Olfactiory and Gustatory Systems
Olfaction
Olfactory Receptors
- located in a 5 cm2 area of olfactory epithelium.
- well protected by location deep in the nose.
- permanent loss of olfaction can occur as a result of accidental whiplash if the cribriform plate is fractured or as a result of hemorrhaging at the base of frontal lobes that may tear the olfactory filaments.
Olfactory Mucosa
- several layered epithelium consisting primarily of olfactory receptors supporting cells, and basal cells.
- basal cells can divided and form new olfactory receptors including their axons which reinnervate the neurons in the olfactory bulb.
- about 100 million olfactory receptors in the olfactory mucosa.
- each olfactory receptor is a bipolar neuron with cilia extending from its apex into the mucus covering the olfactory mucosa.
- basal end of the olfactory receptor constricts to form a small diameter (0.2m ) unmyelinated axon.
- hundreds of the axons are bundled together by a Schwann sheath.
- primary olfactory nerve bundle, which contains the axons of all the receptors, passes through the thin and bony cribriform plate before entering the olfactory bulb.
- olfactory discriminations do not appear to be based upon morphologically distinct types of olfactory receptors.
- olfactory cells exhibit characteristic response spectra … each cell is excited to different levels by many odor substances, but relative sensitivity to different effective odor substances varies from one cell to another.
- each odor substance elicits a particular pattern of neural activity involving many sensory cells.
Olfactory Pathways
- activation of olfactory receptors involves mechanisms similar to other biological transducers
- Transducers … specialized cells that transduce one type of energy (chemical…neural activity).
- absorption of an odorant molecule by a cilium… change in membrane conductance … generator current
- ionic current results in a graded change in potential across the membrane called a generator potential.
- generator potential is graded as a function of stimulus intensity… modulates the frequency of action potential discharges in the olfactory receptor cell axon.
- olfactory receptor CAN support the generation of action potentials… the graded potential produced by odorants is called a generator potential.
- other sensory systems such as taste… the receptor cell synaptically modulates impulse discharge (action potential frequency) in a second-order neuron.
- graded potential in the taste or gustatory system is referred to as a receptor potential.
- Second-order neurons may also be referred to as primary afferents.
- individual generator currents summate resulting in a difference in potential that can be recorded across the olfactory epithelium in response to an odor.
- This type of extracellular potential (recorded from the surface of the olfactory epithelium) is called an electro-olfactogram (EOG).
- amplitude of the EOG…dependent upon the concentration of the odor.
- transduction process takes place in the area of the cilia and olfactory vesicle or knob.
Neural Coding
- olfactory bulb has a distinct laminar organization.
- axons of olfactory receptors enter the olfactory bulb… converge to form glomeruli.
- Each glomerulus contains the terminal endings of about 25,000 receptor axons which make synaptic contact with about 25 mitral cells.
- mitral cell is a second-order neuron in the olfactory system and main output neuron of the olfactory bulb.
- mitral cell axons proceed to the depths of the bulb and then project posteriorly to emerge together to form the lateral olfactory tract.
- Surrounding the glomeruli are cell bodies of intrinsic inhibitory interneurons called periglomerular cells.
- odorant excites the olfactory receptors… synapse on the mitral cell dendrites… producing an excitatory postsynaptic potential (EPSP).
- periglomerular cell forms a reciprocal synapse with the mitral cell dendrites.
- periglomerular cells are INHIBITORY… inhibit mitral cells in surrounding glomeruli… surround or lateral inhibition.
- Primary olfactory cortex consists of the lateral olfactory gyrus and part of the rostral olfactory region of the uncus called piriform cortex.
- rest of the olfactory area of the uncus is called periamygdaloid cortex.
- some olfactory fibers project directly to the corticomedial part of the amygdala.
- olfactory association cortex is in the anterior part of the parahippocampal gyrus.
- this cortical area also receives some direct projections from the olfactory tract.
GUSTATION
Morphology and Innervation of Taste Receptors
- gustation … chemical sense.
- taste buds
- receptor organs for taste.
- widely distributed throughout the oral cavity… most concentrated on the tongue
- associated with specialized papillae: folds in the skin of the tongue.
- three basic types of papillae: 1) fungiform papillae
- mushroomlike
- located on the anterior two thirds of the dorsal tongue surface
- contains one to five taste buds that are innervated by the chorda tympani
- readily exposed they respond quickly to substances taken into the mouth
- respond well to sweet and salty substances
2) foliate papillae
- each side of the tongue near its posterior region
- innervated by the glossopharyngeal nerve
3) circumvallate papillae.
- on the posterior part of the tongue
- contain taste buds trenches that surround each papilla
- innervated by the glossopharyngeal nerves
- substances must diffuse into the trenches before stimulating the taste buds
- very sensitive to bitter stimuli
- few vagal fibers innervate taste buds in the epiglottis.
- chorda tympani also innervates buds in soft palate
- each taste bud is a collection of 40 to 50 cells.
- Microvilli extend from the apices of the taste cells and are in direct contact with the saliva covering the tongue.
- thought that the taste molecules interact with the microvilli.
- taste nerve fiber divides many times before approaching the base of the taste bud.
- single fiber of the chorda tympani nerve bundle may innervate several taste buds & many cells within each taste bud… electrical activity recorded from one fiber represents the activity from many taste cells.
- Individual cells within the taste bud … continually replaced by daughter cells of dividing epithelial cells surrounding the taste bud (basal cells).
- cells enter the taste bud…differentiate…function as normal taste cells for about 8 days… degenerate and are replaced.
- integrity of the taste bud is maintained even though the individual cells have limited lives
- dynamics require that the taste nerve endings within the taste bud continuously move and innervate new taste cells.
Gustatory Pathways
- fibers from the vallate and foliate papillae run predominantly in the glossopharyngeal nerve
- those from fungiform in the anterior two thirds of the tongue enter the chorda tympani nerve.
- cell bodies of cells with axons in the chorda tympani nerve are located in the geniculate ganglion of the facial nerve and synapse in the medulla in the ipsilateral rostral portion of the nucleus tractus solitarius.
- cell bodies of cells with axons in the glossopharyngeal nerve are in the petrosal ganglion and terminate on different neurons in the rostral nucleus tractus solitarius.
- cells in the nucleus of the solitary tract project via the central tegmental tract to the medial part of the VPM.
- Neurons of the nucleus of the solitary tract ALSO project to a nucleus in the rostral pons near the superior cerebellar peduncle (parabrachial nuclei).
- cells in this area of the pons project to the lateral hypothalamus and the central nucleus of the amygdala.
- VPM projections are to gustatory cortex…either paritetal operculum in the post-central gyrus near the representation of the tongue, OR to opercular-insular cortex overlying the claustrum.
Taste Qualities
- in contrast to the olfactory system… four quite distinct basic taste sensations: sweet, sour, salty, and bitter.
Neural Coding
- area postrema… located rostral to the obex on each side of the fourth ventricle.
- major afferent input is from the nucleus of the solitary tract.
- one of several specialized regions that lie outside the blood-brain barrier
- referred to as circumventricular organs.
- stimulation of an emetic chemotaxic center in the area postrema triggers vomiting.
Tags: basal cells, chorda tympani, cribriform plate, electro-olfactogram, fungiform papillae, glomerulus, olfactory receptor, Olfactory Receptors, parahippocampal gyrus, periglomerular, Transducers
