The senses

The body senses through receptors that turn a stimulus into a signal the brain can read. The receptors of the general senses, such as pain, are scattered through the body and are told apart by what they respond to. The special senses sit in the eye, the ear, the nose and the tongue, and each turns light, sound or chemicals into that signal in its own way.

Sensory systems

Sensory systems are responsible for hearing, sight, touch, pain, taste and smell. ¹ Through the senses, people transduce information from one modality, such as sound, into another, the action potential, to perceive the external and internal environments. ² In each sensory system, highly specialised receptors transduce the stimulus, such as light, sound or food, into a perceptible event. ¹ The sensory systems contain highly specialised sensory neurons that send signals to the brain about both the type of stimulus and its strength. ²

General and special senses

Receptors for the general senses, such as pain, cold, heat and touch, are scattered throughout the body. ³ The special senses, by contrast, are located in relatively small and specialised areas, such as the eyes, the inner ears and the tongue. ⁴

Sensory receptors

Receptors may be grouped into exteroceptors, interoceptors and proprioceptors. ³ Exteroceptors sense changes in the external environment. ⁵ Interoceptors are cells in internal organs that respond to stimuli within the body, such as blood pressure. ⁶ Proprioceptors are cells in muscles, tendons and other organs that respond to internal stimuli about body position and movement. ⁷ A more detailed breakdown groups receptors into nociceptors, chemoreceptors, thermoreceptors, mechanoreceptors and photoreceptors. ^{3, 8}

A receptor may be a free nerve ending, an encapsulated nerve ending, an accessory cell or a modified neuron. ³ Differences in membrane structure account for differences in receptor sensitivity, and each receptor responds best to certain stimuli and not to others. ³

Chemoreceptors respond to water-soluble and lipid-soluble compounds that contact their cell membranes. ⁹ Diffuse chemoreceptive neurons in the central nervous system respond to changes in the pH and carbon dioxide concentration of the cerebrospinal fluid, and those along arterial trunks monitor the oxygen content of the blood. ⁹ Taste buds are an example of chemoreceptors, specialised cells that sense chemical stimuli. ¹⁰

Thermoreceptors detect changes in temperature. ¹¹ Warm receptors outnumber cold receptors. ⁹ Thermoreceptors have a phasic response to stimuli, which allows humans to adapt quickly to a new environmental temperature. ⁹ Their anatomy is not well understood. ⁹

Photoreceptors are the specialised cells in the eye that respond to changes in light, colour and movement. ^{9, 12}

Mechanoreceptors respond to mechanical pressures or physical distortions of the cell membrane, such as those created by sound, touch and muscular contractions. ^{9, 13} They range from free nerve endings to complex structures. ⁹ Tactile receptors provide touch, pressure and vibration sensations, baroreceptors monitor pressures within the walls of blood vessels, and proprioceptors report on the positions of joints and muscles. ⁹

Touch and body position

Information about touch, pain and temperature is transmitted by the somatosensory nervous system. ¹⁴ Touch receptors in the skin, also called mechanoreceptors, allow people to determine the size, shape and texture of objects. ¹⁴ Specialised classes of mechanoreceptors respond preferentially to light touch, to textured surfaces moving over the skin and to constant pressure. ¹⁴

Different parts of the body differ in how finely they discriminate touch. ¹⁴ On two-point discrimination, a test of how well a person can distinguish two items touching the skin, the fingers have the highest sensitivity. ¹⁴ The trunk and proximal limbs discriminate touch poorly: on the trunk, two stimuli must be 3 to 4 cm apart to be perceived as distinct. ¹⁴

Proprioceptors are internal mechanoreceptors that give information about where the body and limbs are located in space. ¹⁴ Those associated with skeletal muscle give information about muscle length, and those associated with tendons give information about muscle tension. ¹⁴ Movements that require a high degree of precision, such as fine manipulation with the fingers, correlate with a higher density of proprioceptors in the corresponding muscles. ¹⁴

Pain

Nociceptors are a class of skin receptor, different from the touch receptors, that transmits pain information. ¹⁴ They are free nerve endings sensitive to temperature, mechanical trauma and chemicals in extracellular fluid, and they produce prickling and burning sensations. ^{9, 8} Their subtypes transmit pain information that originates from different sources, such as noxious heat, chemicals or touch. ¹⁴ The brain interprets the signals from nociceptors to form a fuller experience of, and response to, the painful stimulus. ¹⁴ As with touch, different parts of the body have different sensitivities to painful stimuli. ¹⁴

Very few nociceptors are solely responsible for transmitting pain information from the internal organs, the viscera. ¹⁴ Visceral pain information is usually transmitted by neurons that also carry information from skin nociceptors. ¹⁴ The colocation of the two types of pain information results in referred pain: pain signals originating in the heart, for example, are perceived as pain in the arm. ¹⁴

Patients with the rare hereditary sensory and autonomic neuropathy type 5 report insensitivity to pain and touch. ¹⁵ The lack of pain is caused by mutations in the gene for neuronal growth factor (NGF) that leave the protein unable to function properly. ¹⁵ NGF is essential for the development and survival of the neurons that transmit pain, touch and temperature, and certain NGF mutations lead to the absence of these neurons. ¹⁵ Unable to receive information about potentially damaging stimuli and dull pain, affected patients suffer repeated trauma to the joints, skin and other tissues. ¹⁵ Pain insensitivity is rare but very dangerous. ¹⁵

NGF has a prominent role in the onset and development of acute and chronic pain states, because it binds TrkA receptors. ¹⁶ In the adult nervous system, TrkA receptors are expressed on the sensory nerves that transmit pain and touch information. ¹⁶ Volunteers who received NGF injections have reported enhanced responses to pain. ¹⁶

The structure of the eye

Humans rely most heavily on vision to transmit information about their surroundings. ¹⁷

The eye has three layers, or tunics: a fibrous tunic, a vascular tunic and a neural tunic. ^{18, 9} The fibrous tunic comprises the sclera and the cornea. ¹⁹ The sclera is the tough, opaque white overcoat of the posterior of the eye. ^{19, 20} The cornea is the transparent outer portion of the eye, through which light passes. ^{19, 21}

The vascular tunic contains the iris, the ciliary body and the choroid. ¹⁹ The iris is the circular, coloured part of the eye, suspended in the aqueous humor and perforated by the pupil. ²² The ciliary body is the thick part of the vascular tunic that joins the iris and the choroid. ¹⁰ The choroid is a very vascular membrane between the retina and the sclera, and it contains the blood supply for the neural tunic. ^{10, 19} Its blood vessels give the inner eye a dark colour, which prevents light reflection within the eye that would lower visual acuity. ¹⁹

The neural tunic contains the retina, the multilayered light-sensitive layer of the eye. ^{19, 23}

The eye can also be divided into an anterior chamber and a posterior chamber. ¹⁹ The anterior chamber is the third of the eye in front of the lens. ^{19, 9} It contains the aqueous humor, a clear, watery fluid, together with the cornea, iris, ciliary body and lens. ^{9, 24} The posterior chamber is the two-thirds of the eye behind the lens. ⁹ It contains the vitreous humor, a transparent, semigelatinous substance, together with the retina, choroid and optic nerve. ^{9, 25}

Focusing and refraction

Light passes through the cornea, the lens and the vitreous gel and is absorbed by the photoreceptors in the retina at the back of the eye. ^{17, 26} Light enters through the cornea and the pupil, which can contract and relax like a camera shutter to let in more or less light as needed for clear focusing of an image on the retina. ⁹ The lens then focuses the image onto the retina. ^{9, 22} It is a transparent, crystalline structure attached to the ciliary body by a ring of suspensory ligaments and the ciliary muscle. ^{22, 9}

Distant objects require no change in the shape of the lens, but close objects require the ciliary muscle to contract, pull on the suspensory ligaments and change the shape of the lens. ⁹ This muscle action is part of the reason that reading text tires the eyes more than gazing at the horizon for the same length of time. ⁹

Vision occurs when light focuses on the retina, and clear vision requires clear refraction through the cornea, the lens and the humors. ²⁷ For several reasons, refraction may not be clear, which leads to one of the common refraction errors: ^{27, 9}

• Presbyopia: the inability to focus on nearby objects because of an inflexible lens
• Myopia: the inability to focus on distant objects because of a long eyeball
• Hyperopia: the inability to focus on nearby objects because of a short eyeball
• Astigmatism: blurry vision because of irregularities in the cornea, the lens or both

²⁷

These categories are general, and many other factors, including disease, age and genetics, can be involved. ²⁷ As humans age, the lens becomes less flexible, which leads to presbyopia. ⁹

The retina

The retina's photoreceptors are of two kinds, rods and cones. ^{9, 17} Rods are pigment-containing, very sensitive photoreceptors for movement and shades of grey, and they are responsible for seeing in low light, such as at night. ^{23, 17, 9} Cones are the light-sensitive cells responsible for colour and visual sharpness, and they allow humans to see detailed images. ^{21, 17, 9} There are three types of cone, each most sensitive to a particular colour: red, green or blue. ¹⁷

Rods and cones generate a graded neural potential that is proportional in size to the amount of light absorbed. ¹⁷ This response differs from the traditional all-or-none action potential. ¹⁷ The photoreceptors send electrical signals to retinal neurons, which connect to the visual centres of the brain. ¹⁷ The impulses travel along the optic nerve to the part of the brain that interprets vision. ⁹ The visual image arrives at the retina upside down, and the brain adjusts for this without any conscious knowledge of the adjustment. ⁹

The density of photoreceptors is greatest in the fovea, the central part of the retina. ^{17, 26} The fovea is also the area of the retina on which the lens focuses incoming light, so it is crucial for seeing detail. ¹⁷ It is responsible for central vision. ²⁶ An image is formed in part by the amount of light focused on a particular photoreceptor and in part by the amount focused on its neighbours. ¹⁷ The brain structures for vision take the retinal neurons' information about the quantity of light absorbed and process it to extract shape, movement and colour. ¹⁷

Age-related macular degeneration

Age-related macular degeneration (AMD) is the most common cause of vision impairment in adults older than 50 years. ²⁸ Vision is lost when the retinal pigment epithelium, the non-neuronal layer of the retina, begins to degenerate, which leads to degeneration of the neighbouring layer of rods and cones. ²⁸ This degeneration occurs only in the macula, the region near the centre of the retina that includes the fovea. ²⁸ So AMD results in the loss of detailed central vision and the preservation of peripheral vision. ²⁸ To a person with AMD, a scene might appear with the central vision lost and the remaining peripheral vision blurred. ²⁶

As of 2014, there was no cure for AMD, but therapies existed for "wet" AMD, in which abnormal blood vessels growing behind the epithelium leak blood or fluid that damages the photoreceptors. ²⁸

The ear

The ear is responsible for both hearing and equilibrium. ²⁷ Hearing and balance are very important senses that can be underappreciated until they are not functioning normally. ²⁹

The external ear consists of the pinna, the outer visible part of the ear or earflap, and the external acoustic meatus, or ear canal. ^{27, 30} The ear canal ends at the tympanic membrane, or eardrum, a thin, semitransparent membrane that separates the outer ear from the middle ear. ^{27, 31}

The middle ear holds three auditory ossicles, or ear bones: the malleus (hammer), the incus (anvil) and the stapes (stirrup). ²⁷ The ossicles are attached to the tympanic membrane and to the oval window of the inner ear. ²⁷ The eustachian tube, a tube lined with mucous membrane that connects the nasopharynx to the middle ear, opens into the middle ear and equalises pressure on both sides of the tympanic membrane. ^{27, 32}

The inner ear holds the cochlea, the vestibule and the semicircular canals. ²⁷ The cochlea is a snail-shaped structure whose sensory cells are stimulated by sound waves, and it contains the organ of Corti, the actual organ of hearing. ^{21, 27} The vestibule contains the organs of equilibrium, housed in two chambers, the saccule and the larger utricle. ^{27, 33, 34}

Hearing

The auditory system facilitates communication with the world by allowing humans to perceive sounds and interpret their significance. ²⁹ Sound travels through the air as a series of pressure waves, which are funnelled through the external and middle ear to the tympanic membrane. ^{27, 29} The membrane vibrates the ossicles, which amplify the degree of motion and convey the movement to the oval window. ^{27, 29} The oval window is a thin membrane at an oval opening in the inner ear, attached to the base of the stapes, and it transmits the vibrations from the tympanic membrane to the cochlea. ^{35, 29}

The vibrations of the oval window pass through the fluid of the inner ear into the cochlea, where sound energy is converted into neural signals. ^{27, 29, 26} The moving fluid distorts the cochlea's sensory receptors, the hair cells, by pushing them against the tectorial membrane. ^{27, 26} The tectorial membrane is a gelatinous membrane that overlies the spiral organ and connects with the hair cells. ³⁶ In response to the fluid movement, the hair cells transmit electrical signals to cochlear neurons. ²⁶ Sensory neurons on the tectorial membrane pick up the vibrations and pass them along the cochlear branch of the acoustic nerve to the brain, which interprets them as voices, noise or music. ^{27, 37}

The cochlea has been described as a frequency analyser, because neurons at different points along its length, as though the snail shell were unwound, respond maximally to different sound frequencies, or pitches. ²⁶ This frequency organisation is maintained throughout many of the auditory brain structures. ²⁶ Cochlear neurons transmit information about the intensity, or loudness, of a sound to central auditory regions in part by the number of action potentials they generate. ²⁶

Sensorineural hearing loss

Patients with sensorineural hearing loss (SNHL) have lost the function of most cochlear hair cells, although some cochlear neurons remain and are functional. ³⁸ Their severe hearing impairment is not helped by hearing aids, which amplify sound and rely on a complete sensory pathway in the cochlea. ³⁸ Cochlear implants, devices surgically implanted into the cochlea, can restore sound perception to most patients with SNHL. ³⁸ The implants deliver electrical impulses directly to the remaining cochlear neurons, with the goal of mimicking the input the dysfunctional hair cells would have received. ³⁸

Balance

The vestibular system has a primary role in the senses of balance and body position. ³⁸ Like the auditory system, it relies on fluid movement to initiate action potentials. ³⁸ The two systems share a fluid connection, which is evident when vertigo, a vestibular disorder, accompanies an ear infection, an auditory disorder. ^{38, 39} The vestibular system has two main components, the semicircular canals and the otolith organs. ³⁹

The three semicircular canals are housed in the inner ear but play no part in hearing. ³⁷ They are bony, fluid-filled loops concerned with sensing and maintaining balance, and they are arranged so as to respond to movement in a single plane. ^{20, 37} Together they detect rotations of the head in three dimensions. ³⁹ Fluid within the inner ear distorts hair cells that pick up messages about gravity, acceleration or position in space and pass them along the vestibular branch of the acoustic nerve to the brain for interpretation. ³⁷ When the body is stationary, the vestibular neurons that innervate the semicircular canals generate a constant rate of action potentials. ³⁹ So they transmit the speed and direction of a rotation by increasing or decreasing that rate. ³⁹

The otolith organs detect head tilt and linear movement, such as the sensation of riding an elevator. ³⁹

Smell

The senses of taste and smell are closely related, and they are responsible for most human interaction with chemicals in the environment. ⁴⁰ The nose is part of both the respiratory system and the olfactory system. ⁴¹ Its olfactory receptors lie in the olfactory mucosa of the nasal cavity. ⁴¹ They are chemoreceptors, and they are very sensitive but become fatigued very easily. ⁴¹ The receptors are almost hidden in the nasal cavity, away from the general inflow of air during breathing, so humans do not have as good a sense of smell as many other animals. ⁴¹ To get a good whiff of a smell, people usually inhale deeply or sniff repeatedly. ⁴¹

Smell occurs when molecules strike the olfactory cilia of the olfactory cells. ⁴¹ These cells are embedded in the mucous membrane lining the nasal cavity and respond to chemical particles dissolved in the mucus. ⁴¹ The resulting impulse passes along the olfactory nerve to the brain for interpretation. ⁴¹

An odour is detected when an odorant, an odour molecule, comes into contact with the olfactory epithelium, the sheet of cells lining the inside of the nose. ⁴⁰ Sensory cells housed within the olfactory epithelium have receptors that cause a neural response when certain odorants bind them. ⁴⁰ The particular pattern of neural responses arising from different olfactory cells provides a neural signature that is unique to each odorant. ⁴⁰ Exposure to certain odorants can cause physiological responses, such as salivation and hormonal fluctuations. ⁴⁰ The olfactory system is one of the least studied sensory systems in humans, so much less is known about its organisation and function. ⁴⁰

Smell does have a survival component, and it is not a sense concerned only with esoteric things such as flowers and perfume. ⁴² People without a sense of smell are unable to note changes in the environment, such as smoke, toxic chemicals, fire or food gone rancid. ⁴²

Taste

Taste is the sense that occurs in response to the contact of dissolved material with specialised nerve receptors. ⁴³ The receptors are the taste buds, found mainly on the tongue, where they sit on papillae, but also scattered in the pharynx and larynx. ^{43, 40} Taste buds are special sensory nerve endings that respond to different materials. ⁴⁴ Each taste bud contains gustatory cells with sensory microvilli, or taste hairs. ⁴³ Each person is estimated to have 5,000 to 10,000 taste buds. ⁴⁰

When a tastant, a chemical in food, interacts with taste buds, neural signals are transmitted along the gustatory neuron to the brainstem. ⁴⁰ Taste impulses are sent along cranial nerves VII, IX and X to the brain for interpretation. ⁴³ Different locations on the tongue have higher sensitivities to different classes of tastant, but it is a common misconception that certain areas of the tongue are used exclusively to perceive certain tastes. ⁴⁰

A 2007 account counts five taste sensations: sweet, sour, salt, bitter and umami. ⁴³ Umami is the savoury taste imparted by glutamate and by ribonucleotides, including inosinate and guanylate, which occur naturally in many foods such as meat, fish, vegetables and dairy products. ³³ A 2014 account names flavours such as salt, sweet, bitter and sour as those human taste sensation primarily detects, and places umami among other tastes humans can perceive, such as fatty and pungent. ⁴⁰

Much of the sensation of taste is due to the central integration in the brain of both gustatory and olfactory information. ⁴³ Some tastes, such as chocolate, are a combination of the five taste sensations of the 2007 account, plus smell. ⁴³ A much larger range of chemicals can be detected through smelling than through tasting. ⁴⁰