Lecture Notes on Special Senses

Learning Objectives

• Understand the external and internal anatomy of the eye
• Describe pupillary control and lens accommodation
• Explain how visual information is conveyed (i.e., pathway)
• Understand the types of lingual papillae
• Know which cranial nerves are responsible for conveying taste from each region of the tongue
• Understand the external and internal anatomy of the ear, cochlea, and vestibular apparatus
• Describe the auditory and vestibular pathways

The Eye

Anatomy of the Eye

• The eye consists of three concentric layers:
  • Outer Eye:
  • Sclera: Covers the posterior 5/6 of the eye; it is white and vascular.
  • Cornea: Anterior 1/6th of the eye; it is transparent and avascular.
  • Middle Eye (Uvea):
  • Choroid: Pigmented, highly vascular layer.
  • Ciliary Body: Attaches to the lens, controls lens shape, and secretes aqueous humor.
  • Iris: Contains the pupil and muscles that control pupil size.
  • Inner Eye:
  • Retina: Contains photoreceptors (rods and cones).
• Pupillary Control:
  • Sphincter Pupillae:
  • Circular muscles
  • Constricts pupil in bright light (Parasympathetic control)
  • Dilator Pupillae:
  • Radial muscles
  • Dilates pupil in dim light (Sympathetic control)

Visual Pathway

• Visual Pathway Components:
  • Nasal Retina: Detects temporal visual field
  • Temporal Retina: Detects nasal visual field
  • Pathway:
  • Optic nerve → optic chiasm → optic tract → lateral geniculate nucleus (LGN) → optic radiations → calcarine fissure (primary visual cortex)
• Visual Field Deficits:
  • Monocular blindness
  • Bitemporal hemianopsia
  • Homonymous hemianopsia

Taste

Tongue Anatomy

• Regions:
  • Anterior 2/3: Apex & body, mobile
  • Posterior 1/3: Root, non-mobile
• Lingual Papillae:
  • Filiform: No taste buds, mechanical role
  • Fungiform: Contains taste buds, located on the anterior part of the tongue
  • Circumvallate: Largest, contains taste buds, surrounded by deep invaginations
  • Foliate: Parallel ridges, isolated to lateral edges
• Cranial Nerve Innervation:
  • Anterior 2/3: General sensation through CN V3, taste by CN VII
  • Posterior 1/3: Both sensations by CN IX
  • Epiglottis: Both sensations by CN X

The Ear

Anatomy of the Ear

• Divided into three parts:
  • External Ear:
  • Includes the auricle (pinna) and external auditory meatus
  • Tympanic Membrane: Separates external ear from middle ear
  • Middle Ear:
  • Contains ossicles: Malleus, Incus, Stapes
  • Eustachian Tube: Equalizes pressure
  • Tympanic Cavity: Contains auditory ossicles
  • Inner Ear:
  • Cochlea: Involved in hearing
  • Vestibular Apparatus: Involved in balance

Auditory Pathway

• Pathway through ear to the brain:
  • Cochlear nerve (CN VIII) → cochlear nuclei → superior olivary complex → inferior colliculus → medial geniculate body → auditory cortex

Vestibular Function

• Responsible for balance and spatial orientation:
  • Semicircular Canals: Detect rotational motion
  • Utricle and Saccule: Detect linear acceleration

Vestibular Pathway

• Signals transmitted from vestibular apparatus to:
  • Nuclei in the brainstem (nerves III, IV, VI) control eye movements
  • Cerebellum integrates balance information