Skeletal Muscle

Structure and function

  1. Describe the types and major functions of muscle tissue.  

    • Smooth muscle

      • Lack obviously organized myofilament

      • Spindle-shaped mono- to bi-nucleate cells

      • Controlled by autonomic nervous system

      • Lines the outside of tubes in the body and moves contents about

    • Cardiac muscle

      • Has striated myofilaments but not as highly organized as in skeletal

      • Rectangular mono- to bi-nucleate cells

      • Modulated by the autonomic nervous system

      • Forms the thick walls of the heart and pumps blood

    • Skeletal muscle

      • Highly organized myofilaments giving obvious striations

      • Large, elongate, multi-nucleate cells

      • Controlled by the somatic nervous system

      • Most abundant type of muscle and main focus of this lecture 

      • Active component of the musculoskeletal system that moves you about

  2. Describe the organization of skeletal muscle, from cell (skeletal muscle fiber) to whole muscle and justify calling a named muscle an organ. 

    • myofiber/muscle fiber contained within sarcolemma

    • groups of fibers

  3. Name the connective tissue layers that surround each skeletal muscle fiber, fascicle, entire muscle, and group of muscles and indicate the specific type of connective tissue that composes each of these layers. 

    • sarcolemma: plasma membrane of a muscle fiber

    • endomysium: areolar, wraps around single muscle cell/fiber (over sarcolemma)

    • perimysium: dense irregular, wraps around fascicle of skeletal muscle cells

    • epimysium: dense irregular, wraps around all fascicles within a muscle

  4. Describe the main components within a skeletal muscle fiber (e.g., sarcolemma, transverse [T] tubules, sarcoplasmic reticulum, myofibrils, thick [myosin] myofilaments, thin [actin] myofilaments, troponin, tropomyosin). 

    • sarcolemma

      • plasma membrane surrounds muscle fibers

    • t tubules

      • plasma membrane invaginates to surround each myofibril

    • sarcoplasmic reticulum

      • smooth endoplasmic reticulum that wraps around each myofibril and activates/relaxes shortening

    • myofibrils

      • formed by synthesis of myofilaments

    • myosin (thick)

      • protein that converts atp to mechanical energy, generating muscle contraction

    • actin (thin)

    • troponin

      • 3 subunits bind to help position and inhibit

    • tropomyosin

      • regulatory protein, along filament with 2 per strand, blocks myosin binding site

  5. Define sarcomere. 

    • shortening unit of a myofibril

  6. Describe the arrangement and composition of the following components of a sarcomere: A-band, I-band, H-zone, Z-disc (line), and M-line. 

    • z line at end

    • m line in middle

    • a band in middle

    • I band at ends

    • h zone in very middle

  7. Describe the structure of the neuromuscular junction (relate back to synaptic transmission form neural tissue). 

    • between axon and somatic motor

  8. Define the sliding filament theory of skeletal muscle contraction. 

    • thin filaments slide over thick filaments in ATP dependant manner, sarcomere shortened

  9. Describe the sequence of events involved in the contraction of a skeletal muscle fiber, including events at the neuromuscular junction, excitation-contraction coupling, and cross-bridge cycling. 

    cross-bridge:

    1. motor with bound atp or adp + pi binds to actin

    2. actin interaction with motor stimulates pull of actin and its filament towards center of sarcomere

    3. Pi and adp disassociate from motor domain

      1. crossbridge remains giving rigor state without ATP

    4. motor domain dissociates from actin, mves back, and recocks motor for another round

    5. if high load: crossbridge cannot fully complete power stroke

    6. if low load: cycle gives max speed giving max shortening (isotonic)

  10. Describe the sequence of events involved in skeletal muscle relaxation. 

    • calcium ions pumped into sarcoplasmic reticulum

    • causes low tension state

Physiology

  1. Define motor unit and muscle twitch and describe the events occurring during the three phases of a muscle twitch.

    motor neuron and muscle cells it innervates

    twitch: max level of force bc calcium saturation

    form tetanus in muscle after increased frequency

    • single action potential gives twitch

      • refractory: delay from Electrochem coupling and beginning crossbridge formation

      • force development: crossbridge cycling

      • force relaxation: calcium pumped back into sr preventing cycling

    • amount of action is determined by number of motor units fired

  2. Explain how smooth, graded contractions of a skeletal muscle are produced.

    • ap gives twitch

      • refractory

      • force development

      • force relaxation

    • increased firing turns twitch into tetanus

  3. Differentiate between isometric and isotonic muscle actions and types of isotonic actions.

    • isometric: no change in length, max force

      • determined by stimulation freq, sarcomere length, cross sectional area

    • isotonic: constant change in length/speed via action

      • concentric: shortening

      • eccentric: lengthening

  4. Describe the force-velocity curve and its X and Y intercepts and the power curve.

    • y is force, x is shortening

    max velocity: cant generate much force

    power = force x speed

    isometric: max force

  5. Describe factors that influence the force, velocity, and duration of skeletal muscle action.

    • inc freq - max thin filament active - max crossbridge cycl

    • myosin type (isoform) determines max atpase rate - crossbridge cycl rate

    • high speed: fast myosin isoforms

    • slow speed: slow myosin isoform

  6. Define lever and explain how a lever operating at a force advantage differs from one operating at a speed advantage.

    most often: second class

    • fulcrum/pivot; joint

    • load/resist: item moved

    • effort/force: result of action

    • force advantage: short distance between load and fulcrum, long between effort and fulcrum

    • speed advantage: long distance between load and fulcrum short between effort and fulcrum

  7. Name the three types of lever systems and indicate the arrangement of effort, fulcrum, and load in each. Also note the advantages of each type of lever system.

    strap like: origin to insert

    pinate: angled fibers, cross section diameter

    • first class

      • fulcrum between load and effort

    • second class

      • fulcrum one end, effort at other, load between

    • third class: load at one end, fulcrum at other, effort between load and fulcrum

  8. Explain how the multicellular organization of muscle fibers in a muscle organ influences its musculoskeletal action (speed vs force).

    • low cross sec area = low force

    • more sarcomeres = high extent of shortening

    • high cross sec = high force

    • fewer sarcomeres = low extent of shortening

  9. Identify and name the function of select muscles of the head.