Neurosurgery: Anatomy & Physiology

Cerebral Vasculature

Circle of Willis

  • Function: Creates Redundancy with Rich Collateral Circulation
  • Carotid Arteries (Supply 85%)
    • Anterior Cerebral Artery (ACA)
    • Middle Cerebral Artery (MCA)
      • Most Common Diseased Intracranial Artery
      • Not Technically Part of the Circle of Willis
  • Vertebral Artery
    • Basilar Artery
      • Posterior Cerebral Arteries (PCA)
  • Communications
    • Anterior Communicating Artery: ACA-ACA
    • Posterior Communicating Artery: MCA-PCA

Cranial Nerves

Cranial Nerve Function Mn

CN Nerve Function
I Olfactory Smell
II Optic Sight
III Oculomotor Eye Muscles (Not SO/LR)
IV Trochlear Eye Superior Oblique Muscle
V Trigeminal Sensation to Face Mastication Muscles
VI Abducens Eye Lateral Rectus Muscle
VII Facial Motor of Face Taste to Anterior 2/3 of Tongue
VIII Vestibulocochlear Hearing
IX Glossopharyngeal Taste to Posterior 1/3 of Tongue Motor of Swallowing & Stylopharyngeus
X Vagus Numerous: *See Individual Sections
XI Accessory Trapezius & SCM
XII Hypoglossal Tongue Muscles Injury Deviates to Same Side

Specific Branches

  • Trigeminal (V):
    • Ophthalmic
    • Maxillary
    • Mandibular
  • Fascial (VII): Mn
    • Temporal
    • Zygomatic
    • Buccal
    • Marginal Mandibular
    • Cervical

Cranium Exit

  • Cribriform Plate: CN I
  • Optic Foramen: CN II
  • Superior Orbital Foramen: CN III-VI
    • V Has Branches Through Other Exits
  • Stylomastoid Foramen: CN VII
  • Internal Acoustic Meatus: CN VIII
  • Jugular Foramen: CN IX-XI
    • XI Enters Cranium in Foramen Magnum
  • Hypoglossal Canal: CN XII

Head & Neck Pathways

  • CN VII (Facial)
    • Travels Through the Parotid Gland
  • CN X (Vagus)
    • Travels Between the Internal Jugular Vein & Carotid Artery
  • CN XI (Accessory)
    • Travels Through the Posterior Triangle

Cranial Nerve Exam

  • CN 1 – Asked to Identify Odors (Soap, Coffee, etc.)
    • Frequently Forgone in Initial Examinations (“CN 2-12 Intact”)
  • CN 2 – Check Visual Fields in All 4 Quadrants & Watch Pupillary Response
    • Possibly Assess Vision with Snellen Chart
  • CN 3/4/6 – Eye Movement Assessed by Following an Examiner’s Finger to All 4 Quadrants & to the Tip of the Nose
  • CN 5 – Test Sensation to the Face
  • CN 7 – Assess Facial Symmetry (Smile & Grimace)
  • CN 8 – Assess Hearing
  • CN 9/10 – Open Mouth & Say “Ah” (Soft Palate Should Elevate Without Lateral Deviation)
  • CN 11 – Turn Head Side-to-Side & Shrug Shoulders Against Resistance
  • CN 12 – Extend Tongue & Evaluate for Atrophy or Lateral Deviation

Spinal Cord & Spinal Nerves

Spinal Cord Tracts

  • Lateral Corticospinal Tract
    • Descending Motor Function
    • Decussates at the Medulla
    • Ipsilateral in Cord
    • Higher (Cervical) Fibers Run Medially & Lower (Lumbar) Fibers Run Laterally
      • Why Cervical “Cape Like” Loss Seen in Central Cord Syndrome
  • Dorsal Column
    • Ascending Proprioception & Deep Touch Sensation
    • Decussates at the Medulla
    • Ipsilateral in Cord
    • Higher (Cervical) Fibers Run Laterally & Lower (Lumbar) Fibers Run Medially
  • Spinothalamic Tract
    • Ascending Pain & Temperature Sensation
    • Decussates in the Anterior White Matter 2 Levels Above the Target
    • Contralateral in Cord
    • Higher (Cervical) Fibers Run Medially & Lower (Lumbar) Fibers Run Laterally
      • Why Cervical “Cape Like” Loss Seen in Central Cord Syndrome

Spinal Nerve Structure

  • Origination:
    • Cervical: Arise Above the Vertebrae
      • Except C8 is Below
    • Thoracic/Lumbar: Arise Below the Vertebrae
  • Nerve Roots:
    • Ventral Root: Efferent Conduction
    • Dorsal Root: Afferent Conduction
  • All Spinal Nerves Carry Motor, Sensory & Autonomics
    • Exception: C1 Only Has Motor
  • Node of Ranvier – Bare Area to Speed Conduction

Phrenic Nerve

  • Origin: C3, C4 & C5 Mn
  • Path:
    • Travels Over the Anterior Scalene Muscle in the Neck
    • Passes Anterior to the Pulmonary Hilum
  • Function:
    • Motor: Ipsilateral Diaphragm
    • Sensory: Pericardium, Diaphragmatic Pleura & Parietal Peritoneum
      • No Sensory Function to Diaphragm
  • Causes “Referred” Shoulder Pain After Abdominal Surgery
  • Injury Causes Diaphragm Paralysis
    • May See an Elevated Hemidiaphragm on Imaging

Brachial Plexus

  • Structure: Mn
    • Roots:
      • C5 Nerve Root
      • C6 Nerve Root
      • C7 Nerve Root
      • C8 Nerve Root
      • T1 Nerve Root
    • Trunks:
      • Superior Trunk (Roots C5 & C6)
      • Middle Trunk (Root C7)
      • Inferior Trunk (Roots C8 & T1)
    • Cords:
      • Lateral Cord (Superior & Middle Trunks)
      • Posterior Cord (All Trunks)
      • Medial Cord (Inferior Trunk)
    • Primary Terminal Branches:
      • Musculocutaneous Nerve (Lateral Cord)
      • Axillary Nerve (Posterior Cord)
      • Median Nerve (Lateral & Medial Cords)
      • Radial Nerve (Posterior Cord)
      • Ulnar Nerve (Medial Cord)
  • Transitions:
    • Roots to Trunks: After Passing the Scalene Muscles
    • Trunks to Cords: In Cervicoaxillary Canal (Posterior to Clavicle)
  • Additional Branches:
    • Off Upper Trunk:
      • Suprascapular Nerve – Supraspinatus & Infraspinatus
      • Nerve to the Subclavius – Subclavius
    • Off Lateral Cord:
      • Lateral Pectoral Nerve – Pectoralis Major
    • Off Posterior Cord:
      • Superior Subscapular Nerve – Subscapularis
      • Thoracodorsal Nerve – Latissimus Dorsi
      • Inferior Subscapular Nerve – Subscapularis & Teres Major
    • Off Medial Cord:
      • Medial Pectoral Nerve – Pectoralis Major & Pectoralis Minor
      • Medial Cutaneous Nerve of the Arm – Medial Skin of Arm
      • Medial Cutaneous Nerve of the Forearm – Medial Skin of Forearm

Peripheral Nerves

Upper Extremity

  • Axillary Nerve
    • Motor: Arm Abduction 15-90 Degrees (Deltoid & Teres Minor)
    • Sensation: Skin Over “Regimental Badge” Area (Inferolateral Shoulder)
  • Musculocutaneous Nerve
    • Motor: Arm Flexion (Biceps & Coracobrachialis)
    • Sensation: Lateral Forearm
  • Median Nerve
    • Motor: Forearm Flexion/Pronation, Lateral 3.5 Digit Flexion & Thumb Abduction
    • Sensation: Lateral 3.5 Digits (Anterior & Tips)
    • Deficits:
      • “Ape Hand Deformity” – Thumb Remains Adducted
      • “Sign of Benediction” – Attempt to Form a Fist & Lateral 3 Finger Remain Extended
  • Radial Nerve
    • Motor: Arm Extension (Triceps) & Forearm Extension
    • Sensation: Posterior Arm/Forearm & Lateral 3.5 Digits (Posterior)
    • Deficits:
      • Wrist Drop – Unable to Extend Hand/Fingers
      • Associated with “Saturday Night Palsy” (Falling Asleep with Chair in Axilla) & “Crutch Palsy” (From the Prolonged Use of Crutches)
  • Ulnar Nerve
    • Motor: Medial 1.5 Digit Flexion & Intrinsic Hand Muscles (Cross/Abduct Fingers)
    • Sensation: Medial 1.5 Digits (Anterior & Posterior)
    • Deficits:
      • “Claw Hand” – Attempt to Extend Fingers & Medial 2 Fingers Remain in Flexion
        • Able to Form a Fist (Often Confused with “Sign of Benediction”)

Lower Extremity

  • Femoral Nerve
    • Motor: Anterior Thigh (Leg Extension)
    • Sensation: Medial Thigh & Medial Lower Leg (By Saphenous Nerve)
  • Sciatic Nerve
    • Tibial Nerve
      • Motor: Posterior Thigh (Leg Flexion) & Posterior Lower Leg (Plantar Flexion)
    • Sural Nerve (Branch of Tibial Nerve)
      • Sensation: Posterolateral Lower Leg & Lateral Foot
    • Common Peroneal Nerve
      • Deep Peroneal Nerve
        • Motor: Ankle Dorsiflexion
        • Sensation: Foot First Dorsal Webspace
      • Superficial Peroneal Nerve
        • Motor: Foot Eversion & Some Plantar Flexion (Peroneus Longus & Brevis)
        • Sensation: Anterolateral Lower Leg, Dorsal Foot & Medial Foot

Mnemonics

Cranial Nerves

  • “Oh, Oh, Oh, To Touch And Feel Very Good Vagina’s, Ah Heaven”
    • I – Olfactory
    • II – Optic
    • III – Oculomotor
    • IV – Trochlear
    • V – Trigeminal
    • VI – Abducens
    • VII – Facial
    • VIII – Vestibulocochlear
    • IX – Glossopharyngeal
    • X – Vagus
    • XI – Accessory
    • XII – Hypoglossal
  • There are More Clean Mnemonics Out There, but This is the Classic

Branches of the Facial Nerve

  • “To Zanzibar By Motor Car” (T.Z.B.M.C.)
    • Temporal
    • Zygomatic
    • Buccal
    • Marginal Mandibular
    • Cervical

Origin of the Phrenic Nerve

  • C3-4-5 Keeps the Diaphragm Alive

Brachial Plexus

  • “Rod Tidwell Drinks Cold Beer”
  • Roots > Trunks > Divisions > Cords > Branches