The Muscular System: An Overview

1. Muscle Classification:

The muscular system can be classified based on various criteria:

a) Based on Function:

• Skeletal Muscles: Attached to bones, responsible for movement and locomotion.
  This is the only type of muscle that humans can consciously control.
• Cardiac Muscle: Found only in the heart, functions involuntarily to pump blood throughout the body.
  Cardiac muscle has a unique structure that allows it to contract rhythmically, continuously, and powerfully.
• Smooth Muscle: Found in the walls of internal organs like the stomach, intestines, blood vessels, etc. It functions involuntarily to regulate organ activity.
  Smooth muscle contracts more slowly than skeletal muscle but has the ability to sustain contractions for long periods, ensuring the proper functioning of internal organs.

b) Based on Structure:

• Striated Muscle: Composed of muscle fibers with cross-striations, including skeletal and cardiac muscle.
• Smooth Muscle: Composed of muscle fibers lacking cross-striations, only including smooth muscle.

c) Based on Nervous Control:

• Voluntary Muscles: Controlled by the central nervous system, allowing conscious control. Example: Skeletal muscle.
  Voluntary muscles are typically used for purposeful movements like walking, running, grasping, etc.
• Involuntary Muscles: Controlled by the autonomic nervous system, not subject to conscious control. Example: Cardiac muscle, smooth muscle.
  Involuntary muscles maintain essential physiological functions like heart rate, digestion, excretion, etc.

2. Muscle Structure:

Each muscle consists of two main parts:

• Muscle Belly: Also known as the muscle body, this is the primary part of the muscle that performs muscle contraction.
  The muscle belly comprises muscle fibers arranged along the length of the muscle.
• Tendon: This part connects the muscle to the bone, transmitting the force of muscle contraction.
  Tendons are white, composed of connective tissue, and very strong and flexible.

a) Muscle Belly:

• Muscle Fibers: The muscle belly is made up of muscle fibers, which are small muscle filaments arranged parallel to the long axis of the tendon. They can be arranged parallel, angled to one side, or to both sides of the tendon.
  Muscle fibers are the basic contractile unit of a muscle.
• Arrangement of Muscle Fibers: The arrangement of muscle fibers influences the force and range of motion of a muscle.
  • Parallel: Muscle fibers run parallel to the long axis of the muscle, attaching to the tendon at both ends.
    This type of muscle has strong contractile force but a small range of motion.
  • Pennate: Muscle fibers run parallel to the long axis of the muscle, attaching to the tendon at both ends, with the muscle belly narrowing towards both ends.
    This type of muscle has strong contractile force and a moderate range of motion.
  • Circular: Muscles arranged in a circular shape around a natural opening of the body.
    This type of muscle is responsible for opening and closing natural openings.
  • Triangular: Muscle fibers converge on one side of the tendon.
    This type of muscle has strong contractile force in one specific direction.
  • Feather-like: Muscle fibers attach to the tendon at an angle, forming a feather-like shape.
    • Unipennate: Fibers attach along one side of the tendon.
    • Bipennate: Fibers attach along both sides of the tendon.
    This type of muscle has strong contractile force but a small range of motion.

b) Tendon:

• Tendon Fibers: Tendons are composed of white, strong fibers that attach to bone.
• Arrangement of Tendon Fibers: They can be arranged parallel or in a spiral pattern.
  Spiral arrangement provides greater elasticity to the tendon, reducing injury while still allowing for effective muscle contraction.
• Aponeurosis: This is a flat tendon of broad muscles.
  Example: The aponeurosis on the scalp between the occipital and frontal bellies of the occipitofrontalis muscle.

3. Muscle Functions:

• Body Movement: Skeletal muscles are the primary organs responsible for body movement and locomotion.
• Maintaining Posture: Muscles work in coordination with bones to maintain balance and posture while standing, sitting, lying down, etc.
• Storage and Transport of Substances: Contraction of skeletal muscle can increase lymphatic flow and venous blood return.
  This function helps eliminate waste products, deliver oxygen, and transport nutrients throughout the body.
• Heat Production: Muscle contraction generates heat, which helps the body maintain a stable internal temperature.

4. Naming Muscles:

Muscle names are typically derived from several criteria:

• Muscle Fiber Direction: Rectus abdominis, transverse abdominis, oblique abdominis…
• Muscle Size: Gluteus maximus, gluteus minimus…
• Muscle Shape: Deltoid, trapezius, orbicularis oculi, serratus anterior…
• Number of Points of Origin: Biceps, triceps…
• Function: Flexor, extensor, rotator, levator…
• Location: Temporalis, mentalis, masseter…
• Origin and Insertion: Sternocleidomastoid.

5. Blood Supply and Nerve Distribution:

• Blood Supply:
  • Each muscle is typically supplied by one artery and one or two accompanying veins.
  • Patterns of Blood Supply:
    • Type 1: Supplied by one artery.
    • Type 2: Supplied by one dominant artery and several smaller arteries.
    • Type 3: Supplied by two dominant arteries.
    • Type 4: Supplied by numerous smaller arteries.
    • Type 5: Supplied by one dominant artery and numerous smaller arteries in different parts of the muscle.
• Nerve Supply:
  • Nerve distribution depends on the muscle’s shape.
  • Muscles of the upper and lower limbs and face are typically innervated by one nerve, although its axon may originate from different spinal cord segments.
  • Muscles of the trunk originate from different sources and are often innervated by multiple nerves.

6. Muscle Fascia:

• Fascia: This is connective tissue that encloses a single muscle or a group of muscles.
• Types of Fascia:
  • Epimysium: This surrounds each individual muscle, providing protection and allowing the muscle to contract independently.
  • Deep Fascia: This surrounds a group of muscles, separating them from other muscle groups or neighboring organs.
    Deep fascia divides muscle regions into compartments.
  • Superficial Fascia: This lies directly beneath the skin, closely associated with subcutaneous tissue, ensuring the elasticity of the skin.

7. Synovial Sheaths of Tendons:

• Synovial Sheath: This is a lubricating sheath that surrounds tendons, allowing for smooth movement and preventing friction against bone.
• Structure: Consists of two layers:
  • Visceral Layer: Adheres directly to the tendon.
  • Parietal Layer: Adheres to the fibrous sheath.
• Synovial Cavity: Located between the two layers, contains synovial fluid, which lubricates tendon movement.
• Mesotendon: The two layers of the synovial sheath form the mesotendon along the sides of the tendon.
• Fibrous Sheath: This surrounds tendons and anchors them to the bone, forming a fibrous tube that allows the tendon to glide over bone.
• Retinaculum: In the wrist and ankle, the fibrous sheath thickens and becomes very strong to form the retinaculum.

8. Synovial Bursae:

• Synovial Bursa: These are closed sacs containing synovial fluid located between two muscles, between a muscle and bone, or between a tendon and bone, reducing friction between these structures.

9. Muscles of the Head, Face, and Neck:

• Muscles of the Head:
  • Facial Muscles: Include muscles of the scalp, ear, eyes, nose, and mouth.
  • Masticatory Muscles: Include the temporalis, masseter, medial pterygoid, and lateral pterygoid muscles.
    Masticatory muscles are innervated by the mandibular nerve.
  • Rectus Muscles: Include the rectus capitis anterior, rectus capitis posterior major, and rectus capitis posterior minor.
  • Oblique Muscles: Include the obliquus capitis superior and obliquus capitis inferior.
    Oblique muscles are innervated by the posterior branch of C1.
• Muscles of the Neck:
  • Suprahyoid Muscles: Include the digastric, stylohyoid, mylohyoid, and geniohyoid muscles.
  • Infrahyoid Muscles: Include the sternohyoid, sternothyroid, thyrohyoid, and omohyoid muscles.
  • Lateral Vertebral Muscles: Include the anterior, middle, and posterior scalene muscles.
  • Lateral Neck Muscles: Include the platysma and sternocleidomastoid muscles.

10. Layers of the Scalp:

• Skin: The outermost layer, consisting of the epidermis and dermis.
• Connective Tissue: This thick layer contains numerous blood vessels and nerves.
• Aponeurosis: A thick, tough layer of connective tissue that covers the entire scalp.
• Loose Connective Tissue: A thin layer connecting the aponeurosis to the skull bone.

11. Triangles of the Neck:

• Anterior Cervical Triangle:
  • Boundaries:
    • Superior: Mandible.
    • Anterior: Midline of the neck.
    • Lateral: Sternocleidomastoid muscle.
  • Contains:
    • Submandibular triangle.
    • Carotid triangle.
    • Muscular triangle.
• Submandibular Triangle:
  • Boundaries:
    • Superior: Mandible.
    • Anterior: Anterior belly of the digastric muscle.
    • Posterior: Posterior belly of the digastric muscle and stylohyoid muscle.
  • Contains:
    • Submandibular salivary gland.
    • Facial artery and vein.
• Carotid Triangle:
  • Boundaries:
    • Superior: Posterior belly of the digastric muscle.
    • Posterior: Sternocleidomastoid muscle.
    • Inferior: Omohyoid muscle.
  • Contains:
    • Carotid artery and vein.
    • Vagus nerve.
    • Cervical sympathetic trunk.
• Muscular Triangle:
  • Boundaries:
    • Superior: Omohyoid muscle.
    • Posterior: Sternocleidomastoid muscle.
    • Anterior: Midline of the neck.
  • Contains:
    • Inferior thyroid artery and vein.
    • Inferior laryngeal nerve.
    • Trachea.
    • Esophagus.
    • Thyroid gland.
• Posterior Cervical Triangle:
  • Boundaries:
    • Anterior: Sternocleidomastoid muscle.
    • Posterior: Trapezius muscle.
    • Inferior: Clavicle.
  • The omohyoid muscle divides the posterior cervical triangle into two.
  • Contains:
    • Occipital triangle.
    • Supraclavicular triangle.

12. Fascia of the Head, Face, and Neck:

• Fascia of the Head and Face:
  • Temporal Fascia: Bounded by the superior temporal line and the zygomatic arch.
  • Masseteric Fascia: Continuous with the temporal fascia, encloses the masseter muscle, and attaches to the mandible.
  • Parotid Fascia: Formed by the superficial layer of the cervical fascia.
• Fascia of the Neck:
  • Superficial Layer: Lies beneath the skin, connected to the facial muscles.
  • Pretracheal Layer: Encloses the trachea.
  • Prevertebral Layer: Encloses the cervical vertebral column.
  • Carotid Sheath: Encloses the blood vessels and nerves of the neck.