Muscular System Study Notes

Muscle Function: Overview

• Types and Functions of Muscles
• Skeletal Muscle:
  • Striated and voluntary
  • Functions:
  • Produces movement
  • Maintains posture
  • Stabilizes joints
  • Generates heat
• Smooth Muscle:
  • Non-striated and involuntary
  • Location:
  • Walls of viscera, tubes, and passageways
  • Functions:
  • Maintains tone
  • Allows for stretch
• Cardiac Muscle:
  • Striated and involuntary
  • Located in the heart
  • Function: Pumps blood

Structure of Whole Muscle and Single Muscle Fiber

• Whole Muscle Structure:

• Fascia: Layers of connective tissue surrounding muscle

  • Epimysium: Outermost layer
  • Perimysium: Surrounds fascicles (bundles of fibers)
  • Endomysium: Surrounds individual fibers

• Tendons:

  • Cord-like structures that attach muscles to bones

• Single Muscle Fiber Structure:

• Sarcolemma: Muscle fiber membrane

• Transverse Tubules (T-tubules): Invaginations of the sarcolemma

• Sarcoplasmic Reticulum (SR): Specialized endoplasmic reticulum that stores calcium

• Myofibrils: Long cylindrical structures within muscle fibers

• Sarcomeres: Contractile units containing actin (thin filaments) and myosin (thick filaments)

Sliding Filament Mechanism of Muscle Contraction

• Overview:
• Thin filaments (actin) slide past thick filaments (myosin), shortening the sarcomere
• Steps in Contraction:

1. Nerve impulse stimulates the muscle fiber
2. Calcium is released from the SR
3. Calcium binds to troponin, exposing myosin binding sites on actin
4. Myosin heads bind to actin, forming cross-bridges
5. Myosin heads pull actin filaments toward the sarcomere center
6. ATP provides energy for myosin head detachment and re-cocking

Role of Calcium and ATP in Muscle Contraction

• Calcium:
• Binds to troponin, initiating the sliding filament mechanism
• ATP:
• Provides energy for myosin head movement and detachment
• Required for calcium reuptake into the SR during relaxation

Skeletal Muscles and Nerves

• Motor Unit:
• Single motor neuron and all the muscle fibers it innervates
• Neuromuscular Junction (NMJ): Site of communication between motor neuron and muscle fiber
• Action Potential: Nerve impulse travels down the motor neuron
• Acetylcholine (ACh): Neurotransmitter released at NMJ, binding to muscle membrane receptors
• Muscle Membrane Stimulation: ACh binding leads to electrical impulse for contraction
• Recruitment: Activation of additional motor units to increase muscle force

Single-Fiber and Whole-Muscle Responses

• Twitch: Single muscle fiber response to a single stimulus
• Tetanus: Sustained contraction from rapid, repetitive stimulation
• Recruitment: Increasing active motor units to increase force
• Muscle Tone: State of partial contraction maintained by different groups of fibers

Energy Sources for Muscle Contraction

• Creatine Phosphate: Provides rapid energy for short-term activity
• Glycolysis: Anaerobic metabolism of glucose producing ATP and lactic acid
• Aerobic Metabolism: Metabolism of glucose and fatty acids in the presence of oxygen, yielding large ATP amounts

Muscle Terminology and Naming

• Origin and Insertion:
• Origin: Attachment to stationary bone
• Insertion: Attachment to movable bone
• Synergists and Antagonists:
• Synergists: Assist muscle movement
• Antagonists: Oppose muscle movement
• Muscle Naming: Based on size, shape, fiber orientation, location, number of origins, origin and insertion, and action

Major Skeletal Muscles

• Head:
• Facial muscles (e.g., frontalis, orbicularis oris)
• Chewing muscles (e.g., masseter, temporalis)
• Trunk:
• Breathing muscles (e.g., diaphragm, intercostals)
• Abdominal muscles (e.g., rectus abdominis, obliques)
• Shoulder and Arm:
• Deltoid, pectoralis major, latissimus dorsi, biceps brachii, triceps brachii
• Wrist, Hand, and Fingers:
• Flexor and extensor carpi/digitorum groups
• Thigh, Leg, and Foot:
• Gluteal muscles, quadriceps femoris, hamstrings, gastrocnemius

Key Terms

• Acetylcholine (ACh): Neurotransmitter that stimulates muscle contraction
• Actin: Thin filament contractile protein
• Antagonist: Muscle opposing another muscle’s action
• Creatine Phosphate: Energy storage form in muscles
• Myofibrils: Structures within muscle fibers that contain sarcomeres
• Neuromuscular Junction (NMJ): Communication site between neuron and muscle fiber
• Sarcomere: Basic contractile unit of muscle
• Sliding Filament Mechanism: Process of muscle contraction by filament sliding