Sensory Systems Flashcards
What the hierarchical organisation?
All sensory systems follow a hierarchical organisation…
Association cortex
Secondary sensory cortex
Primary sensory cortex
Thalamic nuclei
Receptors
Each level of the sensory hierarchy receives input from lower levels and adds another layer of analysis
With increasing level neurons respond optimally to stimuli of greater and greater specificity and complexity
Whats sensory organisation?
Hierarchical organisation also reflected in the differentiation between sensation and perception
- Sensation – process of detecting the presence of a stimulus
- Perception – higher-order process of integrating, recognizing, and interpreting sensations
How does the eye work?
Light enters the eye through the pupil
- Size of the pupil regulated be the iris
- High illumination – small pupil
Light then goes through the lens
- Focuses incoming light on the retina
- Turns everything upside down
Retina
- Contains the receptors and four other types of neurons
- Neurons are arranged in layers, the receptor cells are the innermost layer
Axons of the retinal ganglion cells leave the eye in one bundle
- Bundle – optic nerve
- Resulting gap in the receptor layer – blind spot
- Blind spot is filled by the visual system from receptors that surround it
Fovea
- Area in the centre of the retina 0.33 cm
- Specialized for high-acuity vision
How does the retina process?
Receptors in the retina: rods and cones
Rods
- Periphery of the retina
- Respond best to faint light
- Outnumber cones (20x)
- Many rods converge to one ganglion cell
Cones
- Fovea, also around the fovea
- Respond best to bright light
- Essential for colour vision
- in the fovea, each cone associated with one ganglion cell; generally low convergence
Rods and cones contain photopigments
- When struck by light these photopigments release energy
- Energy release activates second messengers
- Second messengers inhibit the activity of the receptor cell, which initiates the signal transduction across the other neurons to the brain
How does it go from retina to primary visual cortex?
Signals from left visual field (not eye!!) reach the right visual cortex and vice versa:
- Light from left visual field meets the right retina and vice versa
- Axons from the nasal part of the retinas cross (optic chiasm)
- Axons from the temporal part of the retinas do not cross
After optic chiasm, signals go through thalamus (lateral geniculate nucleus, LGN) and then to the primary visual cortex (V1, striate cortex)
Why do we need to move our eyes?
Eye: ~3 brief movements per second – saccades
Critical for a high-acuity, wide-angled, coloured perception
Without eye movements retinal images disappear after a few seconds
- Reason: Neurons in the visual system respond to change rather than steady input
Visual system temporally integrates information from saccades
- Reason for our high-quality perception
- Reason why world does not vanish when we blink
What’s component processing/trichromatic theory?
3 different types of cones
- The photopigment in each type of cone makes it particularly responsive to short/ medium/ long wavelengths
- Perceived colour depends on the relative activity of the three types of cones
Whats opponent processing?
Neurons respond in opposite directions to complementary colours (red-green, blue-yellow, black-white)
At all levels of the visual pathway except for the receptors
Explains why complementary colours cannot exist together – no bluish yellow or reddish green
What is colour blindness?
Certain colours cannot be distinguished
- Results from a deficiency or absence of photopigments responsive to a certain wavelength
- Most deficiencies are red or green deficiencies
Gene for colour blindness is carried on the X-chromosome, mainly men affected
Test: Ishihara colour test
What’s colour constancy?
Our perception of colour does not only depend on the wavelength of light that reaches our retina
We use context to interpret what we think colour should be
Top-down processing
Also brightness constancy, size and shape constancy
What are cortical mechanisms of vision?
Main visual areas in the cerebral cortex are the striate cortex, prestriate cortex, inferotemporal cortex, and posterior parietal cortex
What are receptors?
Receptors with different characteristics
Perception of a range of different touch and pain sensations
What are the sensory pathways
Pathway I (dorsal-column medial lemniscus system)
- Touch and proprioception (position of the body)
- Axons ascend ipsilaterally, cross over to the other side in the brain stem, pathway continues to thalamus
- Most neurons project to the primary somatosensory cortex (SI)
Pathway II (anterolateral system)
- Pain and temperature
- Most neurons cross over to the contralateral side when they enter the spinal cord, part of the pathway continues to the thalamus, other part terminates in brain stem
- Neurons that reach the thalamus project to SI, SII, posterior parietal cortex & other areas
What are cortical areas?
SI – postcentral gyrus
- Somatotopic – organised according to a map of the body surface (somatosensory homunculus)
- Input mainly from contralateral
- Output to secondary somatosensory areas and association cortex
Secondary somatosensory cortex (SII and other areas)
- Input mainly from ipsilateral and contralateral SI
- Output to association cortex
•Association cortex
- Posterior parietal lobe
- Input from SI and secondary somatosensory areas
- Extensive damage to the right posterior parietal lobe can result in asomatognosia
- Failure to recognise parts of one’s own body
- Often accompanied by anosognosia, the failure to recognize one’s symptoms
How do we feel pain?
Axons carrying pain information release two neurotransmitters at their synapses in the spinal cord
- Mild pain: Glutamate
- Strong pain: Glutamate and substance P
Two pathways to the brain
