Neurophysiology: Special Senses

Olfactory System

• The olfactory system is essential for localizing food/prey, danger detection, and reflex-stimulated secretion of digestive enzymes.
• The system consists of the olfactory bulb, olfactory tract, lateral olfactory gyrus, and the piriform lobes.
• Olfactory cells are part of the specialized epithelium found in the ethmo-turbinate bones of the nasal cavity, which are receptor cells for smell sensation.
• Dogs have 220 million olfactory receptors, while humans have 5 million.
• The olfactory cells give rise to the olfactory nerve fibers that terminate at the olfactory bulb.
• These cell receptors are bipolar neurons derived originally from the CNS (central nervous system).

Olfactory Cell Stimulation

• Odorant molecules that enter the nasal cavity diffuse into the mucus.
• The odorant molecules bind to the GPCR in the membrane of each cilium.
• The alpha subunit activates adenylyl cyclase, which produces cAMP.
• cAMP activates the A-gated sodium-calcium channel, allowing for the influx of cations (which are mostly calcium).
• The influx of calcium activates the chloride channels to open, and chloride leaves the cell.
• Chloride leaving the cell results in membrane depolarization, which generates an EPSP (excitatory post synaptic potential) in the cilia.
• The EPSP travels from the cilia to the trigger zone of the olfactory cells to generate an action potential.

Central Pathway for Olfaction

• The olfactory nerve fibers terminate and synapse in the olfactory bulb to form the olfactory tract (cranial nerve 1).
• Tuft cells and Mitral cells are present in the olfactory bulb.
• Dendrites of these cells synapse with the terminal end of olfactory cells, forming the glomeruli of the olfactory bulb.
• The olfactory tract reaches the ipsilateral piriform lobe (olfactory area) and non-olfactory portions of the brain (part of the limbic system).
• The limbic system is responsible for forming olfactory memories.

Gustation System

• Taste buds have the function of taste.
• Dogs have about 1700 taste buds, while humans have 9000, and cats have 470.
• Dogs have a sense of taste for sweet, salt, sour, and bitter.
• In dogs, taste buds are located in various types of papillae, which are protrusions on the dorsal and lateral surface of the tongue.
• Taste buds are composed of groups of columnar taste receptor cells which are bundled together like bananas.
• Taste receptors are arranged such that their tips form a small taste pore, and microvilli extend through this pore.
• Each taste cell has receptors for only one type of flavor.
• Chemical molecules that trigger the sense of taste are dissolved by saliva.
• These molecules enter the taste buds through pores and bind to the receptors located in the membrane of the microvilli, and this binding results in the depolarization of the taste receptor cells' membrane.

Central Pathway for Gustation

• The taste cells are innervated by bipolar neurons that contribute axons to 2 cranial nerves (CN): facial (CN7), and glossopharyngeal (CN9).
• Afferent fibers send the message to the ipsilateral cerebral cortex (insular area), which also projects to the amygdala of the limbic system.

Auditory System

• The auditory system is designed to detect and analyze sounds in the environment, and much of animal communication relies on this system.
• Hearing requires at least 1 ear, but localization of sound requires 2 ears because the auditory system must detect the difference in time arrival or intensity of sounds in 2 ears.
• An animal's sense of hearing is enhanced by their ability to move their ears around, scanning the environment for different sounds, and localizing sounds.
• Hearing involves the external ear, middle ear, and inner ear, with the sensory receptors being located in the inner ear.
• The cochlear microphonic potential is a type of oscillatory pattern that has intermittent release of glutamate due to the intermittent firing of afferent nerves.
• The cochlear microphonic potential mirrors the waveform of the acoustic stimulus.
• Different auditory cells are activated by different frequencies.
• Hair cells located at the base of the basilar membrane respond best to high frequencies.
• Hair cells located at the apex respond best to low frequencies.
• The tonotopic map is the spatial mapping of frequencies present in the brain.
• Information is transmitted from the hair cells along the cochlear nerve in an ascending pathway.
• The cochlear nerve relays auditory impulses to the cochlear nuclei in the medulla oblongata.
• Axons ascend the brainstem (making several synapses) and reach the thalamus.
• The information is processed in the auditory cerebral cortex.

External Ear

• The external ear directs sound waves into the ear.
• The external ear is composed of a pinna (the fleshy part), and the ear canal, which is L-shaped.
• The tympanic membrane (eardrum) separates the external ear from the middle ear.

Middle Ear

• The middle ear is an air-filled cavity in the temporal bone that is connected to the nasopharynx by the auditory tube (eustachian tube).
• The eustachian tube drains the middle ear cavity.
• The middle ear contains 3 tiny bones that are connected together called "ossicles".
• The malleus (ossicle) is connected to the eardrum.
• The incus (ossicle) is located between the malleus and the stapes.
• The stapes (ossicle) is connected to the oval/round window, acting as a membrane separation between the middle and inner ear.
• The ossicles function to transfer vibrations of the eardrum to the oval window, avoiding significant loss of vibration as the sound wave is transferred from the air-filled outer ear to the fluid inner ear, where the sensory receptor is located.

Inner Ear (Labyrinth)

• The inner ear contains the sensory organs for the auditory system and the vestibular system.
• The vestibular system detects acceleration and static tilt of the head.
• The cochlea is the auditory portion of the inner ear, which is spiral shaped and filled with perilymph fluid.
• The cochlea contains the cochlear duct (which is filled with endolymph fluid), and the organ of corti (which has hair receptors).
• The organ of corti utilizes hair sensory cells that are mechanoreceptors.
• The organ of corti has 50-100 stereocilia in their apical surface which is connected by tip links at their tips.
• Tip links are attached to potassium channels and open when the bending of the stereocilia pulls the tip links apart.
• Stereocilia do not regenerate and can cause hearing loss.
• Hearing loss can happen when there are excessively loud sounds moving the stereocilia excessively, destroying them.