Chapter 17 - Special senses

• Sensory receptors: specialized structures that respond to stimuli
• Majority of sensory receptors are classified as "general senses" which are relatively simple in structure
• General senses include 1) Receptors for Somatic Sensations 2) Receptors for Visceral Sensations
• Receptors for Somatic Sensations: detect 4 types of sensations (tactile, thermal, pain, proprioceptive)
• Tactile: any sort of touch
• Proprioceptive: how we perceive body position and body movement
• Receptors for Visceral Sensations: detect information from internal organs that is not consciously perceived (changes in blood pressure, blood chemistry, gut stretch, etc.)
• Special senses: involve complex sensory organs found in specific areas of the head and they have complex neural pathways
• Special senses are innervated by cranial nerves and have specific sensory areas located in cerebral cortex
• The special senses include vision, hearing, equilibrium, smell and taste
• The special senses of vision, hearing and equilibrium are located in complex sensory organs: eye and ear
• The special senses of smell and taste are located in distinct epithelial structures: olfactory epithelium and taste buds
• Sense organs: structure composed of nervous tissue along with other tissues that enhance its response to a certain type of stimulus
• Perception: interpretation of the meaning of a stimulus
• Accessory structures of the eye include eyebrows, eyelids, conjunctiva and extrinsic eye muscles
• Eyebrows shade eye from sunlight and catches perspiration from forehead
• Eyelids are skin-covered folds that protect the eyes from foreign objects and excess light
• Eyelids spread lubricating secretions over the eye and the eyelashes trap and sweep away foreign particles
• Conjunctiva is the transparent vascular mucous membrane that lines the inner surfaces of the eyelids and covers the anterior surface of the sclera
• Sclera: white part of the eye
• Conjunctiva secretes mucous to prevent desiccation of the eyes
• Desiccation: extreme dryness
• Extrinsic eye muscles are 6 skeletal muscles that originate on the bony orbit and insert on the sclera
• Extrinsic eye muscles maintain the shape of the eyeball, hold it in orbit and provide precise eye movements
• The cranial nerves that help extrinsic eye muscles are the oculomotor, trochlear and abducens
• Accessory structures of the eye

  
• Sty: infected hair follicle at base of eyelash
• Blood shot eyes: vessels in conjunctiva over sclera get irritated and dilated
• Conjunctivitis: inflammation of the conjunctiva (by bacteria or virus). if severe it is a highly contagious infection known as pinkeye
• Problems with extrinsic eye muscles: diplopia and strabismus
• Diplopia: double vision; eyeballs can't be focused on same visual field due to paralysis, weak eye muscles, alcohol consumption
• Strabismus: cross eyed; due to congenital weakness in eye muscles in which the affected eye rotates medially or laterally
• Lacrimal (tear) gland: located in the superior lateral region of each orbit and secretes lacrimal fluid (tears) continuously
• The tears of the lacrimal gland contain water, salt, mucous, antibodies and lysozyme. It clears, lubricates and moistens the eye
• Lysozyme: antibacterial enzyme that prevents infection
• Pathway of tears: 1. lacrimal gland, 2. lacrimal ducts, 3. lacrimal fluid flows over the eye, 4. lacrimal punctum, 5. lacrimal canaliculus, 6. lacrimal sac, 7. nasolacrimal duct, 8. nasal cavity, 9. down the throat and we swallow them
• Flow of tears

  
• Anatomy of the eye

  
• Layers of the wall of the eye (out to in): fibrous tunic, vascular tunic, retina
• Sclera + Cornea = Fibrous tunic