Muscles and Muscle Tissue
What type of tension is being produced when the overall length of the muscle is increasing while the muscle is functioning?
Eccentric conctraction
What type of tension is developed most efficiently (i.e. requires less ATP expenditure per unit of work accomplished)? least efficiently?
Eccentric contraction; concentric contraction
Name the CT sheath that surrounds individual muscle fibers. Of what type of CT is this sheath composed?
Endomysium; made of areolar CT
creatine kinase
Enzyme that catalyzes the reaction between ADP and creatine phosphate
Name the CT sheath that holds bundles of fascicles together to create a muscle. Of what type of CT is this sheath composed?
Epimysium: made of dense irregular CT
Describe the relationship between resting length and tension development in smooth muscle.
Even when smooth muscle is stretched, it can still generate considerable force during contraction.
Which fiber type has the fastest contraction speed? the slowest?
Fast oxidative fibers; slow oxidative fibers
What is the role of gap junctions and pacemaker cells in single-unit smooth muscle?
Gap junctions causes synchronized contraction of smooth muscle, because it allows smooth muscle to transmit action potentials from fiber to fiber. Pacemaker cells set the pace of contraction for all cells.
What are graded muscle responses? What are the two general ways of producing a graded muscle response?
Graded muscle responses are variations in the intensity of muscle contractions. Graded muscle responses can be produced by increasing the strength or frequency of stimulation.
creatine phosphate (CP)
High-energy molecule stored in muscles that reacts with ADP to produce ATP and creatine during short intervals of maximum muscle power
Which myofibril structures or regions change length during the contraction process?
I bands, H zones, and the distance between successive Z lines
What type of tension is being produced when internal tension is being developed, but no movement occurs?
Isometric contraction
What happens to the A band, I band, H zone, and Z discs during contraction? to sarcomere length?
Length of the A band does not change. The I bands shorten, the H zones disappear, the distance between Z discs shortens, and overall, the sarcomere length shortens.
Describe the gross anatomy of a muscle, beginning with the myofibrils.
Myofibrils are essentially bundles of myofilaments that are arranged into sarcomeres lined end on end. Bundles of myofibrils make up a muscle fiber (or muscle cell), while groups of muscle fibers make up a fascicle. Together, the fascicles make up the entire muscle organ. Muscle fibers are covered by the endomysium; fascicles are covered by the perimysium; and the entire muscle is lined by the epimysium.
What is the function of myoglobin?
Myoglobin is the red pigment found in muscle that helps transfer oxygen from the blood to the mitochondria.
What is the role of myosin light chain kinase (MLCK)?
Myosin light chain kinase enzyme must be activated in order to activate the myosin ATPase of the myosin heads.
Identify the two primary types of myofilaments found in myofibrils.
Myosin, which make up the thick filaments, and actin, which make up the thin filaments
What is the location of the blood vessels and nerves which supply the skeletal muscle tissue?
Near the center of the muscle, where they branch through the connective tissue sheaths.
What is acetylcholine? What role does it play at the neuromuscular junction?
Neurotransmitter that is stored in the synaptic vessels of the axon terminal; when ACh is released via exocytosis from the synaptic vessels as a result of calcium ion buildup n the axon terminal, it binds to receptors on the motor end plate that opens the ligand-gated sodium ion channels and allows sodium ions to enter the cell
What happens to force production in a muscle that is in extreme stretch or contraction?
No force will be produced.
Describe the entire process of excitation-contraction coupling, beginning with a nerve impuse in a motor neuron.
Once the AP reaches the sarcolemma, it propagates down through the T tubules and causes a change in shape of the voltage-gated calcium ion channels found in the terminal cisternae. As a result, the calcium ion channels open and release calcium ions from the SR into the sarcoplasm. The calcium ions then bind to troponin, causing troponin to change shape and pull tropomyosin off the active binding sites on actin for the myosin heads. Once contraction begins, the excitation-contraction coupling process has ended.
What factors influence the amount of force that a muscle can generate?
Optimal resting length, number of muscle fibers activated, size of the muscle factors, frequency of stimulation
What is meant by the term oxygen deficit or oxygen debt (which is more correctly called "recovery oxygen uptake")?
Oxygen debt refers to the amount of oxygen needed to restore a muscle. The restorative processes that take place include replenishing oxygen reserves, converting lactic acid back to pyruvate, replacing glycogen stores and resynthesizing ATP and creatine phosphate.
What is the advantage of the parallel arrangement of fibers? of the pennate arrangement? the convergent?
Parallel arrangement allows fibers can shorten (or contract) the most. Pennate arrangement of fibers provides more strength, because there are more muscle fibers. Convergent arrangement of fibers allow for greater pushing and pulling force, because the muscle fibers are shorter.
Name the CT sheath that surrounds a fascicle. Of what type of CT is this sheath composed?
Perimysium; made of fibrous CT
At what point does the power stroke occur?
Power stroke occurs when the activated myosin head attaches to an active site on actin and releases (ADP + Pi)
What causes each of the bands, zones and lines in a myofibril?
1) A band: region of overlap between the thick and thin filaments 2) I band: region of thin filaments only 3) H zone: region of thick filaments only that is found in the center of each A band 4) M line: dark line bisecting the A band formed by the myomesin protein 5) Z line: point of intersection between two sarcomeres
Name and describe the four arrangements of fascicles/fibers within skeletal muscles.
1) Circular: occurs when the fascicles are arranged in concentric rings 2) Convergent: occurs when the muscle has a broad origin, and its fascicles converge toward a single tendon of insertion 3) Parallel: occurs when the length fascicles runs parallel to the long axis of the muscle 4) Pennate: occurs when the fascicles are short and attach obliquely to a central tendon that runs the length of the muscle
Name and describe the muscle fiber's three avenues of ATP production.
1) Direct phosphorylation: reaction of creatine phosphate with ADP to form creatine and ATP 2) Anaerobic pathway: involves glycolysis and lactic acid formation 3) Aerobic pathway: involves aerobic cellular respiration (including Acetyl CoA, Krebs cycle, and electron transport chain)
Name and describe the four functional characteristics of muscle.
1) Excitability: ability to receive and respond to a stimulus 2) Contractility: ability to shorten forcibily 3) Extensibility: ability to be stretched or lengthened by an outside force without damaging tissue 4) Elasticity: ability to recoil and resume resting length after stretching
Classify the three types of muscle tissue (according to location, appearance, and type of nervous system control).
1) Skeletal muscle: found on bones; multinucleated with a striated appearance; exhibits voluntary control 2) Cardiac muscle: found only in the heart; striated, branched, and uninucleated; exhibits involuntary control 3) Smooth muscle: found in the walls of hollow visceral organs; non-striated, spindle-shaped cells with a centrally-located nucleus; exhibits involuntary control
Classify each muscle fiber type with respect to its predominant pathway for ATP synthesis, the amount of myoglobin present, and the activity of its myosin ATPase.
1) Slow oxidative fibers: synthesizes ATP aerobically; high myoglobin content; slow myosin ATPase activity 2) Fast oxidative-glycolytic fibers : synthesizes ATP aerobically; intermediate myoglobin content; fast myosin ATPase activity 3) Fast oxidation fibers: synthesizes ATP anaerobically, low myoglobin content; fast myosin ATPase activity
Describe the difference between a direct and indirect muscle attachment.
A direct muscle attachment occurs when the epimysium directly fuses to the periosteum of a bone or the perichondrium of cartilage. An indirect muscle attachment occurs when the connective tissue sheaths extend beyond the muscle and anchors indirectly to the CT covering a skeletal element or to the fascia of other muscles.
How does the size of a motor unit influence the strength of contraction and the control of a movement?
A larger motor unit will produce a stronger contraction but cannot produce as precise of a movement as a smaller motor unit.
What benefit is provided by a lever that operates at a mechanical advantage? at a mechanical disadvantage?
A lever operating at a mechanical advantage allows a small force exerted over a large distance to move a heavy load over a small distance. A lever operating at a mechanical disadvantage requires a larger force but can move a load rapidly over a large distance with a wide range of motion.
Describe the structure of a myosin molecule. How are myosin molecules arranged to make up a thick filament?
A myosin molecule is made of two heavy and four light polypeptide chains. The two heavy chains twist together to form the tail, and the tail attaches to two heads by a flexible hinge. In a thick filament, the myosin molecules bundle together so that the tails form the smooth H zone of the A band, while the myosin heads protrude towards either end of the H zone.
When does a muscle begin to develop (or exert) external tension?
After internal tension develops in the SEE
What are the series elastic elements (SEE) of the muscle and how do they influence force production?
All the non-contractile parts of the muscle. In the initial part of contraction, crossbridge cycling creates internal tension in the SEE before the SEE begin to pull on the tendons and generate external tension that causes movement.
What two factors permit relaxation?
Amount of calcium ions and ATP molecules
Describe the structure of an actin molecule. How are individual actin monomers arranged to make up a thin filament?
An actin molecule is composed of two intertwined long actin filaments, called F actin, that is formed from polymerized G actin. It also contains two regulatory proteins, tropomyosin and troponin. Tropomyosin is the rod-shaped protein that spirals around the actin core, while troponin is the globular three-polypeptide complex composed of TnI, which binds to actin, TnT, which binds to tropomyosin and positions it to actin, and TnC, which binds to calcium ions.
Which sources of ATP is/are anaerobic? aerobic?
Anaerobic sources of ATP are produced from direct phosphorylation and glyclolysis/lactic acid formation are. Aerobic sources of ATP are produced from aerobic cellular respiration.
What is the significance of the calcium ion release channels and the calcium ion active transport pumps in the SR?
Calcium ion release channels allow calcium ions to flow from the SR to the sarcolemma, therefore allowing calcium ions to bind to troponin and cause muscle contraction. Calcium ion transport pumps remove calcium ions once contraction is finished, therefore allowing the muscle to relax.
What is the function of the T tubules?
Conduct electrical signals to the deepest regions of the muscle cell and every sarcomere, which allows every myofibril to contract at the same time
What is the function of the Z disc?
Connects adjacent sarcomeres together
What is a motor unit?
Consists of one motor neuron and all the muslce fibers it innervates
What specializations of the muscle fiber (at the motor end plate) enable it to respond to signals from the motor neuron?
Contains receptor binding sites for acetylcholine (ACh), ligand-gated sodium ion channels, and junctional folds to increase the surface area of the sarcolemma
Which properties of the muscle is the muscle's most distinguishing, or unique, characteristic?
Contractility
What structures anchor smooth muscle fibers and transmit the tension they develop?
Dense bodies and intermediate filaments
What is the difference between unfused tetanus and complete tetanus? How is complete tetanus achieved in a muscle fiber?
Unfused tetanus occurs when the tension reaches a vacillating level. Complete tetanus occurs when the tension reaches a non-vacillating peak level, and is achieved when the stimulus occurs so frequently that the relaxation phase of the muscle is completely eliminated.
Which type of smooth muscle exhibits rhythmicity and self-excitation?
Unitary smooth muscle
What are the two types of smooth muscle?
Unitary smooth muscle (i.e. visceral muscle) and multi unit smooth muscle
Where is each type of smooth muscle located?
Unitary smooth muscle is found in all hollow organs except the heart. Multi unit smooth muscle is found in intrinsic eye muscles, airways and large arteries.
What is the most prevalent chemical compound (by weight) in muscle?
Water
Explain the all-or-none law as it relates to skeletal muscle fibers and motor units.
When a motor unit is stimulated by a threshold stimulus, the muscle fibers will either contract to their fullest extent or not contract at all. (In other words, the amount of tension produced by a muscle fiber is independent of the intensity of the stimulus. Rather, a stronger stimulus produces greater tension because it recruits more muscle fibers, according to the principle of recruitment.)
What causes myosin to detach from actin after the power stroke is completed?
When a new ATP molecule binds onto the myosin head
Describe the process of signal transmission at the neuromuscular junction (i.e., communication of nerve AP to the muscle fiber).
When an action potential reaches the axon terminal, voltage-gated calcium ion channels are opened and release calcium ions into the axon terminal. As a result of calcium ion buildup inside the axon terminal, acetylcholine (ACh) is released via exocytosis from the synaptic vesicles. ACh then travels through the synaptic cleft and binds onto receptors found along the motor end plate. Consequently, ligand-gated sodium ion channels are activated, allowing sodium ions to enter the cell and potassium ions to exit the cell. The influx of sodium ions then triggers an electrical impulse (or action potential) in the sarcolemma of the cell. The process of signal transmission is terminated when acetylcholinesterase (AChE) breaks down acetylcholine.
What is the role of the calcium ion and the regulatory proteins in the sliding filament mechanism?
When calcium ions bind to troponin, the troponin molecule changes shape and pulls tropomyosin off the active binding sites on actin.
When is the (ADP + Pi) which fueled the power stroke released from the myosin head?
When the myosin head attaches to an active site on actin
At what point in crossbridge cycling does ATP become hydrolyzed and provide the energy for activation of the myosin head?
When the myosin head returns to its cocked position
phosphorylation
adding a phosphate group
glycolysis
anaerobic breakdown of glucose to form pyruvate
insertion (of a muscle)
bony attachment that moves when an action occurs
origin (of a muscle)
bony attachment that remains fixed or stationary when an action occurs
What factors contribute to onset of muscle fatigue?
Problems in the excitation-contraction coupling process or ionic imbalances
What are the four general functions of the muscular system?
Producing movement, maintaining posture, stabilizing joints, thermogenesis
What happens to pyruvic acid when there is not enough oxygen present to completely catabolize it?
Pyruvic acid is converted to lactic acid.
What is the function of the SR and terminal cisternae?
Regulate intracellular levels of calcium, as well as store and release calcium when the muscle is stimulated to contract
How is smooth muscle contraction regulated?
Regulated by nerves, hormones or local chemical changes. (Smooth muscle can respond differently depending on the neurotransmitters released by the nerves. Hormones and other local chemical changes can affect calcium ion entry into the sarcoplasm.)
What prevents prolonged stimulation of the muscle fiber by acetylcholine (ACh)?
Release of acetylcholinesterase (AChE)
What is a muscle twitch?
Response of an isolated muscle or motor unit to a single, brief threshold stimulus
What is a sarcomere?
Smallest and functional unit of the skeletal muscle that is composed of an A band flanked on either side by an I band
Identify and compare the two types of indirect muscle attachments.
Tendon and aponeurosis; a tendon is rope-like, whereas an aponeurosis is sheet-like.
In indirect attachments, what tissues are involved in transmitting pulling (i.e. tension forces generated during contraction) forces from the muscle fiber to the bone?
Tendons, aponeuroses, Sharpey's fibers
Relative to muscle function, what is the significance of Sharpey's fibers?
The Sharpey's fibers secure the periosteum and tendons to the underlying bone, so that when the bone contracts, the periosteum will not be stripped off the bone.
What is the role of ATP and the myosin crossbridge in the sliding filament mechanism?
The attachment of an ATP molecule to a myosin head allows the myosin head to detach from the actin filament. ATP also provides energy for the myosin head to return back to its high-energy, activated and cocked position.
Describe the structure of the neuromuscular junction.
The neuromuscular junction is the site of contact between the axonal endings of a motor neuron and the muscle fiber it innervates. The axonal endings are called synaptic terminals and contain mitochondria and synaptic vesicles that release acetylcholine (ACh). The region of sarcolemma directly opposite the synaptic terminal is called the motor end plate, and it contains receptors onto which ACh binds to allow for the passage of sodium and potassium ions.
What is the refractory period? Why is it significant?
The refractory period is the period of repolarizing the sarcolemma by closing the sodium ion channels, reopening the potassium ion channels and releasing the potassium ions out of the cell. The refractory period is important because it restores the polarized state of the sarcolemma (negative charge inside, positive charge outside).
Describe the location of the sarcoplasmic reticulum (SR) with respect to the myofibrils.
The sarcoplasmic reticulum (SR) forms a tubular network similar to a sweater sleeve around each myofibril. It also forms terminal cisternae which run longitudinally along the myofibril and adjacent to the T tubules.
Describe the stress-relaxation response.
The stress-relaxation response allows a hollow organ to expand slowly to accommodate a greater volume without causing strong contractions that would otherwise expel its contents.
What gives skeletal muscle fibers a striated appearance?
The striatons are a result of the alternating dark and light bands, or the A bands and I bands respectively, of the myofibril.
What happens to the strength of contraction when two or more identical stimuli are delivered to the muscle in rapid succession (i.e. before complete relaxation occurs)? What is this phenomenon called?
The subsequent contractions are stronger than the previous contractions; this phenomenon is called wave summation.
What is the most common lever system in the body? Where is the fulcrum with respect to the load and effort in this lever system?
Third-class lever system; the fulcrum is located closer to the effort than it is to the load.
What are the T tubules? Where are they located?
Tunnel-like invaginations of the sarcolemma that extend deep into the cell interior; located at the level of the A band-I band junction
myoblast
mesodermal embryonic cell that fuses together to form mature muscle fibers
prime mover (agonist)
muscle that directly brings about a specific action
antagonist
muscle that directly opposes a specific action of the prime mover
synergist
muscle that helps a prime mover by adding a little extra force to the same movement or rudcing unnecessary movements that might occur as the prime mover contracts
hydrolysis
removing a phosphate group from ATP to release energy and form ADP + Pi)
aponeurosis
sheet-like extension of connective tissue sheaths that attaches muscle indirectly to a bone or fascia of another muscle
threshold stimulus
stimulus at which the first observable contraction occurs
maximal stimulus
strongest stimulus at which all a muscle's motor units are recruited
sarcoplasm
cytoplasm of a muscle fiber (or muscle cell)
dense bodies
cytoplasmic structures on the sarcolemma of smooth muscle that attach to intermediate filaments and act as enchoring points for thin filaments
action potential
electrical current
myosin ATPase
enzyme responsible for hydrolyzing ATP into (ADP + Pi)
acetylcholinesterase (AChE)
enzyme that terminates the action of acetylcholine by breaking it down
intermediate filament
filament found in smooth muscle that attaches to dense bodies on the sarcolemma
triad
group made of a T tubule and the two terminal cisternae that lie adjacent to it
syncytium
immature muscle cells that do not fuse with the muscle cells but live between the muscle fibers
axon
threadlike part of a nerve cell that conducts electrical signals
gap junctions
type of cell adhesion in smooth muscle that causes synchronized contraction by allowing smooth muscle to transmit action potentials from fiber to fiber
graded responses
variations in strengths of muscle contractions
synaptic vesicles
vesicles in the axon terminal that contain the neurotransmitter acetylcholine
recruitment
process of activating more motor units
muscle endurance
refers to the efficiency of force production in muscle fibers
muscle power (strength)
refers to the size of the muscle fibers
What type of tension does a muscle develop when it is acting against an agonist?
Concentric contraction
Describe the chemical composition of muscle fibers.
75% water (by weight), 25% proteins, and 5% salts, pigments and substrates
What role does complete tetanus play in voluntary movements?
Complete tetanus almost never happens but theoretically occurs during bouts of superhuman strength.
Which source of ATP provides energy to fuel prolonged, submaximal activity?
ATP produced from aerobic cellular respiration
Which source of ATP provides energy to fuel brief, high-intensity activity?
ATP produced from direct phosphorylation
Physiologically speaking, what is the value of ATP's high-energy phosphate bonds?
ATP's high-energy phosphate bonds allow for the myosin heads to return to their activated, high-energy and cocked positions to continue sliding the actin filaments towards the M line.
Identify the proteins which make up thin filaments.
Actin, troponin, tropomyosin
What is recruitment (i.e. multiple motor unit summation)?
Activating more motor units to create more muscle tension
Why is the triad of functional significance?
Both the T tubules and terminal cisternae interact through integral proteins that protrude into the intermembrane spaces. The integral proteins of the T tubules act as voltage sensors, while the integral proteins of the terminal cisternae act as receptors that regulate calcium ion release from the SR.
What is taking place during the relaxation period?
Calcium ions reenter the SR, muscle tension decreases to zero and the sarcomere returns back to its original length.
Identify the regulatory protein in sooth muscle which binds to calcium ions to activate the contraction process.
Calmodulin
Which fiber type receives the richest blood supply?
Slow oxidative fibers
Where is most of the cell's ATP produced?
Mitochondria
Muscle contraction is currently best described by the sliding filament theory. Why is this name appropriate for the contraction process?
Muscle contraction occurs when the thin filaments slide past the thick filaments.
What is muscle tone, and why is it important?
Muscle tone is the slight contraction in relaxed skeletal muscles. It helps maintain the integrity of muscle.
What are the sources of the sarcoplasmic calcium ions which trigger smooth muscle contraction? Which of these is the main source?
Sarcoplasmic reticulum and extracellular fluid (ECF) sequestered from the caveolae; main source of calcium ions comes from ECF
Which contracts more quickly - smooth or skeletal muscle? Which contracts more efficiently? Which is most fatigue-resistant?
Skeletal muscle contracts more quickly. Smooth muscle contracts more efficiently and is the most fatigue-resistant.
Describe how the following organelles are modified or adapted for function in skeletal muscle tissue: nucleus, mitochondria, and smooth endoplasmic reticulum.
Skeletal muscle tissue are multinucleated, and they contain many more mitochondria that are positioned in rows near the contractile proteins. The smooth endoplasmic reticulum of skeletal muscle tissue is specialized into the sarcoplasmic reticulum (SR), which forms a tubular network around individual myofibrils (like a sweater sleeve) as well as enlarged chambers called terminal cisternae that lie adjacent to the T tubules.
Describe the major structural and functional differences between smooth and skeletal muscle.
Smooth muscle is non-striated and consists of spindle-shaped cells with centrally-located nuclei. They lack T tubules, sarcomeres, connective tissue sheaths (besides the endomysium) and terminal cisternae. Instead, they are supported by intermediate filaments that attach to dense bodies, as well as contain calveolae, which sequester calcium ion-containing extracellular fluid into the sarcoplasm. Unlike skeletal muscle fibers, which are innervated by axon terminals via the neuromuscular junction, smooth muscle fibers are involuntarily controlled by autonomous varicosities. Main sources of calcium ions come from the sarcoplasmic reticulum and the extracellular fluid (ECF). Smooth muscle contraction is dependent on calmodulin, which serves as the binding site for calcium ions, and myosin light chain kinase (MLCK) protein, which activates the myosin ATPase to allow the myosin heads to activate.
What is the synaptic cleft?
Space that separates the axon terminal and muscle fiber and is filled with extracellular substance rich in glycoproteins and collagen fibers
What is muscle fatigue?
State of physiological inability to contract even though the muscle may still be receiving stimuli
Describe the relative positions of actin and myosin when the muscle is at its optimal length for strength production.
The ends of the actin filaments are positioned just along the regions of thick filaments where myosin heads are present.
What is the latent period and what is taking place during the latent period?
The latent period is the initial period of a muscle twitch between the onset of stimulus and the actual muscle contraction (i.e. the period where no muscle activity is apparent). During the latent period, the stimulus is received at the neuromuscular junction, an AP is transmitted to the sarcolemma, and the excitation-contraction coupling process occurs.
What is the relationship between the initial length of the sarcomere (or muscle fiber) and the amount of tension it can produce?
The longer the sarcomere, the more tension it can produce. Also, the closer the sarcomere is to its optimal or resting length, the more tension it can produce when it contracts. (This relationship is known as the length-tension relationship.)
Which protein covers the active sites on the actin filament in a resting muscle fiber?
Tropomyosin
Specifically, how do troponin and tropomyosin help regulate the contraction process?
Tropomyosin covers the active binding sites on actin to prevent myosin heads from latching onto the actin filament. When activated, troponin is responsible for pulling the tropomyosin off the active binding sites.
Which of the thin filament proteins are regulatory proteins?
Troponin and tropomyosin
pacemaker cells
cells found in smooth muscle that set the pace for smooth muscle contraction
neurotransmitter
chemical substance released at the axon terminal as a result of a nerve impulse
axon terminals
nerve endings that attach to the muscle fiber and contain mitochondria and synaptic vesicles containing acetylcholine
acetylcholine (ACh)
neurotransmitter stored in the synaptic vesicles of the axon terminals that bind on to receptors in the motor end plate and activate ligand-gated sodium ion channels
sarcolemma
plasma membrane of a muscle fiber (or muscle cell)
