Posterior triangle of the neck

🖇 I. MUSCLES OF THE NECK

📌 A. Suprahyoid Muscles (Above the Hyoid Bone)

Function: Suspend the Hyoid bone from above, positioned between the Mandible and Hyoid bone.

Four Muscles:

1. Digastric Muscle
2. • Named for two gaster (two heads/bellies)
   • Origin: One head from Mandible, other from Mastoid Process
2. Stylohyoid Muscle
3. • Origin: Styloid Process
   • Insertion: Hyoid bone
3. Geniohyoid Muscle
4. • Origin: Mandible (specifically Genial Tubercle)
   • Insertion: Hyoid bone
4. Mylohyoid Muscle
5. • Attachment: Entire length of Mandible to Hyoid bone

📌 B. Infrahyoid Muscles (Below the Hyoid Bone)

Function: Connect Hyoid bone downwards to Sternum, Clavicle, and Scapula, passing the Thyroid cartilage (Adam's apple).

Arrangement: Two layers

Four Muscles:

1. Thyrohyoid Muscle
2. • Connection: Thyroid cartilage ↔ Hyoid bone
2. Sternothyroid Muscle
3. • Connection: Thyroid cartilage ↔ Sternum
3. Sternohyoid Muscle
4. • Connection: Hyoid bone directly ↔ Sternum
4. Omohyoid Muscle
5. • Connection: Hyoid ↔ Scapula
   • Structure: Two bellies (superior and inferior), similar to Digastric

🖇 II. GENERAL ANATOMY AND TRIANGLES OF THE NECK

📌 A. Other Muscle Groups

Prevertebral Muscles:

• Located anteriorly, related to vertebral column
• Extensive details not covered in this lecture

Lateral Muscles:

• Scalene muscles: Scalenus Anterior, Scalenus Medius, Scalenus Posterior

Sternocleidomastoid Muscle (SCM):

• Oblique muscle
• Course: Sternum and Clavicle → Mastoid process

📌 B. Visceral Spaces

Location: Between Hyoid bone and surrounding muscles

Contents:

• Fascial space containing viscera
• Thyroid gland
• Carotid sheath with its contents

🚨 C. Posterior Triangle Subdivision (Exam Point)

Key Dividing Structure:

• Inferior Belly of Omohyoid Muscle runs horizontally across Posterior Triangle

Division Result: Two smaller triangles:

1. Superior Triangle = Occipital Triangle
2. Inferior Triangle = Supraclavicular Triangle (located above clavicle)

Complexity Note:

• Anterior Triangle is MORE complex than Posterior Triangle
• Reason: Complexity created by Hyoid bone and related musculature

📌 D. Boundaries, Roof, and Floor of Posterior Triangle

🟠 1. The Roof

Superficial Layer:

• Skin
• Superficial Fascia beneath skin

Superficial Fascia Contents:

• Platysma muscle (muscle of facial expression)
• Cutaneous nerves

Cutaneous Nerve Origin:

• ALL cutaneous nerves in neck = branches of Cervical Plexus

Deep Fascia (Cervical Fascia): Complex structure = three tubes nested within each other:

1. Investing Layer (outermost)
2. • Envelops entire neck
2. Pretracheal Fascia (middle tube)
3. • Contains viscera (e.g., Thyroid gland)
3. Prevertebral Fascia (innermost tube)
4. • Surrounds vertebral column and attached muscles

🟠 2. The Floor

Composition: Four muscle layers resting laterally on vertebral column

Muscles (superficial to deep):

1. Semispinalis Capitis
2. Splenius Capitis
3. Levator Scapulae
4. Scalenus Medius
5. • Note: Scalenus Posterior may be present (varies with dissection)

🚨 Fascial Covering: Prevertebral Fascia covers muscular floor (Exam Point)

📌 E. Contents of Posterior Triangle (Neurovascular)

🟣 Veins:

• External Jugular Vein (EJV)

🟣 Arteries:

Subclavian Artery:

• Third Part of Subclavian Artery present in triangle
• Division mechanism: Scalenus Anterior Muscle passes in front of Subclavian Artery
• Creates three parts:
• • 1st part: medial to Scalenus Anterior
  • 2nd part: posterior to Scalenus Anterior
  • 3rd part: lateral to Scalenus Anterior

Thyrocervical Trunk (TCT):

• Branch of first part of Subclavian Artery
• Branches:
• • Superior branch: Inferior Thyroid Artery
  • Two anterior branches:
  • • Transverse Cervical Artery
    • Suprascapular Artery

🖇 III. SPINAL NERVES AND PLEXUSES (CRUCIAL CONCEPT)

📌 A. Spinal Nerve Organization

Total: 31 pairs of spinal nerves

Immediate Division upon exiting spinal column:

1. Dorsal Ramus:

• Immediately enters Epaxial Muscles (Intrinsic back muscles)
• Straightforward course
• Do NOT form plexuses

2. Ventral Ramus (Larger):

• Supplies all other muscles (Hypaxial Muscles)
• Includes: trunk muscles, limb muscles, muscles lying directly on vertebrae (even if posterior)

🚨 B. Mechanism and Purpose of Plexuses (Exam Point)

Definition:

• Nerve networks formed when multiple nerve roots merge, branch, merge again, branch again

Purpose (Triple Redundancy):

1. Single muscle supplied by multiple nerve roots
2. Single nerve root supplies multiple muscles
3. If one innervation route damaged → guaranteed supply maintained via another route

📌 C. Spinal Cord Enlargements

Reason: Segments where ventral rami contribute to limb plexuses are enlarged due to increased neuronal volume needed to supply limbs (structures "added on trunk")

Two Enlargements:

1. Cervical Enlargement (C5–T1)
2. • For Brachial Plexus (Upper Limb)
2. Lumbosacral Enlargement (L1–S3)
3. • For Lumbosacral Plexus (Lower Limb)

📌 D. Rules for Plexus Formation

Question: Do ALL Ventral Rami form plexuses? Answer: NO

🚨 Thoracic Exception (T1–T12):

• Thoracic ventral rami do NOT form plexuses
• Reason: Ribs act as "concrete walls" preventing nerve communication/mixing
• Result: Nerves remain isolated, travel individually in intercostal spaces

Plexuses Above and Below Thorax:

1. Cervical Plexus (C1–C4)
2. • Supplies: neck, part of face, Phrenic nerve (diaphragm), deep neck muscles
2. Brachial Plexus (C5–T1)
3. • Supplies: upper limb
3. Lumbar Plexus (L1–L4)
4. • Supplies: pelvis, perineum, lower limb
4. Sacral Plexus (L4, L5, S1, S2, S3, part of S4)
5. • Supplies: pelvis, perineum, lower limb
5. Coccygeal Plexus (Part of S4, S5, Coccygeal Nerve)
6. • Supplies: perineum

🖇 IV. CRANIAL NERVE FIBERS CLASSIFICATION

🚨 CRITICAL EXAM POINT: This section MORE IMPORTANT than entire lecture material

📌 A. Classification Table
🟠 B. Special Visceral Efferent (SVE) System - Detailed

Distinct Class: Separate from other motor systems

Target Muscles:

• Muscles derived from Branchial Arches (Pharyngeal Arches)
• Includes: Pharynx muscles, Larynx muscles, Mastication muscles

Functional Nature (Hybrid):

• Part somatic-like: Can be initiated consciously
• Part visceral-like: Involuntary
• Example: Swallowing
• • First stage: voluntary (somatic-like)
  • Subsequent stages: involuntary (visceral-like)

Motor Nuclei:

• Nucleus Ambiguus (medulla oblongata)
• • Responsible for many SVE fibers
  • Supplies: pharynx/larynx muscles
• Motor Nucleus of Trigeminal Nerve
• • Supplies: mastication muscles (from 1st branchial arch)

🖇 V. STERNOCLEIDOMASTOID MUSCLE (SCM) - DETAILED STUDY

📌 A. Anatomical Details

Origin (Two Heads):

• Sternal head
• Clavicular head

Insertion:

• Outer surface of Mastoid Process

🚨 Nerve Supply: Spinal Accessory Nerve (CN XI) (Exam Point)

🚨 Unique Feature: SCM and Trapezius = ONLY two skeletal muscles in neck supplied by cranial nerve (Exam Point)

📌 B. Spinal Accessory Nerve (CN XI) Composition

Two Components:

1. Cranial Root:

• Origin: Nucleus Ambiguus

2. Spinal Root (Spinal Accessory N.):

• Origin: Central nucleus in spinal cord (C1–C5 segments)

Course:

• Two roots travel together without mixing
• Exit via Jugular Foramen
• Then separate:

Spinal Root Distribution (CN XI spinal portion):

• Supplies ONLY:
• • Sternocleidomastoid muscle
  • Trapezius muscle

Cranial Root Distribution (CN XI cranial portion):

• Joins Vagus nerve (CN X)
• Supplies: muscles from branchial arches (Pharynx and Larynx)
• Name "Accessory" = because it adds these fibers to Vagus

📌 C. Functional Significance and Coordination

Primary Function: Largely involved in reflex action

• Justifies cranial nerve supply

Example: Visual-Audio Spinal Reflex

• Coordination needed between:
• • Eyes
  • Facial muscles
  • Neck muscles
• Situation: Reacting to stimuli or tracking objects (e.g., watching scenery from moving train)

Coordinating Structure: Medial Longitudinal Fasciculus (MLF)

• Functions as "electrical cable"
• Connects nuclei from Midbrain → Spinal Cord:
• • Oculomotor nucleus
  • Abducens nucleus
  • Trochlear nucleus
  • Nucleus Ambiguus
  • Facial nucleus
• Result: Coordinated action between:
• • Extraocular muscles
  • Facial muscles
  • SCM (via Spinal Accessory Nerve)

🚨 D. Actions of SCM (Exam Points)

Bilateral Contraction (Both SCMs):

• Action: Extension of Head

🚨 Unilateral Contraction (One SCM): (Exam Point)

1. Lateral Flexion (tilting ear toward shoulder on same side)
2. Face rotates to OPPOSITE side (outward rotation)

🚨 SCM and Contralateral Movement Principle: (Exam Point)

• SCM works contralaterally to trunk muscles
• Brain-body relationship:
• • Right brain controls left body
  • Left arm active = Right brain working
  • To look toward Left arm = Right SCM must contract