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- Main article: Perception
Sensation is the irreducible elements of experience out of which perceptions and concepts are constructed. Sensations depend on the organ stimulated and the efficiency of the sensory system not on the object stimulating them.
It is the first stage in the biochemical and neurologic events that begins with the impinging of a stimulus upon the receptor cells of a sensory organ, which then leads to perception, the mental state that is reflected in statements like "I see a uniformly blue wall."
A sensation that might lead to that statement could include the excitation of cone cells in the retina, spatially varying in the proportion of "blue" and "green" cone excitation due to portions of the wall receiving different proportions of yellowish artificial and bluish sky-light; it is common for these variations to be compensated for, within the brain, so that the non-uniform sensation yields a perception of uniform color.
In psychophysics the characteristics of sensations are explored in terms of sensory thresholds (absolute threshold and difference threshold), using methods of measurement of sensitivity, and signal detection theory.
The ways in which senses are divided from one another in concept, and combined in varying ratios in perceiving the world, differs based on individual physiology, social and cultural context, and physical surroundings. The whole sensory system, including both physical sensation and interpretation (or cognition) of information from the senses, is referred to as a sensorium.
In the West, the human body sensations are considered in terms of eightsenses:
- 1 Senses
- 2 Visual sense
- 3 Auditory sense
- 4 Gustatory sense
- 5 Olfactory sense
- 6 Touch
- 7 Cutaneous Sense
- 8 Kinesthetic Sense
- 9 Vestibular Sense
- 10 Organic Sense
- 11 Receptors
- 12 Senses and receptors
- 13 Sensory cortex
- 14 Loss of sensation
- 15 See also
- 16 References
- Main article: Visual perception
Light enters to the eyes through cornea. It then passes through the pupil, and is refracted by the crystalline lens of the eyes. Light is then channeled through the vitreous humour and then on to the retina. In the retina, there are two kinds of cells, rods and cones. Rods see black-and-white colors, and is dominant in the night (because, as Physics state, there are no colors in the night, because what we see is the colors reflected from the atmosphere). Cones then, see colored structures. Cones are exceptionally abundant in the fovea. Cones are reactive to the three colors of red, blue, and green. Other colours are sensed as combinations of these.
- Main article: Auditory perception
Sound is received by the ear via the pinna, the outer ear structure, which then leads the sound inside through the external auditory meatus. After the sound passes through the meatus, it goes to the eardrum, or tympanus, then vibrates its way through the tiny ossicles, the hammer (malleus), anvil (incus), and stirrup (stapes), then to the cochlea. The cochlea converts vibration into electical impulses which are transmitted to the brain.
- Main article: Taste perception
Taste, or gustation, is the ability to detect sensory changes in the tongue, through the use of taste buds, situated deep into the papillae. Intriguingly, the sense called gustation is if fact comprised of varying ratios of multiple sensory systems, shifting in importance and attention as food is chewed, tasted and swallowed. These include the taste buds, the sense of touch in the structures of the mouth and digestive system, chemical sensation of irritation in the trigeminal nerve system, and unique receptors for sensing the properties of water located at the rear of the oral cavity.
- Main article: Olfactory perception
Smell, or olfaction, is received by the olfactory bulb and the connected to the brain by the olfactory nerve, the 1st cranial nerve of the brain, just after the nasal turbinates of the nose warm, strain and filter the air.
- Main article: Tactual perception
- Main article: Cutaneous sense
Please see the skin article for more details.
- Main article: Kinesthetic perception
The kinesthetic sense is the sense of posture and movement. It is also referred to as proprioception.
- Main article: Equilibrioception
The vestibular sense is the sense of balance. It is mediated by the action of the fluid inside the Semicircular canals in the ear.
The organic sense, per se, refers only to sensation from the internal organs, or viscera, but can, however, be expanded to include certain physiological processes, such as hunger, thirst, drowsiness and air hunger. It is also referred to as interoception.
Senses and receptors
While there is debate among neurologists as to the specific number of senses due to differing definitions of what constitutes a sense, Aristotle classified five ‘traditional’ human senses which have become universally accepted: touch, taste, smell, sight, and hearing. Other senses that have been well-accepted in most mammals, including humans, include nociception, equilibrioception, kinaesthesia, and thermoception. Furthermore, some non-human animals have been shown to possess alternate senses, including magnetoception and electroreception.
The initialization of sensation stems from the response of a specific receptor to a physical stimulus. The receptors which react to the stimulus and initiate the process of sensation are commonly characterized in four distinct categories: chemoreceptors, photoreceptors, mechanoreceptors, and thermoreceptors. All receptors receive distinct physical stimuli and transduce the signal into an electrical action potential. This action potential then travels along afferent neurons to specific brain regions where it is processed and interpreted.
- Distance chemoreceptors are integral to receiving stimuli in the olfactory system through both olfactory receptor neurons and neurons in the vomeronasal organ.
- Direct chemoreceptors include the taste buds in the gustatory system as well as receptors in the aortic bodies which detect changes in oxygen concentration.
Photoreceptors are capable of phototransduction, a process which converts light (electromagnetic radiation) into, among other types of energy, a membrane potential. There are three primary types of photoreceptors: Cones are photoreceptors which respond significantly to color. In humans the three different types of cones correspond with a primary response to short wavelength (blue), medium wavelength (green), and long wavelength (yellow/red). Rods are photoreceptors which are very sensitive to the intensity of light, allowing for vision in dim lighting. The concentrations and ratio of rods to cones is strongly correlated with whether an animal is diurnal or nocturnal. In humans rods outnumber cones by approximately 20:1, while in nocturnal animals, such as the tawny owl, the ratio is closer to 1000:1. Ganglion Cells reside in the adrenal medulla and retina where they are involved in the sympathetic response. Of the ~1.3 million ganglion cells present in the retina, 1-2% are believed to be photosensitive.
Mechanoreceptors are sensory receptors which respond to mechanical forces, such as pressure or distortion and give rise to mechanosensation. While mechanoreceptors are present in hair cells and play an integral role in the vestibular and auditory system, the majority of mechanoreceptors are cutaneous and are grouped into four categories: Slowly Adapting type 1 Receptors have small receptive fields and respond to static stimulation. These receptors are primarily used in the sensations of form and roughness. Slowly Adapting type 2 Receptors have large receptive fields and respond to stretch. Similarly to type 1, they produce sustained responses to a continued stimuli. Rapidly Adapting Receptors have small receptive fields and underlie the perception of slip. Pacinian Receptors have large receptive fields and are the predominant receptors for high frequency vibration.
Thermoreceptors are sensory receptors which respond to varying temperatures. While the mechanisms through which these receptors operate is unclear, recent discoveries have shown that mammals have at least two distinct types of themoreceptors:
- The End-Bulb of Krause, or bulboid corpuscle, detects temperatures above body temperature
- Ruffini’s end organ detects temperatures below body temperature
All stimuli received by the receptors listed above are transduced to an action potential, which is carried along one or more afferent neurons towards a specific area of the brain. While the term sensory cortex is often used informally to refer to the somatosensory cortex, the term more accurately refers to the multiple areas of the brain at which senses are received to be processed. For the five traditional senses in humans, this includes the primary and secondary cortexes of the different senses: the somatosensory cortex, the visual cortex, the auditory cortex, the primary olfactory cortex, and the gustatory cortex.
Located in the parietal lobe, the somatosensory cortex is the primary receptive area for the sense of touch. This cortex is further divided into Brodmann areas 1, 2, and 3. Brodmann area 3 is considered the primary processing center of the somatosensory cortex as it receives significantly more input from the thalamus, has neurons highly responsive to somatosensory stimuli, and can evoke somatic sensations through electrical stimulation. Areas 1 and 2 receive most of their input from area 3.
The visual cortex refers to the primary visual cortex, labeled V1 or Brodmann area 17, as well as the extrastriate visual cortical areas V2-V5. Located in the occipital lobe, V1 acts as the primary relay station for visual input, transmitting information to two primary pathways labeled the dorsal and ventral streams. The dorsal stream includes areas V2 and V5, and is used in interpreting visual ‘where’ and ‘how.’ The ventral stream includes areas V2 and V4, and is used in interpreting ‘what.’
Located in the temporal lobe, the auditory cortex is the primary receptive area for sound information. The auditory cortex is composed of Brodmann areas 41 and 42, also known as the anterior transverse temporal area 41 and the posterior transverse temporal area 42, respectively. Both areas act similarly and are integral in receiving and processing the signals transmitted from auditory receptors.
Primary olfactory cortex
Located in the temporal lobe, the primary olfactory cortex is the primary receptive area for olfaction, or smell. Unique to the olfactory and gustatory systems, at least in mammals, is the implementation of both peripheral and central mechanisms of action. The peripheral mechanisms involve olfactory receptor neurons which transduce a chemical signal along the olfactory nerve, which terminates in the olfactory bulb. The central mechanisms include the convergence of olfactory nerve axons into glomeruli in the olfactory bulb, where the signal is then transmitted to the anterior olfactory nucleus, the piriform cortex, the medial amygdala, and the entorhinal cortex, all of which make up the primary olfactory cortex.
The gustatory cortex is the primary receptive area for taste, or gustation. The gustatory cortex consists of two primary structures: the anterior insula, located on the insular lobe, and the frontal operculum, located on the frontal lobe. Similarly to the olfactory cortex, the gustatory pathway operates through both peripheral and central mechanisms. Peripheral taste receptors, located on the tongue, soft palate, pharynx, and esophagus, transmit the received signal to primary sensory axons, where the signal is projected to the nucleus of the solitary tract in the medulla, or the gustatory nucleus of the solitary tract complex. The signal is then transmitted to the thalamus, which in turn projects the signal to several regions of the neocortex, including the gustatory cortex.
Loss of sensation
Many types of sense loss occur due to a dysfunctional sensation process, whether it be ineffective receptors, nerve damage, or cerebral impairment. Unlike agnosia, these impairments are due to damages prior to the perception process.
Degrees of vision loss vary dramatically, although the ICD-9 released in 1979 categorized them into three tiers: normal vision, low vision, and blindness. Two significant causes of vision loss due to sensory failures include media opacity and optic nerve diseases, although hypoxia and retinal disease can also lead to blindness. Most causes of vision loss can cause varying degrees of damage, from total blindness to a negligible effect. Media Opacity occurs in the presence of opacities in the eye media, distorting and/or blocking the image prior to contact with the photoreceptor cells. Vision loss due to media opacity often results despite correctly functioning retinal receptors. Optic Nerve Diseases such as optic neuritis or retrobulbar neuritis lead to dysfunction in the afferent nerve pathway once the signal has been correctly transmitted from retinal photoreceptors.
Similarly to vision loss, hearing loss can vary from full or partial inability to detect some or all frequencies of sound which can typically be heard by members of their species. For humans, this range is approximately 20 Hz to 20 kHz at ~6.5 dB, although a 10 dB correction is often allowed for the elderly. Primary causes of hearing loss due to an impaired sensory system include long-term exposure to environmental noise, which can damage the mechanoreceptors responsible for receiving sound vibrations, as well as multiple diseases, such as HIV or meningitis, which damage the cochlea and auditory nerve, respectively.
Primary causes of anosmia, a loss of smell, due to sensory damage involve the death of olfactory receptor neurons, often from nasal polyps or upper respiratory tract infections, as well as damage to the olfactory nerve, often from hypothyroidism or physical trauma. Unfortunately, anosmia is strongly correlated with a loss of taste.
Insensitivity to somatosensory stimuli, such as heat, cold, touch, and pain, are most commonly a result of a more general physical impairment associated with paralysis. Damage to the spinal cord or other major nerve fiber may lead to a termination of both afferent and efferent signals to varying areas of the body, causing both a loss of touch and a loss of motor coordination. Other types of somatosensory loss include hereditary sensory and autonomic neuropathy, which consists of ineffective afferent neurons with fully functioning efferent neurons; essentially, motor movement without somatosensation.
Taste loss can vary from true aguesia, a complete loss of taste, to hypogeusia, a partial loss of taste, to dysgeusia, a distortion or alteration of taste. The primary cause of ageusia involves damage to the lingual nerve, which receives the stimuli from taste buds for the front two-thirds of the tongue, or the glossopharyngeal nerve, which acts similarly for the back third. Damage may be due to neurological disorders, such as Bell’s palsy or multiple sclerosis, as well as infectious diseases such as meningoencephalopathy. Other causes include a vitamin B deficiency, as well as taste bud death due to acidic/spicy foods, radiation, and/or tobacco use.
- Sensation seeking
- Sensory adaptation
- Sensory deprivation
- Sensory disabilities (attitudes toward)
- Sensory feedback
- Sensory gating
- Sensory integration
- Sensory neglect
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