Physiology of Nerve Impulse Transmission

Transmission of Nerve Impulse to Muscle

• Neurotransmitters are chemicals released upon nerve impulse, specifically acetylcholine for skeletal muscles.
• Acetylcholine binds to receptors on the sarcolemma, making it permeable to sodium ions (Na+).
• The influx of sodium generates an action potential which initiates muscle contraction, an irreversible process once started.

The Sliding Filament Theory of Muscle Contraction

• Activation from the nerve causes myosin heads (crossbridges) to attach to thin filament binding sites.
• Myosin heads continue to bind to consecutive sites along the thin filament, resulting in the sliding of myosin along actin.
• This sliding mechanism causes muscle shortening (contraction).

Contraction of Skeletal Muscle

• Muscle fiber contraction operates on an "all or none" principle; not all fibers may contract simultaneously.
• Different combinations of fiber contractions create graded responses with varying degrees of shortening.
• Rapid stimuli can lead to continuous contraction or tetanus due to increased muscle fiber activation.

Muscle Response to Strong Stimuli

• Muscle force correlates with the number of fibers stimulated; more active fibers result in greater muscle tension.
• Muscles maintain contraction until energy reserves are depleted.

Energy for Muscle Contraction

• Muscles initially utilize stored ATP for energy, sufficient for 4-6 seconds.
• After depleting ATP, creatine phosphate (CP) facilitates energy transfer to ADP, regenerating ATP before CP runs out in about 20 seconds.
• Anaerobic glycolysis breaks down glucose without oxygen, yielding ATP quickly but inefficiently, producing lactic acid which leads to fatigue.
• Aerobic respiration, occurring in mitochondria, breaks down glucose into carbon dioxide and water, generating energy while requiring continuous oxygen.

Muscle Fatigue and Oxygen Debt

• Muscle fatigue results in an inability to contract, primarily due to oxygen debt that needs to be repaid to detoxify lactic acid.
• Increased lactic acid acidity and ATP depletion diminish muscle contractility.

Types of Muscle Contractions

• Isotonic contractions involve myofilaments sliding past each other, resulting in muscle shortening.
• Isometric contractions cause muscle tension to increase without shortening.

Muscle Tone

• Some muscle fibers remain contracted even at rest, providing muscle tone through involuntary control of various fibers.

Muscles and Body Movements

• Movement is achieved as muscles pull on bones at least two attachment points: the origin (movable bone) and insertion (immovable bone).

Effects of Exercise on Muscle

• Enhanced muscle usage results in increased muscle size, strength, efficiency, and fatigue resistance.

Smooth Muscle Characteristics

• Lacks striations, composed of spindle-shaped cells, and operates involuntarily; found in hollow organ walls and contracts slowly and consistently.

Cardiac Muscle Characteristics

• Striated muscle with typically one nucleus per cell, connected by intercalated discs, operates involuntarily and is exclusive to the heart, maintaining steady contractions.

Functions of Muscles

• Muscles produce movement, maintain posture, stabilize joints, and generate heat.

Microscopic Anatomy of Skeletal Muscle

• Skeletal muscle fibers are multinucleate with nuclei located underneath the sarcolemma.
• Sarcolemma is a specialized plasma membrane; sarcoplasmic reticulum is a specialized smooth endoplasmic reticulum.
• Myofibrils contain aligned bundles of myofilaments creating distinct I (light) and A (dark) bands.

Types of Ordinary Body Movements

• Common movements: flexion (decreases joint angle), extension (opposite of flexion), rotation (bone movement around its axis), abduction/adduction (moving away/toward the midline), and circumduction (cone-shaped movement).

Types of Muscles

• Prime mover: primary muscle for a movement.
• Antagonist: opposing muscle to the prime mover.
• Synergist: assists prime mover and stabilizes movement.

Naming of Skeletal Muscles

• Muscles are named based on:
  • Direction of fibers (e.g., rectus for straight)
  • Size (e.g., maximus for largest)
  • Location (e.g., temporalis for located near bone)
  • Number of origins (e.g., triceps for three heads)
  • Shape (e.g., deltoid for triangular)
  • Action (e.g., flexor for muscle that flexes a bone).