Revisiting major anatomical risk factors of maxillary sinus lift and soft tissue graft harvesting for dental implant surgeons.

The anatomical variations of the maxillary sinus septa, greater palatine artery, and posterior superior alveolar arteries might cause unexpected complications when they are damaged. Dentists who know these structures well might hope to learn more practical knowledge to avoid and assess injury preoperatively. Therefore, this review paper aimed to review the reported anatomy and variations of the maxillary sinus septa, greater palatine artery/nerve, and posterior superior alveolar artery, and to discuss what has to be assessed preoperatively to avoid iatrogenic injury. To assess the risk of injury of surgically significant anatomical structures in the maxillary sinus and hard palate, the operator should have preoperative three-dimensional images in their mind based on anatomical knowledge and palpation. Additionally, knowledge of the average measurement results from previous studies is important.

Supernumerary incisive canals in a cadaver: a rare anatomical variation.

We present a rare anatomical variation in which an additional incisive canal was observed during the routine dissection of the palatal region in a cadaver. The diameter of the incisive and additional incisive canals measured 4.59 mm and 0.91 mm, respectively. Even though this variation is rare, oral surgeons should be aware of such anatomical variations for a better understanding.

Morphological observation and CBCT of the bony canal structure of the groove and the location of blood vessels and nerves in the palatine of elderly human cadavers.

PURPOSE: The greater and lesser palatine nerves and vessels supply the hard and soft palates, and the roots of these vessels and nerves run through a bony structure. However, the arrangement of blood vessels in the maxilla requires attention during clinical treatments, but detailed morphological information about changes in the greater and lesser palatine arteries and nerves during aging is unavailable. We therefore need detailed investigations of the morphology of the donor cadaver palatine using cone-beam computed tomography (CBCT) and macroscopic observations. METHODS: We investigated 72 donor cadavers using macroscopic segmentation and CBCT. The results' analysis examined differences in skull measurement parameters and differences between dentate and edentulous cases. RESULTS: The greater palatine artery and nerve showed different macroscopic arrangements in dentate and edentulous cadavers. We also classified three types of bony structures of the nerve and vessel roots in the molar regions of the palatine using CBCT images: the shallow groove, deep groove, and flat groove. The deep groove is the deepest of the three and is remarkable in edentulous elderly cadavers. CONCLUSION: This study of macroscopic and CBCT data provides information useful for planning dental implant surgeries and autogenous bone harvesting.

A Hypothesis and Pilot Study of Age-Related Sensory Innervation of the Hard Palate: Sensory Disorder After Nasopalatine Nerve Division.

BACKGROUND The nasopalatine nerve may be injured during extraction of teeth embedded in the anterior hard palate. The neural recovery process and its impact on sensation in the anterior hard palatal region are controversial. In our clinical practice, we noticed a distinct recovery process in children compared with adolescents or adults after surgery. We hypothesized that the sensory innervations of the anterior palate might shift during later childhood and pre-adolescence, which is due to the development of the nasopalatine nerve along with the maxillary growth and permanent teeth eruption. MATERIAL AND METHODS Forty patients (20 females and 20 males, mean age 11.8±2.2) with impacted supernumerary teeth in anterior palatine area were included into our study, and were divided into 3 groups according to their age. A 24-week follow-up was conducted and the sensation in the anterior hard palate region was examined at every check point. All the data were collected and analyzed by Kaplan-Meier analysis. RESULTS Fourteen children did not complain of any numbness immediately after anesthetization, and other children with sensory disorders had shorter healing periods compared to adolescent/adult patients. CONCLUSIONS The results indicated that the dominant nerve of the anterior hard palate region was dramatically changed from the greater palatine nerve to the nasopalatine nerve, which is important in deciding when to operate and in selection of anesthesia method.

Computerized analysis of the greater palatine foramen to gain the palatine neurovascular bundle during palatal surgery.

OBJECTIVE: Investigation of the computerized dimensional anatomic location of the greater palatine foramen (GPF) and lesser palatine foramens (LPF) is important indicating site to collect palatal donor tissue, reconstructioning the orofacial area of the oncology patient and applying the greater palatine nerve block anesthesia. The aim of this study is to determine a patient-friendly landmark and to specify the precise location of the GPF in order to standardise certain anatomical marks of safe neurovascular bundle. MATERIALS AND METHODS: 120 bony palates were examined to detect the position of the GPF and the LPF related to adjacent anatomical landmarks using a computer software program. The GPF was assessed regarding the position, the diameter and the distances between each foramen and the midline maxillary suture (MMS), the inner border of alveolar ridge (AR), posterior palatal border (PBB), and incisive foramen (IF). RESULTS: The GPF was identified as single in 81 %, double in 16 %, triple in 2 % and absent in 2 % of the specimens. The mean distances between the GPF and the MSS, the GPF and the AR, the GPF and the PPB, the GPF and the IF were 16, 4, 4, and 40 mm, respectively. In majority of the cases, the GPF was seen between the distal surfaces of the third maxillary molar (78 %). Single LPF was observed in 53.45 % of the skulls, two LPF were observed in 31 % of the skulls bilaterally and five LPF were rare in 2.1 % of the specimens. The LPF was most commonly at the junction of the palatine bone and the inner lamella of the pterygoid plate (71.9 %). CONCLUSIONS: This study made possible to investigate the variability of the GPF and the feasibility of the greater palatine neurovascular bundle, and to calculate the lengths of some parameters with the help of certain software. To collect the donor tissue of the neurovascular greater palatine network, each distance among the AR-GPF-PPB were equal to 4 mm. To estimate the possible length of the graft, the incision was made along the third and the second molar to the IF as 4 cm. The data we obtained within this study have been presented to help the surgeons avoid unexpected hemorrhage during the palatinal procedures such as posttraumatic dental reconstruction, maxillofacial tumor resections, palatal micro-implants, and dentofacial orthopedic surgery.

Topographical relationship of the greater palatine artery and the palatal spine. Significance for periodontal surgery.

AIM: The aims of this study were to (1) identify the branching pattern and course of the greater palatine artery (GPA), (2) carry out a morphological analysis of the palatal bony prominence that divides the medial and lateral grooves and (3) characterize the topographical relationships between these two structures. METHODS: Thirty-six hemimaxillae were studied with the aid of a surgical microscope to elucidate the GPA. A further 25 dry skulls were examined to establish the morphology of the palatal spine. RESULTS: The most common GPA branching pattern was type I (41.7%, 15 sides), which gave off the medial and canine branches after the bony prominence. The distances from the CEJ to the lateral branch of the GPA were 9.04 ± 2.93 mm (canine), 11.12 ± 1.89 mm (first premolar), 13.51 ± 2.08 mm (second premolar), 13.76 ± 2.86 mm (first molar) and 13.91 ± 2.20 mm (second molar). The palatal spine was frequently observed as the bony prominence (66.3%, 57 sides), and was located at 6.49 ± 1.76 mm from the greater palatine foramen, with a length of 10.42 ± 2.45 mm. There was no a correlation between the bony prominence shape and the GPA branching pattern. CONCLUSIONS: These results could provide the reference data regarding the topography of the GPA for periodontal surgery.

Neurovascular anatomical variations in the anterior palate observed on CBCT images.

OBJECTIVE: This study aimed to assess the presence of additional foramina and canals in the anterior palate region, through cone beam computed tomography (CBCT) images, describing their location, direction, and diameter. MATERIALS & METHODS: CBCT exams of 178 subjects displaying the anterior maxilla were included and the following parameters were registered: gender; age group; presence of additional foramina in the anterior palate (AFP) with at least 1 mm in diameter; location and diameter of AFP; and direction of bony canals associated with AFP. RESULTS: Twenty-eight patients (15.7%) presented AFP and in total 34 additional foramina were registered. No statistical differences between patients with or without AFP were found for gender or age. The average diameter of AFP was 1.4 mm (range from 1 to 1.9 mm). Their location was variable, with most of the cases occurring in the alveolar process near the incisors or canines (n = 27). In 18 cases, AFP was associated with bony canals with upward or oblique direction toward the anterior nasal cavity floor. In 14 cases, the canal presented as a direct extension of the canalis sinuosus, in an upward direction laterally to the nasal cavity aperture. In two cases, the canal was observed adjacent to the incisive and joined the nasopalatine canal superiorly. DISCUSSION: CBCT images have a crucial role in the recognition of anatomical variations by allowing detailed tridimensional evaluations. Additional foramina and canals in the anterior region of the upper jaw are relatively frequent. Practitioners should be aware and trained to identify these variations. CONCLUSIONS: Over 15% of the population studied had additional foramina in the anterior palate, between 1 mm and 1.9 mm wide, with variable locations. In most cases the canals associated with these foramina either presented as a direct extension of the canalis sinuosus, or coursed towards the nasal cavity floor.

Clinical measurements of hard palate and implications for subepithelial connective tissue grafts with suggestions for palatal nomenclature.

PURPOSE: The objectives of the present study were to 1) identify a reliable measuring technique for a palatal graft, 2) observe the patterns of the neurovascular bundle, and 3) identify the morphology of the hard palate. Subepithelial connective tissue grafts are performed at an increasing rate to improve esthetics and oral health. Palatal graft techniques began in 1963, and today the subepithelial connective tissue graft is the most widely accepted technique. The greater palatine nerve and artery are critical neurovascular structures to identify. Their pattern and palate morphology are not well defined. MATERIALS AND METHODS: Anatomy texts, atlases, and specialty texts were analyzed. We dissected 17 palates (17 left and right halves) from embalmed human cadavers, implementing a measuring technique to locate the most coronal structure of the greater palatine artery and greater palatine nerve bundle, observing the patterns and palatal morphology. Electronic digital calipers and a periodontal probe were used for data collection. RESULTS: The dissection results revealed a reliable measuring technique, a common pattern of the bundle, and osseous palatal landmarks not clearly defined in contemporary texts. The dissections also demonstrated a medial and lateral groove, along with a crest in the palatine process of the maxillary bone. The greater palatine artery traversed the lateral groove, and the greater palatine nerve traversed the medial groove consistently. The crest was located anteroposteriorly between the grooves. CONCLUSIONS: The results of our study suggest a useful measurement technique, a consistent neurovascular pattern, and the need to reconsider the palatal nomenclature.

Three-dimensional analysis of maxillary anatomic landmarks for greater palatine nerve block anesthesia.

Greater palatine nerve block anesthesia (GPNBA) is a local anesthetic procedure used for maxillary and nasal treatment. Investigation of the three-dimensional anatomic location of the greater palatine foramen (GPF) is important for successful local anesthesia. The study aim was to provide standards for anatomic structures in the oral cavity that can be easily referred to in GPNBA. Maxillary computed tomography data were obtained from patients between 8 and 16 years of age whose maxillary incisors and first molars had already erupted (the growth group, n = 103); changes in the maxilla were observed over time in this group. Reference values for GPNBA in adults were measured in 107 patients older than 18 years. Maxillary computed tomography images were reconstructed three-dimensionally. Regression analysis demonstrates that all maxillary measurements in the growth group except for the distance from the posterior nasal spine to the GPF in the coronal plane correlated significantly with age. In adults, the mean perpendicular distance from the interdental alveolar bone between the left and right central incisors (1alvB) to the GPF in the coronal plane was 46.16 mm, and the mean distance from 1alvB to the GPF was 51.05 mm. The mean distance from the maxillary central incisor to the GPF was 57.58 mm. The mean angle between the line from the maxillary central incisor to each GPF and the sagittal plane was 16.49 degrees. The mean perpendicular distance from the anterior nasal spine to the GPF in the coronal plane was 43.49 mm, whereas the mean perpendicular distance from the GPF to the bone plane was 12.67 mm, and the mean perpendicular distance from the GPF to the occlusal plane was 22.13 mm. These measurements can be used to find the height of the GPF. In adults, the measured perpendicular distance from the incisive foramen to the GPF in the coronal plane was 32.04 mm, and the perpendicular distance from the median of the line that connects both of the contact points between the maxillary tuberosity and the pterygoid plate to the GPF in the coronal plane was 5.23 mm. Three-dimensional reference values relative to the anatomic structures in the oral cavity may increase the success rate of GPNBA and reduce complications. Although the maxillary growth pattern was analyzed, a limitation of this study is that maxillary anatomic measurements were not analyzed with regard to race or ethnicity.

5-HT(1D) receptor immunoreactivity in the sphenopalatine ganglion: implications for the efficacy of triptans in the treatment of autonomic signs associated with cluster headache.

OBJECTIVE: To determine if 5-HT(1D) receptors are located in the sphenopalatine ganglion. BACKGROUND: While the 5-HT(1D) receptor has been described in sensory and sympathetic ganglia in the head, it was not known whether they were also located in parasympathetic ganglia. METHODS: We used retrograde labeling combined with immunohistochemistry to examine 5-HT(1D) receptor immunoreactivity in rat sphenopalatine ganglion neurons that project to the lacrimal gland, nasal mucosa, cerebral vasculature, and trigeminal ganglion. RESULTS: We found 5-HT(1D) receptor immunoreactivity in nerve terminals around postganglionic cell bodies within the sphenopalatine ganglion. All 5-HT(1D) -immunoreactive terminals were also immunoreactive for calcitonin gene-related peptide but not vesicular acetylcholine transporter, suggesting that they were sensory and not preganglionic parasympathetic fibers. Our retrograde labeling studies showed that approximately 30% of sphenopalatine ganglion neurons innervating the lacrimal gland, 23% innervating the nasal mucosa, and 39% innervating the trigeminal ganglion were in apparent contact with 5-HT(1D) receptor containing nerve terminals. CONCLUSION: These data suggest that 5-HT(1D) receptors within primary afferent neurons that innervate the sphenopalatine ganglion are in a position to modulate the excitability of postganglionic parasympathetic neurons that innervate the lacrimal gland and nasal mucosa, as well as the trigeminal ganglion. This has implications for triptan (5-HT(1D) receptor agonist) actions on parasympathetic symptoms in cluster headache.

Somatosensory innervation of healthy human oral tissues.

The oral somatosensory system relays essential information about mechanical stimuli to enable oral functions such as feeding and speech. The neurochemical and anatomical diversity of sensory neurons across oral cavity sites have not been systematically compared. To address this gap, we analyzed healthy human tongue and hard-palate innervation. Biopsies were collected from 12 volunteers and underwent fluorescent immunohistochemistry (≥2 specimens per marker/structure). Afferents were analyzed for markers of neurons (ßIII tubulin), myelinated afferents (neurofilament heavy, NFH), and Merkel cells and taste cells (keratin 20, K20). Hard-palate innervation included Meissner corpuscles, glomerular endings, Merkel cell-neurite complexes, and free nerve endings. The organization of these somatosensory endings is reminiscent of fingertips, suggesting that the hard palate is equipped with a rich repertoire of sensory neurons for pressure sensing and spatial localization of mechanical inputs, which are essential for speech production and feeding. Likewise, the tongue is innervated by afferents that impart it with exquisite acuity and detection of moving stimuli that support flavor construction and speech. Filiform papillae contained end bulbs of Krause, as well as endings that have not been previously reported, including subepithelial neuronal densities, and NFH+ neurons innervating basal epithelia. Fungiform papillae had Meissner corpuscles and densities of NFH+ intraepithelial neurons surrounding taste buds. The differing compositions of sensory endings within filiform and fungiform papillae suggest that these structures have distinct roles in mechanosensation. Collectively, this study has identified previously undescribed neuronal endings in human oral tissues and provides an anatomical framework for understanding oral mechanosensory functions.

Augmentation in proximity to the incisive foramen to allow placement of endosseous implants: a case series.

PURPOSE: To assess whether augmentation in the proximity of the incisive foramen with an intraoral bone graft to allow for reliable placement of implants is achievable, not jeopardizing the nasopalatine nerve and vessels in a way causing patients' distress. PATIENTS AND METHODS: Five patients who had lost a central maxillary incisor due to trauma, and in whom a deficiency of bone at the palatal side was present in the proximity of the incisal canal, were augmented with autogenous cancellous bone harvested from the retromolar region. After a healing period of 3 months, implants were inserted. Patients' acceptance, complications, and postoperative morbidity of the procedure were prospectively evaluated by standardized clinical and radiographic examinations up to 12 months after augmentation. RESULTS: At the time of implant surgery, in all cases there was sufficient bone for insertion of the implants with adequate primary stability. Up to now (follow-up of 12-15 months) no fixtures have been lost and all peri-implant tissues have a healthy appearance. All patients were satisfied. CONCLUSION: Augmentation in the proximity of the incisive foramen to enable implant placement appears to be feasible, both from the perspective of the patient and the professional.

Addition of dexmedetomidine to bupivacaine for greater palatine nerve block prolongs postoperative analgesia after cleft palate repair.

BACKGROUND AND OBJECTIVE: The effect of dexmedetomidine on the duration of sensory blockade has not been studied in humans. We evaluated the effect of adding dexmedetomidine to bupivacaine on the duration of postoperative analgesia in children who underwent repair of a cleft palate. METHODS: Thirty children who were scheduled for repair of a complete cleft palate using a combination of general anaesthesia and greater palatine nerve block were allocated randomly into one of two equal groups (n = 15). In both groups, the greater palatine nerve block was performed bilaterally using 0.5 ml of solution on each side. The B group received bupivacaine 0.25%, whereas the BD group received bupivacaine 0.25% with 1 microg kg(-1) dexmedetomidine. Heart rate, systolic blood pressure, pain score, the time to the first request for analgesia, and the degree of sedation were recorded. RESULTS: There was no difference in haemodynamic variables between the two groups. The pain score was significantly higher in the B group as compared with the BD group. The time to the first request for analgesia was significantly longer in children in the BD group (mean 22 h, range 20.6-23.7 h) as compared with those who received bupivacaine alone (14.2 h, 13-15 h). Sedation scores in the postoperative period did not differ between the study groups. CONCLUSION: Greater palatine nerve block with a combination of dexmedetomidine and bupivacaine increased the duration of analgesia after repair of a cleft palate by 50% with no clinically relevant side effects.

Perineural Invasion and Perineural Tumor Spread in Head and Neck Cancer.

Perineural invasion (PNI), the neoplastic invasion of nerves, is a common pathologic finding in head and neck cancer that is associated with poor clinical outcomes. PNI is a histologic finding of tumor cell infiltration and is distinct from perineural tumor spread (PNTS), which is macroscopic tumor involvement along a nerve extending from the primary tumor that is by definition more advanced, being radiologically or clinically apparent. Despite widespread acknowledgment of the prognostic significance of PNI and PNTS, the mechanisms underlying its pathogenesis remain largely unknown, and specific therapies targeting nerve invasion are lacking. The use of radiation therapy for PNI and PNTS can improve local control and reduce devastating failures at the skull base. However, the optimal volumes to be delineated with respect to targeting cranial nerve pathways are not well defined, and radiation can carry risks of major toxicity secondary to the location of adjacent critical structures. Here we examine the pathogenesis of these phenomena, analyze the role of radiation in PNI and PNTS, and propose guidelines for radiation treatment design based on the best available evidence and the authors' collective experience to advance understanding and therapy of this ominous cancer phenotype.

The anatomy of a horizontally impacted maxillary wisdom tooth.

A completely horizontally impacted upper third molar was revealed after routine dissection of a 62-year-old human cadaver of a Caucasian male. The molar was penetrating into the maxillary sinus and there was antral dehiscence of its bony alveolus. The bony alveolus was immediately in front of the greater palatine canal contents, and the bottom of the alveolus was dehiscent towards the greater palatine foramen. Within the greater palatine canal and foramen the greater palatine artery was duplicated and the nerve was found. Such antral relations of an impacted upper third molar predispose to oroantral communications if extraction is performed, while the close neurovascular relations represent a risk factor for postextractional haemorrhage and neurosensory disturbances and must be borne in mind when deciding on or performing the extraction.

Localization of palatal area in human somatosensory cortex.

To determine the 'hard palate representing' area in the primary somatosensory cortex, we recorded somatosensory-evoked magnetic fields from the cortex in ten healthy volunteers, using magnetoencephalography. Following electrical stimulation of 3 sites on the hard palate (the first and third transverse palatine ridges, and the greater palatine foramen), magnetic responses showed peak latencies of 15, 65, and 125 ms. Equivalent current dipoles for early magnetic responses were found along the posterior wall of the inferior part of the central sulcus. These dipoles were localized anterior-inferiorly, compared with locations for the hand area in the cortex. However, there were no significant differences in three-dimensional locations among the 3 selected regions for hard palate stimulation. These results demonstrated the precise location of palatal representation in the primary somatosensory cortex, the actual area being small.

Anatomical study of the pterygopalatine fossa using an endoscopic endonasal approach: spatial relations and distances between surgical landmarks.

OBJECT: The pterygopalatine fossa is an area that lies deep within the skull base. The recent extensive use of the endoscopic endonasal approach has provided neurosurgeons with a method to reach various areas of the skull base through a less invasive approach than traditional transcranial or transfacial approaches. This study aims to provide neurosurgeons with new data concerning direct endoscopic measurements and precise anatomical topography features of the pterygopalatine fossa. METHODS: An anatomical dissection of six fixed cadaver heads (12 pterygopalatine fossae) was performed to analyze spatial relationships and distances between the most important neurovascular structures in this region, and to estimate the size of the endoscopic surgical field for operations in this area. The endoscopic endonasal approach offers direct access to the pterygopalatine fossa through its anteromedial walls. CONCLUSIONS: Using an endoscopic endonasal approach makes it possible to identify all of the anatomical landmarks of the pterygopalatine fossa and almost all of the contiguous skull base areas.

Treatment of a subtype of trigeminal neuralgia with descending palatine neurotomy in the pterygopalatine fossa via the greater palatine foramen-pterygopalatine canal approach.

PURPOSE: To discuss an effective surgical treatment of a subtype of trigeminal neuralgia with descending palatine neuralgia of the maxillary division. METHOD: Nine patients, who suffered from trigeminal neuralgia with descending palatine neuralgia of the maxillary division, received neurotomy and avulsion of the descending palatine nerve in the pterygopalatine fossa via the greater palatine foramen-pterygopalatine canal approach. Seven of the patients had a recurrence of descending palatine neuralgia after they received treatment of maxillary neuralgia with neurotomy and avulsion of the infraorbital nerve; two patients were diagnosed with descending palatine neuralgia of the maxillary division in our department. Postoperative follow-up was conducted. RESULTS: Pain in the palate disappeared; all patients felt numb and paresthetic in the area innervated by the trigeminal nerve, with no pain. During the 3-36 months of follow-up, no recurrence occurred. CONCLUSIONS: Descending palatine neurotomy in the pterygopalatine fossa via the greater palatine foramen-pterygopalatine canal approach is a simple, safe and effective way to treat a subtype of trigeminal neuralgia--descending palatine neuralgia.

MRI study of pharyngeal airway changes during stimulation of the hypoglossal nerve branches in rats.

The medial branch (Med) of the hypoglossal nerve innervates the tongue protrudor muscles, whereas the lateral branch (Lat) innervates tongue retractor muscles. Our previous finding that pharyngeal airflow increased during either selective Med stimulation or whole hypoglossal nerve (WHL) stimulation (coactivation of protrudor and retractor muscles) led us to examine how WHL, Med, or Lat stimulation affected tongue movements and nasopharyngeal (NP) and oropharyngeal (OP) airway volume. Electrical stimulation of either WHL, Med, or Lat nerves was performed in anesthetized, tracheotomized rats while magnetic resonance images of the NP and OP were acquired (slice thickness 0.5 mm, in-plane resolution 0.25 mm). NP and OP volume was greater during WHL and Med stimulation vs. no stimulation (P < 0.05). Ventral tongue depression (measured in the midsagittal images) and OP volume were greater during Med stimulation than during WHL stimulation (P < 0.05). Lat stimulation did not alter NP volume (P = 0.39). Our finding that either WHL or Med stimulation dilates the NP and OP airways sheds new light on the control of pharyngeal airway caliber by extrinsic tongue muscles and may lead to new treatments for patients with obstructive sleep apnea.

Interactive web-based programs to teach functional anatomy: the pterygopalatine fossa.

Certain areas of the body contain structures that are difficult to envision in their proper spatial orientations and whose functions are complex and difficult to grasp. This is especially true in the head, where many structures are relatively small and inaccessible. To address this problem, we are designing Web-based programs that consist of high-resolution interactive bitmap illustrations, prepared using Adobe Photoshop, and vector-based animations, prepared via Macromedia Flash. Flash action script language is used for the animations. We have used this approach to prepare a program on the pterygopalatine fossa, an important neurovascular junction in the deep face that is especially difficult to approach by dissection and to depict in static images in an atlas. The program can be viewed online at http://cds.osr.columbia.edu/anatomy/ppfossa/. A table of contents simplifies navigation through the program and a menu enables the user to identify each of the vascular and neuronal components and either to insert or to remove each from its position in the fossa. The functional anatomy of the nerves in the fossa is animated. For example, users can activate and subsequently follow action potentials as they course along axons to their targets. This high degree of interactivity helps promote learning.