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Chapter 12 Muscles Part 3 For Monday, start with slide # 26 Exam 3 will be on Monday November 21 Will cover chapters 11, 12, 13 May cover more (depends on how far we get)

Figure 12-7

Excitation-Contraction (E- C) Coupling 1. Acetylcholine (ACh) is released from the somatic motor neuron 2. ACh initiates an action potential in the muscle fiber 3. The muscle AP triggers calcium ion release from the sarcoplasmic reticulum 4. Calcium ion combines with troponin and initiates contraction

Figure 12-11a, part 1

1. Acetylcholine (ACh) is released from the somatic motor neuron – ACh binds to the ACh receptor-channels on the motor end plate of the muscle fiber – When the channels open, Na+ and K+ enter the channels. Na+ influx exceeds K+ efflux – Na+ influx depolarizes the membrane, creating an end-plate potential (EPP) – EPPs always reach threshold and always initiate an AP

Figure 12-11a, overview

2. ACh initiates an action potential in the muscle fiber – The AP is conducted across the surface of the muscle fiber and into the T-tubules – This is accomplished by the sequential opening of voltage-gated Na+ channels – Similar to the AP conduction in an axon, but a lot slower in skeletal muscle

3. The muscle AP triggers calcium ion release from the sarcoplasmic reticulum – Free cystolic Ca++ levels in a resting muscle are normally quite low – After an AP, Ca++ levels increase 100 fold – When cystolic Ca++ levels are high, Ca++ binds to troponin, tropomyosin moves to the “on” position, and contraction occurs

Figure 12-11b, overview

Molecular Events Transduction of the electrical signal into a calcium signal requires 2 different membrane proteins – DHP receptor – RyR receptor

Molecular Events DHP (Dihydropyridine) Receptor – Found in T-tubule membrane – It is a voltage-sensing L-type calcium channel – The DHP receptors do not form open channels – Rather, they are mechanically linked to Ca++ release channels in the sarcoplasmic reticulum RyR (ryanodine) receptors – These are the Ca++ release channels

Molecular Events When the depolarization of the AP reaches the DHP receptor, the receptor changes conformation (shape) This change opens the RyR Ca++ release channels in the sarcoplasmic reticulum Stored Ca++ ions then flow down their electrochemical gradient into the cytosol where the Ca++ ions initiate contraction

Figure 12-11b, overview

Molecular Events To end the contraction, the sarcoplasmic reticulum pumps Ca++ ions back into its lumen, using a Ca++ ATPase As the free Ca++ ion concentration decreases, Ca++ releases from troponin, tropomyosin slides back into place and blocks actin's myosin-binding site As crossbridges release, the muscle fiber relaxes

Molecular Events The discovery that Ca++ and not the AP is the signal for contraction led to lots of research on Ca++ Ca++ was once thought to only be a signalling molecule in muscle cells, but now has been found to be an almost universally used second messenger molecule

Timing of events in E-C coupling (Fig. 12-12, p. 419) Somatic motor neuron AP arrives (1 st graph) Skeletal muscle AP ( 2 nd graph) Muscle Contraction (3 rd – Latent period • graph) Short delay between muscle AP and beginning of muscle tension development • Delay represents timing for the E-C coupling to occur – Twitch: a single contraction-relaxation cycle in skeletal muscle

Figure 12-12

Muscle Contraction-Relaxation Cycle 3 rd graph Once contraction begins, muscle tension increase steadily up to a maximum value as crossbridge interaction increases Tension then decreases (relaxation phase) During relaxation, elastic elements of the muscle return the sarcomeres to their resting length

Figure 12-12

Muscle Contraction-Relaxation Cycle 3 rd graph A single AP evokes a single twitch The speed with which a muscle twitch develops tension (rising slope of curve, bottom graph), the maximum tension (height of the curve), and the twitch duration (width of the curve) all vary, depending on the muscle fiber

Figure 12-12

Skeletal Muscle and ATP Requirements (p. 420-421) The amount of ATP in a muscle fiber at any one time is sufficient for approximately 8 muscle twitches Phosphocreatine – Backup energy source in muscle – Has high-energy phosphate bonds – These are made from creatine + ATP when muscle is resting

Phosphocreatine – When muscles are active, the high-energy P is transferred to an ADP molecule to make more ATP Creatine kinase (phosphokinase) – The enzyme used to make the above transfer – Muscle cells contain lots of this enzyme – Elevated blood levels of this enzyme indicate muscle damage (can differentiate between skeletal and cardiac muscle damage)

Figure 12-13

Carbohydrates, particularly glucose, are the most rapid and efficient source of energy for ATP production Glucose gets metabolized to pyruvate through glycolysis When adequate oxygen is present, then pyruvate is further broken down in the citric acid cycle, yielding 30 ATP per molecule of glucose

During strenuous exercise, when oxygen levels fall in the muscle tissue, glucose is metabolized to lactate (via fermentation), and yields only 2 ATP per molecule of glucose Fatigue – When muscle energy demands outpace the available ATP – Most studies show that, even intense exercise only uses 30% of the muscle ATP

Fatigue (fig. 12-14, p. 421) Central Fatigue – Psychological effects – – Subjective feelings of tiredness, etc.

May be a protective mechanism • Psychological fatigue precedes muscle fatigue Peripheral Fatigue – May be at any level from neuromuscular junction to muscle contractile elements – Most studies indicate that this type of fatigue usually has to do with excitation-contraction failure (E-C coupling)

Figure 12-14

Muscle Fiber Types Slow-Twitch Oxidative (ST or type I) – Red muscle (lots of myoglobin) – Slowest speed Fast-Twitch Oxidative-Glycolytic (FOG or type IIA) – Red Muscle (lots of myoglobin) – Intermediate speed Fast-Twitch Glycolytic (FG or type IIB) White Muscle (not much myoglobin) Fastest speed

Muscle Fiber Types Myoglobin: red O 2 – binding pigment The more myoglobin that is present, the faster O 2 can get transported into the muscle fiber Human muscles are a mixture of fiber types Ratio of the types in an individual muscle will vary from muscle to muscle and also from one individual to another

Muscle Fiber Types: speed Fast-twitch muscle fibers (type II) develop tension 2-3 times faster than the slow-twitch fibers The speed is determined by the isoform of myosin ATPase present in the thick filament Fast-twitch fibers can split ATP very rapidly and therefore can complete multiple contractile cycles more rapidly than slow-twitch fibers

Muscle Fiber Types: Duration Duration of contraction – Varies according to fiber type – Determined by how fast Ca++ is removed from the cytosol – Sarcoplasmic reticulum removes the Ca++ Fast-twitch fibers – Pump Ca++ back into the sarcoplasmic reticulum faster than slow-twitch fibers do

Muscle Fiber Types: Duration Fast-twitch fibers – Twitches last approximately 7.5 msec – – Useful for fine, quick movements Playing the piano, etc.

Slow-twitch fibers – Twitches may last up to 10 times longer than slow twitch – These muscles are used almost constantly for maintaining posture, standing, walking, etc.

Muscle Fiber Types: Fatigue Resistance Glycolytic fibers (fast-twitch type IIB) – Rely primarily on anaerobic glycolysis (fermentation) for ATP – H+ accumulation from this contributes to acidosis (extracellular pH less than 7.38) – Acidosis contributes to development of fatigue – Glycolytic fibers fatigue more easily than oxidative fibers

Oxidative fibers (slow-twitch, fast-twitch type IIB) – Rely primarily on oxidative phosphorylation for ATP – Have more mitochondria than glycolytic fibers do – Have more blood vessels than glycolytic fibers also – Have lots more myoglobin (helps oxygen diffuse into the muscle fiber) – Have a smaller fiber diameter

Oxidative fibers (slow-twitch, fast-twitch type IIB) compared to the glycolytic fibers – Oxidative fibers maintain a better supply of O 2 the other type because of • Smaller diameter, shorter distance for O 2 to diffuse than • Presence of more myoglobin molecules which can then transport more O 2 • More capillaries to bring in O 2 and nutrients

Glycolytic fibers compared to Oxidative fibers – White muscle, due to lots less myoglobin – Muscle fibers of glycolytic type are larger in diameter – Fewer blood vessels present in the glycolytic fibers – Glycolytic fibers are more likely to run out of O 2 after repeated contractions – Also, they fatigue more rapidly than the other type

Fast-twitch oxidative-glycolytic fibers These have properties of both oxidative and glycolytic fibers: – Smaller than fast-twitch glycolytic fibers – Use both oxidative and glycolytic metabolism to produce ATP – More fatigue-resistant than the fast-twitch glycolytic fibers

Figure 12-15

Table 12-2

Muscle Tension and Sarcomere Length In a muscle fiber, the tension developed during a twitch is a direct reflection of the length of the individual sarcomere before contraction Sarcomere optimum length: neither too short or too long Normal resting length of skeletal muscles usually has sarcomeres at their optimum length before a contraction

Figure 12-8

Muscle Tension and Sarcomere Length At the molecular level, the tension a muscle fiber can generate is directly proportional to the number of crossbridges formed between the thick and thin filaments If the fibers start a contraction at a very long sarcomere length (fig. 12-16e), then the thick and thin filaments barely overlap and very few crossbridges can form The sliding filaments would only have minimal interaction and could not generate much force

Figure 12-16

Muscle Tension and Sarcomere Length At optimum sarcomere length (fig. 12-16c), the filaments would be able to form lots of crossbridges, generating optimum force If the sarcomere is shorter than optimum length (fig. 12-16b), there is too much overlap between thick and thin filaments The thick filaments can move the thin filaments only a short distance before the thin filaments from opposite ends of the sarcomere start to overlap

Muscle Tension and Sarcomere Length If the sarcomere is so short that the thick filaments run in to the Z-disks (fig. 12-16A), then myosin can't find binding sites for crossbridge formation and muscle tension would decrease very rapidly Summary: Development of single-twitch tension in a muscle fiber is a passive property that depends on both filament overlap and sarcomere length

Figure 12-16

Summation of Muscle Twitches The force generated by the contraction of a single muscle fiber can be increased by increasing the rate (frequency) at which muscle action potentials stimulate the muscle fiber Typical muscle action potential lasts 1-3 msec A muscle contraction may last 100 msec Tension: force created by a contracting muscle

Figure 12-17a

Figure 12-17b

Tetanus If action potentials continue to stimulate the muscle fiber repeatedly at short intervals (high frequency), relaxation between contractions diminishes until the muscle fiber reaches a state of maximal contraction or tetanus Incomplete (unfused) tetanus – Stimulation rate not maximal, fiber relaxes slightly between stimuli Complete (fused) tetanus – No relaxation, reaches maximum tension and remains there

Figure 12-17c

Figure 12-17d

Figure 12-17

Motor Unit Consists of one somatic motor neuron and the muscle fibers it innervates When the motor neuron sends an action potential, all of the fibers in the motor unit contract One somatic motor neuron innervates many muscle fibers, but each individual fiber is only innervated by a single neuron

Figure 12-18

Motor Unit Fine motor actions (eye, hand movements) – Motor unit has few fibers (as few as 3-5) – – Muscle response can be quite small Allows for fine gradations of movement Gross motor actions (standing, walking) – Each motor unit can have hundreds to thousands of fibers – Gastrocnemius (calf muscle) has 2,000 fibers in each motor unit

Motor Unit Within each motor unit, all the muscle fibers are of the same type – Fast-twitch motor unit – Slow-twitch motor unit The neuron controls the type of muscle fiber that develops – During development, each somatic motor neuron secretes a growth factor that directs the differentiation of all the muscle fibers in its motor unit so that they all become the same type

Inheritance and Muscle Type Endurance athletes (distance runners, cross-country skiers) have lots of slow-twitch fibers Sprinters, hockey players, weightlifters have a lot of fast-twitch fibers Athletic training can modify muscle fiber composition because muscle fiber types show some plasticity Endurance training can enhance the aerobic capacity of some fast-twitch muscle fibers until they are almost as fatigue-resistant as slow-twitch fibers

Table 12-2

Endurance Training (continued) The conversion of fast-twitch to slow-twitch fibers occurs only in the muscles being trained Endurance training also increases the number of mitochondria and capillaries, thus increasing the aerobic capacity of the fibers even more High altitude training increases aerobic capacity also by increasing the number of RBCs in the blood This allows the blood to carry more oxygen

Contraction Force and Recruitment Muscles are composed of multiple motor units of different types (fig. 12-18, p. 426) This diversity allows the muscle to vary contraction by – Changing the types of motor units that are active – Changing the number of motor units that are responding at any one time

Figure 12-18

Contraction Force and Recruitment Recruitment – The force of contraction in a skeletal muscle can be increased by recruiting additional motor units – Recruitment is controlled by the nervous system – Proceeds in a standardized manner • A weak stimulus activates only neurons with the lowest thresholds • These control the slow-twitch fibers – Fatigue-resistant – Generate minimal force

As the stimulus increases in strength, additional motor neurons, with higher thresholds begin to fire – These stimulate motor units of fast-twitch oxidative-glycolytic fibers – – Fatigue-resistant Since more motor units are now contracting, the force being generated is greater than before

As the stimulus continues to increase, additional motor neurons, with the highest thresholds, begin to fire – These stimulate motor units of the glycolytic fast twitch fibers – At this point, the muscle contraction is approaching its maximum force

Asynchronous Recruitment The nervous system avoids muscle fatigue in sustained contractions by asynchronous recruitment of motor units This means that the different motor units take turns maintaining muscle tension The alternation of active motor units allows some of the motor units to rest between contractions, thus preventing fatigue

Asynchronous Recruitment This type of recruitment prevents fatigue only in sub maximal contractions In high-tension, sustained contractions, the individual motor units may reach a state of unfused tetanus in which the muscle fibers cycle between contraction and partial relaxation Usually don't notice this cycling since the different motor units are contracting and relaxing at slightly different times

Asynchronous Recruitment The contracting and relaxing average out and seem to be one smooth contraction As different motor units start to fatigue, we become unable to maintain the same amount of tension in the muscle As this happens, the force of the contraction gradually decreases

Table 12-3

Next: Chapter 13 Integrative Physiology I: Control of Body Movement (in the syllabus as Chapter 13: Reflex and Motor Control)

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