contractile units of muscle

*5 different types:

o   Actin filaments (composed of actin)

o   Myosin filaments (composed of myosin)

o   Sarcolemma: plasma membrane

o   Sarcoplasm: cytoplasm

o   Sarcoplasmic reticulum: ER

·      Excitable: receive and respond to stimuli

·      Contractile: shorten forcibly when adequately stimulated

·      Extensible: can be stretched out

·      Elastic: can recoil after being stretched

skeletal muscle 

organization

muscle -> fascicle -> muscle fiber (cell) 

skeletal muscle:

connective tissue sheaths

o   Endomysium: surrounds each muscle fiber

o   Perimysium: surrounds each fascicle (bundle of muscle fibers)

o   Epimysium: surrounds entire muscle  

*sarcolemma is deep to endomysium                 

skeletal muscle:

nerve and blood supply

skeletal muscle 

attachments

*insertion: end of muscle attached to bone that moves

*origin: end of muscle attached to bone that moves less

-Note: most muscles attache to bone, cartilage, or other muscles by tendons (cord-like) or aponeuroses (flat)

o   Sarcoplasm

o   multiple nuclei

o   glycosomes (glycogen storing granules)

o   Mitochondria (supply ATP)

o   Myoglobin (red pigment; stores Oxygen)

o   Myofibrils

o   Sarcoplasmic reticulum

o   T tubules

-extends lengths of muscles

-takes up 80% of muscle cell

-composed of sarcomeres (repeating units)

-contain actin and myosin myofilaments

Actin Myofilaments

o   Thin filaments composed of:

-  2 strands of round actin subunits

-2 regulatory proteins: tropomyosin and troponin

o   Thick filaments composed of:

*  Bundles of myosin molecules

-  Each myosin molecule has a rod-like tail and two globular heads

*Arrangement of actin and myosin filaments give fibers their “striated” appearance.

* I-bands: light regions

-contain actin filaments only and Z discs (anchors actin filaments)

*A-bands: dark regions

-  contains Actin and myosin filaments [h zone has myosin only] and M line (anchors myosin filaments)

                                               i.     Each fiber (cell) is innervated by 1 motor neuron

                                              ii.     each motor neuron innervates several fibers.

·      Region where axon terminal of motor neuron comes into close contact with muscle cell is neuromuscular junction.

·      Resting muscle cells are polarized (interior of cell negative in relation to outside of cell)

·      **fiber is a cell**

·      ACh receptors are chemically gated Na+/K+ ion channels

·      More Na+ enders than K+ exits ->causes a local depolarization (interior of cell becomes positive)

·      If local depolarization is strong enough, voltage-gated Na+ channels open ->  further depolarizes sarcolemma

->initiates on action potential.

·      action potential propagated along sarcolemma

excitation: contraction coupling

·      Action potential propagates along sarcolemma and down T tubules

·      Triggers sarcoplasmic reticulum to dump Ca2+ into sarcoplasm.

·      Ca2+ bind to troponins -> shifts tropomuyosin stands exposing active sites on actin.

·      cocked myosin heads attach to active sites forming cross bridges.

1.   cross bridge formation

  2. the power (working) stroke

3. cross bridge detachment

4. cocking of myosin head

o   motor neuron stops stimulating muscle fiber

o   Ach remaining in synaptic cleft Is degraded by acetylcholinesterase.

o   dumped Ca2+ is pumped back in to sarcoplasmic reticulum

o   lack of ca2+ blocks myosin binding sites on actin (tropomyosin re-covers active sites on actin)

o   muscle cell relaxes.

**  (Mentally learn how to judge and react.)

·      Each muscle supplied by1 or more nerves (bundle of neurons).

·      each motor neuron stimulates many muscle fibers (4à several hundred/motor unit).

·      muscle fibers of a motor unit are spread throughout entire muscle

·      stimulation of a single motor neuron causes weak contraction of entire muscle

o   Response of 1 motor unit to a single stimulus.

o   3 distinct phases:

- Latent period: time between application of stimulus and start of contraction

-  Contraction: cross bridge cycle; muscle tension increases

-relaxation: SP reabsorbs Ca 2+ from sarcoplasm; muscle tension decreases

o   varying muscle contraction depending on demand.

o   achieved by altering either frequency of stimulus or strength of stimulus

o   Changes in stimulus frequency

-  applying multiple stimuli causes wave summation (successively stronger contractions)

-  partial relaxation between contractions (unfused tetanus)

o   Changes in stimulus strength

-Increasing Strength of stimulus increases the force of muscle contraction

- force increases because a greater number of motor units are stimulated [recruitment]

Isotonic vs. Isometric 

Contractions

Isotonic: ("same tension") muscle length changes to move blood

-ex: all jumping & throwing activities

Isometric: ("same length") muscle neither shortens nor lengthens [load is too heavy]; hoever internal tension increases.

-ex: trying to lift a piano; the increase in tension just before a muscle moves

o   stores only 4-6 seconds worth of ATP

o   makes ATP in 3 ways (1. Direct phosphorylation of ADP via creatine phosphate.

o   lactic acid fermentation (anaerobic respiration)

o   Aerobic respiration

-  Activities require a surge of power for a few seconds (diving, weight lifting, etc) rely on ATP and creatine phosphate stores.

- activities requiring burst-like activities (tennis, soccer etc) reply on lactic acid fermentation

-activities requiring endurance (marathon running, jogging, etc) rely on aerobic respiration

Aerobic (endurance) exercise

-increases number of mitochondria and capillaries in muscles

-increases synthesis of myoglobin (allows muscles to store oxygen)

-results in increase efficiency, greater endurance, strength, & resistance to fatigue

-increase in size of muscle fibers

-increase in # of mitochondria & glycogen storage

-results in increase muscle strength and definition