Chapter 9 - Muscles and Muscle Tissue Flashcards
(70 cards)
1
Q
Types of muscle tissue
A
- Skeleton
- Cardiac
- Smooth
- Skeletal and smooth muscle cells are elongated and called muscle fibers.
- Myo or mys (roots for muscle) and sacro are in reference to muscle.
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Q
Skeletal muscle
A
- Skeletal muscle tissue is packaged into the skeletal muscles, organs that attach to and cover the skeleton.
- Skeletal muscle cells are the longest and have stripes/striations.
- Skeletal muscles are voluntary muscles.
- Skeletal muscles can contract rapidly but tire easily, and can exert tremendous power.
3
Q
Cardiac muscle
A
- Cardiac muscle cells are striated, but involuntary.
- Neural controls can allow the heart to speed up for brief periods (i.e. running a race).
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Q
Smooth muscle
A
- Found in the walls of hollow visceral organs
- Its role is to force fluids and other substances through internal body channels.
- Smooth muscle forms valves to regulate the passage of substances through internal body openings, dilates pupils of eyes, forms arrector pilli.
- Smooth muscle consists of elongated cells NO striations; Involuntary muscle
5
Q
Characteristics of muscle tissue: What enables muscle tissue to perform its duties?
A
- Excitability/responsiveness: the ability of a cell to receive and respond to stimulus by changing its membrane potential.
- Contractility: the ability to shorten forcibly when adequately stimulated. This ability sets muscle apart from all other tissue types.
- Extensibility: the ability to extend or stretch. Muscle cells can be stretched even beyond their resting length when relaxed.
- Elasticity: is the ability of a muscle cell to recoil and resume its resting length after stretching.
6
Q
Muscle Functions
A
- Produce movement: skeletal muscles responsible for all locomotion and manipulation. Eg. Walking, digestion, pumping blood.
- Maintain posture and body position
- Stabilize joints
- Generate heat as they contract
7
Q
Nerve and blood supply of skeletal muscles
A
- One nerve, one artery and one or more veins serves each muscle.
- Every skeletal muscle fiber is supplied with a nerve ending that controls its activity.
- Contracting muscle fibers use huge amounts of energy and require almost continuous delivery of oxygen and nutrients via the arteries.
- These muscles also need waste products removed frequently.
8
Q
Photo: Connective tissue sheaths of skeletal muscle: epimysium, perimysium, and endomysium
A
9
Q
Connective tissue sheaths of skeletal muscles
A
- Sheaths support each cell and reinforce and hold together the muscle, preventing muscles from bursting.
- 3 types of sheaths:
1. Epimysium - an overcoat of dense irregular connective tissue that surrounds the whole muscle.
2. Perimysium and fasicles - Within each muscle, muscle fibers are grouped into fasicles resembling bundles of sticks. Surrounding each fasicle is the perimysium (dense irregular connective tissue).
3. Endomysium - sheath of connective tissue that surrounds each individual muscle fiber (fine areolar connective tissue).
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Q
Attachments of skeletal muscles
A
- Muscle attachments, whether origin or insertion, may be direct or indirect.
- In direct, or fleshy attachments, the epimysium is fused to the periosteum of a bone or perichondrium of cartilage.
- In indirect attachments, the muscle’s connective tissue wrappings extend beyond the muscle usually as a tendon
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Q
Skeletal muscle fiber
A
- Each skeletal muscle fiber is a long cylindrical cell with multiple oval nuclei just beneath its sarcolemma (plasma membrane).
- These muscle fibers are huge cells.
- A muscle cell contains 3 specialized structures (other than its other organelles):
1. Myofibrils
2. Sarcoplasmic reticulum
3. T tubules
12
Q
Sarcoplasm
A
The cytoplasm of a muscle cell.
Usually contains large amounts of:
- glycosomes (granules of stored glycogen that provide glucose during muscle cell activity for ATP production).
- myoglobin (a red pigment that stores oxygen).
13
Q
Myofibrils
A
- Rod-like, these run parallel to the length of the muscle fiber (each fiber contains 100s to 1000s of myofibrils).
- Myofibrils are very densely packed and account for 80% of the cell volume.
- Myofibrils are made of a chain of sacromeres linked end to end. Sacromeres contain very small rodlike structures called myofilaments.
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Q
Striations
A
- Striations are a repeating series of dark and light bands along the length of each myofibril.
- A bands are dark
- I bands are light
- These bands give the cell its striated appearance.
15
Q
Sarcomeres
A
The region between the 2 successive Z discs is a sarcomere.
- Sarcomere is the smallest contractile unit of a muscle fiber (the functional unit of a skeletal muscle).
- Contains an A band flanked by half an I band at each end.
- In each microfibril, sarcomeres align end to end.
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Q
Myofilaments
A
- The banding patten of a myofibril arises from the arrangement of even smaller structures within the sarcomeres, known as myofilaments.
- Myofilaments - actin-containing microfilaments and myosin motor proteins.
- Actin and myosin play a role in motility and shape change in cells.
- 2 types of contractile myofilaments in a sarcomere:
1. Thick filaments
2. Thin filaments
17
Q
Photo: Myofilament
A
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Q
Thick filaments in myofilament
A
- Contain myosin (red)
- Extend the length of the A band.
- Connected in middle of sarcomere at the M line.
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Q
Thin filaments in myofilament
A
- Contain actin (blue)
- Extend across the I band and partway into the A band.
- The Z disc anchors the thin filaments.
20
Q
Skeletal muscle fibers contain 2 sets of intracellular tubules that help regulate muscle contraction:
A
- The Sarcoplasmic Reticulum
- T Tubules
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Q
Sarcoplasmic Reticulum
A
- The SR is an elaborate smooth endoplasmic reticulum.
- SR regulates intracellular levels of ionic calcium.
- Stores calcium and releases it on demand when muscle fiber is stimulated to contract.
- SR surrounds each myofibril
- SR tubules run along the myofibril, and communicate with each other at the Hzone.
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Q
Terminal cisterns of the SR
A
- End sacs
- Form larger, perpendicular cross channels at the A band - I band junctions, occurring in pairs.
- There are also mitochondria and glycogen granules, all of which are involved in producing energy used during contractions.
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Q
Photo: Relationship of the SR and T Tubules to myofibrils of skeletal muscle…
A
24
Q
T Tubules
A
- An elongated tube formed at each A band - I band junction, where the sarcolemma of the fiber protrudes deep into the cell interior.
- The lumen (cavity of the T tubule) is continuous with the extracellular space so that T tubules greatly increase the fiber’s surface area, allowing changes in the membrane potential to quickly penetrate deep into the muscle fiber.
- T tubules are continuations of the sarcolemma, hence they conduct impulses to the deepest regions of the muscle cell and every sarcomere.
- These impulses trigger calcium release from adjacent terminal cisterns.
- T tubules ensure that every myofibril in a muscle fiber contracts at the same time.
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Triads
- A formation that occurs when the T tubule runs between the paired terminal cisterns of the SR.
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The sliding filament model of contraction
- Contraction: The activation of myosin's cross bridges, which are force generating sites.
- If the cross bridges generate enough tension on the thin filaments to exceed opposing forces, then shortening occurs.
- Contraction ends - cross bridges become inactive, muscle fibers relax.
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Sliding filament model of contraction states that...
During contraction, the thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree. Neither the thick or thin filaments change length during contraction.
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How the sliding filament model of contraction works
1. Nervous system stimulates muscle fibers, myosin heads on thick filaments latch onto myosin-binding sites on actin in thin filaments, and sliding begins.
2. These cross bridge attachments form and break several times during contraction (tiny ratchets), generating tension and propelling the thin filaments toward the center of the sarcomere.
3. Simultaneously in the sarcomeres throughout the cell, the muscle cell shortens.
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4 steps must occur for a skeletal muscle to contract...
1. Nerve stimulation
2. Action potential - an electrical current must be generated in the sarcolemma.
3. The action potential must be propagated along the sarcolemma.
4. Intracellular calcium levels must rise briefly.
Steps 1 and 2 occur at the neuromuscular junction.
Steps 3 and 4 link electrical signals to contraction, referred to as excitation-contraction coupling.
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The phases leading to muscle fiber contraction…
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Somatic motor neurons
- Somatic motor neurons are motor neurons that activate skeletal muscle fibers.
- They reside in the spinal cord.
- Each neuron has a long threadlike extension called an axon that extends from the cell body to the spinal cord to the muscle fiber it serves.
- Each axon divides into many branches as it enters the muscle.
- Axon branches end on muscle fiber, forming the neuromuscular junction or motor end.
- Each muscle fiber has one neuromuscular junction with one motor neuron.
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Excitation - Contraction (E-C) Coupling
- E-C Coupling: events that transmit AP along the sarcolemma (excitation) are coupled to the sliding of the myofilaments (contraction).
- AP is propagated along the sarcolemma, and down into the T-Tubules, where voltage sensitive proteins in tubules stimulate Ca release from the SR (Ca release leads to the contraction).
- AP is brief and ends before the contraction is seen.
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Neuromuscular Junction Excitation…
- A nerve impulse travels down the axon of a motor neuron.
- When the impulse reaches the axon terminal, voltage-gated calcium channels open up, and calcium enters the axon terminal.
- The calcium influx causes the synaptic vesicle to excytose Ach into the synaptic cleft.
- ACh binds to receptors on the sarcolemma, causing chemically gated Na and K channels to open and initiate an end plate potential.
- When the threshold is reached, voltage-gated Na channels open, initiating an AP.
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Muscle fiber contraction - Cross bridge cycling
- At a low intracellular Ca concentration:
1. Trypomyosin blocks active sites on actin.
2. Myosin heads cannot attach to actin.
3. The muscle fiber remains relaxed.
- Voltage-sensitive proteins in T Tubules change shape, causing the SR to release Ca to the cytosol.
- At higher intracellular Ca concentrations, Ca binds to troponin.
- Troponin changes shape and moves trypomyosin away from myosin-binding sites.
- Myosin heads are then allowed to bind to actin, forming cross bridges.
- Cycling is initiated, causing sarcomere shortening and muscle contraction.
- When nervous stimulation ceases, Ca is pumped back into the SR, and contraction ends.
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4 steps to the cross bridge cycle…
1. Cross bridge formation: a high energy myosin head attaches to an actin thin filament active site.
2. The working (power) stroke: the myosin head pivots and pulls the thin filament toward the M line.
3. Cross bridge detachment: ATP attaches to the myosin head, causing the cross bridge to detach.
4. Cocking of the myosin head: Energy from hydrolysis of ATP “cocks” the myosin head into a high energy state (this energy will be used for the power stroke in the next cross bridge cycle).
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Whole Muscle Contraction
- The same principles apply to contraction of whole muscles and single fibers.
- Contraction produces muscle tension, the force exerted on the load or object to be moved.
- Contraction may or may not shorten the muscle.
1. Isometric contraction: no muscle shortening; muscle tension increases but does not exceed the load.
2. Isotonic contraction: the muscle shortens because the muscle tension exceeds the load.
- The force and duration of contraction vary in response to stimuli of different frequencies and intensities.
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Each muscle is served by at least one muscle nerve…
- Motor nerves contain axons of up to 100s of motor neurons.
- Axons branch into terminals, each of which forms a NMJ with a single muscle fiber.
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Motor Unit
- The motor unit is the nerve-muscle functional unit.
- The motor unit consists of the motor neuron, and all muscle fibers (4 - several 100) it supplies. (The smaller the fiber # the greater the fine control).
- Muscle fibers from a motor unit are spread throughout the whole muscle, so stimulation of a single motor unit causes only a weak contraction of the entire muscle.
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The muscle twitch
- A muscle twitch is the simplest contraction resulting from a muscle fiber’s response to a single action potential from a motor neuron.
- The muscle fiber contracts quickly and then relaxes.
- Twitches can be observed and recorded by myograms. “Tracing” is the line recording the contraction activity.
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3 phases of the muscle twitch:
1. Latent period: events of excitation-contraction coupling (No muscle tension seen)
2. Period of contraction: cross bridge formation ( Tension increases)
3. Period of relaxation: Ca reabsorbed into SR (tension declines to 0)
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Graded muscle responses
- Normal muscle contraction is relatively smooth, and strength varies with need.
- Graded muscle responses vary in strength of contraction for different demands: This is required for proper control of skeletal movement.
- Responses are graded by:
1. Changing frequency of stimulation
2. Changing strength of stimulation
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Photo: Single stimulus
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Photo: Wave(temporal) summation
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Photo: High Stimulation Frequency
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Muscle Tone
Muscle tone is the constant, slightly contracted state of all muscles.
Muscle tone is due to spinal reflexes: Groups of motor units are alternately activated in response to input from stretch receptors in muscles.
Muscle tone keeps muscles firm, healthy and ready to respond.
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Isotonic contractions
- In isotonic contractions, a muscle changes in length and moves a load.
- Isotonic contractions can be either concentric or eccentric.
- Concentric contractions: a muscle shortens and does work (i.e. bicep contracts and picks up a book).
- Eccentric contractions: muscle lengthens and generates force (i.e. laying a book down causes biceps to lengthen while generating a force).
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Photo: Isotonic contraction: Concentric
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Isometric contractions
In isometric contractions, the load is greater then the maximum tension muscle can generate, so the muscle neither shortens nor lengthens.
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Photo: Isometric contraction
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Energy for contraction and ATP
- ATP supplies the energy needed for the muscle fiber to…
1. Move and detach cross bridges
2. Pump calcium back into the SR
3. Pump Na out and K back into the cell after excitation-contraction coupling.
- Available stores of ATP are depleted in 4-6 seconds.
- ATP is the only source of energy for contractile activities, therefore it must be regenerated quickly.
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Anaerobic pathway
- In anaerobic pathways there is glycolysis and lactic acid formation
- ATP can also be generated this way by breaking down and using the energy stored in glucose.
- Glycolysis is the first step in glucose breakdown. This step does not require oxygen. Glucose is broken into 2 pyruvic acid molecules. 2 ATPs are generated for each glucose broken down.
- The low O2 levels prevent the pyruvic acid from entering the aerobic respiration phase.
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Aerobic respiration
- Provides 95% of ATP during rest and light-to-moderate exercise.
- Slower than the anaerobic pathway.
- Consists of a series of chemical reactions that occur in mitochondria and require O2.
- Aerobic respiration breaks glucose into CO2, H2O and a large amount of ATP (32)
- Fuels used include glucose from glygogen stored in muscle fiber, bloodborne glucose, and free fatty acids.
- Fatty acids are the main fuel after 30 minutes of exercise.
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There are 2 energy systems used during sports:
1. Aerobic endurance: Length of time muscle contracts using aerobic pathways; light to moderate activity can continue for hours.
2. Anaerobic threshold: Point at which muscle metabolism converts to anaerobic pathway.
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Muscle fatigue
- The physiological inability to contract despite continued stimulation.
- Usually occurs when there are ionic imbalances.
- Lack of ATP is rarely a reason for fatigue except in severely stressed muscles.
- Imbalanced levels of K, CA and P can interfere with E-Coupling.
- Prolonged exercise could damage SR and interfere with CA regulation and release.
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Excess post exercise o2 consumption
- For a muscle to return to its preexisting state:
1. O2 reserves need to be replenished.
2. Lactic acid is reconverted to pyruvic acid.
3. Glycogen stores are replaced.
4. ATP and creatin phosphate reserves are resynthesized.
All replenishing steps require extra O2. This is referred to as an excess post exercise O2 consumption (EPOC).
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What are the 4 factors that affect the force of a contraction?
The force of a contraction depends on the number of cross bridges attached, which is affected by 4 factors:
1. Number of muscle fibers stimulated.
2. Relative size of fibers.
3. Frequency of stimulation
4. Degree of muscle strength
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Number of muscle fibers stimulated (muscle contraction factor)
The more motor units recruited, the greater the force.
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Relative size of fibers (muscle contraction factor)
The bulkier the muscle, the more tension it can develop. Muscle cells can increase in size with regular exercise (Hypertrophy)
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Frequency of stimulation (muscle contraction factor)
The higher the frequency, the greater the force. Stimuli are added together.
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Degree of muscle strength (muscle contraction factor)
Muscle fibers with sacromeres that are 80-120% their normal resting length generate more force.
- If a sarcomere is less than 80% resting length, filaments overlap too much and force decreases.
- If a sarcomere is greater than 120% of resting length, filaments do not overlap enough, so force decreases.
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How fast a muscle contracts and how long it can stay contracted is influenced by:
Muscle fiber type
Load
Recruitment
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Muscle fiber type (velocity and duration of contraction)
Classified according to 2 characteristics:
1. Speed of contraction: Slow or fast fibers according to - speed at which myosin ATPases split ATP - pattern of electrical activity of motor neurons.
2. Metabolic pathways used for ATP synthesis: Oxidative fibers use aerobic pathways . . . Glycolitic fibers use anaerobic pathways.
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Photo: Contractile velocity, contractile duration
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Structural and functional characteristics of 3 types of skeletal muscle fibers
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Load and recruitment (velocity and duration of contraction)
- Load: muscles contract fastest when no load is added.
- The greater the load, the shorter the duration of contraction.
- The greater the load, the slower the contraction.
- Recruitment: The more motor units contracting, the faster and more prolonged the contraction.
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Adaptation to exercise
- Aerobic endurance exercise, such as jogging, swimming, biking leads to increased muscle capillaries, number of mitochondria, and myoglobin synthesis.
- This results in greater endurance, strength and resistance to fatigue.
- May convert fast glycolitic fibers into fast oxidative fibers.
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Resistance exercise
Resistance exercise (typically anaerobic) such as weight lifting or isometric exercises, leads to:
- muscle hypertrophy (due primarily to increase in fiber size)
- increased mitochondria, myofilaments, glycogen stores, and connective tissue
- increased muscle strength and size
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Photo: Comparison of muscles #1
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Photo: Comparison of muscles #2
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Photo: Comparison of muscles #3
