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Lecture notes

NHB (Neurobiology and Human Behaviour) Sensory Systems (without vision and olfaction/taste)
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University of Cambridge (Medicine 1st class): Summary of key points of NHB (Neurobiology and Human Behaviour) Sensory system lectures (without vision and olfaction/tastes) for 2nd year.

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  • June 29, 2023
  • 9
  • 2022/2023
  • Lecture notes
  • Ian winter, ewan st. john smith
  • All classes

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NHB (Neurobiology and Human Behaviour) Lecture
Notes: Sensory Systems
Perception is context dependent (see mouthing Daa but hear Baa will perceive
Daa)
Lateral inhibition enhances acuity
Highest area of acuity has highest cortical representation

Hearing
Pinna
● Spectral notch (at higher frequencies for higher elevation)
○ Since sound can have direct pathway or indirect pathway

(reflected off pinna)
● Can have multiple pinna representations in the brain
EAM
● Amplification of frequencies common in speech (~3 kHz)
○ Canal is 2.3cm and is an open cylinder so has resonance of

wavelengths (2.3 x 4) cm
Middle ear
● Impedance matching -> 30 dB gain
○ Area of ear drum to stapes, lever of malleus and incus
● Protection from loud sounds
● Anti-masking of high frequencies by low frequencies (high pass filter)
External and middle ear mean air conduction is normally better than bone

Basilar membrane base is stiffer than apex so needs more energy (higher
frequency) to vibrate
● Logarithmic
● Fourier transform
○ Splitting up complex form into a series of sine waves of different

frequencies

OHC
● Responsible for Hz sensitivity and selectivity (sharpness of frequency
tuning (without lateral inhibition))
○ Sensori-motor (since they also transduce like IHC but additionally

change length)
● 3x as many as IHC
● Hyperpolarise -> lengthen; depolarise -> shorten
● Medial cochlear complex sends efferents to it
● V shaped stereo cilia (microvilli)
● More sensitive to acoustic and chemical (e.g. aminoglycoside) damage
than IHC

IHC
● 10-20 Type 1 afferent fibres from each IHC

, ● IHC potential oscillates with the sound (sine wave)
● Release glutamate at the frequency of the sound up to 5 kHz (low-
pass filter of IHC membrane, the cell can’t repolarise fast enough),
then the IHC is permanently depolarised
○ IHC potential has an alternating component (AC but misnomer

since it’s potential not current) from sine wave oscillations, and DC
since stereocilia have more gain in one direction so equal
movement results in net depolarisation
○ At low Hz, DC is masked by AC, since the IHC repolarises quickly
○ But at high Hz, AC is lost (completely at 5 kHz) but DC remains
○ So leads to depolarising block, Hz is not conveyed to auditory

nerve fibres

Tip-links mechanism, sigmoid curve is steep indicating high sensitivity to
bundle movement




● High [K+] in endolymph (in scala media) creates a positive driving
force (+80 mV in addition to -40/50 mV resting potential) for
depolarisation upon sterocilia activation (K+ current)
○ No GPCRs for speed
● Maintained by stria vascularis which lines wall of scala media
● Presbycusis is due to scala vascularis degeneration with age

Auditory nerve (only discussing type I afferents)
● Bank of parallel, overlapping, bandpass filters in nerve (1 per neurone)
○ Characteristic (or best) Frequency

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