The Muscular System

1. The Muscular System “Muscles come and go; flab lasts.” -Bill Vaughan Muscle Tissue: Group of cells that shorten during contraction which creates tension and results in movement.

2. Types of Muscles • There are over 650 muscles in the human body • There are three types of muscle in the human body: • Smooth • Cardiac • Skeletal

3. Types of Muscles • Skeletal Muscle • Most prevalent • Attach bone to bone, via Ligaments. • Voluntary • We have conscious control over these muscles • The brain tells them what to do • “Striated” or Striped Appearance • Series of light and dark • I bands = Light (Actin), A bands = Dark (Myosin)

4. Types of Muscles • Cardiac Muscle • Found only in one place, the heart • Responsible for pumping blood from heart to the rest of the body • Involuntary • Directed by autonomic nervous system • Also “Striated” or Striped appearance • Series of light and dark like skeletal muscle

5. Types of Muscles • Smooth Muscle • Surrounds the body’s internal organs • Including blood vessels, hair follicles, intestines • Contracts more slowly than skeletal muscle, but can stay contracted for a longer time • Involuntary • Directed by autonomic nervous system, like cardiac muscle • Spindle shaped fibres arranged in dense sheets

6. Types of Muscles Larger than Cardiac Smaller than Skeletal Not Striated Skeletal Muscle Smooth Muscle Cardiac Muscle

7. Properties of Muscle Fibres • Irritability • Ability to respond to a stimulus • Contractibility • Ability to shorten in length • Elasticity • Ability to stretch and return to normal • Extensibility • Ability to extend in length • Conductivity • Ability to transmit nerve impulses

8. Muscles PULL!!! • Muscles PULL (by contracting and relaxing) by working in opposing pairs or groups, they cannot push. • Sometimes muscles attached directly to bone, but more often they are joined to bone by tendons.

9. Basic Muscular Principles • Skeletal muscles contract only if stimulated to do so • Skeletal muscles produce movements by pulling on bones • Bones serve as levers and joints serve as fulcrums on these levers • Muscles that move a body segment do not usually lie over that part (e.g. bicep flexes elbow but is in upper arm) • Skeletal muscles almost always act in groups rather than individually (prime movers/agonists, antagonists, stabilizers/fixators, synergists)

10. Muscle Actions • Prime mover or Agonist -a muscle principally involved in a movement is an agonist • Antagonist - muscle with an action opposite to a prime mover is an antagonist • Fixators (stabilizers) increase tension to stabilize joints • Synergists – assist in the action of the agonists but are not primarily responsible for action; known as guiding muscles they assist in refined movements and rule out undesired motions • Reciprocal inhibition – involves muscles working in pairs so as one contracts the other relaxes

11. Muscle Terms • Origin – is least moveable part or the part of the muscle that attaches closest to the midline. It is the proximal attachment. • Insertion – is the most moveable part or the part of the muscle that attaches farther from the midline. It is the distal attachment. • Function (action, motion) is what the muscle does when activated. • Innervate – the nerve(s) that stimulate or activate that muscle. • Blood Supply– the artery that gives the muscle it’s blood.

12. Muscle Functions • Muscles can be grouped according to function (action, motion) • Flexors and extensors • Abductors and adductors • Rotators • Levators and depressors • Tensors • Supinators and pronators

13. The Neuromuscular System Linkage between the Muscular system and the Nervous system (nerve impulses from the brain or spinal cord). Motor Unit • Motor Unit = Motor Neuron + Axon (Pathway) + Muscle (Stimulated) • Motor Units range from small (eye movements) to large (quadriceps of leg). Muscle Twitch • A single nervous impulse and the resulting contraction. • Nerves transmit impulses in “waves” that ensure smooth movements.

14. Neuromuscular Junction • Junction point between the nervous and muscular systems. • A chemical neurotransmitter is released (Ach - acetylcholine) • Detected by receptors on surface of muscle fibre. • Results in muscle contraction

15. The “All-or-None” Principle • One motor unit can stimulate several muscle fibres. • When a motor unit is stimulated to contract, it will do so to it’s fullest potential (maximum) • Whether a motor unit consists of 10 muscle fibres or 800, either all the fibres will contract or none will contract

16. Outside Anatomy of Skeletal Muscle • Skeletal muscle makes up a substantial portion of human body weight • The perimysium(outer connective tissue) binds groups of muscle fibres called fasciculi together which are then bound by the larger/stronger epimysium • The epimysiumenvelopes the entire muscle, it changes its properties as it becomes one with the tendon • Remember the periosteum of the long bone? • This is where the tendon of the muscle attaches to the bone • Muscle is attached to bone at two locations: • Origin = Least movable part of attachment. • Insertion = Most moveable part of attachment.

17. Outside Anatomy: Skeletal Muscle

18. Inside Anatomy of Skeletal Muscle • The endomysium (sheath of connective tissue) surrounds each individual muscle fibre • Beneath the endomysium is the sarcolemma a plasma membrane containing the muscle cell’s cytoplasm (sarcoplasm) • The endomysium is surrounded by a rich blood supply and, therefore, a good system for O2 and CO2 removal • Nerves also supply the muscle • Thread like structures, myofibrils, run the length of the muscle • Within the myofibril are thin (Actin) and thick (Myosin) filaments • Actin and Myosin are contained within Sarcomeres (compartments)

19. Actin and Myosin: The Sliding Filament Theory • Myosin contains a head and a tail • The head attaches to actin, binding them together • Actin has 2 other proteins; • Troponin – has a binding site for calcium • Tropomyosin – Cord like structure that covers the binding site on actin

20. Actin and Myosin: The Sliding Filament Theory Together, actin and myosin, have a swivel-locking mechanism • Myosin head will not attach to actin unless calcium is released by the sarcoplasmic reticulum • This sequence is referred to as the “Excitation Contraction Coupling” • ACh (acetylcholine) initiates this • Tropomyosin swivels, allowing the binding sites on actin filament (troponin) to be exposed • During contraction, protein filaments interact, causing them to slide across each other (the sarcomere shortens).

21. Sliding Filament Theory REST • the cross-bridges extend toward actin • The actin and myosin are now in a coupled position. • The level of calcium concentration is low STIMULATION: • myosin cross-bridges form a type of bond with selected sites on the actin filament • Acto-myosin formation takes place • There is an immediate increase of intracellular Ca2+ • This is brought about by the arrival of the action potential at the transverse tubules, which causes Ca2+ to be released by the SR • The inhibitory action of troponin that prevents actin-mysoin interaction is released when Ca2+ions bind rapidly with troponin in the actin filament. • Now the muscle is “turned on”

22. Sliding Filament Theory CONTRACTION • Cross-bridges swivel or collapse • The muscle shortens and the actin slides over the myosin • Tension develops • The ATP is broken down to ADP plus Pi plus energy • Contraction will continue as long as Ca2+ions remain at a level that inhibits the troponin-tropomyosin system

23. Sliding Filament Theory RELAXATION • nerve stimulus to the muscle is removed, Ca2+ ions move back into the SR • The retrieval of Ca2+from the troponin-tropomyosin proteins “turns off” the active sites on the actin filaments • This deactivation accomplishes two things: • It prevents any mechanical link between the myosin cross-bridges and the actin filaments • It reduces the activity of myosin ATPase so there is no more splitting. Muscle now returns to resting state.

25. Role of TROPONIN and TROPOMYOSIN Calcium (Ca2+) needs to bind troponin which alters the location of tropomyosin Under resting conditions, tropomyosin, BLOCKS the myosin binding site on actin With Ca2+binding to troponin, the mtosin binding site on actin is available!

27. Homework • Explain the “Sliding Filament Theory” in your own words, include the terms: Myosin cross bridges, ATP, Calcium release, Troponin, Tropomyosin • Discuss “Excitation Contraction Coupling” • Summarize in your notes the key roles that ATP and Ca2+ play in the development and execution of a muscle contraction.