topic 2

• skeletal muscle tissue


  attached to bones and skin

  striated

  voluntary

  powerful
• skeletal muscle location and shape

  attached to bones or the skin

  single, very long, cylindrical, multinucleate cells with obvious striations
• cardiac muscle location and shape

  walls of the heart

  branching chains of cells; uni or binucleate; striations
• smooth muscle location and shape

  unitary muscle in walls or hollow visceral organs; multi unit muscle in intrinsic eye muscles, airways, large arteries

  single, fusiform, uninucleate; no striations
• sarcomere


  smallest contractile unit of a muscle fiber

  the region of a myofibril between 2 successive z discs

  it contains one a banded flanked by half an i band at each end

  composed of thick and thin myofilaments
• z disc


  coin shaped sheet of proteins that anchors the thin filaments and connects myofibrils to one another
• h zone


  lighter midregion where filaments do not overlap
• m line


  line of protein myomesin that words adjacent thick filaments together
• thick filament


  many myosin molecules whose heads protrude at opposite ends of the filament
• thin filaments


  two strands of actin subunits twisted into a helix plus two types of regulatory proteins
• muscle contraction


  the generation of force

  does not necessarily cause shortening of the fiber

  shortening occurs when tension generation by cross bridges on the thin filaments exceeds forces opposing shortening
• in the relaxed state thin and thick


  filaments overlap only slightly
• during contraction


  myosin heads bind to actin, detach, and bind again, to propel the thin filaments toward the m line
• as h zones shorten and disappear

  sarcomeres shorten, muscle cells shorten, and the whole muscle shortens
• during a muscle contraction

  the h zone disappears, the z discs get closer, the i zones shrink, the m line and a zone do not change
• requirements for skeletal muscle contraction

  1. events at the neuromuscular junction

  2. excitation contraction coupling

  3. cross bridge cycles
• events at the neuromuscular junction

  -skeletal muscles are stimulated by somatic motor neuronal

  -axons of motor neurons travel from the central nervous system via nerves to skeletal muscles

  -each axon forms several branches as it enters a muscle

  -each axon ending forms a neuromuscular junction with a single muscle fiber
• events at neuromuscular junction steps

  1. action potential arrives at axon terminal of motor neuron

  2. voltage gated ca2+ channels open and ca2+ enters the axon terminal

  3. ca2+ entry causes some synaptic vesicles to release their contents by exocytosis

  4. acetylcholine, a neurotransmitter, diffuses across the synaptic cleft and binds to receptors in the sarcolemma

  5. ach binding opens ion channels that allow simultaneous passage of na+ into the muscle fiber and k+ out of the muscle fiber

  6. ach effects are terminated by its enzymatic breakdown in the synaptic cleft by acetylcholinesterase
• events in the generation of an action potential

  1. local depolarization: generation of end plate potential on the sarcolemma

  2. generation and propagation of the action potential

  3. repolarization
• excitation-contraction coupling


  sequence of events by which transmission of an AP along the sarcolemma leads to sliding of the myofilaments

  latent period: time when e-c coupling events occur, time between ap initiation and the beginning of contraction

  ap is propagated along sarcolemma to t-tubules

  voltage sensitive proteins stimulate ca2+ release from sr; ca2+ is necessary for contraction
• excitation contraction coupling steps

  1. action potential is propagate along the sarcolemma and down the t-tubules

  2. calcium ions are released

  3. calcium binds to troponin and removes the blocking action of tropomyosin

  4. contraction begins
• role of calcium in contraction at low intracellular concentrations

  tropomyosin blocks the active sites on actin because troponin is not bound by ca2+

  myosin heads cannot attach to actin

  muscle fiber relaxes
• cross bridge cycle steps

  continues as long as the ca2+ signal and adequate atp are present

  cross bridge formation

  working (power) stroke

  cross bridge detachment

  "cocking" of the myosin head
• cross bridge formation


  high energy myosin head attaches to thin filaments
• working stroke


  myosin head pivots and pulls thin filament toward m line
• cross bridge detachment


  atp attaches to myosin head and the cross bridge detaches
• cocking of the myosin head

  energy from hydrolysis of atp cocks the myosin head into the high energy state
• sarcoplasmic reticulum


  a network of smooth endoplasmic reticulum surrounding each myofibril

  its modified into pairs of terminal cisternae which form perpendicular cross channels

  it functions in the regulation of intracellular calcium levels

  it communicates with other regions at the H-zone
• t-tubules allow or the

  conduction of motor nerve impulses deep into the muscle fiber. integral proteins protrude into the intermembrane space between the t-tubules and the terminal cisternae
• in a muscle contraction


  z discs and thin filaments get closer, h line disappears, m line doesnt move, i bands get narrower and move closer together