6 Special Senses

• Auditory system


  • Transduce sound
  • Recognize environmental cues
  • Communicate with other organisms
  • Involved in language
• Vestibular system


  • Provide CNS with information related to the position and movements of the head in space
  • Control of eye movement
• Sound


  • Compression and decompression waves in air or in other elastic media such as water
  • Propagates at about 335 m/sec in air
  • Frequency measured in cycles per second, or hertz (Hz)
  • Amplitude and phase
• Sound pressure


  • Waves associated with pressure changes
  • Expressed as sound pressure level (SPL) or in decibel (dB)
• Noise


  Unwanted sounds that may have any composition of pure tones
• Normal young human ear sensitivity


  • Pure tones with frequencies between 20 and 20,000 Hz
  • Lowest threshold for human hearing are approximately 3000 Hz
  • Normal speech has an intensity of 65 dB
  • Sounds exceeding 100 dB can damage peripheral auditory apparatus
  • 120 dB can cause pain and permanent damage
• Presbycusis


  As people age, their thresholds at high frequencies rise, reducing their ability to hear such tones
• Peripheral auditory apparatus


  • Subdivided into: external, middle, and inner ear

  
• External ear


  • Includes the pinna and external auditory meatus (auditory canal)
  • Pinna helps direct sounds into the auditory canal and plays a role in sound localization
  • Auditory canal transmits pressure waves to the tympanic membrane
  • Auditory canal has a resonant frequency of about 3500 Hz
  • Cerumen: a waxy protective substance secreted by the canal

  
• Middle ear


  • Separated from the external ear by the tympanic membrane
  • Contains air
  • Ossicles link the tympanic membrane to the oval window of the inner ear
  • Adjacent to the oval window is the round window, another membrane-covered opening between the middle and inner ear
  • Ossicles: malleus, incus, stapes

  
• Inner ear


  • Includes the bony and membranous labyrinths
  • Bony labyrinth is a complex but continuous series of spaces in the temporal bone of the skull
  • Membranous labyrinth consists of a series of soft tissue spaces and channels lying inside the bony labyrinth
• Cochlea


  • Spiral-shaped organ consisting of 234 turns
  • Bony labyrinth component is subdivided into several chambers
  • Scala vestibuli, scala tympani, and scala media
• Perilymph


  • Fluid in the bony labyrinth, including the scala vestibuli and scala tympani, which closely resembles cerebrospinal fluid
  • Low [K+]; High [Na+]
• Endolymph


  • Fluid in the membranous labyrinth, including the scala media, which is generated by the stria vascularis
  • High [K+] (145 mM); Low [Na+] (2 mM)
• Organ of Corti


  • Neural apparatus responsible for transduction of sound, located within the cochlear duct
  • Lies on the basilar membrane and consists of: 3 rows of outer hair cells, single row of inner hair cells, gelatinous tectorial membrane, supporting cells
  • Contains 15,000 outer and 3500 inner hair cells
  • Rods of Corti help provide a rigid scaffold
  • Stereocilia on the apical surface contact the tectorial membrane
  • Innervated by nerve fibers of the cochlear division of the vestibulocochlear nerve (CN VIII)

  
• Sound transduction


  1. Sound waves reach the ear and cause the tympanic membrane to oscillate

  2. Oscillations transmitted to the scala vestibuli by the ossicles

  3. Shear forces set up by the relative displacement of the basilar and tectorial membranes cause the stereocilia of the hair cells to bend

  4. Upward displacement bends the stereocilia toward the tallest cilium, which depolarizes the hair cells

  5. Downward deflection bends the stereocilia in the opposite direction, which hyperpolarizes the hair cells

  
• Cochlear nerve fibers


  • Neurotransmitter release (glutamate) by hair cells in the organ of Corti can evoke action potentials in the primary afferent fibers of the cochlear nerve
  • Afferent fibers in the vestibulocochlear nerve (CN VIII) are bipolar cells with a myelin sheath around the cell bodies and axons
  • Cell bodies are in the spiral ganglion, peripheral processes synapse at the base of hair cells, and central processes synapse in the cochlear nuclei of the brainstem
  • Encodes duration by the duration of activity and intensity by the amount of neural activity and number of fibers that discharge
• Central auditory system


  1. Cochlear afferent fibers synapse on neurons of the dorsal and ventral cochlear nuclei

  2. Main ascending auditory tract: lateral lemniscus

  3. Ipsilateral or contralateral nuclei: superior ovary nuclei

  4. Each lateral lemniscus ends in an inferior colliculus

  5. Neurons of the inferior colliculus project to the medial geniculate nucleus of the thalamus, which gives rise to the auditory radiation

  6. Auditory radiation ends in the primary auditory cortex (Brodmann areas 41 and 42), located on the superior surface of the temporal lobe
• Receptive fields and tonotopic maps


  • "Tuning curves"
  • Neurons ordered according to their "best" frequencies

  
• Binaural interactions


  • Most auditory neurons at levels above the cochlear nuclei respond to stimulation of either ear
  • Contribute to sound localization

  
• Cortical organization


  • Isofrequency columns (neurons in the column have the same characteristic frequency)
  • Alternating summation and suppression columns
• Vestibular system


  • Detects angular and linear accelerations of the head
  • Signals allow body to make adjustments in posture that maintain balance and trigger head and eye movements to stabilize the visual image on the retina
  • Composed of: 3 semicircular canals, 2 otolith organs
  • Each structure contains endolymph and is surrounded by perilymph
  • Ampulla of each semicircular canal contains a sensory epithelium called a crista ampullaris
  • Sensory epithelia of the otolith organs are called the macula utriculi and the macula sacculi

  
• Vestibular transduction


  • Like cochlear hair cells, vestibular hair cells are functionally polarized
  • When the stereocilia are bent toward the longest cilium (kinocilium), the vestibular hair cell is depolarized
  • When the cilia are bent away from the kinocilium, the hair cell is hyperpolarized
• Vestibular system


  • Allows body to make adjustments in posture that maintain balance and trigger head and eye movements to stabilize the visual image on the retina
• Components of vestibular system on each side


  • 3 Semicircular canals (Horizontal, Anterior, Posterior)
  • 2 Otolith organs (Utricle, Saccule)
  • Contain endolymph and are surrounded by perilymph
  • Each semicircular canal has a swelling called an ampulla at the point where it joins the utricle
  • The ampulla of each of the semicircular canals contains a sensory epithelium called a crista ampullaris or ampullary crest
  • The sensory epithelia of the otolith organs are called the macula utriculi and the macula sacculi
• Vestibular transduction


  Like cochlear hair cells, vestibular hair cells are functionally polarized
• Vestibular transduction


  1. When the stereocilia are bent toward the longest cilium (kinocilium), the vestibular hair cell is depolarized

  2. When the cilia are bent away from the kinocilium, the hair cell is hyperpolarized

  3. The hair cell releases an excitatory neurotransmitter (glutamate or aspartate) tonically, so that the afferent fiber on which it synapses has a resting discharge

  4. When the hair cell is depolarized, more neurotransmitter is released, and the discharge rate of the afferent fiber increases

  5. When the hair cell is hyperpolarized, less neurotransmitter is released, and the firing rate of the afferent fiber slows
• Semicircular canals


  1. As acceleration to the left begins, the inertia of the endolymph in the horizontal canals increases pressure toward the right side

  2. This causes the cilia to bend on hair cells of the ampulla of the left horizontal canal toward the utricle and bends the cilia of the right canal away from the utricle

  3. These actions increase the firing rate in the afferent fibers on the left and decrease the firing rate of the afferent fibers on the right

  4. Once the head is moving at a constant velocity of rotation (i.e., no acceleration), there would be no force on either cupula, and therefore the hair cells of both canals would be firing as they do at rest
• Otolith organs


  • Not all the hair cells in the otolith organs are oriented in the same direction. Instead, they are oriented in relation to a ridge, called the striola
  • In the utricle, the hair cells on either side of the striola are polarized toward the striola, whereas in the saccule they are polarized away from the striola

  
• Location of primary afferent fibers of the vestibular nerve


  • Scarpa ganglion
• Location of vestibular nuclei


  • Rostral medulla
  • Caudal pons
  • Cerebellum (specifically Nodulus)

  
• Projections from vestibular nuclei

  • Medial longitudinal fasciculus to the oculomotor nuclei, for controlling eye movements (vestibule-ocular reflex)
  • Lateral and medial vestibulospinal tracts, which, respectively, activate the trunk and neck muscles for equilibrium and head movements (vestibulocolic reflex)

  
• Gustation (taste)


  Sense that helps detect chemical stimuli present in food
• Olfaction (smell)


  Sense that helps detect chemical stimuli present in air

  
• In the evolution of humans, the chemical senses (taste and smell) apparently did not have the survival value of some of the other senses, but they contribute considerably to quality of life and food selection, and they are important stimulants of digestion
• Elementary taste qualities


  • Salty (Sodium chloride)
  • Sweet (Sucrose)
  • Sour (Hydrochloric acid)
  • Bitter (Quinine)
  • Umami (Monosodium glutamate)
• Taste receptors


  Chemoreceptors located in the taste buds
• Components of a taste bud

  • 50-150 receptor cells
  • Supporting cells
  • Basal cells
• Chemoreceptor cells


  Synapse at their bases with primary afferent nerve fibers, and their apices have microvilli that extend toward a taste pore
• Chemoreceptor cells live only about 10 days