Section 1, Chapter 9 Muscular System . Muscle is derived from Musculus , for “Mouse”. Imagine a mouse running beneath the skin . Functions of Muscles: Body movement Maintain posture Produces heat Propel substances through body Heartbeat. Types of muscles include: Smooth muscle
Imagine a mouse running beneath the skin.
Functions of Muscles:
Propel substances through body
Types of muscles include:
There are two types of smooth muscles
Figure 9.3 Scanning electron micrograph of a fascicle surrounded by its perimysium. Muscle fibers within the fascicle are surrounded by endomysium.
Skeletal muscle organization
Openings into t-tubules
Figure 9.4 Organization of actin and myosin filaments
Z Line is the attachment site of actin filaments (center of I bands)
Figure 9.5 thin and thick filaments in a sarcomere.
When a muscle is at rest, myosin heads are extended in the “cocked” position.
During a contraction, myosin heads bind to actin, forming a cross-bridge and the myosin head pivot forward (Power Stroke) and back (Recovery stroke)
The troponin-tropomyosin complex prevents cross-bridge formation when the muscle is at rest.
End of section 1, chapter 9
Synapse: Functional (not physical) junction between an axon of a neuron and another cell
The two cells are separated by a physical space, called the synaptic cleft.
Neurotransmitters are stored within synaptic vesicles of the presynaptic cell and they’re released into the synapse.
Neuromuscular Junction (NMJ) refers to the synapse between an axon and a muscle fiber.
Motor End Plate is a highly folded region of muscle fiber at NMJ that contain abundant mitochondria
Figure 9.8a. General NMJ
Motor neurons innervate effectors (muscles or glands)
A motor unitincludes a motor neuron and all of the muscle fibers it controls
1 motor unit may control between 1 and 1000 muscle fibers
Figure 9.9 two motor units. The muscle fibers of a motor unit are innervated (controlled) by a single motor neuron.
Acetylcholine (ACh) is the only neurotransmitter that initiates skeletal muscle contraction
Sequence of Actions
Sequence of Actions…Continued
ACh diffuses across synaptic cleft & binds to receptors on motor endplate.
ACh opens Na+ channels on muscle
Na+ floods into the muscle, initiating a muscle impulse.
A muscle impulse (action potential) is propagated across the entire muscle.
Stimulus for a muscle impulse. Corresponds to steps 1-7 in the previous slides.
The muscle impulse causes the release of calcium from the SR. Calcium binds to troponin and tropomyosin is repositioned exposing the actin filaments.
8. The muscle impulse diffuses across sarcolemma and down the t-tubules into the cisternae of sarcoplasmic reticula.
9. The sarcoplasmic reticula release their calcium supplies into the sarcoplasm.
10. Calcium binds to troponin and the troponin repositions the tropomyosin, so the myosin can bind to actin.
11. Cross-bridge cycling causes the muscle to contract.
Calcium released from sarcoplasmic reticulum binds to troponin.
Troponin moves tropomyosin, exposing actin filaments to myosin cross-bridges.
myosin heads bind to actin, forming a cross bridge and cross-bridge cycling causes the muscle to contract.
End of section 2, chapter 9
section 3, chapter 9
Sliding Filament Theory of Contraction
Figure 9.11a. Individual sarcomeres shorten as thick and thin filaments slide past one another.
When a muscle is relaxed, tropmyosincovers the binding sites on actin.
A molecule of ADP and Phosphate remains attached to myosin from the previous contraction.
Myosin heads bind to actin filaments.
The phosphate is released.
Myosin heads spring forward “Power Stroke” pulling the actin filaments.
ADP is released from Myosin
ATP is broken down, providing the energy to “cock” the myosin filaments (recovery stroke).
Steps 1-6 are repeated several times.
Figure 9.10. The cross-bridge cycle. The cycle continues as long as ATP is present, and nerve impulses release Acetylcholoine.
Watch the You-Tube video “Sliding Filament” to view cross-bridge cycling in action.
* Notice that ATP is required for muscle relaxation!
section 4, chapter 9
Energy Sources for Contraction
New ATP molecules are synthesized by
Hydrolysis of Creatine Phosphate
Glycolysis (anaerobic respiration)
Creatine Phosphate can be hydrolyzed into Creatine, releasing energy that is used to make new ATP.
The energy from creatine phosphate hydrolysis cannot be used to directly power muscles.
Instead, it’s used to produce new ATP.
Anaerobic respiration (glycolysis) occurs in the cytosol of the cell and does not require oxygen.
Glucose molecules are partially broken down producing just 2 ATP for each glucose.
If there isn’t sufficient oxygen available, glycolysis produces lactic acid as a byproduct.
Exercise and strenuous activity depends on anaerobic respiration for ATP supplies.
During exercise anaerobic respiration causes lactic acid to accumulate in the cells.
After exercise, when oxygen is available the O2 is used to convert lactic acid back to glucose in the liver.
Oxygen debt is the amount of oxygen needed by liver cells to convert accumulated lactic acid back to glucose.
Aerobic respiration is used primarily at rest or during light exercise.
Muscles that rely on aerobic respiration have plenty of mitochondria and a good blood supply.
Figure 9.13. The oxygen required for aerobic respiration is carried in the blood and stored in myoglobin. In the absence of oxygen, anaerobic respiration uses pyruvic acid to produce lactic acid.
End of Chapter 9, Section 4
section 5, chapter 9
A muscle contraction can be observed by removing a single skeletal muscle and connecting it to a device (myograph) that senses and records changes in the overall length of the muscle fiber.
A threshold stimulus is the minimum stimulus that elicits a muscle fiber contraction
A threshold stimulus will cause the muscle fiber to contract fully and completely.
A stronger stimulus does not produce a stronger contraction!
The muscle fiber will not contract at all if the stimulus is less than threshold.
A twitch is a single contractile response to a stimulus
A twitch can be divided into three periods.
series of twitches
If the muscle is allowed to relax completely before each stimulus than the muscle will contract with the same force.
If the muscle is stimulated again before it has completely relaxed, then the force of the next contraction increases.
i.e. stimulating the muscle at a rapid frequency increases the force of contraction. This is called summation
Tetanic Contraction (c)
If the muscle is stimulated at a high frequency the contractions fuse together and cannot be distinguished.
A tetanic contraction results in a maximal sustained contraction without relaxation
A muscle that is stimulated with threshold potential contracts completely and fully.
A stronger stimulus does not produce a stronger contraction!
Instead, the strength of a muscle is increased by recruitment of additional motor units.
Recruitment – progressive activation of motor units to increase the force of a muscle contraction.
As the intensity of stimulation increases, recruitment of motor units continues until all motor units are activated.
Muscle tone is produced because some muscles are in a continuous state of partial contraction in response to repeated nerve impulses from the spinal cord.
Figure 9.18. muscle contractions
Fast & Slow twitch refers to the contraction speed, and to whether muscle fibers produce ATP oxidatively (by aerobic respiration) or glycolytically (by glycolysis)
Slow-twitch fibers are best suited for endurance exercise over a long period with little force.
Slow-twitch fibers rely on aerobic respiration for energy (ATP) and are resistant to fatigue.
Slow-Twitch fibers contain abundant myoglobin for oxygen storage “red fibers” and mitochondria to carry out aerobic respiration.
Because of their oxygen demands, slow-twitch fibers have a good blood supply.
Fast-twitch glycolytic fibers contract rapidly and with great force, but they fatigue quickly.
They are best suited for rapid contractions over a short duration.
Fast-twitch glycolytic fibers (type IIa) contain very little mitochondria and myoglobin and are “white fibers”
Fast-twitch intermediate or fast oxidative fibers contain intermediate amounts of myoglobin.
They contract rapidly but also have the capacity to generate energy through aerobic respiration.
Migrating birds have abundant slow-twitch fibers for flying long distances, which is why their flesh is dark.
Chickens that can only flap around the barnyard have abundant fast-twitch muscles and mostly white flesh.
End of Chapter 9