Muscular System

The Muscular System Three basic muscle types are found in the body Skeletal muscle Cardiac muscle Smooth muscle

Characteristics of Muscles (muscle cell = muscle fiber) Excitability (responsiveness or irritability): Contractility: Extensibility: Elasticity:

Skeletal Muscle Characteristics Most are attached by tendons to bones Cells are multinucleate Striated Voluntary surrounded and bundled by connective tissue

Smooth Muscle Characteristics Has no striations Single nucleus Involuntary Found mainly in the walls of hollow organs

Cardiac Muscle Characteristics Has striations Usually has a single nucleus Joined to another muscle cell at an intercalated disc Involuntary Found only in the heart

Table 9.3

Connective Tissue Wrappings of Skeletal Muscle Endomysium Dense regular Perimysium fibrous Epimysium loose Fascia

(wrapped by perimysium) Epimysium Epimysium Bone Perimysium Tendon Endomysium Muscle fiber in middle of a fascicle (b) Blood vessel Fascicle (wrapped by perimysium) Endomysium (between individual muscle fibers) Perimysium Fascicle Muscle fiber (a) Figure 9.1

Table 9.1

Skeletal Muscle Attachments Sites of muscle attachment Bones Cartilages Connective tissue coverings

Function of Muscles Produce movement Maintain posture Stabilize joints Generate heat

Microscopic Anatomy of a Skeletal Muscle Fiber Cylindrical cell 10 to 100 m in diameter, up to 30 cm long Multiple peripheral nuclei Many mitochondria Glycosomes for glycogen storage, myoglobin for O2 storage Also contain myofibrils, sarcoplasmic reticulum, and T tubules

Microscopic Anatomy of Skeletal Muscle

Microscopic Anatomy of Skeletal Muscle Myofibril Bundles of myofilaments

Features of a Sarcomere Contractile unit Thick filaments: run the entire length of an A band Thin filaments: run the length of the I band and partway into the A band Z disc: coin-shaped sheet of proteins that anchors the thin filaments and connects myofibrils to one another H zone: lighter midregion where filaments do not overlap M line: line of protein myomesin that holds adjacent thick filaments together

Thin (actin) filament Z disc H zone Z disc Thick (myosin) filament I band A band Sarcomere I band M line (c) Small part of one myofibril enlarged to show the myofilaments responsible for the banding pattern. Each sarcomere extends from one Z disc to the next. Sarcomere Z disc M line Z disc Thin (actin) filament Elastic (titin) filaments Thick (myosin) filament (d) Enlargement of one sarcomere (sectioned lengthwise). Notice the myosin heads on the thick filaments. Figure 9.2c, d

Sarcomere

Longitudinal section of filaments within one sarcomere of a myofibril Thick filament Thin filament In the center of the sarcomere, the thick filaments lack myosin heads. Myosin heads are present only in areas of myosin-actin overlap. Thick filament Thin filament Each thick filament consists of many myosin molecules whose heads protrude at opposite ends of the filament. A thin filament consists of two strands of actin subunits twisted into a helix plus two types of regulatory proteins (troponin and tropomyosin). Portion of a thick filament Portion of a thin filament Myosin head Tropomyosin Troponin Actin Actin-binding sites Heads Tail Active sites for myosin attachment ATP- binding site Actin subunits Flexible hinge region Myosin molecule Actin subunits Figure 9.3

Microscopic Anatomy of Skeletal Muscle

Microscopic Anatomy of Skeletal Muscle Sarcolemma – (carry electrical current and connect to tendons Sarcoplasmic reticulum- store Ca.

Sarcoplasmic Reticulum (SR) Network of smooth endoplasmic reticulum surrounding each myofibril Pairs of terminal cisternae form perpendicular cross channels Functions in the regulation of intracellular Ca2+ levels

T Tubules Continuous with the sarcolemma Penetrate the cell’s interior at each A band–I band junction Associate with the paired terminal cisternae to form triads that encircle each sarcomere

Part of a skeletal muscle fiber (cell) I band A band I band Z disc H zone Z disc Myofibril M line Sarcolemma Triad: • T tubule • Terminal cisternae of the SR (2) Sarcolemma Tubules of the SR Myofibrils Mitochondria Figure 9.5

Nerve Stimulus to Muscles Skeletal muscles must be stimulated by a nerve to contract

Nerve Stimulus to Muscles Neuromuscular junctions Synaptic cleft

Transmission of Nerve Impulse to Muscle Neurotransmitter for skeletal muscle is acetylcholine Neurotransmitter attaches to receptors on the sarcolemma Sarcolemma becomes permeable to sodium (Na+) Sodium rushing into the cell generates an action potential

Figure 9.8 1 2 3 4 5 6 Myelinated axon of motor neuron Action potential (AP) Axon terminal of neuromuscular junction Nucleus Sarcolemma of the muscle fiber Action potential arrives at axon terminal of motor neuron. 1 Ca2+ Synaptic vesicle containing ACh Ca2+ Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal. 2 Mitochondrion Synaptic cleft Axon terminal of motor neuron Ca2+ entry causes some synaptic vesicles to release their contents (acetylcholine) by exocytosis. 3 Fusing synaptic vesicles Junctional folds of sarcolemma ACh Acetylcholine, a neurotransmitter, diffuses across the synaptic cleft and binds to receptors in the sarcolemma. 4 Sarcoplasm of muscle fiber ACh binding opens ion channels that allow simultaneous passage of Na+ into the muscle fiber and K+ out of the muscle fiber. 5 Na+ K+ Postsynaptic membrane ion channel opens; ions pass. ACh effects are terminated by its enzymatic breakdown in the synaptic cleft by acetylcholinesterase. 6 Ach– Degraded ACh Postsynaptic membrane ion channel closed; ions cannot pass. Na+ Acetyl- cholinesterase K+ Figure 9.8

2 1 1 Axon terminal Open Na+ Channel Closed K+ Channel Synaptic cleft Action potential + + + o + + Na+ K+ a t i z r i 2 a Generation and propagation of the action potential (AP) o l p d e o f v e W a 1 1 Local depolarization: generation of the end plate potential on the sarcolemma Sarcoplasm of muscle fiber Figure 9.9, step 2

1 2 Action potential is propagated along the sarcolemma and down the T tubules. 1 Steps in E-C Coupling: Sarcolemma Voltage-sensitive tubule protein T tubule Ca2+ release channel 2 Calcium ions are released. Terminal cisterna of SR Ca2+ Figure 9.11, step 4

3 4 Actin Troponin Tropomyosin blocking active sites Ca2+ Myosin Calcium binds to troponin and removes the blocking action of tropomyosin. 3 Active sites exposed and ready for myosin binding Contraction begins 4 Myosin cross bridge The aftermath Figure 9.11, step 7

Cross bridge formation. Actin Ca2+ Thin filament ADP Myosin cross bridge Pi Thick filament Myosin 1 Cross bridge formation. Figure 9.12, step 1

The power (working) stroke. ADP Pi 2 The power (working) stroke. Figure 9.12, step 3

Cross bridge detachment. ATP 3 Cross bridge detachment. Figure 9.12, step 4

Sliding Filament Model of Contraction In the relaxed state, thin and thick filaments overlap only slightly During contraction, myosin heads bind to actin, detach, and bind again, to propel the thin filaments toward the M line As H zones shorten and disappear, sarcomeres shorten, muscle cells shorten, and the whole muscle shortens

The Sliding Filament Theory of Muscle Contraction

The Sliding Filament Theory

Contraction of a Skeletal Muscle Muscle fiber contraction is “all or none” not all fibers may be stimulated during the same interval Graded responses

Types of Graded Responses Twitch Not a normal muscle function

Types of Graded Responses Tetanus (summing of contractions) One contraction is immediately followed by another The muscle does not completely return to a resting state The effects are added

Types of Graded Responses Unfused (incomplete) tetanus Fused (complete) tetanus Figure 6.9a, b

Muscle Response to Strong Stimuli force depends upon the number of fibers stimulated can continue to contract unless they run out of energy (ATP)

Energy for Muscle Contraction Initially, muscles used stored ATP for energy Only 4-6 seconds worth of ATP is stored by muscles After this initial time, other pathways must be utilized to produce ATP

Energy for Muscle Contraction Direct phosphorylation Muscle cells contain creatine phosphate (CP) After ATP is depleted, ADP is left CP transfers energy to ADP, to regenerate ATP CP supplies are exhausted in about 20 seconds

Energy for Muscle Contraction Aerobic Respiration This is a slower reaction that requires continuous oxygen

Energy for Muscle Contraction Anaerobic glycolysis Pyruvic acid is converted to lactic acid This reaction is not as efficient, but is fast Huge amounts of glucose are needed Lactic acid produces muscle fatigue Figure 6.10b Slide 6.26a

Muscle Fatigue and Oxygen Debt When a muscle is fatigued, it is unable to contract The common reason for muscle fatigue is oxygen debt Oxygen is required to get rid of accumulated lactic acid Increasing acidity (from lactic acid) and lack of ATP causes the muscle to contract less

Types of Muscle Contractions Isotonic contractions The muscle shortens Isometric contractions The muscle is unable to shorten

Muscle Tone Some fibers are contracted even in a relaxed muscle under involuntary control The more muscle is used the more tone it becomes.

Muscles and Body Movements

Effects of Exercise on Muscle Results of increased muscle use Increase in muscle size Increase in muscle strength Increase in muscle efficiency Muscle becomes more fatigue resistant

Large number of muscle fibers activated Muscle and sarcomere stretched to slightly over 100% of resting length Large muscle fibers High frequency of stimulation Contractile force Figure 9.21

Muscle Fiber Type Classified according to two characteristics: Speed of contraction: slow or fast, according to: Speed at which myosin ATPases split ATP Pattern of electrical activity of the motor neurons

Muscle Fiber Type Metabolic pathways for ATP synthesis: Oxidative fibers—use aerobic pathways Glycolytic fibers—use anaerobic glycolysis

Muscle Fiber Type Three types: Slow oxidative fibers Fast oxidative fibers Fast glycolytic fibers

Table 9.2

Longitudinal layer of smooth muscle (shows smooth muscle fibers in cross section) Small intestine Mucosa (a) (b) Cross section of the intestine showing the smooth muscle layers (one circular and the other longitudinal) running at right angles to each other. Circular layer of smooth muscle (shows longitudinal views of smooth muscle fibers) Figure 9.26

Peristalsis Alternating contractions and relaxations of smooth muscle layers that mix and squeeze substances through the lumen of hollow organs Longitudinal layer contracts; organ dilates and shortens Circular layer contracts; organ constricts and elongates

Microscopic Structure Spindle-shaped fibers: thin and short compared with skeletal muscle fibers Connective tissue: endomysium only SR: less developed than in skeletal muscle Pouchlike infoldings (caveolae) of sarcolemma sequester Ca2+ No sarcomeres, myofibrils, or T tubules

Myofilaments in Smooth Muscle Ratio of thick to thin filaments (1:13) is much lower than in skeletal muscle (1:2) Thick filaments have heads along their entire length No troponin complex; protein calmodulin binds Ca2+

Myofilaments in Smooth Muscle Myofilaments are spirally arranged, causing smooth muscle to contract in a corkscrew manner Dense bodies: proteins that anchor noncontractile intermediate filaments to sarcolemma at regular intervals

Figure 9.28a

Figure 9.28b