BIOL 109n 1st Edition Lecture 10Outline of Last Lecture I. List the functions of Skeletal systemII. Human Bone III. Bone is a living tissueOutline of Current Lecture I. Describe the three kinds of musclesII. List the four traits that all muscle types have in commonIII. Attachment Current LectureI. Muscle cells: there are 3 types of muscles: - Skeleton - Cardiac- Smooth II. Characteristics of Muscles; - Excitable- respond to stimuli- Contractile- can shorten - Extensible- can stretch- Elastic- return to their original length after being shortened or stretched 1. Skeletal Muscles: Function- Skeletal muscles are voluntary muscles responsible for moving our body and maintaining posture. - Skeletal muscles work in pairs: each muscle is attached to a bone by a tendon (a band of connective tissue) - The original of the muscle is the end attached to the bone that remains stationary during movement- The insection is the end attached to the bone that moves- Bones act as levels in working with skeletal muscles to produce movement - The body has more than 600 skeletal muscles: Most muscles are arranged in pairs, called antagonistic pairs, that work in opposite to one another. Muscles These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.that must contract at the same time to cause movement are called synergistic muscles- Flexion: the biceps contracts and pulls the forearm up, flexing the arm, the relaxed triceps is stretched. Extention: the triceps contracts and pulls the forearm down, extending the arm. The relaxed biceps is stretched 2. Muscle- Skeletal muscle is made up of many fascicles= bundles of muscle cells (muscle fiber) - Muscle cell= bundles of myofibrils(specialized bundles of protein)- Each contains groups of long myofilaments. - Each is composed of myosin(thick) and action (thin) filaments (actin are more numerous) Muscle Cell –> Muscle Fiber (Myocyte) - Plasma membrane- sarcolemma- Cytoplasma- sarcoplasm- Endoplasmic reticulum - sacroplasma reticulum Muscle-> Fascicle-> Muscle Cell -> Myofibril - Sacrolemma – plasma membrane - Myofilaments: actin (thin) myosin (thick) 3. Sacromeres are the contractile unites of muscle. The ends of each sacromere are marked by dark protein bands called Z lines. Within each sarcomere the actin and myosin filaments are arranged in a specific manner. Sacromeres attach to form myofibril - One end of each actin filament is attached to a Z line- Myosin filaments lie in the middle of the sacromere, and their ends partially overlap with surrounding actin filaments - The degree of overlap increases when the muscle contracts- Sacromeres shorten as the actin filaments slide along the myosin filaments- Muscle contraction occurs at the molecular level. According to the sliding filament model, muscle contracts when actin filaments slide past myosin filaments. The myosin head attaches to the actin filament forming a cross-bridge.Then it bends and swivels, pulling the actin filament toward the midline of the cell4. Sliding Filament Theory/Cross Bridge Cycle Myosin heads split ATP and become reoriented and energized ->Myosin heads bind toactin, forming crossbridges->Myosin heads rotate toward center of the sarcomere (power stroke) -> As myosin heads bind ATP, the crossbridges detach from actin5. Rigor Mortis The movement of the myosin filament is powered by ATP- adenosine triphosphate. Muscle contraction will occur as long as ATP is present. Without ATP, cross-bridges cannot be broken. Within 3 to 4 hours after death, the muscles become stiff. Actin and myosin gradually break down and muscles relax again after 2 to 3 days. 6. Neuromuscular Junction Contraction is triggered when a nerve impulse travels down a motor neuron until it reaches the neuromuscular junctionA nerve impulse reaches the neuromuscular junction acetylcholine is released at the neuromuscular junction-> when acetylcholine binds to receptors on the plasma membrane of the muscle cell, an electrochemical message is generated. 7. Voluntary Movement A whole muscle contraction can vary due to:  Different number of muscle cells contracting at a given moment  Different force of contraction in individual cells because of the differences in the frequency of stimulation  A motor unit= a motor neuron and all the muscle cells it stimulates. All muscle cells in a given motor unit contract together On average there are 150 muscle cells in a motor unit. Muscles responsible for precise, finely- controlled movements have small numbers of muscle cells in each motor unit, in contrast to muscles for less precise movements.  The strength of muscle contraction depends on the number of motor units that are stimulated. More motor unit stimulated -> stronger muscle contraction-> can lift heavier things or kick stronger 8. Muscular Dystrophy  If too many calcium ions enter the cell, proteins may be destroyed causing the death of the cells. Dead muscle cells are replaced by fat and connective tissue, resulting in muscular weakness.  Duchenne Muscular Dystrophy is one of the most common forms9. Muscle stimulation increases the strength of contraction  Muscle twitch- contraction of a muscle in response to a single stimulus. Shortcontraction and longer relaxation -> twitches are very brief and typically not part of normal movement. If a second stimulus is received before the muscle is fully relaxed, the second twitch will be stronger than the first, due to wave summation-> more muscle cells are stimulated to contract Taken to the extreme, a sustained powerful contraction is called tetanus.  Fatigue sets in when a muscle is unable to contract even when stimulated  Slow-twitch cells: - Loaded with mitochondria, and therefore deliver prolonged, strong contractions- Rich in blood and myoglobin (oxygen-binding pigment)-> dark red colors Fast-twitch cells: - Contract rapidly and powerfully but with much less endurance- They rely on lactic acid fermentation as their source of energy, and therefore tire quickly- Contain more actin & myosin -> larger cell diameter Muscle cramps – low blood level of calcium, magnesium, or potassium may cause forceful, involuntary muscle