The muscle
The muscles are excitable cells; they are machines to convert the chemical energy to mechanical energy.It differs from the nervous system by the fact that it has a contractile mechanism which is activated by A.p. The muscle can be excited electrically, mechanically, chemically → action potential (A.p.).
Types of muscle: Skeletal muscles: These are voluntary muscles attach to bone. Smooth muscles: Involuntary muscle. It is Muscle of the viscera (e.g., in walls of blood vessels, intestine, & other 'hollow' structures and organs in the body). Cardiac muscles: Muscle of the heart. Involuntary. 40% of the body is skeletal muscles and 10% are smooth and cardiac muscles.
Characteristics of muscle: 1-excitability - responds to stimuli (e.g., nervous impulses) 2-contractility - able to shorten in length 3-extensibility - stretches when pulled 4-elasticity - tends to return to original shape & length after contraction or extension Functions of muscle: 1-motion 2-maintenance of posture 3-heat production
The skeletal muscle: It is that type of the muscles that is attached to bones & moves skeleton, also called striated muscle.
It lacks anatomical and functional connection between individual muscle fibers. It is voluntary.
Morphology:
It is composed of numerous fibers. Each muscle fiber extends along the length of .the muscle. The muscle fibers are arranged in parallel between the two tendon ends, so that the force of contraction is additive.The striations
The myosin and actin filaments interdigitate and cause the myofibrils to have alternate light and dark bands. The light bands are only Actin filaments called I .bands. The dark bands contain Myosin called A bands, .overlapping with Actin filaments So the striations are due to difference in the refractive index of the parts of the muscle fibers.-In the middle of The I band there is darker Z line. The portion of the myofibrils that is between 2 successive Z discs is called Sarcomere which is the smallest functional unit of the muscle . -The A band is divided by the lighter H band, in the middle of it a transverse line called M line.
Molecular charectristics of the contractile filaments
Myosin filaments: It is composed of 6 polypeptide chains, a/ 2 heavy (wrap spirally around each other to form double helix called tail, one end of these chains is folded bilaterally into a globular polypeptide structure called head (2 heads) with 2N terminals, they contain actin binding sites and a catalytic site that hydrolyse ATP and, b/ 4 light chains (are parts of the head (help control the function of the head during contraction).Part of the body + the head extends to form arms (called the cross bridges). The cross bridges are flexible at 2 points one where the arm leaves the body, the other where the head attaches to the arm, these called hinges.
The thin filament is made of actin ,torponin and tropomyosin.
Actin molecules: Actin filament is made up of 2 chains of globular unit that form a long double helix and contain binding sites for myosin. Each strand is composed of polymerized G actin molecule ,attached to each one molecule of ADP , these are the active sites with which the cross bridges of myosin interact.:Tropmyosin molecule They are located in the groove between the two chains forming long filaments overlying the binding sits of myosin. So in the resting state they lie on the top of the active sites of the actin strands so no attraction between actin and myosin.
Troponin It is a protein attached intermittently at regular intervals along the sides of tropomyosin molecules. It is a complex of 3 loosely bound protein subunits: Troponin I has a strong affinity for actin inhibits the interaction between myosin to actin
The sarcotubular system:
The T system (transverse tubules): it is a system of transverse tubules in the form of letter T which is continuous with the membrane of the muscle fiber. It starts from one side of the cell membrane to the opposite side, so it is continuous with the extracellular space, and they contain extracellular fluid inside ; they are present along the whole length of the muscle fiber and is responsible for spreading of action potential from the cell membrane to the interior of the muscle fiber, the electrical currents around them create the muscle contraction.The sarcoplasmic reticulum : it forms an irregular system of tubules surrounding the myofibrils it has an enlarging ends or chambers called terminal cisterns in close contact with the T system at the junction between A and I bands. The arrangement of the T system with the ciatern of the endoplasmic reticulum at either side called Traid
Electrical characteristics of skeletal muscles:
1- The resting membrane potential is – 80 to – 90 mill volt in skeletal muscle fiber (same as in large mylinated nerve fiber).2- The electrical changes of the ion fluxes are similar to those of the nerve fiber during action potential. 3- Duration of the action potential is 1 to 5 milliseconds (5 times longer than that in mylinated nerve fiber). 4- The conduction velocity is 3 to 5 m/ second (less than that in large mylinated nerve fiber).5-Due to the slight difference in the threshold between muscle fibers of the same muscle and the difference in the distance between the stimulation site and different muscle fibers, the action potential recorded from the whole muscle after direct stimulation is proportional to the intensity of the stimulus between threshold and maximum intensity (do not obey all or none law).6- Each single contraction is followed by a single relaxation in response to a single action potential (simple muscle twitch).
Excitation contraction coupling The process by which depolarization of the muscle fiber initiates contraction is called excitation- contraction coupling.
1- Sliding filament theory: 1 – The discharge of motor neuron.2- An action potential travels along the motor nerve to its ending in the muscle fiber.3- Secretion of small amounts of neurotransmitter substance Acetylcholine (Ach) at the motor end plate.4-Ach binds to nicotinic receptors on muscle fiber membrane to open Ach gated channels.
5- Increase in Na and K ions conductance (Na ions diffuse to the interior of the muscle fiber membrane) and this will initiate a local end plate potential, and when firing level is reached, action potential is generated and spread along the whole muscle fiber. 6- The inwards spread of the action potential by the T system of tubules. 7- Release of calcium ions from the terminal cisterns of the sarcoplasmic reticulum. 8-Calcium will bind to Troponin C molecule this will lead to conformational changes:
The binding of Troponin I to actin will be weakened.This allows Tropomyosin to move laterally outside the groove and uncover the binding sites for the myosin heads.So Ca ions will act as an inhibitory factor on troponin –tropomyosin attachment to actin. The formation of cross bridges between actin and myosin heads → sliding of thin on thick filaments producing shortening (the sarcomere will be shortened).
So during muscle contraction 1- the Z lines move closer to each other, 2- the I band becomes shorter and 3- the A band stays at the same length.
2- The walk- along or Rachet theory of contraction: This theory suggests that the sliding during muscle contraction is produced by attaching, breaking and reforming of the cross linkages between actin and myosin heads, the intensity of the interaction depends on the number of cross linkages .
-After uncovering of the active sites of the actin. - Myosin head link to actin at 90 degrees angle, the head tilts towards the arm to drag the actin filament along with it, - Producing movement by swiveling(pulling). - Then disconnect(breaks away from the active site, then the head returns to its extended direction , then it combines with a new active site farther down along the actin filament and the process is repeated).
Steps in relaxation: 1- After a fraction of a second, the calcium ions are pumped actively back into the sarcoplasmic reticulum by a Calcium membrane pump, they are going to diffuse into the terminal cisterns to be released by the next action potential. 2- The release of calcium ions from Troponin C, 3- Then cessation of binding between actin and myosin (i.e. tropomyosin returns to its site. 4- Contraction stops.
Types of contraction: 1- Isomertic contraction: is when the muscle does not shorten during contraction i.e. no change in muscle length, but the tension will increase. e.g. trying to lift a heavy object. The work done here is zero, because no movement.
2- Isotonic contraction: It is the contraction that causes shortening of the muscle length and the muscle has the same tension. e.g. lifting an object by contracting the biceps muscle. Here there is work done because there is movement. The muscle shortens against a fixed load, and its characteristics depends on the load against which the muscle contracts and on the inertia of the load.
The summation of contraction
It means the adding together of individual twitch contractions to increase the intensity of overall contraction. Because the contractile mechanism has no refractory period. Repeated stimulation, before relaxation can produce additional activation of the contractile elements . The response will be added to that already present, this is called "summation of contraction".It depends on the frequency of stimulation. It occurs in 2 ways:
1 – Multiple fiber summation (increasing the number of motor units contracting at the same time).2- Frequency summation and tetanization, with rapid repeated stimulation, before any relaxation occurs and the response fuses into one continuous contraction and the whole contraction appears to be smooth called Tetanus (by increasing the frequency of contraction).During tetanus the tension developed is 4 times than the individual contraction.
At slightly higher frequencies, the strength of the muscle contraction reaches maximum, so any additional increase in frequency beyond that point has no further effect in increasing the force of contraction, This is because enough Ca ions are maintained in the sarcoplasm ,even between action potentials so will not allow relaxation to happen. But if a lower frequency is used, there will be a period of incomplete relaxation between the summated stimuli; this condition is called incomplete tetanization or clonus.3- The CNS sends weak signals, so smaller motor units contract than the larger ones, the strength will increase, larger and larger motor units will be excited (large motor units have more contractile force than the smaller ones).
Effect of muscle length on the force of contraction:
The tension develops in a muscle when it is stimulated to contract isometrically; this is called the total tension. When the length of the unstimulated muscle fiber is changed this is called passive tension..Passive tension curve is the curve plotted to include the changes in tension against the changes in the muscle length. It shows that, as the length increases, it rises sharply. The total tension curve shows the total tension against the passive tension, we will see a sharp increase up to a maximum value, then the curve declines. If we measure the distance between the two curves we will see the active tension curve which is similar in shape with the total tension curve but has a lower peak.
Energy sources and metabolism: Contraction of the muscle depends on energy supplied by ATP. In general the source of energy is the metabolism of carbohydrates and lipids.
Most of the energy is required for physical activity (contraction, relaxation) or walk along mechanism, and small amounts are required for: 1- Pumping of Ca ions from the sarcoplasm to the sarcoplasmic reticulum after the contraction is over. 2- Regeneration of ATP. ATP split to ADP, then ADP rephosphorlated to ATP. 3- Removal of lactic acid. 4- Heat production.
Sources of energy for the rephosphorlation:
1- Substance called phosphocreatine (high energy phosphate bond).ADP+P→ATPThis compound supplies energy to the muscle at rest .During exercise this compound hydrolyse at the junction between actin and myosin releasing energy ,this reaction is catalyzed by the enzyme phosphorylcreatinine at the mitochondria and myosin heads.
2- Glucose: it is supplied by the blood and undergoes series of reactions forming finally Co2, H2o and Energy. 3- Glycolysis of glycogen stored in the muscle cells: enzymatic break down of glycogen to pyruvic which has 2 pathways in the presence of O2, it enters citric acid cycle (crips cycle), then the respiratory chain to form co2, H2O and large amount of energy, (aerobic Glycolysis). But in the absence of O2(like in prolong contraction), pyruvate is reduced to lactate and lactic acid and small amount of energy. 4- The free fatty acids(FFA) (gives double the energy that glucose gives) skeletal muscles take the FFA in the blood and oxidized to give Co2, H2o and ATP (the use of FFA mainly at rest and during recovery after contraction
The oxygen debt mechanism: During exercise the blood vessels dilates to provide enough O 2 for the muscle, the energy is supplied by aerobic glycolysis.
If the exercise is sever or continues for longer periods, the anaerobic glycolysis contribute also to provide energy. It is self limiting ,because lactic acid will diffuse to the blood it will lower the PH ,accumulate in the muscle causing muscle exhaustion ,
So after the exercise there will be a period of hyperventilation to produce the extra amount of O2 in order to remove the lactic acid and to rebuild the storage of ATP and phosphorylcreatinin . This extra amount of O2 taken to replace the demand required more than that supplied by the aerobic glycolysis during exercise is called the oxygen debt .mechanism
Trained persons need less period of hyperventilation because they have smaller oxygen debt mechanism (i.e. he has endurance, he can use the muscle to perform the job better).
Heat production in the muscle:
This is produced as: 1- Resting heat: liberated during resting stage. 2- Initial heat which is produced during activity and it is divided into:a- Activation heat: produced during contraction, from the actin –myosin interaction.b - Shortening heat: produced during shortening (isotonic contraction), due to the changes in muscle fiber structure during shortening.In isotonic contraction there are both activation and shortening heat but in isometric there is only activation heat.3- Recovery heat: liberated by the metabolic processes that restore the muscle fiber to its precontraction state and it is rather equal to the initial heat. 4- Relaxation heat: liberated because work should be done to return the muscle to its original length (after isotonic contraction) in addition to the recovery heat.
Types of muscle fibers
3 types according to the differences in enzyme activity, metabolism and contractile properties: 1- Type I fibers: these are darker than other muscles called Red muscles or the slow fibers, they response slowly and have longer duration of action(resist fatigue). They are specialized for long slow sustained contraction, supplied by slow conducting fibers. e.g. muscles in the back and in the lower limbs which are used to maintain posture. They are small fibers, have more extensive blood supply to supply high oxygen, and high number of mitochondria and large number of myoglobulin.The motor units: The motor unit means all the muscle fibers innervated by a single nerve fiber .i.e. the axon of a single motor neuron divides to supply many muscle fibers. There are 2 types of motor units: 1- Small motor units: contain 3-6 muscle fibers, concerned with fine graded, precise movement, like movements of the hand. They are Small muscles that react rapidly and whose control must be exact have more nerve fibers for fewer muscle fibers (2-3 muscle fibers for each motor unit, e.g. laryngeal muscles).
2- large motor units :contain usually 120-165 muscle fibers ,like muscles of the back ,for the sustained form of activity. These are large muscles that do not need fine control (e.g. soleus muscle), may have several hundreds of muscle fibers in the motor unit. Each motor unit is of one type i.e. innervates one type of muscle, but when a nerve to slow muscle is cut and replaced by a nerve to fast muscle ,the slow muscle after a period of time becomes fast.
The muscle fibers in each motor unit overlap other motor units in microbundles of 3-15 fibers. This interdigitation allows the separate motor units o contract in support of one another rather than as individual segment.