Physiological Makeup Essay Research Paper PHYSIOLOGY 525SKELETAL

Physiological Makeup Essay, Research Paper

PHYSIOLOGY 525SKELETAL MUSCLEJANUARY 28, 1998 There are three types of muscle: skeletal, smooth, and cardiac. From histology you know that skeletal and cardiac muscle are striated and smooth muscle is “smooth”- not striated. Muscles are often referred to as “sarco”, from the Greek word meaning strength. Therefore instead of saying cytoplasm one uses sarcoplasm. The endoplasmic reticulum in muscle types is called sarcoplasmic reticulum (SR). Sometimes authors instead of using “sarco” they say “myo”- myoplasm (cytoplasm, sarcoplasm). From histology you should remember that skeletal muscle cells are multinucleated. There are hundreds and hundreds of cells that are fused together into multinucleated structures called myotubes or myofibers. Myotubes, myofibers, and skeletal muscle cells are different words with the same meaning. They are huge cells that are centimeters in length and up to 200 m in diameter. They are attached to bones by tendons. Now if you look inside a single multinucleated muscle cell you will see that it is made up of myofibrils. There are hundreds of myofibrils inside a single muscle cell. Myofibrils are striated not in reality, but when you stain them with stains they become striated. There are dark bands and light bands therefore they are called “striated”. Each myofibril is composed of myofilaments. These myofilaments are either polymers of actin (thin filaments) or polymers of myosin (thick filaments). There is a central myosin filament and six actin filaments around it. Muscles are able to contract because these individual filaments slide on each other and come together. The sarcomere is the fundamental unit of contraction for skeletal and cardiac muscle. It is the area between two adjacent Z lines. The actin filaments come in from each side but they do not overlap. They are on top of each other but not overlapping because there has to space for them to slide in either direction. The H band is the space from where the actin, the thin filaments, end and where they begin again. It is free of actin and is composed only of myosin. It is the band that expands or contracts depending on where the actin filaments are. The dark bands are where the myosin and actin overlap. The light bands are where there are only actin filaments. Myosin, thick filaments, are large proteins consisting of 2 heavy chains and 4 light chains organized in a globular head called a cross-bridge and a long tail. The thick filaments are located in the center of the sarcomere and form the A band. The thin filaments are anchored into Z lines on both sides forming the I bands. Actin filaments have a polarity; there is a minus end and a plus end. Thin filaments are composed of actin, tropomyosin and troponin. Troponin is made up of 3 subunits: troponin I, troponin T and troponin C. Troponin is the regulatory protein that controls muscle contraction. Troponin I is inhibitory, troponin C binds calcium and troponin T interacts with tropomyosin. The sarcolemma which is the plasma membrane has invaginations which go into the muscle cell and T-tubules. T-tubules interact with the cisternae of the sarcoplasmic reticulum. There is one T-tubule in the middle and two cisternae on each end. This is called a TRIAD. The cisternae of the SR continues and if you imagine in 3D the SR surrounds each myofibril in its entirety. The T-tubules make specialized contact with these cisternae of the SR which allows communication between these two structures. In order for muscle cells to contract the Z lines have to come together. Muscle contraction is produced by the sliding of actin-rich filaments over the myosin-rich thick filament. The first thing to remember about skeletal muscles is that they are able to produce skeletal movement. In order to do this they have to connect to structures; skeletal muscles usually connect to bones by tendons. They are able to do this because each muscle cell is surrounded by a endomysium, perimysium and epimysium, connective tissues that allow skeletal muscle to connect to the bones. Dr. Hirsch told you that the physiologic control of skeletal muscles is simple because we only have motoneurons controlling contraction of skeletal muscles. There is a single neurotransmitter, acetylcholine. The nerve controls skeletal muscle contractions via acetylcholine. Skeletal muscles are innervated by myelinated motoneurons, usually the A type, 12-20 diameter and 70-120m/sec conduction velocity. We do not have the ability to make each muscle cell to contract slowly or quickly; each muscle cell contracts at a given speed. In order to go slow or fast I either have to use a few muscle cells or every muscle cell that I have here. Speed of contraction is proportional to the number of muscle cells engaged in contraction. This means that we can actually control them (voluntary control). Each motoneuron coming from the ventral horn of the spinal cord splits its axon into several endings. Each of these endings make contact with single muscle cells (motor unit). If I want to contract or move my arm slowly then I am using very few motor units. Fine movements (eye, hand, etc.) use 3-6 muscle fibers per motor unit. Each motor unit controls only a few muscle cells. For gross movements(back, posture) up to 100-150 muscle fibers/motor unit can be used. A single motor neuron controlling hundreds of muscle cells. When a muscle is excited with a single stimulus, the resulting contrac