C2006/F2402 '05 Outline for Lecture 21 --  (c) 2005 D. Mowshowitz  -- Lecture updated 04/19/05

Useful Web Sites & Animations

Handouts: 21A (muscle structure) & 21B (cross bridge cycle) -- not on web; very similar to pictures in texts. Extra copies will be available after class in boxes on 7th floor Mudd.

I. Muscle

    A. Three main types -- smooth, cardiac and skeletal (See handout 21 A or Purves 47.1 (47.5) or Becker Chap. 23 (compare figs. 23-10, 23-22, 23-23).

    B. Common Features --

1. Have actin MF and myosin

2. Actin and myosin slide past each other; neither shortens. (For skeletal muscle, see Becker fig. 23-16.)

3. Electrically excitable

    C. Some Major Differences

1. Cells Fused? Single cells (smooth) vs fused at ends (cardiac) vs fused, multinucleate (skeletal)

2. Striated vs. not.

3. Role of ATP. Need ATP to remain contracted (skeletal) vs don't (smooth) vs never remain contracted (cardiac)

4. Pacemaker activity or not?  (cardiac always does; some smooth muscle does) .

5. Control of bridge cycle -- all use Ca++, but details differ, as explained in detail below.

a. Use calmodulin or tropomyosin/troponin to control cross bridge formation?

b. Mask actin or myosin?

    D. Details of skeletal muscle structure & overview of how filaments slide-- see handout 21A or Purves fig. 47.3 (47.7) or Becker Ch. 23, fig 23-10 to 23-15.

At this point, it helps to start making a table that summarizes all the similarities and differences between the 3 types of muscle. Fill it in as you go -- add to it as you find out more about the structure and function of the different types.

II. Skeletal Muscle Contraction  -- see animations listed at start of lecture.

    A. Bridge Cycle -- How ATP is used to power sliding of thick and thin filaments -- steps (1 to 4) & states (A to E) match those on handout 21B. See also Becker 23-18; Purves 47.6 (47.10 -- this includes role of Ca++ as well as ATP). Cycle can start anywhere, but description below assumes you start with state B on handout and carry out step 1 first. Role of Ca++ is omitted in this go round.

1. ADP-myosin binds to actin -- Pi released ( converting state B --> state C)

2. Power stroke -- myosin, actin slide relative to one another. ADP released (converting state C --> state D).

3. ATP binds to myosin -- Myosin detaches from actin (converting state D --> state E = state A)

4. ATP is split -- form high energy form of myosin (ADP & Pi remain bound) -- Converting state A/E to state B.

5. Continue with step 1. Note ATP must continue to be split to maintain cross bridges and therefore tension in fiber.

    B. Role of Ca++, troponin and tropomyosin (see handout 21B or Purves 47.4 (47.8) or Becker 23-19)

1. Tropomyosin and troponin are part of the thin filaments

2. Tropomyosin blocks myosin binding sites on actin

3. Ca++ binds to troponin (not tropomyosin)

4. Ca++ binding to troponin --> movement of tropomyosin, exposing actin binding sites, so bridge cycle can go

5. How bridge cycle is blocked/regulated

a. In absence of Ca++, bridge cycle is blocked at step 1 above.

b. In absence of ATP (& presence of Ca++), cycle blocked at step 3 (rigor mortis).

Try problem 9-1, parts A, B & E, 9-11, and 9-12.

    C. Where does Ca++ come from? How is it released? See Becker fig. 23-20 & 21 or Purves 47.5 (47.9)

1. Overall: Ca++ is released from SR (ER).

2. Presynaptic side: AP comes down motor neuron --> releases transmitter (AcCh)

3. Post synaptic side -- events at membrane/motor endplate:

a. AcCh binds to nicotinic receptors on motor endplate

b. Depolarization of muscle membrane = EPP (end plate potential)

c. One AP in neuron --> One EPP = sufficient depolarization to trigger AP in membrane of muscle fiber (One EPSP is not sufficient to trigger an AP in postsynaptic neuron.)

4. T tubules & SR

a. AP in muscle plasma membrane (sarcolemma) spreads to T tubules

b. AP in T tubule --> Ca++ release from SR. Changes in membrane potential in T tubule ---> change in shape of protein in T tubule membrane --> opening of channels in SR (ER)  --> release of stored Ca++

(Coupling probably mechanical between voltage sensitive protein in T tubule membrane and channel in SR membrane. The coupling system is similar, but not exactly the same, in smooth & cardiac muscle, as will be discussed.)

Try problems 9-2 to 9-4.

   D. Twitches and Contractions

1. What's a twitch = 1 contraction = response to one EPP; measured by force exerted by muscle fiber when it contracts.

2. Twitches are summed. See Purves 47.7 (47.11)

a. Twitch lasts longer than muscle membrane AP

b. Second AP can trigger twitch before first is over --> more contraction (shortening)

c. Tetanus: Multiple AP's can ---> fully contracted muscle that stays contracted = tetanus (requires continual splitting of ATP to maintain contraction)

    E. Types of skeletal muscle fibers and contractions -- See Purves 47.8 (47.12).

1. Muscle can be fast twitch or slow

  Fast Twitch Slow Twitch
ATPase of Myosin Higher Lower
Ca++ Pump (returns Ca++ to ER) Faster Slower
Speed of Bridge Cycle Faster Slower
Reach max. tension (after EPP) Rel. quickly Rel. slowly
Size/ max. possible tension Usually larger Usually smaller
Overal Properties of Muscle "flash in the pan" "slow but steady"
Used for quicker response,
bursts of activity (sprinters)
slower response,
sustained activity (long distance runners)

2. Muscle can be oxidative or glycolytic

  Glycolytic Oxidative
Color paler ("white meat") Red color due to myoglobin to store oxygen ("red meat")
# of capillaries to deliver oxygen Rel. low Relatively high
# of mitochondria for oxidative metabolism Rel. low Relatively high
Need for oxygen Low High. Need more oxygen but less glucose -- "Oxygen dependent" but energy metabolism is more efficient
Ease of Fatigue  rel. quickly relatively slowly -- Do not accumulate lactic acid
Glycolytic enzymes Higher Lower

3. Usually Fast twitch fibers are glycolytic; fatigue easily etc.; Slow twitch fibers are oxidative.  Some muscle fibers are fast but oxidative.

4. Exercise changes enzyme content and glycolytic/oxidative differences but not slow vs. fast or # fibers. (Does change fiber size.) Slow and fast are innervated differently, and that can't be changed.

III. Smooth muscle

    A. Ca++ triggers contraction

1. Role of Ca++: State of thick filaments, not state of thin, are affected by Ca++.

2. Where Ca++ comes from
: Most Ca++ comes from outside of cell, not SR. AP opens voltage gated Ca++ channels and Ca++ comes in from extracellular fluid (ECF).

3. Ca++ binding protein: Calmodulin, not troponin, controls response to Ca++. Details of cycle next time.

    B. Nature of contraction -- contraction is slower than with skeletal, but smooth muscle can maintain stable cross bridges    for a long time without breaking ATP. Often used to maintain tension in hollow tubes (GI tract, blood vessels, etc.).

    C. Regulation of contraction

1.  Autonomic system and/or hormones control contraction, not somatic system (as for skeletal).

a. Stimulus can be excitatory or inhibitory. Hormones and autonomic neurotransmitters from postganglionic neurons affect channels & pumps indirectly using 2nd messengers. (Compare to situation with nerve/muscle synapse.)

b. Stimulus can generate an action potential (which in turn affects Ca++ channels) or act through a second messenger to affect Ca++ levels (w/o going through an AP).  Examples in previous lectures: epinephrine can cause smooth muscle contraction (through IP3) or relaxation (through cAMP and Ca++ pump). Response depends on receptors on smooth muscle.

c. Some smooth muscles have pacemaker cells that generate an AP spontaneously. Autonomic and/or hormonal stimulation modulate effects of internal signals from pacemakers.

2. Structure of nerve/muscle synapse is different -- neurons have multiple varicosities (points of contact with smooth muscle -- contain vesicles of neurotransmitter), and smooth muscle has no complex structure at synapse (no motor endplate). One smooth muscle cell (or group of cells) can get input from both PS and S. Details of structure next time.

Next Time: wrap up of smooth muscle, then cardiac muscle; heart structure & function; gas exchange.