Auditory and Vestibular Physiology (Physiology 2)

Auditory and Vestibular Physiology - Dr Penny Murphy Lecture Outline The sensory apparatus of both auditory and vestibular systems is found in the inner ear, and uses the same basic type of receptor. The auditory system plays a vital role in human communication, as well as being a source of great pleasure for many. Loss of hearing, especially late in life, can restrict the ability to socialise with others, to gain information via normal communication routes, and to enjoy some forms of entertainment. The resulting isolation can have detrimental effects on both physical and psychological health, daily activity and economic well-being. The vestibular system is best known as the system responsible for our sense of equilibrium and the ability to maintain upright posture. However, it is also vital for the accurate control of eye movements. The vestibular system provides the afferent drive for a set of reflexes that compensate for self-movement, allowing us to achieve stable visual fixation on a stationary target and to accurately follow a moving target with our eyes. Damage to the vestibular system can lead to dizziness and a sense of disequilibrium, persistent oscillatory eye movements (nystagmus) and an associated sensation that the world is spinning around the head or that the head itself is spinning persistently (vertigo). A mismatch between vestibular and other sensory signals is thought to mimic the effects of poisoning, leading to nausea and vomiting. This lecture will cover the structure and function of the hair cell receptors that serve both systems, and the tunnels and membranes of the inner ear that channel the stimuli in such a way that the different groups of hair cells respond to diverse stimuli (sound waves of different frequencies, rotational movements of the head, linear movements of the head, the direction of gravity). It will cover the basics of auditory system function and stimulus encoding, as well as some vulnerabilities in the system that lead to loss of hearing. Likewise it will cover how the vestibular system responds to and hence encodes different types of head movement and gravity, and how it integrates this information with visual input in order to ensure accurate eye movements. Desired learning outcomes Describe the structure of a generic hair cell receptor, and explain how it converts stimulation of the stereocilia into a receptor potential. Describe how the structure of the ear channels sound waves of different frequencies to activate specific populations of auditory receptor cells (hair cells) Explain how the auditory system captures and encodes information about the loudness, frequency, origin and timing of a sound, and how this is passed on to and used by the cerebral cortex Explain the distinction between inner and outer hair cells and their respective roles in hearing Explain the vulnerabilities in the system that can lead to conductive and sensorineuronal deafness. Describe how the structure of the inner ear creates fluid movement in response to different types of head movement and changes in the direction of gravity, such as to generate responses in different populations of vestibular hair cells. Describe the neural pathways for vestibulo-ocular reflexes, including the role of the cerebellum for short- and long-term modulation of these responses Session description This is an on-line, self-learning session sub-divided into three sections. 1. This lecture connects with a number of previous sessions, which you should identify and review before starting some sections. Don't move on without ensuring that you are confident in the relevant background knowledge. 2. Each section has a powerpoint and Panopto presentation, which include outlines of the contents and a check-list of things you should be able to do before moving on to the next section To see the slides properly, use the download icon and run in powerpoint presentation view. Session Checklist The three sections will take approximately 1 hour to listen to, but studying them properly will take 2-3 times as long: Part a - 15 mins Part b - 25 mins Part c - 20 mins Session Recording and Powerpoints Part a: Hair Cells This segment introduces the hair cell receptors that serve both systems, and gives an overview of the auditory and vestibular sensory apparatus. Preparation - before going further: review your previous teaching on membrane potentials, and ensure that you understand the changes that will occur if cation (ie +vely charged ion) channels are opened in a nerve cell membrane. Once you've done that, go ahead with the lecture. Part a powerpoint slidesDownload Part a powerpoint slides Part b: Auditory System This segment goes back through the structures of the outer, middle and inner ears, adding functional information, and then concentrates on the cochlea of the inner ear and the neural apparatus needed to transduce and encode sound. The cochlea is made up of three chambers, separated by flexible membranes that channel and vibrate in time with the sound waves. One of these is the basilar membrane, which supports the spiral organ which in turn houses the auditory receptors. The auditory receptors are hair cells - so called because they have a tuft of sterocilia sprouting from their apical surface. Thanks to the arrangement of chambers, the body of each hair cell is surrounded by a normal extracellular fluid while the stereocilia stick out into a compartment filled with potassium rich endolymph. Endolymph has an excess of positive charge compared with the surrounding tissue, which creates an electrical gradient that drives K+ into the hair cell when sound waves open the mechanically-gated channels in the stereocilia. This depolarises the cell and causes it to release glutamate from the synapse. The structure of the cochlea, the location and function of the inner hair cells, and further processing in the superior olivary nuclei capture and encode key characteristics of the sound. Their signal is transmitted to the auditory cortex, where these characteristics are used to reconstruct and understand the sound. The outer hair cells act as an amplification system (and famously dance to Elvis). Preparation - before going further: Make sure that you understand how hair cells function and that you've grasped the overall anatomy of the auditory part of the system. Once you've done that, go ahead with the lecture. Part b powerpoint slidesDownload Part b powerpoint slides Part c: Vestibular System This segment will cover the two systems that are found within the vestibular labyrinth. One picks up and encodes linear movements of the head and changes in the direction of gravity. The other deals with rotational movements. In both cases it is changes in the speed of movement that matter - ie acceleration or deceleration - because tilting of the hair cell stereocilia depends on the effect of inertia on the structures that are associated with them. Both systems feed into the motor system, providing compensation for externally generated movements of the body. The semi-circular canals are particularly important for the control of eye movements, and this will be covered in a little more detail. Preparation - before going further: Make sure that you've grasped the overall anatomy of the vestibular part of the system. Note that vestibular hair cells function in much the same way as those in the auditory system, so if anything isn't clear review parts a and b. Note that I assume you have some understanding of the role of extraocular eye muscles and the importance of foveal vision. Part c powerpoint slidesDownload Part c powerpoint slides Well done, you've reached the end of the session If you want to test test yourself, either now or during exam revision, this link will take you to a quiz that uses exam-style questions and provides feedback on incorrect answers. Auditory & Vestibular quiz Follow up activities This is where the real learning takes place. Discuss the material in this lecture with others You need to practice using the information, and to do so in different contexts. Answering quiz questions is one way to do that, but any quiz will cover only a small part of the material. Active discussion is more wide-ranging and very effective. Each PowerPoint presentation includes a check-list of things you should be able to do before moving on to the next part. If you are struggling with any of the concepts and the reading material hasn't helped, then post questions on the "Discussion Forum". Use the forum to discuss your questions with other students and to offer suggestions to others who have posted questions of their own. I will monitor the forum and add explanations where needed. Some suggested reading material: You need to read around to consolidate your knowledge and clear up any difficulties in understanding the lecture material. NB - use the reading to back up and better understand the information given in the lecture, but don't get bogged down in the extra information given or the additional fine detail: "Neuroscience: Exploring the Brain" Bear, Connors and Paradiso, 3rd ed or higher; Chapter 11. The Cochlear Amplifier - Ashmore & Gail (2004) Current Biology 14:R403-4.Links to an external site. Short review article, for interest. J. Ashmore dancing hair cell , including a brief explanation of the role of outer hair cells in the "cochlear amplifier". These video resources are relatively simplistic in their content but the animations may clarify the mechanics of the systems. If you can't access them, then ask the library for help (we have a subscription, but it depends on how you're logged onto the University intranet): JoVE - Overview of hearing JoVE - The cochlea JoVE - Hair cells JoVE Vestibular system Some important terms for sensory neuroscience: Receptor - has multiple meanings, especially in sensory neuroscience: a) protein complex that binds to and is activated by a signalling molecule b) nerve cell, often highly specialised, that is adapted to detect and respond to an external (ie external to the receptor) event, converting the energy in that event into a change in membrane potential (the receptor potential / generator potential) Primary afferent - first cell in the chain that carries a sensory signal from the sensory apparatus to the cerebral cortex. In some systems the receptive apparatus is part of the primary afferent, but in the "special senses" the receptors are separate and highly specialised nerve cells. Threshold - has multiple meanings, especially in sensory neuroscience: a) action potential threshold, which is the level of depolarisation necessary to allow voltage-gated sodium channels to open and hence trigger an action potential. Roughly -50mV. b) minimum sensory input necessary to activate a sensory receptor, and hence potentially generate a sensation. Different types of receptor have varying sensitivity and hence activation threshold (NB action potential threshold does not vary). c) perceptual threshold, which is the minimum sensory input needed to generate conscious perception of the stimulus. Depends not only on the strength of activation of the receptors, but also the attention of the subject (see modulatory systems lecture). Sensitivity - the relative responsiveness of a sensory system (or component thereof) to stimulation. Higher sensitivity correlates with lower response thresholds, larger response magnitude to a given stimulus, and a tendency to saturate at lower stimulus strengths. Specificity - a measure of how finely a system (or component thereof) differentiates between similar stimuli. Receptor adaptation - in sensory physiology this refers to a common feature of receptor cells, which respond strongly to changes in stimulus strength but reset their membrane potentials back to rest if a stimulus remains constant. Some adapt very rapidly (eg Pacinian corpuscles in the skin, which only respond repeatedly to rapid vibration), others very slowly (eg cone photoreceptors adapt over several seconds). C fibre nociceptors are probably the only ones that don't obviously adapt. Adaptation damps down neural responses to homogeneous stimuli while allowing small differences in stimulus strength to be signaled strongly. This extends dynamic range and as well as saving energy. Lateral inhibition - a key mechanism in sensory neurophysiology, typified by inhibition of one part of a sensory pathway by excitation of a neighbouring part. It is seen for example in the somatosensory system, where the cells carrying information from neighbouring patches of skin inhibit on another, and in the visual system where cells carrying information from neighbouring patches of retina inhibit one another. Less obviously, cells carrying information about similar qualities rather than location (eg sound frequencies) may inhibit one another. In each case the inhibition damps down neural responses to homogeneous stimuli while allowing small differences in stimulus strength to be signaled strongly. This inhibition extends dynamic range as well as saving energy. Dynamic range - the range of stimulus strengths over which a sensory system can respond. A single receptor type without the ability to adapt would either have to have very poor sensitivity (it could only afford to respond weakly, even to big jumps in stimulus strength) or a very limited response range (if it responded strongly to even small changes in stimulus strength, it would rapidly reach its maximum response magnitude and be unable to signal changes in stimulus strength beyond that point). Sensory systems overcome this by 1) having receptors that vary in sensitivity and 2) through adaptation of those receptors to extend their individual dynamic ranges. Saturation - point at which a sensory system (or component thereof) has reached its maximum response magnitude, beyond which it cannot signal increases in stimulus strength or differentiate between stimuli that differ in strength. Synaptic plasticity - the changes in the strength and effectiveness of synapses in response to patterns of use. The best understood mechanism is long-term potentiation (LTP), which increases the strength of fast excitatory (ionotropic) synapse by increasing transmitter availability, receptor numbers and receptor effectiveness. The trigger for LTP is that the synapse releases glutamate and immediately afterwards the postsynaptic cell strongly depolarises. The strong depolarisation indicates that many other synapses activated simultaneously, suggesting that they are all involved in a useful circuit. Specific circuit / pathway - neural machinery performing "computations" linked to sensory, motor and cognitive functions. Modulatory pathway - neural inputs that adjust how a cell responds to its specific inputs, which control the sleep-wake cycle, attention and concentration, mood and some emotional states. Related inputs are essential to the function of some motor and cognitive pathways.