Biology 12 Unit 1:Maintaining Dynamic Equilibrium

Special Senses  Input to the nervous system is in the form of our senses: pain, vision, taste, smell, hearing, balance, and others.  Vision, taste, smell, and hearing input are the special senses.  Pain, temperature, and pressure are known as somatic senses. Sensory input begins with sensors that react to stimuli in the form of energy that is transmitted into an action potential and sent to the CNS.

Special Senses

Taste & Smell Specialized receptors to detect presence of chemicals in the external environment. Specific chemicals combine with receptors causing an action potential. There are five types of specialized taste receptors - salty, sweet, bitter, sour, savoury (glutamate). Taste and smell combine to give foods the variety of flavours we associate with them.

The Eye - Structure

 Anterior chamber- A cavity of the eye filled with aqueous humour, a fluid that provides oxygen, glucose, and proteins.  Choroid Layer-rich in blood vessels that supply the eye tissues with oxygen and nutrients.  Ciliary body-Changes the shape of the lens, which adjusts the eye's focus.  Cornea and lens-Focuses light.  Fovea-A depression in the retina where cones (photoreceptor cells) are concentrated and vision is most acute.

Iris-Colored part of the eye. Optic disc-Round, flat structure where nerve fibres from the retina converge, blind spot. Optic nerve-Transmits information about images to the brain. Retina-Contains light-sensitive nerve cells. Sclera-White, outer layer of the eyeball. Vitreous-Transparent gel that fills the main cavity of the eye.

The Eye - Photoreceptors  The human eye can detect light in the nanometre (nm) range, the visible light spectrum.  Light with wavelengths shorter than 400 nm is termed ultraviolet (UV) light.  Light with wavelengths longer than 700 nm is termed infrared (IR) light.

 In the eye, two types of photoreceptor cells are clustered on the retina.  Rods detect differences in light intensity.  Cones detect colour.  Rods are more common in a circular zone near the edge of the eye.  Cones occur in the centre (or fovea centralis) of the retina.  Light reaching a photoreceptor causes the breakdown of the chemical rhodopsin, which in turn causes a membrane potential that is transmitted to an action potential. The action potential transfers to synapsed neurons that connect to the optic nerve. The optic nerve connects to the occipital lobe of the brain.

 Humans have three types of cones, each sensitive to a different colour of light: red, blue and green.  Opsins are chemicals that bind to cone cells and make those cells sensitive to light of a particular wavelength (or colour).  Humans have three different forms of opsins coded for by two genes on the X chromosome and one gene on chromosome 7.  Defects in one or more of these opsin genes can cause colour blindness, usually in males.

The Eye - Pathway The Visual Pathway: from visual spectrum to the brain. From Nelson Biology

The Eye - Defects  Astigmatism-is a hereditary impediment. It occurs when the curvature of the cornea does not permit the image to project itself correctly on the retina.  Myopia-The person who suffers from myopia has short- sightedness. Myopia is a condition in which the image is focused in front of the retina.  Glaucoma-is a progressive blindness that causes severe damage to the optic nerve. It is caused by an increased build- up of aqueous humour in the anterior chamber of the eye. This disease can cause total blindness.  Far-Sightedness (hyperopia)-The image is focussed behind the retina. The person who suffers from farsightedness cannot focus on near objects.  Cataracts-A loss of transparency of the lens of the eye, or of its capsule. Surgery, to remove the cataract, can restore the sight.

The Eye - Technologies  Corrective Lenses  Glasses, contacts, surgical implants  Radial Keratotomy  Using a knife to reshape the cornea  Laser Eye Surgery  Using a laser to reshape the cornea  Other Surgery  Cornea transplants, cataract surgery

The Ear

The Ear - Structure  External  Pinna  Auditory Canal  Middle  Ossicles (malius, incus, stapes)  Tympanic membrane  Eustachian tube  Oval Window  Round Window  Inner  Vestibule (utricle, saccule)  Semicircular canals  Cochlea

The Ear - Hearing  Hearing involves the actions of the external ear, eardrum, ossicles, and cochlea.  In hearing, sound waves in air are passed from vibrations of a gas (air outer ear) to a solid (ossicles of middle ear) and then a liquid (fluid of the inner ear).  Next these vibrations cause movement of hair cells in the cochlea.  Finally they are converted into action potentials in a basilar membrane connected to the auditory nerve.  Very loud sounds can cause violent vibrations in the membrane under hair cells, causing a shearing or permanent distortion to the cells, resulting in permanent hearing loss.

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The Ear - Orientation and Gravity  Orientation and gravity are detected in the vestibular apparatus made up of the utricle and saccule.  Calcium carbonate crystals shift in response to gravity causing hair cells to bend thus providing sensory information about gravity.  Acceleration in three different planes of movement is detected by the semicircular canals. Hair cells respond to shifts of liquid within the canals, providing a sense of acceleration.

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