Muscle Fibers. Muscle Tissue and Muscles. Muscle Fiber Anatomy. Fiber Structure. Anatomy

Transcription

1 Muscle Tissue and Muscles Anatomy Muscle Fibers Each fiber is surrounded by connective tissue called endomysium Fibers are closely associated with blood vessels (capillaries), lymph vessels, and nerves Each fiber has at least one neuromuscular junction Contraction is monitored by proprioceptors (such as Golgi tendon organs and muscle spindles) All-or-none principle applies to contraction Fiber recruitment (or the lack of) allows for changes in degree of tension produced by the whole muscle Muscle Fiber Anatomy Identify A-band Axon Axon terminals I-band Motor end-plate Myofibrils Neuromuscular junction Sarcolemma Sarcomere Z disc (line) Identify Horizontal tubules of SR Myofibrils T-tubule Terminal cisterna (lateral sacs) of SR Triad Fiber Structure 1

2 Sarcoplasm Sacroplasm Cytoplasm of the muscle fiber Contains typical organelles with modifications of: Glycogen deposits are immediate energy reserves Myoglobin - oxygen binding proteins (similar to hemoglobin of red blood cells) Myofibrils - thick protein filaments which function mostly in support, contraction regulation, and contraction Sarcolemma Sarcolemma, the cell (plasma) membrane Excitable membrane contains electric (voltage) gated channels Extends into cell to form transverse tubules, or T tubules, contains electric (voltage) gated channels Has at least one junction with an axon at neuromuscular junction Has receptors for neurotransmitter (acetylcholine, or ACh) on its portion of the neuromuscular junction (called the motor end plate) Sarcoplasmic Reticulum (SR) Sarcoplasmic reticulum (SR) Modified endoplasmic reticulum Mostly found as a network of horizontal tubules which wrap around the myofibrils and fuse at each end to form a vertical tubule called the terminal cisterna, or lateral sac A T-tubule with its associated terminal cisterna (lateral sacs) is referred to as a triad Sarcoplasmic Reticulum (SR) Functions mostly in the regulation of ionic calcium High concentration of Ca++ is maintained in the SR by ATP calcium ion pumps Ca++ can be released into the sarcoplasm by opening voltage gated Ca++ channels of the terminal cisternae Ca++ channels are opened by depolarization 2

3 Ca ++ Concentration If Ca++ is released, it is quickly pumped back into the SR, unless the voltage gated channels are reopened before Ca++ is removed from the sarcoplasm (another depolarization quickly follows) the Ca++ channels remain open (rapid or continuous depolarization occurs) Nuclei Nuclei Fibers are multinucleate (fiber is produced by fusion of many cells during development) Location of nuclei is typically between sarcolemma and myofibrils Myofibrils Thick protein filaments Extend the length of the muscle fiber (cell) Their proteins function mostly in support, contraction regulation, and contraction A-bands (anisotropic) are dark regions I-bands (isotropic) are light regions Organization of Myofibrils A-Bands Are observed as dark regions and consist mostly of the thick filaments and the overlapping portions of the thin filaments H-zone is light region in middle of A-band M-line Is line in middle of H-zone I-Bands Are observed as light regions and consist mostly of the thin filaments and their associated Z-discs (lines) Sarcomere Region of the myofibril which is defined as the unit of contraction Region from middle of each I-band, the Z-discs (lines) to next Z-disc; thus includes an A Band 3

4 Fiber Structure Identify A-band H-zone I-band M-line Sarcomere Thick filament Thin filament Thin Filaments Thin filaments Consist of the protein actin and two associated proteins, troponin and tropomyosin One end anchored to Z disc (line) other extends into A-band Actin G (globular) actin molecules bonded to form double chain F (filament) actin Each G actin has binding site for head (cross-bridge) of myosin In resting muscle binding sites are blocked by regulator proteins, troponin and tropomyosin Troponin and Tropomyosin Troponin (consists of three subunits) Binds Ca++ Attached to tropomyosin Attached to G-actin Tropomyosin Long filamentous protein which covers actin s myosin binding sites Attached to troponin Troponin-tropomyosin moves away from the myosin binding sites when troponin binds with Ca++ 4

5 Thick Filaments Consist of molecules of myosin. Each molecule has a head (cross-bridge) region and tail region Molecules form the thick filaments Located in A-band Region of thin-filament overlaps thick filament Myosin Molecules form thick filament Heads (cross-bridges) associate with binding sites on actin (in resting muscle actin s binding sites are blocked) Myosin binds ATP ATP serves as energy for movement of crossbridge which results in shortening of sarcomere (muscle contraction) Myosin Energy Configurations High energy configuration Bonding of ATP moves myosin head (cross-bridge) to a high energy configuration Low energy configuration Transfer of energy when the pivoting movement of the myosin head (cross-bridge) occurs results in a low energy configuration Myosin Head (cross-bridge) Movement Chemical energy (from ATP) is converted to mechanical energy when the myosin head (crossbridge) moves. Since the head (cross-bridge) is attached to actin, actin also moves resulting in the shortening of the sarcomere (muscle) ATP is required to reconfigure the head to the high energy position. 5

6 Relationship of Thin and Thick Filaments Identify Actin Cross-bridges Thick filament Tropomyosin Troponin Z-disc (line) Receptor A Neural Pathway Integration Effectors Somatic reflex arc Monitoring of Muscle Contraction Proprioceptors Receptors which are associated with muscle, tendons, joints, and the vestibular apparatus used to monitor degree of muscle tension (tone) and spatial position Muscle spindle Golgi tendon organ Muscle spindles Proprioceptors which monitor degree of muscle fiber length (stretch) Golgi tendon organs Proprioceptors which measure length (stretch) of tendon associated with muscle Control of Muscle Contraction Somatic division of motor (efferent) nervous system controls contraction of skeletal muscle Autonomic division of motor (efferent) nervous system controls contraction of cardiac and smooth muscle (motor neurons originate from lateral horns in thoracic region of spinal cord) 6

7 Motor Unit Motor unit A motor unit is the motor neuron and the fibers it controls Motor neuron may branch (collaterals) and control a few to many muscle fibers. Precise movements are controlled by small motor units (hand) whereas more gross movements are controlled by large movements (thigh) Skeletal Muscle Contraction Excitation-Contraction Coupling Page 300 Excitation is both chemical and electrical Contraction results because of physical movement Neuromuscular Junction Excitation Phase of Skeletal Muscle Depolarization occurs at neuromuscular junction, sarcolemma, T-tubules, and terminal cisternae - which leads to release of Ca ++ Synapse called neuromuscular junction Consists of Axon terminals Endings of axon Synaptic cleft Space between presynaptic and postsynaptic membranes Motor end plate Region of sarcolemma (postsynaptic membrane) which contains receptors for neurotransmitter, acetylcholine (ACh) 7

8 Neural Synapse (EPSP) 1. Action potential arrives 2. Calcium ion channels open 3. Calcium ions promote exocytosis of neurotransmitter acetylcholine (ACh) from presynaptic membrane. Calcium ions are quickly removed 4. Acetylcholine (ACh) binds to its receptors on postsynaptic membrane 5. Receptors promote opening of associated Na+ channels Na+ moves inward. 6. Inward movement of Na+ at postsynaptic membrane produces depolarization (EPSP) 7. Acetylcholine (ACh) is deactivated by enzyme acetylcholinesterase (AChE) Threshold and Action Potentials Threshold Point of depolarization (stimulation) which initiates an effect (action potential) In this case the electrically-gated Na+ channels open, (which are adjacent to the active mechanicallygated channels). The mechanically gated Na+ channels become inactive Action potential Not local; travels great distance Involves electrically-gated channels Propagated along fiber (axon) Repolarization as K+ Moves Out Local current opens adjacent electrically-gated K+ channels K+ moves outward and repolarization occurs Local currents open adjacent Na+ channels Action potential is propagated to adjacent forward section 8

9 Na+ / K+ Pump The Na+ / K+ re-establishes the extracellular and intracellular ionic gradients Pump requires ATP Na+ is pumped outward K+ is pumped inward Depolarization at Neuromuscular Junction Neuromuscular junctions Depolarization as Na+ Moves Inward Receptor s Na+ channels become inactive Local current opened adjacent electrically-gated Na+ channels These channels produce local current Adjacent Na+ channels open Depolarization Begins at motor end plate Spreads to surrounding sarcolemma Propagated to T tubules Results in depolarization of terminal cisternae 9

10 Ca ++ Release Depolarization of terminal cisternae results in the opening of calcium channels and Ca++ release into sarcoplasm Ca++ bonds to troponin and results in the movement of tropomyosin away from the myosin binding sites Ca++ is pumped back and stored in SR. (Ca++ channels will open again when another depolarization occurs) Movement of Regulator Proteins What does Ca++ bond to? What happens when Ca++ bonds? What site is exposed on actin? When do the regulator proteins (troponintropomyosin) move back into position? Relationship of Thin and Thick Filaments Identify Actin Cross-bridges Thick filament Tropomyosin Troponin Z-disc (line) Contraction Phase of Skeletal Muscle Starts with binding of myosin with actin and the results in the inward movement of the thin filaments (shortening of the sarcomere) 10

11 Myosin and Actin Attach The attachment of myosin and actin results in the power stroke, the movement of the myosin head (cross-bridge) to the low-energy position Power Stroke Myosin head conforms to low energy configuration Actin is moved inward (sarcomere shortens) ATP ---> ADP + P ATP Binds Myosin and Actin Detach Before the myosin head (cross-bridge) configures to the high-energy position it detaches from the G-actin molecule 11

12 Myosin Configures to High- Energy Position Binding of ATP produces a change in the shape of the myosin head (cross-bridge) Myosin and Actin Attach Myosin can now attach to the next G-actin molecule Attachment will produce the next power stroke Contraction Continues For as long as Ca++ concentration is high and troponin-tropomyosin complex does not block myosin binding sites ATP is supplied to permit Detachment of myosin from G-actin Energize myosin head Fatigue does not occur ph change (lactic acid) Decrease in ATP Ionic imbalance Nutritional imbalance Contraction Ends If another rapid depolarization does not occur Ca++ pumped into SR Troponin-tropomyosin complex moves back into blocking position Thus, myosin can not bind with actin 12

13 Resting Fiber Myosin can not interact with actin until Ca++ bonds to troponin resulting in removal of troponin-tropomyosin blockade Review Animations Myosin Binds with Actin The tension (force of contraction) which the fiber (cell) develops depends upon? What causes the movement of the head (crossbridge)? Myosin Releases from Actin When does myosin release from actin? When does myosin configure to its high energy position? 13

14 Sliding of Thin Filament List the steps involved in the ratcheting inward of thin filament: Muscle Contraction Myograms 14