Neck and Trunk

Vertebral Column Structure and Overview

Functions of the Vertebral Column:
- Supports the head.
- Transmits weight from the head, shoulder girdle, upper extremities, and trunk.
- Protects the spinal cord.
- Allows trunk movement.
- Absorbs and transmits shock.
Biomechanical Principle:
- Individual vertebrae move only a small amount, but their combined motion creates substantial overall spinal movement.
Vertebral Curves:
- Vertebral curves make the spine stronger and significantly more resilient than a straight spine.
Regional Breakdown of the Vertebral Column:
- Cervical Region:
  - Number of vertebrae: $7$
  - Curve type: Lordosis / convex
  - Key characteristic: Most mobile region of the spine
- Thoracic Region:
  - Number of vertebrae: $12$
  - Curve type: Kyphosis / concave
  - Key characteristic: Ribs restrict and limit motion
- Lumbar Region:
  - Number of vertebrae: $5$
  - Curve type: Lordosis / convex
  - Key characteristic: Primary weight-bearing region
- Sacral Region:
  - Number of vertebrae: $5$ (fused)
  - Curve type: Kyphosis / concave
  - Key characteristic: Fully fused structural segment
- Coccyx Region:
  - Number of vertebrae: $3$ (fused)
  - Curve type: Not applicable
  - Key characteristic: Tailbone region

Spinal Movements and Planes

Planes and Axes of Movement:
- Flexion / Extension / Hyperextension:
  - Plane: Sagittal plane
  - Axis: Frontal axis
- Lateral Flexion / Side Bending:
  - Plane: Frontal plane
  - Axis: Sagittal axis
- Rotation:
  - Plane: Transverse (horizontal) plane
  - Axis: Vertical / longitudinal axis
Role of Facet Joints:
- The orientation of facet joints determines the amount and direction of movement available in each spinal region.
- Cervical Region:
  - Facets allow movement across all $3$ planes and axes, resulting in the greatest overall spinal mobility.
- Thoracic Region:
  - Facets permit mainly lateral flexion with reduced rotation; ribs restrict overall movement.
- Lumbar Region:
  - Facets allow increased flexion and extension, slight lateral flexion, and essentially no rotation.

Vertebral Bony Landmarks

Vertebral Body:
- Anterior, major weight-bearing portion composed mainly of cancellous bone.
- Vertebral bodies progressively enlarge from C3 toward S1.
- C1 and C2 possess special anatomical configurations distinct from typical vertebral bodies.
Neural (Vertebral) Arch:
- Posterior portion surrounding the spinal cord.
Pedicle:
- Connects the body to the posterior neural arch.
Lamina:
- Posterior part of the neural arch situated between the transverse and spinous processes.
Transverse Process:
- Lateral projection serving as an attachment site for muscles and ligaments.
Articular Processes / Facets:
- Form facet joints and guide the direction of spinal movement.
Spinous Process:
- Most posterior projection; acts as a muscle and ligament attachment site and can be palpated along the midline spine.
Vertebral Foramen:
- Opening formed by the combination of the body and neural arch; functions as a passage for the spinal cord and cauda equina.
Intervertebral Foramen:
- Opening formed by adjacent vertebral notches; functions as a passage for spinal nerves.

Intervertebral Discs

General Characteristics:
- Total count: $23$ intervertebral discs.
- Starting location: Begins between C2 and C3.
- Proportion: Discs account for approximately $25\%$ of total spinal-column length.
Primary Functions:
- Shock absorption.
- Maintenance of overall spinal flexibility.
Anatomical Components:
- Annulus Fibrosus:
  - Outer portion composed of concentric fibrocartilaginous rings.
  - Encloses and surrounds the inner nucleus pulposus.
- Nucleus Pulposus:
  - Inner, pulpy, gelatinous center.
  - Maintains a very high water content.

Special Cervical Vertebrae and Upper Cervical Joints

Special Cervical Vertebrae:
- Atlas (C1):
  - Ring-shaped vertebra with no traditional body and no spinous process.
  - Supports and articulates with the cranium.
- Axis (C2):
  - Serves as a pivot for atlas rotation.
  - Features the odontoid process (dens), which projects vertically into the anterior ring of C1.
- Vertebra Prominens (C7):
  - Characterized by a long spinous process that is easily palpated when the neck is flexed.
  - Resembles a thoracic vertebra in structural appearance.
Upper Cervical Joints:
- Atlanto-occipital (AO) Joint:
  - Articulation: Between C1 superior facets and the occipital condyles.
  - Primary Movements: Flexion and extension with minimal lateral flexion; supports the head.
  - Memory Trick: Performs the "yes" motion (nodding yes via flexion/extension).
- Atlantoaxial (AA) Joint:
  - Articulation: Between C1 anterior arch and the dens of C2.
  - Stabilization: Dens is held posteriorly by the transverse ligament.
  - Primary Movement: Cervical rotation.
  - Memory Trick: Performs the "no" motion (shaking head no via rotation).

Key Ligaments of the Vertebral Column

Anterior Longitudinal Ligament (ALL):
- Location: Attached to the anterior surface of vertebral bodies.
- Function: Prevents hyperextension of the spine.
Posterior Longitudinal Ligament (PLL):
- Location: Attached to the posterior surface of vertebral bodies inside the vertebral canal.
- Function: Helps prevent excessive flexion.
Ligamentum Flavum:
- Location: Connects adjacent laminae along the posterior border of the vertebral canal.
- Properties and Function: Elastic ligament that limits excessive flexion.
Nuchal Ligament:
- Location: Extends from the occiput to the cervical spinous processes.
- Function: Limits excessive cervical flexion and continues inferiorly into the supraspinal and interspinous ligaments.
Supraspinal Ligament:
- Location: Extends continuously from C7 to the sacrum.
- Attachment: Attaches directly to the tips of spinous processes.
Interspinous Ligament:
- Location: Runs between successive spinous processes.
- Function: Resists excessive flexion.
Transverse Ligament:
- Location: Located at C1.
- Function: Secures the dens against C1 and protects the spinal cord from dens impingement.

Clinical High-Yield and Injury Susceptibility

Lumbar Spine:
- Most frequently injured spinal region due to absorbing much of the body's weight.
- Most lumbar movement occurs at L4–L5 and L5–S1.
- L4–L5 and L5–S1 represent common sites for intervertebral disc herniation.
Cervical Spine:
- Freely moveable and the second most frequently injured region.
- Supports the head and permits extensive range of motion.
Thoracic Spine:
- Exhibits significantly reduced motion due to the presence of ribs, the shape of thoracic vertebral bodies, and long spinous processes.

Cervical Musculature

Sternocleidomastoid (SCM):
- Location / Insertion Clue: Mastoid process.
- Bilateral Action: Neck flexion.
- Unilateral Action: Ipsilateral side-bending and contralateral rotation.
- Nerve Supply: CN XI (Spinal Accessory nerve), C2–C3.
Scalenes:
- Location / Insertion Clue: Cervical transverse processes to ribs 1 and 2.
- Bilateral Action: Assists neck flexion.
- Unilateral Action: Ipsilateral lateral flexion.
- Nerve Supply: C3–C8.
Prevertebral Group:
- Location / Insertion Clue: Deep group located anterior to the cervical vertebrae.
- General Action: Postural control, "chin tuck" maneuver, and head/neck flexion.
- Instructor Focus: Emphasize group concept, location, and overall function rather than memorizing individual muscle names.
Suboccipital Group:
- Location / Insertion Clue: Base of the skull.
- General Action: Moves the head in extension, rotation, and lateral bending depending on individual muscle pull; stabilizes AO and AA joints.
- Instructor Focus: Emphasize group concept, location, and general function.
Splenius Capitis:
- Location / Insertion Clue: Lower nuchal ligament + C7–T3 spinous processes extending to the occipital/mastoid region.
- Bilateral Action: Head extension.
- Unilateral Action: Ipsilateral rotation and lateral flexion.
- Nerve Supply: Middle and lower cervical nerves.
Splenius Cervicis:
- Location / Insertion Clue: T3–T6 spinous processes extending to C1–C3 transverse processes.
- Bilateral Action: Neck extension.
- Unilateral Action: Ipsilateral neck rotation.
- Nerve Supply: Middle and lower cervical nerves.

Anterior Trunk Musculature

Rectus Abdominis:
- Attachments: Pubic crest to xiphoid process and costal cartilages 5–7.
- Action: Trunk flexion and abdominal compression.
- Nerve Supply: T7–T12 intercostal nerves.
External Oblique:
- Attachments: Iliac crest, pubis, and linea alba to the lower 8 ribs.
- Bilateral Action: Trunk flexion and abdominal compression.
- Unilateral Action: Lateral flexion and rotation to the opposite side.
- Nerve Supply: T8–T12, iliohypogastric, and ilioinguinal nerves.
- Rotation Direction: Rotates the trunk to the opposite side (e.g., Right shoulder toward Left hip).
Internal Oblique:
- Attachments: Inguinal ligament, iliac crest, and thoracolumbar fascia to ribs 8–12 and linea alba.
- Bilateral Action: Trunk flexion and compression.
- Unilateral Action: Lateral flexion and same-side rotation.
- Nerve Supply: T8–T12, iliohypogastric, and ilioinguinal nerves.
- Rotation Direction: Rotates the trunk to the same side (e.g., Right shoulder toward Right side).
Transverse Abdominis:
- Attachments: Inguinal ligament, iliac crest, thoracolumbar fascia, and lower rib cartilages to linea alba and pubis.
- Action: Compresses abdomen; essential for coughing, sneezing, forced expiration, laughing, and "bearing down."
- Nerve Supply: T7–T12, iliohypogastric, and ilioinguinal nerves.
- Functional Concept: Functions like a "corset" — horizontal fibers compress the abdomen rather than creating trunk displacement.

Posterior Trunk Mechanics and Erector Spinae

General Structural Mechanics for Posterior Trunk Muscles:
- Spinous process to Spinous process: Creates a vertical line of pull resulting in extension.
- Transverse process to Transverse process: Creates a vertical line of pull resulting in unilateral side bending and bilateral extension.
- Rib to Rib: Similar to transverse-to-transverse, but positioned more laterally, creating more effective side bending.
- Spinous process to Transverse process (or vice versa): Creates an oblique line of pull resulting in bilateral extension and unilateral rotation.
Erector Spinae Group (Arranged Medial to Lateral: S-I-L Mnemonic):
- Spinalis:
  - Position: Medial muscle column.
  - Key Attachment Pattern: Spinous process to spinous process.
  - Action: Trunk extension.
- Longissimus:
  - Position: Intermediate muscle column.
  - Key Attachment Pattern: Transverse process to transverse process; spans from occiput to sacrum.
  - Action: Bilateral extension and unilateral side bending.
- Iliocostalis:
  - Position: Most lateral muscle column.
  - Key Attachment Pattern: Rib to rib; also attaches to transverse processes.
  - Action: Bilateral extension and unilateral side bending.

Deep Back Extensors and Short Spinal Muscles

Transversospinalis Group:
- Structural Orientation: Oblique line of pull attaching from a transverse process below to a spinous process above.
- Group Action: Bilateral extension and unilateral rotation toward the opposite side.
- Innervation: Spinal nerves.
- Subdivisions by Vertebral Span:
  - Semispinalis: Spans $5+$ vertebrae.
  - Multifidus: Spans $2$ to $4$ vertebrae.
  - Rotatores: Shortest and deepest layer; spans $1$ vertebra.
Interspinalis:
- Structure: Deep vertical pull running between adjacent spinous processes.
- Action: Trunk extension.
- Innervation: Spinal nerves.
Intertransversarii:
- Structure: Runs between adjacent transverse processes.
- Action: Trunk lateral bending.
- Innervation: Spinal nerves.

Quadratus Lumborum

Anatomical Location: Deep muscle of the posterior/lateral trunk.
Origin: Iliac crest.
Insertion: Last rib and transverse processes of L1–L4.
Action: Trunk lateral bending; elevates/hikes the hip when origin is pulled toward insertion.
Nerve Supply: T12 and first lumbar nerves.

Must-Know Quick Reference Summary

AO Joint: C1 + occiput -> Performs flexion/extension (nodding "yes").
AA Joint: C1 + C2 -> Performs rotation (shaking head "no").
C1 Atlas: Ring shape, no body, no spinous process -> Supports skull.
C2 Axis: Contains dens -> Pivot for C1 rotation.
C7 Vertebra Prominens: Prominent spinous process -> Palpation landmark when neck is flexed.
ALL (Anterior Longitudinal Ligament): Positioned anteriorly -> Prevents hyperextension.
PLL (Posterior Longitudinal Ligament): Positioned posteriorly -> Limits excessive flexion.
Erector Spinae: S-I-L medial to lateral -> Extension and side bending.
External Oblique: Rotates to opposite side -> Right shoulder to Left hip.
Internal Oblique: Rotates to same side -> Right shoulder to Right hip.
Transverse Abdominis: Horizontal "corset" fiber arrangement -> Abdominal compression.

Self-Test Questions and Answers

1. What are the $3$ spinal planes of movement?
- Answer: Sagittal, frontal, and transverse planes.
2. Flexion/extension occur in what plane and around what axis?
- Answer: Sagittal plane; frontal axis.
3. What are the $5$ vertebral regions and their counts?
- Answer: Cervical ( $7$ ), Thoracic ( $12$ ), Lumbar ( $5$ ), Sacral ( $5$ fused), and Coccyx (~ $3$ fused).
4. What is the main job of the vertebral body?
- Answer: Serves as the major weight-bearing structure.
5. What passes through the vertebral foramen?
- Answer: Spinal cord and cauda equina.
6. What passes through the intervertebral foramen?
- Answer: Spinal nerves.
7. What makes C1 unique?
- Answer: Ring-shaped with no traditional body or spinous process.
8. What is the dens and what does it do?
- Answer: Odontoid process of C2; acts as a pivot for C1 rotation.
9. What is the primary movement difference between AO and AA joints?
- Answer: AO = flexion/extension; AA = rotation.

Which ligament prevents hyperextension?

Answer: Anterior longitudinal ligament (ALL).

Which ligament holds the dens against the anterior arch of C1?

Answer: Transverse ligament.

Which spinal region is most often injured?

Answer: Lumbar spine.

What is the unilateral action of the SCM?

Answer: Ipsilateral side-bending + contralateral rotation.

What is the difference between external and internal oblique rotation?

Answer: External oblique rotates to the opposite side; internal oblique rotates to the same side.

What muscle acts like a corset?

Answer: Transverse abdominis.

What is the order of the erector spinae from medial to lateral?

Answer: Spinalis, longissimus, iliocostalis (S-I-L).

What is the order of transversospinalis muscles by span length?

Answer: Semispinalis ( $5+$ vertebrae), multifidus ( $2$ – $4$ vertebrae), rotatores ( $1$ vertebra).

What is the main action of quadratus lumborum?

Answer: Lateral bending; can hike/elevate the hip.

Study Strategy Recommendations

Step 1: First, memorize movement rules and the primary features of the three main spinal regions (cervical, thoracic, and lumbar).
Step 2: Drill muscle actions systematically using the pattern: location -> attachment direction -> action.
Step 3: Finish studying by taking the self-test without looking at the provided answers.