Notes

collected by

Asmaa Salah

Asmaa Salah

Neuroplasticity of the Auditory System Definition: refers to the nervous system’s ability to reorganize and adapt in response to internal and external changes.  This process continues throughout life and reflects the ability to acquire new skills and behavior. Neuroplasticity has replaced the formerly-held position that the brain is a physiologically static organ, and explores how - and in which ways - the brain changes throughout life  The orgniztion of CANS can be modified or reorganized by cochlear pathology, central lesion, maturation, experience ,learning or rehabilitation

 One of its mechanisms is the increase in synaptic activity and efficacy following strong and repeated stimulation of the sensory system called Long Term Potentiation.  Neuroplasticity can work in two directions; it is responsible for deleting old connections as frequently as it enables the creation of new ones. Through this process, called “synaptic pruning,” connections that are inefficient or infrequently used are allowed to fade away, while neurons that are highly routed with information will be preserved, strengthened, made even more synaptically dense Classification of Neuroplasticity Clifford (1999) classification: 1. Experience independent plasticity, the brain sculpts itself as a result of spontaneous internally generated activities. 2. experience expectant plasticity, the brain uses inputs from the external environment to guide its structural developmental changes. 3. experience dependent plasticity. responsible for the improvement after remediation. It is encountered if a modification in external stimuli produces changes in the feature of the brain Another classification by Shinn-Cunningmam (2001)  developmental, the brain ability to organize its own circuits and related function during maturation. Asmaa Salah

 compensatory the brain ability to recover and use another cues to perform a certain task when its realistic cues are not available.(after lesion or damage)  experience dependent plasticitythe neuro-chemical and structural change occurring in the brain as consequences of external stimulation such as training (learning related) Mechanisms of Plasticity The organization in neuroplasticity may involve 1. the activation of neurons and neural connections that were previously in a state of rest (the reserve neurons replace the non functional neurons). 2. the formation of new connections between neurons (arborization).

3. increasing neuronal thickness and increasing the size of synapses intensify signal transduction, enhances the neurotransmitter release from pre-synaptic terminals to neurotransmitter 4. Long term potentiation - Definition: the incrase in synaptic activity and efficacy following strong and repeated stimulation of asensory system - it occurs in amygdala and hippocampus regions of the brain which generally linked to memory. - Basis: changes in synaptic morphology and chemical neurotransmission (strong transmission is associated with both chemical and anatomical changes at the synaptic level) Factors affecting plasticity 1. Maturation: the younger the brain ,the more plastic it is. - the ability of the auditory cortex to reorganize continues throughout life ,this reflects the ability to acquire new skills and behaviour 2. Type of experience:  active neural input to the brain is essential during very early stages of develpment so auditory experienc has apowerful influence on central fuction throughout life  auditory deprivation e.g -conductive hearing loss:early history of chronic otitis media with effusion is associated with higher incidence of learning difficulties , abnormal MLD . Asmaa Salah

- cochlear hearing loss: plasticity occurs here by tonotopic reorganization of the auditory cortex( the deprived region of the cortex become responsive to fequencies of adjacent auditory region)  auditory stimulation and training have been shown to facilitate improvement in auditory processing abilities - stimulation and experience activate nd strenghten neural pathways whereas unstimulated pathways atrophy

Significance Plastic Changes in the Auditory System  In the auditory system, the sensory epithelium is re-represented in all successive nuclei within the auditory pathway up to and including the auditory cortex. Because sound frequency is space mapped in the cochlea, the cochleotpic representation is usually named tonotopic representation. 

that central sensory maps are subject to reorganization as a result of manipulation of the peripheral sensory input particularly during early development.

Plasticity Resulting from Aging:  Age-related hearing deficits in humans have usually been attributed to changes in the cochlea, including a loss of sensory cells, atrophy of stria vascularis, and a loss of spiral ganglion cells. However, an important part of presbycusis consists of changes in the central auditory system Presbycusis is accompanied by a loss of spiral ganglion cells, which usually comprises the loss of peripheral dendrites as well as central projections, i.e., auditory nerve fibers. The efferent system seems to be more robust and does not degenerate to the same extent as the afferent system. In the central auditory system a marked reduction in the cell size of neurons in the ventral cochlear nucleus was found in subjects with profound hearing loss Neural Plasticity in Tinnitus  reduced input to the auditory nervous system, as occurs in hearing loss, may increase the sensitivity of the ear and certain parts of the auditory nervous systems.  Studies in humans and animals have shown indications that tinnitus may be associated with reorganization of auditory nuclei and the auditory cerebral cortex

Asmaa Salah

Experience Dependent Plasticity in the Central Auditory System:  Long-term plasticity has been confirmed n the human auditory cortex. Increased auditory cortical representation was observed in highly skilled musicians. Dipole moments for piano tones, but not for pure tones. Enlargement was correlated with the age at which musicians began to practice and did not differ between musicians with absolute and relative pitch Auditory perceptual learning  Two forms of auditory perceptual learning that are of great practical significance relate to speech perception 1. the effect of language experience on the perception of speech sounds, and thus on language acquisition, during a critical period of development. 2. the improvements in speech perception shown by cochlear implantees 3. over the post-implantation period; there appears to be no evidence to indicate the extent to which this form of learning reflects plasticity in auditory cortex itself or in higher-level areas. Neural Plasticity in Cochlear Implant:  It is common experience that all subjects wearing cochlear implants show continuous improvement in their ability to hear several years after implantation  that most gains in performance with CI occur in the first 9-12 mo of use. This holds true particularly for postlingually deaf adults, although some patients show continued improvement during 4 or 5 years of implant use.  Adults who have been deaf for many years and who received their implant when they were older tend not to perform as well as adults who have been deaf for only a few years or who received their implant when they were younger.

Asmaa Salah

 The first PET imaging studies demonstrated a revival of brain metabolic activity in auditory cortex areas of deaf people after implantation  In another study cerebral blood flow in postlingually deafened CI users was compared with that in normal subject using PET .hearing speech activated more cortical areas in cochlear implant users than in normal subject. A comparison of speech activation in these two groups revealed higher activation in CI users not only in the temporal cortices but also in Broca's areas and its right hemisphere homolog, the supplementary motor area and the anterior cingulated gyrus

Asmaa Salah

Neurotransmitters  Neurotransmitters chemicals that cross a synapse and mediate nerve impulse transmission from one neuron to the next neuron, or from sensory cells (hair cells in the cochlea) to neurons, or from neurons to effector cells (in this case, the same sensory hair cells of the cochlea) • Neurotransmission is the process of transferring a message from the axon of one nerve cell, across a small space or synapse, to the dendrites of a nearby neuron. This message is transmitted via a chemical substance called a neurotransmitter

Neurotransmitters of the auditory system • Neurotransmission in the auditory system largely involves a balanced release of excitatory and inhibitory neurotransmitters Excitatory neurotransmission is mediated primarily by glutamate, while GABA and glycine mediate inhibitory neurotransmission

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Simplified diagram of neurotransmission in the Auditory System The first few connections are mainly rapid relays. IHCs form excitatory synapses (red color; +) with spiral ganglion neurons (SGN), and these form excitatory synapses with neurons in the cochlear nuclei. Within the cochlear nuclei, spherical bushy cells (SB) send excitatory input to LSO in the SOC. Beginning at this level, some more complicated connections allow neurons to compare sound input from both ears (i.e., for sound localization •

LSO neurons receive ipsilateral excitatory input from SB and contralateral input from globular bushy cells (GB) via an excitatory synapse with neurons of the MNTB (MN). MNTB neurons make glycinergic inhibitory (green color; –) synapses with LSO neurons

•

In turn, cochlear nuclei and LSO and other SOC nuclei make excitatory and inhibitory connections with higher centers. Combination of excitatory (probably mainly glutamatergic) and inhibitory (GABA {orange color} or glycine {green color}) connections allows binaural integration of sound information at these higher centers (DNLL; IC).

Asmaa Salah

Neurotransmitters of the cochlea the afferent and efferent neurotransmitters in the cochlea A. Afferent to the IHCs are from type I spiral neuron. Glu is the putative neurotransmitters at this synapse. •

Efferent of the IHCs are from the lateral olivocochlear bundle, which project onto the IHCs ,Ach, DYN, ENK and CGRP are putative neurotransmitters at this synapse.

• Afferent to the OHCs are from type II spiral ganglion neurons with Glu as the putative neurotransmitters at this synapse. • Efferent of the OHCs re from the medial olivocochlear bundle, which projects directly onto the OHCs. Ach, GABA, Enk, and CGRP are likely neurotransmitters at this synapse

 neurotransmitters in the ascending and descending bran stem auditory pathway • GLU is the putative neurotransmitters at the auditory nerve to the CN, the projection from CN to SOC is also glutamatergic. • Inhibitory neurotransmitters in the pathway from the SOC to IC is mediated by glycine. • Glycine is the neuotransmitters projecting from contralateral CN • Descending projections to the CN arise from the SOC. The medioventeral periolivary nucleus (MVPC), or lateral superior olive (LSO) use GABA and Ach respectively Asmaa Salah

Asmaa Salah