Sensory nervous system

Source: Wikipedia, the free encyclopedia.
The visual system and the somatosensory system are active even during resting state fMRI
Activation and response in the sensory nervous system

The sensory nervous system is a part of the

transducers that convert data from the outer physical world to the realm of the mind where people interpret the information, creating their perception of the world around them.[1]

The receptive field is the area of the body or environment to which a receptor organ and receptor cells respond. For instance, the part of the world an eye can see, is its receptive field; the light that each rod or cone can see, is its receptive field.[2] Receptive fields have been identified for the visual system, auditory system and somatosensory system.

Senses and receptors

While debate exists among neurologists as to the specific number of senses due to differing definitions of what constitutes a

electroreception.[3]

Receptors

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:

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.[4]

Chemoreceptors

Chemoreceptors, or chemosensors, detect certain chemical stimuli and transduce that signal into an electrical action potential. The two primary types of chemoreceptors are:

  1. Distance chemoreceptors are integral to receiving stimuli in
    gases in the olfactory system through both olfactory receptor neurons and neurons in the vomeronasal organ
    .
  2. Direct chemoreceptors that detect stimuli in
    aortic bodies which detect changes in oxygen concentration.[5]

Photoreceptors

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.

phototransduction, a process which converts light (electromagnetic radiation) into, among other types of energy, a membrane 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 as well as 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.[8][9]
The three primary types of photoreceptors are: 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).[10]
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.[10]
photosensitive ganglia.[11] These photosensitive ganglia play a role in conscious vision for some animals,[12] and are believed to do the same in humans.[13]

Mechanoreceptors

Mechanoreceptors are sensory receptors which respond to mechanical forces, such as

cutaneous
and are grouped into four categories:

  1. 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.
  2. 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.
  3. Rapidly adapting receptors have small receptive fields and underlie the perception of slip.
  4. Pacinian receptors have large receptive fields and are the predominant receptors for high-frequency vibration.

Thermoreceptors

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 thermoreceptors:[15][failed verification]

  1. The end-bulb of Krause or bulboid corpuscle detects temperatures above body temperature.
  2. Ruffini's end organ
    detects temperatures below 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.[16]

Nociceptors

Nociceptors respond to potentially damaging stimuli by sending signals to the spinal cord and brain. This process, called nociception, usually causes the perception of pain.[17] 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.

  1. Thermal nociceptors are activated by noxious heat or cold at various temperatures.
  2. Mechanical nociceptors respond to excess pressure or mechanical deformation.
  3. 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.

Sensory cortex

All

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 cortices of the different senses: the somatosensory cortex, the visual cortex, the auditory cortex, the primary olfactory cortex, and the gustatory cortex.[18] Other modalities have corresponding sensory cortex areas as well, including the vestibular cortex for the sense of balance.[19]

Somatosensory cortex

Located in the

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. There are also pathways for proprioception (via the cerebellum), and motor control (via Brodmann area 4). See also: S2 Secondary somatosensory cortex
.

The human eye is the first element of a sensory system: in this case, vision, for the visual system.

Visual cortex

The visual cortex refers to the primary visual cortex, labeled V1 or

task-negative activity are observed in the ventral attention network, after abrupt changes in sensory stimuli,[22] at the onset and offset of task blocks,[23] and at the end of a completed trial.[24][relevant?
]

Human ear

Auditory cortex

Located in the

posterior transverse temporal area 42, respectively. Both areas act similarly and are integral in receiving and processing the signals transmitted from auditory receptors
.

Human nose

Primary olfactory cortex

Located in the temporal lobe, the

G protein-coupled receptors. 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.

In contrast to vision and hearing, the olfactory bulbs are not cross-hemispheric; the right bulb connects to the right hemisphere and the left bulb connects to the left hemisphere.

Human tongue

Gustatory cortex

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.[25]

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.[26]

Human sensory system

The human sensory system consists of the following subsystems:

  1. Visual system (Vision)
  2. Auditory system (Hearing)
  3. Somatosensory system (Touch/Temperature/Kinesthesia/Pain)
  4. Gustatory system
    (Taste)
  5. Olfactory system (Smell)
  6. Vestibular system (Balance)

Diseases

Disability-adjusted life year for sense organ diseases per 100,000 inhabitants in 2002.[27]
  no data
  less than 200
  200-400
  400-600
  600-800
  800-1000
  1000-1200
  1200-1400
  1400-1600
  1600-1800
  1800-2000
  2000-2300
  more than 2300
  1. Amblyopia
  2. Anacusis
  3. Color blindness
  4. Deafness

See also

  1. Multisensory integration
  2. Neural adaptation
  3. Neural coding
  4. Sensor
  5. Sensory augmentation
  6. Sensory neuroscience
  7. Sensory systems in fish

Quiescent state

Most sensory systems have a quiescent state, that is, the state that a sensory system converges to when there is no input.

This is well-defined for a linear 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.

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 as otoacoustic emissions in healthy ears, and tinnitus in pathological ears.[28] 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".

Some sensory systems can have multiple quiescent states depending on its history, like

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.[29]

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.[30]

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 its default mode network.

References

  1. ^ Krantz, John. "Experiencing Sensation and Perception - Chapter 1: What is Sensation and Perception?" (PDF). p. 1.6. Retrieved May 16, 2013.
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  4. ^ [1] Archived January 12, 2009, at the Wayback Machine
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  6. ^ "Photoreceptors - an overview | ScienceDirect Topics". www.sciencedirect.com. Retrieved 2024-01-25.
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  10. ^ a b "eye, human." Encyclopædia Britannica. Encyclopædia Britannica Ultimate Reference Suite. Chicago: Encyclopædia Britannica, 2010.
  11. S2CID 1124159
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  15. ^ Krantz, John. Experiencing Sensation and Perception. Pearson Education, Limited, 2009. p. 12.3[permanent dead link]
  16. ^ Julius, David. "How peppers & peppermint identified sensory receptors for temperature and pain". iBiology. Retrieved 12 May 2020.
  17. ^ Sherrington C. The Integrative Action of the Nervous System. Oxford: Oxford University Press; 1906.
  18. OCLC 457057287
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  24. ^ Purves, Dale et al. 2008. Neuroscience. Second Edition. Sinauer Associates Inc. Sunderland, MA.
  25. PMID 22593893
  26. ^ "Mortality and Burden of Disease Estimates for WHO Member States in 2002" (xls). World Health Organization. 2002.
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External links