement, isotonic movement, concentric shorter muscle, and eccentric
is when the muscle lengthen under tension.
Muscle Metabolism
47. ATP sources , muscle requires a continuous source of ATP to function, which can be produced
from organic compounds such as glucose, fatty acids, glycogen and other molecules. It can be
produced by one of three mechanism : (1) Creatine Phosphate (2) anaerobic cellular
respiration and (3) aerobic cellular respiration. (Production of ATP through creatine phosphate isunique to muscle)
48. Immediate Energy - Immediate energy requirements of the skeletal system, about six seconds
of intense activity or a minute of brisk walking, until aerobic metabolic processes act to produce
ATP. Myokinase and creatine kinasewithin the muscle provide for this type of energy. Myokinase - Transfers phosphate group from an ADP to another ADP forming an ATP.[ ADP +ADP + Myokinase = ATP + AMP + Myokinase]
Creatine kinase - Creatine Phosphate molecules are energy storing molecules with aphosphate groups that are synthesized during energy surplus. This energy storage unit
transfers a phosphate group to form ATP. ATP is used in times of abundant ATP to form
creatine phosphate so that the creatine phosphate can be used in times of need.
[ ADP + Creatine Phosphate + Creatine kinase = ATP +Creatine + Creatine kinase]
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Slow Oxidative Fibers, these fibers are rich in myoglobin and capillaries and are thesmallest of the fibers, they are fatigue resistance but are not able to deliver large
amounts of force. They are mostly components of muscle used for posture and aerobic,
endurance type activities.
Fast Oxidative - Glycolytic Fibers - Like slow oxidative fibers they can produce ATPaerobically, they are medium sized and faster in comparison to slow oxidative fibers interms of the rate of hydrolysis of ATP in myosin heads. These fibers also known as type
II B are rare in human body and are prevalent only in people with endurance training.
Fast Glycolytic Fibers - They are the largest of the three and contain the most amountof myofibrils and generate the greatest amount of force. They generate ATP mainly
through glycolysis and so they fatigue faster.
A major cause for distinction is the rate at which ATP is hydrolyzed in the myosin head ofeach of the fibers.
Most skeletal muscle are a combination of the three fibers, depending whether they are
used for posture (SO fibers), for generating force (upper limbs) or a combination of two
(legs). The amount of fibers present in an
individual is largely dependent on genetic
predisposition but FOG can become FG or
vise versa to accommodate physical
activity over time.
The textbook used for class onlymentions two of the three fibers since
fast-glycolytic fibers or intermediate
fibers are rare in human body and are
only abundant in trained athletes. Thetable below shows just the two muscle
fibers.
54. Muscle Strength and Conditioning Many factors affect the efficiency of muscle such as the
size of the muscle, fascicle arrangement (pennate muscles are stronger in comparison to
parallel muscle which in turn is stronger than ocular muscle). Size of motor unit, multiple motor
unit summation, temporal summation (delivery rate by the neuron, stronger the rate=stronger
force), optimum length of muscle and fatigue.
Elasticity of a muscle changes largely due to connective tissues that surrounds each
muscle.
55. Resistance Exercise is weight training where by increasing the size of the muscle, which
makes them stronger, Endurance (aerobic) Exercise makes the muscle fibers less subject to
fatigue. A combination of the two known as Cross trainingwhich is a combination of the two.
56. Delayed onset muscle soreness results from microtrauma to the muscle.
57. Cramps are initiated by the central nervous system, they are painful spasmodic contractions
that are usually the result of extreme cold, heavy exercise, lack of blood supply, electrolyte
depletion, dehydration and low blood glucose.
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Cardiac Muscle
58. Cardiac Muscle fibers share much in common with skeletal muscle tissue in that they both
have the same zones, sarcomere structure but differ in that the ends of the cardiac muscles are
joined to one another through Intercalated Disks which are unique to cardiac muscle fibers. In response to action potential
cardiac muscles contract 10-15
times longer for effective emptying
of chambers.
The ends of cardiac cells are
irregular shaped (intercalated
disks) with desmosomes for better
protection against stress and gap
junction gap junction to facilitate
transfer of materials. They have larger mitochondria and
more of it since it relies on constant
and abundant amount of ATP
almost exclusively from aerobic
respiration. (2% mitochondrial
volume in muscle, 25 % in cardiac
cells.
59.Cardiac cells can contract without action potential from the nervous system, that is they are
autorhythmic in that they have their own pacemakers that set the rate at which they beat.
Smooth Muscle
60.Smooth muscle have single centrally located nucleus with no visible striations since the
myofilaments (actin, myosin) are thinner in comparison and are randomly distributed. They lack
T-tubules and their sarcoplasmic reticulum is not as functional or developed as skeletal muscle
or cardiac muscle.
61.Some smooth muscles are not
innervated instead and communicate
with each other through gap junction,
pressure or other stimuli. When a nerve
is connected to them they are periodic
swelling that contains
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neurotransmitters, norepinephrine from the sympathetic fibers and acetylcholine from the
parasympathetic nervous system. A single nerve can have up to 20,000 swellings and
innervates large number of smooth muscle cells.
62.Smooth muscles layers of hollow organs can be formed from a single layer or from many layers,
such as small arteries with only one layer of cells to multilevel and layered stomach formation
with smooth muscle. (piloerector muscle is made up smooth muscle )63. Excitation of Smooth Muscles: Excitation of smooth muscle can take place from a number of
different stimuli specific to function unlike the skeletal muscle which is stimulated by the somatic
nervous system.
Autonomic nerve fibers and neurotransmitters, parasympathetic = acetylcholine,sympathetic = norepinephrine,
Chemicals, chemicals can be anything from hormones, carbon dioxide, low pH, nitricoxide to oxytocin during labor contractions.
Temperature, some of the smooth musclesin response to cold contract and dilate inresponse to heat.
Stretch Mechanically gated ion channels open in response to physical stress as in thestomach and bladder.
Autorhythmicity periodic contraction contribute to peristalsis.64. Two types of smooth muscle are
Single unit (visceral)smooth muscle where all of the smooth muscle are connected to
each other through gap junctions so that the activation of a smooth
muscle travels downwards activating other muscles. They are
usually found in smooth muscle of hollow organs.
Multi unit smooth muscle major difference between the two isthat the activation of a muscle stimulates just the muscle that was
stimulated. Makes up the smooth muscle of large arteries,piloerector muscle, muscle of the eye that focuses and adjusts the
size of the pupil.
65. Contraction of the smooth muscle, Smooth muscles contract through similar mechanisms as
skeletal muscle.
Smooth muscle excitation open calcium channels, the entering
calcium binds to calmodulin which starts a reaction that adds a
phosphate group to the head of myosin which makes it attracted
to G-actin of the thin filament.
Actin molecules are attached to dense bodies which are
connected to intermediate filaments that form the cytoskeleton of
the smooth muscle cells, and so as a result of contraction the
entire muscle contracts and becomes smaller.
Smooth muscle has a prolonged latent period of about 50-100
milliseconds due to slow absorption of calcium ions, and to a
mechanism called Latch mechanism (even after the phosphate
group is removed through calcium resorption crossbridge
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continues), these contribute to smooth muscle using less ATP and being less
susceptible to fatigue.
66. Response to stretch, peristalsi and bladder functions are maintained through the
stress-relaxation response, that is the smooth muscle builds up tension in response to stretch
(stress) but soon relaxes in order to accommodate the increasing stress.67. Smooth muscles maintain the ability to contract without the issue of optimum length due to the
fact there are no z-disks, there are myosin heads throughout the thick filament and so there is no
H-zone or bare zone. The sarcomeres are arranged in all directions so that despite formation the
muscle can contract.
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