topic 4

• the major site of refraction is always at the air-cornea interface.
• rods have more photopigment molecules than cones
• light adaptation depends on blocking the influx of Ca++ into photoreceptors
• Signalling increasing stimulus intensity by activating more neurons is possible because different neurons in the same sensory modality have different thresholds for the same stimulus
• receptor potentials and EPSPs spread electrotonically
• A hole in the cochlear partition near the round window would affect our sensitivity to high frequency sounds but not affect to low frequency sounds
• A conductive block in hearing is one which reduces the perception of loudness. This would be caused by fluid in the middle ear and Wax in the external ear canal
• The auditory system can transduce sounds in the kiloHertz range through Direct mechanical effects on ion channels
• Frequency selectivity in responses at the cochlea arises From frequency selectivity of cochlear vibration along its length
• Skin receptors that have free nerve endings lack capsules on the end of the receptor nearest the epidermis
• All A-β nerve fibres have bare nerve endings
• each 20-dB increase represents a 10-fold increase in SPL (sound pressure level) but a 100-fold increase in sound intensity.
• The density of touch receptors in the calf is less than that in the finger tip
• We were asked to test glabrous areas of skin because body hair has receptors for touch but not every part of our skin has hair
• In the swinging flashlight test, the examiner swings the beam of a penlight back and forth from one eye to the other to examine for asymmetrical abnormality in pupil size
• cones and rods are Electromagnetic receptors
• Humans can hear a very large range of sound pressures and the logarithmic decibel scale expresses this range more conveniently
• Tactile perception of the form of an object is most likely to depend on the use of nerve fibres with merkel's discs
• Each afferent fibre receives input from only one type of mechanoreceptor
• Pupils decrease in size when looking at a closer object
• In the simulation, women's hearing at any age was better than men's hearing. This was most likely Due to the two populations being exposed to different levels of occupational or recreational noise
• In the simulation on vision, the blind spot that you measured was higher than it was wide
• In the simulation on hearing loss you would have found that the following factors were most important in predicting your susceptibility to hearing damage your age, the loudness of sounds to which you are exposed in life, and the duration for which you are exposed to such sounds
•  the great majority of afferent neurons are connected to Inner Hair Cells rather than the numerically greater Outer Hair Cells & that therefore Inner Hair Cells are responsible for signalling information that leads to the percept of sound.
• stapes displacement causes cyclical displacement of the cochlear partition, that this displacement commences from the base of the cochlea and increases in size as it moves along, reaches a peak at some point along the cochlea and then very rapidly dies down.
• Peripheral sensory systems

  Consist of peripheral components and central components
• Peripheral systems


  1. Convert physical energy into a biological response

  2. Carry the biological response into the CNS

  3. CNS interprets the input and responds to it
• Sensory modalities


  • Contain specialised receptor cells to respond to a specific range of stimuli
  • Range is related to a species' evolutionary niche
• Peripheral systems consist of

  • Accessory structures
  • Receptor surface
  • Output neurons
• Receptor surface

  • Cells located within this surface, convert stimulus energy into a biological response called receptor potential (Transduction - energy conversion)
  • Specialised receptors for each sensory system
• Types of receptors

  • Specialised dendrites (ending of nerve fibres) - Somatosensory, proprioception, olfactory
  • Specialised receptor cells to nerve fibres - auditory, taste, vestibular
  • Specialised receptor cells to intermediate cell to nerve fibres - vision
• Receptor potential

  Receptors are electrically polarised cells, less so than neurons (-30 - -60 mV) <-- more 'leaky' than nerve cells so losing negative charge (lots of Na+ entering the cell) at a higher capacity than neurons
• Transduction of stimuli

  1. Stimuli has to change RMP to signal the need for a transduction, either by having a direct effect on the ion channels, or by indirectly effecting the ion channels

  2. Direct --> ion channels are stretched open and signal is directly received

  3. Indirect --> activation of molecule cascade, where the final molecule acts upon ion channels (slower), used GPCRs
• Direction of the change in RMP does not matter
• Types of movement to the nerve fibres

  • Type I --> triggered at a site near the neuron (like the axon hillock but not the same) where the threshold is the lowest, and information flows
  • Type II --> transmitted via chemical transmitters and then generation of the AP on nerve fibres
  • Type III --> additional step, must be transmitted twice, then generation of the AP on nerve fibres
• Nerve fibres

  Classified according to axon diameter and presence of methylation
• Action potentials (APs)


  • Used because hormones are too slow and current is fast but leaky (regenerating it = solution)
  • Require a lot of metabolic energy to perform, usually quite small to promote efficiency
  • Chemical signalling allows for reliability when communicating
  • Neurons can transmit and integrate many neurons (excitatory & inhibitory) --> altogether allows for sophisticated intercell signalling
• More information is transmitted through

  • Size and patterns of AP
  • The brain learns this code through an organism's development and can spend its energy doing more sophisticated things
• Intensity/Strength

  • As stimulus intensity increases, a receptor sensitive to that stimuli will respond more strongly by opening more channels/activating more molecules --> Larger change in RMP
  • Activation of more neurons and more APs are activated
• Spacing of stimuli


  • Bursts of AP closer together/further apart
  • Receptor potentials can sum, APs can't