Basics of the Nervous System

The Nervous System

• Two main divisions:

  • Central Nervous System (CNS)

  • Peripheral Nervous System (PNS)

    • Contains 12 paired nerves

    • All connected to the brain at its base and pass through the skull via fissures or foramina

    • Responsible for communication between the brain and the body.

Peripheral Nervous System Divisions

• Divided into two divisions:

  • Afferent Nervous System (Sensory Nervous System)

    • Carries information from receptors to the brain or spinal cord

    • Approaches the brain, returning messages (e.g., touching something hot and feeling pain)

  • Efferent Nervous System (Motor Nervous System)

    • Carries information from the brain or spinal cord to muscles or glands

    • Exits the brain to send messages (e.g., picking up a pen)

    • Autonomic Nervous System

      • Considered part of the Efferent system, divided into:

        • Sympathetic Nervous System

          • Involved in "fight-or-flight" responses (e.g., shutdown of salivary gland secretion)

        • Parasympathetic Nervous System

          • Involved in "rest-or-digest" responses (e.g., stimulation of salivary gland secretions)

Cranial Nerves Overview

• CN I: Olfactory Nerve

  • Afferent nerve (Sensory) responsible for smell

  • Transmits information from smell receptors in the nose to the brain

  • Enters through nasal mucosa, exits via cribriform plate

• CN II: Optic Nerve

  • Afferent nerve (Sensory) responsible for sight

  • Enters through retina, exits via optic canal/foramen

• CN III: Oculomotor Nerve

  • Efferent nerve (Motor) responsible for eye movement

  • Enters into extrinsic muscles of the eye, exits via superior orbital fissure of sphenoid bone

• CN IV: Trochlear Nerve

  • Efferent nerve (Motor) responsible for eye movement

  • Enters into a muscle of the eye, exits via superior orbital fissure of sphenoid bone

• CN V: Trigeminal Nerve

  • Mixed nerve (Afferent and Efferent)

    • Efferent: Motor to muscles of mastication

    • Afferent: Sensory to facial/head skin, teeth, oral cavity, and general tongue sensations

• CN VI: Abducens Nerve

  • Efferent nerve (Motor) responsible for eye movement

  • Enters into muscle of the eye, exits via superior orbital fissure of sphenoid bone

  • Clinical note: First nerve affected by sinus infection due to its position

• CN VII: Facial Nerve

  • Mixed nerve (Afferent and Efferent)

    • Efferent: Motor to muscles for facial expression, lacrimal, and salivary glands

    • Afferent: Sensory for taste sensation in the anterior 2/3 of the tongue and skin of the ear

• CN VIII: Vestibulocochlear Nerve

  • Afferent nerve (Sensory)

  • Enters via internal auditory (acoustic) meatus of temporal bone, exits into the eardrum

  • Components:

    • Cochlear: Responsible for hearing

    • Vestibular: Responsible for equilibrium and balance

• CN IX: Glossopharyngeal Nerve

  • Mixed nerve (Afferent and Efferent)

    • Afferent: Sensations from oropharynx, skin around the ear, and posterior 1/3 of the tongue

    • Efferent: Motor for parotid gland secretion and pharyngeal muscles involved in swallowing

  • Exits via jugular foramen between occipital and temporal bones

• CN X: Vagus Nerve

  • Mixed nerve (Afferent and Efferent)

    • Afferent: Sensory for a small area around the ear and taste for the epiglottis

    • Efferent: Motor to muscles of the soft palate, involved in swallowing

  • Widest distribution of all cranial nerves, exits via jugular foramen

• CN XI: Accessory Nerve

  • Efferent nerve (Motor)

    • Contains cranial and spinal roots

      • Cranial: Exits via jugular foramen, innervates trapezius and sternocleidomastoid muscles

      • Spinal: Exits via foramen magnum

• CN XII: Hypoglossal Nerve

  • Efferent nerve (Motor)

    • Responsible for movements of intrinsic and extrinsic muscles of the tongue except palatoglossus

  • Exits via hypoglossal canal in occipital bone.