Muscle Tissue Overview

Muscle Tissue

• Three types of muscle tissue: skeletal, smooth, and cardiac
• Skeletal muscle: voluntary, responsible for movement
• Smooth muscle: involuntary, found in internal organs (e.g., stomach, intestines)
• Cardiac muscle: involuntary, found in the heart
• Cardiac muscle cells are connected by intercalated discs allowing for rapid electrical communication and coordinated contractions. These intercalated discs consist of gap junctions

Skeletal Muscle Tissue

• Functions of skeletal muscle: movement, posture, heat production
• Gross structure of skeletal muscle: epimysium (outer layer), perimysium (surrounds fascicles), endomysium (surrounds individual muscle fibers), fascicles (bundles of muscle fibers), muscle fibers (individual cells), myofibrils (bundles of protein filaments), myofilaments (actin and myosin).
• Skeletal muscle cell structure:
  • Sarcolemma (cell membrane)
  • Triad (combination of transverse tubules and terminal cisternae)
  • Transverse tubules (invaginations of sarcolemma), allow for rapid transmission of action potentials
  • Terminal cisternae of sarcoplasmic reticulum (store calcium ions critical to muscle contraction)
  • Myofibrils (bundles of protein filaments responsible for contraction)

Myofilaments

• Actin – thin filament, with binding sites for myosin
• Myosin – thick filament, with heads that bind to actin and form cross-bridges
• Troponin – protein complex associated with actin, regulates the position of tropomyosin
• Tropomyosin – protein that blocks the myosin-binding sites on actin when the muscle is at rest
• Sarcomere – repeating unit of myofibril, extends from one Z-disc to the next
• Z-disc/line – defines the boundaries of the sarcomere
• M line – supporting structure in the middle of the sarcomere
• A band – entire length of the myosin filament
• I band – region containing only actin filaments

Sliding Filament Model and Triggers

• Action Potential

  • An action potential (AP) causes the release of acetylcholine (Ach) from the motor neuron.
  • At the neuromuscular junction (NMJ), Ach binds to receptors on the muscle cell, initiating an AP
  • Ach is released as a result of the graded potential in the motor neuron. Motor neuron is presynaptic, the muscle cell is postsynaptic. Ach causes an excitatory postsynaptic graded potential.
  • Ach causing to cause an excitatory postsynaptic potential which ultimately leads to muscle cell contraction.
  • The motor end plate is specialized region of the sarcolemma, with a high density of Ach receptors.

• Calcium Role

  • Release of calcium ions (Ca2+) from the sarcoplasmic reticulum (SR) is triggered by the muscle AP.
  • Ca2+ binds to troponin, causing a conformational change that moves tropomyosin away from the myosin-binding sites on actin.
  • Myosin heads bind to actin, generating a power stroke and causing muscle contraction.
  • Ca2+ is necessary to initiate and sustain muscle contraction

• Filament Interactions

  • Myosin heads bind to exposed actin-binding sites, forming cross-bridges.
  • The power stroke occurs, pulling the actin filament toward the center of the sarcomere.
  • ATP binds to myosin, causing the release of the myosin head from actin.
  • The cycle repeats as long as Ca2+ is present.

Sarcomere and Muscle Tension

• Sarcomere length affects the amount of tension a muscle can generate.
• Optimal sarcomere length allows for maximum overlap of actin and myosin filaments.
• Twitch: a single contraction-relaxation cycle in a muscle fiber
• Myogram elements:
  • Latent period: time between stimulus and contraction
  • Contraction phase: muscle shortening
  • Relaxation phase: muscle returning to resting length

Energy for Working Muscles

• Creatine phosphate: provides rapid energy for short bursts of activity
• Aerobic metabolism: provides sustained energy for longer duration activities, using oxygen
• Glycolysis alone: provides energy for short, high-intensity activities (anaerobic)

Muscle Fiber Types

• Different fiber types: fast-twitch, slow-twitch, intermediate. Benefits and costs vary between these different fiber types

Brain and Spinal Cord

• Protection: meninges (dura mater, arachnoid mater, pia mater)
• Blood-Brain Barrier (BBB): formed by astrocytes, protects the brain from harmful substances and protects CNS neurons
• Cerebrospinal Fluid (CSF): circulates within ventricles, provides cushioning, buoyancy, and removes waste products
• Brain and Spinal Cord Organization: white matter (axons), gray matter (neuron cell bodies), different paths for sensory and motor information
• Major Brain Structures:
  • Brainstem (midbrain, pons, medulla oblongata)
  • Cerebellum
  • Diencephalon (thalamus, hypothalamus)
  • Pineal gland
  • Cerebrum
• Spinal Cord Structure: gray matter in the center and white matter surrounds it, motor, fine touch/proprioception, pain and temperature path information
• Reflexes and Reflex Arcs: innate, acquired, monosynaptic, polysynaptic reflexes, patellar tendon reflex

This quiz covers the three types of muscle tissue: skeletal, smooth, and cardiac. It explores their functions, structures, and the unique features of cardiac muscle cells that facilitate coordinated contractions. Test your knowledge on the roles and properties of each muscle type.