A sensory system is the part of the nervous system that consists of neural pathways, sensory receptors, and brain parts that process sensory information. Sensory receptors recognize stimuli from both internal and external environments, neural pathways conduct information from the stimuli to the brain, and the brain processes the information. The human sensory system is further subdivided into somatosensory system, visual system, auditory and vestibular system, gustatory system, and olfactory system.
Sensory receptors are specialized nerve endings that encode type, spatial location, intensity, duration, threshold, and frequency of stimuli. Stimuli may come from the internal or external environment, and they excite sensory receptors. Sensory receptors are classified according to the stimulus type they respond to. In humans, the different types of sensory receptors in the somatosensory system include the mechanoreceptors, thermoreceptors, nociceptors, and muscle spindle stretch receptors. The sensory systems for vision, hearing and balance, taste, and olfaction, each have their own set of sensory receptors.
Mechanoreceptors detect changes in pressure or touch. Thermoreceptors recognize changes in temperature. Nociceptors detect excessive heat and severe pressure that cause tissue damage and release of chemicals by damaged tissue. Muscle spindle stretch receptors detect posture and movement.
Photoreceptors are found in the visual system. They contain photopigments that absorb light energy. Hair cells of the organ of Corti are found in the auditory and vestibular system. They detect pressure waves from sound stimulus as well as changes in posture and movement. Gustatory and olfaction systems rely on chemoreceptors that detect tastes and odors, respectively.
These sensory receptors convert the stimuli into electrical energy, which is conducted along the neural pathways. The neural pathways of the sensory system are afferent neurons that diverge and synapses that conduct information, the electrical energy, from a single stimulus. In the somatosensory system, the most important neural pathways are the dorsal column-medial lemniscus pathway and the spinothalamic tract. A neural pathway in the visual system involves an optic nerve, while those of the auditory and vestibular systems involve the cochlear nerve and vestibular nerve, respectively. Receptors of the gustatory system involves facial, glossopharyngeal, and vagus nerves that synapse in the nucleus of the solitary tract found in the brain, and finally, receptors of the olfactory system involves olfactory nerves that connect with olfactory glomeruli found at the base of the cranial cavity, just below the frontal lobe of the brain.
Except for the olfactory system, the neural pathways conduct information to the thalamus, and information is transmitted from it to the appropriate location in the cerebral cortex. Information from visual stimuli is transmitted to the occipital lobe, while that from auditory and vestibular stimuli is transmitted to the temporal lobe, and brain stem and cerebellum, respectively. Gustatory and somatosensory stimuli information is transmitted to the parietal lobe. Although information from the olfactory stimuli can reach the thalamus, its neural pathway does not need to relay information to the thalamus. The information is transmitted directly to the frontal lobe of the cerebral cortex.