7. Muscular Tissue NOTES

MUSCULAR SYSTEM

TYPES OF MUSCLE TISSUE

• Three Types of Muscle

  • A. Skeletal Muscle

  • B. Cardiac Muscle

  • C. Smooth Muscle

COMPARING THREE TYPES OF MUSCULAR TISSUE

• Skeletal Muscle

  • Location: Attached to bones

  • Shape: Long cylinder

  • Nucleus: Multiple, located under sarcolemma

  • Striations: Striated

  • Function: Skeletal movement, heat production

  • Control: Voluntary

• Cardiac Muscle

  • Location: Heart

  • Shape: Branched

  • Nucleus: Single, centrally located

  • Striations: Striated

  • Function: Pumping action of heart

  • Control: Involuntary

• Smooth Muscle

  • Location: Lines some Internal organs & walls of blood vessels

  • Shape: Spindle

  • Nucleus: Single, centrally located

  • Striations: No striations

  • Function: Propels contents of organs, changes diameter of blood vessels

  • Control: Involuntary

FUNCTIONS OF SKELETAL MUSCLE

1. Movement

2. Maintaining posture

3. Stabilizing joints

4. Generating body heat

ORGANIZATION OF A SKELETAL MUSCLE

• Muscle

• Muscle Fascicle

• Muscle Fiber (Cell)

• Myofibril

• Myofilament

CONNECTIVE TISSUE COVERINGS OF MUSCLE

• Epimysium: Covers muscle

• Perimysium: Covers fascicle

• Endomysium: Covers muscle fiber

MUSCLE FIBER (CELL)

• Surrounded by plasma membrane called sarcolemma (sarcolemma is inside the endomysium, inner layer CT)

• Contains sarcoplasm: Cytoplasm rich in mitochondria

• Multiple nuclei located under sarcolemma

• Composed of myofibrils

3 STRUCTURES OF THE MYOFIBRILS

1. Sarcoplasmic Reticulum (SR): Network of tubes all around/surrounding each myofibril.

2. Terminal Cisterns - when the SR enlarges into a pouch. It stores calcium.

3. Transverse Tubule (T-tubule): Fold/tube coming from the sarcolemma that transmits electrical signals deep into muscle fiber (action potential)

   → Triad: Formed by 2 terminal cisterns on either side of 1 T-tubule.

SARCOMERE

• Sarcomeres are what makes up the myofibrils.

• Contain two main types of myofilaments Thick and Thin

• Functional unit of muscle fiber

→ Z Discs: The Boundaries of a Sarcomere

• Z discs are the boundaries of a sarcomere, marking where one sarcomere ends and the next begins.

• They only anchor/attached to the thin (actin) filaments and help maintain structure.

• When a muscle contracts, sarcomeres shorten as actin and myosin slide past each other, pulling the Z discs closer together.
  
  Where Sarcomeres Fall in the Muscle Structure

  • Muscle → Made of muscle fascicles

  • Muscle fascicle → Made of muscle fibers (cells)

  • Muscle fiber → Filled with myofibrils

  • Myofibrils → Made up of repeating sarcomeres

  • Sarcomeres → In the Myofibrils and Contain myofilaments (Thick (myosin & Thin (actin) )

ARRANGEMENT OF SARCOMERE

• M Line: Middle line of sarcomere

• A Band: Contains thick filaments (dark). red thick lines

• I Band: Contains thin filaments (light). blue thin lines

BANDS (STRIATIONS) OF SARCOMERE

• ONLY Thin filaments are attached to Z discs

• A Band (appear in dark striations) contain thick filaments only. RED thick lines

• I Band (appear in light striations) contain thin filaments only. BLUE thin lines.

ARRANGEMENT OF MYOFILAMENTS

• 2 types of myofilaments, THICK & THIN

• Six thin filaments surround one thick filament

• Thick filaments do not move; thin filaments slide towards the midline when the muscle contracts

MYOFILAMENTS: THICK FILAMENT

• Composed of myosin proteins

  • Each myosin molecule head 1 tail and 2 heads.

  • The actin binding sites are on the 2 heads.

  • When muscle contraction begins, Actin-Binding sites on myosin heads will attach to actin(on the thin filaments) and form a cross bridge.

    • Myosin → bind to actin (on thin filaments) Z discs.

MYOFILAMENTS: THIN FILAMENTS

• Composed of three proteins:

  1. Actin: Contains myosin-binding sites. Its where the myosin heads from the thick filament binds to.

  2. Tropomyosin: long strand of protein that Covers binding sites on actin when muscle is relaxed.

  3. Troponin: Anchors tropomyosin in place, but when ready to contract it binds to calcium and moves/pulls the tropomyosin exposing the binding sites .

CONTROL OF SKELETAL MUSCLE MOVEMENT

• Motor Area in the Brain: Sends action potential through neurons

• Neurons transmit action potential to skeletal muscle at a synapse

NEUROMUSCULAR JUNCTION

• Comprises the junction where neurons communicate with muscles

• Motor Unit: Consists of one somatic motor neuron and all muscle fibers it activates

• Neuromuscular Junction: Contact point between motor neuron and skeletal muscle

STRUCTURES AT NEUROMUSCULAR JUNCTION

• Synaptic Knob: Contains synaptic vesicles with acetylcholine (ACh)

• Motor End-Plate: Sarcolemma with junctional folds containing ACh receptors

• Synaptic Cleft: Space between neuron and muscle

EXCITATION AT THE NEUROMUSCULAR JUNCTION

1. Action potential arrives, opening Ca++ channels

2. Ca++ influx causes ACh release

3. ACh binds to receptors, opening ligand-gated Na++ channels

4. Na++ enters, creating an action potential

5. Action potential travels to T-tubules

EXCITATION-CONTRACTION COUPLING

1. Action potential travels down T-tubule, releasing Ca++ from terminal cisterns

2. Ca++ binds to troponin

3. Troponin pulls tropomyosin, exposing myosin-binding sites

4. Cross-bridge forms and thin filaments slide inward

ROLE OF CALCIUM IN SLIDING FILAMENT MECHANISM

1. Binds to troponin

2. Tropomyosin moves away to expose myosin-binding sites

3. Cross-bridges form, power stroke occurs

4. Sarcomere shortens

SLIDING FILAMENT MECHANISM

• Sarcomere contraction leads to thin filaments sliding inwards

• Z lines move closer, I band shrinks, A band remains same width

ROLE OF ATP IN SKELETAL MUSCLE CONTRACTION

• ATP enables myosin head to attach and bend

• New ATP detaches cross-bridge, allowing cycle to repeat

RELAXATION OF A MUSCLE FIBER

1. Cessation of action potential, stopping ACh release

2. Acetylcholine-esterase removes ACh from cleft

3. Ca++ reabsorbed into terminal cisterns

4. Troponin and tropomyosin reposition, blocking myosin-binding sites

5. Cross bridges detach, muscle elongates

MUSCLE TWITCH

• Myogram: Recording of muscle contraction

• Phases of Muscle Twitch:

  1. Latent Period: Action potential propagation

  2. Contraction Phase: Ca++ binds to troponin, cross-bridges form

  3. Relaxation Phase: Ca++ reabsorbed, muscle returns to original length

WAVE (TEMPORAL) SUMMATION

• Rapid nerve impulses increase muscle tension

• Second twitch stronger than first due to increased cross-bridges

TETANUS

• Continuous sustained contractions due to high stimulus frequency

TREPPE

• Gradual increase in muscle tension resembling a staircase pattern, basis for warm-up

TYPES OF MUSCLE CONTRACTIONS

• Isotonic Contractions: Muscle length shortens to move load

• Isometric Contractions: Muscle length remains unchanged due to heavy load

MUSCLE ATTACHMENTS

• Origin: Less movable attachment, more proximal

• Insertion: More movable, distal; closer to origin during contraction

PATHWAYS FOR REGENERATING ATP DURING MUSCLE ACTIVITY

• Short-duration Exercise:

  • Anaerobic pathway using creatine phosphate (15 seconds)

• Prolonged-duration Exercise:

  • Aerobic pathway using glucose and oxygen (30 minutes)

1. Creatine Phosphate

• Provides immediate ATP for 15 seconds:

  • CP + ADP → Creatine + ATP

2. Anaerobic Pathway

• Utilizes glucose through glycolysis

• Energy provided for 30-40 seconds:

  • Glucose → 2 Pyruvic acid + 2 ATP

3. Aerobic Pathway

• Utilizes glucose and oxygen for 32 ATP (30 minutes)

  • Glucose + O2 → CO2 + H2O + ATP

• After 30 minutes, fatty acids and amino acids provide energy for hours.