Page 86 - Alaska A & P Primer

14.1 Sensory Perception
14.2 Central Processing
14.1 OBJECTIVES
1. Describe different types of sensory receptors
14.2 OBJECTIVES
1. Describe the pathways that sensory systems follow into the central nervous system
The senses are olfaction (smell), gustation (taste), somatosensation (sensations associated with the skin and body), audition (hearing), equilibrium (balance), and vision. With the exception of somatosensation, this list represents the special senses, or those systems of the body that are associated with specific organs such as the tongue or eye. Somatosensa- tion belongs to the general senses, which are those sensory structures that are distributed throughout the body and in the walls of various organs. The general senses can be divided into somatosensation, which is commonly considered touch, but includes tactile, pressure, vibration, temperature, and pain perception. The general senses also include the visceral senses, which are separate from the somatic nervous system function in that they do not normally rise to the level of conscious perception.
The cells that transduce sensory stimuli into the electrochemical signals of the nervous sys- tem are classified on the basis of structural or functional aspects of the cells. The structural classifications are either based on the anatomy of the cell that is interacting with the stimu- lus (free nerve endings, encapsulated endings, or specialized receptor cell), or where the cell is located relative to the stimulus (interoceptor, exteroceptor, proprioceptor). Thirdly, the functional classification is based on how the cell transduces the stimulus into a neural signal. Chemoreceptors respond to chemical stimuli and are the basis for\ olfaction and gustation. Related to chemoreceptors are osmoreceptors and nociceptors for fluid balance and pain reception, respectively. Mechanoreceptors respond to mechanical stimuli and are the basis for most aspects of somatosensation, as well as being the basis of audition and equilibrium in the inner ear. Thermoreceptors are sensitive to temperature changes, and photoreceptors are sensitive to light energy. The nerves that convey sensory information from the periphery to the CNS are either spi-
nal nerves, connected to the spinal cord, or cranial nerves, connected to the brain. Spinal nerves have mixed populations of fibers; some are motor fibers and some are sensory.
The sensory fibers connect to the spinal cord through the dorsal root, which is attached to the dorsal root ganglion. Sensory information from the body that is conveyed through spi- nal nerves will project to the opposite side of the brain to be processed by the cerebral cor- tex. The cranial nerves are connected to the same side of the brain from which the sen- sory information originates.
Sensory input to the brain enters through pathways that travel through either the spinal cord (for somatosensory input from the body) or the brain stem (for everything else, ex- cept the visual and olfactory systems) to reach the diencephalon. In the diencephalon, sen- sory pathways reach the thalamus. This is necessary for all sensory systems to reach the cerebral cortex, except for the olfactory system that is directly connected to the frontal and temporal lobes. The two major tracts in the spinal cord, originating from sensory neurons in the dorsal root ganglia, are the dorsal column system and the spinothalamic tract. The major differences between the two are in the type of information that is relayed to the brain and where the tracts decussate.
The dorsal column system primarily carries information about touch and proprioception and crosses
the midline in
the medulla.
The spinotha- lamic tract is primarily re- sponsible for pain and tem- perature sen- sation and crosses the midline in the spinal cord at the level at which it en- ters.
The trigemi- nal nerve adds similar sensa- tion informa- tion from the head to these pathway
s.
This content is available for free at https://cnx.org/content/col11496/1.7
State of Alaska EMS Education Primer - 2016
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