Sensory nervous system
The sensory nervous system is a part of the
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
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:
Chemoreceptors
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 in gases in the olfactory system through both olfactory receptor neurons and neurons in the vomeronasal organ.
- Direct chemoreceptors that detect stimuli in
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.
Mechanoreceptors
Mechanoreceptors are sensory receptors which respond to mechanical forces, such as
- 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
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]
- The end-bulb of Krause or bulboid corpuscle detects temperatures above body temperature.
- Ruffini's end organdetects 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.
- 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.
Sensory cortex
All
Somatosensory cortex
Located in the
Visual cortex
The visual cortex refers to the primary visual cortex, labeled V1 or
Auditory cortex
Located in the
Primary olfactory cortex
Located in the temporal lobe, the
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.
Gustatory cortex
The
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:
- Visual system (Vision)
- Auditory system (Hearing)
- Somatosensory system (Touch/Temperature/Kinesthesia/Pain)
- Gustatory system(Taste)
- Olfactory system (Smell)
- Vestibular system (Balance)
Diseases
- Amblyopia
- Anacusis
- Color blindness
- Deafness
See also
- Multisensory integration
- Neural adaptation
- Neural coding
- Sensor
- Sensory augmentation
- Sensory neuroscience
- 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
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
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- ^ "Photoreceptors - an overview | ScienceDirect Topics". www.sciencedirect.com. Retrieved 2024-01-25.
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- ^ Krantz, John. Experiencing Sensation and Perception. Pearson Education, Limited, 2009. p. 12.3[permanent dead link]
- ^ Julius, David. "How peppers & peppermint identified sensory receptors for temperature and pain". iBiology. Retrieved 12 May 2020.
- ^ Sherrington C. The Integrative Action of the Nervous System. Oxford: Oxford University Press; 1906.
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