Sensory Reception

(M. Waugh, M. Lavigne, D. Facey)

(presented by Saint Michael's College, Colchester, VT  USA )

Animals respond to the messages they receive from the world aound them. Their reactions to the outside world depend on how the data collected from their surroundings are correctly coded into signals that can be received and processed by neurons in the brain. The sensory organs provide the only means of communication from the environment to the nervous system. Sensations arise when signals are detected by sensory receptor cells are transmitted through the nervous system to the designated part of the brain. Various organs and cells are designated to receive specific stimuli.  The major categories of sensory reception addressed here are chemoreception, mechanoreception, and photoreception.


What are the general properties of sensory reception and how are these messages transmitted to the central nervous system?

Animals require a constant detection of information from their surroundings. Such information is the animal’s link to the outside world. Sensory input is initially detected by sensory receptors. Some receptors are very complex, with many individual receptors along with other structures being organized into sensory organs such as the vertebrate eye.

Sensory receptors are transducers; they convert stimuli into electric signals. In most cases, they do not directly generate action potentials.  Instead, sensory receptors generate receptor potentials, which vary in intensity with the intensity of the stimulus.   These changes in membrane potential are passed to adjacent sensory neurons, which may generate an action potential if the incoming stimuli are sufficient for the neuron to reach threshold (see section on Communication - the nervous system for further details about how this occurs).  Increases in receptor potential intensity are translated into a higher frequency of action potentials in the sensory neurons.

Sensory receptors are specialized to respond to only certain stimuli, which will activate the receptor with weak or moderate levels of intensity. The signal is then chemically amplified within the receptor cells. In order for the signal to be effective the intracellular chemical signal must cause membrane channels to open. This produces an electrical signal that will be transmitted to the central nervous system.


How can sensory systems detect such a broad range of stimuli intensity?

The sensitivity range of a sensory organ is much broader than the range of a single receptor cell. This is because individual afferent fibers of the sensory system cover different parts of the sensitivity spectrum. For example, only the most sensitive receptor cells will respond to a low level stimulus.  As the stimulus intensity continues to increase, the receptors become fully activated (saturated), but a group of less sensitive receptor becomes stimulated.  This recruitment of additional receptors continues as the stimulus intensity increases, until all receptors are fully saturated.  This subdivision of the total range of response by receptor cells of different sensitivities is called range fractionation because individual receptors cover only a fraction of the total range of the sensory system.  When all receptors are fully active, the system is not capable of detecting any further increase in stimulus intensity.  

Some sensory systems (receptors and their neurons) generate a rather constant "background" rate of action potentials.  If stimulated further, the rate of action potential generation increases due to increase levels of depolarization of the neurons.  Therefore, the system does not rely on a minimal level of sensory input in order to respond, which  makes the system more sensitive.


What is chemoreception?

Chemoreception is the ability to perceive specific molecules in the air or in water. These molecules are important clues to the presence of specific objects in the environment. It is essential to many animals in finding food, locating a mate, and avoiding danger.

Chemoreception is divided into two main categories: gustation (taste) and olfaction (smell). Gustatory receptors respond to dissolved molecules that come in contact with the receptors. Olfactory receptors respond to airborne molecules from sources a distance away.


What are the differences in chemoreception in invertebrates and in vertebrates?

Vertebrates detect chemicals using general receptors and two types of specialized receptors, gustatory and olfactory. Many aquatic vertebrates have generalized chemical receptors scattered over their body surface. Vertebrates usually accomplish chemoreception by moving chemically rich air or water into a canal or sac that contains the chemical receptors.

Chemoreception is much different in invertebrates than in vertebrates. For example, planarians find food by following chemical gradients in their surroundings. Their simple chemoreceptors are found in pits on their bodies, over which they move water with cilia. Insects have chemoreceptors in their body surface, mouthparts, antennae, forelegs, and, in some cases, the ovipositor. Moths, for example, smell with thousands of sensory hairs on their antennae. About 70 percent of the adult male receptors are made to respond to one molecule called bombkyol, a sex attractant released by females of the species. The molecules enter the tiny pores of the hair, or sensillum, where the olfactory receptors are found.


What are the olfactory (smell) mechanisms?

The receptors for the olfactory nerves are located in the upper part of the nasal cavitity. The olfactory sense organ consists of hair-like cells at the end of a neuron and is simple compared to the complex visual and auditory organs. The olfactory receptors are very sensitive to stimuli; however, they also become very fatigued. This explains why odors seem to go away after being easily noticeable. Canals lined with sheets of receptors with the nasal cavity are called turbinates. Protruding from the end of the nerve are thin cilia that are covered by mucus. Molecules are absorbed into the mucous layer and passed to the cilia where the chemical is detected. Notice the chemicals must be dissolved in the mucus and absorbed in order for the olfactory receptors to react. This is a lot like the gustatory mechanisms.


What are the gustatory (taste) mechanisms?

The receptors for the gustatory nerves are known as taste buds located on the tongue and the roof of the mouth. Sweet, sour, bitter, and salty are the four basic taste sensations resulting from stimulation of the taste buds and the stimulation of the olfactory receptor. This is why it is harder to taste when one has a cold. These four basic tastes may evolutionarily developed to show some basic food properties. Sweet taste signals foods high in calories, salty foods signal for food that helps maintain water balance, sour tastes may help to signal foods that could be dangerous if eaten in excess, and bitter taste sensations signal toxic foods.

Taste is also referred to as contact chemoreception for obvious reasons. For example, insects have contact receptors called taste hairs or sensilla. At the tip of each sensillum is a tiny pore that allows molecules to reach the sensory cells. Each cell is sensitive to a different chemical.  Sensilla can be located in a variety of locations on the body.  Flies, for example, have sensilla on their tarsi (feet).


What is mechanoreception?

Mechanoreception is sensing physical contact on the surface of the skin or movement of the surrounding environment (such as sound waves in air or water). The simplest mechanoreceptors are nerve endings of skin’s connective tissue. The most complex example of mechanoreception occurs in the middle and inner ear of vertebrates. The hair cell is the basic unit of vertebrate mechanoreception.


What are the structural mechanisms of the vertebrate ear?

Sound waves enter the external ear of a vertebrate aided by the pinna and the tragus. The entire external structure has a function similar to that of a funnel, amplifying and then concentrating sound waves. Vibrations from sound waves cause changes in air pressure, which travel from the external ear, down the auditory canal, and then move the eardrum (tympanum). This energy is then conducted through the malleus, incus, and stapes, the three small bones that constitute the rest of the middle ear. These three bones are key in the conversion from airborne vibrations to fluid movements. Beneath the stapes is a membrane called the oval window, which opens into the choclea of the spiral shaped, fluid filled inner ear. This entire process serves to amplify sound stimuli up to 22 times before it reaches the cochlea.


How does the ear then change vibration waves to mechanical sound?

The ear converts energy of sound into nerve impulses. This process begins at the tympanic membrane. The vibrations that move the eardrum, and then consequently the three additional bones of the middle ear, are transmitted to the oval window. These vibrations in turn move the fluid of the cochlea. The cochlea is divided into three longitudinal chambers. The two outer chambers are called the scala tympani, and the scala vestubuli, and they are both filled with a liquid perilymph that contains high sodium concentrations. The scala media is the compartment located between these outer two chambers. The scala media is filled with a fluid endolymph that had high concentrations of potassium. It also contains the organ of corti.

The sound vibrations that pass by the oval window into the chochlear chambers and vibrate the tectorial and basilar membranes, eventually dissipate through the membrane of the round window.

The floor of the chochlea contains the previously mentioned basilar membrane, and the scala media, containing the organ of corti is where these vibrations undergo the conversion to neuronal impulses. The organ of corti contain sensory hair cells, and the waves of fluid in the cochlea press the hair cells against an overhanging tectorial membrane, and then pull them away. These hair cells are just across synapses from sensory neurons, and this action provides a stimulus that opens sodium channels in the sensory cell membranes. This provides for an action potential in the environment of high potassium concentrations that the endolymph has. Auditory nerves located in a spiral ganglion carry the action potential to the brain.

The frequency of impulses from action potentials relays information on sound to the brain. The louder a sound is the greater height or amplitude of the vibrations in the sound wave, the more movement of hair cells, and thus the more action potentials. Pitch can be distinguished through differences in sound wave frequencies. Different areas of the basilar membrane are sensitive to different pitches due to different levels of flexibility along the membrane. Higher frequencies stimulate the basilar membrane closest to the oval window, lower frequencies stimulate areas further along. These regions then stimulate neurons to send the sound signals to specific areas of the brain, and that leads to the perception of a certain pitch.


How does the insect’s system of mechanoreception compare to that of the vertebrate’s?

Most insects have ‘ears’ in their legs. A common structure consists of respiratory tracts called tracheae that lead to a membrane stretched over an internal air chamber. Similar to a mammal, sound waves stimulate the membrane to vibrate, but in the insect, this directly activates nerve impulses in attached receptor cells. These nerve impulses then travel to the central nervous system. Some insects also have a related tracheal system that directs information on air pressure changes, inside the insect, to the eardrum. If the right tympanum is stimulated, it will send the signal through the tracheae to the left tympanum. The delay in stimulus between the left and the right ear helps the insect locate the direction from which the sound came.

Some insects such as the noctuid moths have ears specially adapted to avoid their predators; such as bats. The ear structure consists of a tympanic cavity, a membrane, and three neurons in a scolopida formation. The system is stimulated by the ultrasonic vibrations of bat cries. One specific neuron is sensitive to the low intensity vibrations picked up from distant predators, and a different neuron is sensitive to the strong vibrations of a nearby predator. When the neurons are stimulated they send an action potential along the tympanic nerve, and the moth can move according to which neurons have been stimulated.

Some insects also use their sense of mechanoreception to attract mates. At dusk, their setae stand upright, and they vibrate in accordance to the sound waves sent out by the hum of the female.


What is the system of mechanoreception in a fish?

Many fishes and amphibians have a lateral line system enabling them to experience mechanoreception. Pores run up both sides of the fish, through which moving water enters the lateral line system. This leads to stimulation of neuromasts, the receptor cells of fishes. These neuromasts are located throughout the skin, in channels beneath the scales of the main body, and in the dermal bones of the head. These function like sensory hair cells, and vibrations in the water indicating nearby objects or organisms can be detected. Similar to in the vertebrate, their stimulation leads to an action potential. Eventually these nerve impulses travel to the brain through sensory neurons.

Fish also have inner ears systems to extend their hearing to higher frequencies. Sound waves in the water surrounding the fish are conducted as vibrations through the skull. Then, they travel to chambers similar to those located in the cochlea of the vertebrate, and move small granules called otoliths. These granules then stimulate sensory hair cells. Most fish tissue has the same approximate density as water, so vibrations in the water travel right through a fish’s body. Any structures in the fish that have a significantly different density vibrate differently. The otolith provides this sensory detection in the inner ear of a fish. It’s membranes pass the signal on the neighboring sensory hair cells, and eventually trigger action potentials in the neurons of the auditory nerve.

The gas bladders of some fish also provide an area of variable density. Vibrations pass through the gas bladder, and travel through a pathway of small bones called Weberian ossicles. These serve to connect the gas bladder directly with the inner ear of the fish.


What is photoreception?

Photoreception is the translation of photons of light into electrical and then neuronal signals.


What are the structural mechanisms of the vertebrate eye?

In vertebrates such as humans, the surface of the eyeball is made up of the sclera, a white connective tissue, and under that a thin pigmented layer called the choroid. The sclera contains the cornea which is transparent, and is where light initially enters the eye, and the choroid contains the iris which contracts and expands to regulate the amount of light entering the hole in its center, known as the pupil. The rear internal surface of the eye is the retina, which contains the actual photoreception cells. Between the cornea and the rest of the eyeball is a clear protein lens. The rest of the eyeball consists of a mass of ‘jelly like’ vitreous humor, which functions as an additional liquid lens through which to focus light images.


How does the vertebrate eye operate?

Visual Perception in all animals is based on a conserved mechanism. Specific protein molecules make up an optical pathway in which light is directed towards a certain photoreceptive surface in which photoreceptors capture photons. Light initially enters the eye through the cornea. During this process, light rays are bent, and are then further refracted upon passage through the lens to form an inverted image on the retina. To focus on images, most vertebrates change the curve and thickness of the lens. This action is controlled through ciliary muscles surrounding the lens. They relax, and the lens flattens out when the organism is viewing a distant object, and they contract to provide a rounded lens through which to view closer images. Strong ocular muscles direct both left and right eyes so that images received by each eye travel to the same spots on the two retinas; producing binocular convergence.

In the retina, there are two types of receptor cells, rods and cones. Rods are for dim light, and cones are for bright light and color. Rods and cones contain visual pigments made up of light absorbing retinal molecules. These are bound to proteins called opsins, which control which pigments are absorbed by each receptor cell. In the rods, this protein is called rhodopsin, and in bright light, the opsin and retina separate thus making the rods inactive. Rods are more sensitive to light than cones, which is why they work better in dim light. This sensitivity is due to the connections with neuronal cells that are significantly closer than the receptor cell/ neuronal cell connection in cones. This increased convergence leads to greater magnification of a weak stimuli. When only our rods are stimulated, such as in dim light, we only see in black and white. Additionally, because our rods are not part of the fovea, where images are best focused, we can see images better at night when we don’t look directly at them. In the human there are three kinds of cones; blue, green, and orange. Each type of cone has specific photopigment molecules, and each molecule experiences maximum absorption at a different wave length. All other colors are perceived by the stimulation of two or more cone types.

In vertebrates such as human beings, there is a specialized portion of the retina called the fovea. This area provides for our high visual acuity. This region only contains cones, and this enables the human to see in great detail. This area of the eye is most efficiently taken advantage of during the day when the photoreceptor cells of the cones that dominate the fovea, are best able to absorb light. The light hitting the receptor cells, rods or cones, produces a charge gradient across the membrane, but this is not an action potential. These cells, however, synapse with neurons that then synapse with ganglion cells. These convey the image message as an action potential to the brain along optic nerves. The optic nerves from the right and left eyes meet at an optic chiasma in the brain.

To demonstrate the blind spot, cover your left eye and look at the "O" below directly with your right eye.  You should also be able to see the "+" even though you aren't looking directly at it.  Now slowly move closer to the screen (or page), keeping you right eye focused on the "O".  The image of the "+" should disappear, then reappear as you continue to move closer.  This is because the image of the "+" moved across your blind spot as you moved closer..

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Why do some animals see in black and white?

Many animals are nocturnal, and have increased amounts of rods in their optical systems. The cones that control color vision, are really unnecessary or are needed in extremely small quantities.


What kind of photoreception systems do insects have?

Vertebrate and insect eyes have vastly different morphology and structure, although they operate under very similar photochemical systems. The compound eye of most insects has many facets. Behind the corneal lens of each facet, there are functional units called ommatidium. The receptor cells within the ommatidium each detect a very small fraction of the spectrum of light that the eye as a whole is exposed to; like the rods and cones of the vertebrate eye. In compound eyes, the photoreception cells are called retinular cells, and they surround a single eccentric cell. The receptor cells have a specific portion of membrane, designated as a rhabdomere, which has a high density of microvilli. Rhodopsin, a photoreceptive pigment molecule, is contained in this rhabdomere, and this protein absorbs the photons of light energy that enter the eye. This then provides for amplification of this light signal through a G-Protein directed reaction. Ion channels in the cells open, allowing calcium ions to enter the cell. This is the basis for a current traveling down the receptor cell axon, which crosses gap junctions and reaches the dendrite of the adjacent eccentric cell. This eccentric cell then depolarized and generates action potentials. These travel through the optic nerve to the Central Nervous System.

Within each ommatiduim, different retinular cells are sensitive to different colors due to protein variations with in the rhodopsin. Most insects are equipped to see further along the short wavelength end of the color spectrum, towards ultra-violet, however, they don’t see into the reds, which make up the longer wavelengths that vertebrates can see.


How do fish see?

The optical system in fish is very similar to that of the land vertebrates, however, there are some important differences. The fish has a more spherical shaped lens than the land dwellers. Fish focus by changing the relative distance between the lens and the retina, where as other vertebrates change the curvature of their more flexible lens. Fish have choroids which contain a special structure, the tapetum lucidum, and this contains very reflective guanine crystals to aid in dim light vision. This is very important because of the lowered amount of light that penetrates the fish’s watery environment. Additionally, many deep-sea fish have only these and rods, for increased low light sensitivity. They even have epithelial layers for the specific purpose of protection from bright light.

Fish with cones generally have four types, red, green, blue, and ultraviolet. Some only have two or three of these possibilities; fish with all four usually live close to the water’s surface, and may have further special adaptations. Some fish have upwardly directed eyes, especially those who are preyed upon by birds.  Some deep sea fishes have tubular eyes, which help to concentrate the limited light that penetrates to great depths.  The South American "four-eyed fish " swims along the surface, with it's eyes protruding partly out of the water.  Each of its two eyes is split into an upper half for vision in the air and a lower half for underwater vision.


What are the specific mechanisms of the conversion of light stimulation to neuronal impulses?

Visual pigment molecules are the specific structures that absorb photons of light. These pigment molecules are made up of an opsin such as rhodopsin, and an actual light absorbing component, which is usually retinal. When a photon of light hits this molecule, the normal cis-configuration of the retinal is isomerized into a trans-configuration. This in turn leads to a separation between the opsin and the retinal molecule, and eventually to changes in the opsin’s conformation too. When the light is absorbed by the retinal, proteins that are associated with the cell membrane are activated. This alters the cell's  membrane potential, and can eventually lead to an action potential, which is carried to the brain via a sensory neuron.

At a cell’s resting state, there is a certain concentration of each ion in and outside of the cell membrane. This provides for potential diffusions across the membrane. However, there are also charges on each of these ions, and there are polar gradients that do no necessarily correlate to the diffusion gradients. There is usually a stronger negative charge within the cell and a stronger positive charge outside of the cell. Positive ions are attracted to the cell membrane because of the negative interior, and vice versa. The only way that these ions can travel through the selectively permeable membrane, though, is through specific ion channels. Some of the channels are normally open, and stimulation of the cell closes them, and others are closed at resting state, and open in response to stimulation. Such a stimulus could be the absorption of a photon of light by retinal, and when this occurs, there are two possible results. The cell can become hyperpolarized, or depolarized. In retinal for example, sodium channels close in the presence of light, and potassium continues to move out of the cell. This makes the cell environment even more positive, and the inside more negative. This is hyperpolarization. A lessening in the charge of difference between the in and outside of the cell would conversely be depolarization. Each of these conditions leads to charges that travel across synapses to neuronal cells. This alters the firing rate of action potentials in the adjacent neurons.


What is thermoreception in snakes?

Temperature receptors in some snakes can be extremely sensitive. The infrared detectors in the facial pits of rattlesnakes are an excellent example. The sensory axons from the pit organs increase the rate of action potentials when the temperature inside the facial pit increases by 0.002 0 C. A rattlesnake can detect the body heat of a mouse standing 40cm away if the mouse’s body temperature is at least 10 0C above the surrounding air temperature. Because the snakes have a pit on each side of its head, they can tell the direction of the source of heat.