Facial-mandibular region receptors
Nowadays term “analizator” has been changed in the term “sensor system”. Sensor system is an integrity of peripheral (receptive) and central structures of different levels the management of which is realized by means of direct and indirect connections.
Classification:
1. According to information character coming into CNS from peripheral structures:
– gustatory;
– temperature;
– tactile;
– nociceptive;
– proprioceptive.
2. On functioning specificity:
A. Somato-sensor:
– tactile;
– temperature;
– nociceptive.
B. Chemoreceptors:
– gustatory.
C. Proprioreceptors.
Lingual receptors investigations demonstrated that tactile receptors gives answer reactions first, temperature – second. The latest ones are chemoreceptors.
I.P.Pavlov called all receptors of oral cavity “oral analizator”.
PROPERTIES OF RECEPTORS
1. Specificity of response - each type of receptor gives response to its own specific sensation. Stimulation of pain receptors produces pain sensation. Similarly, stimulation of touch receptors produces touch sensation. Synonym: adequacy or monomodality. Receptors order (from maximum to minimum):
– distant exteroreceptors;
– contact exteroreceptors;
– proprioreceptors;
– interoreceptors do not possess because these receptors must not react to specific stimuli but must act to any stimuli coming inside organism.
2. Polymodality is opposite to specificity. It means possibility to act to all stimuli.
Receptors order (from maximum to minimum):
– interoreceptors;
– proprioreceptors;
– contact exteroreceptors;
– distant exteroreceptors (they do not have).
3. Adaptation or desensitization - when a receptor is continuously stimulated with the same strength of stimulus, after sometime the receptor stops sending impulses through the afferent nerve. Depending upon this property, the receptors are divided into two types:
a) phasic receptors, which get adapted rapidly - touch and pressure receptors;
b) tonic receptors, which are adapted slowly – muscle spindle, pain receptors and cold receptors.
Maximal adaptation have exteroreceptors (more expressed – contact ones: first touching or kiss, clothes; then – distant – phono- and photoreceptors).
4. Response to increase in the strength of stimulus - during stimulation of a receptor, if response given by receptor is to be doubled, strength of stimulus must be increased 100 times. This phenomenon is called Weber-Fechner law, which states that the change in response of a receptor is directly proportional to logarithmic increase in the intensity of stimulus.
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5. Electrical property - ability to generate receptor potential and generator potential.
When a receptor is stimulated, a nonpropagated transmembrane potential difference is developed. This is called receptor potential. Receptor potential is not action potential. It is similar to excitatory postsynaptic potential (EPSP) in synapse, endplate potential in neuro-muscular junction and electro-tonic potential in the nerve fiber.
Receptor potential has such important properties.
a) It is non-propagated (local).
b) It does not work according to the law “everything or nothing”.
Receptor potential is receptor cell membrane depolarization (in complex receptor) or free nervous fiber (in simple receptor) at irritator action to the receptor. Receptor potential is local one. It coincides generator potential in simple receptor. But it differs from it in complex receptor. Generator potential is depolarization of free nervous fiber membrane at mediator portion action. Mediator releasing is caused by receptor formation generation. Generator potential is an action potential. It is equal to action potential and works according to law “everything or nothing” (receptor potential undergoes law of force correlation).
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 ofAction 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.
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6. 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.
7. After-action - receptor action continuation after stimulus action stoppage; it is connected with rhythmical activity.
8. Rhythmical activity – receptor is working despite mediator down-releasing and stimulus absence.
9. 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).
10. Higher excitability comparatively to neurons and nervous fibers.
11. 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.
12. Simular-grouped receptors have non-equal excitability because of different threshold. It allows to rest to one receptors.
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