Nursing Entrance Exam

7.2 The Sensory System Human senses can be divided into two categories: general and special. The “general senses” provide information about the body and its environment, while the “special senses” refer to smell, taste, sight, equilibrium (also called balance), and hearing. Sensory information originating from all parts of the body is sent to the somatosensory cortex. Sensory receptors are specialized cells that monitor specific conditions in the external environment or within the body. A receptor is stimulated by a change in conditions; it passes sensory information to the CNS along the axon of a sensory neuron in the form of action potentials. Axons are discrete parts of sensory pathways that deliver somatic and visceral sensory information to the CNS; the nerves, nuclei, and tracts deliver somatic and visceral sensory information to their final destinations inside the CNS. The afferent division of the nervous system is comprised of receptors, sensory neurons, and sensory pathways. Somatic and visceral sensory information often travel on the same route. Processing centers located in the brain receive somatic sensory information, which is delivered to either the primary sensory cortex of the cerebral hemispheres or to the appropriate areas of the cerebellar hemispheres. Visceral sensory information is distributed primarily to the reflex centers in the diencephalon and the brainstem. 7.2.1 Sensory Receptors Sensory receptors link the nervous system with our internal and external environments. Sensory receptors are specialized cells or cell processes that provide your central nervous system with information about conditions inside or outside the body. The term “general senses” is used to describe our sensitivity to temperature, pain, touch, pressure, vibration, and proprioception. General sensory receptors, which are relatively simple in structure, are dispersed throughout the body. Some of the information they send to the CNS reaches the primary sensory cortex and our awareness. The arriving information is called a sensation. The conscious awareness of a sensation is called a perception. The special senses are olfaction (smell), vision (sight), gustation (taste), equilibrium (balance), and hearing. These sensations are provided by receptors that are structurally more complex than those of the general senses. Special sensory receptors are found in the sense organs, such as the eye or ear, where the receptors are protected by surrounding tissues. The information these receptors provide is distributed to specific areas of the cerebral cortex (i.e., the auditory cortex, the visual cortex, etc.) and to centers in the brainstem. 7.2.2 The Detection of Stimuli Each receptor has a characteristic sensitivity. Specificity is determined by the receptor cell structure, from the presence of accessory cells, or from structures that shield the receptor cell from other stimuli. The dendrites of sensory neurons are the simplest receptors. These dendrites have branching tips that are not protected by accessory structures; thus, they are called free nerve endings.

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