Axial Skeleton, Articulations, and Muscles: Identification and Function

Axial Skeleton: Bones and Articulations

Overview of the Axial Skeleton

The axial skeleton forms the central axis of the body and includes the skull, vertebral column, and thoracic cage. It provides support, protection for vital organs, and attachment points for muscles.

• Skull: Protects the brain and forms the structure of the face.

• Vertebral Column: Supports the head and trunk, protects the spinal cord.

• Thoracic Cage: Protects the heart and lungs, supports the shoulder girdle.

Skull Bones and Features

• Parietal Bones (2): Form the superior and lateral aspects of the skull.

• Sutures: Immovable joints between skull bones. Key sutures include:

  • Coronal

  • Squamous

  • Sagittal

  • Lambdoid(al)

• Frontal Bone (1): Forms the forehead; contains frontal sinuses.

• Occipital Bone (1): Contains the foramen magnum (passage for spinal cord), occipital condyles (articulate with atlas), external occipital protuberance (inion), and hypoglossal canal (CN XII).

• Temporal Bones (2): Features include:

  • Zygomatic arch and process

  • Mandibular fossa (articulates with mandible)

  • Mastoid and styloid processes

  • Stylomastoid foramen

  • External and internal auditory canals (meatus)

  • Jugular foramen (CN X, internal jugular vein)

  • Carotid canal (internal carotid artery)

• Sphenoid Bone (1): Central bone of the skull base; contains sella turcica (houses pituitary gland), hypophyseal fossa, sinus, optic canal (CN II), superior orbital fissure, foramina rotundum, ovale, spinosum.

• Ethmoid Bone (1): Forms part of nasal cavity and septum; features cribriform plate (CN I), crista galli, middle & inferior nasal conchae, perpendicular plate, sinus.

• Maxilla(e) (2): Upper jaw; contains sinuses, palatine process (hard palate), infraorbital foramen, inferior orbital fissure.

• Mandible (1): Lower jaw; features mandibular angle, ramus, head (condyle), mental foramen.

• Nasal (2) and Vomer (1): Form the nasal septum (vomer + perpendicular plate of ethmoid).

• Palatine (2), Lacrimal (2), Zygomatic (2): Contribute to the orbit and face structure.

• Fontanelles (Fetal Skull): Soft spots in infant skulls; include anterior, posterior, sphenoidal, and mastoidal fontanelles.

Vertebral Column

• Typical Vertebra Features:

  • Body (centrum)

  • Vertebral foramen

  • Intervertebral foramen

  • Transverse process (2)

  • Pedicle (2)

  • Lamina (2)

  • Spinous process (1)

  • Superior & inferior articular facets/processes (2 each)

• Special Vertebrae:

  • Atlas (C1): Supports the skull, allows nodding motion.

  • Axis (C2): Contains the dens (odontoid process), allows rotation of the head.

• Vertebral Types and Characteristics:

  • Cervical (7): Have transverse foramen.

  • Thoracic (12): Have costal facets for rib articulation.

  • Lumbar (5): Large bodies for weight bearing.

  • Sacrum (1): Fused vertebrae; features median sacral crest, anterior/posterior sacral foramina.

  • Coccyx (1): Tailbone, fused vertebrae.

• Hyoid Bone: U-shaped bone in the neck, does not articulate with other bones.

Thoracic Cage: Ribs and Sternum

• Sternum: Consists of manubrium, jugular notch, body, and xiphoid process.

• Ribs (12 pairs):

  • Anatomy of a rib: Head, neck, tubercle, angle.

  • Classification:

    • True ribs: Attach directly to sternum (via costal cartilage).

    • False ribs: Attach indirectly or not at all to sternum.

    • Floating ribs: No anterior attachment (synchondrosis joint).

Axial Articulations

• Intervertebral Joints: Cartilaginous joints (symphysis) between vertebral bodies.

• Intervertebral Disc: Composed of anulus fibrosus (outer ring) and nucleus pulposus (gel-like center).

• Intervertebral Foramen: Openings for spinal nerves to exit the vertebral column.

Muscle Histology and Muscle Fiber Structure

Types of Muscle Tissue

• Skeletal Muscle: Voluntary, striated, multinucleated; responsible for body movement.

• Cardiac Muscle: Involuntary, striated, branched, with intercalated discs; found in the heart.

• Smooth Muscle: Involuntary, non-striated; found in walls of hollow organs.

Muscle Fiber Structure

• Striations: Alternating light (I bands) and dark (A bands) areas due to arrangement of actin and myosin.

• Intercalated Discs: Specialized connections between cardiac muscle cells for synchronized contraction.

• Nuclei: Skeletal muscle fibers are multinucleated; cardiac and smooth muscle have one nucleus per cell.

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

• Motor End Plate: Specialized region of muscle fiber membrane at the neuromuscular junction.

• T Tubules: Invaginations of the sarcolemma that help transmit action potentials.

• Sarcoplasmic Reticulum: Stores and releases calcium ions for muscle contraction.

• Triad: Structure formed by a T tubule and two terminal cisternae of the sarcoplasmic reticulum.

• Sarcomere: Functional unit of muscle contraction; contains myosin (thick) and actin (thin) filaments.

Muscles of the Head, Face, and Neck

Muscles Acting on the Head and Neck

Muscle	Action	Additional Info
Epicranius Frontalis	Raises eyebrows	Epicranius aponeurosis
Epicranius Occipitalis	Elevates posterior scalp
Sternocleidomastoid	Single: Rotates head to opposite side; Both: Flexion	O: sternum & medial clavicle; I: mastoid process
Scalenes (Anterior, Middle, Posterior)	Flexion & lateral flexion of neck
Splenius Capitis	Extension of head/neck

Muscles Acting on the Mouth, Lips, and Jaw

Muscle	Action	Additional Info
Temporalis	Elevates & retracts jaw (closes mouth)
Masseter	Elevates & closes jaw (chewing)	O: zygomatic arch; I: mandible
Orbicularis oculi	Closes eyelids (squinting)
Orbicularis oris	Whistling/kissing
Zygomaticus minor	Elevates upper lip (speech, smiling)
Zygomaticus major	Elevates upper lip (speech, smiling)
Levator labii superioris	Elevation of upper lip
Depressor labii inferioris	Depresses lower lip (frowning)
Depressor anguli oris	Depresses corner of lower lip (frowning)
Mentalis	Protrudes lower lip (pouting)

Muscles of the Back and Trunk

Muscle	Action	Additional Info
Erector Spinae (Spinalis, Longissimus, Iliocostalis)	Extension of back (posture); Longissimus also extends head; Iliocostalis also rotates trunk
External Intercostals	Elevate ribs for inhalation (deeper breaths)
Internal Intercostals	Depress ribs for exhalation (deeper breaths)
Diaphragm	Contracts (flattens) for inhalation; relaxes for exhalation (normal breathing)
Transverse Abdominis	Compresses abdominopelvic cavity	Deepest abdominal muscle; aponeurosis
External Oblique	Compresses abdominopelvic cavity; single: rotates trunk/twists (elbow to knee)	Most superficial abdominal muscle; aponeurosis
Internal Oblique	Compresses abdominopelvic cavity; single: rotates trunk/reverse twist (knee to elbow)	Aponeurosis
Rectus Abdominis	Compresses abdominopelvic cavity; flexes trunk (sit up)	O: pubic symphysis; I: xiphoid process & lower ribs

Muscles Acting on the Scapula

Muscle	Origin	Insertion	Action at Scapula
Trapezius	Occipital bone, spinous processes C1-T12, ilium	Scapula	Upper: elevate; Middle: adduct (retract); Lower: depress
Rhomboid minor	Spinous processes C7 & T1	Medial border of scapula	Adduct & fixate
Rhomboid major	Spinous processes T2-T4	Medial border of scapula	Adduct & fixate
Levator scapulae	Transverse processes C1-C4	Medial border of scapula	Elevate
Pectoralis minor	Ribs 3-5	Coracoid process of scapula	Abduct (protract)
Serratus anterior	Ribs	Scapula	Abduct (protract)

Key Definitions

• Origin (O): The fixed attachment point of a muscle.

• Insertion (I): The movable attachment point of a muscle.

• Action: The movement produced by muscle contraction.

Example: Intervertebral Disc Structure

• Anulus fibrosus: Tough outer ring of fibrocartilage.

• Nucleus pulposus: Gel-like center that absorbs shock.

Additional info:

• Muscle actions are often paired (agonist/antagonist) for coordinated movement.

• Fontanelles in the fetal skull allow for brain growth and passage through the birth canal.

• Synchondrosis and symphysis are types of cartilaginous joints; synchondrosis is immovable, symphysis allows slight movement.