Download The Muscular System: Structure, Function, and Contraction and more Schemes and Mind Maps Anatomy in PDF only on Docsity!
UNIT- 2
THE MUSCULAR SYSTEM
Muscular System uscular System deals with the study of various types of muscles of human body. The structure, location ,functions , muscle metabolism and types of muscular movements. There are three types of muscle in the human body: 1.Skeletal Muscle 2.Cardiac Muscle 3.Smooth Muscle
- Skeletal muscle is so named because most skeletal muscles move bones of the skeleton. Skeletal muscle tissue is striated : Alternating light and dark bands ( striations ) are seen when the tissue is examined with a microscope. Skeletal muscle tissue works mainly in a voluntary manner. Its activity can be consciously controlled by neurons (nerve cells) that are part of the somatic (voluntary) division of the nervous system.
- Cardiac muscle is also striated, but its action is involuntary. The contraction and relaxation of the heart is not consciously controlled but it is controlled by autonomic nervous system.
- Smooth muscle is located in the walls of hollow visceral organs, such as blood vessels, airways, and most organs in the abdominopelvic cavity. It is also found in the skin, attached to hair follicles. This tissue lacks the striations of skeletal and cardiac muscle tissue. For this reason, it looks nonstriated, which is why it is referred to as smooth. The action of smooth muscle is usually involuntary.
- Note: Both cardiac muscle and smooth muscle are regulated by neurons that are part of the autonomic (involuntary) division of the nervous system and by hormones released by endocrine glands.
M
Functions of Muscular Tissue
1. Body movements. Movements of the whole body such as walking and running by attachments with bones.
- Body position stability. Skeletal muscle contractions stabilize joints and help maintain body positions, such as standing or sitting. 3. Internal body substances movements and storage. For example- Sphincters (cardiac and pyloric ), - which prevent outflow of the food contents from stomach and provide temporary storage. 4. Thermogenesis. As muscular tissue contracts, it produces heat, a process known as thermogenesis, which maintain normal body temperature.
Microscopic Anatomy of a Skeletal Muscle Fiber
- The diameter of a mature skeletal muscle fiber ranges from 10 to 100 micro m.
- The typical length of a mature skeletal muscle fiber is about 10 cm (4 in.), although some are as long as 30 cm (12 in.).
- Sarcolemma is the plasma membrane of a muscle cell beneath which multiple nuclei of a skeletal muscle fiber are located.
- Transverse (T) tubules are thousands of tiny invaginations of the sarcolemma just like tunnels on the surface toward the center of each muscle fiber opening upward. Note: Muscle action potentials travel along the sarcolemma and through the T tubules, quickly spreading throughout the muscle fiber.
- Sarcoplasm is the , the cytoplasm of a muscle fiber which contains glycogen.Glycogen can be used for synthesis of ATP.
- Within the sarcoplasm a red-colored protein called myoglobin is also present. This the muscle protein which releases oxygen when needed by the mitochondria for ATP synthesis.
- The myofibrils are the contractile organelles of skeletal muscle .Myofibrils are about 2 micro meter in diameter and extend the entire length of a muscle fiber.
- A fluid-filled system of membranous sacs called the sarcoplasmic reticulum or SR encircles each myofibril. Dilated end sacs of the sarcoplasmic reticulum called terminal cisterns present against the T tubule from both sides. A transverse tubule and the two terminal cisterns on either side of it form a triad .SR stores calcium during muscle is in relaxed state. Filaments and the Sarcomere
not change. Shortening of the sarcomeres causes shortening of the whole muscle fiber, which in turn leads to shortening of the entire muscle,that is why it is called as sliding filament theory. The Contraction Cycle:
- At the onset of contraction, the sarcoplasmic reticulum releases calcium ions (Ca2+) into the cytosol. There, they bind to troponin. Troponin then moves tropomyosin away from the myosin binding sites on actin. Once the binding sites are “free,” the contraction cycle —the repeating sequence of events that causes the filaments to slide—begins. The contraction cycle consists of four steps : 1.Hydrolysis of ATP The myosin head includes an ATP-binding site and an ATPase enzyme which readily hydrolyzes ATP into ADP+Pi. This hydrolysis reaction energizes the myosin head. 2. Formation of Crossbridges : The energized myosin head attaches to the myosin-binding site on actin and releases the previously hydrolyzed phosphate group. When the myosin heads coupled to a ctin during contraction, they are referred to as **crossbridge formation.
- Phase of Power stroke.** During the power stroke, the site on the crossbridge where ADP is still bound with actin at myosin binding site opens as a result, the crossbridge rotates and releases the ADP. The crossbridge generates force as it rotates toward the center of the sarcomere, sliding the thin filament past the thick filament toward the M line. 4.Detachment of myosin from actin. When power stroke terminates, the myosin head remains firmly attached to actin till the next molecule of ATP binds over it for another cycle of event for contraction. As soon as the ATP binds on myosin head the myosin head detached from actin,and prepared
for next cycle. Note: This cycle continues again and again whenever the new molecule of ATP hydrolyzes,myosin head activated and cross bridge formation takes place result in power stroke and eventually contraction. THE NEUROMUSCULAR JUNCTION (NMJ)
Synapse: Communication between two neurones or communication between neurons and muscle end plate. Synaptic cleft: Gap between presynaptic and post synaptic membrane at junction point. Neurotransmitter: Chemical messenger released from axonal terminal of a motor neuron. Description:
- At the NMJ, the terminal of the motor neuron which is also called the axonal terminal contains hundreds of membrane-enclosed sacs called synaptic vesicles which contains molecules of acetylcholine (Ach), the neurotransmitter released at the NMJ. Within each motor end plate there are 30 to 40 million acetylcholine receptors which are ligand-gated ion channels. At NMJ the release of Acetylcholine occurs due to the process of exocytosis,when the voltage gated of Ca2+ ion channels opens at axonal terminal and influx of Ca2+ causes fusion of vesicles at axon terminal and Ach released out in the synaptic cleft from pre synaptic end. As soon it released,get bind on the cholinergic receptor at motor end plate(post synaptic membrane ) and series of events happened.All the events are summarized as follows: 1. Release of Acetylcholine. Arrival of the nerve impulse at the synaptic end bulbs causes many synaptic vesicles to undergo exocytosis. During exocytosis, the synaptic vesicles fuse with the motor neuron’s plasma membrane, liberating Acetylcholine into the synaptic cleft. The ACh then diffuses across the synaptic cleft between the motor neuron and the motor end plate. 2. Binding&activation of ACh receptors. Binding of two molecules of ACh to the receptor (ligand gated ion channels)on the motor end plate opens an ion channel in the ACh receptor. Once the channel is open, Na+, influx in the muscle.
- Activation of muscle and production of muscle action potential. The influx of Na+ (down its electrochemical gradient) makes the inside of the muscle fiber more positively charged( depolarization ). This change in the membrane potential triggers a muscle action potential.
Note: The muscle action potential then propagates along the sarcolemma into the T tubule system. This causes the sarcoplasmic reticulum to release its stored Ca2+ into the sarcoplasm, resulting the increased level of cytosolic calcium. Ca2+ displaces troponin from tropomyosin and causes myosin binding site free for binding activated myosin head on actin filament,which is the necessary step in contraction in muscle.
- Enzymatic termination of ACh. After release the free ACh is rapidly broken down by an enzyme called acetylcholinesterase (AChE). This enzyme is attached to collagen fibers in the extracellular matrix of the synaptic cleft. AChE breaks down ACh into acetyl and choline, products that cannot activate the ACh receptor. NERVE ACTION POTENTIAL ELICITS A MUSCLE ACTION POTENTIAL IN THE FOLLOWING WAY
Muscle Metabolism: It is the process of ATP generation by muscle for its own activity from various sources. Production of ATP in Muscle Fibers ➢ A huge amount of ATP is needed to power the contraction cycle, to pump Ca2+ into the sarcoplasmic reticulum, and for other metabolic reactions involved in muscle contraction. Muscle fibers have three ways to produce ATP:
- From creatine phosphate
- By anaerobic cellular respiration
- By aerobic cellular respiration. Note: The use of creatine phosphate for ATP production is unique to muscle fibers, but all body cells make ATP by the reactions of anaerobic and aerobic cellular respiration only. 1.Creatine Phosphate
- Creatine is a small, amino acid–like molecule that is synthesized in the liver, kidneys, and pancreas and then transported to muscle fibers.
- Creatine phosphate is five to six times more plentiful than ATP in the sarcoplasm of a relaxed muscle fiber.The enzyme creatine kinase (CK) catalyzes the transfer of one of the high-energy phosphate groups from ATP to creatine, forming creatine phosphate
and ADP. While muscle fibers are relaxed, they produce more ATP than they need for resting metabolism. The excess ATP is used to synthesize creatine phosphate, an energy-rich molecule that is found only in muscle fibers. Note: This amount of energy is sufficient for maximal short bursts of activity — for example, to run a 100-meter dash. 2.Anaerobic Cellular Respiration (In absence of Oxygen)
- Anaerobic cellular respiration is a series of ATP-producing reactions that do not require oxygen. When muscle activity continues and the supply of creatine phosphate within the muscle fiber is depleted, glucose is catabolized to generate ATP.
- Glycolysis is a series of 10 reactions quickly breaks down each glucose molecule into two molecules of pyruvic acid. These reactions use two molecules of ATP but produce four, for a net gain of two molecules of ATP.Eventually, the pyruvic acid formed by glycolysis in the cytosol enters mitochondria, where it undergoes a series of oxygen-requiring reactions called aerobic cellular respiration that produce a large amount of ATP.
- Anaerobic cellular respiration can provide enough energy for about 30 to 40 seconds of maximal muscle activity.
- Together, conversion of creatine phosphate and glycolysis can provide enough ATP to run a 400-meter race. Aerobic Cellular Respiration 3.Aerobic cellular respiration (in presence of Oxygen) Muscular activity that lasts longer than half a minute depends increasin gly on aerobic cellular respiration, a series of oxygen-requiring reactions that produce ATP in mitochondria. By this process 36 ATP if formed in mitochondria from pyruvic acid,aminoacid and fatty acids and produced CO2 and water and heat as byproduct
contractions are concentric and eccentric. Example- Holding a bucket full of water for a while,gradually you feel a tension in your hand,due to isometric contraction. CARDIAC MUSCLE TISSUE
- Cardiac muscle ( heart muscle ) is striated muscle (striped muscle ) in the walls of the heart. It makes up the tissue called the myocardium. It is involuntary: a person cannot control it consciously. Cardiac muscle is one of three main types of muscle , the others being skeletal and smooth muscle .Cardiac muscle fibers have the same arrangement of actin and myosin and the same bands, zones, and Z discs as skeletal muscle fibers. However, intercalated discs are unique to cardiac muscle fibers. The discs contain desmosomes, which hold the fibers together, and gap junctions, which allow muscle action potentials to spread from one cardiac muscle fiber to another .Cardiac muscle tissue has an endomysium and perimysium, but lacks an epimysium. In cardiac muscle fibers, Ca2+ enters the sarcoplas m both from the sarcoplasmic reticulum (as in skeletal muscl e fibers) and from the interstitial
fluid. Because the channels that allow inflow of Ca2+from interstitia l fluid stay open for a relatively long time, a cardiac muscl e contraction lasts much longer than a skeletal muscle twitch. Smooth Muscle Tissue
- Smooth muscle tissue, unlike skeletal or cardiac tissues, does not have clearly defined striations visible on the cells. This is because smooth muscle cells are organized in a different way than other muscle cells. The actin and myosin filaments in smooth muscle are arranged in a stacked pattern across the cell. This “staircase” arrangement of actin and myosin is much different than the structure in skeletal and cardiac muscle. The actin filaments (red lines) in smooth muscle run from one side of the cell to the other, connecting at dense bodies and at the cell membrane. In skeletal and cardiac muscle, the actin filaments are attached to Z plates, which hold many actin filaments and show up as dark bands under the microscope. In smooth muscle, the actin and myosin fibers are arranged an angles to each other as they run through the cell. Smooth muscle contracts under certain stimuli as ATP is freed for use by the myosin. The amount of ATP released depends on the intensity of the stimuli, allowing smooth muscle to have a graded contraction as opposed to the “on-or-off” contraction of skeletal muscle. The function of smooth muscle is to contract.Smooth muscle lines many parts of the circulatory system, digestive system, and is even responsible for raising the hairs on your arm.
