Chapter Nine: Muscles and Muscle Tissue. M.C. Shamier BSc Shenzou University

Transcription

1 Chapter Nine: Muscles and Muscle Tissue M.C. Shamier BSc Shenzou University

2 Three Types of Muscle Skeletal associated with the bony skeleton, and consists of large cells that bear striations and are controlled voluntarily. Cardiac only in the heart, and consists of small cells that are striated and under involuntary control. Smooth in the walls of hollow organs, and consists of small elongated cells that are not striated and are under involuntary control

3 Table 9.3

4 Table 9.3

5 Table 9.3

6 Table 9.3

7 Muscle tissue is Excitable: able to receive and respond to a stimulus. Contractable: able to contract when stimulated. Extensible: can be stretched. Elastic: able to resume the cells original length once stretched.

8 Functions of Muscle Body movement, pumping blood, propelling substances through hollow organs Propriocepsis: maintaining posture by adjusting with respect to gravity Stabilizing joints by exerting tension Generate heat (shivering)

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

10 Gross Anatomy of Skeletal Muscle Nerve and blood supply: neural control and adequate nutrient delivery and waste removal. Connective tissue sheaths endomysium around each muscle fiber perimysium around groups of muscle fibers epimysium around whole muscles Movement occurs from the movable bone (the muscle s insertion) toward the less movable bone (the muscle s origin).

11 Muscle Terminology Muscle cell = Muscle fiber Muscle cell membrane = sarcolemma Cytoplasm = Sarcoplasm Smooth endoplasmatic reticulum = Sarcoplasmatic reticulum Myo- and sarco- refer to muscle

12 Microscopic Anatomy of Skeletal Muscle Muscle fibers Huge cells Sarcoplasm contains large amounts of glycosomes and myoglobin (to store glucose and oxygen) Multiple nuclei beneath the sarcolemma Myofibrils 80% of cellular volume The contractile element of skeletal muscle cells

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14 Even more microscopic Myofibrils are composed myofilaments Actin (thin) filaments Myosin (thick) filaments This is where the actual contraction takes place

15 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

16 More terms Sarcomere: smallest functional unit (contractile unit) of muscle

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18 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 Each thick filament consists of many myosin molecules whose heads protrude at opposite ends of the filament. Portion of a thick filament Myosin head Thin 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 thin filament Tropomyosin Troponin Actin Actin-binding sites ATPbinding site Heads Tail Flexible hinge region Myosin molecule Actin subunits Actin subunits Active sites for myosin attachment Figure 9.3

19 Microscopic Anatomy of Skeletal Muscle Sarcoplasmatic reticulum is a smooth endoplasmatic reticulum surrounding each myofibril T tubules are infoldings of the sarcolemma that conduct electrical impulses

20 Physiology of a Skeletal Muscle Cell

21 Spinal cord Motor unit 1 Motor unit 2 Axon terminals at neuromuscular junctions Nerve Motor neuron cell body Motor neuron axon Muscle Muscle fibers Axons of motor neurons extend from the spinal cord to the muscle. There each axon divides into a number of axon terminals that form neuromuscular junctions with muscle fibers scattered throughout the muscle. Figure 9.13a

22 Physiology of a Muscle Cell Neuromuscular junction

23 Physiology of a Skeletal Muscle Cell The neuromuscular junction connection between an axon terminal and a muscle fiber. A nerve impulse causes the release of acetylcholine to the synaptic cleft, which binds to receptors on the motor end plate. Acetylcholine binding generates an action potential across the sarcolemma It involves the influx of sodium ions, which makes the membrane potential slightly less negative. Excitation-contraction coupling sequence of events by which an action potential on the sarcolemma results in the sliding of the myofilaments.

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27 Muscle Metabolism ATP production Aerobic (using air, oxygen) Mitochondria absorb oxygen, ADP, phosphate and organic substances. In a series of chemical reactions, energy is released and used to produce large amounts of ATP Anaerobic (not using oxygen) Glycolysis: breakdown of glucose into 2 ATP and 2 pyruvic acid molecules If this process goes on too long, the acid leads to pain sensation

28 Muscle Metabolism At rest: production of ATP exceeds requirement ATP + creatine ADP + creatine phosphate Creatine Phosphate is a high-energy compound When ATP is required, the enzyme creatine phosphokinase can reverse this reaction.

29 Muscle Fiber Types Slow oxidative (long distance running) High myoglobin, many mitochondria and capillaries, fatigue resistant, slow contraction velocity Fast oxidative (middle distance running) High myoglobin, many mitochondria and capillaries, (less) fatigue resistant, high contraction velocity Fast glycolytic (sprinting) Low myoglobin, few mitochondria and capillaries, much glycogen, fatigue easily

30 Isotonic Contraction Muscle changes in length and moves a load

31 Isometric Contraction The load is greater than the tension the muscle is able to develop Tension increases, but the muscle neither shortens nor lengthens

32 Contraction of a Skeletal Muscle A motor unit consists of a motor neuron and all the muscle fibers it innervates. Small motor units: fine control of movement Muscle fibers from a motor unit are spread throughout the muscle so that a single motor unit causes weak contraction of entire muscle

33 Muscle Twitch The response of a muscle to a single action potential The strength of twitch depends on the number of motor units activated Three phases of a twitch: Latent period: events of excitation-contraction coupling Period of contraction: cross bridge formation; tension increases Period of relaxation: Ca 2+ reentry into the SR; tension declines to zero

34 Latent period Period of contraction Period of relaxation Single stimulus (a) Myogram showing the three phases of an isometric twitch Figure 9.14a

35 Graded Muscle Responses Variations in the degree of muscle contraction Required for proper control of skeletal movement Responses are graded by: 1. Changing the frequency of stimulation 2. Changing the strength of the stimulus

36 Graded Muscle Responses Frequency

37 Graded Muscle Responses Stimulus Strength

38 Force of Muscle Contration As the number of muscle fibers stimulated increases, force of contraction increases. Large muscle fibers generate more force than smaller muscle fibers. As the rate of stimulation increases, contractions sum up, ultimately producing tetanus and generating more force. There is an optimal length-tension relationship when the muscle is slightly stretched and there is slight overlap between the myofibrils.

39 Length-strength

40 Velocity and Duration of Contraction Dependent on: Load As load increases, the slower the velocity and shorter the duration of contraction. Recruitment Recruitment of additional motor units increases velocity and duration of contraction. Muscle Fiber type

41 Effect of Exercise Aerobic (endurance) exercise increase in capillary penetration, the number of mitochondria, and increased synthesis of myoglobin, leading to more efficient metabolism, but no hypertrophy. Resistance exercise increase in the number of mitochondria, myofilaments and myofibrils, and glycogen storage, leading to hypertrophied cells.

42 Smooth Muscle

43 Smooth Muscle Small, spindle-shaped cells with one central nucleus, no connective tissue coverings No striations, no sarcomeres, no troponin and a lower ratio of thick to thin filaments compared with skeletal muscle. Arranged into sheets of opposing fibers, forming a longitudinal layer and a circular layer Peristalsis propels substances through the organs by contraction of the opposing layers

44 Peristalsis

45 Contraction of Smooth Muscle Slow and synchronized due to gap junction electrical coupling Actin and Myosin sliding filament mechainsm Calcium is the trigger Enters the cell, binds to calmodulin and activates myosin light chain kinase. ATP is the source of energy Requires less energy than skeletal muscle contraction

46 Regulation Nervous Autonomic By neurotransmitters acetylcholine or norepinephrine Stimulating or inhibiting Endocrine/Metabolic Hormones Local factors: lack of oxygen, histamine, excess carbon dioxide, or low ph

47 Smooth Muscle Contraction Smooth muscle contracts briefly in response to stretch, then cells relax to accommodate. Not only hypertrophy (increase in individual cell size) but also hyperplasia (increase in cell number) is possible.

48 Types of Smooth Muscle Single-unit smooth muscle (visceral muscle) most common contracts rhythmically as a unit electrically coupled by gap junctions spontaneous action potentials Multiunit smooth muscle large airways to the lungs, large arteries, arrector pili muscles in hair follicles, the iris of the eye cells are structurally independent of each other has motor units capable of graded contractions.

49 End of Chapter Nine