Sensory receptor

In the gastrointestinal tract, drugs or toxins, as well as mechanical stimulation, induce emesis by activation of sensory receptors on afferent neurons in the vagus and sympathetic nerves. Information is relayed to the vomiting centre via the nucleus tractus solitarius... 

Sensory receptors that structurally and functionally belong to the G protein coupled receptor superfamily. Olfactory receptors are a large GPCR family with >300 members in human that are expressed in neurons of the nasal olfactory epithelium where they sense mostly volatile olfactory molecule. The overall number of olfactory receptors differs widely between species and an expansion of different recqrtors is in particular obvious in species that depend on their olfactory sense for survival. 

Sensory receptors expressed in particular in taste receptor cells of the taste buds that sense the five basic tastes salt, sour, sweet, bitter and umami (glutamate taste). Sodium type ion channels sense salty taste whereas sour taste is transduced by potassium type ion channels. The underlying cause of sweet, bitter, and umami tastes is the selective activation of different groups of G protein coupled receptors that discriminate between sweet, bitter, and umami tasting molecules. 

As early as 1848, it had been suggested that sensory receptors transduce only one sensation, independent of the manner of stimulation. Behavioral experiments tend to support this theory. In 1919, Renqvist proposed that the initial reaction of taste stimulation takes place on the surface of the taste-cell membrane. The taste surfaces were regarded as colloidal dispersions in which the protoplasmic, sensory particles and their components were suspended in the liquor or solution to be tested. The taste sensation would then be due to adsorption of the substances in the solution, and equal degrees of sensation would correspond to adsorption of equal amounts. Therefore, the rate of adsorption of taste stimulants would be proportional to the total substances adsorbed. The phenomenon of taste differences between isomers was partly explained by the assumption that the mechanism of taste involves a three-dimensional arrangement for example, a layer of fatty acid floating on water would have its carboxylic groups anchored in the water whereas the long, hydrocarbon ends would project upwards. 

Many different types of sensory receptors are located throughout the body. These receptors monitor the status of the internal environment or that of the surroundings. Sensory receptors are sensitive to specific types of stimuli and measure the value of a physiological variable. For example, arterial baroreceptors measure blood pressure and chemoreceptors measure the oxygen and carbon dioxide content of the blood. The information detected by these sensors then travels by way of afferent neuronal pathways to the central nervous system (CNS). The CNS is the integrative portion of the nervous system and consists of the (1) brain and the (2) spinal cord. 

Afferent neurons lie predominantly in the PNS (see Figure 6.1). Each has a sensory receptor activated by a particular type of stimulus, a cell body located adjacent to the spinal cord, and an axon. The peripheral axon extends from the receptor to the cell body and the central axon continues from the cell body into the spinal cord. Efferent neurons also lie predominantly in the PNS. In this case, the cell bodies are found in the CNS in the spinal cord or brainstem and the axons extend out into the periphery of the body where they innervate the effector tissues. By way of convergence, the centrally located cell bodies may receive inputs from several different regions of the brain that will influence their activity. 

A reflex is initiated by stimulation of a sensory receptor located at the peripheral ending of an afferent or first-order sensory neuron. This afferent neuron transmits impulses to the spinal cord. Within the gray matter of the spinal cord, the afferent neuron synapses with other neurons. As such, the spinal cord serves as an integrating center for the sensory input. The afferent neuron must ultimately synapse with an efferent or motor neuron. When the afferent neuron synapses directly with the motor neuron, it forms a monosynaptic reflex. An example of this type of reflex is the stretch reflex. When the afferent neuron synapses with an intemeuron that then synapses with the motor neuron, it forms a polysynaptic reflex, e.g., the withdrawal reflex. Most reflexes are polysynaptic. The motor neuron then exits the spinal cord to innervate an effector tissue, which carries out the reflex response. 

An example of the mechanism of the withdrawal reflex is illustrated in Figure 7.4. When a painful stimulus activates a sensory receptor on the right foot, action potentials are transmitted along the afferent neuron to the spinal cord. By way of divergence, this neuron synapses with several other neurons within the gray matter of the spinal cord ... 

An example of this type of reflex is the baroreceptor reflex (see Figure 1.2). Baroreceptors located in some of the major systemic arteries are sensory receptors that monitor blood pressure. If blood pressure decreases, the number of sensory impulses sent from the baroreceptors to the cardiovascular control center in the brainstem also decreases. As a result of this change in baroreceptor stimulation and sensory input to the brainstem, ANS discharge to the heart and blood vessels is adjusted to increase heart rate and vascular resistance so that blood pressure increases to its normal value. 

Explain how each of the three types of sensory receptors within the digestive tract is stimulated chemoreceptors, osmoreceptors, and mechanoreceptors... 

The digestive tract contains three types of sensory receptors that are sensitive to chemical or mechanical changes within the system. These include ... 

Each sensory afferent neuron connects with an interneuron or accessory neuron. These interneurons are located entirely within the CNS, with the majority occurring in the cerebral cortex. They form numerous interconnections and are the means by which all cognitive information, thoughts and feelings, are processed. It should be emphasised that the main role of this processing of information is inhibitory. The sensory receptors provide the CNS with a massive amount of data. The interneurons process and filter this into a limited amount of useful and important informa tion. Conscious information processing forms just one part of this activity. A great deal of brain activity is concerned with routine processes, which continue without conscious awareness. 

O Sensory receptor Cell body (perikaryon or soma) Synaptic bouton/button... 

Chemoreceptor A sensory receptor responding to a chemical stimulus (e.g., smell or taste) or change in the concentration of a chemical (e.g., H+ ions in the blood or PH). 

Mechanoreceptor A sensory receptor responding to mechanical stimuli, such as touch and air pressure. 

There are many other kinds of ion channels with different structural backbones and topologies. The channels used in the action potential contrast with those generating slow potentials at synapses and sensory receptors by having strongly voltage-dependent gating. The other... 

By means of intracellular recording and staining methods, we have examined the responses of AL neurons to stimulation of the ipsilateral antenna with each of the sex pheromone components as well as partial and complete blends (75). In accordance with results of behavioral and sensory-receptor studies, components A and B are the most effective and potent sex pheromone components for eliciting physiological responses in the male-specific AL neurons. On the basis of these responses, we classified the neurons into two broad categories pheromone generalists and pheromone specialists (76). Pheromone generalists are neurons that respond similarly to stimulation of either the component A input channel or the component B input channel and do not respond differently when the complete, natural pheromone blend is presented to the antenna. In contrast, pheromone specialists are neurons that can discriminate between antennal stimulation with component A and stimulation with component B. There are several types of pheromone specialists. Some... 

The EEG does have some disadvantages, or, more correctly, some limitations. It cannot provide information about the effects of toxicants on the integrity of sensory receptors or of sensory or motor pathways. As a corollary, it cannot provide an assessment of the effects of toxicants on sensory system capacities. Finally, the EEG does not provide specific information at the cellular level and therefore lacks the rigor to provide detailed mechanisms of action. 

The sensory receptors and spinal tracts that convey these sensations are multiple, and the sensations often reach consciousness, in the parietal lobe, as blends of the primary components. Because of the number of receptors and hence of distinct tactile feelings and blends, they are best lumped together. 

For acute exposure in a specific airway, the average rate of flux to the epithelial tissue or mucus layer may be the critical quantity and is measured in micrograms per square centimeter per second (in the steady state the unit is micrograms per square centimeter per breath). Chronic effects are probably related to the time integral over the period of exposure. When sensory receptors are involved in the acute response, the local flux to the small surface areas containing the receptor sites may be crucial. 

The underlying intrinsic smooth muscle motility is modulated by neurohormonal influences. Afferent sensory neurons, extrinsic motor neurons, and intramural neurons innervate the gut. It also has mucosal sensory receptors for monitoring chemical, osmotic, or painful stimuli and muscle receptors to monitor degrees of stretch. 

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