Power Point Lecture Slides prepared by Meg Flemming
Power. Point® Lecture Slides prepared by Meg Flemming Austin Community College CHAPTER 7 The Muscular System © 2013 Pearson Education, Inc.
Steps of Contraction (7 -3) 1. Calcium released from SR 2. Calcium binds to troponin 3. Change of troponin shape causes tropomyosin to move away from active sites 4. Myosin heads bind to active site, creating crossbridges, rotate and cause actin to slide over myosin © 2013 Pearson Education, Inc.
Sliding Filament Theory (7 -3) • Based on observed changes in sarcomere • I bands get smaller • Z lines move closer together • H bands decrease • A bands don't change, indicating that the thin filaments are sliding toward the center © 2013 Pearson Education, Inc.
Figure 7 -3 Changes in the Appearance of a Sarcomere during Contraction of a Skeletal Muscle Fiber. I band Z line A band H band Z line A relaxed sarcomere showing locations of the A band, Z lines, and I band. © 2013 Pearson Education, Inc. I band A band H band Z line During a contraction, the A band stays the same width, but the Z lines move closer together and the I band gets smaller.
The Neuromuscular Junction (7 -4) • Where a motor neuron communicates with a skeletal muscle fiber • Axon terminal of the neuron • An enlarged end that contains vesicles of the neurotransmitter • Acetylcholine (ACh) • The neurotransmitter that will cross the synaptic cleft © 2013 Pearson Education, Inc.
The Neuromuscular Junction (7 -4) • ACh binds to the receptor on the motor end plate • Cleft and the motor end plate contain acetylcholinesterase (ACh. E) • Which breaks down ACh • Neurons stimulate sarcolemma by generating an action potential • An electrical impulse © 2013 Pearson Education, Inc.
Figure 7 -4 Skeletal Muscle Innervation. The cytoplasm of the axon terminal contains vesicles filled with molecules of acetylcholine, or ACh. Acetylcholine is a neurotransmitter, a chemical released by a neuron to change the permeability or other properties of another cell’s plasma membrane. The synaptic cleft and the motor end plate contain molecules of the enzyme acetylcholinesterase (ACh. E), which breaks down ACh. Vesicles ACh Synaptic cleft Motor end plate © 2013 Pearson Education, Inc. ACh. E Slide 1
Figure 7 -4 Skeletal Muscle Innervation. Slide 2 The stimulus for ACh release is the arrival of an electrical impulse, or action potential, at the axon terminal. The action potential arrives at the NMJ after traveling along the length of the axon. Arriving action potential © 2013 Pearson Education, Inc.
Figure 7 -4 Skeletal Muscle Innervation. When the action potential reaches the neuron’s axon terminal, permeability changes in the membrane trigger the exocytosis of ACh into the synaptic cleft. Exocytosis occurs as vesicles fuse with the neuron’s plasma membrane. Motor end plate © 2013 Pearson Education, Inc. Slide 3
Figure 7 -4 Skeletal Muscle Innervation. Slide 4 ACh molecules diffuse across the synaptic cleft and bind to ACh receptors on the surface of the motor end plate. ACh binding alters the membrane’s permeability to sodium ions. Because the extracellular fluid contains a high concentration of sodium ions, and sodium ion concentration inside the cell is very low, sodium ions rush into the sarcoplasm. ACh receptor site © 2013 Pearson Education, Inc.
Figure 7 -4 Skeletal Muscle Innervation. Slide 5 The sudden inrush of sodium ions results in the generation of an action potential in the sarcolemma. ACh. E quickly breaks down the ACh on the motor end plate and in the synaptic cleft, thus inactivating the ACh receptor sites. Action potential ACh. E © 2013 Pearson Education, Inc.
The Contraction Cycle (7 -4) • Involves the triads • Action potential travels over the sarcolemma, down into the T tubules • Causes release of calcium from the SR • Calcium binds to troponin and the contraction cycle starts © 2013 Pearson Education, Inc.
Figure 7 -5 The Contraction Cycle Slide 1 Contraction Cycle Begins Myosin head Troponin Tropomyosin © 2013 Pearson Education, Inc. Actin
Figure 7 -5 The Contraction Cycle Slide 2 Active-Site Exposure Sarcoplasm Active site © 2013 Pearson Education, Inc.
Figure 7 -5 The Contraction Cycle Slide 3 Cross-Bridge Formation © 2013 Pearson Education, Inc.
Figure 7 -5 The Contraction Cycle Slide 4 Myosin Head Pivoting © 2013 Pearson Education, Inc.
Figure 7 -5 The Contraction Cycle Slide 5 Cross-Bridge Detachment © 2013 Pearson Education, Inc.
Figure 7 -5 The Contraction Cycle Slide 6 Myosin Reactivation © 2013 Pearson Education, Inc.
Table 7 -1 Steps Involved in Skeletal Muscle Contraction and Relaxation © 2013 Pearson Education, Inc.
Contraction Produces Tension (7 -5) • As sarcomeres contract, so does the entire muscle fiber • As fibers contract, tension is created by tendons pulling on bones • Movement will occur only if the tension is greater than the resistance • Compression is a force that pushes objects • Muscle cells create only tension, not compression © 2013 Pearson Education, Inc.
Contraction Produces Tension (7 -5) • Individual fibers • Are either contracted or relaxed • "On" or "off" • Tension is a product of the number of cross-bridges a fiber contains • Variation in tension can occur based on: • The amount of overlap of the myofilaments • The frequency of stimulation • The more frequent the stimulus, the more Ca 2+ builds up, resulting in greater contractions © 2013 Pearson Education, Inc.
Contraction Produces Tension (7 -5) • Whole skeletal muscle organ • Contracts with varying tensions based on: • Frequency of muscle fiber stimulation • Number of fibers activated © 2013 Pearson Education, Inc.
A Muscle Twitch (7 -5) • A single stimulus-contraction-relaxation cycle in a muscle fiber or whole muscle • Represented by a myogram © 2013 Pearson Education, Inc.
Three Phases of a Muscle Twitch (7 -5) 1. Latent period • Starts at the point of stimulus and includes the action potential, release of Ca 2+, and the activation of troponin/tropomyosin 2. Contraction phase • Is the development of tension because of the cross-bridge cycle 3. Relaxation phase • Occurs when tension decreases due to the re-storage of Ca 2+ and covering of actin active sites © 2013 Pearson Education, Inc.
Figure 7 -6 The Twitch and Development of Tension Maximum tension development Stimulus Time (msec) 0 5 10 Resting Latent Contraction phase period phase © 2013 Pearson Education, Inc. 20 30 Relaxation phase 40
Summation and Tetanus (7 -5) • Summation • Occurs with repeated, frequent stimuli that trigger a response before full relaxation has occurred • Incomplete tetanus • Near peak tension with little relaxation • Complete tetanus • Stimuli are so frequent that relaxation does not occur PLAY ANIMATION Frog Wave Summation © 2013 Pearson Education, Inc.
Figure 7 -7 Effects of Repeated Stimulations. Maximum tension (in tetanus) Tension = Stimulus Time Summation of twitches occurs when successive stimuli arrive before the relaxation phase has been completed. © 2013 Pearson Education, Inc. Time Incomplete tetanus occurs if the stimulus frequency increases further. Tension production rises to a peak, and the periods of relaxation are very brief. Time Complete tetanus. During complete tetanus, the stimulus frequency is so high that the relaxation phase is eliminated; tension plateaus at maximal levels.
Varying Numbers of Fibers Activated (7 -5) • Allows for smooth contraction and a lot of control • Most motor neurons control a number of fibers through multiple, branching axon terminals © 2013 Pearson Education, Inc.
Motor Unit (7 -5) • A single motor neuron and all the muscle fibers it innervates • Motor units are dispersed throughout the muscle • Fine control movements • Use motor units with very few fibers per neuron • Gross movements • Motor units have a high fiber-to-neuron ratio © 2013 Pearson Education, Inc.
Recruitment (7 -5) • A mechanism for increasing tension to create more movement • A graded addition of more and more motor units to produce adequate tension © 2013 Pearson Education, Inc.
Figure 7 -8 Motor Units. Axons of motor neurons Motor nerve KEY Motor unit 1 Motor unit 2 Motor unit 3 © 2013 Pearson Education, Inc. SPINAL CORD Muscle fibers
Muscle Tone and Atrophy (7 -5) • Muscle tone • Some muscles at rest will still have a little tension • Primary function is stabilization of joints and posture • Atrophy • Occurs in a muscle that is not regularly stimulated • Muscle becomes small and weak • Can be observed after a cast comes off a fracture © 2013 Pearson Education, Inc.
Types of Contraction (7 -5) • Isotonic contraction • When the length of the muscle changes, but the tension remains the same until relaxation • For example, lifting a book • Isometric contraction • When the whole muscle length stays the same, the tension produced does not exceed the load • For example, pushing against a wall © 2013 Pearson Education, Inc.
Elongation of Muscle after Contraction (7 -5) • No active mechanism for returning a muscle to a pre-contracted, elongated state • Passively uses a combination of: • Gravity • Elastic forces • Opposing muscle movement © 2013 Pearson Education, Inc.
ATP and CP Reserves (7 -6) • At rest, muscle cells generate ATP, some of which will be held in reserve • Some is used to transfer high energy to creatine forming creatine phosphate (CP) © 2013 Pearson Education, Inc.
- Slides: 35