Muscle contraction : lectures notes

 

Steps of contraction:

 

1. When action potential reach the end of motor neuron (nerve terminal) in the neuromuscular junction. The neurotransmitter released and will elicit action potential in muscle fiber.

2. Action potential spread all muscle cell membrane, in addition, to deepest parts of muscle fiber by special network called (T – tubules), which is extension of cell membrane to the inside and contain extracellular fluid.

3. Action potential will cause release of Ca+2 from it’s stores (Sarcoplasmic reticulum) and especially in lateral sacs, and Ca+2 will diffuse in intracellular (cytosol) fluid to contractile filaments [myosin & Actin]

4. Ca+2 bind Troponin C [Troponin present in Actin filaments], and will result in conformational changes in Troponin molecule that result in pulling of (Tropomyosin) away from active sites of Actin (myosin binding sites on Actin).

5. When active site of Actin is exposed, it will react with active site of myosin and they will attach to each other.

6. After attachment, ATP energy is released [by hydrolyzed ATP] and this energy will make the cross bridges of myosin bend forward, toward the center of Sarcomere, by this, pulling Actin filaments toward center of Sarcomere [as to say, only Actin move, while myosin is fixed].

Notice

ATP has already bind to myosin head and splitted to ADP and Pi (but the energy has stored in myosin head waiting for binding between myosin and Actin to occur, so that the energy released for doing movement of the head of myosin (power stroke)

7. This movement of cross bridges, pulling Actin filaments that caused by energy released from hydrolyzed ATP is called (power stroke).

8. Up to now movement is achieved (shortening), now, if Ca+2 still available [as to say there is action potential] then the process will continue as follow; after bending completed, second ATP will bind to myosin head (ATP site), after myosin release the ADP and Pi, this binding will result in detachment of cross bridge from Actin active site and cross bridge will return back to it’s position, facing another Actin active site (which is next to the first) and again; attachment, bending by released energy from hydrolyzes ATP, pulling of action filament toward center, and then, by binding another ATP, returning of cross bridge to normal position, will face another Actin active site and so on….

9. If Ca+2 is no more available [no action potential], which is reabsorbed by Sarcoplasmic reticulum by active pumping of Ca+2 to inside of reticulum, then the Troponin will loose it’s Ca+2 then conformational changes return to previous shape, by this, making Tropomyosin again cover the active sites of Actin and no more exposure of active sites between Actin and myosin, no more shortening will happen, and length of Sarcomere will return to it’s previous length (before contraction) by the force of elasticity of muscle. And whole this process from the detachment of cross bridge (by another ATP), pumping of Ca+2 back to reticulum, and recovering of active sites of Actin … all are called relaxation.

* Look at the following figures

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Important notes:

1. When cross bridge bind Actin (active site), the Myosin head (cross bridge) has already hydrolyzed ATP to (ADP) and (Pi) before attachment, and energy (from this hydrolysis) is stored in Myosin cross bridge, this hydrolysis of ATP (to ADP and Pi) and storage of energy in Myosin head is called … Energized cross bridge (this stored energy wait for attachment between Actin and myosin to occur so that released and power stroke will happen)

2. After bending of cross bridge (by energy stored in Myosin head), then the cross bridge will release (ADP) and (Pi), to be ready to receive another ATP [as to say, ADP, Pi are released during power stroke].

3. Imagine if there is No another ATP, as to say detachment will not occur, the muscle will remain contracted; this is called rigor complex or rigor mortis as happen in death.

4. During contraction: A- band remain constant, I- band will shorten, H- zone will shorten and may disappear, and the whole Sarcomere will shorten in length in comparison with Sarcomere at rest.

5. Please notice that energy (ATP) is also needed to cross bridge so that can detach from Actin.

6. Sarcolemma = cell membrane of muscle cell

Length of muscle or (Sarcomere) – Strength relationship: مهم

· When we talk about length of Sarcomere, we are talking indirectly about the degree of interdigitation (overlap) between Actin & myosin filaments.

· When we stretch the muscle (or Sarcomere) [i.e. increasing the length], we are making less overlap between Actin & myosin, and when we decrease the length, we are making more overlap between Actin and myosin, and further decrease in length will make Actin from one side reach Actin of another side and they may overlap on each other instead of  overlapping with myosin.

· We mean by strength as a force or tension of contraction.

· If we increased the length to the degree that there is no overlap, so the strength (tension) will be zero (point C in following figure).

· Each muscle is created ((in optimal length, as to say, the length that produce maximum strength, if this length changed (increased or decreased) the strength will decrease.

So, we have 2 type relationships regarding length-strength relationship:

1.Direct relationship:  when the length of muscle is less than optimal (i.e. there is overlap between Actin and Actin), in this case, increase in length will produce increase in strength until we reach optimal length where maximum strength is achieved. (Look at point D in following figure).

2.inverse relationship:  when the length of muscle is longer than optimal length (there is few overlap between Actin & Myosin), in this case, decreasing length of muscle will produce increase in strength (Tension) and that is (reverse relationship)… (Look at point B in following figure)

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*We conclude from figure, that 100% of muscle length is optimal length (Lo),and between 70% -100% length the relationship would be direct , and between 100% – 170% the relationship would be inverse proportional

 

Notes:

1) Above information is applied in heart failure (failure of pumping properly)

Explanation: Some conditions will make heart enlarge , as to say Sarcomere is longer than optimal length, and by this the overlap between Myosin and Actin is less, in this case, the force of contraction of heart is reduced (reduced pumping), and as result the heart will fail pumping properly [heart failure].

2) The relationship between Length and Strength in heart is called… Frank – Starling Law

 

Mechanics of Muscle contraction:

· When single action potential reaches muscle fiber it will produce single muscle contraction followed by relaxation, this contraction is called (muscle twitch); this muscle twitch has some tension.

· The whole events of contraction and relaxation in a skeletal muscle fiber take time, which is about 100 msec.

· Contraction will occur after some period from the arrival of action potential to the neuromuscular junction, and this period is called (latent period)

· Remember that action potential in nerves has duration of 1-2 msec. and after they finish (complete action potential) contraction will occur after latent period, and contraction will last much longer than action potential (100msec) look at following figure:

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· If there are multiple action potentials (separated by time) it will be several twitches produced (which are separated) and have same tension.

· If the action potentials come to muscles fiber very successive as to say one action potential after each refractory period of the previous action potential, what will happen for the muscle twitches??

ANSWER: before the first twitch finish there will be another twitch (come from another action potential), the second twitch tension will be added to first twitch tension (before it finish) and so on… for third and forth and fifth… twitches… until the total tension reach the maximum value which is called (Tetanus)

So the tetanus is maximum tension produced by the muscle in response to frequency of stimuli which is successive enough so that the twitches are added to each other and no time for each twitch to finish, but the second twitch come on the top of first and added to it.

What we conclude ??

ANSWER: we conclude that there was summation in muscle twitches, one twitch added to another before if finish it’s contraction, and notice (please) that we can add another twitch tension to the previous twitch tension not like action potentials (we cannot), as to say there is No refractory period in adding muscle twitch tensions [because they are mechanics not electricity] look at following figure.

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So, the Tetanus is maximum force or tension produced by muscle fiber in response to rapid, successive stimulations (frequency of action Potentials) and no time is given for relaxation between contractions.

 

Muscle fatigue:

When muscle are in sustain contraction (Tetanus) it will takes few minute (1-5), then the tension will decrease until ceases ….Why?

ANSWER: for one or two of following reasons:

Causes of fatigue:

1.Decrease ATP that is available in muscle fiber, so that, Myosin head can not further attach and detach with Actin.

2.Decrease or depletion of ACH from neuromuscular junction, because rapid stimulation through neuromuscular junction gives No time for re-building new ACH.

As summery:

 

How we can increase the force of contraction or (the conditions that produce maximum tension) ?

1.Increasing number of motor units that are working (Increasing number of contracting muscle fibers.

2.Increasing length of Sarcomereuntil reach optimal length.

3.Producing Tetanus state by (Giving rapid, repetitive stimulations (frequent action potentials) to muscle so that we can have summation of twitches tension to produce maximum sustained tension.

4.Producing Isometric contraction (as to say maximum contraction by the muscle without change in length or without shortening), in this case there is maximum tension or force produced from the muscle.

5.Increase the number of myofibrils in each muscle fiber or number of muscle fibers (as the athletics do )

Nerve Fiber Types:

Regarding the size of fibers, they are classified to large and small fibers regarding diameter, they are as large as 20 micrometer and as small as 0.3 micrometer in diameter.

1. The largest is called (A) type, and the smaller than A is called (B) type and the smallest of both is called (C).

2. Type A has subtypes they are; alpha, Beta, gamma, delta (and they are from large to small…alpha is largest)

3. When the diameter of fiber increase, the velocity of conduction increase.

4. Type (A) has largest diameter about (12-20Mm) and thus largest velocity about 150m / second.

5. Type B less than A about 3Mm in diameter and velocity of conduction about 15m /second.

6. Type C about (1Mm) diameter and velocity about 1m /second is the slowest of all.

 

 

Examples of fiber type functions:

1.A/ Alpha = conduct proprioception sensation, somatic motor

Proprioception is sensation of position in space, our normal position is vertical on ground, if it changed for example when we about to fall, proprioception nerves will send information to brain (very fast) so that give order to correct position

 

2. A/ beta = conduct Touch, pressure sensations.

3. A/ Gamma = motor to muscle spindle.

4. A/ Delta = conduct pain, Temperature, Touch sensations

5. B type = preganglionic autonomic.

6. C type that found in (dorsal root) = pain, reflex responses

That found in (sympathetic) = postganglionic sympathetic.clip_image014

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