Articulations – Anatomy

Introduction to Articulations

Articulations, or joints, are locations where two or more bones meet. They allow for varying degrees of movement and are classified based on their structure and function. In this section, we focus on major articulations of the knee, hip, and shoulder, and the ligaments that support them.

• Definition: An articulation is a joint, the point of contact between bones, cartilage, or teeth.

• Function: Joints provide mobility and stability to the skeleton.

Major Articulations and Ligaments

• Knee:

  • Anterior Cruciate Ligament (ACL)

  • Posterior Cruciate Ligament (PCL)

  • Lateral Meniscus

  • Medial Meniscus

  • Fibular (Lateral) Collateral Ligament (LCL)

  • Tibial (Medial) Collateral Ligament (MCL)

• Hip:

  • Pubofemoral Ligament

  • Iliofemoral Ligament

  • Ischiofemoral Ligament

• Shoulder:

  • Coracoacromial Ligament

  • Glenohumeral Ligament

  • Coracoclavicular Ligament

  • Acromioclavicular Ligament

Example: The ACL in the knee prevents anterior displacement of the tibia relative to the femur, providing stability during movement.

Muscle Cell – Microscopic Anatomy

Introduction to Skeletal Muscle Microscopic Structure

Skeletal muscle contraction depends on the precise organization of microscopic structures within muscle fibers. Understanding these components is essential for grasping how muscles generate force and movement.

• Myofibril: Cylindrical organelles within muscle fibers, composed of repeating units called sarcomeres.

• Sarcomere: The functional contractile unit of muscle, defined as the segment between two Z lines.

• Sarcolemma: The plasma membrane of a muscle cell.

• Sarcoplasm: The cytoplasm of a muscle cell.

• Sarcoplasmic Reticulum: Specialized endoplasmic reticulum that stores and releases calcium ions for muscle contraction.

• Nuclei: Skeletal muscle fibers are multinucleated.

• Neuromuscular Junction: The synapse between a motor neuron and a muscle fiber.

• Myofilaments: Protein filaments (actin and myosin) responsible for muscle contraction.

• Actin Myofilaments: Thin filaments involved in contraction.

• Myosin Myofilaments: Thick filaments that interact with actin to produce contraction.

Key Sarcomere Bands and Zones:

• A band: Dark area; length of the thick (myosin) filaments.

• I band: Light area; contains only thin (actin) filaments.

• H zone: Central region of the A band; contains only thick filaments.

• Z line: Defines the boundary of each sarcomere.

• M line: Center of the sarcomere; holds thick filaments together.

Example: During muscle contraction, the sarcomere shortens as actin and myosin filaments slide past each other, a process described by the sliding filament theory.

Skeletal Muscles – Gross Anatomy and Functions

Overview of Skeletal Muscle Groups

Skeletal muscles are responsible for voluntary movements and are organized into groups based on their location and function. Each muscle has a specific action, often moving bones at joints or supporting body posture.

Muscles of Facial Expression

• Occipitofrontalis (Frontalis, Occipitalis)

• Buccinator

• Orbicularis Oculi

• Orbicularis Oris

• Mentalis

• Platysma

• Zygomaticus Major

• Zygomaticus Minor

Muscles of Mastication

• Temporalis

• Masseter

Muscles That Move the Head and Neck

• Splenius Capitus

• Sternocleidomastoid

• Scalene

Muscles of Ventilation

• External Intercostals

• Internal Intercostals

• Scalene

Abdominal Muscles

• Rectus Abdominus

• Internal Obliques

• External Obliques

• Transversus Abdominus

Muscles That Move the Vertebral Column

• Erector Spinae Group

Muscles That Move the Scapula at the Pectoral Girdle

• Subclavius

• Pectoralis Minor

• Levator Scapulae

• Serratus Anterior

• Trapezius

• Rhomboid Major

• Rhomboid Minor

Connective Tissue Elements

• Epimysium

• Aponeurosis of External Oblique

Muscle Functions Table

The following table summarizes the primary functions of selected muscles as required for lab identification and exam preparation.

Study and Exam Preparation Tips

• Be able to identify the listed joints, ligaments, and muscle cell structures on models and diagrams.

• Use only the definitions and functions provided in the lab manual for exam answers.

• Spelling and complete naming are required for full credit.

Additional info:

• Muscle contraction is explained by the sliding filament theory, where myosin heads bind to actin filaments and pull them toward the center of the sarcomere, shortening the muscle fiber.

• Calcium ions released from the sarcoplasmic reticulum initiate contraction by binding to regulatory proteins on actin.

• Muscle groups often work in opposition (agonist/antagonist pairs) to control movement.