Neural Tube Defects Overview
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Questions and Answers

Which condition is characterized by the herniation of only meninges through a bony defect?

  • Meningocele (correct)
  • Myelomeningocele
  • Meningomyelocele
  • Myeloschisis

    • What is a major contributor to morbidity in patients with meningiomyelocele?

  • Incomplete neural tube closure
  • Maternal hypertension
  • Chiari II malformation (correct)
  • Increased intracranial pressure

    • Which condition results in exposed unfused neural tissue without any skin or meningeal covering?

  • Myelomeningocele
  • Holoprosencephaly
  • Anencephaly
  • Myeloschisis (correct)

    • What is typically elevated in both anencephaly and myeloschisis?

    <p>AFP and AChE</p> Signup and view all the answers

    Which environmental factor is associated with holoprosencephaly?

    <p>Fetal alcohol syndrome</p> Signup and view all the answers

    Which imaging finding is associated with holoprosencephaly?

    <p>Monoventricle appearance</p> Signup and view all the answers

    In which condition is the failure of the anterior neuropore closure most notably linked?

    <p>Anencephaly</p> Signup and view all the answers

    What typically characterizes the 'frog-like' appearance of a fetus?

    <p>Anencephaly</p> Signup and view all the answers

    What prevents axonal regeneration in the central nervous system?

    <p>Dense glial scarring and myelin debris</p> Signup and view all the answers

    What role do Schwann cells play in the peripheral nervous system following axonal injury?

    <p>They degrade myelin and promote macrophage recruitment.</p> Signup and view all the answers

    Which type of cells are responsible for the slower recruitment of phagocytic functions in the central nervous system after injury?

    <p>Macrophages/microglia</p> Signup and view all the answers

    What is a significant factor in the failure of axonal growth in the central nervous system post-injury?

    <p>Apoptosis of myelin-producing cells</p> Signup and view all the answers

    What initiates the formation of a growth cone in the peripheral nervous system?

    <p>Myelin debris clearance and Schwann cell activity</p> Signup and view all the answers

    What type of staining is observed in CNS tumors of neuronal origin?

    <p>Positive for synaptophysin</p> Signup and view all the answers

    Which of the following is true regarding the resting membrane potential of neurons?

    <p>It represents a difference in charges across the cell membrane at rest.</p> Signup and view all the answers

    Which cells contribute to glial scarring following central nervous system injury?

    <p>Astrocytes</p> Signup and view all the answers

    What type of fibers are C fibers characterized as?

    <p>Unmyelinated and slow</p> Signup and view all the answers

    Which type of sensory receptors is associated with slow, unmyelinated fibers?

    <p>Free nerve endings</p> Signup and view all the answers

    Which statement about C fibers is true regarding their susceptibility to local anesthetics?

    <p>They are more easily blocked by local anesthetics.</p> Signup and view all the answers

    Where are Meissner corpuscles primarily located?

    <p>Hairless skin</p> Signup and view all the answers

    Which of the following fiber types is primarily responsible for the sensation of pressure?

    <p>Merkel discs</p> Signup and view all the answers

    What anatomical feature surrounds a fascicle of nerve fibers?

    <p>Perineurium</p> Signup and view all the answers

    Which sensory receptor adapts quickly and responds to vibration and pressure?

    <p>Pacinian corpuscles</p> Signup and view all the answers

    Which type of sensory receptor adapts slowly and is involved in detecting the slippage of objects?

    <p>Ruffini corpuscles</p> Signup and view all the answers

    What neurotransmitter is primarily synthesized in the ventral tegmentum and substantia nigra?

    <p>Dopamine</p> Signup and view all the answers

    Which of the following drugs is classified as a GABA receptor anesthetic?

    <p>Etomidate</p> Signup and view all the answers

    What is the effect of norepinephrine levels in anxiety compared to depression?

    <p>Increased in anxiety, decreased in depression</p> Signup and view all the answers

    What is the key sign associated with Normal Pressure Hydrocephalus (NPH)?

    <p>Papilledema</p> Signup and view all the answers

    Which of the following best describes the pathophysiological mechanism of Normal Pressure Hydrocephalus?

    <p>Reduced reabsorption of CSF leading to gradual accumulation</p> Signup and view all the answers

    Which of the following is NOT a function associated with GABA?

    <p>Promotion of wakefulness</p> Signup and view all the answers

    What role does vitamin B6 play in GABA synthesis?

    <p>It acts as a cofactor for GABA transaminase and glutamate decarboxylase.</p> Signup and view all the answers

    What are the symptoms included in the classic triad of Normal Pressure Hydrocephalus?

    <p>Urinary incontinence, gait apraxia, cognitive dysfunction</p> Signup and view all the answers

    What neurological condition is associated with decreased levels of dopamine?

    <p>Parkinson's disease</p> Signup and view all the answers

    What imaging finding is considered the hallmark for Normal Pressure Hydrocephalus?

    <p>Total ventricular dilation</p> Signup and view all the answers

    Which location is identified as the main source of norepinephrine?

    <p>Locus ceruleus</p> Signup and view all the answers

    What is the most common form of treatment for Normal Pressure Hydrocephalus?

    <p>Ventriculoperitoneal shunt</p> Signup and view all the answers

    In Noncommunicating Hydrocephalus, what occurs as a result of structural blockage?

    <p>Ventricles cannot communicate with each other</p> Signup and view all the answers

    What happens to the membrane potential when GABA binds to its receptor?

    <p>It becomes more negative, leading to hyperpolarization.</p> Signup and view all the answers

    Which part of the nervous system is responsible for bladder contraction during micturition?

    <p>Sacral micturition center</p> Signup and view all the answers

    Which of the following statements regarding Normal Pressure Hydrocephalus is true?

    <p>Cognitive dysfunction may sometimes be reversible</p> Signup and view all the answers

    Study Notes

    Meningocele

    • Meninges herniate through a bony defect.
    • Neural tissue is not present in the herniation.

    Meningomyelocele

    • Meninges and neural tissue (e.g., cauda equina) herniate through a bony defect.
    • Almost always associated with Chiari II malformation.
    • Hydrocephalus is a major cause of morbidity due to obstruction of the 4th ventricular outflow caused by cerebellar herniation in Chiari II malformation.

    Myeloschisis

    • Also known as rachischisis
    • Exposed unfused neural tissue without skin or meningeal covering.
    • Elevated alpha-fetoprotein (AFP) and acetylcholinesterase (AChE) levels.

    Anencephaly

    • Failure of rostral (anterior or cranial) neuropore closure.
    • Results in absence of the forebrain and an open calvarium.
    • 'Frog-like' appearance of the fetus.
    • Elevated AFP and AChE levels.
    • Associated with maternal polyhydramnios due to impaired fetal swallowing of amniotic fluid.

    Holoprosencephaly

    • Failure of left and right hemispheres to separate during embryonic development.
    • Usually occurs between weeks 5-6 of gestation.

    Etiology

    • Genetic:
      • Mutations in sonic hedgehog signaling pathway.
      • Seen in trisomy 13.
    • Environmental:
      • Fetal alcohol syndrome

    Clinical Features

    • Wide range of phenotypic findings:
      • Mild: Closely set eyes (hypotelorism), cleft lip/palate
      • Severe: Single midline eye (cyclopia), primitive nasal structure (proboscis), midfacial clefts.

    Imaging

    • MRI shows a monoventricle and fusion of basal ganglia.

    Posterior Fossa Malformations

    Chiari I Malformation

    • Ectopia (displacement) of the cerebellar tonsils.
    • Axonal regeneration does not occur in the central nervous system.
    • Macrophages remove debris and myelin.
    • Tumors of neuronal origin frequently stain positively for synaptophysin on immunohistochemistry.
    • Synaptophysin is a protein found in the presynaptic vesicles of neurons, neuroendocrine, and neuroectodermal cells.

    Peripheral Nervous System Axonal Regeneration

    • Schwann cells sense axonal degeneration and begin to degrade their myelin.
    • Schwann cells secrete cytokines and chemokines to recruit macrophages.
    • Effective clearance of myelin debris by macrophages helps in nerve regeneration.
    • Trophic factor secretion by Schwann cells stimulates formation of a growth cone from the proximal axon, facilitating nerve regeneration.

    Central Nervous System Axonal Regeneration

    • Phagocytic macrophages/microglia are recruited more slowly due to the blood-brain barrier.
    • Myelin-producing oligodendrocytes become inactive or undergo apoptosis and do not assist with phagocytosis.
    • Slow removal of myelin debris suppresses axonal growth.
    • Astrocytes release inhibitory molecules and proliferate, forming a glial scar that acts as a barrier to axon regeneration.

    Neuron Action Potentials

    • Resting Membrane Potential:
      • Difference in charge across the cell membrane at rest.
      • Prevents free diffusion of ions.

    C Fibers

    • Unmyelinated fibers with a diameter of 0.5 to 2 microns.
    • Involved in the perception of warm sensations and pain.
    • Easier to block with local anesthetics compared to larger fibers.

    Sensory Receptors

    Receptor Type Sensory Neuron Fiber Type Location Senses
    Free Nerve Endings C fibers (slow, unmyelinated) All skin, epidermis, some viscera Pain, temperature
    Free Nerve Endings Aδ fibers (fast, myelinated) Pain, temperature
    Meissner Corpuscles Large, myelinated fibers Glabrous (hairless) skin Dynamic, fine/light touch, position sense
    Pacinian Corpuscles Large, myelinated fibers Deep skin layers, ligaments, joints Vibration, pressure
    Merkel Discs Large, myelinated fibers Fingertips, superficial skin Pressure, deep static touch, position sense
    Ruffini Corpuscles Dendritic endings with capsule Fingertips, joints Pressure, slippage of objects on skin, joint angle change

    Peripheral Nerve

    • Endoneurium:
      • Surrounds single nerve fiber layers.
      • Infiltrated with inflammatory cells in Guillain-Barré Syndrome.
    • Perineurium:
      • Surrounds a fascicle of nerve fibers.
      • Acts as a blood-nerve permeability barrier.
      • Must be rejoined in microsurgery for limb reattachment.
    • Most sensitive neurons: Hippocampus, Purkinje cells (Cerebellum), Neocortex, Striatum (Basal ganglia).

    Changes After Infarction

    • Neurons in the hippocampus, Purkinje cells in the cerebellum, neocortex, and striatum are the most sensitive to ischemic damage.

    Neurotransmitters

    Norepinephrine

    • Stress/panic hormone.
    • Increased levels during anxiety.
    • Decreased levels in depression.
    • Some antidepressants increase norepinephrine levels (e.g., SNRIs).
    • Main source is locus ceruleus (in the posterior pons near the 4th ventricle).

    Dopamine

    • Synthesized in:
      • Ventral tegmentum (midbrain)
      • Substantia nigra (midbrain)
    • Increased levels in schizophrenia.
    • Decreased levels in Parkinson's and depression.

    GABA

    • Largely inhibitory.
    • Synthesized in the nucleus accumbens.
    • Important for pleasure/reward.
    • Activated in:
      • Drug addiction
      • Fear
    • Decreased levels in anxiety and Huntington's disease.
    • GABA Receptor Anesthetics:
      • Etomidate, Propofol, Benzodiazepines, Barbiturates.
      • These drugs activate the GABA receptor causing sedation.
    • GABA Receptor:
      • GABA binds to the receptor, allowing Cl- into the cell which hyperpolarizes the membrane.
      • Hyperpolarization makes it harder for the neuron to become depolarized, decreasing neuronal excitation and causing inhibition.
    • GABA Synthesis:
      • Synthesized via glutamate decarboxylase in neurons.
      • Broken down by GABA transaminase.
      • Both enzymes require B6 as a cofactor.
      • B6 Deficiency can lead to seizures.

    Normal Pressure Hydrocephalus

    • Gradual decline in reabsorptive capacity of the arachnoid villi.
    • Slow accumulation of CSF.
    • Pressure remains normal due to gradual ventricular distention to accommodate the CSF increase.

    Hallmark of NPH

    • Enlarged ventricles on imaging.
    • Compression of the corona radiate leading to the classic triad:
      • Urinary incontinence (urge incontinence).
      • Gait apraxia (magnetic gait).
      • Cognitive dysfunction (sometimes reversible).

    Clinical Presentation

    • "Wet, wobbly, and wacky"

    • Headache

    • Papilledema

    • Dilation of all ventricles on CT scan.

    • Herniation may occur.

    • Normal opening pressure on lumbar puncture (LP):

      • CSF pressure is elevated only episodically.
      • Does not result in an increase in subarachnoid space volume.
      • Gradual ventricular distention.

    • Affects the elderly; idiopathic.

    • Impaired CSF absorption is the suspected mechanism.

    Treatment

    • Ventriculoperitoneal (VP) Shunt:
      • Drains CSF to the abdomen.

    Micturition Reflex

    • Regulated by three centers:
      • Sacral Micturition Center:
        • Located at S2-S4 level.
        • Responsible for bladder contraction.
        • Parasympathetic fibers from the sacral micturition center travel via pelvic nerves and stimulate cholinergic receptors in the bladder wall.
      • Pontine Micturition Center:
        • Located in the pontine reticular formation.
        • Coordinates relaxation of the external urethral sphincter with bladder contraction during voiding.
      • Cerebral Cortex:
        • Inhibits the sacral micturition center.

    Noncommunicating (Obstructive) Hydrocephalus

    • Ventricles cannot communicate with each other.

    Noncommunicating Hydrocephalus

    • Structural blockage of CSF flow within ventricles.

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    Description

    This quiz covers various neural tube defects including meningocele, meningomyelocele, myeloschisis, anencephaly, and holoprosencephaly. Each defect is described in terms of its characteristics, associated anomalies, and clinical implications. Test your knowledge about these critical conditions in neurology.

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