The Nervous System is continuously receiving information about the physical-chemical conditions of the environment and its internal organs. This information is collected by sensory receptors and transformed into nerve stimuli, a process known as sensory transduction. Receptor and generator potentials are the membrane potentials that occur at sensory receptors during this process.
Sensory receptors are highly specialized nervous system cells that are able to react to certain stimuli and generate nerve impulses. The captured information is sent via afferent pathways to different areas of the central nervous system, where it is processed to create a response according to perceived conditions.
All sensory stimuli are physical phenomena, for example pressure, temperature or light. Depending on the type of stimulus to which they are sensitive, sensory receptors can be:Mechanoreceptors: respond to mechanical deformation of the cell itself or adjacent tissue. For example, touch receptors, sound receptors, and balance or pressure receptors in blood vessels. thermoreceptors: respond to temperature variations, for example, skin thermoreceptors. nociceptors: respond to potentially harmful stimuli, both physical and chemical. There are thermal, chemical and mechanical nociceptors. photoreceptors: respond to light stimuli, for example the rods and cones of the eye. Chemoreceptors: They respond to certain conditions and chemicals, for example the receptors for smell, taste or glomus cells and peripheral chemoreceptors that sense the concentration of oxygen in the blood.
Sensory receptors can also be classified into primary and secondary:primary receptors: they are endings of sensory neurons, the neurons themselves respond to physical stimuli. For example, the Meissner bodies of the skin. Secondary receptors: These are specialized cells connected to sensory neurons by synapses. For example, ear hair cells.
Transduction and receptor/generator potentials
In the process of sensory transduction, the physicochemical energy of a stimulus is transformed into an action potential, the basic unit of information in the Nervous System, but before the action potential is produced in the neuron and a nerve impulse actually occurs, the membrane receptor depolarizes, generating potentials known as receptor potential and generator potential.
The transduction process at primary receptors takes place in a specialized area called a sensor. The energy of the sensory stimulus induces a change in membrane permeability in this area, causing voltage-gated ion channels to open or close, either directly or through intracellular messengers (cAMP, cGMP).
With the opening of the ion channels, there is a flow of positive charges, which if it goes inward, it will produce depolarization (mainly sodium, Na+), and if it goes outward, it will produce hyperpolarization (mainly sodium, Na+). of K+). This change in membrane potential at the sensor is the receptor potential or receptor potential.
The receptor potential generates an electrical current that spreads from the sensor to the neuronal axon membrane. The receiving potential reaching the first node of Ranvier (breaks in the myelin sheath) is known as a generating potential and if it is strong enough it will generate an action potential and with it a nerve impulse.
In secondary sensory receptors, the stimulus is transmitted to the nerve fiber through the synapse with a specialized cell that makes the intermediate receptor.
For example, hair cells in the ear have membrane projections that react to endolymph movement by opening ion channels. The opening of these channels initiates the voltage change until the voltage-gated channels open and trigger the receptor potential. Unlike primary receptors, the receptor potential does not transmit the stimulus directly; in secondary receptors The receptor potential triggers the release of neurotransmitters into the synaptic space, and it is the neurotransmitters that will generate the action potential in the nerve fiber.
A feature common to all types of sensory receptors is that they can add spatially and temporally receptor potentials. This makes it possible for the action potential threshold to be reached more quickly and for repetitive potentials to be produced while the stimulus is maintained.