Sensory neuron
Sensory neurons are typically classified as the neurons responsible for converting external stimuli from the environment into internal stimuli. They are activated by sensory input (vision, touch, hearing, etc.), and send projections into the central nervous system
Central nervous system
The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...

 that convey sensory information to the brain or spinal cord. Unlike neurons of the central nervous system, whose inputs come from other neurons, sensory neurons are activated by physical modalities such as light, sound, and temperature.

In complex organisms, sensory neurons relay their information to the central nervous system
Central nervous system
The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...

 or in less complex organisms, such as the hydra
Hydra (genus)
Hydra is a genus of simple fresh-water animal possessing radial symmetry. Hydras are predatory animals belonging to the phylum Cnidaria and the class Hydrozoa. They can be found in most unpolluted fresh-water ponds, lakes, and streams in the temperate and tropical regions and can be found by...

, directly to motor neurons and sensory neurons also transmit information (electrical impulses) to the brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

, where it can be further processed and acted upon. For example, olfactory sensory neurons make synapse
In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell...

s with neurons of the olfactory bulb
Olfactory bulb
The olfactory bulb is a structure of the vertebrate forebrain involved in olfaction, the perception of odors.-Anatomy:In most vertebrates, the olfactory bulb is the most rostral part of the brain. In humans, however, the olfactory bulb is on the inferior side of the brain...

, where the sense
Senses are physiological capacities of organisms that provide inputs for perception. The senses and their operation, classification, and theory are overlapping topics studied by a variety of fields, most notably neuroscience, cognitive psychology , and philosophy of perception...

 of olfaction
Olfaction is the sense of smell. This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, and, by analogy, sensory cells of the antennae of invertebrates...

 (smell) is processed.

At the molecular
Molecular biology
Molecular biology is the branch of biology that deals with the molecular basis of biological activity. This field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry...

 level, sensory receptor
Sensory receptor
In a sensory system, a sensory receptor is a sensory nerve ending that responds to a stimulus in the internal or external environment of an organism...

s located on the cell membrane
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...

 of sensory neurons are responsible for the conversion of stimuli into electrical impulses. The type of receptor employed by a given sensory neuron determines the type of stimulus it will be sensitive to. For example, neurons containing mechanoreceptor
A mechanoreceptor is a sensory receptor that responds to mechanical pressure or distortion. There are four main types in the glabrous skin of humans: Pacinian corpuscles, Meissner's corpuscles, Merkel's discs, and Ruffini corpuscles...

s are sensitive to tactile stimuli, while olfactory receptor
Olfactory receptor
Olfactory receptors expressed in the cell membranes of olfactory receptor neurons are responsible for the detection of odor molecules. Activated olfactory receptors are the initial player in a signal transduction cascade which ultimately produces a nerve impulse which is transmitted to the brain...

s make a cell sensitive to odors.

Somatic sensory system

The somatic sensory system includes the sensations of touch, pressure, vibration, limb position, heat, cold, and pain.

The cell bodies of somatic sensory afferent fibers lie in ganglia throughout the spine. These neurons are responsible for relaying information about the body to the central nervous system. Neurons residing in ganglia of the head and body supply the central nervous system with information about the aforementioned external stimuli occurring to the body. Pseudounipolar neurons are located in the dorsal root ganglia (the head).


Specialized receptor cells called mechanoreceptors often encapsulate afferent fibers to help tune the afferent fibers to the different types of somatic stimulation. Mechanoreceptors also help lower thresholds for action potential generation in afferent fibers and thus make them more likely to fire in the presence of sensory stimulation.

Proprioceptors are another type of mechanoreceptors which literally means "receptors for self." These receptors provide spatial information about limbs and other body parts.

Nociceptors are responsible for processing pain and temperature changes. The burning pain and irritation experienced after eating a chili pepper (due to its main ingredient, capsaicin), the cold sensation experienced after ingesting a chemical such as menthol or icillin, as well as the common sensation of pain are all a result of neurons with these receptors.

Problems with mechanoreceptors lead to disorders such as:
  • Neuropathic pain
    Neuropathic pain
    Neuropathic pain results from lesions or diseases affecting the somatosensory system. It may be associated with abnormal sensations called dysesthesia, which occur spontaneously and allodynia that occurs in response to external stimuli. Neuropathic pain may have continuous and/or episodic ...

     - a severe pain condition resulting from a damaged sensory nerve
  • Hyperalgesia
    Hyperalgesia is an increased sensitivity to pain, which may be caused by damage to nociceptors or peripheral nerves. Temporary increased sensitivity to pain also occurs as part of sickness behavior, the evolved response to infection.-Types:...

     - an increased sensitivity to pain caused by sensory ion channel, TRPM8
    Transient receptor potential cation channel subfamily M member 8 , also known as the cold and menthol receptor 1 , is a protein that in humans is encoded by the TRPM8 gene.-Function:...

    , which is typically responds to temperatures between 23 and 26 degrees, and provides the cooling sensation associated with menthol and icillin
  • Phantom limb syndrome - a sensory system disorder where pain or movement is experienced in a limb that does not exist


Vision is one of the most complex sensory systems. The eye has to first "see" via refraction of light. Then, light energy has to be converted to electrical signals by photoreceptor cells and finally these signals have to be refined and controlled by the synaptic interactions within the neurons of the retina. The five basic classes of neurons within the retina are photoreceptor cells, bipolar cells, ganglion cells, horizontal cells, and amacrine cells.
The basic circuitry of the retina incorporates a three-neuron chain consisting of the photoreceptor (either a rod
Rod cell
Rod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of visual photoreceptor, cone cells. Named for their cylindrical shape, rods are concentrated at the outer edges of the retina and are used in peripheral vision. On...

 or cone
Cone cell
Cone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light. If the retina is exposed to an intense visual stimulus, a negative afterimage will be...

), bipolar cell, and the ganglion cell.
As the picture shows, the first action potential occurs in the retinal ganglion cell. This pathway is the most direct way for transmitting visual information to the brain.
Problems and decay of sensory neurons associated with vision lead to disorders such as:
  • Macular degeneration
    Macular degeneration
    Age-related macular degeneration is a medical condition which usually affects older adults and results in a loss of vision in the center of the visual field because of damage to the retina. It occurs in “dry” and “wet” forms. It is a major cause of blindness and visual impairment in older adults...

     – degeneration of the central visual field due to either cellular debris or blood vessels accumulating between the retina and the choroid, thereby disturbing and/or destroying the complex interplay of neurons that are present there.
  • Glaucoma
    Glaucoma is an eye disorder in which the optic nerve suffers damage, permanently damaging vision in the affected eye and progressing to complete blindness if untreated. It is often, but not always, associated with increased pressure of the fluid in the eye...

     – loss of retinal ganglion cells which causes some loss of vision to blindness.
  • Diabetic retinopathy
    Diabetic retinopathy
    Diabetic retinopathy is retinopathy caused by complications of diabetes mellitus, which can eventually lead to blindness....

     – poor blood sugar control due to diabetes damages the tiny blood vessels in the retina.


The auditory system is responsible for converting pressure waves generated by vibrating air molecules or sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...

 into signals that can be interpreted by the brain.
This mechanoelectrical transduction is mediated with hair cells within the ear. As the picture shows, depending on the movement, the hair cell can either hyperpolarize or depolarize. When the movement is towards the tallest stereocilia
In the inner ear, stereocilia are the mechanosensing organelles of hair cells, which respond to fluid motion in numerous types of animals for various functions, including hearing and balance. They are about 10–50 micrometers in length and share some similar features of microvilli...

, the K+ cation channels open allowing K+ to flow into cell and the resulting depolarization causes the Ca2+ channels to open, thus releasing its neurotransmitter into the afferent auditory nerve. There are two types of hair cells: inner and outer. The inner hair cells are the sensory receptors while the outer hair cells are usually from efferent axons originating from cells in the superior olivary complex
Problems with sensory neurons associated with the auditory system leads to disorders such as:
  • Auditory Processing Disorder
    Auditory processing disorder
    Auditory Processing Disorder , also known as Central Auditory Processing Disorder is an umbrella term for a variety of disorders that affect the way the brain processes auditory information. It is not a peripheral hearing disorder as individuals with APD usually have normal peripheral hearing...

     – auditory information in the brain is processed in an abnormal way. Patients with auditory processing disorder can usually gain the information normally, but their brain cannot process it properly, leading to hearing disability.
  • Pure word deafness
    Pure word deafness
    Pure Word Deafness is caused by bilateral damage to the posterior superior temporal lobes or disruption of connections between these areas. It exhibits itself as inability to comprehend the meaning of speech, but still being able to hear, speak, read, and write.-Presentation:The underlying...

     – comprehension of speech is lost but hearing, speaking, reading, and writing ability is retained. This is caused by damage to the posterior superior temporal lobes, again not allowing the brain to process auditory input correctly.


There are many drugs currently on the market that are used to manipulate or treat sensory system disorders. For instance, Gabapentin
Gabapentin is a pharmaceutical drug, specifically a GABA analogue. It was originally developed for the treatment of epilepsy, and currently is also used to relieve neuropathic pain...

 is a drug that is used to treat neuropathic pain by interacting with one of the voltage-dependent calcium channels present on nociceptive neurons. Some drugs may be used to combat other health problems, but can have unintended side effects on the sensory system. Ototoxic drugs are drugs which affect the cochlea
The cochlea is the auditory portion of the inner ear. It is a spiral-shaped cavity in the bony labyrinth, making 2.5 turns around its axis, the modiolus....

 through the use of a toxin like aminoglycoside
An aminoglycoside is a molecule or a portion of a molecule composed of amino-modifiedsugars.Several aminoglycosides function as antibiotics that are effective against certain types of bacteria...

 antibiotics, which poison hair cells. Through the use of these toxins, the K+ pumping hair cells cease their function. Thus, the energy generated by the endocochlear potential
Endocochlear potential
The endocochlear potential is the main resting potential in the cochlea. It is a positive direct current of 80mV which can be recorded from the endolymph with electrodes. When a sound is presented, the endocochlear potential changes either positive or negative in the endolymph, depending on the...

 which drives the auditory signal transduction process is lost, leading to hearing loss.

Plasticity (Neuroplasticity)

Ever since scientists observed cortical remapping in the brain of Taub’s Silver Spring monkeys
Silver Spring monkeys
The Silver Spring monkeys were 17 wild-born macaque monkeys from the Philippines who lived inside the Institute of Behavioral Research in Silver Spring, Maryland...

, there has been a lot of research into sensory system plasticity. Huge strides have been made in treating disorders of the sensory system. Techniques such as constraint-induced movement therapy
Constraint-induced movement therapy
Constraint-induced movement therapy is a form of rehabilitation therapy that improves upper extremity function in stroke and other Central Nervous System damage victims by increasing the use of their affected upper limb....

 developed by Taub have helped patients with paralyzed limbs regain use of their limbs by forcing the sensory system to grow new neural pathways. Phantom limb syndrome is a sensory system disorder in which amputees perceive that their amputated limb still exists and they may still be experiencing pain in it. The mirror box
Mirror box
A mirror box is a box with two mirrors in the center , invented by Vilayanur S. Ramachandran to help alleviate phantom limb pain, in which patients feel they still have a limb after having it amputated....

 developed by V.S. Ramachandran, has enabled patients with phantom limb syndrome to realign their body map, the somatosensory system’s perception of where the body is in space with physical reality. It is a simple device which uses a mirror in a box to create an illusion in which the sensory system perceives that it is seeing two hands instead of one, therefore allowing the sensory system to control the "phantom limb". By doing this, the sensory system can gradually get acclimated to the amputated limb, and thus alleviate this syndrome.

Fiber types

Peripheral nerve fibers can be classified based on axonal conduction velocity, mylenation, fiber size etc. For example, there are slow-conducting unmyelinated C fibers and faster-conducting myelinated Aδ fibers
A delta fiber
A delta fibers, or Aδ fibers, are a type of sensory fiber.They are associated with cold and pressure, and as nociceptors stimulation of them is interpreted as fast/first pain information....

. These nerve fibers work with neurons to form the nervous system

See also

  • Sensory nerves
    Sensory nerves
    Sensory nerves are nerves that receive sensory stimuli, such as how something feels and if it is painful, smooth, rough, etc.They are made up of nerve fibers, called sensory fibers .Sensory neurons are neurons that are activated by sensory input Sensory nerves are nerves that receive sensory...

  • Afferent nerve
    Afferent nerve
    In the nervous system, afferent neurons , carry nerve impulses from receptors or sense organs towards the central nervous system. This term can also be used to describe relative connections between structures. Afferent neurons communicate with specialized interneurons...

  • Pseudounipolar neuron
  • Efferent nerve
    Efferent nerve
    In the nervous system, efferent nerves, otherwise known as motor or effector neurons, carry nerve impulses away from the central nervous system to effectors such as muscles or glands...

  • Sensory system
    Sensory system
    A sensory system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory receptors, neural pathways, and parts of the brain involved in sensory perception. Commonly recognized sensory systems are those for vision, hearing, somatic...

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