Significance of Receptor Potential
When the receptor potential is sufficiently strong (when the magnitude is about 10 mV), it causes development of action potential in the sensory nerve.
Mechanism of Development of Receptor Potential and Generation of Action Potential in the Nerve Fiber
The deformation of the nerve fiber causes opening of sodium channels. So, positively charged sodium ions enter interior of nerve fiber and a mild depolarization, i.e. the receptor potential occurs (Fig.17). This receptor potential spreads along the non-myelinated part of the nerve fiber.
Fig.17. Development of receptor potential in Pacinian corpuscle.
When this current reaches the first node of Ranvier within the corpuscle, it cause development of action potential in the nerve fiber.
Sensory transduction – the process, which helps the receptor to give response to a stimulus is called sensory transduction (transduction = conversion of one form of energy into another). Sensory transduction depends on type of receptor. For example, chemoreceptor converts chemical energy into action potential in sensory nerve fiber. Touch receptor converts mechanical energy into action potential in sensory nerve fiber.
After-action - receptor action continuation after stimulus action stoppage; it is connected with rhythmical activity.
Rhythmical activity – receptor is working despite mediator down-releasing and stimulus absence.
Ability to transform force in frequency – receptor is increasing transformer because the more is stimulus action the bigger is generator potential duration (synapse is decreasing transformer because 3-5 EPSP give only 1 action potential).
Higher excitability comparatively to neurons and nervous fibers.
Excitabilily fluctuation in one and the same receptor – it is fluctuated from high level to low excitability and finally its absence because of receptor rest.
Simular-grouped receptors have non-equal excitability because of different threshold. It allows to rest to one receptors.
4.Materials for auditory self-work.
4.1. List of study practical tasks necessary to perform at the practical class.
Materials and methods: istruments preparation set, metronom, stand, acids set (sulfuric acid 0,1%, 0,3%, 0,5%, 1,0% solutions), Ringer’s solution; glass, threads, novocain solution.
Investigation object: frog.
Task 1. Receptive field definition.
Every reflex has its own reflex field, i.e. body locus at irritation of which this reflex occurs. Response answer character at reflex field irritation depends not only on its localization on body surface but also on irritation force and duration.
Frog’s brain must be removed. After that you receive spinal frog’s preparation it’s necessary to wait 2-3 minutes for spinal shock phenomena disappearance. Then the investigators must hang the frog by his inferior jaw on the hook fixated in a stand. They wash filter paper piece in 0,1% sulfuric acid solution and put it on inferior leg tibia external skin surface. To observe flexory reaction of corresponding leg. To wash the leg from acid by means of leg’s plunging into water. To realize the irritation of the same leg with 0,3%, then with 0,5% of acid solutions. To choose those concentration at which one can see maximal flexory reflex. Paper with sulfuric acid of this concentration put on lateral abdomen surface. After some minutes you can observe defensive reflex: frog takes the paper off with the nearest leg. To put the paper to the external surface of anterior leg, on the abdomen near to the thoracic part, between superior and inferior legs. You must registrate answer reaction every time. The intervals between irritations must be at least 2-3 min. After each irritation you should put the frog to the glass with water and wash the animal from acid residues.
In second experiment you should put your attention to the correlation of reflex time to stimulus force (students must perform the experiments with all solutions (time must be fixed with metronom or watch with second pointer).