Chapter 15

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Olfactory:
Small patch of olfactory epithelium located on the superior nasal concha that contains three types of tissue:
Olfactory receptors
Supporting cells (sustentacular)
Basal cells
1- Olfactory receptors: bipolar neurons
Dendrites have olfactory hairs (cilium) hanging from the dendrites

2- Supporting cells: mucous membrane lining the nose containing columnar epithelium
3- Basal cells continually produce new olfactory receptors (neurons). They regenerate every month. Exception to the rule that mature neurons are not replaced.
Olfactory bulb and tract.
Taste and smell centers lie very close together in the brain (primary olfactory cortex)

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Physiology of olfaction.
Odors are made up of multiple odorant molecules and each odorant molecule will activate several olfactory receptors. This system can allow us to recognize and remember some 10,000 odors.
Odor molecule dissolves on the mucous membrane. The molecules will bind to Gproteins located on the plasma membrane of the receptor cells. This will activate the enzyme cAMP (adenylate cylase or cyclic AMP) and cause the opening of sodium channels, depolarization and nerve impulse.

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From the olfactory tracts impulse will travel to two main destinations.
1- thalamus to the pyriform (pear shaped) area
- uncus
- entorhinal area
- limen insula (junction between the cortex of the frontal and insula lobes)
- frontal lobe above the orbits where smell is interpreted and identified

2- Subcortical pathway to the amygdala, hypothalamus and other regions of the limbic system. Here is where smells are associated with danger and other emotional responses. i.e. smoke, natural gas, skunk. Sympathetic responses are initiated.

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Olfactory terms:
Anosmia: loss of smell
Hyposmia: decr. smell
Hyperosmia: incr. smell
Dysosmia: distorted smell

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Gustation:
Gustatory receptor cells are located in the taste buds
- papillae (elevations scattered on tongue, roof of mouth and walls of pharynx)
Primary taste sensations: sour, bitter, salty and sweet.
Recently discovered is umami. Sensed in response to a salt (monosodium glutamate) found in the amino acid glutamate.

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Sour, salty and sweet are carried via CN
VII (facial nerve) and bitter carried via CN
IX (glossopharyngeal).
All taste sensations are relayed to the ventral posterior (vp) medial nuclei of the thalamus from the solitary nucleus
(gustatory nucleus) then on to the primary gustatory cortical area of the inferior postcentral gyri of the parietal lobes.
Taste is 80% smell

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Gustatory terms:
Ageusia: loss of taste
Hypogeusia: decr. taste
Hypergeusia: incr. taste
Dysgeusia: distorted taste

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Hearing:
External, middle and inner ear
External ear:
Outer auricle or pinna will funnel sound towards the ear drum. Remember external auditory meatus (EAM) through the temporal bone
Sound waves are transmitted through the EAM until they reach the tympanic membrane (ear drum)
- semitransparent membrane covered by skin on the outside and mucous membranes on the inside.

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Middle ear: Petrous portion of temporal bone
The ear drum magnifies the vibrations of sound waves mechanically. One of the main purposes of the ear is to transform these air pressure waves to an electrical nerve signal that the brain can understand
The ossicles are
- hammer (malleus) attaches to drum
- short and long handle (processes)
-manubrium
- anvil (incus)
- stirrup (stapes). The stirrup attaches to the oval window of the inner ear or cochlea

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Inner ear: labyrinth
Vibration of the stapes on the oval window moves fluid (perilymph) within the inner ear. Vibrations will stimulate the hearing receptors.
Pressure on the oval window is 22x greater than the wave pressure exerted on the ear drum.

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Pressure waves are transmitted from the scala vestibuli to the scala tympani and then into the round window
Pressure waves deform walls of scala vestibuli and scala tympani, collectively causing the vestibular membrane to move (pressure in endolymph increases & decreases)
Pressure fluctuations in endolymph move the basilar membrane
Hairs of the organ of Corti, connected to the basilar membrane, move against the tectorial membrane and cause the generation of nerve impulses

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Decibels (dB) measures sound intensity.
0 dB is least perceived by the human ear
10 dB is 10x louder than 0 dB
20 dB is 100X as loud as 0 dB etc..
Whisper: 40 dB
Normal speech: 60-70 dB
Rock concert: 120 dB
Jet plane: 140 dB (nociception)
Prolonged periods of 90 dB will cause permanent hearing loss

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Hearing nerve fibers are located in the cochlear branch of the vestibulocochlear nerve (VIII).
Auditory pathway for hearing:
- cochlear branch enters b/w the medulla oblongata and the pons to end in the cochlear nuclei both dorsal and ventral.
- fibers cross and ascend into thalamus
(medial geniculate body) to terminate in the primary auditory cortex of the temporal lobe

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Equilibrium: feeling of the position of the head at rest and movement
Static (rest): organ is located in the vestibule between the semicircular canals and the cochlea. Specifically, the utricle and saccule.
Helps keep head still and balanced.
Dynamic (motion): organ located in the ampullae of the semicircular canals.
- anterior
- posterior
- lateral

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The semicircular canals contain fluid and are responsible for detecting changes in motion. The three semicircular canals (all at different angles), are responsible for detecting motion on a different plane.
Within the canals are nerve hairs which sense changes in the movement of the perilymph fluid and depolarize, thus sending a message to the brain. Ex: spinning in a circle- (remember the sit and spin toy)

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Vestibular portion of CN VIII enters b/w the medulla oblongata and the pons and ends in the vestibular nuclear complex.
The vestibular nuclei is located in the rostral m.o. and caudal pons.
Works closely with the cerebellum to signal appropriate skeletal muscles in response to changes in equilibrium.

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Vision:
Eyeball has three layers:
Outer: Fibrous tunic
Middle: Vascular tunic
Inner: Nervous tunic
Fibrous tunic:
Anterior portion contains the clear avascular cornea which helps focus light rays. Very pain sensitive
It is continuous with the white sclera that surrounds the eyeball and is the attachment site for the extrinsic ocular muscles.
Dura mater is continuous with the sclera at the site of the optic nerve that exits the back of the eyeball.

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Vascular tunic (uveal layer) includes the iris, ciliary body and choroid coat.
Choroid coat contains blood vessels that nourish the eye and is loosely joined to the sclera.
Ciliary body located in the anterior part of the eye has ciliary muscles that are attached to suspensory ligaments that hold the lens
(transparent structure located behind the iris) in place. When the muscles contract these ligaments are able to change the shape of the lens (ligaments relax causing lens to thicken) aiding in focusing close objects.
(accommodation- remember parasympathetic division: CN III)

iris:
- colored smooth muscles portion of eye
- thin diaphragm allowing light to enter.
- lies b/w the cornea and the lens
- separates the eyeball in an anterior
(b/w the iris and cornea) and posterior chamber (b/w the iris and vitreous humor) ant.: aqueous humor post.: vitreous humor

- Pupil
- hole in the iris
- pupil size is controlled by smooth muscles in the iris called circular and radial sets.
- circular set acts as sphincter to constrict pupil in the presence of bright light.
(parasympathetic)
- radial set increases diameter of pupil causes it to dilate. (sympathetic.)

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Nervous tunic:
The retina is a thin, delicate structure that contains specialized photoreceptor cells and is continuous with the optic nerve.
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Five types of neuron located in the retina
Receptor cells (rods and cones)
Bipolar neurons
Ganglion cells
Horizontal cells
Amacrine cells

In the center of the retina is yellowish spot called the macula lutea. A depression in the center called the fovea centralis is the region where the sharpest vision is produced.
Medial to the fovea is the optic disc. The optic disc is where the optic nerve exits the eyeball. No receptors are present for vision so it is also called the blind spot.

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Refraction of light rays (bending of light) by the anterior and posterior surfaces of the cornea as well as refraction by the lens cause light rays to come into exact focus on the retina .
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Images focused on the retina are inverted

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Photoreceptors and visual pigments
- Rods and cones
- stimulated when light reaches them
- Rods are more sensitive to light than cones and provide vision in dim light.
- Cones provide sharp images and color vision.
Fovea centralis contains high concentration of cones but no rods.
- Light sensitive pigment in rods is called rhodopsin (visual purple). Rhodopsin, in the presence of bright light, will break down into opsin and retinal (made from Vit. A).

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With rhodopsin broken down, cones are activated and we see in color.
In dim light, rhodopsin is made faster than it is broken down and cones remain unstimulated, therefore we see in shades of gray in dim light.
Light sensitive pigment in cones is called iodopsin.
Three sets of cones within retina
1- erythrolabe- red light waves
2- chlorlabe- green light waves
3- cyanolabe – blue light waves

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Extrinsic eye muscles
1- Superior rectus: turns eye upward and towards the midline (III)
2- Inferior rectus: turns eye downward and towards the midline (III)
3- Medial rectus: turns eye towards midline (III)
4- Lateral rectus: turns eye away from midline (VI)
5- Superior oblique: turns eye downward and away from midline (IV)
6- Inferior oblique: turns eye upward and away from midline. (III)

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Visual pathways:
Axons of the ganglion cells in the retina leave the eye by way of the optic nerve
Some fibers of the optic nerve will cross just anterior to the pituitary gland at a junction called the optic chiasma
- fibers coming from the medial (nasal) retina will cross while fibers coming from lateral (temporal) retina will not.

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Posterior to the optic chiasma is the optic tracts
- right optic tract is composed of
- right temporal and left nasal
- left optic tract is composed of
- left temporal and right nasal
Some fibers branch off to the superior colliculus
(visual reflex)
- Optic tracts synapse in the posterior portion of thalamus at the lateral geniculate body.
- from the geniculate body the pathway travels through optic radiations into the visual cortex of the occipital lobe.