The Muscular System

1. The Muscular System Chapter 7

2. http://www.youtube.com/watch?v=RsWNyqnHQ2I

3. Muscle • One of the 4 basic tissues of body • Made up of cells that can shorten (contract) • Three different types of muscle • skeletal muscle Controlled by conscious mind and moves bones of skeleton. • cardiac muscle Found only in heart • smooth muscle Found throughout body but controlled by unconscious mind

4. Terminology • Myo- generally refers to muscle • Sarco- muscle cells

5. Skeletal Muscle • Moves the bones of the skeleton • Also may generate heat • Also called voluntary striated muscle • Obvious striped pattern microscopically of light and dark bands.

6. Skeletal Muscle Gross Anatomy • Generally have a thick central portion called a belly. • Have two attachment sites that join muscle to whatever tissues they move when they contract. • Generally attach via tendons. • May be attached via aponeuroses- broad sheets of connective tissue (example: linea alba of ventral midline) • Origin- the end that is generally more stable than the other. Does not move when muscle contracts. • Insertion- site that undergoes most of the movement during contraction.

9. Muscle Actions • Only function is to contract when stimulated to do so by a nerve impulse. • Usually work in groups • Some muscles provide most of the movement • Others provide stability to nearby joints • A prime mover (agonist) describes a muscle or muscle group that directly produces a desired movement. • An antagonist is a muscle or muscle group that directly opposes the action of a prime mover.

11. More terms • Synergist- a muscle that contracts at the same time as a prime mover and assists it in carrying out action. • Fixator- stabilized joints to allow other movements to take place.

12. Muscle Naming Conventions • Muscles are generally named by: • Action • Function (flexors and extensors) • Shape • What it looks like (deltoid muscle) • Location • Where it is at in body (biceps brachii) • Direction of Fibers • Rectus muscles- means straight • Number of heads or Divisions • Number of attachment sites (biceps, triceps, etc) • Attachment sites • Origin and insertion sites

14. Selected Muscles • Cutaneous Muscles • Muscles of the skin • Have little or no attachment to bone • Actually see fascia (connective tissue) causing movement • Cutaneous trunci- makes back twitch, used during neurological exams. • Head and Neck Muscles • Control facial expressions, enable chewing and move sensory structures • Include: • Masseter- chewing muscle • Splenius-extend head and neck • Trapezius-extend head and neck • Brachiocephalicus-extends head and neck and pulls front leg forward • Sternocephalicus-extends from sternum to the base of the skull and lowers neck. (flexor)

17. Abdominal Muscles • Function to support the abdominal organs • Help to flex back, aid in defecation, urination, vomiting, respiration and parturition. • Layers from outside in: • External abdominal oblique • Internal abdominal oblique • Rectus Abdominis • Transversus abdominis • Thoracic Limb Muscles • Function mainly in locomotion • Latissimus dorsi- extends from spinal cord to humerus and flexes shoulder • Pectoral muscles- extend from sternum to humerus and act as adductors of front leg to keep them under the animal • Deltoid muscles-abducts and flexes shoulder joint • Biceps brachii-flexes elbow joint • Triceps brachii- extends elbow joint • Extensor carpi radialis-extends the carpus • Deep digital flexor-flexes the digit

21. Pelvic Limb Muscles • Involved mainly in locomotion • Gluteal muscles- extend from bones of pelvis to trochanters of femur • Hamstring Muscles- Help to extend hip joint and flex stifle joint • Biceps Femoris • Semimembranosus • Semitendinosus • Quadriceps Femoris- main extensor of the stifle joint • Gastrocnemius muscle- calf muscle, extensor of the hock.

23. Muscles of Respiration • Increase and decrease size of thoracic cavity • Inspiratory Muscles • Diaphragm • External intercostal muscles • Expiratory muscles • Internal intercostal muscles • Abdominal muscles

25. Microscopic Anatomy of Skeletal Muscle • Muscle cells • Are very large in size • Have a threadlike or fiberlike shape. • Usually are multi-nucleated • Made up of smaller myofibrils composed of actin (thin) and myosin (thick) • Network of sarcoplasmic reticulum (similar to ER) • Stores Ca++ for muscle contraction

27. A band- large dark band made up of myosin filaments • I Band- large light band made up of actin filamaments • Z line- dark band in center of I band, disk that is viewed as a line and is attachment site for actin filaments. • Sarcomere- area from one z line to the next z line. Basic contracting unit of skeletal muscle. When all sarcomeres contract, leads to overall muscle fiber shortening.

30. Neuromuscular Junction • Skeletal muscle is under voluntary control • If nerve supply is interrupted for long period of time, muscle will atrophy (shrink down) • Neuromuscular junctions- sites where the ends of motor nerve fibers connect to muscle fibers. Space is actually a synaptic space.

31. Neuromuscular Junction Continued • Synaptic vesicles - sacs at end of a nerve fiber; contain neurotransmitter (e.g., acetylcholine) • Acetylcholine—neurotransmitter chemical that travels across synapse to activate muscle fiber • Attaches to receptor on sarcolemma • Acetylcholinesterase—enzyme in the synaptic space that removes acetylcholine • If muscle is to contract again, another impulse must be sent

33. Motor Unit • One nerve fiber and all muscle fibers it innervates • Muscles that make small, delicate movements have only a few muscle fibers per nerve fiber in each motor unit • Large, powerful muscles may have a hundred or more muscle fibers per motor unit

34. Connective Tissue Terminology • Endomysium- each individual skeletal muscle fiber is surrounded by this delicate connective tissue layer. • Fascicles- groups of skeletal muscle fibers • Perimysium- connective tissue that binds together fascicles. • Epimysium- fibrous connective tissue that surrounds groups of fascicles.

36. Physiology of Skeletal Muscle • Initiation of Muscle Contraction and Relaxation • Nerve impulse comes down motor nerve fiber, reaches neuromuscular junction and acetylcholine is released into synaptic space. • Acetylcholine binds to receptors on surface of sarcolemma (cell membrane) of the muscle fiber. • This starts impulse that travels along sarcolemma and through the T tubules to the interior of the cell. • Once impulse reaches sarcoplasmic reticulum it causes release of stored calcium ions (Ca++) into the sarcoplasm (cytoplasm). • As Calcium diffuses into myofibrils, initiates contraction process which is powered by ATP. • As contraction occurs, Calcium is pumped back out of myofibrils which shuts down contraction process. • Both relaxation and contraction requires energy.

40. Mechanics of Muscle Contraction • When a muscle fiber is relaxed, actin and myosin overlap a little. • When stimulated cross bridges (levers on the myosin filaments) ratchet back and forth and pull the actin filaments on both sides toward center of the myosin filaments. • Sliding of filaments shortens sarcomere, thereby causing contraction.

42. Characteristics of Muscle Contraction • All or nothing principle • An individual muscle fiber either contracts completely when it receives an impulse or not at all. • Movements vary in strength due to number of muscle fibers stimulated. • Nervous system sends out impulse based on muscle memory- or idea of how many fibers need to be stimulated for that particular activity.

43. Phases of twitch contraction • 1. Latent phase • Brief pause between nerve stimulus and beginning of actual contraction (lasts about 0.01 seconds) • 2. Contracting phase • Actual contraction is taking place (lasts about 0.04 seconds) • 3. Relaxation phase • When cells go back to a relaxed state (lasts about 0.05 seconds)

44. Chemistry of Muscle Contraction • Primary source of energy is ATP • When ATP loses a phosphate group (remember cellular respiration?) energy is released. • CP (creatine phosphate) is responsible for converting ADP back to ATP so that it is ready to provide energy again. • ATP and CP require glucose and oxygen to operate effectively. • Muscle fibers may store glucose and oxygen in glycogen or myoglobin (oxygen attached to a protein) • Myoglobin can release oxygen during strenuous exercise (process is called aerobic metabolism) • Anaerobic metabolism occurs when not enough oxygen is stored. By product is lactic acid, a by-product of incomplete glucose breakdown.

45. Heat Production • Considerable amount of energy produced in muscles is in form of heat. • This heat is used to help maintain internal temperature. • Shivering.

46. Cardiac Muscle • Also known as involuntary striated muscle • Only found within the heart • Cardiac muscle cells are much smaller than skeletal muscles cells and only contain one nuclei per cell (size) • Are longer than wide and have multiple branches. • This forms a branching network of cells. • Contain intercalated disks- where cells attach one to another. • Also transmit impulses from cell to cell. • This allows entire groups of cells to contract together at the same time.

49. Physiology of Cardiac Muscle • No external stimulation required. • Rate and rhythm of contraction is due to SA (sinoatrial node) of heart located in the wall of the right atrium. • Impulse follows a controlled path through the conduction system of the heart. • This helps to transmit, delay, and redirect each impulse so that the cardiac muscle cells in the walls of the heart chambers contract in coordinated manner. • Groups of cardiac cells adopt the contraction rate of the most rapid cell in the group. • Cardiac cells contract in rapid, wavelike fashion. • This helps to squeeze blood out of chambers of the heart