sense organ

A structure which is a receptor for external or internal stimulation. A sense organ is often referred to as a receptor organ. External stimuli affect the sensory structures which make up the general cutaneous surface of the body, the exteroceptive area, and the tissues of the body wall or the proprioceptive area. These somatic area receptors are known under the general term of exteroceptors. Internal stimuli which originate in various visceral organs such as the intestinal tract or heart affect the visceral sense organs or interoceptors. A receptor structure is not necessarily an organ; in many unicellular animals it is a specialized structure within the organism. Receptors are named on the basis of the stimulus which affects them, permitting the organism to be sensitive to changes in its environment.

Photoreceptors are structures which are sensitive to light and in some instances are also capable of perceiving form, that is, of forming images. Light-sensitive structures include the stigma of phytomonads, photoreceptor cells of some annelids, pigment cup ocelli and retinal cells in certain asteroids, the eye-spot in many turbellarians, and the ocelli of arthropods. The compound eye of arthropods, mollusks, and chordates is capable of image formation and is also photosensitive. See also Photoreception.

Phonoreceptors are structures which are capable of detecting vibratory motion or sound waves in the environment. The most common phonoreceptor is the ear, which in the vertebrates has other functions in addition to sound perception. See also Phonoreception.

Statoreceptors are structures concerned primarily with equilibration, such as the statocysts found throughout the various phyla of invertebrates and the inner ear or membranous labyrinth filled with fluid.

The sense of smell is dependent upon the presence of olfactory neurons, called olfactoreceptors, in the olfactory epithelium of the nasal passages among the vertebrates. See also Olfaction.

The sense of taste is mediated by the taste buds, or gustatoreceptors. In most vertebrates these taste buds occur in the oral cavity, on the tongue, pharynx, and lining of the mouth; however, among certain species of fish, the body surface is supplied with taste buds as are the barbels of the catfish.

The surface skin of vertebrates contains numerous varied receptors associated with sensations of touch, pain, heat, and cold. See also Chemical senses; Cutaneous sensation; Sensation.

Our sensory experiences, conscious and unconscious, derive from sensory receptors distributed throughout the body. But the five main senses — sight, hearing, taste (gustation), smell (olfaction), and touch — are associated with specialized structures known as sense organs (the skin being the ‘organ’ for the classical sense of touch). In addition, a sense organ in the inner ear known as the vestibular apparatus provides our sense of balance and equilibrium. It detects movement, and gravity (strictly speaking, angular and linear acceleration of the head).

The variety of specialized sensory experiences is very much greater than the classical five. The skin, as well as being sensitive to tactile contact, can also detect and distinguish warmth and cold, vibrations at various frequencies, and a whole range of different kinds of pain, itch, and tickle. Each of these definable ‘sub-modalities’ depends on the activation of a particular type of specialized nerve ending in the skin (or of more than one type of nerve ending). Visual and auditory senses also have a range of ‘sub-modalities’, each associated with a distinctly different sensory experience — for instance colour, movement, and brightness for vision; loudness, pitch, and position in space for hearing. These different aspects of sensory experience depend not only on receptor cells with different receptive characteristics in the eye and ear, but also on the way in which information from the sense organs is processed in the brain. Indeed, much of our sensory experience is determined not by the basic characteristics of our sense organs, but by the analysis of sensory messages in the brain, especially in the sensory areas of the cerebral cortex. The perception of the distances of objects in space through stereoscopic vision is a good example. This depends on the detection of tiny differences in the two retinal images resulting from the fact that the two eyes are separated in the head and therefore view the world from slightly different angles. Obviously this striking aspect of our visual experience is not represented in the signals from either eye alone, and can be derived only in the brain, where the signals from both come together.

At the heart of all sense organs are sensory receptors — specialized nerve cells or the endings of nerve fibres, which detect particular physical or chemical events outside the cell membrane. Much of the variety of sensory experience depends on the physical characteristics of the tissues that make up the rest of the sense organs. For instance, receptors sensitive to mechanical stimulation (mechanoreceptors) have been put to an extraordinary range of tasks in the body. The hair cells in the cochlear of the inner ear are mechanoreceptors: they respond selectively to sound because the rest of the ear delivers sound energy directly to them and protects them from any other form of mechanical stimulation. The hair cells of the vestibular apparatus are very similar to those of the cochlea, but they respond to tilt or rotation of the head, not sound, because those are the physical forces to which they are exposed. All the receptors in the skin that respond to touch and vibrations, those in muscles, tendons, and joints that detect stretch and tension, and even various receptive endings in the heart, lungs, and blood vessels that signal changes in blood pressure and inflation of the chest, are basically mechanoreceptors. Their particular sensitivities are determined by where they are placed in the body.

The non-neural components of sense organs serve, then, to protect the sensory cells and also to deliver particular forms of stimulation to them. The nasal passages support the delicate olfactory epithelium and direct the airflow across it. Chewing, tongue movement, and swallowing force a flow of macerated food, dispersed in saliva, over the taste buds on the tongue. The cornea and lens of the eye ensure that the light rays are focused on the rods and cones of the retina. The eye is unique among sense organs in that it not only houses the receptor cells, forms an image on them, and directs them towards objects of interest (by means of eye movements), but it also contains a large number of other interconnected nerve cells, in the retina. These process and analyse signals from the rods and cones and then transmit processed messages to the brain, along the optic nerve.

Sense organs are our windows on the world. But, like windows, they restrict our perception to what passes through them. We blissfully imagine that, if our senses are normal, we know everything that there is to know of the world around us. But our environment is full of physical energy and chemicals that our sense organs cannot detect. Many other animals have sense organs that can detect stimuli beyond the confines of the human senses. Our wonderful vision is restricted to the narrow band of wavelength within the electromagnetic spectrum that our photopigments can catch, and our eyes are blind to the ultraviolet and infra-red wavelengths that lie just beyond this visible band of the spectrum. But the eyes of many invertebrates, fish, and birds can detect ultraviolet light — male and female blue tits distinguish each other by brilliant ultraviolet feathers whose ‘colour’ is invisible to our eyes. The family of snakes called pit vipers, which includes rattlesnakes, have a second set of ‘eyes’, in the form of pinhole cameras set in the cheeks, each consisting of a cavity lined with thousands of heat receptors. Like a thermal imaging camera, these ‘pit organs’ sense infra-red radiation, enabling the snake to detect the positions of warmblooded prey in its vicinity. The ears of even a young child can detect frequencies only between about 50 and 20 000 cycles per second (Hz) ; but natural events, from thunderstorms to snapping twigs, generate much higher and lower frequencies. Whales and pigeons can hear frequencies of sound far below the capacity of the human ear. And many animals can detect sound frequencies up to 100 000 Hz: the vocal production and detection of such ultrasound is the basis of the radar-like echo location of bats.

Even more impressive (and more humbling to human beings) are sensory capacities found in many animals that differ from ours not just in degree but in kind. For instance, bees and many other insects can detect the plane of polarization of light (the axis of vibration of the light photons). This enables them to recognize the position of the sun, even when below the horizon or partially obscured by cloud, and hence it helps them to navigate. Pigeons (and probably many other animals) have magnetic sensory receptors through which they can detect the direction of the earth's magnetic field. A homing pigeon with a small bar magnet attached to the back of its head takes much longer to fly home! And certain fish have electroreceptive organs that are sensitive to weak electric fields in the water around them. Some emit electric pulses and use them to communicate. Others, such as sharks, use their electroreceptors when they attack other fish, sensing the minute electrical fields emitted by the gills of their prey.

— Frances M. Ashcroft, Colin Blakemore

See also eyes; hearing; proprioception; sensory receptors; somatic sensation; taste and smell; vestibular system; vision.

An organ in the body containing cells that respond to particular external and internal stimuli. Messages from a sense organ are conveyed by sensory neurones to the central nervous system where they are processed and where perception takes place.

Typical sensory system: the visual system, illustrated by the classic Gray's FIG. 722– This scheme shows the flow of information from the eyes to the central connections of the optic nerves and optic tracts, to the visual cortex. Area V1 is the region of the brain which is engaged in vision.

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 sensation (touch), taste and olfaction (smell). In short, senses are transducers from the physical world to the realm of the mind.

The receptive field is the specific part of the world 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.^[1] Receptive fields have been identified for the visual system, auditory system and somatosensory system, so far.

Contents 1 Stimulus 2 Modality 2.1 V1 (vision) 2.2 A1 (auditory - hearing) 2.3 S1 (somatosensory - touch and proprioception) 2.4 G1 (gustatory - taste) 2.5 O1 (olfactory - smell) 3 Human sensory system 4 See also 5 References

Stimulus

Sensory systems code for four aspects of a stimulus; type (modality), intensity, location, and duration. Arrival time of a sound pulse and phase differences of continuous sound are used for localization of sound sources. Certain receptors are sensitive to certain types of stimuli (for example, different mechanoreceptors respond best to different kinds of touch stimuli, like sharp or blunt objects). Receptors send impulses in certain patterns to send information about the intensity of a stimulus (for example, how loud a sound is). The location of the receptor that is stimulated gives the brain information about the location of the stimulus (for example, stimulating a mechanoreceptor in a finger will send information to the brain about that finger). The duration of the stimulus (how long it lasts) is conveyed by firing patterns of receptors.

Modality

A stimulus modality (sensory modality) is a type of physical phenomenon that can be sensed. Examples are temperature, taste, sound, and pressure. The type of sensory receptor activated by a stimulus plays the primary role in coding the stimulus modality.

In the memory-prediction framework, Jeff Hawkins mentions a correspondence between the six layers of the cerebral cortex and the six layers of the optic tract of the visual system. The visual cortex has areas labelled V1, V2, V3, V4, V5, MT, IT, etc. Thus Area V1 mentioned below, is meant to signify only one class of cells in the brain, for which there can be many other cells which are also engaged in vision.

Hawkins lays out a scheme for the analogous modalities of the sensory system. Note that there can be many types of senses, some not mentioned here. In particular, for humans, there will be cells which can be labelled as belonging to V1, V2 A1, A2, etc.:

V1 (vision)

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

Visual Area 1, or V1, is used for vision, via the visual system to the primary visual cortex.

ear

A1 (auditory - hearing)

Auditory Area 1, or A1, is for hearing, via the auditory system, the primary auditory cortex.

S1 (somatosensory - touch and proprioception)

Somatosensory Area 1, or S1, is for touch and proprioception in the somatosensory system. The somatosensory system feeds the Brodmann Areas 3, 1 and 2 of the primary somatosensory cortex. But there are also pathways for proprioception (via the cerebellum), and motor control (via Brodmann area 4).

tongue

G1 (gustatory - taste)

Gustatory Area 1, or G1, is used for taste.

O1 (olfactory - smell)

Olfactory Area 1, or O1, is used for smell. 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 sensory system

The Human sensory system consists of the following sub-systems:

• Visual system consists of the photoreceptors, optic nerve, and V1.
• Auditory system
• Somatosensory system consists of the receptors, transmitters (pathways) leading to S1, and S1 that experiences the sensations labelled as touch or pressure, temperature (warm or cold), pain (including itch and tickle), and the sensations of muscle movement and joint position including posture, movement, and facial expression (collectively also called proprioception).
• Gustatory system
• Olfactory system

Human sensory receptors are:

• Chemosensor
• Mechanoreceptor
• Nociceptor
• Photoreceptor
• Thermoreceptor

See also

• Sensor
• Sensory neuroscience

References

1. ^ Kolb & Whishaw: Fundamentals of Human Neuropsychology (2003)

Links to related articles v • d • e Nervous system (TA A14, GA 9) Central nervous system Meninges Spinal cord Brain: Rhombencephalon (Medulla oblongata, Pons, Cerebellum) • Mesencephalon • Prosencephalon (Diencephalon, Telencephalon) Peripheral nervous system Somatic Sensory nerve • Motor nerve • Cranial nerves • Spinal nerves Autonomic Sympathetic • Parasympathetic • Enteric nervous system central nervous system navs: anat/physio/dev, noncongen/congen/neoplasia, symptoms+signs/eponymous, proc peripheral nervous system navs: anat/histo/physio/dev, noncongen PNS somatic/autonomic/congen/neoplasia, symptoms+signs/eponymous, proc v • d • e Nervous system: Sensory systems / senses (TA A15) Special senses Visual system/sight Auditory system/hearing Chemoreception (Olfactory system/smell • Gustatory system/taste) Touch Pain • Heat • Balance (Equilibrioception) • Mechanoreception (Pressure, vibration, proprioception) Other Sensory receptor v • d • e Sensory system: Visual system and eye movement Vision Retina → Optic nerve → Optic chiasm → Optic tract → Thalamus (Lateral geniculate nucleus) → Optic radiation → Visual cortex (Cuneus, Lingual gyrus) → Blobs → Globs Muscles of orbit Tracking Smooth pursuit: Parietal lobe · Occipital lobe Saccade: Frontal eye fields Physiologic nystagmus → Fixation reflex → PPRF Horizontal gaze Abducens nucleus → PPRF → MLF → Oculomotor nucleus → Medial rectus muscle Vertical gaze Rostral interstitial nucleus → Oculomotor nucleus, Trochlear nucleus → Muscles of orbit Vestibulo-ocular reflex Semicircular canal → Vestibulocochlear nerve → Vestibular nuclei → Medial rectus muscle Pupil Pupillary dilation Ciliospinal center → Superior cervical ganglion → Long ciliary nerves → Iris dilator muscle Pupillary light reflex (constriction) Retina → Optic nerve → Optic chiasm → Optic tract → Pretectal area → Edinger-Westphal nucleus → Oculomotor nerve → Ciliary ganglion → Short ciliary nerves → Iris sphincter muscle Accommodation Retina → Optic nerve → Optic chiasm → Optic tract → Visual cortex → Brodmann area 19 → Pretectal area → Edinger-Westphal nucleus → Short ciliary nerves → Ciliary ganglion → Ciliary muscle Circadian rhythm Retina → Hypothalamus (Suprachiasmatic nucleus) eye navs: anat/adnexa anat/pathways/physio/dev, noncongen/congen/neoplasia, eponymous signs, proc v • d • e Sensory system: Auditory and Vestibular systems (TA A15.3, GA 10.1029) Auditory Outer ear Pinna (Helix, Antihelix, Tragus, Antitragus, Earlobe) • Ear canal Middle ear Eardrum (Umbo) • Ossicles (Malleus, Incus & Stapes) • muscles (Stapedius, Tensor tympani) • Eustachian tube (Torus tubarius) Inner ear/Labyrinth Vestibule • Membranous labyrinth Oval window → Scala vestibuli / Reissner's/vestibular membrane, Helicotrema and Scala tympani / Basilar membrane (Perilymph) → Round window Cochlea: Modiolus • Cochlear duct / scala media (Endolymph, Stria vascularis, Spiral ligament), Olivocochlear system Organ of Corti: Stereocilia • Tectorial membrane • Sulcus spiralis (externus, internus) • Limbus spiralis To brain Hair cells → Spiral ganglion → Cochlear nerve VIII → Cochlear nuclei → Trapezoid body → Superior olivary nuclei → Lateral lemniscus → Inferior colliculi → Medial geniculate nuclei → Acoustic radiation → Primary auditory cortex Vestibular Inner ear/Labyrinth Static/translations: Utricle (Macula) · Saccule (Macula, Endolymphatic sac, Endolymphatic duct) · Kinocilium · Otolith Kinetic/rotations: Semicircular canals (Superior, Posterior, Horizontal) • Cupula • Ampullae (Crista ampullaris) To brain Vestibular nerve VIII → Vestibular nuclei, Flocculonodular lobe Vestibular nuclei → Vestibulospinal tract, Ventral posterolateral nucleus → Vestibular cortex ear navs: anat/physio/dev, noncongen/congen, eponymous signs, proc v • d • e Sensory system: Gustatory system (TA 15.4, GA 10.991) Tongue Taste bud (Circumvallate papillae, Filiform papilla, Fungiform papilla) · Foliate papillae Path CN (VII, IX, X) → Solitary nucleus (Gustatory nucleus) → Central tegmental tract → Ventral posteromedial nucleus → Gustatory cortex Other Basic tastes v • d • e Sensory system: Olfactory system / Olfaction / Rhinencephalon Path Olfactory epithelium / Olfactory receptor / Olfactory mucosa / Olfactory receptor neurons → Olfactory nerve → Olfactory bulb (Mitral cells, Glomeruli) Olfactory tract → Piriform cortex → Entorhinal cortex → Amygdala → Brodmann area 34 Region Olfactory trigone v • d • e Systems and systems science Systems categories Systems theory · Systems science · Systems scientists (Conceptual · Physical · Social) Systems Biological · Complex · Complex adaptive · Conceptual · Database management · Dynamical · Economical · Ecosystem · Formal · Global Positioning System · Human anatomy · Information systems · Legal systems of the world · Systems of measurement · Metric system · Multi-agent system · Nervous system · Nonlinearity · Operating system · Physical system · Political system · Sensory system · Social structure · Solar System · Systems art Theoretical fields Chaos theory · Complex systems · Control theory · Cybernetics · Living systems · Sociotechnical systems theory · Systems biology · System dynamics · Systems ecology · Systems engineering · Systems psychology · Systems science · Systems theory Systems scientists Russell L. 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