Neuroscience Special Senses

Special Senses

• Olfaction, Gustation, Vision, Audition, Equilibrium

Olfaction: Olfactory Nerve (CN1)

Olfactory receptors

• Cilia (hair cells) of nostrils and nasal membranes

 Olfactory pathway

• Nasal membrane

• Olfactory bulb and tract

• Olfactory cortex of temporal lobe (part of the limbic system)

• Takes information from hippocampus and projects it to hypothalamus

• Goes from hypothalamus to thalamus to orbitofrontal cortex for conscious association of odor with previous memories

Therapeutic significance of olfactory stimulation 

• Olfaction can be used with comatose patients to increase CNS arousal 

• Aromatherapy can have moos enhancement properties 

• Bilateral lesion will lead to Anosmia (loss of sense of smell)

• Lesion to olfactory cortex may lead to seizure activity of pre-seizure aura scent hallucinations 

Gustation: 

• Facial Nerve (CN7), Glossopharyngeal (CN9), Vagus Nerve (CN10)

• Receptors = taste buds on the surface of the tongue

• Papillae: bumps on the tongue (contains taste receptors)

Physiology 

• Saliva dissolves food in mouth

• Dissolved food enters pores of papillae and taste buds

• A membrane potential is activated

• If membrane potential is strong enough, causes action potential

Gustatory Pathway

Taste receptors → CN 7,9,10 → Medulla → Thalamus → Gustatory cortex (in insula)

• Sense of smell is critical to taste (loss of smell will lead to loss of taste)

Therapeutic Significance

• Gustatory stimulation used for comatose patients

• Can facilitate oral motor function in children and adults with oral musculature dysfunction

Vision: Optic Nerve (CN 2)

Visual Receptor Pathway

1. Light Waves

 Fovea or periphery →  Action Potential

2. Visual Signals

Optic Nerve → Optic Chiasm → Optic Tracts → Superior colliculi of midbrain or thalamus

3. Occipital lobe

• Detection: Primary visual cortex

• Interpretation: Visual association cortex

Visual fields pathways

 Nasal field - lateral optic nerve

 Temporal field - medial optic nerve (cross at optic chiasm)

Visual Pathway Pathology

Contralateral homonymous hemianopia: Loss of either right 

or left visual field

Bitemporal Hemianopia: lose both temporal fields of vision

Optic Nerve Lesion: lose whole visual field on that side

Complete severance at optic chiasm: Fully blind

Visual field blind spot: damage to the retina

Nystagmus: Involuntary, rapid, repetitive eye movement

• Normal: Response to rotation, tracking a moving target, caloric testing

• Pathological: Damage to labyrinths, CN8 damage, cerebellar damage

Audition: Vestibulocochlear nerve (CN8)

• Sensory receptors = Hair cells

• Stimulus = sound (vibrations) mechanical input

• Vibrates tympanic membrane (ear drum)

• Hammer, Anvil, Stirrup (inner ear bones)

Hearing Impairment

Sensorineural: Damage to inner ear, CN8, or brain

Conductive: Damage to outer/inner ear structure

Lesion to auditory portion of CN8:  Deafness or tinnitus

Lesion to primary auditory cortex: Cortical deafness

Lesion to auditory association cortex: Auditory agnosia or inability to interpret sounds

The Vestibular System

Function: Equilibrium and Balance

Input= Vestibulocochlear nerve

• Relates to cerebellum (balance)

• Vestibular apparatus in the middle ear

Vestibular pathway

Sensory input → Vestibulospinal tract → motor neurons synapse with spinal nerves → Antigravity muscles → feedback loop to cerebellum

Reticular Formation–Brainstem 

Autonomic nervous system- Vagus nerve

Signs and Symptoms of Vestibular Dysfunction

• Nystagmus (involuntary, rapid, eye movement)

• Vertigo

• Tinnitus

• Hearing loss

• Loss of balance and possible falls

• Broad-based stance

• Sweating, nausea and vomiting