Sensing the world I

what is sensation?

the capacity to detect a particular physical or chemical stimulus

(involves the sensory organs and afferent nerves)

what is perception?

the conscious experience and interpretation of sensory information

(involves neurons in the central nervous system)

what is a stimulus?

a thing or event that evokes a specific functional reaction in an organ or tissue (stimuli can be physical or chemical)

what are some physical stimuli?

* sound
* visible light
* heat
* magnetic field
* UV light

what are some chemical stimuli?

* taste
* odour
* flavour
* pheromone

what is stimulus transduction?

converting the stimulus into the nervous system ‘language’ i.e. neuronal activity

(each sensory organ deploys a specific mechanism to transform chemical or physical attributes of stimuli to neuronal activity)

which organ(s) and stimuli and related to somatosensation?

* organ: skin and other tissues
* stimuli: pressure, warmth, cold, pain

which organ(s) and stimulus and related to sight?

* organ: eyes
* stimulus: light

which organ(s) and stimulus and related to hearing?

* organ: ears
* stimulus: air vibration

which organ(s) and stimulus and related to smell?

* organ: nose
* stimulus: volatile chemicals

which organ(s) and stimulus and related to taste?

* organ: mouth
* stimulus: soluble chemicals

how do pigeons use magnetoreception?

they use the magnetic field of the earth to navigate and find the direction they want to go

what do elephants (and other animals) use infrasound?

* infrasounds are sounds that humans can’t hear because the frequency is too low (they can also travel ling distances)
* they use it to communicate with each other and about things that are happening

how do bats use ultrasounds?

* ultrasounds are sounds humans can’t hear because the frequency is too high
* they use it to navigate and localise everything that is around them including prey

how do fish (e.g. elephant fish) use electrolocation?

* they can detect the electrical fields around them
* if the electrical field is disrupted they know an object is there
* they use electrical signals to communicate with each other

how does vision differ in humans and bees?

* humans have cones that detect blue, green and red
* bees have cones that can detect blue, green and ultraviolet

describe how asymmetric vision in the cockeyed squid works

* the large eye looking upwards detects predators against the dim sunlight
* the small eye looking downward detects prey’s bioluminescent signals

what are sensory receptors?

cells that specialise in converting external stimuli (physical or chemical) into neural (electrical) activity

types:

* photoreceptors
* mechanoreceptors
* chemoreceptors
* nociceptors

what do photoreceptors do?

detect light (vision)

what do mechanoreceptors do?

detect movement (sound, texture, blood pressure, muscle stretch)

what do chemoreceptors do?

detect chemical compounds (smell. taste)

what do nociceptors do?

detect tissue damage (pain)

how do photoreceptors differ?

each photoreceptor points to a unique direction, sensing the light present in this two dimensional space

how do mechanoreceptors vary in the ear?

each inner hair cell responds to specific sound frequencies

how do odorant receptors differ?

each one detects only one chemical attribute

how are receptors distributed in sensory organs?

receptor distribution is not always homogenous (e.g. in the retina, tongue)

why does receptor sensitivity vary?

receptors have different thresholds (i.e. the minimum stimulus intensity that is detected)

what do neural relays do?

* all senses connect to the cortex through a series of neural relays
* sensory info is modified at each relay, allowing for the construction of different aspects of the sensory experience
* they also allow for sensory systems to interact (e.g. visual modification of sound - the McGurk effect)

why is perception more complex than sensation?

the context, emotional state or memories can affect how we perceive the same sensory experience

why is the visual system important for our daily lives?

through seeing and perceiving the world around us we assess reality but can also produce adaptive behaviours

what is visible light?

the section of the electromagnetic spectrum that the human eye can detect

(light can enter the eye from a source of light, or after bouncing on an object)

anatomy of the human eye

what are the two types of photoreceptors contained in the retina?

* cones (detect colour)
* rods (detect light)

what type of photoreceptor is the fovea mainly covered in?

cones (actute vision during daylight)

what type of photoreceptor covers the periphery?

rods (which are more sensitive to light than cones)

how does light affect the release of glutamate in photoreceptors?

* they are depolarised in the dark, releasing glutamate
* light makes them hyperpolarise, reducing glutamate

what is a bipolar cell?

one of the main retinal interneurons and provide the main pathways from photoreceptors to ganglion cells

what affects the probability of neurotransmitter release?

photoreceptors and bipolar cells changing their membrane potential (they do not fire action potentials)

what are the two types of bipolar cells (in vertebrate retinas)?

on and off

how many photoreceptors doe bipolar cells connect to in the fovea and the peripheral retina?

* fovea: one bipolar cell connects to one photoreceptor
* peripheral retina: one bipolar cell connects with several photoreceptors

where is visual information from each side of the visual field analysed?

* right side: left hemisphere
* left side: right hemisphere

what are the three neural routes to the visual brain?

* retinohypothalamic tract
* geniculostriate pathway
* tectopulvinar pathway

what is the retinohypothalamic tract involved in?

* regulating the circadian rhythm
* controls pupillary reflex that expands or contracts the pupil to regulate the amount of light reaching the retina

what is the geniculostriate pathway involved in?

the conscious experience of vision

contains:

* dorsal stream: analyses *how*; guides movement to relative objects
* ventral system: analyse the *what*: object identification

what is the tectopulvinar pathway involved in?

analyses spatial information of objects

* from ganglion cells from retina periphery, with no colour info
* explains the visual ability of patients with blindsight