| Auditory System |
Article Index for Auditory |
Website Links For Auditory |
Information AboutAuditory System |
| CATEGORIES ABOUT AUDITORY SYSTEM | |
| auditory systemauditory system | |
| sensory system | |
| hearing | |
|
EAR ''See Main Article Ear '' Outer Ear The folds of cartilage surrounding the ear canal are called the Pinna . Sound waves are reflected and attenuated when they hit the pinna, and these changes provide additional information that will help the brain determine the direction from which the sounds came. The sound waves enter the Ear Canal , a deceptively simple tube. The ear canal amplifies sounds that are between 3 and 12 KHz . At the far end of the ear canal is the Eardrum , which marks the beginning of the Middle Ear . Middle Ear Sound waves travelling through the ear canal will hit the (hammer), Incus (anvil) and Stapes (stirrup). These Ossicles act as a lever and a teletype, converting the lower-pressure eardrum sound vibrations into higher-pressure sound vibrations at another, smaller membrane called the Oval Window . Higher pressure is necessary because the inner ear beyond the oval window contains fluid rather than air. The sound is not amplified uniformly across the ossicular chain. The Auditory Reflex of the middle ear muscles helps protect the inner ear from damage. The middle ear still contains the sound information in wave form; it is converted to nerve impluses in the cochlea. Inner Ear The inner ear consists of the Cochlea and several non-auditory structures. The cochlea has three fluid filled sections. Strikingly, one of these sections contains an extracellular fluid similar in composition to Endolymph , which is usually found inside of cells. The organ of Corti forms a ribbon of sensory epithelium which runs lengthwise down the entire cochlea. The hair cells of the organ of Corti transform the fluid pressure waves into nerve signals. The journey of a billion nerves begins with this first step; from here further processing leads to a panoply of auditory reactions and sensations. Hair Cell Hair cells are columnar cells, each with a bundle of 100-200 specialized Cilia at the top, for which they are named. These cillia are the mechanosensors for Hearing . Lightly resting atop the longest cilia is the tectorial membrane, which moves back and forth with each cycle of sound, tilting the cilia and allowing electric current into the hair cell (2). Hair cells, like the photoreceptors of the eye, show a Graded Response , instead of the Spikes typical of other neurons. These graded potentials are not bound by the “all or none” properties of an action potential. At this point, one may ask how such a wiggle of a hair bundle triggers a difference in membrane potential. The current model is that cilia are attached to one another by “tip links”, structures which link the tips of one cilium to another. Stretching and compressing the tip links may open an ion channel and produce the receptor potential in the hair cell. Neuron to hair relationship There are far fewer hair cells than afferent nerve fibers in the cochlea. The nerve that innervates the cochlea is the Vestibulocochlear Nerve , or cranial nerve number VIII. Neuronal dendrites innervate cochlear hair cells. The neurotransmitter itself is thought to be glutamate. At the presynaptic juncture, there is a distinct “presynaptic dense body” or ribbon. This dense body is surrounded by synaptic vesicles and is thought to aid in the fast release of neurotransmitter. Efferent projections from the brain to the cochlea also play a role in the perception of sound. Efferent synapses occur on outer hair cells and on afferent dendrites under inner hair cells. NEURAL AUDITION CIRCUIT This sound information, now re-encoded, travels down the Auditory Nerve , through parts of the Brainstem (for example, the Cochlear Nucleus and Inferior Colliculus ), further processed at each waypoint. The information eventually reaches the Thalamus , and from there it is relayed to the cortex. In the Human Brain , the Primary Auditory Cortex is located in the Temporal Lobe . Associated anatomical structures include:
BIBLIOGRAPHY 2) Kandel, et al Principles of Neuroscience. Fourth ed. pp 591-624. Copyright 2000, by McGraw-Hill Co. SEE ALSO EXTERNAL LINKS
|
|
|