Gross Anatomy, Cranial and Spine
Fig. 1.1. For abbreviations, see Table 1.1 and Table 1.2. The middle frontal gyrus (MFG) is usually more sinuous than the IFG or SFG, and it often connects to the pre-central gyrus via a thin isthmus.1 The central sulcus joins the Sylvian fissure in only 2% of cases (i.e., in 98% of cases there is a “subcentral” gyrus). The intraparietal sulcus (ips) separates the superior and inferior parietal lobules. The IPL is composed primarily of the AG and SMG. The Sylvian fissure terminates in the SMG (Brodmann’s area 40). The superior temporal sulcus terminates in the AG.
Fig. 1.1 also identifies the clinically significant areas of Brodmann’s (Br.) map of the cytoarchitectonic fields of the human brain. Functional significance of these areas is as follows:
Br. area 44: (dominant hemisphere) Broca’s area (classically “motor speech area” see speech & language)
Wernicke’s area: (dominant hemisphere) most of Br. area 40 and a portion of Br. area 39 (may also include ≈ posterior third of STG). Significant in speech & language
the striped portion of Br. area 8 in Fig. 1.1 (frontal eye field) initiates voluntary eye movements to the opposite direction
Fig. 1.2. The cingulate sulcus terminates posteriorly in the pars marginalis (pM) (plural: partes marginales). On axial imaging, the pMs are visible on 95% of CTs and 91% of MRIs,2 they are usually the most prominent of the paired grooves straddling the midline, and they extend a greater distance into the hemispheres.2 On axial CT or MRI, the pM is posterior to the widest biparietal diameter.2 The pMs curve posteriorly in lower slices and anteriorly in higher slices (here, the paired pMs form the “pars bracket”—a characteristic “handlebar” configuration straddling the midline).
The “AC-PC line” connects the anterior commissure (AC) and the posterior commissure (PC) on a midline sagittal image. The AC is the horizontally positioned white matter tract that crosses in front of the fornix. The PC is the white-matter band at the level of the pineal that crosses at the posterior third ventricle. The AC-PC line is used in functional neurosurgery and is also used as the baseline for axial MRI scans (and for recent CT scanners). In the more entrenched Talairach definition,3 it passes through the superior edge of the AC and the inferior edge of the PC (as illustrated in Fig. 1.2). Alternative Schaltenbrand definition4 : a line passing through the midpoint of the AC & PC, allowing both AC & PC to be imaged on a single thin axial MRI slice. These definitions differ by 5.81° ± 1.07°.5 The orbitomeatal line (used in older CT scanners) is ≈ 9° steeper than the Talairach AC-PC line.5
The primary motor cortex (AKA “motor strip”) and primary (somato)sensory cortex are organized somatotopically so that specific regions of the brain map correspond to specific areas of the body as shown in Fig. 1.3.
Some key points: the representation of the arm and face are draped over the convexity of the brain, while the foot and leg areas are located along the upper aspect of the medial surface. Areas with fine motor or sensory function (e.g. fingers, tongue) have a larger area of representation.
See Fig. 1.4. Identification of the central sulcus is important to localize the motor strip (contained in the PreCG). The central sulcus (CS) is visible on 93% of CTs and 100% of MRIs.2 It curves posteriorly as it approaches the interhemispheric fissure (IHF), and often terminates in the paracentral lobule, just anterior to the pars marginalis (pM) within the pars bracket (see above)2 (i.e., the CS often does not reach the midline).
parieto-occipital sulcus (pos) (or fissure): more prominent over the medial surface, and on axial imaging is longer, more complex, and more posterior than the pars marginalis6
post-central sulcus (pocs): usually bifurcates and forms an arc or parenthesis (“lazy-Y”) cupping the pM. The anterior limb does not enter the pM-bracket and the posterior limb curves behind the pM to enter the IHF
Hand “knob”: The alpha motor neurons for hand function are located in the superior aspect of the precentral gyrus7 which appears as a knob-like protrusion (shaped like an inverted greek letter omega Ω) projecting posterolaterally into the central sulcus on axial imaging8 (Fig. 1.4). On sagittal imaging it has a posteriorly projecting hook-like appearance and is even with the posterior limit of the Sylvian fissure.8
See Fig. 1.5.
For basion, see Fig. 12.1.
Sutures: cs = coronal, fn = frontonasal, fz = frontozygomatic, ls = lambdoid, nm = nasomaxillary, om = occipitomastoid, pm = parietomastoid, sm = squamomastoid, sp = sphenoparietal, sq = squamosal, ss = sphenosquamous, sz = sphenozygomatic, tz = temporozygomatic, zm = zygomaticomaxillary.
Pterion: region where the following bones are approximated: frontal, parietal, temporal and sphenoid (greater wing). Estimated location: 2 finger-breadths above the zygomatic arch, and a thumb’s breadth behind the frontal process of the zygomatic bone (blue circle in Fig. 1.5).
Asterion: junction of lambdoid, occipitomastoid and parietomastoid sutures. Usually lies within a few millimeters of the posterior-inferior edge of the junction of the transverse and sigmoid sinuses (not always reliable9 —may overlie either sinus).
Sagittal suture: midline suture from coronal suture to lambdoid suture. Although often assumed to overlie the superior sagittal sinus (SSS), the SSS lies to the right of the sagittal suture in the majority of specimens10 (but never by > 11 mm).
The most anterior mastoid point lies just in front of the sigmoid sinus.11
Abbreviations: (F = frontal horn, B = body, A = atrium, O = occipital horn, T = temporal horn) of lateral ventricle. FM = foramen of Monro. Aq = Sylvian aqueduct. V3 = third ventricle. V4 = fourth ventricle. cs = coronal suture. Dimensions D1–4 see Table 1.3.
In the non-hydrocephalic adult, the lateral ventricles lie 4–5 cm below the outer skull surface. The center of the body of the lateral ventricle sits in the midpupillary line, and the frontal horn is intersected by a line passing perpendicular to the calvaria along this line.23 The anterior horns extend 1–2 cm anterior to the coronal suture.
The midpoint of Twining’s line (• in Fig. 1.8) should lie within the 4th ventricle.
Estimates of cervical levels for anterior cervical spine surgery may be made using the landmarks shown in Table 1.4. Intraoperative C-spine X-rays are essential to verify these estimates.
Intercristal line: a line drawn between the highest point of the iliac crests across the back will cross the midline either at the interspace between the L4 and L5 spinous processes, or at the L4 spinous process itself.
See Table 1.5.
Cr. Nn. III, IV, VI, all 3 branches of V1 (ophthalmic division divides into nasociliary, frontal, and lacrimal nerves); superior ophthalmic vv.; recurrent meningeal br. from lacrimal a.; orbital branch of middle meningeal a.; sympathetic filaments from ICA plexus
Cr. N. VII (facial); Cr. N. VIII (stato-acoustic)—see text & Fig. 1.9
AKA internal auditory canal (Fig. 1.9).
The filaments of the acoustic portion of VIII penetrate tiny openings of the lamina cribrosa of the cochlear area.25
Transverse crest: separates superior vestibular area and facial canal (above) from the inferior vestibular area and cochlear area (see below).25
Vertical crest (AKA Bill’s bar—named after Dr. William House): separates the meatus to the facial canal anteriorly (containing VII and nervus intermedius) from the vestibular area posteriorly (containing the superior division of vestibular nerve). Bill’s bar is deeper in the IAC than the transverse crest.
nervus intermedius: the somatic sensory branch of the facial nerve primarily innervating mechanoreceptors of the hair follicles on the inner surface of the pinna and deep mechanoreceptors of nasal and buccal cavities and chemoreceptors in the taste buds on the anterior 2/3 of the tongue
superior branch of vestibular nerve: passes through the superior vestibular area to terminate in the utricle and in the ampullæ of the superior and lateral semicircular canals (mnemonic superior = LSU (Lateral & Superior semicircular canals and the Utricule))
For normal anatomy of right cerebellopontine angle, see Fig. 1.11.
Fig. 1.11 Normal anatomy of right cerebellopontine angle viewed from behind (as in a suboccipital approach).25
Ligaments of the occipitoatlantoaxial complex
superficial component: cephalad continuation of the posterior longitudinal ligament. A strong band connecting the dorsal surface of the dens to the ventral surface of the FM above, and dorsal surface of C2 & C3 bodies below
alar (“check”) ligaments26
transverse atlantal ligament (TAL) or (usually) just transverse ligament: the horizontal component of the cruciate ligament. Attaches at the medial tubercles of C1. Traps the dens against the anterior atlas via a strap-like mechanism (Fig. 1.14). Provides the majority of the strength (“the strongest ligament of the spine”27 )
The most important structures in maintaining atlantooccipital stability are the tectorial membrane and the alar ligaments. Without these, the remaining cruciate ligament and apical dentate ligament are insufficient.
Fig. 1.15 depicts a cross-section of a typical spinal cord segment, combining some elements from different levels (e.g. the intermediolateral gray nucleus is only present from T1 to ≈ L1 or L2 where there are sympathetic (thoracolumbar outflow) nuclei). It is schematically divided into ascending and descending halves; however, in actuality, ascending and descending paths coexist on both sides.
Number (Fig. 1.15)
aThe terminal fibers of this uncrossed tract usually cross in the anterior white commissure to synapse on alpha motor neurons or on internuncial neurons. A minority of the fibers do remain ipsilateral. Also, the anterior corticospinal tract is easily identified only in the cervical and upper thoracic regions.
Number (Fig. 1.15)
Number (Fig. 1.15)
Fig. 1.15 also depicts some of the laminae according to the scheme of Rexed. Lamina II is equivalent to the substantia gelatinosa. Laminae III and IV are the nucleus proprius. Lamina VI is located in the base of the posterior horn.
1st order neuron: heavily myelinated afferents; soma in dorsal root ganglion (no synapse). Short branches synapse in nucleus proprius (Rexed III & IV) of posterior gray; long fibers enter the ipsilateral posterior columns without synapsing (below T6: fasciculus gracilis; above T6: fasciculus cuneatus).
1st order neuron: large, heavily myelinated afferents (Type II); soma in dorsal root ganglion (no synapse). Some ascend uncrossed in posterior columns (with fine touch); most synapse in Rexed VI & VII.
Lesions in peripheral nerves and lesions in nerve roots may sometimes be distinguished in part by the pattern of sensory loss. A classic example is splitting of the ring finger in median nerve or ulnar nerve lesions, which does not occur in C8 nerve root injuries.
Fig. 1.16 shows anterior and posterior view, each schematically separated into sensory dermatomes (segmental) and peripheral sensory nerve distribution.
3 Talairach J, Tournoux P, Talairach J, et al. In: Practical examples for the use of the atlas in neuroradiogic examinations. Co-planar stereotactic atlas of the human brain. New Tork: Thieme Medical Publishers, Inc.; 1988:19-36.
14 Kido DK, LeMay M, Levinson AW, et al. Computed tomographic localization of the precentral gyrus. Radiology. 1980;135:373-377. DOI: 10.1148/radiology.135.2.7367629
18 Rahmah NN, Murata T, Yako T, et al. Correlation between squamous suture and sylvian fissure: OSIRIX DICOM viewer study. PLoS One. 2011;6 DOI: 10.1371/journal.pone.0018199
19 Chater N, Spetzler R, Tonnemacher K, et al. Microvascular bypass surgery. Part 1: anatomical studies. J Neurosurg. 1976;44:712-714. DOI: 10.3171/jns.1976.44.6.0712
20 Spetzler R, Chater N. Microvascular bypass surgery. Part 2: physiological studies. J Neurosurg. 1976;45:508-513. DOI: 10.3171/jns.1976.45.5.0508