Cranial Nerves, Sensory Perception, and Spinal Nerves

Synapses and Neurons

  • Optic Chiasm: Neurons cross here before synapsing in the thalamus.

Thalamus

  • Functions as a reliever for all sensory perception except for smell (olfaction).

  • Think of it as a "switchboard" for sensory input to the cerebral cortex.

  • Visual Cortex: Located in the occipital lobe; responsible for processing visual information.

    • Injury Impact: A blow to the back of the head can lead to blindness because all visual processing occurs here.

Cranial Nerves Overview

  • Cranial Nerve II (Optic Nerve): Involved only in vision.

  • Eye Movement Muscles:

    • There are six muscles that move the eye. The Oculomotor Nerve (Cranial Nerve III) controls four of them.

    • Trochlear Nerve (Cranial Nerve IV): Moves the superior oblique muscle.

    • Abducens Nerve (Cranial Nerve VI): Moves the lateral rectus muscle.

Cranial Nerve V: Trigeminal Nerve

  • Contains three branches:

    • Ophthalmic (upper face),

    • Maxillary (mid face),

    • Mandibular (lower face).

  • Functions:

    • Sensory: General sensory perception for the face.

    • Somatic Motor: Controls muscles for chewing (temporalis, masseter, pterygoids).

Cranial Nerve VI: Abducens

  • Controls external eye muscle involved in lateral movement.

Cranial Nerve VII: Facial Nerve

  • Involved in:

    • Glandular secretions: Controls lacrimal and salivary glands.

    • Facial expressions: Innervates the muscles responsible for facial expressions.

  • Responsible for taste sensation in the anterior part of the tongue.

Cranial Nerve VIII: Vestibulocochlear Nerve

  • Two branches:

    • Vestibular branch: For balance (equilibrium).

    • Cochlear branch: For hearing.

  • Location: Passes through the internal acoustic meatus of the temporal bone.

Cranial Nerve IX: Glossopharyngeal Nerve

  • Involved in:

    • Salivary secretion from the parotid gland.

    • Sensation from the pharynx and posterior part of the tongue.

  • Monitors blood pressure through the carotid sinus in the carotid triangle (the area near the jaw and neck).

Cranial Nerve X: Vagus Nerve

  • Major parasympathetic innervation of the thoracic and abdominal organs.

  • Important for regulating heart rate, digestion, and respiration.

  • Considered the "wanderer" due to its extensive reach throughout the body.

Cranial Nerve XI: Accessory Nerve

  • Controls neck muscles, specifically the sternocleidomastoid and trapezius (muscles involved in neck and shoulder movements).

Cranial Nerve XII: Hypoglossal Nerve

  • Controls tongue movements important for speech and swallowing.

Spinal Nerves Overview

  • 31 pairs of spinal nerves arising from the spinal cord, with:

    • 8 Cervical, 12 Thoracic, 5 Lumbar, 5 Sacral, and 1 Coccygeal.

  • Sensory (Posterior) and Motor (Anterior) Rootlets.

  • Dorsal Ramus: Innervates back muscles and skin.

  • Ventral Ramus: Forms plexuses to innervate limbs.

Major Plexuses

  1. Cervical Plexus: Contains the phrenic nerve (important for diaphragm control).

  2. Brachial Plexus: Responsible for innervating the arm; includes major nerves like musculocutaneous, median, ulnar, radial, and axillary nerves.

  3. Lumbar Plexus: Contains obturator and femoral nerves, involving leg movement and sensation.

  4. Sacral Plexus: Contains the sciatic nerve (largest in the body), innervating the lower limb.

General Sensory vs. Special Senses

  • General Senses: Widely distributed throughout the body (e.g., touch, temperature).

  • Special Senses: Concentrated in the head (e.g., vision, hearing), with specialized receptor cells.

Taste (Gustation)

  • Supported by Gustatory Epithelium on the tongue.

  • Taste Buds found in different types of papillae: Fungiform, Foliate, and Vallate (only these contain taste receptors).

  • Key Flavor Categories:

    • Sweet (carbohydrates)

    • Sour (acidic substances)

    • Salty (sodium)

    • Bitter (potential toxins)

    • Umami (savory flavors due to amino acids and proteins).

Smell (Olfaction)

  • Olfactory receptors are located in the nasal cavity.

  • Like taste, smell relies on molecules diffusing into mucus and binding to sensory receptors.

  • Humans can detect around 10,000 different odors which make up the flavor experience when combined with taste.