Muscular System

1. Muscle fibers
• specialized structures
• able to contract
• muscles are responsible for movement
• almost all movement in body is result of muscle contraction
2. Skeletal muscle tissue
• usually attached to skeleton
• used to move the body
• generally contract voluntarily (by somatic nervous system) 
• can contract involuntarily through reflexes
3. Smooth muscle tissue
• found within walls of organs and structures
• involuntary muscle: not under conscious control
4. Cardiac muscle
• specialized muscle
• found only within heart
5. Skeletal muscle (in detail) 
• responsible for body movement and body posture
• ~639 skeletal muscles in human body
• under conscious/voluntary control
• muscle cells that have many nuclei
• contains light and dark trips, called striations
• result of orientation of contractile proteins inside cells
• also called striated muscle
• adult males made of ~40-50% skeletal muscle tissue
6. Smooth muscle (in detail) 
• found in walls of hollow internal organs
• blood vessels, intestinal tract, urinary bladder, and uterus
• under control of automatic nervous system
• involuntary muscle (stated above) 
• do not have striations, so non-striated muscle
• spindle-shaped, with one nucleus in each cell
• generally arranged in sheets or bundles
• connected by gap junctions
1. little pores or gaps in cell membrane that link adjoining cells
2. allow quick passage of chemical messages between cells
• contracts slowly and rhythmically
7. Cardiac muscle (in detail) 
• found in walls of the heart
• involuntary muscle
8. characteristics of both smooth muscle and skeletal muscle cell
• one central nucleus, but striated muscle
• rectangular in shape
• contraction is strong and rhythmical
• highly resistant to fatigue
• has largest amount of mitochondria per cell of any other muscle
• more mitochondria = more energy = constant movement for a longer period of time
• similar to skeletal muscle in chemical composition
• however, structure is different
• muscle fibers typically branched like tree branch
• connect to other muscle fivers by inter-calculated discs (gap junction) 
9. Structure of muscle tissue
• skeletal muscle is made of skeletal muscle tissue, connective tissue, nerve tissue, and vascular tissue
• vary considerably in size, shape, and arrangement of fiber
• each skeletal muscle is single large, cylindrical muscle cell
• each nucleus in fiber originated from one single myoblust
• smooth and cardiac do not develop this way 
• individual skeletal muscle might be made of thousands of muscle fibers
• bundled together and wrapped in connective tissue called epimysium
• fascia surrounds and separates skeletal muscles
• parts of epimysium fold inward to create fascicles
• each fascicle contains bundle of muscle fibers
• skeleton muscle fibers are soft and fragile
• connective tissue gives support and protection
• helps them withstand forces of contraction
• also provides pathways for blood vessels and nerves
• blood vessel deliver nutrients and oxygen, and remove waste products
10. Muscles and bones
• muscles can only actively contract
• extend passively
• ability of muscles to move parts of the body in different directions
• requires that they be attached to bones in pairs that work against each other (antagonistic pairs)
• muscles are attached to one end of a bon, skip a joint, and attached to a point on other side of joint
• connective tissue forms tendon
• thick, ropelike structure
• the origin (attachment point) does not move when muscle contracts
• tendons and muscles work together and exert pulling force on joints
• flexor: muscle that causes angel of joint to become smaller
• extensor: muscle that causes a joint to straighten out
• joints in body act like levers that reduce amount of effort to make large movements
11. Muscle contraction
• occurs when muscle fiber generates tension through actin and myosin
• muscle fiber contains cellular proteins and myofibrils
• myofibrilis is a long, cylindrical organelle
• made up of actin and myosin
1. actin is thin and threadlike
2. myosin is thicker
• actin and myosin organized into sarcomeres
• actin filament anchored to Z lines
• one Z line to another is a sarcomere
• all sarcomeres contract at the same time when a muscle fiber contracts
12. Neuromuscular junction
• voluntary muscles 
• brain sends nerve signals in form of action potential
• reflexes
• signal to contract can originate in spinal cord through reflex arc
13. Sliding filament theory
• how muscles contract
• presence of calcium ion allows interaction of actin and myosin
• troponin and tropomyosin act as barriers between actin and myosin, preventing contract
• Ca2+ binds to actin filament, and toponin-tripomyosin complex changes.
• allows actin and myosin to come into contact
• action potential arrives at axon terminal of motor neuron
• arrival of action potential activates voltage-dependent calcium channels at axon terminal
• calcium rushes into neuron
• Ca2+ causes vesicles to fuse with plasma membrane, releasing acetylcholine into synaptic cleft
• activation of acetylcholine receptors on muscle fivers open sodium/potassium channel, triggering action potential in muscle fiber
• spreads through muscle fiber network and depolarizes inner portion of muscle fiber
• activates specialized storage sites throughout muscle (sarcoplasmic reticulum) to release Ca2+
• sarcoplasmic reticulum is a special type of smooth endoplasmic reticulum
• Ca2+ ions bind to actin filaments of myofibrils and activate actin for attachment by myosin head filaments
• activated myosin binds strongly to actin filament
• upon binding, myosin rotates at myosin-actin interface, and bends a region in the myosin head
• shortening of muscle fiber occurs when bending part of myosin pulls actin and myosin filaments across each other. 
• binding of ATP with myosin lets myosin head detach from actin
• ATP breaks down into ADP + Pi, and the breaking of bonds gives energy to myosin bead, which allows it to bind to actin again
• repeats as long as ATP and Ca2+ are available
14. Motor units: groups of individual muscle fibers that are called motor units
• each muscle fiber contracts either all the way or not at all
15. Cardiac muscle contractions
• good blood supply = provides nutrients and oxygen
• highly resistant to fatigue
• heart is unable to pump well when there is a lack of blood to heart muscle tissue, which leads to heart attack
• cardiac muscle can initiate contraction by itself
• heart can still beat properly even after connections to central nervous system are severed
• a single cardiac muscle cell can contract rhythmically
• after heart attack or cardiac arrest, fibrillations can result
• life-threatening
• can be stopped by defibrillation
16. Smooth muscle contractions
• must often ben stretched
• elasticity
• do not depend on motor neurons to be stimulated
• however, automatic nervous system is connected
• also can be affected by hormones
• oxytocin causes contraction of uterus during childbirth
• muscle contraction caused by sliding of myosin and actin filaments
• calcium initiates contractions in a different way in smooth muscle