Sensory Systems

Receptors

Receptors are the peripheral (distal) endings of sensory neurons.
They are used by animals to obtain information about the environment.
Receptors are specific for the type of stimulus that they can detect. For example, photoreceptors can only detect light, heat receptors can only detect heat, pressure receptors can only detect pressure, etc.
Receptors function by depolarizing neurons and producing action potentials.
Click here for information on how neurons conduct information.

Types of receptors

Chemoreceptors detect ions or molecules.  Smell (olfaction) and taste rely on chemoreceptors.
Mechanoreceptors detect changes in pressure, position, or acceleration; include receptors for touch, stretch, hearing, and equilibrium.
Electromagnetic receptors are specialized for infrared radiation, visible light, or magnetic fields.
Thermoreceptors detect hot or cold temperatures.
Pain receptors detect severe heat and pressure and chemicals released by inflamed tissue.

Sensory Organs

Sensory receptors may be arranged into sensory organs.  Sensory organs are better able to detect stimuli. For example, photoreceptors on the surface of an animal can detect the presence of light but the photoreceptors in the eye (an organ) can be used to form an image.
A sensory organ may include receptors (nervous), epithelial, and connective tissues.

Sensory Reception and the Brain

Many of the sense organs discussed below (except smell) conduct information to the midbrain or the thalamus.  From the thalamus, the information is relayed to the appropriate part of the cerebral cortex.
 

Detecting a range of stimuli

Stimulation of the plasma membrane of a receptor cell results in depolarization. Sufficient depolarization produces an action potential.
Action potentials never vary in magnitude; they are all or none; stronger stimuli do not produce stronger action potentials.

A strong stimulus will cause a neuron to fire impulses more rapidly or fire a longer burst of impulses.


A strong stimulus could also stimulate more receptors to fire. In the left part of the diagram below, a weak stimulus causes one receptor to conduct action potentials  but two other receptors are not stimulated. The center part of the diagram shows the same three receptors but in this case, a medium stimulus causes two of them to conduct action potentials. The right side of the diagram shows a strong stimulus producing action potentials in all three receptors.

Skin

A variety of sensory receptors are found in the skin including mechanoreceptors, thermoreceptors, and pain receptors (nociceptors). These enable the individual to detect touch (pressure), temperature, and pain. Some of these receptors are listed below.

Free nerve endings - heat, light pressure, pain

Pacinian corpuscles - firm pressure

Meissner corpuscles- onset and end of continuous light pressure

Ruffini endings - continuous pressure

Chemoreceptors

Chemoreceptors may be be the oldest form of sensory receptor because they are universally found in animals.

Invertebrates

Chemoreceptors are found on the surface of planarians (a flatworm) but are concentrated on the auricles- the enlarged area on each side of the eyespots.
Chemoreceptors are found on or in snail antennae, octopus tentacles, arthropod antennae and mouthparts, insect legs, vertebrate mouths and noses, and the fins of some fish.

Vertebrate Chemoreceptors

Chemoreceptors enable vertebrates to taste their food. These taste receptors are found mostly on the tongue as part of sensory organs called taste buds.
Chemicals stimulate microvilli, leading to depolarization of the receptors.

Taste

Humans perceive five types of taste: bitter, sour, salty, sweet, and umami.
Taste enables vertebrates to distinguish nutritious from noxious substances.
Neurons from the tongue travel to the medulla oblongata and to the thalamus. From here, they are routed to the appropriate areas of the cerebral cortex.

Smell

Olfactory receptors are chemoreceptors that detect odors.
The sense of smell is more acute than that of taste. Most of our sensation of taste comes from smell.
An olfactory receptor has several different receptor proteins embedded within the surface of its membranes. Each protein corresponds to the shape of a specific molecule to be detected. It is theorized that odors can be distinguished from one another because specific odors stimulate certain specific combinations of receptors; different odors stimulate other combinations.
Unlike other sensory information, olfactory information does not go to the midbrain or thalamus.  Instead, it passes directly to the cerebral cortex by the olfactory bulbs.
Humans have about 12 million olfactory receptors compared to 4 billion in a bloodhound.  Bloodhounds can track the path of an animal because they can smell individual cells that have fallen from the animal.

Pheromones

Pheromones are molecules that are used for communication between two or more different animals.
Many animals use pheromones as social signals.
Male silk moths can detect one molecule of bombykol (a sex attractant) in 1 X 10¹⁵ molecules of air.

Photoreceptors

Photoreceptors have pigments in receptor cells that absorb light energy and trigger action potentials.

Invertebrate Photoreception

Ocelli

Ocelli (eyespots) are depressions in the epidermis that allow the presence and intensity of light to be determined.

Planarians

Planarians have ocelli that are covered but have an opening to one side and forward. They can tell the direction of light because shadows fall on some of the receptor cells while others are illuminated. They move away from light.

Mollusks

Mollusks are the simplest animals with eyes.
Some mollusks have lenses and therefore are capable of forming clear images. Cephalopods are fast-moving predators and need to catch prey.  The camera-type eyes of some cephalopods (squid, octopus) are capable of focusing and forming clear images.

Insects and Crustaceans

Insects and crustaceans have compound eyes, each with many units called ommatidia.
Light is focused onto the pigmented portion of photoreceptor cells called rhabdomeres.
Each ommatidium samples a small part of the visual field.
Having multiple ommatidia allows the animal to easily detect motion.
A single ommatidium produces a single action potential, therefore arthropod vision is grainy but they have a large field of view and can see in low light.
Insect eyes are sensitive to different wavelengths than human eyes.

Vertebrate Photoreception

Structure of the Eye

Vertebrates have camera type eyes that are capable of forming clear images.
The eyeball has a lens, sclera (outer white), choroid (middle, dark brown, with blood vessels and pigment that absorbs stray light), a receptor-packed retina, and a transparent cornea covering the front of the eye.
Choroid tissue extends in front of the lens to form the iris.
The ciliary body is an extension of the choroid layer.  It functions with the suspensory ligament to attach and support the lens.
The aqueous humor is a watery material between cornea and lens. The vitreous humor is gelatinous material within the eye.

Focusing

Focusing by the lens is called accommodation.
Fish move their entire lens to focus.
Focusing in humans is done by changing the shape of lens with ciliary muscles.  These muscles are part of the ciliary body.

The ciliary muscles surround the lens. They are attached to the lens by suspensory ligaments. When distant objects are viewed, the ciliary muscles relax, putting tension on the suspensory ligaments. The taut ligaments pull the lens, causing it to become thinner. Nearby objects cause the ciliary muscle to contract, reducing the tension on the suspensory ligaments and allowing the lens to assume a thicker shape.

The cornea bends light but the lens finishes focusing.
As a person ages, their ability to focus their lens decreases.  Glasses can correct for this loss of focusing ability.

Focusing problems

Nearsightedness occurs when the eye is too long and the plane of focus is in front of the retina.
Farsightedness occurs when the eye is too short and the plane of focus is behind the retina.
Astigmatism is an abnormality in the shape of the lens or cornea so that part of it does not focus properly.

Stereoscopic vision

In humans, the visual field of each eye overlaps. Each eye sees the image from a slightly different angle and the brain creates a three-dimensional image. This allows the individual to perceive depth of field.

Many animals do not have overlapping fields of vision. They cannot perceive depth but they have a wider field of view. Some (ex: birds) can perceive depth by moving their heads.

Below: This bird is looking at the camera. Before the photograph was taken, the birds head was oriented forward. As the photographer approached, it turned to the side to get a better view of what was in front of it.

Rods and Cones

Vertebrate eyes contain two kinds of photoreceptors: rods and cones.

Rods

Rods cannot distinguish colors and do not provide sharp vision.   They are more sensitive to dim light and are better at detecting motion than cones.
They are most abundant in the periphery of the retina.
Rods have several hundred stacked membranous disks that contain the pigment rhodopsin.

Cones

Cones function in color vision.
They produce sharp images but require bright light.
Cones are most dense in an area in the back of the eye called the fovea.   This area has the greatest visual acuity.
There are three types of cones, each responds to one color, either red, green, or blue because the visual pigment is slightly different for each color.
Different combinations of red, green and blue produce other colors.  For example, the brain interprets signals that come from red and green cones as yellow. The figure below shows how any of the colors can be produced as a combination of these three colors.

Processing of visual information

Rods and cones signal neurons called bipolar cells, which, in turn, signal neurons called ganglion cells. Therefore, processing of visual information occurs in the retina as well as in the brain.
In the dark, sodium gates are open, the cells are depolarized and release neurotransmitter. Some bipolar cells are inhibited (hyperpolarized) by the  neurotransmitter and other bipolar cells are stimulated (depolarized).
When light strikes rhodopsin molecules, their shape changes, causing sodium gates to close. As a result, the cells become polarized and neurotransmitter is not released. Bipolar cells that are inhibited by the neurotransmitter become stimulated and bipolar cells that are stimulated become inhibited.

Example of Processing in the Retina: An Optical Illusion

The diagram below shows seven gray bars. Each bar is slightly darker than the bar above it. Within each bar, the shade of gray does not change.  The bars appear, however, to be a lighter shade near the bottom boundary with the next bar.  When cone cells in the retina are stimulated, nearby cone cells are inhibited.  The dark edge of a bar inhibits nearby cones that are viewing the lighter bar above it causing the lighter edge to appear lighter than it actually is.  This phenomenon functions to accentuate the edges, making them more visible.

The Optic Nerve and Brain

Axons from ganglion cells pass in front of the retina and then form the optic nerve that brings the signal to the thalamus.  From the thalamus, information passes to the part of the cerebral cortex in the back of the head called the primary visual cortex for further processing.
Before reaching the thalamus, the optic nerves from each eye join at the optic chiasm. The information from the left part of the retina of each eye travels to the left primary visual cortex. Information from the right side of each retina travels to the right primary visual cortex.

The photograph below was obtained by slicing through the head of a cadaver.   The slice was done at the level of the eye.  The lenses, optic chiasm, and left optic nerve can be seen.

The right optic nerve is not visible because it is either above or below the field of view.

Characteristics of Sound

Sound waves are a series of compressed and uncompressed molecules of air. The diagram below shows how the vibration of a loudspeaker produces these waves of compression. The waves are spreading away from the loudspeaker.

The sound waves are graphed below. The pitch of sound depends on the number of waves (vibrations) per second. Deep bass sounds are produced by few vibrations per second while high, shrill sounds are produced by thousands of vibrations per second.

Humans can generally hear sounds that range from approximately 20 to 20,000 Hz (vibrations per second). Animals such as bats, dolphins, and whales use sound to locate prey (called echolocation) and can hear in the range from 100 - 140,000 Hz.
The amplitude (height) of the wave in the graphs represents the amount of compression. Loud sounds have a greater amplitude, that is, they result from more compression. The graphs below show two sounds of the same pitch (frequency) but the louder sound has a greater amplitude.

Mechanoreceptors

Characteristics of Mechanoreceptors

Mechanoreceptors are sensitive to mechanical stimuli such as pressure, sound waves, and gravity.
Some are found in the skin; others are hair cells that contain cilia. Bending of the cilia causes depolarization.

Insects

Some species of insects have receptors associated with the hairs on the body surface. They can detect sound when it vibrates the hairs. For example, the hairs on the antennae of male mosquitoes can detect females flying nearby.
Many species have tympanic membranes (eardrums) located on their legs or abdomen.

Fish

Fish and amphibians have a lateral line system that can detect changes in water pressure.  These allow the animals to detect water currents.
In advanced fish, the receptors are located within a canal that has openings to the outside.
Lateral line receptors are cilia embedded in a mass of gelatinous material called a cupula. The cupula bends the hair cells triggering action potentials.
The ears of fish function mainly as organs of balance.

Vertebrates

Mechanoreceptors in the ear function in hearing, balance, and detecting motion.
Stretch receptors in the muscles of vertebrates provide the body with information on the position of the limbs.

The Vertebrate Ear

Evolution

The ears of humans evolved from the lateral line system of fishes and therefore depend on bending hair cells (mechanoreceptors).

Outer ear

The pinna directs sound to the auditory canal.
The auditory canal contains hair which functions to filter the air and modified sweat glands that produce earwax.

Middle ear

Three small bones called ossicles transfers vibrations from the eardrum (tympanic membrane) to the inner ear. They amplify pressure 20 times.
Structures

malleus (hammer)

incus (anvil)

stapes (stirrup)

round window

oval window

eustachian tube- connected to pharynx; permits equalization of air pressure

Inner ear

Structure of the Cochlea

The cochlea is a coiled structure that resembles a snail shell.  It contains 3 canals.
The floor of the middle canal is called the basilar membrane. It contains at least 24,000 hair cells. Movement of the hairs (cilia) triggers action potentials and results in the sensation of sound.
The tectorial membrane protrudes into the canal just above the hair cells.
The hair cells plus the tectorial membrane are the organ of Corti.

Mechanism of operation:

The stirrup vibrates the oval window.
Pressure in the fluid of inner ear (vestibular and tympanic canals) causes the basilar membrane to move up and down with the vibrations.
The up-and-down motion of the hair cells pushes them against the tectorial membrane, producing action potentials.
Auditory information is carried from the cochlea too the midbrain and thalamus.  The information is  relayed from the thalamus to the temporal lobe of the cerebral cortex.
Click the thumbnail below to view a diagram summarizing the flow of sensory information in the brain.

Pitch and Loudness

The basilar membrane is narrow and rigid near the oval window but widens toward the inside of the cochlea. The narrow, stiff portion resonates (vibrates) at higher frequencies than the broad portion further inside.
High frequencies (bells, whistles) vibrate the narrow, rigid part nearest the oval window. Low frequencies (bass sounds) vibrate the membrane further inside. The stimulation of hair cells closest to the oval window produces the sensation of a high-pitched sound while the stimulation of hair cells further in produces a deep bass sensation.
Louder sound causes greater movement of the basilar membrane.
Loud sounds can cause permanent damage to the hair cells.

Balance

Statocysts

Cnidarians, mollusks and crustaceans use statocysts as an organ of equilibrium.

Vestibule

The vestibule can detect a tilting head or changes in velocity in one direction.
It contains two small chambers called the utricle and the saccule. Within these chambers hair cells have cilia that are embedded in a gelatinous material.   The gelatinous material also is mixed with granules of calcium carbonate.
When the head is tilted or when there is a change in velocity, the granules and gelatin move in response to gravity or motion.  Their movement bends hair cells. When the cilia are bent in one direction, the frequency of action potentials is increased.  When bent in the opposite direction, the frequency decreases.

Semicircular canals

The semicircular canals are filled with a fluid that moves when the head is rotated.  Because each canal is oriented at 90 degrees to each other, they are capable of detecting rotation in any direction.
An ampulla is located at the base of each semicircular canal. The ampulla contains cupula which are hair cells embedded in a gelatinous material.
When the head rotates, fluid in at least one canal moves past the cupula, bending hair cells and triggering action potentials.