a&p2_module3.docx - The Special Senses Five special senses...

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The Special Senses Five special senses: o Vision o Smell o Taste o Hearing o Equilibrium (balance) These senses have specialized organs containing specialized receptor cells, which carry their impulses by way of specialized somatic and visceral afferents. Touch is the other sense – somatic sense – does not have a specialized sense organ. o Uses general receptors composed of modified dendrites of sensory neurons. o Includes pressure, vibration, pain, heat, and the combined info is carried in general somatic afferents and general visceral afferents. Vision and the Eye Organ responsible for sight is the eye – it consists of three layers Outer Layer: o Outer fibrous layer includes the sclera and cornea o Posterior potion is known as the sclera (the white of the eye) o Anterior portion is known as the cornea – is transparent and is where light enters o Aqueous humor fills the spaces between the cornea and lens Middle Layer: o Includes the choroid, ciliary muscle and iris Choroid (posterior) is darkly-colored and prevents light from dispersing through the eye Highly vascular and supplies blood to the other layers of the eye Ciliary body (anterior) changes the shape of the lens, which allows it to focus Iris (anterior to the ciliary body) contains the colored portion of the eye. Controls the amount of light let into the pupil which is the hole in the center of the iris. Iris uses muscle fibers to contract or dilate based on the amount of light in the environment. Inner Layer o Sensory layer – includes the retina o Retina contains 2 types of photoreceptors which are cells that are sensitive to light. o The more numerous cells are rods – which are stimulated in dim light. o Rods are more sensitive to light but do not generate sharp or color images. o Cones operate in bright light which help to generate sharp color images. Lens is located posterior to the iris and pupil Interior of the eye, which is posterior to the lens, is the vitreous body Vitreous body is filled with vitreous fluid which helps to hold the retina firmly to the choroid. Vision Pathway (How we see)
Light passes into the eye moving, moves through the cornea, aqueous humor, lens, and vitreous humor to the posterior surface of the eye on the retina. Photoreceptors in the retina send a signal through the optic nerve on to the optic chiasm which is located at the base of the hypothalamus. The medial fibers of the optic nerve cross to the other side when they read the optic chiasm where the optic tracts are formed. The optic tracts terminate in the thalamus in a region called the lateral geniculate nucleus (LGN). The info moves from the thalamus through the optic radiations to the primary visual area of the occipital lobe.

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