Sense Organs Chapter 38
Chemical Senses Chemoreceptors are the receptors responsible for smell and taste. Because all members of the animal kingdom have developed a sense of taste and/or smell, chemoreceptors are believed to be among the first traits that distinguished animals from other kingdoms Chemoreceptors can be Universally located around the body Annelids, like earthworms In sensory structures like antennae Insects and arthropods Centrally located Vertebrates
Taste Human taste buds are found on the palate, epiglottus, pharynx, but primarily on the tongue. Taste buds begin with a small pore for collecting particles and chemicals from food products. Microvilli collect chemicals on the surface of the bud and pull them into the pore Inside the pore, receptor proteins for specific chemicals bind to the product and initiate an action potential. The impulse is transported to the brain where it interprets the specific flavor for recognition. The taste buds provide the brain with relative information for sweet, salty, bitter, sour, and savory
Smell Olfactory cells in the epithelium of the nasal cavity are responsible for scents Each cell bears cilia which contain one of 1000 s of sensor receptors for aromatic chemicals The combination of which cilia are triggered by the chemical creates the unique scent. Bears, sharks, moths, snakes, hounds, rats, vultures, and T- Rex s are so dependent on their sense of smell that a minimum of 50% of their brain activity is devoted smell sensations alone Old anecdote: A pine needle dropped in the forest. The deer heard it; The eagle saw it; the bear smelled it.
Smell Smell and taste memories combine into one memory in the brain The brain associates smells with taste and taste with smell so much that the loss of one may trigger a lapse in the other How does food taste when you have a cold? How does the kitchen smell when your tongue is burned? To heighten the senses, the tongue and nose also send nerve impulses to each other when one is triggered. The purpose of a smell and taste sense is to help organisms distinguish between edible/not edible, healthy/dangerous.
Vision Photoreceptors are sensory receptors for light rays Eyespots: found on planarians; only able to determine direction of light Compound eyes: cornea s act as lenses in arthropods to direct light into photoreceptors A wider range of the visual light spectrum, but not as wide a depth of field Camera eyes: found in vertebrates; capable of focusing, changing shape, and adjusting to light
Human Eye The outer layer of the human eye is the sclera, which contains the cornea and all eye parts within The middle layer, the choroid, contains the blood vessels The inner layer, the retina, contains rod and cone cells, which detect images The pupil at the front of the eye dilates to control the amount of light that enters the retina The cornea and lens control the focusing ability of the eye
Sight When a rod/cone absorbs light, the pigment rhodopsin releases an action potential. When the action potential reaches the brain, it signals that light was detected at that specific angle in the eye. Rods are activated by minimal amounts of bright light, but cones require heavy doses of bright light Each cone picks up a certain level of red, green, or blue wavelengths of light All signals exit the eye through a hole in the back called the optic chiasm. This is the only area where signals aren t received (hence, the blind spot )
Hearing The human ear has three divisions: outer, middle, and inner. Outer ear The outer ear contains the pinna (external flap) and the opening to the auditory canal where sound waves are collected Middle Ear The middle ear begins with the tympanic membrane (the eardrum). The auditory tube is responsible for equalizing air pressure Inner Ear Filled with fluid, this section contains nerves which trigger sound impulses
Balance Mechanoreceptors in the semicircular canals of the inner ear detect rotational and angular movement Tiny hair cells are brushed by the fluid in the inner ear to one direction or another. This direction provides the brain with details about rotational momentum and direction. A separate series of receptors in the vestibule monitor gravitational direction in a similar way.