USMLE STEP 1 Neurology The notochord induces what to differentiate into what? - Answer: Induces overlying ECTODERM to differentiate into NEUROECTODERM and form NUERAL PLATE Neural plate then gives rise to? - Answer: Neural tube and neural crest cells Notochord becomes what? - Answer: Nucleus pulposus of the intervertebral disks in adults Alar Plate - Answer: Dorsal [Sensory] Same orientation as spinal cord Basal Plate - Answer: Ventral [Motor] Same orientation as the spinal cord How does FGF affect chordin and noggin? - Answer: FGF regulates chordin and noggin to down regulate BMP which leads to neural plate induction Three Primary Vesicles - Answer: 1. Forebrain [Prosencephalon] 2. Midbrain [Mesencephalon] 3. Hindbrain [Rhombencphaln] USMLE STEP 1 Neurology The Forebrain gives rise to: - Answer: 1. Telencephalon [Cerebral hemispheres, lateral ventricles] 2. Diencephalon [Thalamus, Third Ventricle] The Midbrain gives rise to: - Answer: 1. Mesencephalon [Midbrain, aqueduct] The Hindbrain gives rise to: - Answer: 1. Metencephalon [Pons, cerebellum, upper part of the fourth ventricle] 2. Myelencephalon [Medulla, Lower part of the fourth ventricle] CNS/PNS Origins - Answer: Neuroectoderm = CNS neurons, ependymal cell [Inner lining of ventricles, makes CSF], oligodendrocytes, astrocytes Neural crest cells = PNS, schwann cells Mesoderm = Microglia [Like Macrophages, originate from Mesoderm] Neural tube defects - Answer: NEUROPORES fail to fuse [4th week] → Persistent connection between amniotic cavity and spinal canal Associated with: 1. Low folate levels before conception and during pregnancy 2. ↑ a-fetoprotein levels [AFP] in amniotic and maternal serum 3. ↑ AChE in amniotic fluid [Helpful confirmatory test] -- Fetal AChe in CSF transudates across defect into amniotic fluid Spina bifida occulta - Answer: Failure of bony spinal canal to close, NO STRUCTURAL HERNIATION USMLE STEP 1 Neurology Usually seen at lower vertebral levels DURA INTACT Associated w/ tuft of hair or skin dimple at level of bony defect NORMAL AFP Meningocele - Answer: Meninges [BUT NO NEURAL TISSUES] herniates through bony defect Meningomyelocele - Answer: Meninges and neural tissue herniate through bony defect Associated w/ Arnold-Chiari Type II Malformation Anencephaly - Answer: Malformation of the ANTERIOR NEURAL TUBE → 1. No forebrain 2. Open calvarium Clinical Findings: 1. ↑ a-fetoprotein levels [AFP] 2. Polyhydramnios [No swallowing center in brain] 3. Associated w/ MATERNAL TYPE I DIABETES 4. Maternal folate supplementation ↓ risk Holoprosencephaly - Answer: Failure of LEFT AND RIGHT HEMISPHERES TO SEPARATE Usually occurs during the 5th and 6th week May be related to mutations in the SONIC HEDGEHOG signaling pathway USMLE STEP 1 Neurology Moderate form = Cleft lip/palate Severe form = Cyclopia Seen in: 1. PATAU SYNDROME 2. FETAL ALCOHOL SYNDROME Chiari Type II Malformation - Answer: Posterior fossa malformation Significant HERNIATION of the CEREBELLAR TONSILS and VERMIS through FORAMEN MAGNUM with AQUEDUCTAL STENOSIS and HYDROCEPHALUS Patients often present w/: 1. Lumbosacral meningomyelocele 2. Paralysis below the defect Dandy-Walker Malformation - Answer: AGENESIS of CEREBELLAR VERMIS with CYSTIC ENLARGEMENT of the 4th ventricle Fills enlarged posterior fossa Associated w/: 1. Hydrocephalus 2. Spina bifida Syringomyelia - Answer: Cystic cavity [Syrinx] within SPINAL CORD If central canal = Hydromyelia Crossing anterior spinal commissural fibers are typically damaged CAPE-LIKE BILATERAL LOSS OF PAIN and TEMPERATURE in the UPPER EXTREMITIES Fine touch sensation is preserved