Human Anatomy and Physiology Special Senses

All senses work the same way: Receptors collect information stimulate neurons information is sent to the brain the cerebral cortex integrates the information forms a perception (a person’s particular view of the stimulus)

Receptor types: Pain receptors or nociceptors – respond to tissue damage due to mechanical, electrical, thermal or chemical energy Thermoreceptors – respond to temperature change

Receptor types: Mechanoreceptors – respond to mechanical forces, such as pressure or fluid movement; changes usually deform the receptor Proprioceptors – sense changes in muscles and tendons Baroreceptors – in blood vessels – detect changes in pressure Stretch receptors – in lungs – sense degree of inflation

Receptor types: Photoreceptors -respond to light – as little as one photon Chemoreceptors – sensitive to chemical concentration of various substances

Receptors are structured in two basic ways: receptors can be nerve endings other kinds of cells which are associated with nerve endings When these are stimulated, they produce graded potentials. If hit threshold, nerve fires.

A sensation or perception occurs when the brain interprets the incoming nerve impulses. All impulses coming into the brain are alike. The sensation depends on which part of the brain is stimulated. Synesthesia = “joined perception” tasting, colors, etc.

Who has synethesia Women : in the U.S., studies show that three times as many women as men have synesthesia; in the U.K., eight times as many women have been reported to have it. The reason for this difference is not known. Left-handed : synesthetes are more likely to be left-handed than the general population. Neurologically normal : synesthetes are of normal (or possibly above average) intelligence, and standard neurological exams are normal. In the same family : synesthesia appears to be inherited in some fashion; it seems to be a dominant trait and it may be on the X- chromosome.

Developmental Aspects of the Special Senses Special sense organs are formed early in embryonic development. Maternal infections during the first five or six weeks of pregnancy may cause visual abnormalities as well as sensorineural deafness in the developing child. An important congenital eye problem is strabismus. The most important congenital ear problem is lack of the external auditory canal. Vision requires the most learning. The infant has poor visual acuity (is farsighted) and lacks color vision and depth perception at birth. The eye continues to grow and mature until the eighth or ninth year of life, Problems of aging associated with vision include presbyopia, glaucoma (the most common cause of blindness in the U.S.), cataracts, and arteriosclerosis of the eye's blood vessels. The newborn infant can hear sounds, but initial responses are reflexive. By the toddler stage, the child is listening critically and beginning to imitate sounds as language development begins. Sensorineural deafness (presbycusis) is a normal consequence of aging. Taste and smell are most acute at birth and decrease in sensitivity after the age of 40 as the number of receptors decreases.

Sensory adaptation The only receptors that don’t adapt are: pain receptors Somatic Senses: Exteroceptive senses – changes at body surface Proprioceptive senses – changes in muscles and tendons and body position Visceroceptive senses – changes in viscera(The internal organs of the abdomen and thorax; specifically, the hollow Organs such as intestines, bladder, etc.)

Touch and pressure senses: 1. Free nerve endings – touch and pressure 2. Meissner’s corpuscles – light touch receptors are connective tissue 3. Pacinian corpuscles – heavy pressure and vibrations receptors are connective tissue Itch and Tickle: Receptors are free nerve endings

Pain Also free nerve endings Most pain receptors can be stimulated by more than one stimulus Deficiency of blood flow (ischemia) and thus a deficiency of oxygen (hypoxia) can stimulate pain receptors.

Pain fibers are of two types: Acute pain fibers ( A or delta fibers) – thin, myelinated fibers (Conducts up to 30 meters/sec) Sharp, localized pain Seldom continues after stimulus stops Chronic pain fibers (C fibers) – thin, unmyelinated fibers (conduct up to 2 meters per second) Dull, aching and widespread pain May continue for some time after stimulus

Stretch receptors: We know how our body parts are moving through our proprioceptive or kinesthetic sense. These receptors adapt only slightly Keep brain informed of the status of body parts to insure coordination. Use specialized receptors that sense tension in tendons and muscles. No sensation occurs when these are stimulated.

Muscle spindles sense stretching of muscle, and cause contraction Of the muscle to maintain position. Golgi tendon organs sense stretching of tendons and cause the muscle to relax to prevent damage to the tendon.