PPT Notes Chapter 9 Flashcards
Three types of muscle
- skeletal, cardiac, and smooth
- These types differ in structure, location, function, and means of activation
Muscle Similarities
Skeletal and smooth muscle cells are elongated and are called muscle fibers
Muscle contraction depends on two kinds of myofilaments – actin and myosin (both proteins)
Muscle terminology is similar
- Sarcolemma – muscle plasma membrane
- Sarcoplasm – cytoplasm of a muscle cell
Prefixes – myo, mys, and sarco all refer to muscle
Skeletal Muscle Tissue
- Packaged in skeletal muscles that attach to and cover the bony skeleton
- Has obvious stripes called striations
- Is controlled voluntarily (i.e., by conscious control)
- Contracts rapidly but tires easily
- Is responsible for overall body motility
- Is extremely adaptable and can exert forces ranging from a fraction of an ounce to over 70 pounds
Cardiac Muscle Tissue
- Occurs only in the heart
- Is striated like skeletal muscle but is not voluntary
- Contracts at a fairly steady rate set by the heart’s pacemaker
- Neural controls allow the heart to respond to changes in bodily needs
Smooth Muscle Tissue
- Found in the walls of hollow visceral organs, such as the stomach, urinary bladder, and respiratory passages
- Forces food and other substances through internal body channels
- It is not striated and is involuntary
Functional Characteristics of Muscle Tissue
- Excitability, or irritability – the ability to receive and respond to stimuli
- Contractility – the ability to shorten forcibly
- Extensibility – the ability to be stretched or extended
- Elasticity – the ability to recoil and resume the original resting length
Muscle Function
- Skeletal muscles are responsible for all locomotion
- Cardiac muscle is responsible for coursing the blood through the body
- Smooth muscle helps maintain blood pressure, and squeezes or propels substances (i.e., food, feces) through organs
- Muscles also maintain posture, stabilize joints, and generate heat
THE ONLY ACTION A MUSCLE HAS
CONTRACTION
Skeletal Muscle
Each muscle is a discrete organ composed of:
- muscle tissue
- blood vessels
- nerve fibers
- connective tissue
The three connective tissue sheaths are:
- Endomysium – fine sheath of connective tissue composed of reticular fibers surrounding each muscle fiber
- Perimysium – fibrous connective tissue that surrounds groups of muscle fibers called fascicles
- Epimysium – an overcoat of dense regular connective tissue that surrounds the entire muscle
Skeletal Muscle: Nerve and Blood Supply
Each muscle is served by one nerve, an artery, and one or more veins
- Each skeletal muscle fiber is supplied with a nerve ending that controls contraction
- Contracting fibers require continuous delivery of oxygen and nutrients via arteries
- Wastes must be removed via veins
Skeletal Muscle: Attachments
- Most skeletal muscles span joints and are attached to bone in at least two places
- When muscles contract the movable bone, the muscle’s insertion moves toward the immovable bone, the muscle’s origin
Muscles attach:
Directly – epimysium of the muscle is fused to the periosteum of a bone
Examples: deltoid, supraspinatus
Indirectly – connective tissue wrappings extend beyond the muscle as a tendon or aponeurosis
Examples: biceps brachii, gastrocnemius
Microscopic Anatomy of a Skeletal Muscle Fiber
- Each fiber is a long, cylindrical cell with multiple nuclei just beneath the sarcolemma
- Fibers are 10 to 100 micrometers in diameter, and up to hundreds of centimeters long
- Each cell is a syncytium produced by fusion of embryonic cells
- Sarcoplasm has numerous glycosomes [!]and a unique oxygen-binding protein called myoglobin [!]
- Fibers contain the usual organelles, myofibrils, sarcoplasmic reticulum, and T tubules
WHAT DOES EVERY CELL NEED TO FUNCTION??
GLUCOSE AND OXYGEN
Myofibrils
- Myofibrils are densely packed, rodlike contractile elements
- They make up most of the muscle volume
- The arrangement of myofibrils within a fiber is such that a perfectly aligned repeating series of dark A bands and light I bands is evident
Sarcomeres
- The smallest contractile unit of a muscle
- The region of a myofibril between two successive Z discs
- Composed of myofilaments made up of contractile proteins
- Myofilaments are of two types – thick and thin
Myofilaments: Banding Pattern
- Thick filaments – extend the entire length of an A band
- Thin filaments – extend across the I band and partway into the A band
- Z-disc – coin-shaped sheet of proteins (connectins) that anchors the thin filaments and connects myofibrils to one another
- Thin filaments do not overlap thick filaments in the lighter H zone (when muscle is relaxed)
- M lines appear darker due to the presence of the protein desmin
Ultrastructure of Myofilaments: Thick Filaments
- Thick filaments are composed of the protein myosin
- Each myosin molecule has a rod-like tail and two globular heads [anatomy]
- Tails – two interwoven, heavy polypeptide chains
- Heads – two smaller, light polypeptide chains called cross bridges
Cross bridge is most important
Ultrastructure of Myofilaments: Thin Filaments
Thin filaments are chiefly composed of the protein actin
Each actin molecule is a helical polymer of globular subunits called G actin [anatomy]
The G actin subunits contain the active sites to which myosin heads attach during contraction
Tropomyosin and troponin are regulatory subunits bound to actin
Sarcoplasmic Reticulum (SR)
- SR is an elaborate, smooth endoplasmic reticulum that mostly runs longitudinally and surrounds each myofibril
- Paired terminal cisternae form perpendicular cross channels
- Functions in the regulation of intracellular calcium levels [physiology]
- Elongated tubes called T tubules penetrate into the cell’s interior at each A band–I band junction
- T tubules associate with the paired terminal cisternae to form triads [anatomy]
T Tubules
- T tubules are continuous with the sarcolemma
- They conduct impulses to the deepest regions of the muscle [physiology]
- These impulses signal for the release of Ca2+ from adjacent terminal cisternae
Sliding Filament Model of Contraction
- Thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree In the relaxed state, thin and thick filaments overlap only slightly
- Upon stimulation, myosin heads bind to actin and sliding begins
- Each myosin head binds and detaches several times during contraction, acting like a ratchet to generate tension and propel the thin filaments to the center of the sarcomere
- As this event occurs throughout the sarcomeres, the muscle shortens
When filaments slide
muscles contract
Skeletal Muscle Contraction
- In order to contract, a skeletal muscle must:
- Be stimulated by a nerve ending
- Propagate an electrical current, or action potential, along its sarcolemma
- Have a rise in intracellular Ca2+ levels, the final trigger for contraction
- Linking the electrical signal to the contraction is excitation-contraction coupling
Nerve Stimulus of Skeletal Muscle
- Skeletal muscles are stimulated by motor neurons of the somatic nervous system
- Axons of these neurons travel in nerves to muscle cells
- Axons of motor neurons branch profusely as they enter muscles
- Each axonal branch forms a neuromuscular junction with a single muscle fiber
Neuromuscular Junction
The neuromuscular junction is formed from:
- Axonal endings, which have small membranous sacs (synaptic vesicles) that contain the neurotransmitter acetylcholine (ACh)
- The motor end plate of a muscle, which is a specific part of the sarcolemma that contains ACh receptors and helps form the neuromuscular junction
- Though exceedingly close, axonal ends and muscle fibers are always separated by a space called the synaptic cleft
- The axon and sarcolemma never touch
- When a nerve impulse reaches the end of an axon at the neuromuscular junction:
- Voltage-regulated calcium channels open and allow Ca2+ to enter the axon
- Ca2+ inside the axon terminal causes axonal vesicles to fuse with the axonal membrane
- This fusion releases ACh into the synaptic cleft via exocytosis
- ACh diffuses across the synaptic cleft to ACh receptors on the sarcolemma
- Binding of ACh to its receptors initiates an action potential in the muscle
Destruction of Acetylcholine
- ACh bound to ACh receptors is quickly destroyed by the enzyme acetylcholinesterase
- This destruction prevents continued muscle fiber contraction in the absence of additional stimuli
Action Potential
- A transient depolarization event that includes polarity reversal of a sarcolemma (or nerve cell membrane) and the propagation of an action potential along the membrane
- Wig Wag Wig Wag of charges that lead to an electrical charge
Role of Acetylcholine (ACh)
- ACh binds its receptors at the motor end plate
- Binding opens chemically (ligand) gated channels
- Na+ diffuses in and K+ diffuses out, and the interior of the sarcolemma becomes less negative
- This event is called depolarization
Depolarization
- Initially, this is a local electrical event called end plate potential
- Later, it ignites an action potential that spreads in all directions across the sarcolemma
Action Potential: Electrical Conditions of a Polarized Sarcolemma
- The outside (extracellular) face is positive, while the inside face is negative
- This difference in charge is the resting membrane potential
- The predominant extracellular ion is Na+
- The predominant intracellular ion is K+
- The sarcolemma is relatively impermeable to both ions
Salt on a banana
Action Potential: Depolarization and Generation of the Action Potential
- An axonal terminal of a motor neuron releases ACh and causes a patch of the sarcolemma to become permeable to Na+ (sodium channels open)
- Na+ enters the cell, and the resting potential is decreased (depolarization occurs)
- If the stimulus is strong enough, an action potential is initiated
Action Potential: Propagation of the Action Potential
- Polarity reversal of the initial patch of sarcolemma changes the permeability of the adjacent patch
- Voltage-regulated Na+ channels now open in the adjacent patch causing it to depolarize
- Thus, the action potential travels rapidly along the sarcolemma
- Once initiated, the action potential is unstoppable, and ultimately results in the contraction of a muscle
Action Potential: Repolarization
- Immediately after the depolarization wave passes, the sarcolemma permeability changes
- Na+ channels close and K+ channels open
- K+ diffuses from the cell, restoring the electrical polarity of the sarcolemma
- Repolarization occurs in the same direction as depolarization, and must occur before the muscle can be stimulated again (refractory period)
- The ionic concentration of the resting state is restored by the Na+ -K+ pump
3-2-1: Sodium, Potassium, ATP
Excitation-Contraction Coupling
Once generated, the action potential:
Is propagated along the sarcolemma
Travels down the T tubules
Triggers Ca2+ release from terminal cisternae
Ca2+ binds to troponin and causes:
The blocking action of tropomyosin to cease
Actin active binding sites to be exposed
Myosin cross bridges alternately attach and detach
Thin filaments move toward the center of the sarcomere
Hydrolysis of ATP powers this cycling process
Ca2+ is removed into the SR, tropomyosin blockage is restored, and the muscle fiber relaxes
- Action potential generated and propagated along sarcomere to T-tubules
- Action potential triggers Ca2+ release
- Ca++ bind to troponin; blocking action of tropomyosin released
- Contraction via crossbridge formation; ATP hyrdolysis
- Removal of Ca2+ by active transport
- Tropomyosin blockage restored; contraction ends
Sequential Events of Contraction
- Cross bridge formation – myosin cross bridge attaches to actin filament
- Working (power) stroke – myosin head pivots and pulls actin filament toward M line
- Cross bridge detachment – ATP attaches to myosin head and the cross bridge detaches
- “Cocking” of the myosin head – energy from hydrolysis of ATP cocks the myosin head into the high-energy state
Contraction of Skeletal Muscle Fibers
Contraction – refers to the activation of myosin’s cross bridges (force-generating sites)
Shortening occurs when the tension generated by the cross bridge exceeds forces opposing shortening
Contraction ends when cross bridges become inactive, the tension generated declines, and relaxation is induced
Contraction of Skeletal Muscle (Organ Level)
Contraction of muscle fibers (cells) and muscles (organs) is similar
The two types of muscle contractions are:
- Isometric contraction – increasing muscle tension (muscle does not shorten during contraction & joints do not move)
- Isotonic contraction – decreasing muscle length (muscle shortens during contraction & joints move)
Motor Unit: The Nerve-Muscle Functional Unit
- A motor unit is a motor neuron and all the muscle fibers it supplies
- The number of muscle fibers per motor unit can vary from four to several hundred
- Muscles that control fine movements (fingers, eyes) have small motor units
- Large weight-bearing muscles (thighs, hips) have large motor units
- Muscle fibers from a motor unit are spread throughout the muscle; therefore, contraction of a single motor unit causes weak contraction of the entire muscle
Muscle Twitch
A muscle twitch is the response of a muscle to a single, brief threshold stimulus
There are three phases to a muscle twitch
- Latent period
- Period of contraction
- Period of relaxation
Phases of a Muscle Twitch
- Latent period – first few msec after stimulus; EC coupling taking place
- Period of contraction – cross bridges from; muscle shortens
- Period of relaxation – Ca2+ reabsorbed; muscle tension goes to zero
Graded Muscle Responses
Graded muscle responses are:
Variations in the degree of muscle contraction
Required for proper control of skeletal movement
Responses are graded by:
- Changing the frequency of stimulation
- Changing the strength of the stimulus
Muscle Response to Varying Stimuli
- A single stimulus results in a single contractile response – a muscle twitch
- Frequently delivered stimuli (muscle does not have time to completely relax) increases contractile force – wave summation
- More rapidly delivered stimuli result in incomplete tetanus
- If stimuli are given quickly enough, complete tetanus results
Muscle Response: Stimulation Strength
- Threshold stimulus – the stimulus strength at which the first observable muscle contraction occurs
- Beyond threshold, muscle contracts more vigorously as stimulus strength is increased
- Force of contraction is precisely controlled by multiple motor unit summation
- This phenomenon, called recruitment, brings more and more muscle fibers into play
Muscle Tone
Muscle tone:
Is the constant, slightly contracted state of all muscles, which does not produce active movements
Keeps the muscles firm, healthy, and ready to respond to stimulus
Spinal reflexes account for muscle tone by:
- Activating one motor unit and then another
- Responding to activation of stretch receptors in muscles and tendons
Muscle Metabolism: Energy for Contraction
ATP is the only source used directly for contractile activity
As soon as available stores of ATP are hydrolyzed (4-6 seconds), they are regenerated by:
- The interaction of ADP with creatine phosphate (CP)
- Anaerobic glycolysis
- Aerobic respiration
Muscle Metabolism: Anaerobic Glycolysis
When muscle contractile activity reaches 70% of maximum:
Bulging muscles compress blood vessels
Oxygen delivery is impaired
Pyruvic acid is converted into lactic acid
The lactic acid:
- Diffuses into the bloodstream Is picked up and used as fuel by the liver, kidneys, and heart
- Is converted back into pyruvic acid by the liver
Muscle Fatigue
Muscle fatigue – the muscle is in a state of physiological inability to contract
Muscle fatigue occurs when:
- ATP production fails to keep pace with ATP use
- There is a relative deficit of ATP, causing contractures [cramps]
- Lactic acid accumulates in the muscle
- Ionic imbalances are present
Intense exercise produces rapid muscle fatigue (with rapid recovery)
- Na+-K+ pumps cannot restore ionic balances quickly enough
- Low-intensity exercise produces slow-developing fatigue
ATP knocks myosin head off. If head stays on, it leads to cramps from overuse
Oxygen Debt
Vigorous exercise causes dramatic changes in muscle chemistry
For a muscle to return to a resting state:
- Oxygen reserves must be replenished
- Lactic acid must be converted to pyruvic acid
- Glycogen stores must be replaced ATP and CP reserves must be resynthesized
Oxygen debt – the extra amount of O2 needed for the above restorative processes
Heat Production During Muscle Activity
- Only 40% of the energy released in muscle activity is useful as work
- Still more efficient than any machine
- The remaining 60% is given off as heat
- Dangerous heat levels are prevented by radiation of heat from the skin and sweating
Force of Muscle Contraction
The force of contraction is affected by:
- The number of muscle fibers contracting – the more motor fibers in a muscle, the stronger the contraction
- The relative size of the muscle – the bulkier the muscle, the greater its strength
- Degree of muscle stretch – muscles contract strongest when muscle fibers are 80-120% of their normal resting length
Effects of Aerobic Exercise
Aerobic exercise results in an increase of:
- Muscle capillaries
- Number of mitochondria
- Myoglobin synthesis
Effects of Resistance Exercise
Resistance exercise (typically anaerobic) results in:
- Muscle hypertrophy
- Increased mitochondria, myofilaments, and glycogen stores
The Overload Principle
- Forcing a muscle to work promotes increased muscular strength
- Muscles adapt to increased demands
- Muscles must be overloaded to produce further gains
Muscular Dystrophy
Muscular dystrophy – group of inherited muscle-destroying diseases where muscles enlarge due to fat and connective tissue deposits, but muscle fibers atrophy
Duchenne muscular dystrophy (DMD)
- Inherited, sex-linked disease carried by females and expressed in males (1/3500)
- Diagnosed between the ages of 2-10
- Victims become clumsy and fall frequently as their muscles fail
- Progresses from the extremities upward, and victims die of respiratory failure in their 20s
- Caused by a lack of the cytoplasmic protein dystrophin
- There is no cure, but myoblast transfer therapy shows promise
