Thesensory nervous system is a part of thenervous system responsible for processingsensory information. Asensory system consists ofsensory neurons (including the sensory receptor cells),neural pathways, and parts of thebrain involved in sensoryperception andinteroception. Commonly recognized sensory systems are those forvision,hearing,touch,taste,smell,balance and visceral sensation. Sense organs aretransducers that convert data from the outer physical world to the realm of the mind where people interpret the information, creating theirperception of the world around them.[1]
While debate exists among neurologists as to the specific number of senses due to differing definitions of what constitutes asense,Gautama Buddha andAristotle classified five 'traditional' human senses which have become universally accepted:touch,taste,smell,vision, andhearing. Other senses that have been well-accepted in most mammals, including humans, includepain,balance,kinaesthesia, andtemperature. Furthermore, some nonhuman animals have been shown to possess alternate senses, includingmagnetoreception andelectroreception.[3]
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, andthermoreceptors. All receptors receive distinct physical stimuli and transduce the signal into an electricalaction potential. This action potential then travels alongafferent neurons to specific brain regions where it is processed and interpreted.[4]
Chemoreceptors, or chemosensors, detect certain chemical stimuli and transduce that signal into an electrical action potential. The two primary types of chemoreceptors are:
Distance chemoreceptors are integral to receiving stimuli ingases in theolfactory system through both olfactory receptor neurons and neurons in thevomeronasal organ.
Photoreceptors are neuron cells and are specialized units that play the main role in initiating vision function. Photoreceptors are light-sensitive cells that capture different wavelengths of light. Different types of photoreceptors are able to respond to the varying light wavelengths in relation to color, and transduce them into electrical signals.[6] Photoreceptors are capable ofphototransduction, a process which converts light (electromagnetic radiation) into, among other types ofenergy, amembrane potential. There are five compartments that are present in these cells. Each compartment corresponds to differences in function and structure. The first compartment is the outer segment (OS), where it is responsible for capturing light and transducing it. The second compartment is the inner segment (IS), which includes the necessary organelles that function in cellular metabolism and biosynthesis. Mainly, these organelles include mitochondria, Golgi apparatus, and endoplasmic reticulum, among others. The third compartment is the connecting cilium (CC). As its name suggests, CC works to connect the OS and the IS regions together for the purpose of essential protein trafficking. The fourth compartment contains the nucleus and is a continuation of the IS region, known as the nuclear region. Finally, the fifth compartment is the synaptic region, where it acts as a final terminal for the signal, consisting of synaptic vesicles. In this region, glutamate neurotransmitter is transmitted from the cell to secondary neuron cells.[7][8] The three primary types of photoreceptors are:cones are photoreceptors which respond significantly tocolor. 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).[9]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 isdiurnal ornocturnal. In humans, rods outnumber cones by approximately 20:1, while in nocturnal animals, such as thetawny owl, the ratio is closer to 1000:1.[9]Ganglion cells reside in theadrenal medulla andretina where they are involved in thesympathetic response. Of the ~1.3 million ganglion cells present in the retina, 1–2% are believed to bephotosensitive ganglia.[10] These photosensitive ganglia play a role in conscious vision for some animals,[11] and are believed to do the same in humans.[12]
Mechanoreceptors are sensory receptors which respond to mechanical forces, such aspressure ordistortion.[13] While mechanoreceptors are present inhair cells and play an integral role in thevestibular andauditory systems, the majority of mechanoreceptors arecutaneous 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 ofform androughness.
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 varyingtemperatures. While the mechanisms through which these receptors operate is unclear, recent discoveries have shown thatmammals have at least two distinct types of thermoreceptors:[14]
The end-bulb of Krause orbulboid corpuscle detects temperatures above body temperature.
TRPV1 is a heat-activated channel that acts as a small heat detecting thermometer in the membrane which begins the polarization of the neural fiber when exposed to changes in temperature. Ultimately, this allows us to detect ambient temperature in the warm/hot range. Similarly, the molecular cousin to TRPV1, TRPM8, is a cold-activated ion channel that responds to cold. Both cold and hot receptors are segregated by distinct subpopulations of sensory nerve fibers, which shows us that the information coming into the spinal cord is originally separate. Each sensory receptor has its own "labeled line" to convey a simple sensation experienced by the recipient. Ultimately, TRP channels act as thermosensors, channels that help us to detect changes in ambient temperatures.[15]
Nociceptors respond to potentially damaging stimuli by sending signals to the spinal cord and brain. This process, callednociception, usually causes the perception ofpain.[16] They are found in internal organs, as well as on the surface of the body. Nociceptors detect different kinds of damaging stimuli or actual damage. Those that only respond when tissues are damaged are known as "sleeping" or "silent" nociceptors.
Thermal nociceptors are activated by noxious heat or cold at various temperatures.
Mechanical nociceptors respond to excess pressure or mechanical deformation.
Chemical nociceptors respond to a wide variety of chemicals, some of which are signs of tissue damage. They are involved in the detection of some spices in food.
The visual cortex is the part of the cerebral cortex that processes visual information. Processing in the visual cortex starts in the primary visual cortex, labeled V1 orBrodmann area 17, as well as the extrastriate visual cortical areas V2-V5.[19] Located in theoccipital lobe, V1 acts as the primary relay station for visual input, transmitting information to two primary pathways labeled thedorsal andventral 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.'[20] Increases intask-negative activity are observed in the ventral attention network, after abrupt changes in sensory stimuli,[21] at the onset and offset of task blocks,[22] and at the end of a completed trial.[23][relevant?]
In contrast to vision and hearing, theolfactory bulbs are not cross-hemispheric; the right bulb connects to the right hemisphere and the left bulb connects to the left hemisphere.
Thegustatory cortex is the primary receptive area fortaste. The wordtaste is used in a technical sense to refer specifically to sensations coming from taste buds on the tongue. The five qualities of taste detected by the tongue include sourness, bitterness, sweetness, saltiness, and the protein taste quality, calledumami. In contrast, the termflavor refers to the experience generated through integration of taste with smell and tactile information. The gustatory cortex consists of two primary structures: theanterior insula, located on theinsular lobe, and the frontaloperculum, located on thefrontal lobe. Similarly to the olfactory cortex, the gustatory pathway operates through both peripheral and central mechanisms.[clarification needed] Peripheraltaste receptors, located on thetongue,soft palate,pharynx, andesophagus, transmit the received signal to primary sensory axons, where the signal is projected to thenucleus of the solitary tract in themedulla, or the gustatory nucleus of the solitary tract complex. The signal is then transmitted to thethalamus, which in turn projects the signal to several regions of theneocortex, including the gustatory cortex.[24]
The neural processing of taste is affected at nearly every stage of processing by concurrent somatosensory information from the tongue, that is,mouthfeel. Scent, in contrast, is not combined with taste to create flavor until higher cortical processing regions, such as the insula and orbitofrontal cortex.[25]
Most sensory systems have a quiescent state, that is, the state that a sensory system converges to when there is no input.[citation needed]
This is well-defined for alinear time-invariant system, whose input space is a vector space, and thus by definition has a point of zero. It is also well-defined for any passive sensory system, that is, a system that operates without needing input power. The quiescent state is the state the system converges to when there is no input power.[citation needed]
It is not always well-defined for nonlinear, nonpassive sensory organs, since they can't function without input energy. For example, a cochlea is not a passive organ, but actively vibrates its own sensory hairs to improve its sensitivity. This manifests asotoacoustic emissions in healthy ears, andtinnitus in pathological ears.[26] There is still a quiescent state for the cochlea, since there is a well-defined mode of power input that it receives (vibratory energy on the eardrum), which provides an unambiguous definition of "zero input power".[citation needed]
Some sensory systems can have multiple quiescent states depending on its history, likeflip-flops, and magnetic material withhysteresis. It can also adapt to different quiescent states. In complete darkness, the retinal cells become extremely sensitive, and there is noticeable "visual snow" caused by the retinal cells firing randomly without any light input. In brighter light, the retinal cells become much less sensitive, consequently decreasing visual noise.[27]
Quiescent state is less well-defined when the sensory organ can be controlled by other systems, like a dog's ears that turn towards the front or the sides as the brain commands. Some spiders can use their nets as a large touch-organ, like weaving a skin for themselves. Even in the absence of anything falling on the net, hungry spiders may increase web thread tension, so as to respond promptly even to usually less noticeable, and less profitable prey, such as small fruit flies, creating two different "quiescent states" for the net.[28]
Things become completely ill-defined for a system which connects its output to its own input, thus ever-moving without any external input. The prime example is the brain, with itsdefault mode network.[citation needed]
^Kolb, Bryan (2003). Winshaw, Ian Q. (ed.).Fundamentals of human neuropsychology (5th ed.). New York, NY: Worth Publishers.ISBN0-7167-5300-6.OCLC55617319.[page needed]
^Sherrington C.The Integrative Action of the Nervous System. Oxford: Oxford University Press; 1906.
^Brynie, Faith Hickman (2009).Brain sense: the science of the senses and how we process the world around us. New York: American Management Association.ISBN978-0-8144-1326-5.OCLC457057287.
^Downar, J.; Crawley, A. P.; Mikulis, D. J.; Dav (2000). "multimodal cortical network for the detection of changes in the sensory environment".Nature Neuroscience.3 (3):277–283.doi:10.1038/72991.PMID10700261.S2CID8807081.
