Muscle Tissue and Energy Production

Brown Fat in Babies and Adults

  • Babies possess brown fat due to underdeveloped skeletal muscles, and it aids in maintaining body temperature, preventing shivering.
  • Adults can acquire brown fat through exposure to cold environments (e.g., football players).
  • Brown fat boosts ATP production and helps regulate body temperature.

Types of Muscle Tissue: A Detailed Comparison

The following is a detailed comparison of the three types of muscle tissue: skeletal, cardiac, and smooth.

Features

1. Location:

  • Skeletal Muscle: Attached to bones, forming part of the skeletal system.
  • Cardiac Muscle: Exclusively found in the heart.
  • Smooth Muscle: Located in various internal structures (e.g., digestive system, urinary system, iris of the eye, blood vessels).

2. Appearance:

  • Skeletal Muscle: Characterized by parallel, cylindrical, striated fibers, which may be multinucleated. Striations result from contractile proteins, and nuclei are sometimes pushed to the periphery.
  • Cardiac Muscle: Features branched, striated fibers, each containing a single nucleus. Large mitochondria necessitate branching, with fibrous connective tissue (collagen) filling the spaces. Intercalated discs facilitate communication between cells, enabling them to contract as a unit.
  • Smooth Muscle: Composed of spindle-shaped fibers lacking striations, hence the term "smooth."

3. Control:

  • Skeletal Muscle: Voluntary control.
  • Cardiac Muscle: Involuntary control.
  • Smooth Muscle: Involuntary control.

4. Contraction Speed:

  • Skeletal Muscle: Fast contraction speed, essential for survival (fight or flight).
  • Cardiac Muscle: Moderate contraction speed, ensuring efficient blood pumping without overexertion.
  • Smooth Muscle: Slow contraction speed, crucial for processes like digestion and blood pressure regulation.

5. Function:

  • Skeletal Muscle: Movement, heat production, regulating organ volume, and stabilizing body positions
  • Cardiac Muscle: Pumping of blood.
  • Smooth Muscle: Peristalsis, segmentation, internal sphincters, regulating organ volume (e.g., iris controlling pupil size).

Connective Tissue Associated with Skeletal Muscle

  • The muscle is observed in cross-section, revealing individual muscle fibers.

Endomysium

  • Connective tissue that surrounds each individual muscle fiber.
  • \"Endo\" means inner.

Perimysium

  • Surrounds groups of muscle fibers, forming bundles called fascicles.

Epimysium

  • Encloses a group of fascicles, creating the entire muscle.
  • These layers (endomysium, perimysium, epimysium) extend beyond the muscle to form the tendon.

Tendons

  • In the lab, tendons are identified as dense, regular connective tissue.
  • In this content, tendons are defined as extensions of the endomysium, perimysium, and epimysium beyond the muscle.
  • Tendons attach muscle to bone.

Aponeurosis

  • A broad, flat, tendon-like structure that attaches muscles (e.g., scalp muscles to cranial bones, facial muscles).

Deep Fascia

  • Surrounds parts of the perimysium, protecting structures like arteries, veins, and nerves.
  • Supports and protects blood vessels and nerves supplying the skeletal muscle.

Superficial Fascia

  • Merges with deep fascia to form the hypodermis (under the skin).

Nerve Impulses and Muscle Contraction

Axon Potentials

  • Depolarization: The charge starts to flip, becoming more positive on the inside and more negative on the outside.
  • Action Potential: The charge is completely switched, with all positives inside and all negatives outside.
  • Requires sufficient stimulus strength for muscle contraction.
  • Repolarization: Return to the resting potential, with the inside becoming more negative again.

Axon Resting State

  • More negative on the inside and more positive on the outside.
  • Depolarization is needed to flip the charge for muscle contraction.

Action Potential and Muscle Contraction

  • The action potential is the goal because action means contraction.
  • The entire axon must flip the charge.

Muscle Relaxation

  • The axon must return to the resting potential (repolarization).
  • Another impulse is needed for the muscle to contract again.
  • Sufficient strength means reaching the action potential.

Nerve Supply and Neuromuscular Junction

Somatic Motor Neuron

  • The nerve cell that supplies skeletal muscle.

Neuromuscular Junction (NMJ)

  • The area where the nerve meets the muscle.

Motor End Plate

  • The skeletal muscle area at the NMJ.

Neurotransmitter: Acetylcholine (ACh)

  • Released by the neuron and picked up by skeletal muscle cells at the motor end plate, initiating current flow throughout the muscle tissue.
  • Stored in synaptic vesicles within synaptic bulbs.

Process

  1. Impulse reaches synaptic bulbs.
  2. Acetylcholine is released.
  3. Acetylcholine is picked up by receptors at the motor end plate.
  4. The impulse (current) spreads throughout the muscle tissue in both directions.
    • The NMJ is in the middle of the muscle to facilitate this.

Anesthesia

  • General anesthetics work by paralyzing muscles, including the diaphragm.
  • Anesthetic drugs have a similar structure to acetylcholine, binding to receptors and preventing muscle contraction.
  • The body breaks down the drug to restore muscle function post-surgery.

T-Tubules and Calcium Release

  • The current travels throughout the muscle tissue and down the T-tubules.
  • The goal is for the current to reach the sarcoplasmic reticulum to release calcium.

Muscle Relaxation

  • Acetylcholine must be removed from the receptors.
  • Acetylcholine esterase, an enzyme, breaks down acetylcholine into acetate and choline.
  • Acetate and choline are reabsorbed, and the axon repolarizes.

Key Terms for Skeletal Muscle

  • Sarcolemma: Cell membrane of skeletal muscle cells.
  • Sarcoplasm: Cytoplasm of skeletal muscle cells.
  • Sarcoplasmic Reticulum (SR): Stores calcium in skeletal muscle cells.

Myofibrils

  • Muscle fibers within skeletal muscle cells.

Types

  • Actin: Thin filament.
  • Myosin: Thick filament.
  • Contractile proteins enabling muscle contraction.
  • More myofibrils result in stronger contractions.

Actin Binding Site

  • Covered when the muscle is relaxed.
  • Troponin and Tropomyosin: Two proteins associated with actin that regulate the exposure of the binding site.

Myosin Head

  • Locks into the binding site on actin, forming a cross-link or cross-bridge.
  • Causes sliding, shortening the muscle.
  • The myosin head binds to actin as long as the binding site is exposed.

Key Requirements for Muscle Contractions

  • Calcium.
  • ATP.
  • Also need ATP for muscles to relax.

Sarcomere

  • Functional unit of skeletal muscle.
  • Millions of sarcomeres work together for muscle contraction.

ATP and Myosin

  • ATP binds to myosin.
  • ATP is broken down into ADP + phosphate.
  • Changes the shape of the myosin head.
  • Calcium binds to troponin; calcium causes troponin and tropomyosin to move away from actin.
  • Exposes the binding site.
  • Myosin head binds to the binding site on actin, forming a cross-bridge.
  • ADP and phosphate move away from myosin.
  • Actin and myosin slide over each other, shortening the muscle (contraction).

Muscle Relaxation (Requires Additional ATP)

  • A new ATP binds to myosin.
  • Breaks the cross-bridge between actin and myosin.
  • Calcium is released from troponin, and the binding site is covered again.

Summary of Muscle Contraction and Relaxation

  • The nerve impulse is transmitted.
  • Calcium binds to troponin, exposing the binding site.
  • ATP binds to myosin, changing its shape.
  • Myosin head locks into the binding site, forming a cross-bridge, then sliding.
  • Contraction occurs, shortening the muscle.
  • Another ATP molecule is needed to break the cross-bridge for relaxation.

Rigor Mortis: Muscle Stiffening After Death

  • Cause of death: hypoxia

Stages of Death

  • Palomortis: Bluish skin discoloration (cyanosis), indicating no more oxygen.
  • Algomortis: Body temperature goes down because contractions aren't happening with the same force.
  • Livor Mortis: Blood pools.
  • Rigor Mortis: Muscle stiffening.

Rigor Mortis in Detail

  • Typically occurs an hour or two after death and lasts about twelve hours.
  • Caused by muscle fibers still contracting after death due to stored ATP.
  • The muscles stay stiff because there is no new ATP to break the cross-bridges.
  • Proteins eventually break down, and the person comes out of rigor.

ATP Production: Cellular Respiration

Cellular Respiration

  • The process by which a cell uses glucose and oxygen to synthesize ATP.
  • Glucose is always the preferred source to make ATP.

Types of Respiration

  • Anaerobic Respiration: No oxygen required.
  • Aerobic Respiration: Oxygen is required.

Anaerobic Respiration (Glycolysis)

  • Occurs in the cytoplasm of the cell.
  • Glycolysis: breaking sugar
  • Glycolysis is a series of 10 chemical reactions that uses oxidative reactions.
  • The chemical reactions occur in a specific order and are all enzyme regulated.
  • Requires an investment of 2 ATP molecules to start.
End Product of Glycolysis
  • 2 Pyruvate molecules (3 carbon compound, half of a glucose).
  • 4 ATP molecules.
  • Net 2 ATP molecules.

Pyruvic Acid and Oxygen

  • Oxygen is needed in the cytoplasm and in the mitochondria.
  • If oxygen is present:
    • Pyruvic acid gets broken down into a chemical called acetyl CoA.
    • Acetyl CoA enters the mitochondria for aerobic respiration.
  • If there is not enough oxygen present:
    • Pyruvic acid does not get converted into Acetyl CoA.
    • Pyruvic acid is converted into lactic acid instead, the waste product.

Lactic Acid

  • Possibilities
    • Oxygen levels go back up, no major damage done (ideal scenario).
    • Lactic acid gets converted back to pyruvic acid then Acetyl CoA.
    • Or it can be transported to the liver where the liver converts lactic acid to pyruvic acid making some ATP.
    • Another possibility (worst case scenario): Oxygen levels never go back up; lactic acid builds more and more and becomes very fatal.
      • Means the pyruvic acid to CoA is never able to occur.

Fatigue

  • Lactic acid from muscle metabolism contributes to muscle fatigue.