Evolution of senses

TASTE BOTH REQUIRE SOLUTION OF CHEMICALS IN A FLUID – ONE OF TWO CHEMICAL SENSES BOTH REQUIRE SOLUTION OF CHEMICALS IN A FLUID – SALIVA IN THE MOUTH, MUCUS IN THE NASAL PASSAGES. Primary taste receptors are in taste buds on the tongue.

Primary taste receptors are in taste buds on the tongue. dendrites of receptor nerves extend into gustatory cells in the taste buds. gustatory cells are sensitive to one of five basic tastes: sweet, sour, salty, bitter, umami gustatory cells receptors produce neurotransmitters that depolarize local dendrites

SMELL or OLFACTION Related to taste, work together. Dendrites extend into olfactory epithelium on the dorsal surfaces of the nasal passages.

At distal end of dendrites are olfactory cilia. not cilia in a classic sense – they don’t move. Chemicals dissolved in mucus cause dendrites to fire. Unlike taste, not limited to five messages

ONE OF TWO CHEMICAL SENSES Lacrimal gland produces tears that sweep across eyeball towards lacrimal canal, lacrimal sac and nasolacrimal duct.

The eyeball is a fluid-filled sac, the sac consisting of three layers: 1) The fibrous layer or sclera – the white of the eye except anteriorly where it forms the cornea. 2) The vascular layer or choroid – the vascularized part of the sac which anteriorly produces the ciliary body (which holds the lens) and the iris. 3) The sensory layer or retina – is an outgrowth of the brain. Photoreceptor cells of retina are modified neurons.

The eyeball is a fluid-filled sac, with two kinds of fluids Vitreous humor– fills the space posterior to the lens. It provides interocular pressure to hold the retina and lens in place. 2) Aqueous humor – fills the space under the cornea in front of the lens. Light is transmitted through the humors and focused by the cornea and lens on the retina where photoreceptors transduce light energy to electrical energy.

Fibers of the optic nerve enter into the eyeball and traverse the posterior wall in front of the photoreceptor cells. Photoreceptors “see” light that is reflected off of the pigmented layer of the retina after it has passed through the nervous layer of the retina.

Each absorbs light at different wave lengths. Rods and cones are photoreceptors that absorb light energy and produce an electrical signal. Humans have three types of cones and one type of rods. Each absorbs light at different wave lengths. Absorption results in changes in neurotransmitters at synapse with bipolar cells. Sufficient transmission causes ganglion cells to depolarize. nervous layer of retina

HEARING and BALANCE Outer ear includes pinna and auditory canal Middle ear includes tympanum and three ear ossicles in the tympanic cavity Inner ear or labyrinth includes three main structures: the vestibule, the cochlea and the semicircular canals. HEARING and BALANCE

Airborne sound is converted to mechanical energy by the tympanum, and transmitted by the malleus, incus and stapes. It is transmitted through the fenestra ovalis by the stapes and into fluid that fills the labyrinth, the perilymph. The cochlea is a coiled bony chamber, but the chamber is divided into three canals. The receptor cells are in the middle of the three parts, the scala media. They are located in the organs of Corti

Mechanical energy transmitted by fluids in the scalae, deform membranes between them. Resonance will cause neurons at certain points to fire depending on frequency of the sound. Cochlear hair cells are receptors, their deflection causes cation (Na+) channels to open and depolarization to start.