Chapter 8 Muscular System What is a muscle

Chapter 8 Muscular System

What is a muscle? �A muscle is an organ of the muscular system � There are three types of muscles: ◦ Skeletal ◦ Smooth ◦ Cardiac � Only skeletal muscle is under conscious control � A skeletal muscle is composed of: ◦ ◦ Skeletal muscle tissue Connective tissue coverings Nervous tissue (to control contractions) Blood

Connective Tissue Coverings � Outer coverings are called fascia (first blank) ◦ Separate individual skeletal muscles from adjacent muscles ◦ Tapers to form tendons (second blank) that fuse to the periosteum of bones �Muscles are connected to bones via tendons � Aponeuroses connect muscles to each other ◦ These are broad sheets of connective tissue � Each individual muscle fiber is covered by special connective tissue called endomysium

Think about it… � Pretend you are going to play a joke on someone and give them 100 pencils. The pencils will represent muscle fibers. � First you wrap each individual pencil in tissue paper (dense tissue paper of course!). This would be endomysium. � Then you take about 10 pencils in a bundle (a fascicle) and wrap them in paper (perimysium). � Then you take all the bundles and wrap them in gift wrap (epimysium). � But you are going to mail this joke, so you also have to wrap it in brown paper representing the fascia.

Skeletal Muscle Fibers � Actin and myosin are the basis of muscle fibers � The alternating strands of these filaments cause skeletal muscles to have striations (stripes!) � Myosin is the thick filament �Creates the dark “A bands” of the striation � Actin is the thin filament �Creates the light “I bands” of the striation �#4: which one is thick? Which one is thin? GOOD JOB

How do contractions work? � Acetylcholine is a neurotransmitter responsible for stimulating muscle contractions ◦ Stored in vesicles (cellular containers) around the muscle fiber ◦ The vesicles empty their acetylcholine when the brain tells them to ◦ The acetylcholine washes down over the muscle fiber and causes a contraction

How do muscles relax? � Acetylcholine must be neutralized before the contraction will cease � To achieve the cessation, the following two events must occur: ◦ Acetylcholinesterase is released ◦ Acetylcholine is degraded causing the contraction to end

The Cross-bridge Cycle � This contraction/relaxation mechanism is called the cross-bridge cycle ◦ Acetylcholine is released from vesicles ◦ Binding sites on the actin filament open ◦ ATP allows the myosin filament cross-bridge to bind to the actin filament ◦ Contraction occurs ◦ Acetylcholinesterase breaks down the acetylcholine causing the contraction to cease

Cross-bridge clips

Oxygen Debt � When at rest, the respiratory and cardiovascular systems can supply sufficient oxygen for the skeletal muscles to support aerobic respiration � When active, muscles use anaerobic respiration to obtain energy because there is not enough oxygen to perform aerobic respiration (oxygen debt)

Lactic Acid � Anaerobic respiration leads to the accumulation of lactic acid (sore muscles) � Muscles can be trained to increase their aerobic respiration capability (produce less lactic acid)

Clostridium botulinum bacterium Clostridium botulinum (C. botulinum) produces a powerful toxin that prevents acetylcholine from being released from vesicles. � In the absence of acetylcholine, muscles cannot contract, which results in paralysis. � Botox is made from this toxin. � The

Clostridium tetani bacterium Clostridium tetani (C. tetani) causes tetanus which results in ceaseless muscle contraction � C. tetani is a bacterium that is found in soil � Infection is often called “Lockjaw” � The

Anabolic Steroids � Anabolic steroids are muscle builders that mimic male hormones (testosterone) � They have medical uses including the treatment of delayed puberty, cancer and AIDS � Abuse leads to increase in muscle mass as well � Because these steroids mimic natural testosterone, the body stops making it which leads to shrinkage of the testicles, baldness, infertility, and gynecomastia

Muscular Dystrophy � Muscle fibers are held together by a protein called dystrophin � Missing or abnormal dystrophin leads to muscular dystrophy, a disease that can lead to severe disability. � Symptoms include progressive weakness that leads to muscle atrophy and wasting � If the condition is caused by missing dystrophin, death will occur by late teens � If the condition is caused by abnormal dystrophin, life expectancy is longer

Skeletal Muscle Trivia � There are 60 muscles in the face ◦ It takes 20 muscles to smile and 40 to frown � The gluteus maximus is the largest muscle in the body � The sartorius (lifts thigh) is the longest muscle � The deltoid is shaped like a triangle

Skeletal Muscles to Know. Upper Body � Frontalis-forehead � Orbicularis Oculi-open/close eyes � Zygomaticus-over cheeks � Sternocleidomastoid-turns head � Trapezius-sits above shoulder � Deltoid-covers shoulder � Pectoralis-chest muscles � Obliques-side muscles � Abdominals- ‘stomach’ muscles

Skeletal Muscles to Know. Limbs � Biceps-anterior upper arm � Triceps-posterior upper arm � Quadriceps-anterior upper leg � Biceps femoris-posterior upper leg � Gastrocnemius-calf muscle � Sartorius-inner thigh