Peripheral Nervous System and Cranial Nerves

  • Central Nervous System Layers
    • Connective tissues surrounding the CNS serve several functions:
    • Mechanical protection
    • Nourishment to the nervous system via blood vessels
    • Circulation of cerebrospinal fluid (CSF)
    • The three layers from outer to inner:
    • Dura Mater: Tough outer layer, attached to skull bones. In some areas, forms partitions (dural reflections).
    • Arachnoid Mater: Middle layer, contains subarachnoid space filled with CSF.
    • Pia Mater: Inner layer directly covering the brain and spinal cord.
  • Subarachnoid Space
    • True space located between arachnoid and pia mater.
    • Contains CSF, essential for brain protection and nourishment.
    • CSF is produced by the choroid plexus located in the major ventricles of the brain.
  • Cerebrospinal Fluid (CSF) Production and Flow
    • CSF originates from blood plasma processed by the choroid plexus.
    • Production starts in lateral ventricles -> third ventricle -> fourth ventricle
    • Exits the fourth ventricle through:
    • Lateral apertures (pair)
    • Median aperture (single)
    • Flows into the subarachnoid space around CNS.
  • Dural Reflections and Sinuses
    • The dura mater creates partitions, including:
    • Falx Cerebri: Separates the left and right cerebral hemispheres.
    • Sinuses formed between the layers of the dura have irregular shapes:
    • Superior Sagittal Sinus: Runs anterior to posterior in the sagittal plane.
    • Inferior Sagittal Sinus: Slightly lower than the superior sinus.
  • Brain Ventricles Overview
    • Lateral Ventricles: First two ventricles, connected to the third through the interventricular foramen.
    • Third Ventricle: Located between the two halves of the thalamus.
    • Fourth Ventricle: Positioned below the cerebellum and contains CSF exit points.
  • Arachnoid Granulations
    • Extensions of the arachnoid that project into the superior sagittal sinus to drain CSF back into the blood.
    • Act as unidirectional valves, allowing waste material from CSF to exit into venous circulation.
  • Cranial Nerves Overview
    • There are 12 pairs of cranial nerves, categorized by Roman numerals or by specific names.
    • Cranial Nerve Naming:
    • Often numbered using Roman numerals I to XII.
    • Each name typically indicates function or area supplied (e.g., olfactory, optic).
  • Specific Cranial Nerves Highlights
    • Cranial Nerve I: Olfactory
    • Pure sensory nerve for smell.
    • Passes through the cribriform plate.
    • Cranial Nerve II: Optic
    • Pure sensory for vision.
    • Passes through optic canal, crosses at optic chiasm.
    • Cranial Nerve III: Oculomotor
    • Motor nerve controlling most eye movements and pupil constriction.
    • Passes through superior orbital fissure.
    • Cranial Nerve IV: Trochlear
    • Pure motor for one extraocular muscle.
    • Also passes through superior orbital fissure.
    • Cranial Nerve V: Trigeminal
    • Major sensory nerve with three branches:
      • V1 (Ophthalmic): Sensory to forehead and upper face.
      • V2 (Maxillary): Sensory to mid-face and nasal cavity.
      • V3 (Mandibular): Mixed nerve with sensory (lower face, anterior tongue) and motor fibers (muscles of mastication).
    • Branches exit through various openings (superior orbital fissure, foramen rotundum, foramen ovale).
    • Cranial Nerve VII: Facial
    • Mixed nerve for muscles of facial expression, taste (anterior 2/3 of tongue), and parasympathetic control of salivary and lacrimal glands.
    • Exits through internal auditory meatus.
    • Cranial Nerve VIII: Vestibulocochlear
    • Pure sensory for hearing and balance from the inner ear.
    • Cranial Nerve IX: Glossopharyngeal
    • Mixed nerve for taste (posterior 1/3 of tongue), sensory from the pharynx, and parasympathetic to the parotid gland.
    • Cranial Nerve X: Vagus
    • Mixed nerve extending beyond the head to thoracic and abdominal organs, controlling parasympathetic functions.
  • General Notes
    • CSF production is about 500 mL per day, but capacity of the system is roughly 50 mL, leading to continuous cycling of CSF every 8 hours.
    • Diseases such as meningitis can compromise CSF circulation and lead to serious complications.
    • Understanding cranial nerve pathways, functions, and targets is critical for diagnosing neural conditions.