taste and smell & hearing and balance

Name the structures involved in the transduction of olfactory and gustatory stimuli

Olfactory System: Olfactory epithelium, olfactory sensory neurons (OSNs), olfactory cilia, olfactory bulb, mitral and tufted cells, olfactory tract, olfactory cortex.

Gustatory System: Taste buds, gustatory receptor cells, cranial nerves (facial nerve CN VII, glossopharyngeal nerve CN IX, vagus nerve CN X), nucleus of the solitary tract (medulla), thalamus, gustatory cortex.

Explain the structure of the olfactory epithelium and the role of its constituent cell types

The olfactory epithelium, located in the superior nasal cavity, consists of:

1. Olfactory Sensory Neurons (OSNs) – Bipolar neurons with olfactory cilia that detect odor molecules and initiate signal transduction.

2. Supporting (Sustentacular) Cells – Provide structural and metabolic support, detoxify harmful substances, and regulate the extracellular environment.

3. Basal Cells – Stem cells that regenerate OSNs (which have a limited lifespan of ~30-60 days).

4. Bowman’s Glands – Produce mucus to dissolve odorants and maintain a moist environment.

Trace the route of olfactory signals from olfactory sensory neurons to the olfactory cortex

• Olfactory sensory neurons in the olfactory epithelium detect odorants and send signals via their axons.

• Axons pass through the cribriform plate of the ethmoid bone to synapse in the olfactory bulb with mitral and tufted cells.

• Mitral and tufted cells relay signals via the olfactory tract.

• The olfactory tract projects to:

  • Primary olfactory cortex (including the piriform cortex, amygdala, and entorhinal cortex) for odor processing.

  • Limbic system (hippocampus, amygdala) for emotional and memory associations.

  • Orbitofrontal cortex (via the thalamus) for conscious odor perception and integration with taste.

Trace the route of gustatory signals from gustatory sensory neurons to the gustatory cortex

• Gustatory receptor cells in taste buds detect taste stimuli.

• Signals are transmitted via three cranial nerves:

  • Facial nerve (CN VII) – anterior two-thirds of the tongue.

  • Glossopharyngeal nerve (CN IX) – posterior one-third of the tongue.

  • Vagus nerve (CN X) – epiglottis and pharynx.

• These nerves synapse in the nucleus of the solitary tract (NST) in the medulla.

• Signals are relayed to the thalamus (ventral posteromedial nucleus, VPM).

• The thalamus projects to the primary gustatory cortex (insula and frontal operculum) for conscious taste perception.

• Integration with the orbitofrontal cortex allows for multisensory perception of flavor.

Identify factors affecting taste perception and preference

• Genetic Variation – Differences in taste receptor genes (e.g., TAS2R38 for bitter sensitivity).

• Age – Taste sensitivity declines with age due to reduced receptor cell turnover.

• Hormonal Influence – Pregnancy, stress, and hormones like leptin and ghrelin can alter taste perception.

• Cultural and Dietary Habits – Exposure to certain flavors affects taste preferences.

• Adaptation and Desensitization – Repeated exposure can reduce sensitivity (e.g., spicy food tolerance).

• Health Conditions – Conditions like infections, chemotherapy, and neurological disorders (e.g., Parkinson’s, Alzheimer’s) can impair taste.

• Medications – Certain drugs (e.g., antibiotics, antihistamines, chemotherapy agents) can alter taste perception.

• Olfactory Function – Since flavor is a combination of taste and smell, olfactory impairments (e.g., congestion, anosmia) can reduce taste perception.

hearing and balance

To identify structures of the ear and their respective functions

1. Outer Ear

   • Auricle (Pinna) – Collects and directs sound waves into the ear canal.

   • External Auditory Canal – Channels sound waves toward the tympanic membrane.

   • Tympanic Membrane (Eardrum) – Vibrates in response to sound waves, transmitting mechanical energy to the middle ear.

2. Middle Ear

   • Ossicles (Malleus, Incus, Stapes) – Amplify and transmit vibrations from the tympanic membrane to the oval window.

   • Oval Window – Transfers vibrations from the stapes to the cochlea.

   • Eustachian Tube – Equalizes pressure between the middle ear and the external environment.

3. Inner Ear

   • Cochlea – Converts mechanical vibrations into neural signals for hearing.

   • Vestibular System (Semicircular Canals & Otolithic Organs) – Detects head movement and position for balance.

Describe the role of hair cells in the inner ear

• Cochlear Hair Cells

  • Located in the organ of Corti on the basilar membrane.

  • Have stereocilia, which bend in response to sound-induced movement of the basilar membrane.

  • This bending opens ion channels, leading to depolarization and neurotransmitter release onto auditory nerve fibers.

• Vestibular Hair Cells

  • Found in the semicircular canals, utricle, and saccule.

  • Detect head movement and gravitational forces.

  • Send signals to the brain via the vestibular nerve for balance control.

Explain the role of the cochlea in hearing

• The cochlea is a spiral-shaped organ that converts mechanical vibrations into electrical signals.

• Key Structures & Functions:

  1. Basilar Membrane – Vibrates at different frequencies along its length (high frequencies near the base, low frequencies near the apex).

  2. Organ of Corti – Houses hair cells that transduce mechanical energy into neural signals.

  3. Tectorial Membrane – Overlies hair cells, facilitating the bending of stereocilia.

  4. Auditory Nerve (Cochlear Nerve) – Carries signals from hair cells to the brain for sound perception.

Compare and contrast structures of the vestibulum (i.e., semi-circular canals vs. otolithic organs)

Describe the auditory pathway

1. Sound Waves Enter – Collected by the auricle and directed to the tympanic membrane.

2. Middle Ear Amplification – Vibrations are transmitted via ossicles (malleus, incus, stapes) to the oval window.

3. Cochlear Transduction – Fluid movement in the cochlea stimulates hair cells in the organ of Corti.

4. Neural Transmission – Hair cells release neurotransmitters onto the cochlear nerve (part of CN VIII - Vestibulocochlear Nerve).

5. Brainstem Processing – Signals are relayed to the cochlear nuclei (medulla) → superior olivary complex (pons) → inferior colliculus (midbrain).

6. Thalamic Relay – The medial geniculate nucleus (MGN) of the thalamus processes auditory information.

7. Cortical Perception – Signals reach the primary auditory cortex (temporal lobe), where sound is perceived and interpreted.

cranial nerves involved in taste

vagus nerve

glossopharyngeal nerve

facial nerve

• TRANSDUCTION Sensory cells convert stimuli into impulses for the nervous system through a process known as

• taste and smell are with CHEMORECEPTORS molecules in our food

• Olfactory Epithelium:

  • Contains olfactory sensory neurons and supporting cells; detects dissolved odor molecules

• Glomerulus:

  • Structure where olfactory axons connect with mitral cells, processing signals before reaching the brain.

• olfactory tract then olfactory cortex

• taste is 80 % smell

• all tastes register in all part of tongue