7- Neurophysiology (Special Senses).docx

- **Olfactory system** - The olfactory system is essential for: localizing food/prey, danger detection, and reflex-stimulated secretion of digestive enzymes. - The olfactory system consists of the olfactory bulb, olfactory tract, lateral olfactory gyrus, and t...

- **Olfactory system** - The olfactory system is essential for: localizing food/prey, danger detection, and reflex-stimulated secretion of digestive enzymes. - The olfactory 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. - The olfactory mucosa of dogs have 220million 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). - They are located in the olfactory epithelium interspersed among **supporting cells**, which have cilia projecting into the mucus that coats the inner surface of the nasal cavity. - The (olfactory) cilia reacts to odors (olfactory chemical stimuli) in the air and stimulate the olfactory cells. - **Olfactory glands** are spaced among the olfactory cells, and they function to secrete mucus onto the surface of the nasal cavity. - Taste and smell are strongly tied to primitive emotional and behavioral functions of the nervous system. - There are connections between the olfactory and gustatory system, and the regions responsible for emotion and memory. - **Olfactory Cell Stimulation: Steps** - Step 1: Odorant molecules that enter the nasal cavity diffuses into the mucus. - Step 2: The odorant molecules bind to the [GPCR] in the membrane of each cilium. - Step 3: The alfa subunit activates **adenylyl cyclase**, which produces cAMP. - Step 4: **cAMP** activates the A-gated sodium-calcium channel, allowing for the influx of cations (which are mostly calcium). - Step 5: The influx of calcium activates the chloride channels to open, and chloride leaves the cell. - Step 6: Chloride leaving the cell results in membrane [depolarization], which generates an [EPSP] (excitatory post synaptic potential) in the cilia. - Step 7: 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) - Non-olfactory portions of the brain (part of the limbic system) - The following areas [send olfactory signals to the hippocampus and frontal cortex]: Amygdala, entorhinal cortex, hippocampal formation, and septal nuclei. - Olfactory signals do NOT directly project to the thalamus. - The **limbic system** is responsible for forming olfactory memories. - Olfaction can evoke emotional and autonomic responses. - **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. - **Fungiform papillae** can be found throughout the dorsal surface od the rostral 2/3 of the tongue, especially on the later margins and the tip. - **Vallate papillae** occupy the caudal portion of the dorsal tongue. - **Foliate papillae** are found on the dorsolateral part of the caudal part 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. - This mechanism depends on the taste molecule that binds to their specific receptors. - **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. - This map tells where sounds of different frequencies are processed in the brain. - Information is transmitted from the hair cells along the cochlear nerve in an [ascending pathway]. - Step 1: The cochlear nerve relays auditory impulses to the cochlear nuclei in the **medulla oblongata**. - Step 2: Axons ascend the brainstem (making several synapse) and reach the **thalamus**. - Step 3: The information is processed in the **auditor cerebral cortex**. - **External Ear** - The external ear [function] to direct 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. - Ossicles also decrease the amplitude of sound waves, protecting the sensitive sensory cells. - **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 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. - The base of the hair cells sits on the basilar membrane, and the cilia are embedded in the tectorial membrane. - The **tectorial membrane** overlies the sensory cells and acts as an anchored gel-coated ridge in the organ of corti. - The **basilar membrane** makes up the floor of the organ of corti. - The basilar membrane is very elastic and is very important for sound transmission. - The basilar membrane acts as a sound [frequency analyzer]. - **Transduction of Sound Waves to Action Potentials** - Step 1: Sound waves are transmitted to the inner ear and cause vibrations to the organ of corti. - Step 2: Vibrations of the organ of corti causes bending of cilia on the hair cells by a shear force, as the cilia push against the tectorial membrane. - Step 3: The bending of the cilia in ONE direction produces a change in the potassium conductance of the hair cell membrane, resulting in **depolarization**. - Step 4: Depolarization triggers potassium influx from endolymph, resulting in high potassium concentrations. - Step 5: They high potassium concentrations leads to calcium channels opening and releasing the Glutamate neurotransmitter. - The **Glutamate** neurotransmitter functions as an excitatory neurotransmitter, causing action potentials in the cochlear afferent nerve fibers. - Step 6: Bending cilia in the OTHER direction will cause **hyperpolarization**. - Step 7: Hyperpolarization will stop the potassium influx, and stop the release of glutamate, no longer forming the action potential.