Special Senses

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University of Saskatchewan
Anatomy and Cell Biology
ACB 221
Keith Russell

Notice that the olfactory nerve fibers penetrate through the cribiform plate of the ethmoid. This is why there are holes in the cribiform plate. When you inhale, the nasal concha will create a current in the air. This current will bring airborne compounds (scents) into contact with the olfactory nerve cilia that come through the ethmoid. The olfactory nerve cilia are embedded in mucus, the chemical compounds will get caught and diffuse in this mucus. Once they are diffused the olfactory nerve fibers, the nerves will depolarize sending an action potential to the olfactory bulb of CN I. The axons of the olfactory bulb will travel down the olfactory tract of CN I to reach the olfactory cortex, the hypothalamus, and the limbic system. Synapsing in the hypothalamus and limbic system explain the profound emotional and behavioural responses that are produced by certain smells (i.e., perfumes). NOTE: Olfactory sensations are the ONLY sensations that reach the cerebral cortex without first synapsing in the thalamus (remember: the thalamus is the relay center of the brain; therefore, every other sense will pass through the thalamus prior to going to any other part of the brain). Why? Because of the olfactory bulbs. Taste buds are found all over the tongue, protected by little epithelial bumps called papillae. The taste buds will have nerve axons that connect with either the facial, glossopharyngeal, or vagus nerve (cranial nerves VII, IX, X, respectively) depending on the type of food (i.e., salty, bitter, etc.). When enzymes in the mouth (i.e., amylase, etc.) begin to break down the food that you are eating, the taste buds will be able to sense the chemicals of the broken down food – hence you can taste. This sensation (i.e., tasting) will then cause action potentials to travel up the nerves (i.e., cranial nerves) to the thalamus (i.e., the relay center for the brain). From the thalamus, the action potential will then travel to the gustatory cortex, found superior to the temporal lobe in the mediolateral parietal lobe. The ear is divided into three anatomical regions: 1. The external or outer ear (the visible portion of the ear, which collects and directs sound waves to the middle ear) 2. The middle ear (the chamber containing the auditory ossicles, which amplify the sound waves and transmit them to the inner ear) 3. The inner ear (contains the sensory organs for hearing and also for equilibrium – this is where the vestibulocochlear nerve (CN VIII) innervates). Malleus = mallet Incus = anvil Stapes = stirrup The external (outer) ear will direct sounds from the surrounding environment into the external acoustic (auditory) meatus. Sound waves cause the tympanic membrane (eardrum) to vibrate. This vibration causes the three bones in the ear (malleus, incus, and stapes) to move, passing these vibrations on to the cochlea, via the oval window. The oval window is covered with a thin membrane. The vibrations cause this thin membrane to vibrate, which in turn creates waves in the fluid (perilymph) inside the cochlea. The cochlea is a snail shaped, fluid-filled (fluid = perilymph) structure in the inner ear. “Sound” vibrations (or waves) will begin in the cochlea through vibrations created at the oval window. These waves will propagate from the oval window to the round window. From the oval window to the apex of the cochlea (center of the snail), the sound waves will travel through the vestibular duct – this is because the vestibular duct is closest to the tympanic membrane and can therefore stimulate it with greater ease. From the apex of the cochlea to the round window, the sound waves will travel through the tympanic duct (you can see this in Figure 18.17, pg 488). So, the sound waves will enter the inner ear via the oval window and leave the inner ear via the round window. Why do the sound waves need to leave the ear? Well, so long as the sound waves perform their job (i.e., create an action potential to travel down the cochlear branch of the vestibulocochlear nerve to the brain – so we can hear), they can come into the ear and leave. The sound waves need to leave the inner ear so that there is not a build up of vibrations – which will damage the ear; therefore, the round window is an escape drain for sound. Since the brain only speaks in action potentials (i.e., nerve impulses), how do we convert fluid vibrations to action potentials? Essentially, how does the brain know that we are “hearing”? Inside the cochlea is another structure called the organ of Corti. Hair cells are located on the basilar membrane of the cochlea. The cilia (the hair) of the hairs cells make contact with another membrane called the tectorial membrane. When the hair cells are excited by the vibrations in the fluid, the nerve depolarizes, creating an action potential (nerve impulse). These impulses allow for the conversion of fluid vibrations to chemical signals (i.e., nerve impulses) which is a language that the brain understands. Thus, the nerve impulses are first sent to the thalamus of the brain, via the cochlear branch of the vestibulocochlear nerve, then to the temporal lobe. Once the nerve impulses are in this lobe, we can “hear” – or understand that we are hearing. Question: How can you differentiate between high pitched sounds and low pitched sounds? ◊ Low pitched sound waves will peak farthest from the oval window ◊ High pitched sound waves will peak closest to the oval window 1. Incus 2. Cochlea 3. Tympanic Duct 4. Organ of Corti The vestibular complex is part of the inner ear that provides equilibrium by detecting rotation, gravity, and accelerat
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