Hearing, seeing, feeling, smelling and tasting Chapter 9 – The Senses

Summary Introduction The eye and sight The ear and hearing Sense of taste Sense of smell Cutaneous receptors

Introduction Our “senses” continually provide us with information about our surroundings. Sense organs are complex organs like the eye or specialized receptors in areas such as the nasal mucosa or tongue.

Introduction Conversion of a stimulus to a sensation: Stimuli (light, sound, temperature, etc. are changed into an electrical signal or nerve impulse. The signal is then transmitted over a ? neuron to the ? The signal is interpreted and we become consciously aware of a sensation.

The Eye Contains receptors for vision and a refracting system that focuses light rays on the receptors in the retina. The eye sits in the orbit formed by the maxilla, zygomatic, frontal, sphenoid and ethmoid bones. Extrinsic muscles attach the surface of the eyeball to bones.

The Eye Eyelids – contain skeletal muscle that allow us to close them and totally cover the exterior eyeball. Eyelashes – help to keep dust out of our eyes. Tears

The Eye Cranial Nerves The eye contains 3 layers Optic – vision Oculomotor, abducens and trochlear – eye movment The eye contains 3 layers

The Eye Structure of the eyeball Sclera – tough fibrous tissue. Front surface is the “white” of our eyes and the cornea. The cornea is transparent, receives no blood supply and is nourished by the aqueous humor. Sclera is covered by the conjunctiva in the front of the eyeball.

The Eye Structure of the eyeball Choroid - contains a dark pigment to prevent scattering of light that enters the eyeball. Also contains blood vessels and 2 involuntary muscles. Iris – Ciliary body (muscle) –

The Eye Structure of the eyeball Lens – composed of transparent, elastic protein; no blood supply, nourished by the aqueous humor. Retina – contains microscopic receptor cells called rods and cones. Rods – Cones – Fovea –

The Eye Layer of the eye Retina Ganglionic neurons carry impulses generated by the rods and cones until they converge at the optic disc. From the optic disc they form the optic nerve and pass through the wall of the eyeball to the occipital nerve. Optic disc – also know as the “blind spot”, no rods or cones; exit to the optic nerve. Occipital lobe of the cerebrum – visual interpretation.

The Eye Structure - fluids of the eyeball – 2 types: Aqueous humor – watery fluid in front of the lens (anterior cavity) nourishes the lens and cornea. Continually formed by the capillaries in the ciliary body, flow through the pupil and is reabsorbed in the canal of Schlemm. If drainage is blocked, the internal pressure in the eye increases and may damage the eye and lead to blindness = glaucoma.

The Eye Structure - fluids of the eyeball – 2 types: Vitreous humor – jelly-like fluid behind the lens (posterior cavity). Literally holds the retina in place and gives structure to the eyeball.

The Eye Focusing Problems Presbyopia – “old sightedness” or “short arm syndrome”. Ciliary bodies lose their elasticity and can no longer change the shape of the lens to bring near objects into focus. Myopia – nearsightedness, image focuses in front of the retina rather than on it, eyeball is elongated. Corrected by glasses, contacts or radial keratotomy (Lasix).

The Eye Focusing Problems Hyperopia – “farsightedness”, image focuses behind the retina, produces a fuzzy image. Corrected by lenses. Astigmatism – refraction error – fuzzy image, irregular curvature of the cornea or lens, requires special lenses to correct (Toric lenses) or contacts.

The Ear Sense organ associated with hearing and equilibrium and balance. 3 main parts External Middle Inner

The Ear External ear – External Auditory canal – a curving tube about one inch long; extends into the temporal bone and end at the tympanic membrane (eardrum).

The ear Middle ear – tiny epithelium lined cavity which is hollowed out of the temporal bone. Tympanic membrane – separates the external and middle ear and vibrates when sound waves strike it. 3 tiny bones called ossicles (bones) transmit sound waves.

The Ear Middle Ear Bones Malleus – Incus – Stapes –

The Ear Middle Ear Oval Window separates the middle ear from the inner ear. Eustachian tube – connects the throat with the middle ear; allows air to enter and leave the middle ear which equalizes pressure. Why do throat and ear infections occur together?

The Ear Middle Ear - Hearing Sequence Sound waves cause the eardrum to vibrate, and this movement is transmitted and amplified by the ear ossicles. Movement of the stapes against the oval window causes movement of fluid in the inner ear which generates electrical impulses.

The Ear Inner Ear – contains mechanoreceptors that are activated by vibration and generate nerve impulses that result in hearing and equilibrium. The 3 spaces are called the bony labyrinth and contain fluids called perilymph and endolymph. Vestibule – membranous sacs (utricle and saccule) adjacent to the oval window and between the semicircular canals. Contains receptors for equilibrium.

The Ear Inner Ear Cochlea – snail shell; contains the Organ of Corti which holds the receptors for hearing (hair cells). As the hairs bend (vibration) they generate an electrical impulse. Semicircular Canals – contain the crista ampularis which is a specialized receptor that generates a nerve impulse when you move your head. Receptors for equilibrium.

Sense of Taste Taste buds – chemical receptors that generate nervous impulses resulting in the sense of taste. There are about 10,000 microscopic taste buds located on the papillae of the tongue. Gustatory cells –

Sense of Taste Taste Sensations Sweet, sour, bitter, salty, and Umami (=savory). Other flavors results from a combination of taste bud stimulations and olfactory receptor stimulation. i.e. our taste sensations include odors as well.

Sense of Smell Olfactory receptors – chemical receptors responsible for the sense of smell are located in the upper part of the nasal cavity. Olfactory receptors are stimulated by chemicals dissolved in the watery mucus that lines the nasal cavity. We detect about 10,000 different scents. Olfactory receptors are easily fatigued – many odors are not noticeable after a time.

Hunger and Thirst Visceral sensations – receptors are located in the hypothalamus; the stimulus is a change in the body’s water/salt content and levels of nutrients in the blood.

Cutaneous Sensations Receptors of the general sense organs are found in almost every part of the body. Encapsulated nerve endings – located in the dermis; touch and pressure. Free nerve endings – mainly in the dermis of the skin, mucosa, internal organs. They sense pain or crude touch. *Referred pain.

Cutaneous Sensations Receptors Meissner’s corpuscles – skin, fingertips and lips; sens of fine touch and vibration. Ruffini’s corpuscles – skin and sq tissue of the fingers; touch and pressure. Pacinian corpuscles – subcutaneous; deep pressure and vibration.

Cutaneous Sensations Receptors Krause’ end bulbs – skin and sq; touch and maybe cold. Muscle spindles – skeletal muscle; proprioception. Proprioception is the sense of position and movement in various parts of the body.

Characteristics of Sensations Projection – sensation seems to come from the area where the receptors were stimulated; in reality they are being “felt” via the cerebral cortex. Phantom pain – receptors are removed with amputated limbs but severed nerve endings continue to send impulses to the brain.

Characteristics of Sensations Intensity – the intensity of a sensation is related to the strength of the stimulus and/or number of receptors stimulated. Contrast – effect of a previous sensation on a current sensation; brain compares a new sensation to a previous one.

Characteristics of Sensations Adaptation – you become unaware of a continuous stimulus. After image – sensation remains in the consciousness even after the stimulus is gone – flash from a camera.