Summary of Purves Chapter 22: Early Brain Development
Samenvatting Fundamentals of Neuroscience, UvA
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Vrije Universiteit Amsterdam (VU)
Biomedical Sciences
Neuronal Networks and Behavior (AB_1051)
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Neuronal Networks and behavior
Our sensory system help us to survive. We have 5 basic human senses
- 5 basic human senses are =
o Sight
o Taste
o Smell
o Hearing
o Touch (will not be discusses during this course)
- Each animal has its own sensory system
- Our senses show us everything but we cannot process every single input. We need to make selections
(filtering)
- The brain is a prediction machine = predicts what is going to happen based on information it already has
obtained from past experience
o We use higher order association areas in our brain for this and not the sensory system
o We get only limited view of reality outside us therefore it filters
- Our nervous system is interconnected with our body (not separate), so only your brain is not enough to
predict what is going to happen
How does the brain work
- The human brain is build brain areas that are specialized in specific processes
- Within those brain areas there are neurons that collect electrical impulses that come from senses
o A lot of neurons allows for processing of a lot of information
- Synaptic transmission neuron have
o Dendrites = receive information from axons
o Axons = transmits information to the following cell (to
dendrites)
o Synapse = excites the cell leading to action potential or
inhibits
o Spines makes contact with other cells
- Synapse function: electrochemical information transmission
o Neurotransmitters released from presynaptic cell by influx of calcium (Caused by depolarization of
the membrane) --> bind to receptors on postsynapse --> postsynapse can now also depolarize and
transmit the information
- Information that can be measured:
o Excitatory postsynaptic potential (EPSP) induced by neurotransmitters such as glutamate cause
increase in the probability of action potential generation
▪ Receptors for glutamate = AMPA and NMDA
o Inhibitory postsynaptic potential (IPSP) induced by neurotransmitters such as GABA cause a
decrease in the probability of action potential generation
▪ Receptors for GABA = GABAergic receptors
➔ Each neuron will integrate multiple EPSPs and IPSPs, the sum will determine whether the cell
fires an action potential
- Output = action potential
o This is an all-or-nothing response
,General plan of sensory system (what is needed for a complete sensory system?)L
- Sensors that receive and translates information into the language of the machine
o In this case = sensory receptors/axons that translate the energy of the stimulus into electrical signals
(modality, location, intensity and timing)
o A receptor is needed that is activated by a specific input
o These 4 features are encoded in the sensory system:
▪ Modality = types of receptors
• Mechanical = physical stretch or tension on the receptor deforms the membrane
and opens the channel (touch and proprioception, hearing, balance)
o Touch and proprioception (skin’s mechanoreceptors) --> mechanical
displacement is sensed by these receptors and open up so that ions can flow
in
▪ 4 types:
• Meissner corpuscle (fast response) and merkel cell (slow
adapting) = sense very fine motion (very sensitive) because
there at the top of the skin
• Ruffini (slow adapting) and Pacinian corpuscle (rapidly-
adapting) = less precise to touch because they are deeper in
the skin but have big receptors fields (very sensitive)
• Chemical receptors = binding of specific chemicals to these receptors such as pain,
smell, taste, itch
• Photoreceptors = sensitive to light (in retina) --> change conformation when sensing
light
• Thermal receptors
▪ Location of the receptors = where the information comes from relative to the body
• Position of receptor already gives information = topographical arrangement
o Topographical location is on your skin and also other levels of information
processing such as at the level of the spinal cord and brain
o For example: retinotopy in retina, tonotopy in cochlea, gustotopy in
gustatory cortex
• Each receptor has a different size of receptor field (and density of receptors)
o Consequence of small receptor field = less sensitive but very precise (on
finger tips)
o Consequence of big receptor field = more sensitive but less precise
➔ Spatial resolution is determined by the size of the receptive field and density of
receptors
▪ The intensity
• Sensory threshold is determined by the sensitivity of the receptors: How much
stimulation is needed to trigger action potential?
• Change in energy of the stimulus can be interpreted in the cell as a change in
membrane potential
o And once a specific intensity is reached, this is translated into digital code of
action potential
▪ The timing of the stimulus
• How the neuron react is very important = some neurons are
slowly adapting (senses a shape of an object in your hand) and
some are rapidly adapting (only senses changes)
, o Slow adapting = changes in stimulus coded in frequency
o Rapid = start and end of the stimulus are most important
• Adaption = constant stimulus fades from consciousness
o E.g. not feeling your clothes on your body all the time
- Transport of this information
o In the sensory system this is done by axons: they transport the signal to the series or relay nuclei
o Multiple parallel pathways increase the speed of processing
o Topographical representation is maintained
▪ The upper limbs have a lateral position in dorsal column
▪ The lower limbs are positioned more medial
o There is a crossover of information to the opposite hemisphere = decussation
▪ Happens for example in the medulla
o There are also feedback and descending projections from higher order areas of the brain to almost
all levels of information processing
- Some type of integration needs to happen
o In the sensory system this is done by interneurons and local circuitry in nuclei that process the signal
o All sensory information has to pass thalamus before it reaches the cortex
o Thalamus has an Y-shaped division = interior, lateral and medial thalamus
▪ These project to the missile layers of the cortex
o Primary sensory cortices such as visual cortex 1 (V1) give the
information to secondary areas to process it more
▪ Primary cortices are only a small part of the brain
o The somatosensory cortex consists of 6 layers
▪ Information reaches the cortex from periphery (thalamus) into layer 4 (IV) which is the
primary cortical layer. Neurons then distribute the information to upper and lower layers of
the cortex.
▪ Then to layer 2 and 3 that cause intercortical processing: give the information to other parts
of the cortex
▪ Other layers then process the information back to parts of the brain like the basal ganglia
o The sensory cortex consists of distinct columns. Each column (area) represent a part of the body.
▪ Area 3b receives most of its projections from the superficial skin: fast-adapting and slow-
adapting receptors. These projections are represented in separate cortical columns or slabs
▪ Area 3a receives input from receptors in the muscle spindles
Sensory cortex
- (1) All areas have 6 layers. Information 1
comes from layer IV and gives the
information via neurons to layer I, II, III, V
or VI
- Divided in areas that represent parts of
the body (2) : each area is divided into
smaller parts containing sensory neurons
for fast and slow adapting functions (3)
- Gives output 2
3
, Sensory system: vision Part 1 (chapter 11 and 12)
Receptors in the retinal circuitry
Visual system
- Pathway
o Light waves from an object come into the eye via the cornea.
The cornea is a transparent window at the front of the eye. This
causes the bending of the light
o The light then goes through the pupil which is an opening at
the center of the iris
▪ When light becomes brighter, the pupil becomes smaller (constricts) due to the pupillary
light response
▪ When light becomes dimmer, the pupil dilates (becomes larger)
o Behind the pupil, the lens causes the light to focus on a point behind the back surface of the lens. At
this point the image becomes reversed and inverted
o The light then goes through the vitreous humor, to the retina
o Light hits the retina
o Signal crosses over to the other hemisphere
o To the thalamus
o Occipital lobe
o Visual cortex
- Visual perception has often been compared to the operation of a camera. Like the lens of a camera, the lens
of the eye focuses an inverted image onto the retina. This analogy breaks down rapidly, however, because it
does not capture what the visual system really does, which is to create a three-dimensional perception of
the world that is different from the two-dimensional images projected onto the retina. The analogy also fails
to reflect the cognitive function of the visual system, such as our ability to perceive an object as the same
under strikingly different visual conditions, conditions that cause the image on the retina to vary widely.
Our brain amplifies the image for better resolution. It also extract the most important features for us from an image.
- Gestalt psychology
o “What we see represents not just the properties of objects but, more importantly, the organization
of sensations by the brain.”
o “The brain makes certain assumptions about what is to be seen in the world, expectations that seem
to derive in part from experience and in part from the built-in neural wiring for vision.”
- Vision based on interrelationship: when we look at an image, we do not see the absolute
image. We see relationships between certain parts of the image (interrelationship). We see
the bigger picture.
o When organizing dots, we see different patterns and not only the dots themselves
(columns and rows)
- Figure and ground distinction (object vs background)
o The perceptual distinction between figure (or object) and ground is similar to the
communication engineer's distinction between signal and noise. As we focus on one
signal, other information is relegated to background noise.
o In the visual system only part of an image can be selected as the focus of attention the
rest becomes (momentarily) background
- Occlusion = making sense of the patterns
o We organize the part of an image into one whole picture
o This is helped by the central fact of vision: that closer structures cover those that are more distant
- Assumptions about the visual objects: length of an object depending on the shape of an object
o Illusions = “Misreadings” of visual information by the brain --> this illustrates how the
brain applies certain assumptions about the visual world to the sensory information it
receives
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