The Stimulus – Sound waves converted to fluid waves converted to electrical impulses
Anatomy of the Ear
Pinna – composed of cartilage and has relatively poor blood supply. Its presence on both sides of the head allows us to localize the source of sound from front vs. the back.
Ear Canal – Composed of cartilage and bone. Total length is about 1 inch in adults.
Tympanic Membrane – The tympanic membrane is actually three layers. It is continually growing structure.
Malleus – “hammer”
Incus – “anvil”
Stapes – “stirrup” - footplate is seated over oval window.
Round Window – most basal end of scala tympani, allows the release of hydraulic pressure of perilymph that is caused by vibration of the stapes within the oval window.
Cochlea – Snail shaped structure that is the sensory organ of hearing.
Scala Vestibuli – One of the three partitions within the cochlea filled with perilymph. (Same as cerebro spinal fluid –CSF. It has high concentration of Na+ sodium and a low concentration of K+ potassium.)
Scala Media – Also one of three partitions with the cochlea, the scala media is filled with endolymph. (Endolymph has a high K+ concentration and a low Na+ concentration.)
Organ of Corti – within the scala media, the organ of Corti is the sense organ of hearing. The outer and inner hair cells of the organ of Corti change vibrational energy into neural energy that is transmitted via the VII verve to the brain.
Scala Typani – The third partition, the scala tympani also contains perilymph.
Reissner’s Membrane – Separates the endolymph of the scala media from the perilymph of the scala vestibule.
Tectorial Membrane - A delicate, flexible, gelatinous membrane overlying the sensory receptive inner and outer hair cells.
Vestibular Labyrinth – composed of saccule and utricle – sense organs of balance.
VIII Nerve– Auditory nerve – transmits information from the cochlea and vestibular labyrinth to the brain.
Facial Nerve – VII cranial nerve, travels parallel with the VIII cranial nerve through the internal acoustic canal.
Auditory Hair Cells and the Transduction of Auditory Information
Outer Hair Cells – There are three rows of approximately 12,000 outer hair cells. Although they are much greater in number they receive only about 5% of the innervation of the nerve fibers from the acoustic portion of the VIII nerve.
Inner Hair Cells – There is one row of approximately 3500 inner hair cells. These cells receive about 95% of the innervation from the nerve fibers from the acoustic portion of the VIII nerve. These cells have primary responsibility for producing our sensation of hearing.
The Auditory Pathway
Tympanic membrane – Ossicles – Stirrup/oval window – basilar membrane flexes – moves laterally under tectorial membrane – cilia of the outer hair cells cause fluid movement w/in cochlea - inner hair cells wave opening ion channels - form synapse with dendrites of bipolar branch of 8th auditory nerve - cochlear nuclei of medulla – superior olivary complex, also in medulla – inferior colliculus in dorsal midbrain – medial geniculate nucleus (MGN) of thalamus to auditory cortex of temporal lobe
High frequency sounds cause the base of the basilar membrane (near the oval window closest to stapes) to flex; low frequency sounds cause the apex (opposite end) to flex. They stimulate different groups of auditory hair cells.
Cochlear implants – External part is microphone and miniature electronic signal processor. Internal part is very thin flexible array of electrodes which follow the snail-like curl and end up resting along the entire length of the basilar membrane. Primary purpose of cochlear implant is to restore a person’s ability to understand speech.
Perception of Timbre
Perception of Spatial Location
Behavioral Functions of the Auditory System –
Primary function of hearing:
1. Detect sound.
2. Determine location of source of sound.
3. Recognize the identity of these sources.
Lesions of the auditory cortex
Auditory Agnosia – Inability to comprehend the meaning of sounds even if the individual is not deaf.
Left hemisphere – aphasia – poor speech comprehension and production of meaningless speech.
Right hemisphere – inability to recognize the nature or location of non-speech sounds.
Anatomy of the Vestibular Apparatus
Vestibular Sacs Utricle “little pouch” and Saccule “little sack” - Respond to force of gravity and inform the brain about the head’s orientation.
Semicircular Canals – Respond to angular acceleration - changes in rotation of the head or changes in position. Approximate three major planes of the head: saggital, transverse, and horizontal.
Ampulla – contains organ in which sensory receptors reside. Cilia are embedded in a gelatinous mass called the cupula.
The Receptor Cells – These hair cell resemble the auditory hair cells and their transduction mechanism is also similar.
The Vestibular Pathway - Vestibular and cochlear nerves constitute the two branches of the VIII cranial nerve.
Vestibular nuclei to cerebellum, spinal cord, medulla and pons, and to temporal cortex. Responsible for feeling of dizziness.
Also to cranial nerves III, IV and VI that control the eye muscles. Compensate for sudden head movements – Vestibulo-ocular reflex.
Spinal nerves – dorsal root ganglia, to nuclei in lower medulla, through the medial lemniscus to ventral posterior nuclei VPN of Thalamus – primary somatosensory cortex, to secondary somatosensory cortex.
Perception of Pain
Pain Receptors – Nociceptors – “Detectors of noxious stimuli”
1. High threshold mechanoreceptors – free nerve endings that respond to intense pressure.
2. VR 1 Free nerve ending - Responds to extremes of heat, to acids, and to the presence of capsaicin, i.e. chili peppers.
3.ATP. – When blood supply to a region of the body is disrupted - ischemia, which occurs during a spasms of blood vessels that cause angina or migraine or muscle damage. The nociceptor responsible for the pain caused by angina, migraine, damage to muscles and cancer.
Sensory Pain -Spinal cord – Ventral posterolateral thalamus to primary and primary and secondary somatosensory cortex.
Unpleasantness of pain – Anterior cingulated cortex and insular cortex.
Emotional consequences of chronic pain – Prefrontal cortex.
Phantom Limb – Organization of parietal cortex especially the right hemisphere.
Chemical reaction with receptors
Umami – Japanese word means “good taste” refers to the taste of MSG.
Anatomy of the Taste Buds and Gustatory Cells
Tongue, palate, pharynx and larynx contain approximately 10,000 taste buds.
The Gustatory Pathway - Cranial nerves VII, IX and X. – to medulla to Ventral Posteromedial thalamus to primary gustatory cortex to amygdala and hypothalamus and basal forebrain.
The Stimulus – Chemical sense
Seems to be related to memory most effectively
Anatomy of the Olfactory Apparatus
6,000,000 olfactory receptors. Two patches of mucous membrane each made up of an area of about 1 sq. inch.
Transduction of Olfactory Information
Olfactory receptor cells divide into 10 – 20 cilia that penetrate the layer of mucus. 35 bundles of axons, ensheathed by glial cells enter the skull through small holes in the “perforated” plate. The Olfactory bulbs lie at base of brain, Olfactory tract axons project directly to amygdale and to two regions of limbic cortex: Pyriform cortex and entorhinal cortex. Amydala to hypothalamus. Entorhinal cortex to hippocampus and pyriform cortex to hypothalamus and orbitofrontal cortex via dorsomedial nucleus of thalamus.
Perception of Specific Odors
Humans can detect about 10,000 different odorants.