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 human body and includes the skull, vertebral column, ribs, and sternum. It provides support, protection for vital organs, and attachment points for muscles.

• Major Components: Skull, vertebral column, thoracic cage (ribs and sternum), and hyoid bone.

• Functions: Protection of the brain, spinal cord, and thoracic organs; support for the body; and muscle attachment.

Skull Bones and Features

• Parietal Bones (2): Form the superior and lateral aspects of the skull. Sutures: Coronal, squamous, sagittal, lambdoid (immovable joints between skull bones).

• Frontal Bone (1): Forms the forehead and superior orbits. Frontal Sinuses: Air-filled cavities within the frontal bone.

• Occipital Bone (1): Forms the posterior skull base. Key Features: Occipital condyles (articulate with atlas), external occipital protuberance (inion), foramen magnum (spinal cord passage), hypoglossal canal (CN XII).

• Temporal Bones (2): Form the sides and base of the skull. Key Features: Zygomatic arch/process, mandibular fossa, mastoid and styloid processes, stylomastoid foramen, external/internal auditory meatus, jugular foramen (CN X & internal jugular vein), carotid canal (internal carotid artery).

• Sphenoid Bone (1): Central wedge of the cranial floor. Key Features: Sella turcica (pituitary fossa), optic canal (CN II), superior orbital fissure, foramen rotundum/ovale/spinosum.

• Ethmoid Bone (1): Forms part of the nasal cavity and septum. Key Features: Cribriform plate (CN I), crista galli, nasal conchae, perpendicular plate, ethmoid sinus.

• Maxillae (2): Upper jaw bones. Key Features: Maxillary sinuses, palatine process (hard palate), infraorbital foramen, inferior orbital fissure.

• Mandible (1): Lower jaw bone. Key Features: Mandibular angle, ramus, condyle (head), 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 facial structure.

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

Vertebral Column

• Typical Vertebra Features: Body (centrum), vertebral foramen, intervertebral foramen, transverse process, pedicle, lamina, spinous process, superior/inferior articular facets.

• Special Vertebrae: Atlas (C1): Supports the skull, allows nodding. Axis (C2): Has dens (odontoid process) for rotation.

• Vertebral Types and Characteristics:

  • Cervical (7): Have transverse foramina.

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

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

  • Sacrum (1): Fused vertebrae, median sacral crest, sacral foramina.

  • Coccyx (1): Tailbone, fused small vertebrae.

• Hyoid Bone: U-shaped bone in the neck, not articulated with other bones; supports the tongue.

Thoracic Cage: Ribs and Sternum

• Sternum: Manubrium, jugular notch, body, xiphoid process.

• Ribs (12 pairs): Each rib has a head, neck, tubercle, and angle.

• Rib Classifications:

  • True Ribs (1-7): Attach directly to sternum via costal cartilage.

  • False Ribs (8-12): Indirect or no attachment to sternum.

  • Floating Ribs (11-12): No anterior attachment.

Axial Articulations

• Intervertebral Joints: Symphysis joints 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 Tissue and Histology

Types of Muscle Tissue

Muscle tissue is specialized for contraction and movement. There are three main types:

• Skeletal Muscle: Voluntary, striated, multinucleated; attached to bones for movement.

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

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

Muscle Fiber Structure

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

• Intercalated Discs: Specialized junctions in cardiac muscle 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 muscle fiber.

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

• T Tubules: Invaginations of the sarcolemma that conduct impulses deep into the muscle fiber.

• 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 contractile unit of muscle, composed of myosin (thick) and actin (thin) filaments.

Muscles of the Head, Neck, Back, and Trunk

Muscles Acting on the Head, Face, 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/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	Used for normal (quiet) breathing
Transverse Abdominis	Compresses abdominopelvic cavity	Deepest abdominal muscle; aponeurosis
External Oblique	Compresses abdominopelvic cavity; single: rotates trunk/twists	Most superficial abdominal muscle; aponeurosis
Internal Oblique	Compresses abdominopelvic cavity; single: rotates trunk/reverse twist	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
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 scapula
Rhomboid Major	Spinous processes T2-T4	Medial border of scapula	Adduct & fixate scapula
Levator Scapulae	Transverse processes C1-C4	Medial border of scapula	Elevate scapula
Pectoralis Minor	Ribs 3-5	Coracoid process of scapula	Abduct (protract) scapula
Serratus Anterior	Ribs	Scapula	Abduct (protract) scapula

Additional Info

• Synchondrosis: A type of cartilaginous joint where bones are joined by hyaline cartilage (e.g., costal cartilage of ribs).

• Symphysis: A type of cartilaginous joint where bones are joined by fibrocartilage (e.g., intervertebral discs).

• Aponeurosis: A broad, flat tendon that connects muscles to each other or to bone.