Intro to sensation Flashcards
what is difference between sensation and perception?
- Sensation is the process of encoding of events and stimuli by the nervous system
- Sensation depends on low level physical, biochemical and neural events
- Perception is the process by which the brain interprets sensory information
What is transduction?
Conversion of one energy form into another
converting a physical stimulus into an electrical signal
a physical stimulus causes a change in membrane potential
Transmission and transduction- Mechanoreceptor in the somatosensory system
There are a range of mechanoreceptors sensitive to different levels of pressure and vibration transmitted through the skin.
Transduction: Typically the stretch-sensitive channels are Na+ channels. So opening them leads to membrane depolarisation and action potentials.
Transmission and Transduction Photoreceptors in the visual system
Transmission Eyelids, cornea, lens and pupil control: amount of light entering the eye image focus Transduction Opsin molecules in photoreceptor membrane absorb photons of light, producing a graded hyperpolarisation in membrane potential:
What happens to opsin molecule in photoreceptor membrane when there is higher level of light?
Higher level of light –>Higher absorption of photons of light more –> hyperpolarisation in membrane potential
what is the difference between cones and rods?
Rod photoreceptors are sensitive to low light levels (e.g. starlight) (scotopic vision) They do not mediate color vision, and have a low spatial acuity.
Cone photoreceptors require high light levels (e.g. sunlight, indoor lighting)(photopic vision) are capable of color vision and are responsible for high spatial acuity.
Transmission and Transduction - Chemical receptors in taste and smell
- Receptors for salty, sweet, sour, bitter and unami taste sensations.
- Salty sensations => Na+ channels
- Sour sensation => H+ channels (H+ influx closes H-gated K+ channels)
- Other taste sensations => special G-protein coupled receptors
- Many receptor cells are non-spiking (and unmyelinated) they don’t have a clear axon and it does not generate action potential. Neurotransmitter is usually release as result of generator potential which is the membrane potential of the cell. Hence the more depolarised the membrane is the higher rate of neurotransmitter release.
- For non-spiking neurons (photoreceptors, taste receptors, hair cells in cochlea), the rate of neurotransmitter release is proportional to the membrane potential.
Sensory coding:sensory neurons encode information in ….
sensory neurons encode information in the rate and timing of action potentials
a neuron’s responses to repetitions of an identical stimulus are highly variable
neurons are tuned for (can encode) multiple stimulus features
Rate coding theory
Rate coding theories suggest that the precise timing of action potentials doesn’t matter – only the number of spikes within a certain time window is important.
The rate coding model of neuronal firing communication states that as the intensity of a stimulus increases, the frequency or rate of action potentials, or “spike firing”, increases
Temporal coding theory
The temporal coding is a type of neural coding which relies on precise timing of action potentials or inter-spike intervals. Combined with traditional rate coding models, temporal coding can provide additional information with the same rate. There is no precise definition of temporal code.
Adaptation
Neural adaptation or sensory adaptation is a gradual decrease over time in the responsiveness of the sensory system to a constant stimulus. It is usually experienced as a change in the stimulus.
Neural adaptation is the change in neuronal responses due to preceding stimulation of the cell.
Somatosensory: do you notice clothes on your back
Visual: readability of your phone in sunshine versus inside your house
Auditory: air-conditioners
Why adapt?
Limited number of neurons available to encode the properties of the world
Optimise neurons to encode change
Matches neural sensitivity to prevailing environmental statistics
) somatosensory cortex contains a somatotopic map
Adjacent groups of neurons in S1 respond to touch of adjacent regions of the skin surface
Note that there are 3 major regions (face, upper limb, lower limb) and a few discontinuities.
auditory cortex contains a tonotopic map
In auditory cortex, adjacent neurons are optimally activated with the same frequency.
As you move across cortex, this preferred frequency systematically changes.
The tonotopic map is a map of sound frequencies, or pitches.
visual cortex contains a retinotopic map
Adjacent photoreceptors in the retina encode adjacent parts of the visual field
Similarly, adjacent neurons in primary visual cortex (V1) encode adjacent parts of the visual field. Thus, V1 contains a retinotopic “map”.
