Skeletal muscle contraction

[Audio] Welcome! In this lecture session we will be looking at skeletal muscle contraction. You will remember that muscle is one of the excitable tissue and contraction of skeletal muscle is one important means by which the skeletal muscle respond to stimulus. This property of skeletal muscle is no doubt, importance for various body function and indeed, protection of the entire body..

[Audio] Apart from skeletal muscle, other types of muscles includes cardiac muscle and smooth muscle. Each of this muscles have characteristics peculiar to them. The skeletal muscle for example is a striated muscle and is also a voluntary muscle. The cardiac muscle is also striated but unlike the skeletal muscle is involuntary muscle. Also, smooth muscle is a non-striated and voluntary muscle..

[Audio] These muscles are found in different body parts. For instance the bicep, triceps and postural muscles are examples of skeletal muscle. Smooth muscle on the other hand can be found along digestive tract and some other internal organs of the body. Cardiac muscle is generally known as the muscle of the heart..

[Audio] Am sure you would have seen a muscle before. Well, if not human muscle, you would have seen animal muscle before. A typical example of an animal muscle is the " meat" that you eat. You will agree with me that this muscle (meat) has several strands of muscle fibers that forms it. Indeed, the muscle is a tissue and the fibers that forms the muscle this way are known as muscle fibers. Interestingly, this same arrangement and/or characteristic feature of the animal muscle (meat) that we have considered is similar to what is found human. In other words, the skeletal muscle is generally made up of several muscle fibers. Another important characteristic feature of muscle is that they often end in tendon. The tendon is white form tissue which serve to connect muscles to the skeletal system or bones. Lastly, another important feature of skeletal muscle is that the muscle fibers are arranged in parallel between tendon. This form of arrangement is basically to provide additive force of contraction..

[Audio] Indeed, when we take a look at the skeletal muscle using a microscope, the first observation you will see is the muscle fibers as earlier mentioned. The muscle fibers are actually seen as a single cell which are multinucleated, long and cylindrical. Another interesting thing you would see is that the muscle fibers are often surrounded by a type of fluid which is known as sarcolemma. Furthermore, microscopic view of the skeletal muscle have revealed the presence of two myofibrils which includes A thick filament, and A thin filament The thick filaments are composed of myosin while the thin filament are composed of actin. Together, the myosin and the actin filament forms an important structure known as sarcomere, which is the basic functional or contractile unit of the skeletal muscle..

[Audio] The diagram here illustrate all that have been discussed earlier. As you can see, the muscle (A) is a biceps muscle and is attached to the bone of the body via tendon as shown by the arrow. On the left side, when we view the muscle clearly ( B), we would see the different muscle fibers ( one of it is shown ( C)). Another important thing seen in this diagram is the sarcomere. This shows the arrangement of both the actin and myosin filament. Pls appreciate the structure of the sarcomere..

[Audio] As earlier mentioned, the myofibrils that form the skeletal muscle are arranged side by side and the space in between them is filled with fluid that is known as sarcoplasm. The sarcoplasm is similar to the extracellular fluid that surrounds a cell. It is in this sarcoplasm that the skeletal muscle derives its nutrients and other necessary materials for its function. Some of the content of the sarcoplasm includes: potassium, magnessium, phosphate, protein enzymes, mitochondria, sarcoplasmic reticulum (this is similar to the endoplasmic reticulum in cell). The sarcoplasmic reticulum is known as specialized endoplasmic reticulum of skeletal muscle)..

[Audio] Just like the nerve, the skeletal muscle exhibit electrical phenomenon (i.e. in response to stimulus they generate action potential ). However, unlike what you have in nerve, the response time and magnitude of electrochemical energy developed in skeletal muscle is lesser. For example: The resting membrane potential in skeletal muscle is about -90mv and its action potential last 2- 4mins while the velocity of action potential in skeletal muscle is 5m/s..

[Audio] Interestingly, the ion distribution in skeletal muscle is similar to what we have in nerve cell. Here, you can see a high amount of sodium ion in the extracellular fluid than in the intracellular fluid. Also, there are more potassium ion in the intracellular fluid than we have in the extracellular fluid. Similar to what we have in the nerve. These ion distribution is responsible for generation of electrical potential across the membrane; in particular, resting membrane potential at rest..

[Audio] Once the skeletal muscle becomes stimulated, action potential is generated and the skeletal muscle subsequently contract. As earlier mentioned, two important filament make up the contractile unit of the skeletal muscle. One of it is the myosin filament and the other is actin filament. The myosin filament is actually made up of multiple myosin molecules and each of these myosin molecules has a molecular weight of about 480,000. Also, each of the myosin molecule has six polypeptide heavy chain and four light chain..

[Audio] The illustration here shows The structure of myosin molecule (in A), and interaction of the myosin molecule head with the actin filament (in B). Usually, two heavy chains of the myosin molecule wrap round each other as seen in the diagram to form double helix tail of myosin molecule. One each of these chain form globular head structure. The four light chain of the myosin molecule also form part of the head as seen in the diagram. It is important to mention that the head is the one always in close contact with the actin filament and the interaction that exist between the head of the myosin molecule and the actin filament makes contraction possible. Also, the head of the myosin molecule contains ATPase enzyme which provides energy for the contraction process..

[Audio] The actin filament is another important component of the contractile structure of the skeletal muscle. It is composed of three important structures that includes Actin, tropomyosin and troponin..

[Audio] The actin is a double stranded protein molecule. It forms the backbone of the actin filament. Just like in myosin molecule, two actin strands are wound in a helix Each strand contains G-actin molecules on which ADP molecules binds.The ADP molecules are the active site on actin filament on which cross bridges of myosin filament interacts during contraction..

[Audio] Tropomyosin molecule These are protein molecules that weighs 70,000 each and a length of 40 nanometers. They are wrapped spirally round the actin helix. At rest, tropomyosin lies on top of the active site on actin and prevent actin-myosin interaction..

[Audio] Troponin is another important protein molecule in the actin filament. They are complex of three loosely bound protein subunits, namely Troponin I, Troponin T and Troponin C. Each performs specific roles in controlling muscle contraction As follows: Troponin I has strong affinity for actin, Troponin T has strong affinity for tropomyosin Troponin C has strong affinity for calcium. Basically, the complex attach tropomyosin to actin and affinity of troponin c for calcium is believed to initiate contraction..

[Audio] Usually the skeletal muscle responds to stimulus by contracting or relaxing as it may be. Here, we look at the mechanism of muscle contraction. When an action potential travels along the membranes of a nerve fiber to its terminal ending which is in contact with a muscle at the neuromuscular junction, a neurotransmitter is often secreted at the nerve ending. Example of such neurotransmitter is acetylcholine. Acetylcholine then subsequently act on local area of the muscle fiber membrane and cause opening of acetylcholine gated channel and cause entry of sodium ion into the interior of the muscle fiber. The entry of sodium ion this way result into local depolarization..

[Audio] If the potential generated as a result of entry of sodium is up to the threshold of action potential, more sodium ion channel continue to open and the muscle fiber becomes more depolarized. Eventually, depolarization of the muscle fiber cause sarcoplasmic reticulum in the sarcoplasm to release its calcium ion content which then initiates attractive force between the actin and myosin filament, and thus cause contraction. Within few seconds, the calcium ion is again pumped back into the sarcoplasmic reticulum by calcium ion pump and the contraction process and consequently put an end to contraction..

[Audio] The name given to the mechanism of muscle contraction is sliding filament. It involves interaction between the actin filament and the myosin filament of the skeletal muscle. As illustrated in the diagram, at rest, the actin and myosin filament are not closely packed or in strong contact with each other (upper section of the diagram). However, after initiation of contraction process, they become strongly connected that they pull closely..

[Audio] As an assignment, write on the roles of actin, tropomyosin and troponin in muscle contraction..

[Audio] When skeletal muscle contract, the length of the muscle fiber can either shorten ( as seen during isotonic contraction) or is not shortened at all (as seen during isometric contraction). The set up shown in the diagram can be used to measure isotonic contraction ( A) and isometric contraction ( B) in the laboratory..

[Audio] Still on muscle contraction, based on their responses to stimulus and the duration of contraction, muscles have been classified into two. Namely, 1. Fast muscle, and 2. Slow muscle Fast muscle fibers are the muscle fibers that reacts rapidly. Example of this type of muscle is the anterior tibialis muscle Slow muscle fiber on the other hand are one that responds slowly but with prolong contraction. Example is soleus muscle..

[Audio] Other characteristics and comparison between slow and fast muscle fibers are as stated here. Pls appreciate it..

[Audio] When a muscle contract, it usually undergo a form of work against a load. Therefore, the functioning of a muscle results always in loss of energy. You may have at a point in time, wondered why you get fatigued after doing some form of muscle exercise. The reason is because the activity of the muscle is accompanied with energy loss and when energy store becomes exhausted, fatigue set in..

[Audio] The energy used by muscle is often gotten from different sources The first source is stored ATP. The second source is phosphocreatine which supplies energy for contraction up to 5- 8secs. The third is from glycolysis and finally, energy is gotten from oxidative metabolism..

[Audio] Indeed, skeletal muscle is supplied by nerves. The name give to each muscle fibers of the skeletal muscle with the nerve that innervate it is called a motor unit. Usually, small muscles whose reactions are always rapid and definite receives more nerve supply than the muscle fiber content. This mean that they have more motor unit. Large muscles on the other hand have less motor unit. This is because this type of muscle do not require fine control like that of small muscles..

[Audio] We now know that muscle receives nerve supply. Thus, the junction where the nerve terminal meets the muscle fibers is what we know as neuromuscular junction. Some important characteristic feature of neuromuscular junction are as illustrated in the diagram here. First, there is a nerve terminal at this junction and contained in the nerve terminal are synaptic vesicles that contains neurotransmitters. Another important feature at the neuromuscular junction Is the synaptic cleft. This is a space (about 20- 30nm wide) between the nerve terminal and the muscle fiber. The synaptic vesicle in the nerve terminal often release their neurotransmitter content first into the synaptic space before acting on the muscle fiber or postsynaptic membrane..

[Audio] Acetylcholine is a neurotransmitter that promote muscle contraction. Usually, it is released from the synaptic vesicle contained in the terminal of nerve fibers when an action potential reaches this terminal ending of nerve. It has been demonstrated that the action potential at the nerve terminal cause opening of calcium ion channel and subsequently favor the entry of calcium ion into the terminal region. The entry thus promote binding of the synaptic vesicle to the terminal membrane and eventually release its neurotransmitter content via exocytosis into the synaptic cleft. The acetylcholine released then binds to its acetylcholine receptor on the muscle fiber membrane and cause opening of acetylcholine gated channel. Consequently, sodium ion begins to enter into the muscle fiber membrane which causes generation of first a local potential and finally an action potential..

[Audio] One thing that is common to all excitable tissues and/or cells of the body is that once they are excited, they often return back to their resting or initial state. In the case of skeletal muscle. After undergoing contraction it returns back to the initial state. One means through which this is possible is the removal of acetylcholine. This is due in part to the role of acetylcholine in muscle contraction. Apparently, if acetylcholine continue to remain in the synaptic cleft, it will continue to promote contraction. Thus, the removal of acetylcholine from the synaptic cleft will cause contraction to stop. There are two important means for removal of acetylcholine in the synaptic cleft The first is by degradation of acetylcholine by the enzyme known as acetylcholinesterase that is present in the synaptic cleft. The second means is by diffusion from the synaptic cleft..

[Audio] In addition to the action or role of acetylcholine in muscle contraction that was earlier discussed, pls know that acetylcholine is synthesized in the cytosol of nerve terminal and stored in small vesicles which was produced by Golgi apparatus in the cell body of the nerve and transported via axoplasm to the nerve terminal. Every other details here and in the slide that follows have been discussed earlier. Kindly read and ensure you understand the information..

The released acetylcholine cause opening of acetylcholine gated channel which cause sodium to flow into the cell. The inflow of sodium cause excitation in the muscle that leads to contraction..

[Audio] Lastly, we look at the term excitation contraction coupling. This is a process that leads to contraction and relaxation of skeletal muscle. It includes process that cause skeletal muscle excitation (i.e. release of acetylcholine, inflow of sodium and depolarization of skeletal muscle). It also involves the activation of the dihydropyridine receptor and consequent opening of dihydropyridine receptor controlled channel on sarcoplasmic reticulum. The opening of this channel favors release of calcium ion from the sarcoplasmic reticulum. The released calcium then activates myosin – actin interaction and thus cause skeletal muscle contraction..

[Audio] After some time, the calcium ion is pumped back by means of a calcium pump on the sarcoplasmic reticulum membrane into the sarcoplasm and stored. These event then return the skeletal muscle back to its relaxed state..

[Audio] The diagram here illustrates the excitation contraction coupling discussed earlier. Pls appreciate the diagram..

[Audio] Thank you.. Thank you.