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Somatic and Special Senses Anatomy and Physiology The Pathway • Sensory receptor gets stimulated • Triggers a nerve impulse (wave of depolarization) • Impulse sent to the brain • You feel a sensation • Sensation: a feeling that occurs when brain interprets electrical impulses Sensory receptors stimulated by different and specific things • Chemoreceptors: chemicals • Pain receptors: tissue damage • Thermoreceptors: temperature change • Mechanoreceptors: pressure or movement • Photoreceptors: light Chemoreceptors • Taste • Smell • CO2 and O2 levels in the blood Pain Receptors • Consist of free nerve endings • Widely distributed in the skin • Not so accurate in the viscera • No pain receptors in the brain Referred Pain • Feels as though the pain is coming from somewhere else • Example: a person might feel pain in the left arm during a heart attack • Several body parts use the same nerve pathways Thermoreceptors • • • • Two kinds: Heat receptors Cold receptors Hot and cold extremes also stimulate pain receptors Mechanoreceptors • Pressure or movement • Free nerve endings • Meissner’s corpuscles are sensitive to light touch • Pacinian corpuscles are sensitive to heavy pressure Meissner’s corpuscle Pacinian corpuscle Photoreceptors • Sensitive to light • Only found in the eye • Rods and Cones Sensory Adaptation • When receptors continue to be stimulated, they stop sending the nerve impulse • Particularly noticeable with smell • Pain receptors adapt poorly, if at all What makes one sensation different from another? • All nerve impulses are the same • They are simply waves of depolarization— electrical signals • Depends on which area of the brain receives the impulse Sense of Smell • Chemoreceptors • Chemicals dissolved in liquid stimulate them • “Olfactory” is associated with smell • Smell and taste function together Olfactory Receptors • Olfactory organs in the upper part of the nasal cavity contain the olfactory receptors • The receptors are surrounded by columnar epithelial cells • Cilia cover the dendrite ends Olfactory Receptors Olfactory Receptors The nerve pathway • Receptors send impulses to neurons in the olfactory bulb • Olfactory nerve tracts take the impulse from the bulb to temporal and frontal lobes • Brain interprets the receptor combinations as an odor The Brain Areas Not everyone can smell! • 2 million people in the U.S. have no sense of smell • This condition is called anosmia • Usually due to damage in the olfactory nerves, but could also be frontal lobe damage Sense of Taste • Chemoreceptors • Receptors are in the taste buds • Taste buds are part of tiny elevations (bumps) on the tongue called papillae Taste buds are in the folds of the papillae papilla Taste bud Taste Buds The Four Taste Sensations • • • • • Sweet Sour Salty Bitter Taste can be a combination of these The Pathway • Taste receptors send nerve impulses along three cranial nerves into the medulla oblongata • From there it goes to the thalamus (relay center) to the parietal lobe Hearing • Involves mechanoreceptors • Vibration stimulates the tiny hairs mechanically • Chain reaction sends the vibrations to the inner ear Hearing • Ear is divided into three sections: outer, middle, and inner • Outer ear consists of auricle (pinna), external auditory canal (meatus), up to the eardrum (tympanic membrane) Hearing • Function of the auricle is to gather sound waves into the ear • Sound waves travel through the external auditory canal • Sound waves vibrate the eardrum Hearing • In the middle ear, there are three bones called ossicles. • They are lined up end to end. • The vibrating eardrum causes the first bone to vibrate, then the second, then the third. Hearing • The first bone is the malleus (hammer), then the incus (anvil), then the stapes (stirrup) Hearing • The stirrup pushes on another membrane called the oval window • This is the entrance into the inner ear • “Real” hearing takes place in the inner ear Hearing • The oval window is smaller than the tympanic eardrum (eardrum) • So there is more force per unit area, resulting in amplification of the vibrations Hearing • Oval window is touching the cochlea, the real organ of hearing • Chochlea is snail shaped • Inside the cochlea is the Organ of Corti, which contains the actual hearing receptors Hearing • The actual hearing receptors are hair cells inside the cochlea • The hair cells are attached to membranes. When the membranes vibrate, the hair cells are bent. • This triggers the attached neuron to depolarize, sending the impulse to the temporal lobe of the brain Hearing • Different frequencies of sound waves vibrate different parts of the membrane. • This causes different neurons to be stimulated, allowing us to hear different sounds Loss of Hearing • In older people, the membranes become less flexible • Could be damage to the auditory nerve or the cochlea • Loud noises or music can speed up the loss of hearing Ear Infections • Common in young children • Auditory tube connects the throat with the middle ear • Cold, sore throat, infection may spread into ear Normal eardrum Infected eardrum Balance and Equilibrium • Inner ear also controls balance and equilibrium • Static equilibrium senses the position of the head and posture when the body is still • Dynamic equilibrium involves moving Balance and Equilibrium • Static equilibrium is controlled by the vestibule, a structure located between the cochlea and the semicircular canals. Vestibule Balance and Equilibrium • In the vestibule, hairs of sensory neurons stick up into a gel material. Moving the head forward, backward, or to one side stimulates the hair cells. They send a nerve impulse to the brain. The brain controls muscles to maintain balance. Balance and Equilibrium Balance and Equilibrium • Dynamic equilibrium is controlled by the semicircular canals • The three semicircular canals are at right angles to each other, making them in different planes • Contain hair cells (receptors) and fluid • When body moves, the canals move, but the fluid stays stationary, bending the hairs The hair cells (sensory neurons, send impulses to the brain. The brain interprets and adjusts to maintain balance. Seasick Balance and Equilibrium Vertigo: a sensation of dizziness You are not moving, but your eyes send a message that you are. Causes: inner ear infection, or in roller coasters, motion sickness, receptors and brain cannot compensate for abnormal movements Balance and Equilibrium • Other structures help in maintaining equilibrium. Special mechanorecptors called proprioceptors detect body position. The eyes also send information about body position.
