physiology

what is myofibrils

Myofibrils Are Composed of Actin and Myosin Filaments Each muscle fiber contains several hundred to several thousand myofibrils Each myofibril is composed of myosin (thick) filaments and actin (thin) filaments, which are large polymerized protein molecules that are responsible for the actual muscle contraction.

what is the Tropomyosin Molecules.

The actin filament also contains another protein, tropomyosin. These molecules are wrapped spirally around the sides of the F-actin helix. In the resting state, the tropomyosin molecules lie on top of the active sites of the actin strands so that attraction cannot occur between the actin and myosin filaments to cause contraction.

what is the Actin Filaments Are Composed of Actin, Tropomyosin, and Troponin

The backbone of the actin filament is a double-stranded F-actin protein molecule. Each strand of the double F-actin helix is composed of polymerized G-actin molecule The bases of the actin filaments are inserted strongly into the Z discs; the ends of the filaments protrude in both directions to lie in the spaces between the myosin molecules .

explain Muscle Action Potential

1. Resting membrane potential: about −80 to −90 millivolts in skeletal fibers— the same as in large myelinated nerve fibers. 2. Duration of action potential: 1 to 5 milliseconds in skeletal muscle—about five times as long as in large myelinated nerves. 3. Velocity of conduction: 3 to 5 m/sec—about 1/13 the velocity of conduction in the large myelinated nerve fibers that excite skeletal muscle.

what is the three types of muscle proteins

Contractile proteins Regulatory proteins Structural proteins

what is the Structural proteins

Contribute to the alignment, stability, extensibility, and elasticity Titin Alpha-actinin Myomesin Nebulin Dystrophin

what is the Sliding Filament Mechanism of Muscle Contraction

In the relaxed state, the ends of the actin filaments extending from two successive Z discs barely begin to overlap one another. Conversely, in the contracted state, these actin filaments have been pulled inward among the myosin filaments, so their ends overlap one another to their maximum extent. Also, the Z discs have been pulled by the actin filaments up to the ends of the myosin filaments. Thus, muscle contraction occurs by a sliding filament mechanism. What causes the actin filaments to slide inward among the myosin filaments? This is caused by forces generated by interaction of the cross-bridges from the myosin filaments with the actin filaments. Under resting conditions, these forces are inactive. But when an action potential travels along the muscle fiber, this causes the sarcoplasmic reticulum to release large quantities of calcium ions that rapidly surround the myofibrils. The calcium ions in turn activate the forces between the myosin and actin filaments, and contraction begins. But energy is needed for the contractile process to proceed. This energy comes from high-energy bonds in the ATP molecule, which is degraded to adenosine diphosphate (ADP) to liberate the energy.

what is the Contraction cycle

Involves four major steps 1.ATP hydrolysis 2.Attachment of myosin to actin 3.Power stroke 4.Detachment of myosin from actin

Inhibition of the Actin Filament by the Troponin-Tropomyosin Complex; Activation by Calcium Ions.

It is believed that the active sites on the normal actin filament of the relaxed muscle are inhibited or physically covered by the troponin-tropomyosin complex. Consequently, the sites cannot attach to the heads of the myosin filaments to cause contraction. When calcium ions combine with troponin C, the troponin complex undergoes a conformational change. This "uncovers" the active sites of the actin, thus allowing these to attract the myosin crossbridge heads and cause contraction to proceed.

what is the Troponin and Its Role in Muscle Contraction

Troponin molecules are actually complexes of three loosely bound protein subunits: troponin I has a strong affinity for actin, troponin T for tropomyosin, troponin C for calcium ions. This complex is believed to attach the tropomyosin to the actin. The strong affinity of the troponin for calcium ions is believed to initiate the contraction process.

why The Sarcolemma Is a Thin Membrane Enclosing a Skeletal Muscle Fiber

The sarcolemma consists of a true cell membrane, called the plasma membrane, and an outer coat made up of a thin layer of polysaccharide material that contains numerous thin collagen fibrils. At each end of the muscle fiber, this surface layer of the sarcolemma fuses with a tendon fiber. The tendon fibers in turn collect into bundles to form the muscle tendons that then insert into the bones.

What is the neuromuscular junction?

Synapse between a somatic motor neuron and a skeletal muscle fiber Motor end plate Muscle membrane End plate potential (EPP) Action potential generated at the muscle membrane

what is the Muscle structure

About 40% of the body is skeletal muscle, and another 10% is smooth and cardiac muscle. Some of the same basic principles of contraction apply to all three different types of muscle. Muscles functions: Contraction and motion, force Generate heat Provide support muscle properties Electrical excitability A property of both neurons and muscle cells Ability to respond to certain stimuli by producing action potentials Contractility Ability of muscle to contract forcefully when adequately stimulated When a muscle contracts, it generates tension, and possibly movement Extensibility Ability of muscle to stretch without being damaged Chemical composition of the muscle Proteins (contractile proteins and enzymes) 20% ATP, ADP Myoglobin Lactic acid All skeletal muscles are composed of numerous fibers ranging from 10 to 80 micrometers in diameter. Each of these fibers is made up of smaller subunits. In most skeletal muscles, each fiber extends the entire length of the muscle. Whole Muscle (an organ) ↓ Fasciculus (group of fibers ) ↓ Muscle Fiber (a single cell) ↓ Myofibrils (a specialized structure) ↓ Thin & Thick filaments ↓ Actin & Myosin (protein molecules)

what s the sacrolema

Sarcoplasm Is the Intracellular Fluid Between Myofibrils. The spaces between the myofibrils are filled with intracellular fluid called sarcoplasm, containing large quantities of potassium, magnesium, and phosphate, plus multiple protein enzymes. Also present are tremendous numbers of mitochondria that lie parallel to the myofibrils. These supply the contracting myofibrils with large amounts of energy in the form of adenosine triphosphate (ATP) formed by the mitochondria . Sarcoplasmic Reticulum Is a Specialized Endoplasmic Reticulum of Skeletal Muscle. Also in the sarcoplasm surrounding the myofibrils of each muscle fiber is an extensive sarcoplasmic reticulum. This reticulum has a special organization that is extremely important in controlling muscle contraction. The rapidly contracting types of muscle fibers have especially extensive sarcoplasmic reticula.

what is the General Mechanism of Muscle Contraction

The initiation and execution of muscle contraction occur in the following sequential steps. 1. An action potential travels along a motor nerve to its endings on muscle fibers. 2. At each ending, the nerve secretes a small amount of the neurotransmitter substance acetylcholine. 3. The acetylcholine acts on a local area of the muscle fiber membrane to open multiple "acetylcholine-gated" cation channels through protein molecules floating in the membrane. 4. Opening of the acetylcholine-gated channels allows large quantities of sodium ions to diffuse to the interior of the muscle fiber membrane. This causes a local depolarization that in turn leads to opening of voltage-gated sodium channels. This initiates an action potential at the membrane. 5. The action potential travels along the muscle fiber membrane in the same way that action potentials travel along nerve fiber membranes. 6. The action potential depolarizes the muscle membrane, and much of the action potential electricity flows through the center of the muscle fiber. Here it causes the sarcoplasmic reticulum to release large quantities of calcium ions that have been stored within this reticulum. 7. The calcium ions initiate attractive forces between the actin and myosin filaments, causing them to slide alongside each other, which is the contractile process. 8. After a fraction of a second, the calcium ions are pumped back into the sarcoplasmic reticulum by a Ca++ membrane pump and remain stored in the reticulum until a new muscle action potential comes along; this removal of calcium ions from the myofibrils causes the muscle contraction to cease.

what is the actin-myosin filments

The myosin and actin filaments partially interdigitate and thus cause the myofibrils to have alternate light and dark bands. The light bands contain only actin filaments and are called I bands because they are isotropic to polarized light. The dark bands contain myosin filaments, as well as the ends of the actin filaments where they overlap the myosin, and are called A bands because they are anisotropic to polarized light. The ends of the actin filaments are attached to the Z disc. From this disc, these filaments extend in both directions to interdigitate with the myosin filaments. The Z disc, which itself is composed of filamentous proteins different from the actin and myosin filaments, passes crosswise across the myofibril and also crosswise from myofibril to myofibril, attaching the myofibrils to one another all the way across the muscle fiber . The portion of the myofibril (or of the whole muscle fiber) that lies between two successive Z discs is called a sarcomere. When the muscle fiber is contracted, the length of the sarcomere is about 2 micrometers. At this length, the actin filaments completely overlap the myosin filaments, and the tips of the actin filaments are just beginning to overlap one another. At this length the muscle is capable of generating its greatest force of contraction. Titin Filamentous Molecules Keep the Myosin and Actin Filaments in Place. The side-by-side relationship between the myosin and actin filaments is difficult to maintain. This is achieved by a large number of filamentous molecules of a protein called titin. These titin molecules act as a framework that holds the myosin and actin filaments in place so that the contractile machinery of the sarcomere will work.

what is the myosin molecule

The myosin molecule is composed of six polypeptide chains—two heavy chains and four light chains. The two heavy chains wrap spirally around each other to form a double helix, which is called the tail of the myosin molecule. One end of each of these chains is folded bilaterally into a globular polypeptide structure called a myosin head. The myosin filament is made up of 200 or more individual myosin molecules. The tails of the myosin molecules bundled together to form the body of the filament, while many heads of the molecules hang outward to the sides of the body. The protruding heads are called cross-bridges. Each cross-bridge is flexible. Myosin filaments have small projections from the sides of them. These are cross-bridges. It is the interaction between these cross-bridges and the actin filaments that causes contraction. ATPase Activity of the Myosin Head. Another feature of the myosin head that is essential for muscle contraction is that it functions as an ATPase enzyme. This property allows the head to cleave ATP and use the energy derived from the ATP's high-energy phosphate bond to energize the contraction process. The myosin molecule has 2 binding sites: 1. Binding site for ACTIN 2. ATPase site