Sensory receptors initiate action potentials in sensory neurons. 

Sensory receptors convert the energy of the stimulus (light, heat, sound) or the presence of detectable chemicals into action potentials and send these to the central nervous system.  Our perception of that stimulus is determined by the region of the central nervous system to which the sensory neurons synapse. 

Without specific receptors we are blind to potential stimuli.  E.g., we are oblivious to radio waves; also certain chemicals have no smell to us (hence in manufactured odorless chemicals, odorous chemicals (odorants) are added to alert us to their presence.  Many pesticides and even natural gas have odorants added as a safety measure enabling us to detect the presence of the harmful substance). 

Consider three types of receptors based on the type of stimulus they detect: 

1.  Mechanoreceptors – detect bending or stretching of plasma membrane, examples include modified dendrites in skin as touch receptors and hair cells as found in various animal organs.  Hair cells occur in the lateral line of fish and in our inner ear.  “Hairs” (cilia or microvilli) project from the hair cell and are moved by the physical forces of vibration or fluid movement.  The movement of the hairs controls the release of neurotransmitters that cross the synapse to a sensory neuron stimulating action potentials.  

2.  Chemoreceptors – as found in sense organs such as the olfactory epithelium lining the nasal cavity, have plasma membrane proteins that detect the presence of certain chemicals that then stimulate the release of neurotransmitters to sensory neurons. 

Other organs with chemoreceptors –

• antennae of insects (do branched antennae of moths and branch feather stigma of grass)
• taste buds in our tongue or in the feet of flies
• auricles of planarians

Other chemoreceptors are found in the brain (hypothalamus) and these detect blood tonicity (water content) and send nerve signal to the pituitary signaling the release of ADH during times of lower water content in blood (high solute content).  ADH of course travels in the circulatory system to the kidney and you know the rest of that story. 

3.  Electromagnetic receptors – detect various portions of the electromagnetic spectrum (infrared, visible light, uv light) or electrical and magnetic fields.  In humans, these are found in the eyes. 

Examples of animals with various electromagnetic reception

• Pit vipers have pits which detect infrared radiation used to track the footprints of struck prey, they follow a heat trail.
• Whales and birds are known to follow earth’s magnetic field during migrations
• Many insects have eyes that can detect ultraviolet radiation, thus the pattern of color on a flower may look vastly different to an insect than to us.  Nectar guides are colored markings reflecting uv or visible light and lead insects to nectar and thus facilitate pollination.

Human Eye 

The three layers of the human eye (Fig. 47.5) 

1.  The sclera (sclera means hard, in reference here to the tough, fibrous nature of this tissue).  The sclera covers most of the eye in white layer except for the front of the eye where the sclera is clear (transparent).  The transparent, frontal portion of the sclera is called the cornea. 

2.  The choroid, the middle layer.  The front of the choroid just behind the cornea encompasses two notable structures, the iris and the lens. 

Iris - contractile diaphragm that regulates the size of the pupil.  Body of the iris is pigmented so as to keep light out of the eye (“iris” means rainbow and in this sense refers to the multiple colors and shades within your iris).  The pupil allows light to pass beyond, through the lens and to the retina. 

Lens – a clear structure composed of precisely positioned concentric layers of cells.  Lens cells have no nuclei or other organelles.  They are filled with a transparent protein. 

A chamber of aqueous humor (watery fluid) exists in front of the lens and extends through the pupil to the cornea.  This Chamber is known as the anterior compartment.

Glaucoma:  High fluid pressure inside the chamber in the front of the lens can push the lens back towards the large chamber behind the lens (filled with vitreous humor – jelly-like, “glass-like” [compare “in vitro” fertilization].  This large chamber is known as the posterior compartment. The resulting pressure seeks the path of least resistance which happens to be the site where the optic nerve enters the back of the eye.  The optic nerve thus takes the brunt of the force resulting in decreased vision and blindness.  

3.  Retina – contains light sensitive sensory cells.  The retina transmits in the form of action potentials, sensory input from the rod and cone cells stimulated by the qualities of light.  Rod cells initiate action potentials based on any type of light reception and are easily stimulated even by low light levels.  Cone cells initiate action potentials based upon the color (wavelength) of light and require brighter light to operate. 

The optic nerve leads from the back of the eye to the brain.  You can get a sense of the location of the optic nerve within the retina by experiencing the “blind spot.”  [see handout activity] 

Human Ear (see fig. 47.9 & 47.10)

Outer Ear

The outer ear consists of the pinna and auditory canal.  Ear wax is secreted into the auditory canal.   Ear wax is “a substance that helps to guard the ear against the entrance of foreign materials” (Mader, p. 856) 

Middle Ear.  The middle ear consists of:

·        tympanic membrane (ear drum) - converts soundwaves to mechanical movement.

·        opening to eustachian tube (auditory tube) - equalizes with atmospheric pressure the air pressure within the air-filled chamber of the middle ear

·         Auditory Bones

o     Malleus (hammer) - the largest of the three auditory bones; receives mechanical vibrations from  tympanic membrane and tranfers the motion to the incus.

o     Incus (anvil) - the middle bone, transfers motion to the stapes.

o     Stapes (stirrup) - transfers motion to the inner ear. 

Inner Ear.  Components of the inner ear are embedded in the skull bone.  The semicircular canals and the vestibule function for balance, not hearing.

• Semicircular Canals - hair cells in semicircular canals detect the movement of fluid within the canals in response to rotating movements of the head.

• Vestibule - contains otoliths (crystalline granules pulled by the force of gravity or acceleration) that rub against hair cells.  Stimulated hair cells transmit signals to nerves that signal to the brain the body's position with respect to gravity.

• Cochlea - a tubular organ coiled into a spiral; hair cells in the cochlea convert mechanical vibrations transmitted by the stapes into nerve impulses transmitted by neurons to the brain.  The activation of specific neurons transmits action potentials to specific cells within the brain that gives us the perception of sound.