Chapter 9 The Muscular System

Functions Movement Maintain posture Respiration Production of body heat Communication Heart Beat

Properties of Muscle Muscle fibers shorten with a force Contractility Muscle fibers shorten with a force Excitability Ability of muscle to respond to stimuli Extensibility Muscle can be stretched to its normal resting length and beyond to a limited degree Elasticity Ability of a muscle to retain its shape after stretching

Long cylindrical shape Attached to bones Multiple peripheral nuclei Types of Muscle Skeletal Long cylindrical shape Attached to bones Multiple peripheral nuclei Striated/ voluntary 40% of the body’s weight Controlled by the nervous system

Types of Muscle Smooth Spindle shaped Hollow organs and tubes, iris of the eye, walls of blood vessels Single central nucleus No Striations Involuntary

Types of Muscle Cardiac Cylindrical and branched Located ONLY in the heart Single central nucleus Striations Intercalated disks Involuntary

Skeletal Muscle Structure Another name for skeletal muscle is muscle fiber (because they are elongated) Muscle fibers develop from less mature, multinucleated cells called myoblasts (Great extra credit question!!)

Skeletal Muscle Cont. Number of fibers remain constant after birth (You are born with as many as you will have for your lifetime) Hypertrophy (enlargement) of muscle fibers result in an increase in their size (exercise!!) A single muscle fiber can extend from one end of a muscle to another Large muscle contain large diameter fibers and small muscle contain delicate fibers

Connective Tissue of the Muscle Fiber The outermost “wrapping” of a muscle is called the epimysium. The name means “Outside the muscle” This covers the entire muscle

Connective Tissue Within each muscle, cells are grouped into bundle called a fascicle. Each fascicle is surrounded by a fibrous tissue called the perimysium . Perimysium means “around the muscle”

Connective Tissue Within in each fascicle, each muscle fiber (cell) is surrounded by a fine CT layer called the endomysium. The name means “within the muscle.”

Epimysium (Fascicle) Perimysium Endomysium To Summarize: The layers of CT superficial to deep. Epimysium (Fascicle) Perimysium Endomysium

Nerves Specialized nerve cells Motor Neurons Specialized nerve cells Their cell bodies are located in the brainstem or nerve cord and their axons extend into the muscle fibers They stimulate contraction At the perimysium, the axons branch, each toward the center of one muscle fiber

Myofibril Myofibril Threadlike structure that extends from one end of the muscle fiber to the other Composed of 2 proteins filaments called Myofilaments Actin are thin (8 nanometers in diameter) Myosin are thick (12 nanometers in diameter) Sarcomeres are highly ordered units in which actin and myosin are organized http://www.youtube.com/watch?v=Ae0UYxqj3cM and http://highered.mcgraw- hill.com/sites/0072495855/student_view0/chapter10/anima tion__myofilament_contraction.html

Actin Myofilaments Actin Each actin myofilament is composed of 2 strands of fibrous actin (F actin) The two strands of F actin are coiled to form a double helix Each F actin is a polymer of 200 small globular units, or monomers

Globular actin Globular Actin Each has an active site that myosin attaches to during contraction Tropomyosin is an elongated protein that winds along the groove of the F actin double helix

Myosin Filaments Long and shaped like golf clubs Composed of myosin molecules Long and shaped like golf clubs Each myosin molecules consists of heavy myosin molecules wound together to form a rod portion and 2 heads that extend laterally. Each myosin myofilament consists of 300 myosin molecules

Myosin Heads Three important properties The heads can bind to active sites on actin molecules to form cross-bridges The heads are attached to the rod portion by a hinge region The heads have ATPase activity, the enzyme that breaks down ATP for energy

Physiology of Skeletal Muscle Fibers Action Potentials are . . . Electrical signals of the nervous system Transmitted along axons to muscle fibers Stimulates production of action potentials in muscle fibers, causing contraction http://www.youtube.com/watch?v=CepeYFvqmk4

Types of Contraction Isometric Contraction Length of muscle is constant Tension increases Example are the muscle of posture in the spine

Length of muscle changes Tension is constant Types of Contraction Isotonic Contraction Length of muscle changes Tension is constant 2 types (Concentric and Eccentric)

Types of Isotonic Contraction Concentric Contraction Muscle length shortens Tension increases The most common contraction; lifting, walking, running, etc.

Types of Isotonic Contraction Eccentric Contraction Muscle length increases Tension remains constant Example is lowering a heavy object http://www.youtube.com/watch?v=jKygojJgFQE

Length vs. Tension The length of muscle before stimulation influences the force of a contraction or the amount of work a muscle can do This explains why weight lifters have a particular stance before they lift weights

Question?? What happens when a muscle is not stretched? Decrease in tension because actin and myosin are already overlapping

Question?? What happens when a muscle is severely stretched? Decrease in tension because actin and myosin are only overlapping in a small area and fewer connections can be made

Question?? When is a muscle able to perform at its peak? When the muscle is relaxed and actin and myosin are overlapping in every possible place

Fatigue We have all experienced fatigue after a long work-out session It can be due to 3 factors: The nervous system The muscle fibers Neuromuscular junctions

Nervous System Fatigue The most common Called psychological fatigue It happens when your mind “perceives” additional work as being impossible and action potentials are never sent It can be helped with encouragement

Happens when ATPase is depleted Muscle Fiber Fatigue The second most common Happens when ATPase is depleted

Neuromuscular Junction Fatigue It is fairly rare Happens when the release of acetylcholine is greater than acetylcholine synthesis

Inflammation in muscle fibers and connective tissue Muscle soreness Inflammation in muscle fibers and connective tissue Built-up of lactic acid

There are two types of muscle fibers: slow and fast twitch Types of Fibers There are two types of muscle fibers: slow and fast twitch The amount of each in humans is primarily determined by your genes

Slow Twitch Also called high-oxidative or Type 1 Contraction slower Smaller in diameter Better blood supply Large amounts of myoglobin acts as a reservoir for oxygen Fatigue resistant Dark meat in chicken due to increased blood supply

Fast Twitch Less myoglobin Also called low-oxidative or Type 2 Contract fast Larger diameter Less blood supply Less myoglobin Fatigue happens faster Light meat of chicken due to low blood supply

Fiber distribution No clear distinction in humans Sprinters have more fast twitch fibers Long distance runners have more slow twitch fibers

Aging and the muscular system Time to contract increases Action Potentials sent decreases Loss of muscle fibers after 25 years old 50% of muscle fibers gone at 80 The amount of fast-twitch fibers is greatly reduced