lab 11 special senses

• Sensation is the conscious or subconscious awareness of changes in the internal or external environment
• General senses scattered throughout the body include:

  • Somatic senses: touch, pressure, pain, temperature, and proprioception
  • Visceral senses provide information about internal organs
• Special senses are vision, hearing, equilibrium, taste, and smell
• Special senses have distinct receptor cells that are restricted to the head
• Receptor cells for special senses are localized in complex sensory structures (eye and ear) or in distinct epithelial structures (taste buds and olfactory epithelium)
• Adipose tissue functions as padding and insulation within the orbit to protect the eye
• The sclera, also known as the "white of the eye", consists of dense fibrous connective tissue and forms a tough, external coat
• The sclera protects and shapes the eyeball and provides a sturdy anchoring site for the extrinsic eye muscles
• The cornea is the transparent, anterior portion of the sclera that allows light to enter the eye and helps bend (refract) light rays to focus on the photoreceptors in the retina
• The cornea of the dissection specimen may appear opaque due to the preservative
• The optic nerve (II) is located on the posterior surface of the eye and is a solid white cord of sensory nerve fibers
• If possible, identify remnants of the six extrinsic eye muscles, which provide rotary movements of the eye, focus the eye for optimum vision, and anchor the eye in the bony orbit
• The wall of the eye is composed of three layers or tunics:

  • Outermost layer: sclera and cornea
  • Middle layer: iris, ciliary body, and choroid
• The iris is the circular colored portion of the eye that lies anterior to the lens and consists of two layers of muscles:

  • Inner circular muscles
  • Outer radial smooth muscles arranged around the central opening called the pupil
• In close vision and bright light, the circular smooth muscles of the iris contract, causing the pupil to constrict and allowing less light to enter the eye

  In distant vision and dim light, the radial smooth muscles contract, causing the pupil to dilate and allowing more light to enter the eye
• The lens is normally flexible and changes its shape to focus light rays for close and far vision. It has been hardened and has become opaque due to the preservative
• The lens is held in place by suspensory ligaments attached to the ciliary body
• The ciliary body is attached to the iris posteriorly and consists of ciliary processes and ciliary muscles
• Ciliary muscles control the shape of the lens
• Ciliary processes contain capillaries that produce the aqueous humor
• The choroid is continuous with the ciliary body and extends around the posterior of the eye
• Choroid is a highly vascular and darkly pigmented tissue
• Choroid functions to absorb excess light rays to prevent reflection and scattering of light within the eyeball
• The retina forms the innermost layer of the eye and is very thin and easily separated from the choroid
• The retina is held in position up against the choroid by the vitreous body in the intact eye
• The retina consists of two layers:

  • An outer pigmented layer (next to the choroid)
  • The inner neural layer composed of photoreceptors, bipolar cells, and ganglion cells
• Photoreceptor cells (rods and cones) convert light energy into signals that are sent to bipolar cells and then to ganglion cells where action potentials are generated
• Ganglion cell axons leave the back of the eye as the thick optic nerve
• The optic disc or "blind spot" is where the optic nerve leaves the retina and lacks photoreceptors
• Lateral to the optic disc is the macula lutea, a small yellowish area on the retina
• The central portion of the macula lutea is the fovea centralis, which is the area of greatest visual acuity containing only cones
• Cones are stimulated by bright light and produce color vision, while rods are more plentiful on the periphery of the retina, stimulated by dimmer light, allowing us to see in shades of black, white, and grey
• The lens of the eye must become more rounded or convex in shape in order to view objects at close range. This adjustment is called accommodation.
• The closest distance at which an object appears to be in sharp focus is called the near point.
• Visual acuity refers to the degree of "sharpness" of eyesight. It is greatest in that portion of theretina containing a large number of cones, but not rods -- the fovea centralis.
• hyperopia: far-sightedness

  • see far clearly, near object blurry
  • eye ball too short
  • lens too flat
  • need convex/converging lens
  • bend light inwards to increase refraction
• myopia: nearsighted

  • see near lear, far objects blurred
  • eyeball too long" lens too round
  • need concave/diverging lens
• Astigmatism is a condition where the cornea or lens has irregularities in the surface curvature
• The cornea or lens may be spoon-shaped rather than spherical in astigmatism
• Objects may be in focus in one axis and blurred in another axis in astigmatism