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Anatomy of the Nervous System

Common Carotid Artery!!navigator!!

  • 70% of blood flow is delivered to ICA
  • shares waveform characteristics of both internal + external carotid arteries
  • velocity increases toward the aorta (9 cm/sec for each cm of distance from the carotid bifurcation)

Carotid Bifurcation

= physiologic stenosis ← inertial forces of blood flow diverting main-flow stream from midvessel to a path along vessel margin at flow divider

Location: lateral to upper border of thyroid cartilage; at level of C3-4 intervertebral disk

Branches: ECA arises anterior + medial to ICA (95%)

External Carotid Artery Branches!!navigator!!

mnemonic:All Summer Long Emily Ogled Peter's Sporty Isuzu

  • Ascending pharyngeal artery
  • Superior thyroid artery
  • Lingual artery
  • External maxillary = facial artery
  • Occipital artery
  • Posterior auricular artery
  • Superficial temporal artery
  • Internal maxillary artery

Internal Carotid Artery!!navigator!!

  1. CERVICAL SEGMENT
    ascends posterior and medial to ECA; enters carotid canal of petrous bone; NO branches
    Carotid bulb = carotid sinus:
    • = dilated proximal part of ICA with thinner media + thicker adventitia containing many receptor endings of glossopharyngeal nerve
    • Function: baroreceptor responsive to changes in arterial blood pressure
    • hypersensitive carotid sinus
      = slight touch / head movement initiates
    1. vasodilatation with drop in blood pressure
    2. vagal stimulation with sinoatrial / atrioventricular cardiac block
    • stagnant eddy that rotates at outer vessel margin
  2. PETROUS SEGMENT
    ascends briefly in carotid canal → bends anteromedially in a horizontal course (anterior to tympanic cavity and cochlea); exits near petrous apex through posterior portion of foramen lacerum; ascends to juxtasellar location where it pierces dural layer of cavernous sinus

    Branches:
    1. Caroticotympanic a.: to tympanic cavity, anastomoses with anterior tympanic branch of maxillary a. + stylomastoid a.
    2. Pterygoid (vidian) a.: through pterygoid canal; anastomoses with recurrent branch of greater palatine a.
  3. CAVERNOUS SEGMENT ascends to posterior clinoid process → then turns anteriorly + superomedially through cavernous sinus; exits medial to anterior clinoid process piercing dura
    Branches:
    1. Meningohypophyseal trunk
      1. tentorial branch
      2. dorsal meningeal branch
      3. inferior hypophyseal branch
    2. Anterior meningeal a.: supplies dura of anterior fossa; anastomoses with meningeal branch of posterior ethmoidal a.
    3. Cavernous rami supply trigeminal ganglion, walls of cavernous + inferior petrosal sinuses
  4. SUPRACLINOID SEGMENT
    ascends posterior + lateral between oculomotor + optic nerve
    Branches:
    mnemonic: OPA
    • Ophthalmic a.
    • Posterior communicating a.
    • Anterior choroidal a.
    1. Ophthalmic a. exits from ICA medial to anterior clinoid process, travels through optic canal inferolateral to optic nerve
      1. recurrent meningeal branch: dura of anterior middle cranial fossa
      2. posterior ethmoidal a.: supplies dura of planum sphenoidale
      3. anterior ethmoidal a.
    2. Superior hypophyseal a.: optic chiasm, anterior lobe of pituitary
    3. Posterior communicating a. (pCom)
    4. Anterior choroidal a.
    5. Middle + anterior cerebral arteries (MCA, ACA)

Carotid Siphon!!navigator!!

Flow direction: C4–C1

  1. C4 segment = before origin of ophthalmic a.

    Duplex Identification of Carotid Arteries

    CriteriaExternal Carotid ArteryInternal Carotid Artery
    Sizeusually smaller than ICAusually larger than ECA
    Locationoriented medially + anteriorly toward faceoriented laterally + posteriorly toward mastoid process (mnemonic: IAC vis-à-vis ECA positioned like helix vis-à-vis tragus of your ear)
    Branchesgives off arterial branches (superior thyroid artery as 1st branch)NO arterial branches
    Waveformhigh-resistance flow pattern supplying capillary beds in skin + muscle:
    • forward systolic component
    • early diastolic flow reversal, occasionally followed by another component
    • little / no flow in late diastole
    		low-resistance waveform pattern supplying capillary bed in brain:
    • high-velocity forward systolic component
    • sustained strong forward flow in diastole
    • stagnant eddy with flow reversal opposite to flow divider in carotid bulb
    Maneuveroscillations on temporal tap maneuver
  2. C3 segment = genu of ICA
  3. C2 segment = supraclinoid segment after origin of ophthalmic a.
  4. C1 segment = terminal segment of ICA between pCom + ACA

Anterior Cerebral Artery (ACA)!!navigator!!

A1 (horizontal) segment between origin and anterior communicating a. (aCom)

  1. inferior branches
    supply superior surface of optic nerve + chiasm
  2. superior branches
    penetrate brain to supply anterior hypothalamus, septum pellucidum, anterior commissure, fornix columns, anterior inferior portion of corpus striatum
  3. medial lenticulostriate artery (largest striatal artery) = recurrent artery of Heubner for anteroinferior portion of head of caudate, putamen, anterior limb of internal capsule

A2 (interhemispheric) segment after origin of anterior communicating a. (aCom); ascends in cistern of lamina terminalis

Branches:

  1. Medial orbitofrontal a.: along gyrus rectus
  2. Frontopolar a.
  3. Callosomarginal a.: within cingulus gyrus
  4. Pericallosal a.: over corpus callosum within callosal cistern
    1. Superior internal parietal a.: anterior portion of precuneus + convexity of superior parietal lobule
    2. Inferior internal parietal a.
    3. Posterior pericallosal a.
      from callosomarginal / pericallosal artery:
      • anterior + middle + posterior internal frontal aa.
      • paracentral a.: supplies pre- + postcentral gyri

Supply: anterior ⅔ of medial cerebral surface + 1 cm of superomedial brain over convexity

Middle Cerebral Artery!!navigator!!

= largest branch of ICA arising lateral to optic chiasm

M1 (horizontal) segment = courses in lateral direction

Branches: lateral lenticulostriate aa.

Supply: part of head and body of caudate, globus pallidus, putamen, and posterior limb of internal capsule

M2 (sylvian) segment = enters sylvian fissure just ventral to anterior perforated substance; divides into superior and inferior divisions with 2 / 3 / 4 branches

Branches: temporal lobe and insular cortex (sensory language area of Wernicke), parietal lobe (sensory cortical areas), inferolateral frontal lobe

M3 (cortical) segment = distal branches lateral to insular cortex = candelabra [candelabrum, Latin = decorative candlestick / lamp with several arms or branches]

Branches:

  1. Anterior temporal artery
  2. Ascending frontal artery / prefrontal a.
  3. Precentral artery = pre-Rolandic a.
  4. Central artery = Rolandic a.
  5. Anterior parietal artery = post-Rolandic a.
  6. Posterior parietal artery
  7. Angular artery
  8. Middle temporal artery
  9. Posterior temporal artery
  10. Temporooccipital artery

Supply: lateral cerebrum, insula, anterior + lateral temporal lobe

Posterior Cerebral Artery!!navigator!!

originates from bifurcation of basilar artery within interpeduncular cistern (in 15% as a direct continuation of posterior communicating artery); lies above oculomotor nerve and circles midbrain above the tentorium cerebelli

Branches:

  1. Mesencephalic perforating branches: tectum + cerebral peduncles
  2. Posterior thalamoperforating aa.: midline of thalamus + hypothalamus
  3. Thalamogeniculate aa.: geniculate bodies + pulvinar
  4. Posterior medial choroidal a.: circles midbrain parallel to PCA; enters lateral aspect of quadrigeminal cistern; passes laterally and above pineal gland and enters roof of 3rd ventricle; supplies quadrigeminal plate + pineal gland
  5. Posterior lateral choroidal a.: courses laterally and enters choroidal fissure; anterior branch to temporal horn + posterior branch to choroid plexus of trigone and lateral ventricle + lateral geniculate body
  6. Cortical branches:
    1. Anterior inferior temporal artery
    2. Posterior inferior temporal artery
    3. Parietooccipital artery
    4. Calcarine artery
    5. Posterior pericallosal artery

Supply: medial + posterior temporal lobe, medial parietal lobe, occipital lobe

Thalamic Blood Supply!!navigator!!

by multiple small vessels originating from Pcom + P1 and P2 segments of the PCAs

Territories:

  1. anterior: polar / thalamotuberal arteries ← Pcom
  2. paramedian: paramedian / thalamoperforating arteries ← P1 segment of PCA
  3. inferolateral: thalamogeniculate arteries ← P2 segment of PCA
  4. posterior: posterior choroidal arteries ← P2 segment

Anatomic variant (uncommon):

Artery of Percheron = single dominant thalamoperforating artery supplying both medial thalami (with variable contribution to rostral midbrain)

Occlusion:

  • CHARACTERISTIC bilateral paramedian thalamic infarcts ± midbrain involvement

Arterial Anastomoses of the Brain!!navigator!!

Anastomoses via Arteries at the Base of the Brain

  1. Circle of Willis
    1. Right ICA ↔ right ACA ↔ aCom ↔ left ACA ↔ left ICA
    2. ICA ↔ pCom ↔ basilar a.
    3. ICA ↔ anterior choroidal a. ↔ posterior choroidal a. ↔ PCA ↔ basilar a.
  2. Developmental anomaly four transient embryonal carotid-basilar anastomoses named according to their corresponding cranial nerves that regress in the following sequence:
    1. Primitive acoustic (otic) artery
      = arterial connection between petrous portion of ICA within carotid canal + proximal basilar artery / posterior inferior cerebellar a.
      • traverses internal auditory canal (with CN VIII)
    2. Primitive hypoglossal artery
      = arterial connection between the C1–C3 portion of ICA and proximal portion of basilar a.
      • traverses hypoglossal canal (with CN XII)
    3. Persistent primitive trigeminal artery
      Frequency: ~ 1%
      • short wide connection between the cavernous portion of ICA + basilar artery (between anterior inferior cerebellar a. and superior cerebellar a.)
      • penetrates sella turcica (in 50%)




      • enlargement of ipsilateral ICA
      • ectopic vessel crossing the pontine cistern to anastomose with basilar artery
    4. Proatlantal intersegmental artery
      = arterial connection between CCA bifurcation / ECA (57%) / ICA at C2–C4 level (38%) and vertebral a. in suboccipital region
      • traverses foramen magnum

Anastomoses via Surface Vessels

  1. Leptomeningeal anastomoses of the cerebrum: ACA ↔ MCA ↔ PCA
  2. Leptomeningeal anastomoses of the cerebellum: Superior cerebellar a. ↔ AICA ↔ PICA

Rete Mirabile

ECA ↔ middle meningeal a. / superficial temporal a. ↔ leptomeningeal aa. ↔ ACA / MCA

Cerebral Venous System!!navigator!!

Histo:NO smooth muscle / venous valves → bidirectional flow

Dural Venous Sinuses

= major drainage pathway from cerebral veins into internal jugular veins; enclosed by leaves of dura

  1. Superior group draining majority of brain + skull
    1. Superior sagittal sinus (SSS) collects superficial cerebral veins that drain cerebral convexities
      • luminal surface of triangular shape
      • traversed by septa → maintaining laminar flow + preventing venous reflux into cortical veins
    2. Inferior sagittal sinus
    3. Straight sinus (SS)
    4. Occipital sinus small midline vein draining toward foramen magnum / into jugular fossa / suboccipital veins
      Location: at attachment of falx cerebelli
      • may replace an aplastic transverse sinus
    5. Transverse sinus (TS)
      receives blood from the temporal, parietal, and occipital lobes
      • commonly asymmetric with right dominance
    6. Sigmoid sinus
    7. Confluence of sinuses (torcular herophili) formed by union of the SSS +SS + TS
      • often asymmetric in appearance (see below)
  2. inferior group drains superficial cerebral veins + basal and medial parts of undersurface of the brain + orbits + sphenoparietal sinus + cavernous sinus
    1. Cavernous sinus complex
    2. Superior petrosal sinus
      • arising from junction of transverse with sigmoid sinus
      • extends along petrous ridge
    3. Inferior petrosal sinus
      • arising from distal portion of sigmoid sinus / jugular bulb
      • extends along clivus

Variants of Torcular Herophili

  1. Codominant transverse sinuses (TS)
  2. Dominant right TS
  3. Dominant left TS
  4. Segmental hypo- / aplasia of proximal left TS + inflow to distal segment of TS from vein of Labbé and tentorial tributaries
  5. SSS drains into right TS + SS into left TS
  6. high split of SSS + SS drains into both TS

Superficial Venous System

= SUPERFICIAL CORTICAL VEINS

= great variability in drainage territories

  1. ascending (= superiorly draining) veins named for cortical area that is drained
  2. descending (inferiorly draining) veins
    1. Labbé vein
    2. Sylvian (superficial middle cerebral) vein drains blood from peri-insular region into basal dural sinuses

◊Relative luminal diameters of Trolard vein + Labbé vein + superficial sylvian vein are reciprocal

Location: subarachnoid space; traverse arachnoid mater + meningeal layer of dura mater to drain into dural venous sinus

Deep Venous System

= DEEP CEREBRAL VEINS

= centripetal drainage of hemispheric white matter and basal ganglia

  1. internal cerebral veins:
    1. Internal cerebral vein ← thalamostriate v. ← corpus callosum drainage (from anterior to posterior)
      1. septal vein
      2. anterior caudate vein
      3. terminal vein
    2. Basal vein of Rosenthal
    3. Vein of Galen
    4. Medullary + subependymal veins
  2. transcerebral veins: draining cerebral white matter
    • typically not visualized due to small caliber

Infratentorial Venous System

  1. superiorly into vein of Galen
  2. anteriorly into petrosal sinuses
  3. posteriorly into dural sinuses

Important vascular markers:

  1. Pontomesencephalic v. = anterior border of brainstem
  2. Precentral cerebellar v. = position of tectum
    • Colliculocentral point = midpoint of Twining's line at knee of precentral cerebellar vein
  3. Venous angle = acute angle at junction of thalamostriate with internal cerebral v. = posterior aspect of foramen of Monro
  4. Internal cerebral vv. = demarcate caudad border of splenium of corpus callosum superiorly + pineal gland inferiorly
  5. Copular point = junction of inferior + superior retrotonsillar tributaries draining cerebellar tonsils in region of copular pyramids of vermis

Arachnoid Granulations of Pacchioni

Prevalence: 66% (↑ in number + conspicuity with age)

Function: resorption of CSF

Location: within lacunae laterales of SSS

  • well-defined 2–9 mm focal filling defects within dural sinus
  • produce 13–15 mm calvarial impressions lateral to midline
  • iso- (⅓) / hypoattenuating (⅔) relative to parenchyma

Outline