Muscular System Overview and Structure

Muscular System Overview

• The muscular system is composed of specialized contractile cells called muscle fibers.
• There are three types of muscle tissue: smooth, cardiac, and skeletal.
• Skeletal muscles are attached to bones via tendons and are responsible for moving the skeleton.
• Skeletal muscles fatigue more quickly than other muscle types.
• Skeletal muscles play a crucial role in voluntary movement.

Skeletal Muscle Structure

• Epimysium: Surrounds the entire muscle.
• Perimysium: Surrounds bundles of muscle fibers (fascicles).
• Endomysium: Surrounds individual muscle fibers.
• Muscle fibers: Are cylindrical in shape and lie parallel to each other.
• Mitochondria: Abundant in skeletal muscle fibers, essential for producing ATP from glucose, powering the contractile mechanism.
• Myoglobin: A specialized oxygen-binding protein that stores oxygen within the muscle.
• T-tubules: Extensions of the sarcolemma that transmit action potentials into the muscle fiber.
• Sarcoplasmic reticulum: Stores calcium ions, releasing them when an action potential arrives.
• Myofibrils: Long cylindrical structures within muscle fibers, composed of repeating units called sarcomeres.

Functions of Muscles

• Producing movement: Muscles generate force to move the body.
• Maintaining posture: Muscles support the body against gravity.
• Stabilizing joints: Maintaining joint alignment and preventing excessive movement.
• Generating heat: Heat is a byproduct of muscle metabolic activity.

Muscles of the Face

• Orbicularis oculi: Surrounds the eye, closes the eye, and can "screw up" the eyes when strongly contracted.
• Levator palpebrae superioris: Extends from the posterior part of the orbital cavity to the upper eyelid, raises the eyelid.
• Masseter: Draws the mandible up to the maxilla, closes the jaw, and exerts considerable pressure on food during chewing.
• Temporalis: Passes behind the zygomatic arch, inserting into the coronoid process of the mandible; closes the mouth and aids in chewing.

Muscles of the Neck

• Sternocleidomastoid: Assists in turning the head from side to side. When one side contracts, it draws the head towards the shoulder.
• Trapezius: Covers the shoulder and back of the neck, pulls the head backwards, squares the shoulders, and controls scapula movements when the shoulder joint is utilized.

Muscles of the Back

• Arrangement of back muscles: The muscles are arranged in the same way on both sides of the vertebral column.
• Latissimus dorsi: Originates from the lower thoracic and lumbar vertebrae and the iliac crest; adducts, medially rotates, and extends the arm.
• Teres major: Originates from the inferior angle of the scapula and connects to the humerus; extends, adducts, and medially rotates the arm.
• Quadratus lumborum: Starts from the hip bone, goes up beside the spine, and connects to the 12th rib; works with another muscle to help breathe and bend backward.
• Sacrospinalis (Erector spinae): Lie between spinous and transverse processes of vertebrae; extend the vertebral column.

Muscles of the Anterior Abdominal Wall

• Rectus abdominis: The most superficial muscle, curls the trunk, and supports the abdominal wall during straining, forced expiration, or defecation.
• External oblique: When contracted, helps twist the body, bend sideways, and support the abdomen.
• Internal oblique: Lies deep to the external oblique; when contracted, helps twist the body, bend sideways, and support the abdomen.
• Transversus abdominis: The deepest muscle of the abdominal wall; allows bending, twisting, and lifting, providing a stable base for the body; protects organs in place during activities like lifting, coughing, and breathing.

Muscles of the Thorax

• Intercostal muscles: Are deep muscles between ribs; external intercostals help raise the rib cage during inhalation, while internal intercostals depress the rib cage during forced exhalation.

Muscles of the Pelvic Floor

• Functions of pelvic floor muscles: Support pelvic organs and maintain continence (resistance to intra-pelvic pressure during urination and defecation).
• Organization of pelvic floor muscles: Divided into two identical halves that unite along the midline.
• Levator ani: A group of broad, flat muscles forming the anterior portion of the pelvic floor.
• Coccygeus: Completes pelvic floor formation; perforated in males by the urethra and anus, and in females by the urethra, vagina, and anus.
• Pelvic floor functions: Maintain continence; assist in childbirth.

Muscles of the Shoulder and Upper Limb

• Deltoid: Forms the fleshy contour of the shoulder; responsible for arm movement.
• Biceps brachii: Located on the anterior aspect of the upper arm; stabilizes and flexes the shoulder joint, aids in elbow flexion and supination.
• Triceps: Located on the posterior aspect of the humerus; stabilizes the shoulder joint, aids in adduction of the arm, and extends the elbow joint.
• Brachialis: Located on the anterior aspect of the upper arm, deep to the biceps; the main flexor of the elbow joint.

Muscles of the Hip and Lower Limb

• Quadriceps femoris: A group of four muscles on the front and sides of the thigh; rectus femoris flexes the hip joint; all four muscles extend the knee joint.
• Gluteal muscles: Gluteus maximus, medius, and minimus form the buttock; cause extension, abduction, and medial rotation of the hip joint.
• Hamstrings: Located on the posterior aspect of the thigh; include biceps femoris, semitendinosus, and semimembranosus muscles; flex the knee joint.