Somatic and Special Senses

1. Somatic and Special Senses Chapter 10

2. Sense of Smell • Olfactory receptors are chemoreceptors • Odorant molecules bind to the receptors in different patterns • Enter as gases, but they must dissolve at least partially in the watery fluids before they can be detected • Stimulated olfactory receptor cells send nerve impulses along their axons • Humans have 12 million olfactory receptor cells • Bloodhounds have 4 billion

4. Sense of taste • Taste buds are the special organs of taste • 10,000 taste buds are found on surface of tongue • 1,000 are scattered in the roof of the mouth and walls of the throat • Each taste bud includes modified epithelial cells (taste cells) which function as receptors • Cats: 473 taste buds Dogs: 1,700

5. Before a chemical can be tasted, it must dissolve in the watery fluid (saliva) surrounding the taste buds • Each type is most sensitive to a particular kind of chemical stimulus • Primary taste sensations: • Sweet (tip of tongue) • Sour (margins of tongue) • Salty (widely distributed) • Bitter (back of tongue) • Experiencing flavors involves tasting, smelling, feeling the texture, and temperature • Sensory impulses from taste receptors travel on nerve fibers into the medulla oblongata to thalamus to gustatory cortex within the parietal lobe

6. Sense of hearing • The ear has 3 parts: outer ear, middle ear, inner ear • Outer ear • Auricle (pinna; funnel-like structure which helps collect sound waves and directs them into the external acoustic meatus) • External acoustic meatus (ear canal) • Tympanic membrane (eardrum; sound waves change the pressure on the eardrum and reproduces the vibrations of the sound wave source)

8. Middle ear • Contains auditory ossicles (smallest bones in body) • Malleus • Incus • Stapes • Tiny ligament attach them to the wall of the tympanic cavity • Transmit and amplify vibrations between tympanic membrane to inner ear • Eustachian tube connects middle ear to the throat • Helps to maintain equal air pressure on both sides of the eardrum

9. Inner ear • A complex system of communicating chambers and tubes called a labyrinth • 3 semicircular canals which provide a sense of equilibrium • Cochlea which functions in hearing

10. Decibels – measure of sound intensity • Chart • Factors that cause partial or complete hearing loss • Plugging of the external auditory meatus • Changes to eardrum or auditory ossicles • Loud sounds • Tumors • Brain damage • Use of certain drugs

11. Static equilibrium • Sense the position of the head, maintaining stability and posture when the head and body are still • The head bending stimulates hair cells within the vestibule • Hair releases neurotransmitter which travel to CNS and inform brain of heads new position • Brain tells skeletal muscles to contract or relax to maintain balance • Dynamic equilibrium • Sense the position of the head, maintaining stability and posture when the head and body are in motion

12. Sense of sight • Eyes contain visual receptors • Accessory organs • Eyelid is the thinnest skin of the body and controlled by orbicularis oculi • Conjuctiva is a mucous membrane which lines inner surfaces of the lids • Lacrimal gland secretes tears continuosly • Tears contain lysozyme which is an antibacterial agent

13. Structure of eye • Outer layer • Cornea – transparent helps focus entering light rays • Sclera – white portion of eye • Middle layer • Choroid coat – honeycombed with blood vessels; contains melanin which absorbs excess light • Ciliary body – contain muslce fibers • Lens – transparent; facuses; globular shape; flattens when focusing on distant objects • Iris – thin diaphragm; colored portion • Aqueous humor – watery fluid • Pupil – circular opening; center of iris

14. Inner layer • Retina – contains visual receptor cells (photoreceptors) • Optic disc – nerve fibers from retine leave the eye and jion the optic nerve • Vitreous humor – jellylike fluid which supports the internal parts and helps maintain shape

16. Light refraction • Focusing which bends light waves • If eye shape is normal, light waves focus sharply on the retina • Images is upside down and reversed from left to right • Visual cortex interprets the image in its proper position

18. Visual receptors • Modified neurons called rods and cones • Rods have long, think projections oat their ends and provide black and white vision • Cones have short, blunt projections, and provide color vision

19. Touch and pressure • Senses of touch • Free nerve endings • touch and pressure • common in epithelial tissues • Meissner’s corpuscles • light touch • abundant in hairless portions of skin • Pacinian corpuscles • heavy pressure • Common in deep layers of skin, tendons, ligaments

20. Temperature senses • Warm receptors • Most sensitive to temperatures above 77°F • Become unresponsive at temperatures above 113°F • Temp. near and above 113°F stimulate pain receptors, producing a burning sensation • Cold receptors • Most sensitive to temperatures between 50°F and 68°F • Temperatures below 50°F stimulate pain receptors, producing a freezing sensation • Both warm and cold receptors adapt rapidly. Within about a minute of continuous stimulation, the sensation of warmth or cold begins to fade.

21. Sense of pain • Pain receptors protect the body because tissue damage stimulates them • It signals a person to act to remove the stimulation • Pain receptors adapt poorly, if at all • Once a pain receptor is activated, it may continue to send impulses into the CNS for some time

22. Regulation of Pain Impulses • Awareness arises when impulses reach the thalamus • Cerebral cortex determines pain intensity, locates pain’s source, and mediates emotional and motor responses to the pain • Graymatter in midbrain, pons, and medulla oblongata regulate movement of pain impulses from the spinal cord • Inhibiting substances released (enkephalins, serotonin, endorphins)

23. Sensation • A feeling that occurs when the brain interprets sensory impulses • Depends on which region of the brain receives the impulse • Sensory adaptation – ability to ignore unimportant stimuli