Medsci 206 Module B

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Sensory systems (main 5 and others)

Vision, hearing, taste, smell, touch, vestibular/balance, proprioceptor/posture and movement of body parts relative to each other, other modalities (ie pain, temperature, itch)

What information (external) do sensory system detence and convey?

Lights, sounds, smells, tastes, touch/pressure

What information (internal) do sensory system detence and convey?

O2, CO2, pH, muscle length, body position, motion and equilibrium

Sensory system - detection and conveying of info about external and internal environment leads to…

Awareness of environment and events

Mosquito - sensory information (antennas, mouth)

Antennas - hearing, vibrating hairs
Mouth - detects chemicals in the air
Sensory hairs - sense movements

Bees!

Use vision, odour (phermone emission), hearing, mechanoreceptor and electrical sense (to sense electrical fields of flowers)
- Hairs are responsible for sensing electrical fields, the greater the movement of hair the greater the neuronal firing

Sensory systems (basis)

1. Signal
   2. Collection
   3. Transduction
   4. Processing
   5. Action

Neural coding of information is conveyed to brain centres - diagram

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Where does information from the sensory neurons in the head enter the CNS from?

Cranial nerves 

Specific pathway of information from sensory neurons entering from below the head:

Sensory neurons below the head enter the spinal cord and pass towards the brain via the spinal nerves

All sensory information goes to the thalamus EXCEPT information from the:

Olfactory bulb - goes straight to the olfactory cortex

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Sensory disorders

Blindness, deaf/impaired hearing, anosmia (loss of smell), ageusia (loss of taste)

How many cone receptors do humans have and what colours are they?

3 cone receptors, red/blue/green

Tapetum

The shiny reflective surface immediately behind the retina of most animals (humans do not have this)

Sensory system basis

1. Signal (physical stimulus providing information on our surrounding)
   2. Collection of signal ie. retinal photoreceptors 
   3. Transduction of stimulus to nerve signal
   4. Processing of information by the brain
   5. Generation of resulting action ie an image
   
   (Signal, collection, transduction, processing, action)

Neural coding of information is conveyed to the following brain centres:

Olfactory cortex, auditory cortex, visual cortex, gustatory cortex, somatosensory cortex, vestibular cortex

Attributes of the sensory system

1. Modality - type/class of stimulus
   2. Location - position of stimulus in space
   3. Intensity - measure of amount of stimulus
   4. Timing - onset, duration and offset of stimulus

5 major classic modalities (human)

Main: vision, hearing, taste, smell, touch
Others: vestibular, proprioception, pain/temp/itch

Receptor specificity - 4 classes

1. Mechanoreceptors
   2. Chemoreceptors
   3. Thermoreceptors
   4. Photoreceptors

Transduction

Sensory receptors transform signal into electrical energy

Receptor potential

Leads to an electrical reponse/change in membrane potential

Transduction mechanisms

These vary according to the physical stimulus:
1. Direct - ie mechanoreceptors in skeletal muscle
2. Indirect via 2nd messenger systems - ie chemoreceptors in the olfactory epithelium

Mechanical transduction explaination 

• Mechanoreceptors sense physical deformation of tissue ie pressure on skin/muscles stretching
  - Mechanical stimulation causes opening of stretch sensitive ion channels and an increase in Na+ & Ca2+ conductances that depolarise the receptor neurons
  - Change in receptor potential triggering an action potential 
  
  
  

Example of mechanical transduction: muscle spindle detecting 

As muscle fibres stretch channels open, resulting in a change in receptor potential. As more pressure is applied the channels open more frequently. 

Transduction via 2nd messenger system - chemoreceptors in the olfactory epithelium

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Spatial location of receptors:

• Provides information on location of stimulus source on body

• Enables discrimination of size+shape of object

• Enables resolution of fine detail of the stimulus/environment

Receptive field

The area in which a stimuli can activate a sensory neuron

Example of receptive field of touch receptors

Receptive field sizes

• Small receptive fields = high resolution of fine detail

• Larger receptive fields = more difficulty in localising stimulus

• Ex. receptive fields are small in the fingertips but larger in the palms

The density of receptors define the resolution of the stimulus

Receptor density is not uniform and they may be localised to give a higher definition

• Ex. The fovea of the retina has a high density of photoreceptors and small receptive fields

Intensity

Stimulus intensity is encoded by the frequency of action potentials in sensory nerves

Timing

Firing rates of sensory neurons convey information about the stimulus intensity and time course

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3 basics parts of vision:

Eyes, optic connections, brain

Basic process of vision

1. Light passes through the cornea and lens

2. Light focuses onto retinal photoreceptors

3. Retina sends information to the brain in the form of electrical signals using the optic nerve

4. Brain processes the image and allows us to see

Aqueous humor (anterior chamber)

• Constantly replenished to ensure there is removal of waste, replenishment of nutrients

• Generates intraocular pressure

• Is created by the ciliary body

Vitreous humor (between lens and retina)

• Clear+transparent to help passage of light to retina

• Stagnant (very little exchange/replenishment)

• Contains high levels of vitamin C and keep O2 levels low (lens is hypoxic, O2 results in cataract)

Fovea

Region around the middle of the retina that contains cone photoreceptors

Eye Anatomy

Retinal image projection

Lens is convex-shaped causing light rays to converge on the retina. Image is therefore projected upside-down and back-to-front onto the retina

Retina structure from outer retina to inner retina

1. Pigment epithelium

2. Photoreceptors

3. Outer nuclear layer

4. Outer plexiform layer

5. Inner nuclear layer

6. Inner plexiform layer

7. Ganglion cell layer

8. Optic nerve layer

Pigment epithelium

• Contains melanin to absorb excess light

• Provides nutrition to the photoreceptors

Photoreceptors (cones+rods)

• Light senstive

• Transduces light energy into electrical energy

Outer nuclear layer

Contains cell bodies of rods and cones

Outer plexiform layer

First synaptic interaction between photoreceptors and bipolar cells

Inner nuclear layer

Contains cell bodies of amacrine, horizontal and bipolar cells

Inner plexiform layer

Second synaptic interaction between bipolar and ganglion cells

Ganglion cell layer

• Output cells of retina

• These are the only retinal cells are capable of firing action potentials

Optic nerve layer

Contains the axons of ganglion cells

In short, photoreceptor cells… while inner retina cells …

Photoreceptor cells detect light, inner retina cells process the information

Responses of retinal cells to darkness (D for darkness, D for depolarise)

• ON bipolar cell synapses with ON center ganglion cells

• OFF bipolar cell synapses with OFF center ganglion

ON bipolar cell synapses with ON center ganglion cell (retinal cells response to darkness)

• mGluR

• Presence of glutamate = hyperpolarisation = less glumate

• Results in ON center ganglion to decrease firing

OFF bipolar cell synapses with OFF center ganglion (retinal cells response to darkness)

• AMPAR

• Presence of glutamate = depolarisation = more glutamate

• Results in OFF center ganglion cell to increase firing

Responses of retinal cells to light - ON/ON

• AMPAR

• Presence of glutamate = depolarisation = more glutamate

• Results in ON center ganglion cell to increase firing

Responses of retinal cells to light - OFF/OFF

• mGluR

• Presence of glutamate = hyperpolarisation = less glumate

• Results in ON center ganglion to decrease firing

ON/OFF ganglion cells

ON/OFF refers to the response of the ganglion cells to light

Ganglion cell receptive field is defined as:

• The area of the retina where stimulation with a small spot of light produces a change in ganglion firing rate.

• Area within the receptive field is divided into two regions: center and surround

Receptive field center

Provides direct input from photoreceptors to the bipolar cell

Receptive field surround

Provides indirect input from the photoreceptors to the bipolar cells via horizontal cells

This thing

Turning a spot of light in the centre of an ON-centre ganglion cell receptive field

Produces burst of electrical activity

Turning spot of light on in the centre of an OFF-centre ganglion cell receptive field

Firing decreases

ON-center cell stimulation vs inhibition

• Stimulated when center of receptive field is exposed to light

• Inhibited when surround of receptive field is exposed to light

Light/no light on receptive field center/surround

Uniform illumination of the visual field is less effective in an ON-center ganglion cell compared to a well-placed spot of light on center of cell receptive field.

This organisation makes ganglion cells sensitive to differences in illumination across the receptive field (luminance contrast)

Receptive fields have overlapping distributions

Several ON/OFF ganglion cells analyse every part of the visual space. Receptive fields convey information about cell firing rates in center and surround - combining information from adjacent receptive fields = brain can construct info about edges/shapes/colours

Visual pathway - transmission of sensation from retina to brain

1. Monocular information transmitted by optic nerve

2. Nasal fibres cross at optic chiasm (anatomical crossing point)

3. Axons synapse at the LGN of the thalamus to large magnocellular areas (magnocellular pathway)

4. Optic radition

5. Primary visual cortex (V1)

Visual pathway - steps 1+2

• Around 50% of optic nerves cross over to the contralateral side of the brain

• Left side of brain receives right visual field information and vice versa

Motion detection

Photoreceptors are not sensitive to an object’s motion direction - motion is rather detected by photoreceptors by special class of ganglion cells that are excited by starburst amacrine cells

Directed selectivity ganglion/M/parasol cells

Neurons that respond differentially to direction of visual stimulus. A moving stimulus can elicit strong spiking of action potentials in one direction but not the opposite

DSGC - motion information

• Responds best and has preference to dorsal/ventral/temporal/nasal directions of motion

• Inversion of lens = object movement in visual scene corresponding to movement in opposite direction in retinal plane

DSGC - neuron information

• Concerned with motion detection and alerting animal to threatening+moving visual imagery

• Are large ganglion cells

• AKA as M ganglion cells as they terminate in the magnocellular layer of LGN (make up ~5% of total ganglion cell population

• Pattern is of open, radiating branches

• Fast processing/high-conduction velocity+responds in a transient fashion

Lateral genicular nucleus

Inner two layers (M1 and M2) transmit important information for perception of movement to primary visual cortex

Visual pathway from retina to cortex

Vision for movement —> dorsal stream

Vision for perception —> ventral stream

Summary slide for L2 (vision)

Neural adaptation

Neurons in the brain coding for a particular movement become less sensitive to motion in that direction, therefore looking away from the stimulus results in neurons that detect movement in opposite direction to become more active = appearance of stationary object/image moving. Can see this through the non-adapted eye too due to contralateral crossover of optic nerves

Rods and cones - structure

Differences between rods and cones

1. One type of rod, 3 types of cones

2. Rods ‘see’ black+white, cones ‘see’ colour

3. Rods are located in the periphery, cones located in the fovea

4. Rods 1000x more sensitive to light - can respond to one photon of light while cones require 10-100s of photos to become activated

5. Rods recover slow, cones recover fast and can rapidly adapt to illumination changes

Scotopic vision (low light conditions)

Only rods are functioning: cones have no light to respond to therefore no colour perception

Photopic vision (daylight conditions)

Only cones are functioning as rods bleach out quickly in bright lights, 3 cone cell types provide wide range of colour perception in bright light

Mesopic vision (moderately low light ie right after sunset)

Both rods and cones are functioning therefore cones still have enough light to provide some colour vision

Processing of colour information is done by parvocellular (P) ganglion cells (LGN - parvocellular pathway)

Compared to M cells, P cells have smaller cell body+dendritic fields, sustained responses, slower conduction velocities, sensitive to light wavelength difference and conveys colour information that way

LGN colour perception

Outer 4 layers (P3-P6) transmit information required for colour perception (esp. red and green) to primary visual cortex

Three anatomical pathways of the LGN

• Magnocellular - motion detection

• Parvocellular - colour perception

• Koniocellular - involved in relaying info from S-cones (blue) with contribution from M and L cones

Trichromatic theory

Based on the fact that all colours can be perceived via a combination of 3 cone receptors

Three types of cone photoreceptors

• Red (L-cones): long light wavelengths

• Green (M-cones): medium light wavelengths

• Blue (S-cones): short light wavelengths

Stimulation of cone photoreceptors

The difference in signals received from the 3 cone types allows the brain to perceive continuous range of of colours.

Ex: yellow-green light stimulates L and M cones equally strongly but only stimulates S cones weakly

Opponent Colour Theory

Certain colours don’t exist in ordinary perception - some colours can be seen simultaneously ie red+yellow (orange) and red+blue (purple) but red+green (reddish-green) and yellow+blue (yellowish-blue) cannot be

Colour opponent channels

• 3 opponent channels: blue-yellow, red-green and black-white (brightness or luminosity/achromatic channel)

• One member of colour pair suppresses the other

• P cells comprise of receptive field activated by one colour and receptive field surround activated by another colour

Blue-yellow opponent channel example

Processes information about S cone and combined L+M cone signal differences. If blue light is present S cone is activated and blue+yellow cell receives excitatory input (no L+M cone input) = blue colour perceived

Red-green opponent channel example

Processes information about L+S and M cone firing differences. Red light present results in L cone being activated and red+green cell receives excitatory input (no M cone input) = red colour perceived

Black-white opponent channel example

Transmits info about light intensity

Model for normal colour vision

Trichromatic theory - photoreceptor level

Opponent-process theory - photoreceptor neural interconnection

Protanopia

No red cone cells in retina therefore impairment of long light wavelength perception (reds).

Red-green colour blindness

Deuteranopia

No green cone cells therefore impairment in perceiving medium light wavelengths (greens)

Tritanopia

No blue cone cells in retina (very rare) therefore inability in perceiving short light wavelengths (blues)

• Patients confuse greens and blues, yellows may appear pink

Stage theory

Incorporates trichromatic theory and opponent colour theory into two stages: receptor stage (cone cells - blue, green and red) and neural processing stage (colour opponency)

Congenital Colour Vision Defect (CVD) prevalence

8% of males, 0.5% of females. Deutan (green impairment) is most prevalent.

Anomalous trichromats (all three cones are there but have impaired function) is more prevalent than dichromats.

Protan and deutan have severe impact on daily function whereas tritan is relatively uncommon and has less of an impact

Colour vision tests

• Ishihara test

• Farnsworth D-15

• Farnsworth-Munsell

• The Anomaloscope

• L’anthony Desaturated D-15

• City University Test

• Lantern test

Four classification of tests:

1. Pseudoischromatic plates

2. Arrangement tests

3. Matching tests

4. Naming tests