The senses are olfaction (smell), gustation (taste), somatosensation (sensations associated with the skin and body), audition (hearing), equilibrium (balance), and vision. With the exception of somatosensation, this list represents the special senses, or those systems of the body that are associated with specific organs such as the tongue or eye. Somatosensation belongs to the general senses, which are those sensory structures that are distributed throughout the body and in the walls of various organs. The special senses are all primarily part of the somatic nervous system in that they are consciously perceived through cerebral processes, though some special senses contribute to autonomic function. The general senses can be divided into somatosensation, which is commonly considered touch, but includes tactile, pressure, vibration, temperature, and pain perception. The general senses also include the visceral senses, which are separate from the somatic nervous system function in that they do not normally rise to the level of conscious perception.
The cells that transduce sensory stimuli into the electrochemical signals of the nervous system are classified on the basis of structural or functional aspects of the cells. The structural classifications are either based on the anatomy of the cell that is interacting with the stimulus (free nerve endings, encapsulated endings, or specialized receptor cell), or where the cell is located relative to the stimulus (interoceptor, exteroceptor, proprioceptor). Thirdly, the functional classification is based on how the cell transduces the stimulus into a neural signal. Chemoreceptors respond to chemical stimuli and are the basis for olfaction and gustation. Related to chemoreceptors are osmoreceptors and nociceptors for fluid balance and pain reception, respectively. Mechanoreceptors respond to mechanical stimuli and are the basis for most aspects of somatosensation, as well as being the basis of audition and equilibrium in the inner ear. Thermoreceptors are sensitive to temperature changes, and photoreceptors are sensitive to light energy.
The nerves that convey sensory information from the periphery to the CNS are either spinal nerves, connected to the spinal cord, or cranial nerves, connected to the brain. Spinal nerves have mixed populations of fibers; some are motor fibers and some are sensory. The sensory fibers connect to the spinal cord through the dorsal root, which is attached to the dorsal root ganglion. Sensory information from the body that is conveyed through spinal nerves will project to the opposite side of the brain to be processed by the cerebral cortex. The cranial nerves can be strictly sensory fibers, such as the olfactory, optic, and vestibulocochlear nerves, or mixed sensory and motor nerves, such as the trigeminal, facial, glossopharyngeal, and vagus nerves. The cranial nerves are connected to the same side of the brain from which the sensory information originates.
ampulla
in the ear, the structure at the base of a semicircular canal that contains the hair cells and cupula for transduction of rotational movement of the head
audition
sense of hearing
auricle
fleshy external structure of the ear
basilar membrane
in the ear, the floor of the cochlear duct on which the organ of Corti sits
cochlea
auditory portion of the inner ear containing structures to transduce sound stimuli
cochlear duct
space within the auditory portion of the inner ear that contains the organ of Corti and is adjacent to the scala tympani and scala vestibuli on either side
cupula
specialized structure within the base of a semicircular canal that bends the stereocilia of hair cells when the head rotates by way of the relative movement of the enclosed fluid
equilibrium
sense of balance that includes sensations of position and movement of the head
external ear
structures on the lateral surface of the head, including the auricle and the ear canal back to the tympanic membrane
hair cells
mechanoreceptor cells found in the inner ear that transduce stimuli for the senses of hearing and balance
incus
(also, anvil) ossicle of the middle ear that connects the malleus to the stapes
inner ear
structure within the temporal bone that contains the sensory apparati of hearing and balance
macula
enlargement at the base of a semicircular canal at which transduction of equilibrium stimuli takes place within the ampulla
malleus
(also, hammer) ossicle that is directly attached to the tympanic membrane
middle ear
space within the temporal bone between the ear canal and bony labyrinth where the ossicles amplify sound waves from the tympanic membrane to the oval window
organ of Corti
structure in the cochlea in which hair cells transduce movements from sound waves into electrochemical signals
ossicles
three small bones in the middle ear
otolith
layer of calcium carbonate crystals located on top of the otolithic membrane
otolithic membrane
gelatinous substance in the utricle and saccule of the inner ear that contains calcium carbonate crystals and into which the stereocilia of hair cells are embedded
oval window
membrane at the base of the cochlea where the stapes attaches, marking the beginning of the scala vestibuli
round window
membrane that marks the end of the scala tympani
saccule
structure of the inner ear responsible for transducing linear acceleration in the vertical plane
scala tympani
portion of the cochlea that extends from the apex to the round window
scala vestibuli
portion of the cochlea that extends from the oval window to the apex
semicircular canals
structures within the inner ear responsible for transducing rotational movement information
spiral ganglion
location of neuronal cell bodies that transmit auditory information along the eighth cranial nerve
stapes
(also, stirrup) ossicle of the middle ear that is attached to the inner ear
stereocilia
array of apical membrane extensions in a hair cell that transduce movements when they are bent
tectorial membrane
component of the organ of Corti that lays over the hair cells, into which the stereocilia are embedded
tympanic membrane
ear drum
utricle
structure of the inner ear responsible for transducing linear acceleration in the horizontal plane
vestibular ganglion
location of neuronal cell bodies that transmit equilibrium information along the eighth cranial nerve
vestibule
in the ear, the portion of the inner ear responsible for the sense of equilibrium
Figure 8 The basilar membrane is the thin membrane that extends from the central core of the cochlea to the edge. What is anchored to this membrane so that they can be activated by movement of the fluids within the cochlea?
Figure 8 The hair cells are located in the organ of Corti, which is located on the basilar membrane. The stereocilia of those cells would normally be attached to the tectorial membrane (though they are detached in the micrograph because of processing of the tissue).
Watch this video to learn more about how the structures of the ear convert sound waves into a neural signal by moving the “hairs,” or stereocilia, of the cochlear duct. Specific locations along the length of the duct encode specific frequencies, or pitches. The brain interprets the meaning of the sounds we hear as music, speech, noise, etc. Which ear structures are responsible for the amplification and transfer of sound from the external ear to the inner ear?
The small bones in the middle ear, the ossicles, amplify and transfer sound between the tympanic membrane of the external ear and the oval window of the inner ear.
Watch this animation to learn more about the inner ear and to see the cochlea unroll, with the base at the back of the image and the apex at the front. Specific wavelengths of sound cause specific regions of the basilar membrane to vibrate, much like the keys of a piano produce sound at different frequencies. Based on the animation, where do frequencies—from high to low pitches—cause activity in the hair cells within the cochlear duct?
High frequencies activate hair cells toward the base of the cochlea, and low frequencies activate hair cells toward the apex of the cochlea.
1. What type of receptor cell is involved in the sensations of sound and balance?
A) photoreceptor
B) chemoreceptor
C) mechanoreceptor
D) nociceptor
C