The sensory receptors are highly specialized structures found in the (eyes, ears, tongue, nose, and skin) sensory organs and are responsible for receiving incoming stimuli to the body.
Anatomically, a sensory receptor is the end of a sensory nerve; physiologically, the beginning of the sensory process. The receptor receives information from the stimulus and initiates a process of conducting information to the brain, for the perception and interpretation of information.
The integration of information and its interpretation in a subjective way is called sensory perception. Once this information is received, it is carried through the peripheral nervous system to the central nervous system, where it is processed in specific areas of the cerebral cortex for each receptor. This is where the response is generated.
Sensory receptors are in contact with stimuli. For example, when eating, chemicals in food come into contact with the receptors on the tongue of the taste buds (which are sensory receptors), creating action potentials or nerve signals.
Example of sensory receptors in the human olfactory system. 1: olfactory bulb 2: mitral cells 3: bone 4: nasal epithelium 5: glomerulus 6: olfactory sensory receptor neurons
Another example of sensory receptors are those for smell. The perception of an odor occurs when a fragrance -a chemical substance-, binds to the olfactory sensory receptors located in the nasal cavity (# 6 in the image).
The glomeruli add signals from these receptors and transmit them to the olfactory bulb, which processes and encodes this information and directs it to higher structures in the brain, which identify the smell and relate it to memories and emotions.
Sensory receptor classification
Sensory receptors can be classified in different ways, the most commonly used classification according to the type of stimulus they receive:
- Mechanoreceptors: they receive stimuli from mechanical pressure or distortion, such as vibrations captured by auditory receptors.
- Photoreceptors: they receive light stimuli through the retina. Rods and cones are the only representatives of this type of sensory receptor.
- Thermoreceptors: they receive temperature stimuli from both the internal environment (central thermoreceptors) and the external environment (peripheral thermoreceptors). Some are specific for cold (cold thermoreceptors), such as Krausse corpuscles, and others specific for heat (heat thermoreceptors), such as Ruffini's corpuscles.
- Chemoreceptors: they receive chemical stimuli from the environment. Some pick up chemical stimuli from the internal environment (internal chemoreceptors), such as a change in carbon dioxide concentration, and others pick up external stimuli (external chemoreceptors), such as taste buds.
- Nociceptors: they are the receptors of stimuli that produce pain or are harmful to the body, such as sudden changes in temperature or tissue damage of some kind.
Another way to classify it is according to the medium from which the stimulus comes:
- Exteroceptors: they receive stimuli from the external environment. Touch, sight, smell are some examples.
- Interoceptors: they receive stimuli from the internal body environment. It is associated with the autonomic nervous system, they cannot be controlled. For example hunger, visceral pain, thirst.
- Proprioceptors: receive stimuli from skeletal muscle, tendons, joints, and ligaments. They collect information about their own perception of body position, speed, direction, and range of motion.
Physiology
The general process of all sensory receptors begins with the arrival of a stimulus in the form of a physicochemical impulse, which creates changes in the cell membrane, called receptor potential, increasing its permeability to facilitate an ion exchange that will depolarize the cell.
This depolarization gives rise to a generator potential, which is directly proportional to the intensity of the stimulus, and then the impulse through sensory transduction becomes a purely electrical impulse.
If said electrical impulse is powerful enough to exceed the cell's excitability threshold, then an action potential is generated.
This action potential is conducted through the peripheral nervous system to the central nervous system, from where it is processed in specific areas of the cerebral cortex according to the sensory receptor that was depolarized.
Some afferent pathways from the sensory systems relay in the thalamus before reaching the specific cortex area.
Physicochemical characteristics
- Excitability: refers to the reaction capacity of the receptor. It generates an action potential to transport the stimulus to the central nervous system.
- Specificity: each sensory receptor is selective about the stimulus to be captured and thus specific for the organ in which it is found.
It is impossible for a taste bud to capture the sound of birdsong and is therefore incapable of generating a response to such a stimulus.
The communication pathways with the cerebral cortex, although similar, are totally different in terms of the areas of the cortex that generate the response.
For example, the ciliary cells (auditory receptors) receive the information, send it to the central nervous system, in this case it passes through the inferior colliculus in the midbrain, later it takes over in the medial geniculate nucleus of the thalamus (a different region from that of the relay). visual) and then goes to the temporal lobe, next to the lateral sulcus from where the response to the stimulus occurs.
- Adaptation: it is a characteristic mainly of the neuron that initiates an impulse response, and not of the receptor as such.
The constantly stimulated efferent neuron will increase its firing rate. If this stimulus is maintained over time for a long time, the firing frequency of the efferent neuron will decrease, entering the phase of adaptation to the impulse and therefore the nervous reaction will decrease.
- Coding: refers to the ability to translate the stimulus into electrical current for its cortical interpretation. This includes sending a greater number of impulses to the central nervous system if the stimulus is more intense, or not generating an action potential if the stimulus is not capable of exceeding the membrane threshold.
References
- Cliffs Notes. Sensory Receptors. Recovered from: cliffsnotes.com
- Ted L Tewfik, MD; Auditory System Anatomy. MedScape December 08, 2017 Recovered from: emedicine.medscape.com
- Sarah Mae Sincere. Sensory Receptors. June 6, 2013. Explorable. Recovered from: explorable.com
- Sensory receptors. December 01, 2017. Recovered from: es.wikipedia.org
- School of Medicine. Department of Physiology. Dr. Bernardo LÓPEZ-CANO. Titular Professor at the University of Murcia. HUMAN PHYSIOLOGY. BLOCK 9. NEUROPHYSIOLOGY. Topic 43. Sensory Receptors Recovered from: ocw.um.es