Neuroanatomy

Corpus callosum

Corpus callosum

The corpus callosum (from Latin: “tough body”), also known as the colossal commissure, is a wide, flat bundle of neural fibers beneath the cortex in the eutherian brain at the longitudinal fissure. It connects the left and right cerebral hemispheres and facilitates interhemispheric communication. It is the largest white matter structure in the brain, consisting of 200–250 million contralateral axonal projections.

The band of white matter connecting the two cerebral hemispheres.

Plays a crucial role in interhemispheric communication.

Parts

see Genu.

see Splenium.

Importance

It is particularly important because various tumors and vascular lesions can be located in and around the corpus callosum, and it is a route through which pass several surgical approaches. Performing accurate surgery in this region and avoiding damage to normal structures require that the neurosurgeon have adequate knowledge of the anatomy of the intricate blood supply to this area.

see Callosotomy

Callosal disconnection syndrome, or split brain is an example of a disconnection syndrome from damage to the corpus callosum between the two hemispheres of the brain. Disconnection syndrome can also lead to aphasia, left-sided apraxia, and tactile aphasia, among other symptoms.

Arterial supply

The pericallosal and posterior pericallosal arteries were found to be the main sources of blood supply to the corpus callosum. In 80% of the specimens, the anterior communicating artery gave rise to either a subcallosal artery or a median callosal artery, each of which made a substantial contribution to the blood supply of the corpus callosum 1).

see Pericallosal pial plexus.

Short callosal arteries were present in 58 hemispheres (96.6%) and supplied the superficial surface of the corpus callosum along its midline and were a primary arterial source to this structure. Long callosal arteries were found in 28 hemispheres (46.6%) and contributed to the pial plexus. The cingulocallosal arteries were present in all hemispheres and supplied the corpus callosum, cingulate gyrus, and also contributed to the pericallosal pial plexus. The recurrent cingulocallosal arteries were present in 17 hemispheres (28.3%) and also contributed to the pericallosal pial plexus. The median callosal artery, an anatomical variation, was present in 10 brains (33.3%). This vessel supplied the corpus callosum and the cingulate gyrus 2).

Morphometry

Variations in morphometry exist. There is a paucity of data on CC dimensions in Nigeria, and no standardized reference is available. The study aimed to determine the CC dimensions among the adult population in southeast Nigeria. The result will provide reference ranges and form a benchmark for comparisons of CC-related pathologies. A retrospective study of CC morphometric dimensions in normal subjects who had cranial MRI over two years in Memfys Hospital, Enugu, Southeast Nigeria, using a 1.5T GE© 16 channel machine. The CC was segmentalized into seven subregions using the modified Witelson method with special computer software. All measurements were taken twice from the T1 mid-sagittal image, and the mean was used for computation. The results were analyzed using descriptive and inferential statistics. A total of 200 subjects were recruited for the study. The mean length and height of the CC were 75.58 ± 4.52 mm and 24.64 ± 3.40 mm, respectively. The width dimensions of the genu, body, rostrum and splenium were 10.88 ± 1.81 mm, 5.66 ± 1.32 mm, 3.65 ± 1.25 mm, and 10.02 ± 1.70 mm, respectively. No gender variations were noted among the different dimensions of CC (P = 0.90). The length and height of CC increase gradually with age and show a positive correlation. The width dimensions of the genu and splenium increase till middle age and subsequently decreases in line with brain atrophy (p = 0.0000& p = 0.004). Using Pearson’s correlation test, no correlation was noted in the dimensions of the body and rostrum of the corpus callosum when related to age and sex. (P = 0.92 & p = 0.66). Reference ranges of CC dimensions in the subregion were presented, and variations exist in its different morphometric dimensions which are affected by brain atrophy. Gender does not influence the dimensions in these subpopulations 3)

Corpus callosum abnormality

Corpus callosum abnormality.

Corpus callosum and epilepsies

Epilepsies are reported in up to two thirds of patients with complete or partial CC agenesis (AgCC). However, AgCC per se is not indicative for seizure disorders. Moreover, additional malformations of cortical development (MCD) are causal. Microstructural CC abnormalities are detected by advanced imaging techniques, are part of diffuse white matter disturbances and are related to cognitive deficits. The etiological significance remains unexplained. However, they are also found in non-epileptic benign and transient disorders. In drug-resistant epilepsies with violent drops to the floor (“drop seizures”) callosotomy may be beneficial in seizure reduction. Since the EEG after callosotomy exhibits a single seizure focus in up to 50% of patients, consecutive resective surgical methods might be successful.

CC is part of cerebral white matter and anomalies cannot act per se as seizure onset zone. Imaging techniques demonstrate additional lesions in patients with epilepsies. CC is the major pathway for seizure generalization. Therefore, callosotomy is used to prevent generalized drop seizures 4).

Corpus callosum lesion

Corpus callosum lesion.

Agenesis of the corpus callosum

Agenesis of the corpus callosum.

Corpus callosum dysgenesis

Corpus callosum dysgenesis.

Arteriovenous malformation of the corpus callosum

Arteriovenous malformation of the corpus callosum.

Corpus callosum tumor

Corpus callosum tumor

1) 

Türe U, Yaşargil MG, Krisht AF. The arteries of the corpus callosum: a microsurgical anatomic study. Neurosurgery. 1996 Dec;39(6):1075-84; discussion 1084-5. PubMed PMID: 8938760.

2) 

Kahilogullari G, Comert A, Arslan M, Esmer AF, Tuccar E, Elhan A, Tubbs RS, Ugur HC. Callosal branches of the anterior cerebral artery: an anatomical report. Clin Anat. 2008 Jul;21(5):383-8. doi: 10.1002/ca.20647. PubMed PMID: 18521950.

3) 

Ajare EC, Campbell FC, Mgbe EK, Efekemo AO, Onuh AC, Nnamani AO, Okwunodulu O, Ohaegbulam SC. MRI-based morphometric analysis of corpus callosum dimensions of adults in Southeast Nigeria. Libyan J Med. 2023 Dec;18(1):2188649. doi: 10.1080/19932820.2023.2188649. PMID: 36946121.

4) 

Unterberger I, Bauer R, Walser G, Bauer G. Corpus callosum and epilepsies. Seizure. 2016 Apr;37:55-60. doi: 10.1016/j.seizure.2016.02.012. Epub 2016 Mar 3. Review. PubMed PMID: 27010176.

Nucleus accumbens

Nucleus accumbens

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The nucleus accumbens (NAcc), also known as the accumbens nucleus or as the nucleus accumbens septi (Latin for nucleus adjacent to the septum) is a region in the basal forebrain rostral to the preoptic area of the hypothalamus.

The nucleus accumbens and the olfactory tubercle collectively form the ventral striatum, which is part of the basal ganglia.

Each cerebral hemisphere has its own nucleus accumbens. It is located where the head of the caudate nucleus and the anterior portion of the putamen meet just lateral to the septum pellucidum.

Function

The nucleus accumbens can be divided into two structures—the nucleus accumbens core and the nucleus accumbens shell. These structures have different morphology and function.

Cocaine use followed by withdrawal induces synaptic plasticity in the nucleus accumbens (NAc), which are thought to underlie subsequent drug-seeking behaviors and relapse. Previous studies suggest that cocaine-induced synaptic changes depend on acid-sensing ion channels (ASICs). Gupta et al. investigated the potential involvement of carbonic anhydrase 4 (CA4), an extracellular pH-buffering enzyme. They examined the effects of CA4 in mice on ASIC-mediated synaptic transmission in medium spiny neurons (MSNs) in NAc, as well as on cocaine-induced synaptic changes and behavior. They found that CA4 is expressed in the NAc and present in synaptosomes. Disrupting CA4 either globally, or locally, increased ASIC-mediated synaptic currents in NAc MSNs and protected against cocaine withdrawal-induced changes in synapses and cocaine-seeking behavior. These findings raise the possibility that CA4 might be a previously unidentified therapeutic target for addiction and relapse 1).

Research has indicated the nucleus accumbens has an important role in pleasure including laughter, reward, and reinforcement learning, as well as fear, aggression, impulsivity, addiction, and the placebo effect.

Previous imaging studies independently highlighted the role of the anterior thalamus (ANT) and nucleus accumbens (NAcc) in successful memory retrieval. While these findings accord with theoretical models, the precise temporal, oscillatory and network dynamics as well as the interplay between the NAcc and ANT in successfully retrieving information from long-term memory are largely unknown.

The University of Hamburg, Lübeck and Magdeburg in Germany addressed this issue by recording intracranial electroencephalography in human epilepsy patients from the NAcc (n = 5) and ANT (n = 4) during an old/new recognition test.

The findings demonstrate that differences in event-related potentials between correctly classified old (i.e., studied) and new (i.e., unstudied) images emerged in the NAcc and ANT already between 200 and 600 ms after stimulus onset. Moreover, time-frequency analyses revealed theta (4-8 Hz) power decreases for old compared to new items in the NAcc and the opposite effect in the ANT. Importantly, Granger causality analyses revealed a directional communication from ANT to NAcc suggesting that entrainment from ANT drives successful memory retrieval.

Together, this findings show evidence for the notion that the NAcc and ANT receive memory signals, and that theta oscillations may serve as a mechanism to bind these distributed neural assemblies 2).

Deep brain stimulation of the nucleus accumbens

see Deep brain stimulation of the nucleus accumbens.

1)

Gupta SC, Ghobbeh A, Taugher-Hebl RJ, Fan R, Hardie JB, LaLumiere RT, Wemmie JA. Carbonic anhydrase 4 disruption decreases synaptic and behavioral adaptations induced by cocaine withdrawal. Sci Adv. 2022 Nov 18;8(46):eabq5058. doi: 10.1126/sciadv.abq5058. Epub 2022 Nov 16. PMID: 36383659.
2)

Bauch EM, Bunzeck N, Hinrichs H, Schmitt FC, Voges J, Heinze HJ, Zaehle T. Theta Oscillations underlie Retrieval Success Effects in the Nucleus Accumbens and anterior Thalamus: evidence from human intracranial recordings. Neurobiol Learn Mem. 2018 Jul 4. pii: S1074-7427(18)30154-0. doi: 10.1016/j.nlm.2018.07.001. [Epub ahead of print] PubMed PMID: 29981424.

Insula functions

Insula functions

The insula serves as an integration cortex for multimodal convergence of distributed neural networks such as the somesthetic-limbic, insulo-limbic, insulo-orbito-temporal and the prefrontal-striato-pallidal-basal forebrain 1).

Histologically, the insula is a part of the paralimbic cortex, as it bears in its antero-inferior part an allo and mesocortical area. The insula is functionally involved in cardiac rhythm and arterial blood pressure control, as well as in viscero-motor control and in viscero-sensitive functions. There is considerable evidence for the involvement of the insula as a somesthetic area, including a major role in the processing of nociceptive inputs.

The combined findings of many sleep-related studies have confirmed a close link between the insula and insomnia, sleep deprivation, sleep disorders, and more. Although these results do not directly confirm that the insula is involved in sleep, an overall analysis of the results indicates that the insula may be a potential key brain region involved in sleep 2).

The insular lobe is involved in the gustatory, olfactory, auditory, and vestibular senses, motor integration, and motor planning of speech 3) 4).

Its possible role in some epilepsies may explain some failures of temporal lobectomy 5).

The insulae are believed to be involved in consciousness and play a role in diverse functions usually linked to emotion or the regulation of the body’s homeostasis. These functions include perception, motor control, self-awareness, cognitive functioning, and interpersonal experience. In relation to these, it is involved in psychopathology.

Recent neuroimaging studies have demonstrated that anterior insular cortex activation is associated with accessing interoceptive information and underpinning the subjective experience of emotional state. Only a small number of studies have focused on the influence of insular damage on emotion processing and interoceptive awareness. Moreover, disparate hypotheses have been proposed for the alteration of emotion processing by insular lesions. Some studies show that insular lesions yield an inability for understanding and representing disgust exclusively, but other studies suggest that such lesions modulate arousal and valence judgments for both positive and negative emotions.

In a study, Terasawa et al. examined the alteration in emotion recognition in three right insular and adjacent area damaged cases with well-preserved higher cognitive function. Participants performed an experimental task using morphed photos that ranged between neutral and emotional facial expressions (i.e., anger, sadness, disgust, and happiness). Recognition rates of particular emotions were calculated to measure emotional sensitivity. In addition, they performed heartbeat perception task for measuring interoceptive accuracy. The cases identified emotions that have high arousal level (e.g., anger) as less aroused emotions (e.g., sadness) and a case showed remarkably low interoceptive accuracy. The current results show that insular lesions lead to attenuated emotional sensitivity across emotions, rather than category-specific impairments such as to disgust. Despite the small number of cases, our findings suggest that the insular cortex modulates recognition of emotional saliency and mediates interoceptive and emotional awareness 6).

1)

Shelley BP, Trimble MR. The insular lobe of Reil–its anatamico-functional, behavioural and neuropsychiatric attributes in humans–a review. World J Biol Psychiatry. 2004 Oct;5(4):176-200. Review. PubMed PMID: 15543513.
2)

Wang Y, Li M, Li W, Xiao L, Huo X, Ding J, Sun T. Is the insula linked to sleep? A systematic review and narrative synthesis. Heliyon. 2022 Nov 5;8(11):e11406. doi: 10.1016/j.heliyon.2022.e11406. PMID: 36387567; PMCID: PMC9647461.
3)

Hugues D, Laurent C, Manuel L, et al. The insular lobes: Physiopathological and surgical considerations. Neurosurgery 2000; 4: 801-811
4)

Francesco S, Jacques G, Kost E, et al. Review of current microsurgical management of insular gliomas. Acta Neurochir 2010; 152: 19-26
5)

Guenot M, Isnard J, Sindou M. Surgical anatomy of the insula. Adv Tech Stand Neurosurg. 2004;29:265-88. Review. PubMed PMID: 15035341.
6)

Terasawa Y, Kurosaki Y, Ibata Y, Moriguchi Y, Umeda S. Attenuated sensitivity to the emotions of others by insular lesion. Front Psychol. 2015 Sep 1;6:1314. doi: 10.3389/fpsyg.2015.01314. eCollection 2015. PubMed PMID: 26388817.

3D imaging

3D imaging

The word stereoscopy derives from Greek στερεός (stereos), meaning “firm, solid”, and σκοπέω (skopeō), meaning “to look, to see”.

Stereoscopy (also called stereoscopics or 3D imaging) is a technique for creating or enhancing the illusion of depth in an image by means of stereopsis for binocular vision.

One of the primary restrictions of endoscopic or endoscope assisted surgery is the lack of binocular or stereoscopic vision. Monocular endoscopes and displays create a 2-dimensional (2-D) image that impairs depth perception, hand-eye coordination, and the ability to estimate size 1) 2).

Most stereoscopic methods present two offset images separately to the left and right eye of the viewer. These two-dimensional images are then combined in the brain to give the perception of 3D depth. This technique is distinguished from 3D displays that display an image in three full dimensions, allowing the observer to increase information about the 3-dimensional objects being displayed by head and eye movements.

Operating in a 2D environment requires surgeons to train their hand-eye coordination to respond to visual cues received by the interaction of the operative instruments with the environment to accurately understand the relative depth of structures in the 2-D projection. Surgeons will often move the endoscope in and out or side to side to gain a motion parallax depth cue. This lack of stereoscopic vision has contributed to the steep learning curve in the field of neuroendoscopy. The next obvious step in the evolution of minimal access endoscopic surgery is the development of high-definition stereoendoscopes that produce a 3-dimensional (3-D) image.

Although such stereoendoscopes exist their use in neurosurgery has been limited because of the larger diameter and poor resolution of earlier generations.

Neurosurgery and Stereoscopy

It is easy to make errors in estimating the exact size and positioning of neural structures, especially when only using tomographic methods, as a lot of imagination and little precision is required. Wu and Tang found that combining the use of sectional micro-anatomy and micro-stereoscopic anatomy is much more accurate. They believe that his study makes a significant contribution to the literature because they believe that using improved methods to examine the neural structure is vital in future research on the micro-stereoscopic brain anatomy 3)

In neurosurgery stereoscopy has been used very successfully to demonstrate microsurgical anatomy by Albert L. Rhoton and co-workers 4) and Shimizu et al in 2006 5) , explained the importance of 3D neuroanatomical imaging as a teaching tool in neurosurgical training and comprehensively described original techniques for their production.

The use of 3D imaging in neurosurgical training is not as widespread as it could be, partially due to the complexity of some techniques described in the past, the long preparation time in the pre- and post-operative phases and their interference with the operation, making it impractical in busy neurosurgical theatres.

Companies producing microscopes and endoscopes have developed integrated 3D technology making 3D recording and editing less complex, but these are still very expensive.

Abarca et al performed endoscopic dissection in cadaveric specimens fixed with formalin and with the Thiel technique, both prepared using the intravascular injection of colored material. Endonasal approaches were performed with conventional 2D endoscopes. Then they applied the 3D anaglyph technique to illustrate the pictures in 3D.

The most important anatomical structures and landmarks of the sellar region under endonasal endoscopic vision are illustrated in 3D images.

The skull base consists of complex bony and neurovascular structures. Experience with cadaver dissection is essential to understand complex anatomy and develop surgical skills. A 3D view constitutes a useful tool for understanding skull base anatomy 6).

Advantages

Used effectively, stereoscopic three-dimensional (3D) technologies can engage students with complex disciplinary content as they are presented with informative representations of abstract concepts. In addition, preliminary evidence suggests that stereoscopy may enhance learning and retention in some educational settings. Biological concepts particularly benefit from this type of presentation since complex spatially oriented structures frequently define function within these systems. Viewing biological phenomena in 3D as they are in real life allows the user to relate these spatial relationships and easily grasp concepts making the key connection between structure and function. In addition, viewing these concepts interactively in 3D and in a manner that leads to increased engagement for young prospective scientists can further increase the impact. We conducted two studies evaluating the use of this technology as an instructional tool to teach high school students complex biological concepts. The first study tested the use of stereoscopic materials for teaching brain function and human anatomy to four classes. The second study evaluated stereoscopic images to support the learning of cell structure and DNA in four different high school classes. Most important, students who used stereoscopic 3D had significantly higher test scores than those who did not. In addition, students reported enjoying 3D presentations, and it was among their top choices for learning about these complex concepts. In summary, our evidence adds further support for the benefits of 3D images to students’ learning of science concepts 7).

The teaching of neuroanatomy in medical education has historically been based on didactic instruction, cadaveric dissections, and intra-operative experience for students. Multiple novel 3-Dimensional (3D) modalities have recently emerged. Among these, stereoscopic anaglyphic video is easily accessible and affordable, however, its effects have not yet formally been investigated.

This study aimed to investigate if 3D stereoscopic anaglyphic video instruction in neuroanatomy could improve learning for content-naive students, as compared to 2D video instruction.

A single-site controlled prospective case control study was conducted at the School of Education. Content knowledge was assessed at baseline, followed by the presentation of an instructional neuroanatomy video. Participants viewed the video in either 2D or 3D format, then completed a written test of skull base neuroanatomy. Pre-test and post-test performances were analyzed with independent t-tests and ANCOVA.

249 subjects completed the study. At baseline, the 2D (n=124, F=97) and 3D groups (n=125, F=96) were similar, although the 3D group was older by 1.7 years (p=.0355) and the curricula of participating classes differed (p<.0001). Average scores for the 3D group were higher for both pretest (2D, M=19.9%, SD=12.5% vs. 3D, M=23.9%, SD=14.9%, p=.0234) and posttest (2D, M=68.5%, SD=18.6% vs. 3D, M=77.3%, SD=18.8%, p=.003), but the magnitude of improvement across groups did not reach statistical significance (2D, M=48.7%, SD=21.3%, vs. 3D, M=53.5%, SD=22.7%, p=.0855).

Incorporation of 3D video instruction into curricula without careful integration is insufficient to promote learning over 2D video 8).

Preoperative 3D imaging provides reliable and detailed information about the intraoperative anatomical relationship between the trigeminal nerve and the SPV. This evaluation is useful for preoperative planning 9).

O arm 3D imaging with stereotactic navigation may be used to localize lesions intraoperatively with real-time dynamic feedback of tumor resection.

Stereotactic guidance may augment resection or biopsy of primary and metastatic spinal tumors. It offers reduced radiation exposure to OR personnel as well as the ability to use minimally invasive approaches that limit tissue injury. Further work may be done to assess the utility of stereotactic image guidance in oncological tumor resection, particularly with respect to outcomes for patients 10).

1)

Badani KK, Bhandari A, Tewari A, Menon M: Comparison of two- dimensional and three-dimensional suturing: Is there a difference in a robotic surgery setting? J Endourol 19:1212–1215, 2005.
2)

Taffinder N, Smith SG, Huber J, Russell RC, Darzi A: The effect of a second- generation 3D endoscope on the laparoscopic precision of novices and expe- rienced surgeons. Surg Endosc 13:1087–1092, 1999.
3)

Wu JZ, Tang CH. Correspondence: A combination of sectional micro-anatomy and micro-stereoscopic anatomy is an improved micro-dissection method. J Anat. 2022 Feb 6. doi: 10.1111/joa.13631. Epub ahead of print. PMID: 35128655.
4)

Ribas GC , Bento RF , Rodrigues AJ . Anaglyphic three-dimensional stereoscopic printing: revival of an old method for anatomical and surgical teaching and reporting . J Neurosurg 2001 ; 95 : 1057 – 66.
5)

Shimizu S , Tanaka R , Rhoton AL , Jr ., e t al . Anatomic dissection and classic three-dimensional documentation: a unit of education for neurosurgical anatomy revisited . Neurosurgery 2006 ; 58 : E1000. discussion E .
6)

Abarca-Olivas J, Monjas-Cánovas I, López-Álvarez B, Lloret-García J, Sanchez-del Campo J, Gras-Albert JR, Moreno-López P. [Three-dimensional endoscopic endonasal study of skull base anatomy]. Neurocirugia (Astur). 2014 Jan-Feb;25(1):1-7. doi: 10.1016/j.neucir.2013.02.009. Epub 2014 Jan 18. Spanish. PubMed PMID: 24447642.
7)

Ferdig R, Blank J, Kratcoski A, Clements R. Using stereoscopy to teach complex biological concepts. Adv Physiol Educ. 2015 Sep;39(3):205-8. doi: 10.1152/advan.00034.2014. PubMed PMID: 26330039.
8)

Goodarzi A, EdM SM, Lee D, Girgis F. The effect of stereoscopic anaglyphic 3-dimensional video didactics on learning neuroanatomy. World Neurosurg. 2017 Jul 29. pii: S1878-8750(17)31219-6. doi: 10.1016/j.wneu.2017.07.119. [Epub ahead of print] PubMed PMID: 28765017.
9)

Xiong NX, Zhou X, Yang B, Wang L, Fu P, Yu H, Wang Q, Abdelmaksoud A, Yuan Y, Liu W, Huang Y, Budrytė K, Huang T, Zheng X. Preoperative MRI Evaluation of Relationship between Trigeminal Nerve and Superior Petrosal Vein: Its Role in Treating Trigeminal Neuralgia. J Neurol Surg A Cent Eur Neurosurg. 2019 Mar 26. doi: 10.1055/s-0038-1669399. [Epub ahead of print] PubMed PMID: 30913572.
10)

http://thejns.org/doi/full/10.3171/2016.5.FOCUS16136

Rubber hand illusion

Rubber hand illusion

The rubber hand illusion (RHI) is a perceptual experience that often occurs when an administered tactile stimulation of a person’s real hand hidden from view, occurs synchronously with a corresponding visual stimulation of an observed rubber hand placed in full vision of the person in a position corresponding to where their real hand might normally be. The perceptual illusion is that the person feels a sense of “ownership” of the rubber hand which they are looking at. Most studies have focused on the underlying neural properties of the illusion and the experimental manipulations that lead to it. The illusion could also be used for exploring the sense of limb and prosthetic ownership for people after amputation. Cortical electrodes such as those used in sensorimotor stimulation surgery for pain may provide an opportunity to further understand the cortical representation of the illusion and possibly provide an opportunity to modulate the individual’s sense of body ownership. Thus, the RHI might also be a critical tool for the development of neurorehabilitative interventions that will be of great interest to the neurosurgical and rehabilitation communities 1).

The widely used rubber hand illusion (RHI) paradigm provides insight into how the brain manages conflicting multisensory integration regarding bodily self-consciousness. Previous functional neuroimaging studies have revealed that the feeling of body ownership is linked to activity in the premotor cortex, the intraparietal areas, the occipitotemporal cortex, and the insula. Matuz-Budai et al. from Pécs, investigated whether the individual differences in the sensation of body ownership over a rubber hand, as measured by the subjective report and the proprioceptive drift, are associated with structural brain differences in terms of cortical thickness in 67 healthy young adults. Matuz-Budai et al. found that individual differences measured by the subjective report of body ownership are associated with the cortical thickness in the somatosensory regions, the temporoparietal junction, the intraparietal areas, and the occipitotemporal cortex, while the proprioceptive drift is linked to the premotor cortex and the anterior cingulate cortex. These results are in line with functional neuroimaging studies indicating that these areas are indeed involved in processes such as cognitive-affective perspective-taking, visual processing of the body, and the experience of body ownership and bodily awareness. Consequently, these individual differences in the sensation of body ownership are pronounced in both functional and structural differences 2)

1)

Ramakonar H, Franz EA, Lind CR. The rubber hand illusion and its application to clinical neuroscience. J Clin Neurosci. 2011 Dec;18(12):1596-601. doi: 10.1016/j.jocn.2011.05.008. Epub 2011 Oct 13. PMID: 22000838.
2)

Matuz-Budai T, Lábadi B, Kohn E, Matuz A, Zsidó AN, Inhóf O, Kállai J, Szolcsányi T, Perlaki G, Orsi G, Nagy SA, Janszky J, Darnai G. Individual differences in the experience of body ownership are related to cortical thickness. Sci Rep. 2022 Jan 17;12(1):808. doi: 10.1038/s41598-021-04720-8. PMID: 35039541.

Superior frontal gyrus

Superior frontal gyrus

The superior frontal gyrus (SFG) makes up about one-third of the frontal lobe of the human brain. It is bounded laterally by the superior frontal sulcus.

The superior frontal gyrus, like the inferior frontal gyrus and the middle frontal gyrus, is more of a region than a true gyrus.

Function

Lateral penetration of the Superior frontal gyrus (SFG) in the left hemisphere is associated with worsening phonemic verbal fluency and has greater explanatory power than active contact location. This may be explained by lesioning of the lateral SFG-Broca area pathway, which is implicated in language function 1).

Alagapan et al. combined electrocorticography and direct cortical stimulation in three patients implanted with subdural electrodes to assess if superior frontal gyrus is indeed involved in working memory. They found left SFG exhibited task-related modulation of oscillations in the theta and alpha frequency bands specifically during the encoding epoch. Stimulation at the frequency matched to the endogenous oscillations resulted in reduced reaction times in all three participants. The results provide evidence for SFG playing a functional role in working memory and suggest that SFG may coordinate working memory through low-frequency oscillations thus bolstering the feasibility of using intracranial electric stimulation for restoring cognitive function 2).

The supplementary motor area syndrome is a characteristic neurosurgical syndrome that can occur after surgery in the superior frontal gyrus.

The superior frontal gyrus (SFG) is thought to contribute to higher cognitive functions and particularly to working memory (WM), although the nature of its involvement remains a matter of debate. To resolve this issue, methodological tools such as lesion studies are needed to complement the functional imaging approach.

du Boisgueheneuc et al have conducted the first lesion study to investigate the role of the SFG in WM and address the following questions: do lesions of the SFG impair WM and, if so, what is the nature of the WM impairment? To answer these questions, they compared the performance of eight patients with a left prefrontal lesion restricted to the SFG with that of a group of 11 healthy control subjects and two groups of patients with focal brain lesions prefrontal lesions sparing the SFG (n = 5) and right parietal lesions (n = 4)] in a series of WM tasks. The WM tasks (derived from the classical n-back paradigm) allowed us to study the impact of the SFG lesions on domain (verbal, spatial, face) and complexity (1-, 2- and 3-back) processing within WM. As expected, patients with a left SFG lesion exhibited a WM deficit when compared with all control groups, and the impairment increased with the complexity of the tasks. This complexity effect was significantly more marked for the spatial domain. Voxel-to-voxel mapping of each subject’s performance showed that the lateral and posterior portion of the SFG (mostly Brodmann area 8, rostral to the frontal eye field) was the subregion that contributed the most to the WM impairment. These data led us to conclude that (i) the lateral and posterior portion of the left SFG is a key component of the neural network of WM; (ii) the participation of this region in WM is triggered by the highest level of executive processing; (iii) the left SFG is also involved in spatially oriented processing.

The findings support a hybrid model of the anatomical and functional organization of the lateral SFG for WM, according to which this region is involved in higher levels of WM processing (monitoring and manipulation) but remains oriented towards spatial cognition, although the domain specificity is not exclusive and is overridden by an increase in executive demand, regardless of the domain being processed. From a clinical perspective, this study provides new information on the impact of left SFG lesions on cognition that will be of use to neurologists and neurosurgeons 3).

Superior frontal gyrus tumor.

Quiz

The superior frontal gyrus (SFG) makes up about two thirds of the frontal lobe of the human brain

Y/N

The role of the superior frontal gyrus (SFG) is not yet clear

Y/N

The superior frontal gyrus (SFG) plays a functional role in working memory

Y/N

References

1)

Askari A, Greif TR, Lam J, Maher AC, Persad CC, Patil PG. Decline of verbal fluency with lateral superior frontal gyrus penetration in subthalamic nucleus deep brain stimulation for Parkinson disease. J Neurosurg. 2022 Jan 28:1-6. doi: 10.3171/2021.11.JNS211528. Epub ahead of print. PMID: 35090137.
2)

Alagapan S, Lustenberger C, Hadar E, Shin HW, Frӧhlich F. Low-frequency direct cortical stimulation of left superior frontal gyrus enhances working memory performance. Neuroimage. 2019 Jan 1;184:697-706. doi: 10.1016/j.neuroimage.2018.09.064. Epub 2018 Sep 27. PubMed PMID: 30268847; PubMed Central PMCID: PMC6240347.
3)

du Boisgueheneuc F, Levy R, Volle E, Seassau M, Duffau H, Kinkingnehun S, Samson Y, Zhang S, Dubois B. Functions of the left superior frontal gyrus in humans: a lesion study. Brain. 2006 Dec;129(Pt 12):3315-28. Epub 2006 Sep 19. PubMed PMID: 16984899.

Posterior parietal cortex

Posterior parietal cortex

The posterior parietal cortex (the portion of parietal neocortex posterior to the primary somatosensory cortex) plays an important role in planned movements, spatial reasoning, and attention.

Damage to the posterior parietal cortex can produce a variety of sensorimotor deficits, including deficits in the perception and memory of spatial relationships, inaccurate reaching and grasping, in the control of eye movement, and inattention. The two most striking consequences of PPC damage are apraxia and hemispatial neglect.

Pereira et al. from Geneva showed that perceptual consciousness and monitoring involve evidence accumulation. They performed single-unit recording in a participant with a microelectrode in the posterior parietal cortex, while they detected vibrotactile stimuli around the detection threshold and provided confidence estimates. They find that detected stimuli elicited neuronal responses resembling evidence accumulation during decision-making, irrespective of motor confounds or task demands. They generalized these findings in healthy volunteers using electroencephalography. Behavioral and neural responses are reproduced with a computational model considering a stimulus as detected if accumulated evidence reaches a bound, and confidence as the distance between maximal evidence and that bound. They concluded that gradual changes in neuronal dynamics during evidence accumulation relates to perceptual consciousness and perceptual monitoring in humans 1)

Spatial remapping, the process of updating information across eye movements, is an important mechanism for trans-saccadic perception. The right posterior parietal cortex (PPC) is a region that has been associated most strongly with spatial remapping. The aim of a project of Ten Brink et al. was to investigate the effect of damage to the right PPC on direction specific transsaccadic memory. They compared trans-saccadic memory performance for central items that had to be remembered while making a left- versus rightward eye movement, or for items that were remapped within the left versus right visual field.

They included 9 stroke patients with unilateral right PPC lesions and 31 healthy control subjects. Participants memorized the location of a briefly presented item, had to make one saccade (either towards the left or right, or upward or downward), and subsequently had to decide in what direction the probe had shifted. We used a staircase to adjust task difficulty (i.e., the distance between the memory item and probe). Bayesian repeated measures ANOVAs were used to compare left versus right eye movements and items in the left versus right visual field.

In both conditions, patients with right PPC damage showed worse trans-saccadic memory performance compared to healthy control subjects (for the condition with left- and rightward gaze shifts, BF10 = 3.79; and when items were presented left or right, BF10 = 6.77), regardless of the direction of the gaze or the initial location of the memory item. At the individual level, none of the patients showed a direction specific deficit after leftward versus rightward saccades, whereas two patients showed worse performance for items in the left versus right visual field.

Damage in the right PPC did not lead to gaze direction specific impairments in trans-saccadic memory, but instead caused more general spatial memory impairments 2).

1)

Pereira M, Megevand P, Tan MX, Chang W, Wang S, Rezai A, Seeck M, Corniola M, Momjian S, Bernasconi F, Blanke O, Faivre N. Evidence accumulation relates to perceptual consciousness and monitoring. Nat Commun. 2021 May 31;12(1):3261. doi: 10.1038/s41467-021-23540-y. PMID: 34059682.
2)

Ten Brink AF, Fabius JH, Weaver NA, Nijboer TCW, Van der Stigchel S. Trans-saccadic memory after right parietal brain damage. Cortex. 2019 Jun 28;120:284-297. doi: 10.1016/j.cortex.2019.06.006. [Epub ahead of print] PubMed PMID: 31376588.

Superior longitudinal fasciculus

Superior longitudinal fasciculus

The superior longitudinal fasciculus (also called the superior longitudinal fascicle or SLF) is a pair of long bi-directional bundles of neurons connecting the front and the back of the cerebrum. Each association fiber bundle is lateral to the centrum ovale of a cerebral hemisphere and connects the frontal, occipital, parietal, and temporal lobes. The neurons pass from the frontal lobe through the operculum to the posterior end of the lateral sulcus where numerous neurons radiate into the occipital lobe and other neurons turn downward and forward around the putamen and radiate to anterior portions of the temporal lobe.

The description of human white matter pathways experienced a tremendous improvement, thanks to the advancement of neuroimaging and dissection techniques. The downside of this progress is the production of redundant and conflicting literature, bound by specific studies’ methods and aims. The Superior Longitudinal System (SLS), encompassing the arcuate (AF) and the superior longitudinal fasciculi (SLF), becomes an illustrative example of this fundamental issue, being one of the most studied white matter association pathways of the brain. Vavassori et al. provided a complete illustration of this white matter fiber system’s current definition, from its early descriptions in the nineteenth century to its most recent characterizations. They proposed a review of both in vivo diffusion magnetic resonance imaging-based tractography and anatomical dissection studies, enclosing all the information available up to date. Based on these findings, they reconstructed the wiring diagram of the SLS, highlighting a substantial variability in the description of its cortical sites of termination and the taxonomy and partonomy that characterize the system. They aimed to level up discrepancies in the literature by proposing a parallel across the various nomenclature. Consistent with the topographical arrangement already documented for commissural and projection pathways, they suggested approaching the SLS organization as an orderly and continuous wiring diagram, respecting a medio-lateral palisading topography between the different frontal, parietal, occipital, and temporal gyri rather than in terms of individualized fascicles. A better and complete description of the fine organization of white matter association pathways’ connectivity is fundamental for a better understanding of brain function and their clinical and neurosurgical applications 1).

The aim of a study was to examine the arcuate fasciculus (AF) and superior longitudinal fasciculi (SLF), which together form the dorsal language stream, using fiber dissection and diffusion imaging techniques in the human brain.

Twenty-five formalin-fixed brains (50 hemispheres) and 3 adult cadaveric heads, prepared according to the Klingler method, were examined by the fiber dissection technique. The authors’ findings were supported with MR tractography provided by the Human Connectome Project, WU-Minn Consortium. The frequencies of gyral distributions were calculated in segments of the AF and SLF in the cadaveric specimens.

The AF has ventral and dorsal segments, and the SLF has 3 segments: SLF I (dorsal pathway), II (middle pathway), and III (ventral pathway). The AF ventral segment connects the middle (88%; all percentages represent the area of the named structure that is connected to the tract) and posterior (100%) parts of the superior temporal gyrus and the middle part (92%) of the middle temporal gyrus to the posterior part of the inferior frontal gyrus (96% in pars opercularis, 40% in pars triangularis) and the ventral premotor cortex (84%) by passing deep to the lower part of the supramarginal gyrus (100%). The AF dorsal segment connects the posterior part of the middle (100%) and inferior temporal gyrus (76%) to the posterior part of the inferior frontal gyrus (96% in pars opercularis), ventral premotor cortex (72%), and posterior part of the middle frontal gyrus (56%) by passing deep to the lower part of the angular gyrus (100%).

This study depicts the distinct subdivision of the AF and SLF, based on cadaveric fiber dissection and diffusion imaging techniques, to clarify the complicated language processing pathways 2).

Classification

Superior longitudinal fasciculus classification.

1)

Vavassori L, Sarubbo S, Petit L. Hodology of the superior longitudinal system of the human brain: a historical perspective, the current controversies, and a proposal. Brain Struct Funct. 2021 Apr 21. doi: 10.1007/s00429-021-02265-0. Epub ahead of print. PMID: 33881634.
2)

Yagmurlu K, Middlebrooks EH, Tanriover N, Rhoton AL Jr. Fiber tracts of the dorsal language stream in the human brain. J Neurosurg. 2015 Nov 20:1-10. [Epub ahead of print] PubMed PMID: 26587654.

Medical student

Medical student

For students beginning their medical education, the neuroscience curriculum is frequently seen as the most difficult, and many express an aversion to the topic. A major reason for this aversion amongst learners is the perceived complexity of neuroanatomy 1).

The National Undergraduate Neuroanatomy Competition was established in 2013 as a means for students to display this commitment as well as academic ability.

A bespoke 22 item questionnaire was designed to determine career outcomes and the role of competition attendance in job applications. It was distributed using the SurveyMonkey website to the 87 attendees at the 2013 and 2014 competitions.

Responses were received by 40 competitors (response rate 46.0%). Twenty-four (60.0%) responders intend to pursue a career in either neurosurgery (n=18) or neurology (n=6). This included 10 (25.0%) responders who had successfully entered either neurosurgery (n=9) or neurology (n=1). The performance of these 10 (n=11, 57.0% ± 13.6) was significantly better than the other responders (n=30, 46.5% ± 13.5) (p=0.036). Seventeen (42.5%) responders either included their attendance at NUNC in a post-Foundation job application or intend to.

The National Undergraduate Neuroanatomy Competition provides the opportunity for medical students to demonstrate their interest in neurosurgery. It has the potential to be used as a tool for recognizing medical students suitable for neurosurgery training 2).

History

Osler created the first residency program for specialty training of physicians, and he was the first to bring medical students out of the lecture hall for bedside clinical training. Historically, medical student education in neurological surgery has generally limited student involvement to assisting in research projects with minimal formal clinical exposure before starting sub-internships and application for the neurosurgery match. Consequently, students have generally had little opportunity to acquire exposure to clinical neurosurgery and attain minimal proficiency 3).

Neurosurgery seeks to attract the best and brightest medical students; however, there is often a lack of early exposure to the field, among other possible barriers.

Germany

Medical students show varying clinical practical skills when entering their final year clinical clerkship, which is the final period to acquire and improve practical skills prior to their residency. Behling et al. developed a one-on-one mentoring program to allow individually tailored teaching of clinical practical skills to support final year students with varying skill sets during their neurosurgical clinical clerkship.

Each participating student (n = 23) was paired with a mentor. At the beginning students were asked about their expectations, teaching preferences, and surgical interests. Regular meetings and evaluations of clinical practice skills were scheduled every 2 weeks together with fixed rotations that could be individually adjusted. The one-on-one meetings and evaluations with the mentor gave each student the chance for individually tailored teaching. After completion of the program, each student evaluated their experience.

The mentoring program was well-received by participating students and acquisition or improvement of clinical practical skills was achieved by most students. A varying practical skill level and interest in the field of surgery was seen.

A neurosurgical one-on-one mentoring program is well received by final year medical students and allows for individually tailored learning of clinical practical skills 4).

United States

Lubelski et al. sought to identify successful practices that can be implemented to improve medical student recruitment to neurosurgery.

United States neurosurgery residency program directors were surveyed to determine the number of medical student rotators and medical students matching into a neurosurgery residency from their programs between 2010 and 2016. Program directors were asked about the ways their respective institutions integrated medical students into departmental clinical and research activities.

Complete responses were received from 30/110 institutions. Fifty-two percent of the institutions had neurosurgery didactic lectures for 1st- and 2nd-year medical students (MS1/2), and 87% had didactics for MS3/4. Seventy-seven percent of departments had a neurosurgery interest group, which was the most common method used to integrate medical students into the department. Other forms of outreach included formal mentorship programs (53%), lecture series (57%), and neurosurgery anatomy labs (40%). Seventy-three percent of programs provided research opportunities to medical students, and 57% indicated that the schools had a formal research requirement. On average, 3 medical students did a rotation in each neurosurgery department and 1 matched into neurosurgery each year. However, there was substantial variability among programs. Over the 2010-2016 period, the responding institutions matched as many as 4% of the graduating class into neurosurgery per year, whereas others matched 0%-1%. Departments that matched a greater (≥ 1% per year) number of medical students into neurosurgery were significantly more likely to have a neurosurgery interest group and formal research requirements. A greater percentage of high-matching programs had neurosurgery mentorship programs, lecture series, and cadaver training opportunities compared to the other institutions.

In recent decades, the number of applicants to neurosurgery has decreased. A major deterrent may be the delayed exposure of medical students to neurosurgery. Institutions with early preclinical exposure, active neurosurgery interest groups, research opportunities, and strong mentorship recruit and match more students into neurosurgery. Implementing such initiatives on a national level may increase the number of highly qualified medical students pursuing neurosurgery 5).

A medical student training camp was created to improve the preparation of medical students for the involvement in neurological surgery activities and sub-internships.

A 1-day course was held at Weill Cornell Medicine, which consisted of a series of morning lectures, an interactive resident lunch panel, and afternoon hands-on laboratory sessions. Students completed self-assessment questionnaires regarding their confidence in several areas of clinical neurosurgery before the start of the course and again at its end.

A significant increase in self-assessed confidence was observed in all skill areas surveyed. Overall, rising fourth year students who were starting sub-internships in the subsequent weeks reported a substantial increase in their preparedness for the elective rotations in neurosurgery.

The preparation of medical students for clinical neurosurgery can be improved. Single-day courses such as the described training camp are an effective method for improving knowledge and skill gaps in medical students entering neurosurgical careers. Initiatives should be developed, in addition to this annual program, to increase the clinical and research skills throughout medical student education 6).

Canada

Medical students in Canada must make career choices by their final year of medical school. Selection of students for a career in neurosurgery has traditionally been based on marks, reference letters and personal interviews. Studies have shown that marks alone are not accurate predictors of success in medical practice; personal skills and attributes which can best be assessed by reference letters and interviews may be more important. A study was an attempt to assess the importance of, and ability to teach, personal skills and attitudes necessary for successful completion of a neurosurgical training program.

A questionnaire was sent to 185 active members of the Canadian Neurosurgical Society, asking them to give a numerical rating of the importance of 22 personal skills and attributes, and their ability to teach those skills and attributes. They were asked to list any additional skills or attributes considered important, and rate their ability to teach them.

Sixty-six (36%) questionnaires were returned. Honesty, motivation, willingness to learn, ability to problem solve, and ability to handle stress were the five most important characteristics identified. Neurosurgeons thought they could teach problem solving, willingness to consult informed sources, critical thinking, manual dexterity, and communication skills, but honesty, motivation, willingness to learn and ability to handle stress were difficult or impossible to teach.

Honesty, motivation, willingness to learn, problem solving and Stress management are important for success in a neurosurgical career. This information should be transmitted to medical students at “Career Day” venues. Structuring letters of reference and interviews to assess personal skills and attributes will be important, as those that can’t be taught should be present before the start of training 7).

References

1)

Larkin MB, Graves E, Rees R, Mears D. A Multimedia Dissection Module for Scalp, Meninges, and Dural Partitions. MedEdPORTAL. 2018 Mar 22;14:10695. doi: 10.15766/mep_2374-8265.10695. PubMed PMID: 30800895; PubMed Central PMCID: PMC6342347.
2)

Hall S, Stephens JR, Myers MA, Elmansouri A, Geoghegan K, Harrison CH, E N, D A, Parton WJ, Payne DR, Seaby E, Border S. The career impact of the National Undergraduate Neuroanatomy Competition. World Neurosurg. 2019 Sep 25. pii: S1878-8750(19)32516-1. doi: 10.1016/j.wneu.2019.09.086. [Epub ahead of print] PubMed PMID: 31562974.
3) , 6)

Radwanski RE, Winston G, Younus I, ElJalby M, Yuan M, Oh Y, Gucer SB, Hoffman CE, Stieg PE, Greenfield JP, Pannullo SC. Neurosurgery Training Camp for Sub-Internship Preparation: Lessons From the Inaugural Course. World Neurosurg. 2019 Apr 1. pii: S1878-8750(19)30926-X. doi: 10.1016/j.wneu.2019.03.246. [Epub ahead of print] PubMed PMID: 30947014.
4)

Behling F, Nasi-Kordhishti I, Haas P, Sandritter J, Tatagiba M, Herlan S. One-on-one mentoring for final year medical students during the neurosurgery rotation. BMC Med Educ. 2021 Apr 22;21(1):229. doi: 10.1186/s12909-021-02657-0. PMID: 33882933.
5)

Lubelski D, Xiao R, Mukherjee D, Ashley WW, Witham T, Brem H, Huang J, Wolfe SQ. Improving medical student recruitment to neurosurgery. J Neurosurg. 2019 Aug 9:1-7. doi: 10.3171/2019.5.JNS1987. [Epub ahead of print] PubMed PMID: 31398709.
7)

Myles ST, McAleer S. Selection of neurosurgical trainees. Can J Neurol Sci. 2003 Feb;30(1):26-30. PubMed PMID: 12619780.

Trigeminal nerve

Trigeminal nerve

Johann Friedrich Meckel made the first description of the subarachnoid space investing the trigeminal nerve into the middle fossa.

Possible pathways for facial pain include: trigeminal nerve (portio major as well as portio minor (motor root).

Supratentorial sensory perception, including facial pain, is subserved by the trigeminal nerve, in particular, by the branches of its ophthalmic nerve, which provide an extensive innervation of the dura mater and of the major brain blood vessels. In addition, contrary to previous assumptions, studies on awake patients during surgery have demonstrated that the mechanical stimulation of the pia mater and small cerebral vessels can also produce pain. The trigeminovascular system, located at the interface between the nervous and vascular systems, is therefore perfectly positioned to detect sensory inputs and influence blood flow regulation. Despite the fact that it remains only partially understood, the trigeminovascular system is most probably involved in several pathologies, including very frequent ones such as migraine, or other severe conditions, such as subarachnoid hemorrhage. The incomplete knowledge about the exact roles of the trigeminal system in headache, blood flow regulation, Blood-brain Barrier Permeability, and trigemino-cardiac reflex warrants for an increased investigation of the anatomy and physiology of the trigeminal system 1).

Anatomy

The trigeminal nerve complex is a very important and somewhat unique component of the nervous system. It is responsible for the sensory signals that arise from the most part of the face, mouth, nose, meninges, and facial muscles, and also for the motor commands carried to the masticatory muscles. These signals travel through a very complex set of structures: dermal receptors, trigeminal branches, Gasserian ganglion, central nuclei, and thalamus, finally reaching the cerebral cortex. Other neural structures participate, directly or indirectly, in the transmission and modulation of the signals, especially the nociceptive ones; these include vagus nerve, sphenopalatine ganglion, occipital nerves, cervical spinal cord, periaqueductal gray matter, hypothalamus, and motor cortex. But not all stimuli transmitted through the trigeminal system are perceivable. There is a constant selection and modulation of the signals, with either suppression or potentiation of the impulses. As a result, either normal sensory perceptions are elicited or erratic painful sensations are created 2).

Originating in the posterior fossa of the brain stem, it follows a long and complex course towards its distribution territory, crossing several regions with a complex anatomy and establishing important relationships with several structures.

The nerve fibers originate in the brainstem and are part of several grey matter nuclei occupying all the brainstem and even the first spinal cervical segments.

Each of these sensitive and motor nuclei represents different processing centers, and there is a true systematization of the information this nervous tract is responsible for conducting.

The sensitive nucleus is the largest, comprising 3 true sub-nuclei, each responsible for each aspect of the general sensitivity. The highest is the mesencephalic nucleus, located in the tegmentum close to the midline and to the grey matter close to the Sylvian aqueduct. The neurons that form this nucleus are in charge of the propioceptive integration in the Vth nerve territory, high level information for correct mastication. The main nucleus is in the pons, it is also situated in the depth of the tegmentum, and is responsible for the tactile integration of the territory of this nerve. Finally, the inferior nucleus occupies the tegmentum of the medulla, extending caudally to the first segments of the cervical spine, and is in charge of thermal and pain information. Its location explains the possible appearance of symptoms in the facial territory in patients with a degenerative/inflammatory disorder of the upper cervical spine. There is one single motor nucleus, located in the pons tegmentum supplying mastication muscles, and is correspondingly called mastication nucleus. The fibers related with all these nuclei gather in the pons and emerge through the lateral sector of its anterior aspect, forming a thick nervous tract with two roots: a thicker and lateral sensitive root and a thinner more medial motor root.

The only intra-axial segment of the Vth ends there and initiates its long course to its distribution territory; it is formed by different sub-segments before dividing itself into its terminal branches (the cisternal and Gasserian or transdural segments).

The point where the roots emerge in the brainstem is called “REZ” (Root Entry Zone), an anatomical landmark of great functional hierarchy.

Trigeminal nerve cisternal portion

see Trigeminal nerve cisternal portion.

Branches

The trigeminal nerve as the name indicates is composed of three large branches. They are the ophthalmic nerve (V1, sensory), maxillary nerve (V2, sensory), and mandibular nerve (V3, motor and sensory) branches. The large sensory root and smaller motor root leave the brainstem at the mid-lateral surface of pons.

Function

The trigeminal nerve (the fifth cranial nerve, or simply CN V) is a nerve responsible for sensation in the face and certain motor functions such as biting and chewing. It is the largest of the cranial nerves. Its name (“trigeminal” = tri- or three, and -geminus or twin, or thrice twinned) derives from the fact that each trigeminal nerve, one on each side of the pons, has three major branches: the ophthalmic nerve (V1), the maxillary nerve (V2), and the mandibular nerve (V3). The ophthalmic and maxillary nerves are purely sensory. The mandibular nerve has both cutaneous and motor functions.

Sensory information from the face and body is processed by parallel pathways in the central nervous system. The motor division of the trigeminal nerve is derived from the basal plate of the embryonic pons, while the sensory division originates from the cranial neural crest.

Trigeminal nerve sensory pathways

see Trigeminal nerve sensory pathways.

Trigeminal nerve-related pathology

Trigeminal nerve-related pathology.

Imaging

see Trigeminal nerve imaging.

References

1)

Terrier LM, Hadjikhani N, Velut S, Magnain C, Amelot A, Bernard F, Zöllei L, Destrieux C. The trigeminal system: The meningovascular complex- A review. J Anat. 2021 Feb 18. doi: 10.1111/joa.13413. Epub ahead of print. PMID: 33604906.
2)

Goellner E, Rocha CE. Anatomy of Trigeminal Neuromodulation Targets: From Periphery to the Brain. Prog Neurol Surg. 2020 Oct 6;35:1-17. doi: 10.1159/000511257. Epub ahead of print. PMID: 33022684.