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Sensory information is conveyed to the central nervous system by receptors in the sensory neuron in four steps. Initially, a sensory neuron or structure responds to a stimulus. The stimulus is then transformed into nerve impulses in the dendrites of the sensory neuron. The axon of the sensory neuron carries the impulse to the brain through action potentials. Finally, the brain creates a sensory perception of the impulse that was originally created by the stimulus (Raven & Johnson, 2002).
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Sensory cells respond to stimuli because their membranes consist of stimulus gated ion channels. A sensory stimulus causes the ion channels to open or close depending on the sensory system, causing a change in the membrane potential of the receptor cell. A sensory stimulus that produces a depolarization of the sensory receptor is known as a receptor potential. Receptor potentials are graded. The greater the sensory stimulus, the bigger the depolarization of the sensory receptor, and the greater the frequency of action potentials. Receptor potentials also diminish in size the further they travel away. This helps to prevent minor, unnecessary stimuli from reaching the cell body of the neuron. Once depolarization reaches threshold, the receptor potential produces action potentials which travel to the central nervous system via sensory axons (Raven & Johnson, 2002).
Sensory receptors transform stimuli into nerve impulses. They differ based on the type of stimuli they react to depending on their location in the body. Skin receptors are known as cutaneous receptors and respond to temperature, pain, touch, and pressure. Thermoreceptors which respond to changes in temperature are categorized as cold receptors and warm receptors. Cold receptors are located beneath the epidermis and respond to stimuli connected with drops in temperature, while warm receptors which are located deeper in the dermis, react to stimuli connected with increases in temperature (Raven & Johnson, 2002).
Nociceptors are free nerve endings throughout the body, specifically located near surface areas where damage is more frequent and respond to stimuli perceived as pain. Nociceptors have different thresholds, with some having more sensitivity to pain than others. Receptors relating to light or gentle touch are found mostly on the areas of the face and fingertips, and can be either phasic, responding quickly to a stimulus but stopping after a short period of time, or tonic, slowly adapting to a stimulus and maintaining a continuous response. They also are charged with monitoring the duration of a specific touch and the extent to which it is applied. Hair follicle receptors and Meissner’s corpuscles are examples of phasic receptors. They are located on body surfaces that don’t have hair, such as the palms of the hand. Ruffini endings and Merkel cells are examples of tonic receptors. Ruffini endings sit in the dermis, while Merkel cells are near the surface of the skin (Raven & Johnson, 2002).
For the sense of hearing, sound waves in the ear canal produce vibrations of the tympanic membrane which cause movement of the ossicles which, in turn, causes vibrations against the oval window in the middle ear. These vibrations enable the basilar membrane in the cochlea to vibrate as well. Cilia of sensory hair cells bend in response to the movement of the basilar membrane. This flexibility causes the hair cells to stimulate action potentials in sensory neurons. The frequency of the basilar membrane is higher at the base, which responds to higher pitches, and the frequency at the apex is lower, responding to lower pitches (Raven & Johnson, 2002).
Photoreceptors which respond to sight are located in the retina. The retina consists of three layers of cells and the most external layer closest to the eye is comprised of two photoreceptors – the rods and cones. Light first needs to pass through the first two layers of cells in order to reach the photoreceptors. Rods respond to black and white vision in dim light and cones respond to color vision and high visual perception (Raven & Johnson, 2002).
The sensory pathways carry sensory information to the brain via different pathways depending on the type of information being sent. The dorsal column-medial lemniscal pathway is responsible for sending information relating to touch and skin vibration to the brain. Sensory axons enter the ipsilateral dorsal column of the spinal cord. The dorsal columns relay sensory information to the dorsal column nuclei, located by the spinal cord and medulla, which is where the dorsal column axons terminate. Up to this point, all cell activity in the right dorsal column nuclei represent information from the right side of the body and vice versa. Going forward, axons from dorsal column nuclei travel down and center to the medulla where they decussate, and the sensory system of one side of the brain is now responsible for sensations occurring on the opposite side of the body. From the medulla, the axons ascend through the medial lemniscus, a white matter tract that rises through the medulla, pons and midbrain, synapsing on neurons in the ventral posterior nucleus of the thalamus. The thalamic neurons in the ventral posterior nucleus then venture to different regions in the primary somatosensory cortex (Bear, Connors, & Paradiso, 2016).
The spinothalamic pathway conveys pain and temperature information from the spinal cord to the brain. The axons of second order neurons decussate immediately before traveling through the spinothalamic tract. The axons ascend the spinal cord through the medulla, pons, and midbrain, synapsing only when reaching the thalamus, where they continue on to the cerebral cortex. As the spinothalamic axons travel through the brain stem, they eventually stop next to the medial lemniscus, but each group of axons remain separate. This pathway differs from the dorsal column-medial lemniscal pathway in that it sends information contralaterally as the axons decussate immediately before ascending into the spinothalamic tract. The dorsal column-medial lemniscal pathway sends information ipsilaterally, decussating only once the axons reach the dorsal column nuclei, (Bear et al., 2016).
The trigeminal pain pathway conveys pain and temperature information from the face and head. The fibers in the trigeminal nerve synapse on sensory neurons in the spinal trigeminal nucleus of the brain stem. Axons then decussate and continue to the thalamus in the trigeminal lemniscus where the sensory information is sent to the cerebral cortex. A commonality of the spinothalamic pathway and the trigeminal pain pathwayis that they both supply the brain with sensory information involving pain and temperature. (Bear et al., 2016).
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The trigeminal touch pathwaysupplies the brain with sensory information from the face and head. Sensory axons of the trigeminal nerve carry sensory information from skin mechanoreceptors. These axons synapse on neurons in the ipsilateral trigeminal nucleus, which then decussate and travel into the medial ventral posterior nucleus of the thalamus. From the thalamus, the sensory information is sent to the somatosensory cortex. (Bear et al., 2016).
Somatotopic maps record the body’s surface sensations onto areas in the somatosensory cortex. This is produced by the receptive fields of somatosensory neurons. The size of each body part found on the cortex correlates with the amount of sensory input it receives, as well as the importance of the sensory input of a specific body part (Bear et al., 2016).
The auditory nerve consists of axons of spiral ganglion neurons, which are the first neurons of the auditory pathway to produce action potentials. These neurons provide all of the auditory information to the brain. The axons of the spinal ganglion enter the brain stem via auditory-vestibular nerve. At the medulla in the brain stem, the axons innervate both the dorsal cochlear nucleus and the ventral cochlear nucleus on the same side of the cochlea where the axons originated from. From this point there are many pathways to the cortex. One important pathway sends the axons from the ventral cochlear nucleus to the superior olive on both sides of the brain stem. From here, axons travel upwards to the lateral lemniscus, where they innervate the inferior colliculus in the midbrain which is where all auditory pathways leading to the brain first pass through. Neurons in the inferior colliculus send the axons to the medial geniculate nucleus in the thalamus, and from there, the axons are sent to the auditory cortex (Bear et al., 2016).
Tonotopic representation is manifested through different frequencies of sound in particular regions of the auditory cortex. The middle of the auditory cortex represents frequencies of the basal basilar membrane which converts high frequency sounds into signals, while the sides of the cortex represent frequencies of the apex of the basilar membrane which converts low frequency sounds into signals (Gray, n.d.).
The visual pathway conveys all visual information of the eyes to the brain. The ganglion cell axons of the retina pass through the optic nerve located at the back of the eye, continuing to the optic chiasm which is situated at the base of the brain. At this point, nasal retina axons, axons that originate from the retina half closest to the nose, cross to the opposite side of the brain, and the axons then form the optic tract. Optic tract axons innervate the lateral geniculate nucleus of the dorsal thalamus, which gives rise to axons that send information to the primary visual cortex. The central visual system is organized through retinotopic maps which map cells in the retina which are represented in specific targets in the visual cortex (Bear et al., 2016).
Once sensory information reaches the brain, the brain then interprets the impulse into sensory perception of a stimulus, such as touch or pain. The brain achieves perception through the process of translation and interpretation of neural impulses which help us make sense of the information connected to a stimulus and give meaning to a stimulus. Without sensation, perception would be impossible, and without perception, sensations would be unknown to us since no processing of the information would occur. Although sensation and perception work together, they function differently. Sensation occurs when sensory organs send information to the brain, while perception occurs when the brain interprets sensory information and sends signals to sensory organs to respond to the stimuli. Sensation is the ability to sense stimuli, while perception allows one to take meaning from the sensations. Hearing a sound is a sensation, while understanding that it is music is perception (Sincero, 2013).
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