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.