Muscular System Notes

Muscular System Overview

  • Introduction to Muscles

    • Muscles are defined as organs that generate force to facilitate various types of movement.
    • Examples of muscle actions include:
    • Walking
    • Breathing
    • Pumping blood
    • Moving food in the digestive tract

  • Types of Muscle Tissue

    • There are three primary types of muscle tissue in the body:

    • Skeletal Muscle

      • Major Location: Attached to bones.
      • Major Function: Movement of bones at joints, maintenance of posture.
      • Characteristics:
      • Striations: Present
      • Nuclei: Many nuclei
      • Mode of Control: Voluntary
      • Contraction Characteristics: Contracts and relaxes rapidly when stimulated by a motor neuron.

    • Smooth Muscle

      • Major Location: Walls of hollow viscera (e.g., intestines, blood vessels).
      • Major Function: Movement of viscera, peristalsis, vasoconstriction.
      • Characteristics:
      • Striations: Absent
      • Nuclei: Single nucleus
      • Mode of Control: Involuntary
      • Contraction Characteristics: Contracts and relaxes slowly; single unit type is self-exciting and operates in a rhythmic manner.

    • Cardiac Muscle

      • Major Location: Wall of the heart.
      • Major Function: Pumping action of the heart.
      • Characteristics:
      • Striations: Present
      • Nuclei: Single nucleus
      • Mode of Control: Involuntary
      • Contraction Characteristics: Network of cells contracts as a unit; self-exciting and rhythmic.

Microscopic Anatomy of Muscle Tissue

  • Cell Structure

    • Each muscle fiber is a single, long, cylindrical muscle cell.
    • Fibers respond to stimulation by contracting.
    • Cell Membrane: Known as the sarcolemma.
    • Cytoplasm: Known as the sarcoplasm; contains many mitochondria and nuclei.
    • Sarcoplasm contains parallel myofibrils that are active in muscle contraction.
    • Thick filaments in myofibrils consist of the protein myosin.
    • Thin filaments consist mainly of the protein actin, along with troponin and tropomyosin.

  • Striations

    • Organization of these filaments produces bands called striations.

  • Sarcomeres

    • Myofibrils are made up of many units called sarcomeres, joined end-to-end.
    • A sarcomere extends from one Z line to the next.
    • Striations consist of…
    • I bands (Light bands): Made up of actin filaments, anchored to the Z lines.
    • A bands (Dark bands): Composed of overlapping thick (myosin) and thin (actin) filaments.
    • H zone: A center of the A band, consisting of myosin filaments only.
    • M line: Found in the center of the H zone, comprises proteins holding the myosin filaments in place.

Mechanisms of Muscle Contraction

  • Neuromuscular Junction

    • Skeletal muscle fibers contract only when stimulated by a motor neuron.
    • Each skeletal muscle fiber is functionally connected to the axon of a motor neuron, creating a synapse known as the neuromuscular junction.
    • The neuron communicates with the muscle fiber using chemicals called neurotransmitters, released at the synapse.
    • The cytoplasm of the distal end of the motor neuron contains numerous mitochondria and synaptic vesicles storing neurotransmitters.
    • The muscle fiber membrane contains a region called the motor end plate, which is tightly folded and contains specific receptors for neurotransmitters.

  • Process of Stimulation

    • When an electrical impulse reaches the end of the motor neuron axon, synaptic vesicles release neurotransmitters into the synaptic cleft (the gap between the neuron and muscle fiber).
    • Neurotransmitters diffuse across the cleft, bind to receptors on the motor end plate, and stimulate the muscle fiber to contract.

Functions of the Muscular System

  1. Voluntary Movement
  2. Involuntary Movement
  3. Maintaining Posture
  4. Heat Production
    • Less than half of energy released in cellular respiration forms ATP; the remaining energy becomes heat, which is carried by blood to other tissues, helping to maintain body temperature.

Muscle Fatigue

  • Definition of Muscle Fatigue
    • Refers to the loss of a muscle's ability to contract during strenuous exercise.
  • Causes of Muscle Fatigue
    • Electrolyte imbalances and decreased ATP levels may contribute to fatigue.
    • Decreased pH due to lactic acid accumulation may also play a role.
  • Muscle Cramps
    • A sustained, painful, involuntary contraction that occurs due to changes in extracellular fluid around the muscle fibers, leading to uncontrolled muscle fiber stimulation by motor neurons.

Behavioral Properties of Muscles

  • All muscle tissues have four common behavioral characteristics:
    1. Extensibility: Ability to be stretched.
    2. Elasticity: Ability to snap back to original length after a stretch.
    3. Irritability: Ability to respond to stimuli from the nervous system.
    4. Contractility: Ability to shorten or contract.

Types of Muscle Fibers

  • Skeletal Muscle Fiber Classification

    • Fast Fibers

    • Make up the majority of muscle fibers.

    • Characteristics:

      • Rapid contractions, reaching maximum force quickly but also fatigue quickly.
      • Large diameter and few mitochondria.
      • Store glycogen for anaerobic metabolism, suited for short-term activities.

    • Slow Fibers

    • Characterized by:

      • Smaller diameter, resistant to fatigue, takes longer to reach peak tension.
      • Many mitochondria and capillaries, relying on aerobic metabolism.
      • Provide prolonged contraction.

Exercise and Muscle Use

  • Muscle Response to Activity Levels
    • Hypertrophy: Enlargement of a muscle due to repeated exercise.
    • Atrophy: Decrease in muscle size and strength due to disuse.
    • The type of exercise determines muscle responses:
    • Low intensity increases slow fibers' mitochondria and enhances fatigue-resistance, while maintaining size and strength.
    • Forceful exercise increases fast fibers' actin and myosin filament numbers, enlarging fibers and muscle, thus allowing stronger contractions.
    • Note: The number of skeletal muscle fibers does not change with hypertrophy or atrophy.

Types of Contractions

  • Muscle Contraction Types
    • Muscles can generate force without shortening.
  • Isotonic Contraction
    • Definition: Involves shortening of the muscle.
    • Associated with movement (e.g., lifting a weight).
    • Tension remains the same as the muscle shortens.
  • Isometric Contraction
    • Definition: Involves force generation without shortening.
    • Used to resist overstretching and oppose gravity (e.g., holding a weight in one position).
    • Most movements combine both types of contractions.

Skeletal Muscle Actions

  • Connective Tissue Functions

    • Connective tissue made of collagen helps hold joints together.
    • Ligaments: Connect bones to other bones.
    • Tendons: Connect muscles to bones.

  • Muscle Attachments

    • Origin: The less movable end of a skeletal muscle.
    • Insertion: The more movable end of a skeletal muscle.
    • Muscle contractions pull the insertion toward the origin.
    • Example: Biceps Brachii
    • Name: "Biceps" means 2 origins or heads.
    • Both heads attach to portions of the scapula (coracoid process and tubercle above glenoid cavity).
    • Insertion: Radial tuberosity of the radius.
    • The muscle is located on the anterior surface of the humerus, and its action is flexion of the forearm at the elbow.

Muscle Relationships

  • Common Movements at Joints
    • Flexion: Decrease in the angle between bones at a joint (e.g., bending the arm at the elbow).
    • Extension: Increase in the angle between bones at a joint (e.g., straightening the arm at the elbow).
    • Skeletal muscles generally function in groups.
    • The muscle that causes an action and does the majority of the work is called the agonist (prime mover).
    • Muscles that assist the prime mover are known as synergists.
    • Muscles that oppose an action are referred to as antagonists.
  • The relationships between muscles can vary depending on the action; a muscle can act as a synergist in one scenario and as an antagonist in another.

How Forces Affect the Body

  • Force Generation and Resistance
    • The human body generates and resists forces during daily activities.
    • Internal forces produced by muscles enable bodily movements.
    • External forces, such as air resistance and friction, can hinder movement.
    • The net force is the single force resulting from the summation of all forces acting on a structure.

Basic Kinetic Concepts

  • Kinetics: The analysis of the actions of forces.
  • Pressure: The amount of force spread over an area; for example, the pressure exerted on the floor when shifting weight onto one foot.
    • Example Question: Would you prefer being stepped on by someone wearing a stiletto shoe or a tennis shoe?
  • Torque: A twisting force generated by a contracting muscle.
    • When a muscle contracts, it applies a pulling force on a bone, causing it to move at the nearby joint center.
    • Torque is calculated as:
      extTorque=extForceimesextDistanceext{Torque} = ext{Force} imes ext{Distance}
      where the force is from the muscle and distance is between the muscle attachment and joint center.

Forces and Injury to the Human Body

  • Types of Forces:
    • Compression: A squeezing force, such as when landing from a jump, creating compression forces on skeletal bones.
    • Tension: A pulling force, for example, when hanging from a pull-up bar, resulting in tension in the arms.
    • Shear: A tearing force, often responsible for abrasions, such as when a baseball player slides into a base.

Combined Loads

  • Exist when multiple forces act simultaneously on an object.
  • Examples of Forces:
    • Bending: An off-center force.
    • Torsion: A twisting force along an object's length.

Effects of Force Application

  • Deformation: The change in shape of body tissues due to force.
    • Elastic Deformation: Occurs in soft tissue; when small forces are applied and then removed, the tissue returns to its original size and shape.
    • Plastic Deformation: Occurs in stiffer tissue such as bone; if the force applied exceeds the tissue's elastic limit, permanent deformation may occur.