Muscle Contraction and Muscle Fiber Types

Neuromuscular Junction and Muscle Contraction

  • Three steps lead to muscle contraction, as reviewed in labs.
  • These steps primarily lead to muscle contraction.

Summary of Muscle Contractions

  • Neuromuscular junction connection is always necessary.
  • Junction: space.
  • Neuromuscular: nervous system and muscular system.
  • Somatic motor neurons connect with skeletal muscle.

Excitation Contraction Coupling

  • Excitation: excitability.
  • Contraction: contractility.
  • Conversion of electrical signal into contractile motion.
  • Primary signal: calcium.
  • Increase in intracellular calcium from the SR (sarcoplasmic reticulum) leads to:
    • Sliding filament theory (majority of contractions).
    • Muscle twitch.
  • Muscle twitches are usually signaled by fatigue or stimulus.
EMG (Electromyogram)
  • Used to test skeletal muscle.
  • Generates signals to witness contractions.
EKG/ECG (Electrocardiogram)
  • Used to measure the electrical activity of the heart.
  • Same thing; EKG is German, ECG is American terminology.
  • German uses 'K' for cardio.
  • Muscle twitch can indicate contraction, excessive stimulation, or pathology.
  • Magnesium glycinate can help reduce the effects of stress.
  • Stress depletes magnesium reserves.
  • Low magnesium can cause nerves to be unable to relax, leading to muscle twitch or tetany.
  • Excessive stress depletes magnesium reserves.
  • Lack of magnesium prevents nerves from relaxing, leading to muscle twitches or tetany.

Neuromuscular Junction (NMJ)

  • Connection between neuron and muscle.
  • Purpose: Creating specialized neurotransmitters to connect neurons to muscles.
  • Common neurotransmitter: Acetylcholine (ACh).
  • ACh: A cholinergic molecule responsible for signaling.
NMJ and EC Coupling
  • Acetylcholine needs to be exocytosed for release.
  • Exocytosis: active transport requiring energy.
  • Action potentials open voltage-gated calcium channels.
  • Voltage-gated: Channels open when stimulated by electrostatic force (action potential).
  • Calcium influx (diffusion) through facilitative diffusion.
  • Calcium influx leads to depolarization (cell becomes positive).
  • Unfavorable state; cell converts energy to return to resting potential via exocytosis.
  • Acetylcholine released into the synapse or junction.
  • Acetylcholine binds to its receptor on the cell surface (sarcolemma).
  • Acetylcholine acts as a ligand.
  • Binding of acetylcholine to the receptor initiates depolarization of the sarcolemma. Muscles are excitable and contractible.
  • Muscle can depolarize because it is excitable and can be stimulated.
  • Sodium channels open, and sodium rushes in (more sodium outside).
  • The influx of sodium causes further depolarization.
  • Depolarization is transmitted through T-tubules.
  • T-tubules stimulate DHP (dihydropyridine) calcium channels.
  • DHP channel opening stimulates RYR (Ryanodine receptor), a mechanically gated channel, to release calcium from the sarcoplasmic reticulum (SR).
  • Extracellular calcium sensed by the DHP receptor triggers the opening of the SR to release intracellular calcium.
  • This step is called EC coupling.
  • Responsible for opening calcium release to begin the sliding filament theory.

Muscle Cramps

  • Muscle cramp: excessive contraction.
  • Often due to issues with repolarization, potentially lacking potassium.
  • Potassium is needed for relaxation.
  • Bananas are recommended for cramps, as they are high in potassium.
  • Channel opens in contracted state, intracellular calcium influx occurs, relaxation closes the channel.
  • Calcium returns to the SR via active transport, requiring energy as it moves against the gradient.

Sliding Filament Theory

  • Key elements: Calcium and ATP.
  • ATP is responsible for detachment.
  • Hydrolyzed ATP (ADP) leads to realignment.
  • Myosin head is static and acts as an anchor, pushing actin during contraction/relaxation.
  • Contraction doesn't always mean angle changes; can involve resisting force, like lowering a weight.

Troponin and Tropomyosin

  • Myosin head contains ATPase.
  • ATP is responsible for detachment.
  • Hydrolyzed ATP (ADP) allows reattachment.
  • Contraction: sliding.
  • Tension: Resistant force related to the angle and weight.
Rigor State
  • No ATP, ADP, or inorganic phosphate present in the myosin head.
  • Myosin head tightly bound.
  • Rigor mortis: muscle freeze due to lack of ATP, which is needed for detachment.
G Actin vs F Actin
  • G actin: Globular actin.
  • F actin: Filament actin (e.g., tropomyosin).
Rigor Mortis
  • Skeletal muscle issue.
  • Smooth muscle has different pathways without sarcomeres.
  • Smooth muscle lacks sarcomeres.
  • Nervous system shuts down upon death.
  • Sphincters relax.

Energy for Skeletal Muscle

  • Multiple ways to receive ATP.
  • Aerobic (with oxygen) vs. anaerobic (without oxygen) respiration.
  • Slow oxidative (aerobic) vs. fast glycolytic (anaerobic).
  • Fast glycolytic provides immediate energy.
  • Slow oxidative generates more ATP (35-37 ATP) than fast glycolytic (2 ATP).
  • Anaerobic respiration byproduct: lactic acid (causes muscle soreness).
  • Parts of the body do not have oxygen distributed, causing lactic acid buildup.
Heart Attack (Infarction)
  • Lack of blood flow to heart muscles.
  • Ischemia: Pain due to lack of blood flow and lactic acid buildup.
  • Heart uses glycogen stores when transitioning from oxidative to glycolytic pathways.
Phosphocreatine Pathway
  • Faster way to get ATP.
  • Creatine tests are usually blood tests to assess kidney function (creatine excreted by kidneys).
  • ATP is stored in a resting state by binding a phosphate to creatine, forming ADP and phosphocreatine.
  • During work phase, phosphate is grabbed from phosphocreatine and bound to ADP to create ATP.
  • This bypasses cellular respiration, offering immediate ATP.
  • At rest, excess ATP binds to creatine to form phosphocreatine (energy storage).
  • During activity, phosphocreatine donates phosphate to ADP, forming ATP.
Creatine Supplementation
  • Creatine gives you a good feeling.
  • Problem: Swelling, and you lose gains when you stop using creatine.
  • Creatine can be obtained from meat and liver.
  • Creatine dehydrates you. It converts sugar to energy, so be cautious and stay hydrated.
  • Excess creatine is excreted by the kidneys.
  • If you have kidney problems, you should not take creatine.

Muscle Fiber Types

  • Slow twitch (oxidative) vs. fast twitch (glycolytic) fibers with intermediate fibers.
  • Create a table to compare and contrast the qualities of fast twitch and slow twitching muscles.
Slow Twitch
  • Oxidative, aerobic.
  • Generates most ATP.
  • Marathons (long range).
Fast Glycolytic
  • Anaerobic.
  • Fastest ATP generation.
  • Sprinting, powerlifting.
  • Have the least amount of mitochondria (glycolysis occurs in the cytoplasm).
  • More vascularized.
Intermediate (Fast Oxidative Glycolytic Fibers)
  • Type IIa.
  • Triathletes; bike, swim, running.
  • Pale pigment due to less blood flow and fewer mitochondria.
  • Mitochondria have iron.

Tension Generation Relationships

  • Tetanus: maximum contraction with little or no relaxation.
Motor Unit
  • One motor neuron and multiple muscle fibers.
  • For more precision, one has less muscle fibers.
  • More precise (eyes) = more muscle fibers are required.
Length-Tension Relationship
  • Fixed length provides optimal tension.
  • Too much/little overlap decreases tension.
  • Optimal resting length needed for highest tension (between 2-2.3 micrometers).
Isotonic vs. Isometric Contractions
  • Isotonic: Changes in angle.
  • Isometric: No change in angle.
Isotonic
  • Concentric (shortening).
  • Eccentric (lengthening).
  • Lengthening while the muscles remain contracted.
Isometric
  • No angular change.
  • Planking.
  • No angular change, but connective tissue stretches.
  • Sarcomere shortens.
Tension Contractions
  • Changing distance.
  • Concentric: Shortening.
  • Eccentric: Lengthening.
Muscle Tension
  • Muscle tension plateaus, and is gradually lost over time.
  • Performing a exercise for so long will eventually exhaust the muscle.
  • Muscle tension does not require that much force. Forces are required to move a load to generate movement.

Smooth Muscle and Cardiac Muscle

  • Smooth muscle: involuntary, mononucleated, non-striated.
Smooth Muscle Location
  • Vascular, GI, urinary, respiratory, reproductive, ocular.
  • Ocular: Pupil dilation. Pupils are smooth muscles.
  • Blinking: Skeletal Muscle.
Contractions
  • Phasic: Alternating contractions and relaxation (e.g., eating/swallowing).
  • Tonic: Continuous contractions (e.g., urination/defecation).
Urination/Defecation
  • Psychological aspects.
  • Neurological aspects.
Single vs. Multi Units
  • Looks like NMJ, but with varicosities (big storage of neurotransmitters).
  • Most likely acetylcholine, to regulate smooth muscle.
  • Skeletal muscle usually is not vital (diaphragm is the exception). Other smooth muscle will be vital.
  • Due to specialized organs requires a higher level of regulation for chemical messengers.
Neuron Types
  • Neurons that regulate smooth muscle-all are visceral.
    *Visceral = involuntary response regulation
  • Smooth muscles activation is based off of myosin light chain.
  • Instead of going through NMJ EC coupling they have their own method of light chain here.
Dense Bodies
  • Responsible for smooth muscle shortening and contracting when stimulated by visceral neurons.
Smooth Muscle Regulation
  • Controlled by the autonomic brain.
Paracrine Signalling
  • Grabbing chemical messengers and sending to neigboring cells.
Airways
  • Histamine: released during allergies. Causes vasoconstriction. Dilates blood vessels, causing redness during allergic reactions.
Nitrous Oxide
  • Very potent vasodilator.
  • Brings blood pressure down.
  • Nitroglycerin:
    • Paramedics will use nitroglycerin which will be put under the tongue.
    • Prolongs the blood flow into the heart until operated on by some kind of surgical intervention.
  • Most potency to carry happens when muscles are relaxed. More energy is used for resistant tension.