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Glossary of Exam #3 Muscles Lecture Dr. Harris

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Extensions of deep fascia are:
*Epimysium-covers a whole muscle *Perimysium-surrounds fascicles (bundles) of cells ("fibers") *Endomysium- arround individual cells (fibers)
True or False All of the (CT) deep fasia extentions coalesces at the end of the muscle continuous w/ the tendon
TRUE
Tendon
*dense regular CT *attaches muscle to perioseum
Skeletal Muscle Fiber Ultrastructure
*cells are long & thread-like, thus called FIBERS *typically 1-2" long; up to 12"long *only ~100umin diameter *multinucleated- post-miotic *many mitochondria *special terms: sarcoplasm & sacrolemma *sacroplasmis many myofibrils (arranged parallel) -> striated (banded)
Skeletal Muscle Hierachy
Skeletal muscle to fascicles to muscle fibers to myofibrils to myofilaments (filaments) to chain-like proteins (actin & myosin)
Three Types of Filaments
*thin *thick *elastic
True or False Elastic filaments are structural elements.
TRUE
True or False Thin & thick filaments are not involved in contraction.
FALSE
Thick filaments are:
~200 molecules of myosin
Thin filaments are:
actin tropnin tropomyosin
Muscle is a tissue that is specialized to do what?
To generate mechanical force.
Muscle has the machinery for what?
Energy transduction
Chemical E. (ATP) is converted to what?
Mechanical E. (force) that may result in movement
3 principle types of muscle
Skeletal, Cardiac, Smooth
What are the major differences in the 3 principle types of muscle?
Morphology (appearance) Method of activation
Describe skeletal muscle
Primarily attached to bone-striated--> alternating patter of light & dark bands-voluntary-->activation is under conscious control
Describe Cardiac Muscle
Muscle tissue of the heart -striated -involuntary-->activation is NOT consciously controlled instead heart initiates its own contraction (beats) (although it can be modified by outside influences)
What is automaticity of cardiac muscle?
Initiates its own contraction (beats)
Describe Smooth Muscle
Lines walls of hollow internal structures -non-striated (smooth) -involuntary
3 characteristics of all muscle
1. Excitable: stimulation produces an electrical impulse (action potential) 2. Contractility: can shorten to generate force 3. Elasticity: can be stretched
What % of total human body mass does skeletal muscle make up?
~40-50%
Skeletal muscle can increase the metabolic rate by how much?
10-15 fold
What do the terminal branches of motor neurons do?
Activate muscle
How do we know that skeletal muscle is highly vascular (what shows us)?
Each muscle cell is surrounded by a few capillaries & Reflects need for O2 and CO2 exchange
Muscle (organs) are make up largely of what?
Skeletal muscle tissue held together by CT.
What is a deep fascia in skeletal muscle tissue?
A fibrous membrane (dense irreg. CT) Covers, supports, & separates muscles Binds muscle but allows movement (normally)
What are the 3 extensions of deep fascia?
Epimysium Perimysium Endomysium
What is the function of the epimysium?
Covers a whole muscle
What is the function of the perimysium?
Surrounds fascicles(bundles) of cells ("fibers")
What is the function of the endomysium?
Surrounds each individual cell (fiber)
What is a fascicle?
Bundles of muscle cells or fibers
What does CT do at the ends of muscles?
Coalesces at the end of the muscle and continues on to form the muscle's tendons
What is a tendon?
Dense regular CT that attaches muscle to the periosteum
What shape do muscle cells take on?
Long and thread-like and thus are called fibers
What is the range of lengths of the muscle fibers found in the human body?
Typically 1-2 inches long but some are up to 12 inches or 1 foot long
What is the average diameter of a muscle fiber?
Usually only about 100 micrometers in diameter
When are muscle cells said to be multinucleated?
Post-mitotic
What amount of mitochondria can be found in skeletal muscle fiber? (none, few, several, many ?)
Many
What does the sarcoplasm of skeletal muscle tissue contain?
Many striated myofibrils arranged in parallel with one another
What is the Hierarchy of skeletal muscle?
Skeletal muscle is comprised of fascicles. Fascicles are comprised of muscle fibers. Muscle fibers are comprised of myofibrils. Myofibrils are comprised of myofilaments. Myofilaments are comprised of chain-like proteins called actin and myosin.
What are the 3 types of filaments of skeletal muscle tissue?
Thin Thick Elastic
What is the function of elastic filaments?
Structural elements
What is the function of thin and thick filaments?
Involved in contraction
What are thick filaments comprised of?
~200 molecules of myosin
What are thin filaments comprised of?
Actin Troponin Tropomyosin Contractile proteins Regulatory proteins
What gives skeletal muscle it's striations?
The pattern of overlapping of the thick and thin filaments
What are the 5 main areas within a myofibril?
A-band I-band H-zone M-line Z-disc (line)
What is the compartments within a myofibril where filaments are arranged called?
Sarcomere
What is a sarcomere?
The area between 2 z-discs which is an individual contractile unit within a striated muscle fiber.
What is a Z-disc in a sarcomere?
Thin area of very dense material (very dark)
What is the A-band in a sarcomere?
The dark area from one end of the thick filaments to the other end including the part of the thin filaments that overlap with the thick filaments
What is the I-band in a sarcomere?
The light area where thin filaments do not overlap (no thick filaments) with the Z-disc in its center. (Thus, an I-band is part of 2 adjacent sarcomeres)
What is the H-zone in a sarcomere?
The center of the A-band that contains only thick filaments. The rest of the A-band is called the "zone of overlap"
What is an M-line in a sarcomere?
The center of the H-zone where proteins connect adjacent thick filaments
What are the 2 elastic filaments of a sarcomere?
Connectin: protein at the Z-disc & Titin: which anchors the thick filaments to the Z-disc
What are the 3 organelles of skeletal muscle tissue?
Mitochondria Sarcoplasmic Reticulum (SR) Transverse Tubules
What is the sarcoplasmic reticulum of skeletal muscle and what is it's function?
SR is the network of cisterns and tubues that surround each myofibril and it releases and take up Ca++ through Ca++ release channels
Where is the sarcoplamic reticulum found in muscle fiber?
Surrounds each myofibril
What is the sarcoplamic reticulum's function?
Releases and takes up Ca++ through release channels for Ca++
What are transverse tubules and what are their functions?
T-tubules are minute channels or invaginations of the sarcolemma that are filled with ECF and they carry the electrical impulse deep to myofibrils in the center of the fiber
What molecules are needed for a muscle contraction?
ATP & Ca++
Myosin molecules have globular portions called what?
Myosin heads
What is the job of the myosin heads?
They attach to actin and using energy they swivel or pivot which creates a sliding movement of thin filaments sliding past thick filaments
What blocks the binding sites of actin from the myosin heads during rest?
Troponin-tropomyosin complex
Muscle contraction: What is Step 1?
An action potential (impulse) results in Ca++ release from SR. Ca++ binds to troponin causing a change in shape of the troponin-tropomyosin complex. This exposed the myosin-binding site on actin.
Muscle contraction: What is Step 2?
Attachment (binding) of the myosin head with actin occurs. This actin-myosin complex is called a cross-bridge.
Muscle contraction: What is Step 3?
When myosin attaches, swiveling (rotation) of the myosin head toward sarcomere's center occurs. This is referred to as the power stroke. Thus, the thick and thin filaments slide past one another.
Muscle contraction: What is Step 4?
During the pivot, myosin heads release ADP and Pi. Another ATP molecule can now bind to the myosin head. ATP binding causes the myosin head to detach from actin.
Muscle contraction: What is Step 5?
ATP is broken down. The myosin head is once again energized and is prepared for another cross-bridge cycle.
How often do the 5 steps of a muscle contraction occur during contraction?
The cycle is repeated over and over during a single muscle contraction.
What does a muscle contraction involve?
Interaction of actin and myosin, which slide past one another. Result--> Sarcomere length decreases and overall muscle shortens.
What is the process of the shortening of a sarcomere?
*Thin filaments slide toward M-line. *Width of I-band decreases. * Distance between 2 Z-lines decreases. *Width of H-zone decreases. *Zone of Overlap increases. *Width of A-band stays constant even under contraction.
What initiates contraction?
Ca++ in sarcoplasm So Regulation involves a change in Ca++.
Where is Ca++ sequestered?
SR
What membrane has active transport pumps?
SR
True/False. Muscles use energy while at rest.
True. At rest Ca++ is sequestered in SR which has active transport pumps.
What is the general mechanism of a muscle contraction?
1. Action potential occurs. 2. Opens Ca++ release channels in SR. 3.Ca++ is released into sarcoplasm. 4. Ca++ binds to troponin in thin filaments. 5.Troponin-tropomyosin complex changes shape. 6. Myosin binding sites on actin is exposed. 7. Actin and myosin interact. 8. Contraction takes place.
What are the 2 process of relaxation from contraction?
Motor neuron action potential ceases. Ca++ is pumped back into SR.
During relaxation from contraction what does the cessation of motor neuron action potential cause?
Ach release stops and Ach left in synaptic cleft if broken down by acetycholinsterase (AchE). Muscle action potential stops. SR Ca++ release channels close.
During relaxation from contraction what is the general process involved with pumping Ca++ back into SR?
Pumps can pump against an extreme (Ca++) gradient. Calsequestrin binds to Ca++ inside SR which reduces free Ca++ in solution. Thus more Ca++ can be pumped into SR.
What is a motor neuron?
Neuron (nerve cell) that activates skeletal muscle fiber. Most activate several muscle fiber simultaneously.
What is a motor unit?
A motor neuron & all of the muscle fibers that it innervates. *Basic functional unit of skeletal muscle.
What determines extent of control over skeletal muscle?
The number of muscle fibers in the motor unit
How many fibers are in motor unit(s) in eye muscle?
1 fiber per MU. Since eye muscles must function with much greater precision, it contains much more nerve for a given amount of muscle.
How many fibers are in a motor unit of the thigh muscle?
2,000 fibers per MU
What is on way in which force is modulated?
By changing the number of MU's
What happens if muscle contains smaller MU's?
Increment in force is small.
Motor neurons and muscle fibers are not directly connected so the electrical impulse cannot flow from motor neuron to the muscle. What does the body produce to overcome this problem?
A chemical called a neurotransmitter is released from the motor neuron which diffuses across the synaptic cleft and attaches to the muscle fiber surface allowing the electrical signal to initiate the process of contraction.
What is the specialized region in which neurotransmitters are released between the motor neuron and the muscle fiber called?
The synapse
The synapse between the motor neuron and the muscle fiber is called what?
Neuromuscular Junction (NMJ)
Events at the NMJ involve a specialized region of the muscle membrane called what?
Motor Endplate
The motor neuron's axon branches into clusters of what?
Axon terminals
The neuromuscular junction includes what 2 things?
Axon terminals and Motor endplate
What are the basic steps in which an nerve impulse initiates a contraction?
*Nerve impulse travels down the axon. *Impulse arrives at the axon terminal which causes fusion of synaptic vesicles to the cell membrane. *Ach is released. *Ach binds to Ach receptors at motor endplate and triggers electrical impulse in muscle fiber which initiates contraction.
What is a twitch?
Single contraction resulting from a single motor neuron impulse (action potential)
How do motor units contract during each twitch?
MU's contract simultaneously and to the same extent (if conditions do not change)
What is the All-or-none Principle?
All of the fibers in a MU contract "completely" or not at all. (There is no partial contraction)
What is the latent period and how long does it last?
Time for the Ca++ to be released from SR. 2-3 milliseconds.
What is the contraction period and how long does it take to complete?
The actual contraction of the muscle. 10-100ms depending on fiber type.
What is the relaxation period and how long does it last?
The time that Ca++ is being pumped back into the SR. 10-100ms depending on the fiber type.
How are you able to increase force of a muscle contraction?
Recruiting or activating more MU's and Increasing the frequency of stimulation of each MU.
What is the recruitment (activation) of more MU's called?
Multiple motor unit summation
How does multiple motor unit summation work?
At low force levels: small MU's are added. At higher force levels: larger MU's are added.
What happens if a 2nd stimulus is applied to the MU before the muscle has relaxed from the 1st stimulus?
The 2 twitches will summate and have an additive effect on contraction.
What is the increase in muscle contraction that results when stimuli follow in rapid succession called?
Wave (temporal) summation.
Why is the force of a twitch "limited"?
There is enough Ca++ released to allow complete interaction HOWEVER, Ca++ is removed BEFORE force reaches its maximum and the muscle relaxes.
What does the decreasing of the time interval between twitches result in?
Greater summation of twitch force.
What is tetanus?
A condition where stimuli occur at a high enough frequency that they fuse together completely and a smooth sustained contraction is produced and represents the maximum force output of the MU.
Why is there a limit to how fast twitches in muscle can be generated?
Refractory period
What is the refractory period?
After the muscle action potential begins, there is a brief period of time during which no impulse can activate the muscle.
How long is a refractory period?
5 ms
What are voluntary contractions?
Short-term tetanic contractions
What do short-term tetanic contractions involve?
MU's firing asynchronously (not same time). Each MU may be firing at less than fusion frequency. But added together, contraction of the whole muscle is smooth. Thus by sharing work, each one experiences less fatigue.
What is the importance of muscle in terms of its influence on overall body metabolism?
1. There is a lot of it: ~40-50% of body mass on average. 2. Can increase metabolic rate 10-15 fold.
Cardiac output can increase from ~ ____ liters/min at rest to as much as____ liters/min during maximal exercise which is a ____ fold increase.
6, 30, 5
At rest ___% of the total blood flow (cardiac output) goes to _____ ______. During maximal exercise this increases to ___%.
15, skeletal muscle, 85
Skeletal muscle competes with the thermoregulatory system for _____ _____.
blood flow
Pulmonary ventilation increased from ~__ liters/min to ~ ___ liters/min.
5, 150
Muscle activity involves energy transduction from ________ energy into _______ energy with ______ energy as a by-product.
chemical, mechanical, thermal
Where is energy stored in molecules?
chemical bonds
Energy released in pathways of energy metabolism is used to make what?
ATP
Energy pathways involve many what?
chemical reactions
Energy transfer between chemicals in these pathways involves what?
"Coupling" of chemical reactions
Energy releasing reactions are coupled to what kind of other reactions?
energy absorbing reactions
Muscle contractions require ATP for what 2 functions?
Energize the myosin heads and to pump Ca++.
ATP is converted to what?
ADP + Pi + Energy
How much ATP is stored in the muscle?
Very little, thus ATP must be continuously be produced during activity
What are the 3 energy systems?
Phosphagen (ATP-PC) system. Lactic Acid system (anaerobic system). Aerobic system (cellular respiration).
How do the 3 energy systems differ?
Capacity (amount of E.) and Rate of E. release (power).
Describe the phosphagen system
Very simple=Two components: -ATP -Phosphocreatine (PC) or Creatine Phosphate (CP) Coupled reaction.
What system represents the most immediate source of energy for muscle contraction?
Phosphagen system
Where does all energy for muscle contraction ultimately come from?
The breakdown of food molecules
What are the high energy sources of food that are broken down for energy for muscle contraction?
Lipids, CHO (carbohydrates), Proteins, Ethanol.
Describe the lactic acid system
"Glycolysis that proceeds faster than the supply of O2
How many ATP's are produced by the lactic acid system during glycolysis?
2
Describe the process of glycolysis in a diagram
glucose(glycogen)------> several steps-------------> lactic acid + energy = 2 ATP's
Describe (diagram) how aerobic system works
glucose--->CO2+H2O+Energy= 36 ATP's
Describe how the phosphagen system works
PC ------creatine kinase------>creatine
Describe the process of the phosphagen system
Provides a very rapid E supply. Allows for quick powerful movements. Unfortunately, it is only a very limited E source. Provides E for a maximum of only 15 seconds.
When glucose is completely oxidized in the presence of O2, what percent of E is used to make ATP and where does the rest get lost at?
~40% used to make ATP. Rest is lost as heat.
What is NAD+ ?
A coenzyme that transfers electrons (e-) to other reactions as it carries e-'s as H atoms.
NAD+ is essential for what?
For glycolysis to continue
What are the 2 ways in which NAD+ is regenerated?
Electron transport chain and Lactic acid production.
What are the 3 components of the aerobic system (cellular respiration)?
Glycolysis. Krebs cycle (citric acid cycle). Electron transportation (ETC) and oxidation phosporylation.
What is oxidation phosphorylation?
Production of ATP using E from the ETC
How does NADH carry energy?
In the form of electrons
Describe the anaerobic (lactic acid) system
End product is a strong acid which dissociates into lactate(negative ion) and H+ H+ interferes with contraction thus maximal activity can only be supported for 1-2 minutes.
What does lactic acid do?
Allows glycolysis to continue when O2 is limited
Describe the aerobic system
Completely oxidizes glucose (also fats). Supplies much more E (more efficient). Can support muscle activity for unlimited amount of time. Involves dozens of reactions, so unfortunately E is supplied at a slow rate.
What is the definition of work?
Application of force through a distance. W=FxO
What is the definition of power?
Work per unit of time. P=W/T
Describe the efficiency of the phosphagen system
Extremely high power output for very short period with a very limited capacity
Describe the efficiency of the lactic acid system
Moderate power but still limited capacity
Describe the efficiency of the aerobic system
Limited power but unlimited capacity
What are the 2 types of skeletal muscles?
Slow-twitch and Fast-twitch
How is a muscle classified to be either slow-twitch or fast-twitch?
Based on the speed of contraction and relaxation during a single twitch
How do fast and slow twitch differ?
The difference in myosin ATP-ase activity (fast-twitch split ATP more rapidly) and Difference in Ca++ handling.
What are the characteristics of fast-twitch fibers?
Contraction speed = fast; Power output = high; Endurance = low; Aerobic enzymes = low; Anaerobic enzymes = high; Fatigue resistance = low.
What are the characteristics of slow-twitch fibers?
Contraction speed = slow; Power output = low; Endurance = high; Aerobic enzymes = high; Anaerobic enzymes = low; Fatigue resistance = high.
What are some other common names for slow-twitch fibers?
Red muscle, type I, oxidative fibers, fatigue-resistant muscle, tonic.
What are some other common names for fast-twitch fibers?
White muscle, type II, glycolytic fibers, fatigable muscle, phasic.
Can all muscle fibers perform under both aerobic and anaerobic conditions?
Yes
What are slow-twitch fibers better suited for?
Better equipped biochemically and physiologically to work aerobically
What are fast-twitch fibers better suited for?
Anaerobically equipped
In what order are each muscle fiber type activated and why?
Since the motor neuron of slow-twitch fibers is smaller in diameter, it is activated first. Slow motor units tend to have fewer numbers and smaller fibers. Thus, the motor units activated first do not add much force and they are equipped to resist fatigue.
What 6 factors influence muscle force?
1.Muscle environment. 2.Cross-sectional area. 3.Speed of contraction. 4.Muscle length (amount of stretch). 5.Lever system arrangement. 6.Muscle architecture.
How is muscle environment a factor which influences muscle force?
pH, temp., and available O2
How is cross-sectional area a factor which influences muscle force?
Muscle with greater cross-sectional area produce greater force due to more myofibrils in parallel
How is contraction speed a factor which influences muscle force?
Force and speed are inversely related: As speed of movement increases there is less time for cross-bridge formation, thus force must decrease.
How is muscle length a factor which influences muscle force?
Maximal force occurs at an optimal length that represents maximal overlap of actin and myosin
How is the lever system arrangement a factor which influences muscle force?
Most skeletal muscles attach to a limb very close to the point of rotation of the joint. For example: the biceps brachii operates at a 1:7 mechanical disadvantage so to lift 20 lbs. the muscle must generate 140 lbs. of force.
What are the 2 advantages to the lever system arrangement in the human body?
Big displacement (movement) for short amount of muscle shortening. Much faster speed of motion or contraction.
What are the 2 types of skeletal muscle architecture?
Parallel and Pennate
Describe parallel muscle architecture
Muscle fibers are arranged parallel to long axis and thus the line of action of the muscle. Advantage: all of the shortening goes toward movement.
Describe pennate muscle architecture
Muscle fibers are arranged at an angle to the long axis. Advantage: more fibers can be attached, thus more force is possible. Disadvantage: range of movement (shortening) and speed are compromised.
Describe cardiac muscle tissue
Tissue of the heart wall-striated-involuntary
How does cardiac muscle compare to skeletal muscle?
Cardiac: Fibers are much smaller, 1 or 2 central nuclei, Same striations and actin-myosin arrangement, Many more mitochondria.
Describe the structure of cardiac muscle
Fibers branch and form interconnections through intercalated discs. This allows electrical activity to spread from fiber to fiber. Thus, all of the fibers can work together as a pump.
What are the 4 physiological differences between cardiac and skeletal muscle?
1.Cardiac muscle relies almost solely on aerobic system for ATP. 2.Stimulation of cardiac muscles occurs via specialized fibers within the heart. 3.Cardiac muscle twitches last 10 times longer than those in skeletal muscles. 4.Cardiac muscle has a much longer refractory period.
Describe smooth muscle tissue
Tissue of hollow internal organs, Non-striated (smooth), Involuntary.
How does smooth muscle tissue compare to skeletal muscle?
Cells are very large, Each cell has only 1 central nucleus, There are no striations and no myofibrils, Cells are spindle-shaped.
Describe the structure of smooth muscle tissue
Thin and thick filaments are not arranged in an orderly manner. Thus, there are no sarcomeres. There are no Z-lines, instead, filaments attach to dense bodies
What are the 4 physiological differences between smooth and skeletal muscle?
1.Calcium comes mainly from the ECF. SR is very small. 2.The regulatory protein is calmodulin(not troponin). 3.Smooth muscle contracts much more slowly and each contraction lasts much longer. 4.Contraction can be initiated by factors within the muscle such as changes in certain hormones and ions.
Why does smooth muscle contract much more slowly and last much longer?
There are no T-tubules and Much different organization of fibers.
What does the mechanical force gererated by muscle result in?
It may or may not result in movement.
Muscle has the machinery for…
energy transduction. Chemicat E. (ATP)-->converted-->Mechanical E. (force) --> may result in movement
Three Princeple Types of Muscle
*Skeletal, *Cardiac, *Smooth
Major Difference of the Muscles:
morphology (appearance), -method of activation.
Skeletal Muscle
*primarily attached to bone ; -striated -> alternating pattern of light & dark bands -voluntary -> activation is under conscious control *makes up ~40-50% of the total body mass *Can increase metabolic rate by 10-15 fold
Cardiac Muscle
*muscle tissue of the heart. -striated. involuntary -> activation is NOT consciously controlled instead automaticity
Define automaticality as referes the cardiac muscle.
Heart initiates its own contraction (beats) (although it can be modified by outside influences)
Smooth Muscle
*walls of hollow internal structures, -non-striated (smooth), -involuntary.
Characteristics of all muscle:
*Excitable: stimulation produces an electrical impulse (action potential) *Contractility: can shorten to generate force *Elasticity: can be stretched
True or False Skeletal muscle accounts for much of overall body metabolism
TRUE
Morphology of Skeletal Muscle
*The terminal branches of motor neurons are present:-these activate muscle; *Highly vascular: -each muscle cell is surrounded by a few capillaries -reflects need for O2 & CO2 exchange *Muscle (organs) are made up largely of skeletal muscle tissue held together by connective tissue
The connective tissue that holds together skeletal muscle tissue is called?
Deep Fascia
Deep Fascia is?
*a fibrous membrane (dense irregular CT) *covers, supports, & seperates muscles *binds muscles but allows mov't (normally)
Regulatory proteins are:
troponin tropomyosin
Contractile proteins are:
myosin actin
Areas w/in a myofibril:
*A-band *I-band *H-zone M-line *Z-disc (line)
Filaments are arranged into compartments called a:
Sarcomere
A sarcomere is
>an individual contractile unit w/in a striated muscle fiber >area between two z-discs
Z-disc is
thin area of very dense material (very dark)
A-band is
>dark area >from one end of the thick filaments to the other >includes the part of the thin filaments that overlap w/ thick
I-band is
>light area >area where thin filaments do not overlap (no thick) >a Z-disc is in the center (thus, an I-band is part of 2 adjacent sarcomeres)
H-zone is
>center of the A-band >contains only thick filaments >the rest of the A-band is called the "zone of overlap"
M-line is
>center of the H-zone >proteins connect adjacent thick filaments
"zone of overlap" is
the rest of the A-band that doesn't contain the center and the area where only thick filaments are found.
Elastic filaments are:
connectin titin
Connectin is
protein at the Z-disc
Titin
anchors the thick filaments to the Z-disc
Skeletal Muscle Organelles
*Mitochondria *Sarcoplasmic Reticulum *Transverse tubules
Sarcoplasmic Reticulum
> abbreviated SR >network of cisterns & tubules >surround each myofibril >releases & takes up Ca++ - thru Ca++ release channels
Transverse tubules
>(t-tubules; t-tubular system) >minute channels >invaginations of sarcolemna - filled w/ ECF >carry the electrical impulse deep to myofibrils in the center of the fiber
Muscle contraction involves:
>interaction of actin & myosin, which slide past one another. (Myosin is in a thick filament; actin is part of the thim filament.) -> result - sacromere lengths decreases -thus overall, the muscle shortens
When a sacromere shortens:
^thin filaments slide toward the M-line (can actually slide past it) ^the width of the I-band decreases ^the distances between two Z-lines decreases ^the width of the H-zone decreases ^the zone of overlap increases ^the width of the A-band stays the same ^***the length of filaments do not change
True or False the length of filaments shortens with muscle contraction
FALSE
Contraction Regulations
>calcium (Ca2+) in the sarcoplasm initiates contraction >thus, regulation involves a change in [Ca2+]
When a muscle is at rest where is Ca2+ sequestered?
Sarcoplasmic Reticulum
How does the Sarcoplasmic Reticulum move the Ca2+ back into itself?
By the use of active transport pumps. Thus, muscle uses energy even at rest.
Explain how action potential initiates muscle contraction.
action potential (impulse) -> opens Ca2+ release channels in SR -> Ca2+ is released into sarcoplasm -> Ca2+ binds to tropnin in the thin filaments -> troponin-tropomyosin complex changes shape -> myosin binding site on action is exposed -> actin & myosin interact -> contraction
What are the molecules needed for muscle contraction?
*Calcium *ATP
Myosin molecules have globular portions called:
myosin heads
What can myosin heads do?
They can attach to actin & using energy they can swivel (pivot)- this creates a sliding movement (thin filamenets slide past thick filaments).
Actin has a binding site for myosin heads that are blocked at rest by…
the troponin-tropomyosin complex
The steps that make up the sliding filament mechanism are:
*ATP binds to myosin ATPase, an enzyme located on the myosin head. ATP is broken down. ATP<=ADP +Pi+Energy Energy is transferred to the myosin (step 5) *An action potential (impulse) results in Ca++ release from the S.R. Ca++ binds to troponin causing a change in shape of the troponin-tropomyosin complex. This exposes the myosin-binding site on actin (step 1). *Attachment (binding) of the myosin head w/ actin occurs (step 2). This actin-myosin complex is called a cross-bridge (union). * When myosin attaches, swiveling (rotation) of the myosin head toward the center of the sarcomere occurs. This is referred to as the power stroke (step 3). #Thus, the thick & thin filaments slide past one another.# *During the pivot, the myosin heads release ADP & pi. Another ATP molecule can now bind to the myosin head. #ATP binding causes the myosin head to detach from actin. (step 4)*ATP is broken down. The myosin head is once again energized & is prepared for another cross-bridge cycle (step 5). ##This cycle is repeated over & over during a single muscle contraction ##.
The two processes involved in relaxation of a contraction
*motor neuron action potential ceases *Ca2+ is pumped back into SR
What is involved in the motor neuron action potential ceasation?
*Ach (acetylcholine) releases stops *Ach in synaptic cleft is broken down. (by acetylcholinesterase- AchE) *muscle action-potential stops *SR Ca2+ release channels close
What is involved in Ca2+ being pumped back into SR??
*pumps can pump against an extreme [Ca2+] gradient. (~400 fold difference b/n inside & outside) *aided by calseqestrin -> binds to Ca2+ inside SR -> this reduces free [Ca2+] in solution. -> #thus, more Ca2+ can be pumped into SR
Neuron (nerve cell) that activates skeletal muscle fiber:
Motor neuron
How many muscle fibers does a motor neuron activate in the skeletal muscle?
Most motor neurons activate several muscle fibers simultaneously
What is the significance of the # of fibers/M.U.?
*the # of fibers/M.U. determines the extent of control: i.e., eye muscle -> 1 fiber/M.U., thigh muscle -> 2000 fibers/M.U.*since eye muscles must function w greater precision much more nerve is needed for a given amount of muscle.
How is force modulated?
One way it's modulated is by changing the # of M.U., if you add smaller M.U's -> increment in force is small
True or False: Motor neurons & muscle fibers are not directly connected.
TRUE
How is the electrical impulse transferred from the M.N. to the muscle fiber surface?
Sense the electrical impulse cannot flow from M.N to muscle instead a chemical, the neurotransmitter is released from the M.N. which diffuses across small space (synaptic cleft) and attaches to muscle fiber surface.
What is the name of the region where the neurotransmitter attaches to muscle fiber surface?
Synapse
What is the synapse b/n the M.N. & the muscle fiber called?
Neuromuscular junction (NMJ)
What is a motor endplate?
It is a specialized region of muscle membrane where NMJ events occur.

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