Test 01 - Introduction
Receptor cells use chemical transmitters to communicate with relay cells or afferent nerve fibers. In all sensory systems, accessory structures modify, in some way, the stimulus going to the receptor surface. Receptors are electrically polarized cells and therefore have a resting membrane potential. Stimuli produce, in the appropriate receptors, a change in the electrical properties that is known as the receptor or generator potential. The two ways in which sensory stimuli can act on receptors to change their resting membrane potential (RMP) are by directly acting on ion channels or causing production of second messengers that act on ion channels. The two types of ion channels found in sensory receptors are mechanically gated and chemically gated channels. In receptors, the appropriate stimulus results in a change in the resting membrane potential because of changes in ionic movement through mechanically gated or chemically gated channels. Receptor or Generator potentials and Action potentials are similar in that they are produced by conductance changes due to the opening or closing of channels. Receptor/ Generator Potentials differ from Action Potentials (APs) in that only APs involve the opening of voltage-gated ion channels. The strength of a stimulus can be coded by an increase in the number of APs from a single receptor and an increase in the number of active receptors. The property that allows receptors to code aspects of a dynamic or changing stimulus is fast adaptation The properties of the receptor potential that allow receptors to code stimulus timing and duration are summation and adaptation. Adaptation allows receptors to signal stimulus change.
Receptor tuning allows receptors to signal stimulus quality such as color, pitch, flutter etc. The receptive field (RF) of a sensory neuron refers to the area of the sensory receptor surface providing input to that neuron. Receptors can code (signal) stimulus intensity because the receptor potential is graded to the stimulus intensity. Sensory systems use the following two mechanisms to signal stimulus intensity activate each receptor more strongly and activate more neurons with different thresholds. Test 02 – Hearing
One way in which the middle ear can optimize delivery of sound energy to the cochlea is due to the fact that the area of tympanum is greater than the area of the stapes footplate. The basic role of the middle ear bones is to amplify sound energy to the cochlea. Vibration of the Cochlear Partition is, at the base of the cochlea, most sensitive to high frequencies and, at the apex, to low frequencies. A pure tone containing only one frequency of sound will cause a cyclical vibration of the cochlear partition at the same frequency as the tone. The receptor potential in the cochlear hair cells is produced by change in K+ conductance. Opening of ion channels in the stereocilia of hair cells of the cochlea allows the movement of K+ ions down their electrical gradient. Even in absolute silence there are still APs in all auditory nerve fibers to the brain because there is a standing current flow through some open ion channels in hair cell stereocilia. The afferent nerve fibers leaving the cochlea are frequency selective because the vibration of the cochlea partition is frequency selective. During cyclical vibration of the Cochlear Partition up and down the Hair Cell stereocilia are bent back and forth to open and close ion channels. Outer Hair Cells (OHCs) are responsible for making vibration of the cochlea partition frequency selective and sensitive. Peripheral auditory processing of sensory input is similar to processing in fine touch in that transduction in both cases involves stretch leading to opening of ion channels. The middle ear is made up of the ossicles and the Eustachian tube. The middle ear ossicles act to amplify the energy that is transmitted by the stapes to the oval window. The cochlea is divided into 3 compartments...
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