muscle system 01

The Muscular System

• Muscle is one of the four basic tissues in the body, comprised of cells capable of shortening and lengthening
• Muscles carry out commands from the nervous system
• Cardiac and smooth muscle functions are preprogrammed, not reliant on nerve input, although the nervous system can regulate their activity
• Three types of muscle:
  • Skeletal muscle:
    • Voluntary control
    • Moves the skeleton
    • Striated
    • Somatic nervous system
  • Cardiac muscle:
    • Found only in the heart
    • Involuntary control
    • Striated
    • Autonomic nervous system
  • Smooth muscle:
    • Found throughout the body (e.g., eyes, lungs, stomach, intestines, urinary bladder)
    • Involuntary control
    • Autonomic nervous system

Myo- and Sarco- Prefixes

• Myo- refers to muscle (e.g., myositis is inflammation of muscle)
• Sarco- refers to muscle cells (e.g., sarcoplasm is the muscle cell cytoplasm)

Skeletal Muscle

• Can be consciously controlled, but also has an automatic "cruise control" for functions like breathing
• Gross anatomy:
  • Belly: The thick, central contractile portion of the muscle
  • Tendons: Fibrous bands connecting muscle to bone
  • Aponeuroses: Broad fibrous bands connecting muscle to bone or muscle to other muscle
  • Linea alba: White line running lengthwise between abdominal muscles, connecting them

Muscle Action

• Movement is achieved by pulling on attachment points.
• Prime movers (agonists) are the primary muscles responsible for movement.
• Antagonists oppose the prime mover to smooth the motion
• Synergists assist the prime mover
• Fixators stabilize joints for other movements

Muscle Names

• Names may reflect shape (e.g., deltoid), location (e.g., biceps brachii), fiber direction (e.g., rectus), number of heads (e.g., triceps), or attachment sites (e.g., sternohyoideus)

Microscopic Anatomy of Skeletal Muscle

• Skeletal muscle cells are long, cylindrical, and multinucleated, often called muscle fibers
• Contain myofibrils, which are made of smaller protein filaments
• The basic contractile unit is the sarcomere
  • Contains actin (thin filaments) and myosin (thick filaments
  • I band: Primarily actin filaments (light bands)
  • A band: Thick myosin filaments (dark bands)
  • Z line: Separates sarcomeres
• Sarcomere contraction is the basis for muscle contraction

Connective Tissue Layers

• Endomysium surrounds individual muscle fibers
• Perimysium surrounds fascicles (bundles of muscle fibers)
• Epimysium surrounds the entire muscle
• These tissue layers work together, and provide support and protection to the muscle fibers

Neuromuscular Junction

• The point where nerves connect to muscle fibers
• Synaptic vesicles contain acetylcholine, a neurotransmitter that initiates muscle contraction
• Acetylcholine is quickly broken down by acetylcholinesterase to stop contraction

Mechanics of Contraction

• During muscle relaxation, actin and myosin filaments overlap slightly
• Contraction involves myosin filaments pulling on actin, shortening the sarcomere
• Requires ATP, using chemical energy from nutrients (glucose and oxygen)

Muscle Contraction- other details

• Has a latent phase, contracting phase, and relaxation phase
• A twitch is a single muscle fiber contraction; coordinated contractions yield sustained movement
• The amount and coordination of muscle fibers stimulated controls the force and precision of the movement
• "Muscle memory" reflects the nervous system's training for skilled movements

Chemistry of Muscle Contraction

• ATP provides the energy for muscle contraction
• Creatine phosphate (CP) quickly replenishes ATP
• Breakdown of nutrients (glucose using oxygen) creates ATP and CP
• Anaerobic metabolism occurs in the lack of oxygen and creates lactic acid, which causes muscle fatigue

Heat Production

• Muscle contraction produces substantial heat that needs to be dissipated through sweating or panting
• Shivering is a response to hypothermia

Cardiac Muscle

• Microscopic anatomy: Cardiac muscle cells are smaller, branched, and have a single nucleus.
• Intercalated disks connect cells for coordinated contraction.
• Physiology: Cells contract spontaneously without external stimulation
• Cardiac impulse conduction rate depends on the fastest cell in the group.
• Sinoatrial (SA) node serves as the natural pacemaker.
• Nervous system influences cardiac activity through sympathetic and parasympathetic nerves (adjusting heart rate and strength)

Smooth Muscle

• Microscopic: Spindle-shaped cells with single nuclei, no striations (smooth).
• Visceral smooth muscle: Function in large waves of sustained contraction in the walls of hollow organs
• Multiunit smooth muscle: In eyes and other places where delicate, precise control is necessary

The Integumentary System

• The integumentary system comprises the outermost layers of the body
• Consists of (primarily) skin and related structures (hair, nails, sweat glands)
• Protects, covers, and regulates body temperature