Session 3: Physiology of Skeletal Muscle

Physiology of Skeletal Muscle

Dr. Fanny Casado [email protected]

ING338: Human Physiol. for Engineers. 2017-1

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Learning objectives Explain the relationship between muscle

power with load and muscle type to explain how striated muscle converts action potential into mechanical movement.

Use the Hill’s muscle model to explore length-tension energetics and force-velocity relationships.


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Notas de la presentación

Pressure driven flow; : ELECTRICAL FORCE, POTENTIAL, CAPACITANCE AND CURRENT


Movement and heat

Organ pressure

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Muscles either shorten or produce force When physiologists talk about muscle contraction they actually mean the process of muscle activation into what is known as the cross bridge cycle. Activation of the muscles can produce force without shortening, when holding something heavy. But there is a trade-off between shortening and producing force

Type of contraction

Distance change Function Work

Concentric Shortening (+ D) Acceleration + Isometric No change (0 D) Fixation 0 Eccentric Lenghthening (- D) Deceleration -

Power = Energy / time Energy = Work = Force x(distance) Power = (Force x distance)/time

Muscle power varies with the load and muscle type


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Experimental set-up for measuring isometric force in an isolated muscle


Muscle contraction or Twitch

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Fig. 3.4.2.

The motor unit consists of all of the muscle fibers innervated by a single motor neuron


Cerebrum’s motor cortex or Sensory input

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Fig. 3.4.4.

Increase in the muscle twitch with increased recruitment of motor fibers by increasing the strength of the external stimulus


Despite increased voltage, force plateaus

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Fig. 3.4.3.

External electrode stimulation of a nerve

ING338: Human Physiol for Engineers 2017-1

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Fig. 3.4.5.

The muscle force summates with repetitive stimulation


Despite increased frequency, force plateaus

Tetany has been reached

Tetanus/twitch varies with muscle type

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Fig. 3.4.6

Elastic properties: The length-tension relationship in muscle


Relaxed muscles exert no force. When stretched passively, passive force is produced

To reach tetanus

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Fig. 3.4.7

The basis of strength training in exercise physiology Muscle force can vary by:

Recruitment of muscle fibers Variation of the frequency of activation Variation of the muscle length

Skeletal muscle length is hardest to modify because muscles in physiologically state are arranged near the top of their length.


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Measuring the force and velocity of isotonic muscle contractions


1. Isometric contraction 2. Muscle length changes after

enough afterload force has been produced

3. Afterload is lifted at constant velocity: Isotonic contraction

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Fig. 3.4.8

The speed of muscle contraction depends on the load that must be moved


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Fig. 3.4.9

Type of contraction

Distance change Function Work

Concentric Shortening (+ D) Acceleration + Isometric No change (0 D) Fixation 0 Eccentric Lengthening (- D) Deceleration -

Power = energy / time Energy = Work =Force * Distance Power = (Force * Distance)/time

Power = Force * Velocity

Muscle power varies with the load and muscle type


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Power is about two to three times greater in fast-twitch muscle


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Fig. 3.4.10

Slow- vs. fast-twitch muscles


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Slow-twitch Fast-twitch It is aerobic It is anaerobic It needs steady power (lots of mitochondria)

It needs explosive power

It has endurance It fatigues easily Runners use it Weight-lifters use it

Concentric and eccentric contractions


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Fig. 3.4.11

Arrangement of muscle fibers within a muscle


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Fig. 3.4.12


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Activity Muscle cells are long, multinucleated cells that are shaped something like cylinders that taper near the ends. Consider a 10 cm length of a muscle cell that is 50 mm in diameter. Assume that the cell is a right circular cylinder and that the capacitance of the muscle cell membrane is 1 mf cm-2. Neglect the ends of the cylinder and edge effects where the membrane bends at the edges. A. How many charges have to move across the

membrane in order to produce a potential of -80 mV, negative inside?

B. The total concentration of anions (negatively charged ions) in the cytosol is about 150 x 10-3 M, and the total concentration of cations (positively charged ions) is about the same. About how many cation charges are there in the cytosol of the muscle cell of 10 cm length and 50 mm diameter?


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A. How many charges have to move across the membrane in order to produce a potential of -80 mV, negative inside? The total capacitance of the muscle cell is given as C = Cm x A Where Cm is the capacitance per unit area mem brane and A is the area. The number of charges required to move to produce a membrane potential V is given by the constitutive relation for capacitors: Q = C x V So this is why we need to determ ine C. The surface area, A, of the cell is just the area of the right cylinder is A = Length x 2B r + 2 x B r2 The first term is the surface area along the length of the m uscle cell and the second term is the ends, modeling it as if it were a 90° end. Here length = 10 cm and r = 25 x 10-4 cm. Plugging these in, we find Area = 10 cm x 2 x B x 25 x 10-4 cm + 2 x B x (25 x 10-4 cm)2 = 1571 x 10-4 cm2 + 0.39 x 10-4 cm2 Here we can see that the area of the ends of the cell are in fact negligible compared to the surface area of the bulk of the cell. The total capacitance is just C = 1 :f cm-2 x 1571 x 10-4 cm2 = 1571 x 10-10 farad And the charge to make a potential of -80 mV is Q = 1571 x 10-10 coulomb volt-1 x -.080 volt = 1.257 x 10-8 coulomb B. The total concentration of anions (negatively charged ions) in the cytosol is about 150 x 10-3 M, and the total concentration of cations (positively charged ions) is about the same. About how many cation charges are there in the cytosol of the muscle cell of 10 cm length and 50 :m diameter? The total number of cations is given as N = C x V Where N is the number of moles, C is the concentration and V is the volume. The volume of the muscle cell is length x B r2 = 10 cm x B x (25 x 10-4 cm)2 = 19635 x 10-8 cm3 x 10-3 L cm-3 = 1.96x10-7 L The total number of cations is 150 x 10-3 mol L-1 x 1.96 x 10-7 L = 294.5 x 10-10 mol This is converted to coulombs by multiplying by the Faraday = 96,490 coulomb mol-1 Total charge of cations in the cell = 294.5 x 10-10 mol x 96,490 coulomb mol-1 = 2.842 x 10-3 coulomb C. Compare the calculations in part A and part B. The ions that m ove across the membrane to produce the membrane potential of -80 mV are what fraction of the cytosolic cations? Helpful hints: one mole of charge is 9.649 x 104 coulombs. This is the Faraday. The ions that move across the membrane to produce a membrane potential of -80 mV was calculated to be 1.257 x 10-8 coulomb whereas the total intracellular content of cations was 2.842 x 10-3 coulomb. Thus the fraction of the cytosolic ions that are necessary is just the ratio 1.257 x 10-8 / 2.842 x 10-3 = 4.4 x 10 -6 or 0.00044%.

Session 3: Physiology of Skeletal Muscle

Education

Transcript of Session 3: Physiology of Skeletal Muscle