2. Learning Objectives. By the end of today's session, you should be able to:Describe the organisation of the organ of Corti.Recall how displacement of fluids in the cochlea results in the generation of neural signals in the sensory hair cells of the organ of Corti.Explain how the organ of Cor
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1. 1 The Structure and the Function of the Cochlea
2. 2 Learning Objectives By the end of today’s session, you should be able to:
Describe the organisation of the organ of Corti.
Recall how displacement of fluids in the cochlea results in the generation of neural signals in the sensory hair cells of the organ of Corti.
Explain how the organ of Corti plays a role in frequency analysis.
4. 4 Structure of the Cochlea
Fluid filled tube, coiled to save space
When straightened out, tube 34mm long
Closed at one end (apical cochlea)
Basal end of cochlea contains 2 flexible membranes, the oval window, on which the stapes sits, and the round window that acts like a pressure release surface
Main structural feature of cochlea is basilar membrane (BM)
BM composed of collagen fibres which provide support for sensory cells of inner ear
BM divides cochlea tube into upper (scala vestibuli) and lower compartment (scala tympani)
Third compartment also present, scala media, which is a sub-compartment of scala vestibuli
6. 6 The 3 compartments in the cochlea, the scala media (SM), the scala vestibuli (SV) and the scala tympani (ST) are fluid filled.
The scala media is separated from the scala vestibuli by Reissner’s membrane and from the scala tympani by the basilar membrane.
The scala media provides a special environment for the organ of Corti which rests on the basilar membrane
This structure is responsible for transducing sound waves into neural signals
9. 9 Cochlea fluids
Fluids maintain correct physiological state of cells of cochlea
Have physical properties of water
Fluid in SV and ST termed perilymph
Principle ion is Na+
Perilymph has same composition as CSF as it arises from capillary circulation around cochlea
Fluid in SM different in composition.
Principle ion K+
11. 11 The Structure of the Organ of Corti
Organ of Corti runs the length of the cochlea tube
It consists of sensory hair cells and non sensory cells, supporting cells (Deiter’s cells)
It sits on the basilar membrane and moves with the motion of the basilar membrane
13. 13 There are two types of sensory hair cells – one row of inner hair cells and three rows of outer hair cells.
The apex (top part) of these hair cells contain stereocilia in which are located ion channels
The tectoral membrane overlies the sensory hair cells.
When the stereocilia of the hair cells move against the tectoral membrane , their ion channels are opened.
17. 17 In the human cochlea, there are:
- 3,500 IHCs and
- 12,000 OHCs.
Distributed in rows along length of cochlea
This number is ridiculously low, when compared to the millions of photo-receptors in the retina or chemo-receptors in the nose!
In addition, hair cells share with neurons an inability to proliferate
This means that the final number of hair cells is reached very early in development (around 10 weeks of foetal gestation); from this stage on our cochlea can only lose hair cells.
18. 18 Mechano-electrical Transduction
This describes the process by which the sound waves (mechanical stimuli) are transformed into neural signals (transduction).
Displacement of fluids in the scala tympani cause displacement of the basilar membrane.
As the basilar membrane is displaced, the stereocilia of the sensory hair cells are pushed against the tectoral membrane. This opens the ion channels in the stereocilia.
Sodium ions diffuse into the hair cells to result in the generation of a neural signal that is referred to as a receptor potential.
19. 19 Basilar membrane acts like a mechanical spectrum analyser
It responds to sounds by vibrating in a pattern dependent upon intensity and frequency of incoming sound
Inner hair cells relay information about this pattern to auditory nerve
Different frequencies excite populations of hair cells along cochlear duct
High frequencies excite cells at basal end of cochlea near stapes
Low frequencies excite cells at apical end of cochlea
Larger pressure differences across basilar membrane displace hair cell steriocilia even more
20. 20 Tonotopic mapping
Conversion of sound frequency to coding as position of excitation
Experiments by Von Bekesy in the 1960s established that sounds of different frequencies cause varied displacement of the basilar membrane.
Displacement of the basilar membrane begins at the base and progresses to the apex of the cochlea.
High frequency sound waves peak at the base of the cochlea. Hence sensory hair cells at the base of the cochlea transduce high frequency sounds.
Low frequency sounds peak at the apex of the cochlea.
Hence, sensory hair cells in the apex of the cochlea transduce low frequency sounds.
22. 22 Auditory Area of Brain
The schematic view of some of the auditory areas of the brain shows that information from both ears goes to both sides of the brain
When the auditory nerve from one ear takes information to the brain, that information is directly sent to both the processing areas on both sides of the brain.