Lecture 2.16 - Cadiac/Smooth, MuscIII; Ca2+, Twitch, SO/FOG, Gap Junction, Intercalated Disc, Myogenic

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• A) Neurons : spatial, temporal, or both = more likely to reach AP ; Muscles : recruiting more neurons

• B) Only for neurons

• C) More neurons not necessary for strength - only more frequent action potentials ; is true for neurons

• D) Only for muscles

Summary of skeletal muscle contraction

• NMJ - where neuron meets sarcolemma at motor end plate

• ACh released from motor neuron via exocytosis

• ACh binds to nicotine receptor, opens, allowing Na+ in and causing an AP if large enough

• AP passes down sarcolemma into T-tubules, allowing AP to enter muscle fiber by reaching DHPR / L-type channel

• DHP(voltage-gated Ca2+ channel) connected to RyR channel (in skeletal muscles), allows Ca2+ to rush into sarcoplasm and Ca2+ to be released from intracellular stores

• Ca2+ binds to troponin C, pulls on I, T →moves tropomyosin

• Tropomyosin reveals myosin binding sites

• Cross-bridge cycling

• Myosin flexes, closing distance between actin fibers, causing contraction

• Ca2+ removed via ATP (AChE), goes back into sarcoplasmic reticulum via SERCA, tropomyosin cover binding sites…

Striated muscle

• Due to organization of Actin and Myosin

  • Dark spots - nuclei

• Skeletal muscles = Linear, contracts in same direction

• Cardiac muscles = branch

• Smooth muscles - not striated

Cardiac muscle, striated

• Striated, but branches

• Can be multi-nucleated, sometimes binucleate nucleated, but usually mono-nucleated

• Dark lines - Z

• Light lines - M

Smooth muscle

• NOT STRIATED

  • Mon-linear organization of Actin and Myosin

• Mono-nucleated

Differences - AP + Twitch speed

• Not all skeletal muscle is the same - latent period between AP and Contraction can vary

  • Fast Twitch / FG (Type 2)- contraction right after AP

    • Faster depolarization

    • Shorter latent period

    • Shorter contraction

  • Slow Twitch / SO (Type 1)- contraction a little after AP

    • Slower depolarization

    • Longer latent period

    • Longer contraction

• Muscles can have combinations of slow/fast twitch fibers with some predominating

• Smallest force out of the 3 Twitch types

Fast-twitch muscle (Type 2) / Fast Glycolytic (FG) and Fast Oxidative Glycolytic (FOG)

• PURPOSE:

  • quick bursts

  • large force output

• Reaches depolarization faster

  • Due to number and properties of Na+/K+ channels

  • Short latent period

• Contraction occurs right after AP

• Fatigue quickly

• Fewer mitochondria

• Pale/While color

• High force

• **TWO TYPES:

  • FOG - Fast Oxidative/Glycolytic

    • Medium force

  • FG - Fast Glycolytic

    • Greatest force

EXAMPLES

• Gastrocnemius (jumping calf muscle)

• Biceps brachii

• Finger flexors

• Eye muscles (extraocular muscles — extremely fast)

Slow-twitch muscle (Type 1) / Slow Oxidative (SO)

• PURPOSE:

  • maintain posture

  • repeated activity

  • low-force endurance

    • Can last all day

• Slower depolarization

  • Longer contraction cycle

  • Longer latent period

• Electron Transport chain - Use Oxygen, unlimited ATP

• Resistant to fatigue

• More mitochondria

• Red color

• Lower force

EXAMPLES

• Soleus (calf postural muscle) → standing

• Back extensor muscles (erector spinae) → posture

• Neck stabilizers

• Diaphragm (important!) → continuous breathing

Slow vs Fast Twitch muscle chart

Cardiac Muscle - Twitch

• Normal depolarization phase, but very long strangely shaped repolarization phase, that goes into the contraction

  • Contraction resembles that of the Slow Twitch

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• Predominantly Type I = Slow Twitch?

  • → Think of ALL DAY things

• Answer = B

SO vs FOG/FG Chart (MEMORIZE - Glycogen, Myoglobin, Z-width, Oxidative Enzyme Capacity)

• SO use oxygen via ETC (SLOW), can last all day (continuously generate energy)

• FOG use ETC (slow) and glycolysis (fast!)

  • Hybrid of slow and fast fibers

    • Contract fast, resist fatigue longer than FG

    • Activities:

      • Walking up hill, repeated lifting, middle distance running

• FG uses only glycolysis (FAST)

  • Greatest force, fastest fatigue

    • Activities:

      • Sprinting, jumping, throwing

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Non-obvious stats

• SO and FOG have much higher resistance to fatigue than FG

• FG/FOG have higher glycogen amount than SO (energy)

• SO and FOG have higher myoglobin amount than FG

  • SO and FOG are darker meat, FG is lighter meat

  • Also more fat storage on SO/FOG

• Z-width: FG < SO < FOG

  • FOG require force AND repetition

  • SO only requires repetition

  • FG only requires force

• SO > FOG > FG for oxidative enzyme capacity

Fiber distribution

• Orbicularis - Eyes

  • Majority FOG/FG, less SG

    • Fast movements

• Diaphragm

  • ~ 1:1 SO to FOG/FG, though more SO

• Quadriceps

  • ~1:1 SO to FOG/FG

• Soleus - Calves, walking, all-day muscle

  • All-day walking muscle, mostly SO

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• Cardiac - E

• Heart is always pumping, can speed up, but mostly same rhythm

High frequency stimulation in Cardiac muscles

• High frequency stimulation causes arrhythmic cardiac muscle contractions

• Heart muscles CANNOT stay in tetanus

  • If heart cannot relax, it cannot fill with blood and cause a contraction to send blood out

• Thus : RyR channel is not connected to DHPR channel / L-Type channel

Cardiac muscle: Ca2+ Pathway

• RyR channel is separated from DHPR channels

  • In skeletal, they are physically attached

• 1) AP spreads along sarcolemma and into T-tubules, reaches voltage-gated DHPR channel, opens due to DP

• 2) Ca2+ enters cells through the DHPR channel

• 3) Ca2+ level gets high enough, THEN RyR channels open → DEPOLARIZATION OPENS SARCOPLASMIC RETICULUM (RyR)

  • Ca2+ levels in sarcoplasm sharply rise

• Increases latency period between AP and contraction, which is important for the heart to relax and refill with the next beat

• Cardiac RyR is similar to a voltage-gated channel in Cardiac muscle, but not in skeletal muscle, where it is more like a mechanically gated channel

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• E

Neurogenic muscle

• Muscle is triggered by nerve

  • Nerve sends message to NMJ → depolarization → contraction

    • Skeletal

Myogenic myscle

• Triggered by muscle next to it

  • Sometimes neurogenic muscle

• GAP JUNCTIONS between myogenic muscles

  • Like cardiac muscles

  • Cardiac muscles cause adjacent cardiac muscles to contract, allowing heart to contract as one

    • Pacemaker cells cyclically release AP to cause heart cells to contractions

Neurogenic / Myogenic for Smooth muscles

• Smooth muscle can be myogenic or neurogenic

Cardiomyocyes have unique structures that distinguish them

• Nucleus is inside cardiac cells

• Bi/single nucleated

• Branched; not long and skinny - allows them to contract in multiple directions at the same time

• Intercalated disc - where 2 cardiomyocytes are attached and where gap junctions are

  • Connected via gap junctions (AP spread cell to cell) and desmosomes (velcro)

    • Skeletal muscles are NOT connected by gap junctions

      • Each muscle fiber must be stimulated separately via a motor neuron

Smooth muscles

• Present in cardiovascular system, but not herat - arteries, veins; gut

• Adhesion plates allow them to connect together

  • Myosin/Actin pull on the Adhesion plates on all directions = no pattern

    • Means that cell can contract in multiple directions at same time

• Ex: Allows stomach to get smaller to contract, get smaller, and mechanically grind up food

• Ex: Small intestine can move digested food throughout itself

• Mono-nucleated

• Sometimes have gap junctions

Ca2+ binding to move Actin/Myosin in SM: Calmodulin

• Smooth muscles, instead of using Troponin C, use Calmodulin

  • Ca2+ binds to Calmodulin instead of TnC, causes movement of tropomyosin → contraction

Smooth muscle is in…

• Digestive tract

• Large blood vessels

  • Smooth muscles in organized in 2 layers:

    • Circular muscles, in circles

    • Longitudinal muscles, linearly - peristaltic contraction

      • Shortens and lengthens