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neuronal networks and behaviour summary
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Neuronal networks and behaviorCollege 1
If you want to build a sensing machine, what would you need?
- Receives and translates information into the language of the machine
o Sensory receptors/axons translate the energy of the stimulus into electrical signals
- Transports information for processing
o Axons transport the signal to the series of relay nuclei
- Integrates and processes information
o Interneurons and local circuitry in nuclei
- Gives output
Sensory system
1. Reception: sensory receptors translate the energy of the stimulus into electrical signals
(modality, location, intensity and timing).
2. Transport: axons transport the signal to the series of relay nuclei (parallel processing,
topographical representation, cross-over, feedback connections)
3. Processing: interneurons and local circuitry in nuclei process the signal
Modality: the type of stimulus (type of energy). Sensory receptors don’t have axons that come from
other cells. They need to react to the energy of the stimulus. Sensory neurons look differently. Each
sensory system has some type of receptor that will react only to a certain type of stimulus. → visual
system has rods and cones that will be filled with receptors that only react to light.
In the system of touch there are no cell bodies of these receptor cells in the skin. Only axons in the
skin. The axons react to the changes that happen in the skin by the receptors in the skin membranes
(epithelial cells).
→ 1. Modality of receptors: type of sensory receptors
- Mechanical (touch and proprioception, hearing, balance): physical stretch or tension on the
receptor deforms the membrane and opens the channels
- Chemical (pain, itch, smell, taste): binding of a chemical to the receptor
- Photoreceptors (vision: photoreceptors in retina): change in conformation of a
photosensitive protein
- Thermal (temperature)
- Receptor activation results in the change of postsynaptic potential
skin’s mechanoreceptors (touch)
in the Purves book 4 types of receptors are discussed.
- Merkel cell: these cells are most accurate in understanding in shape
and form of stimulus
- Meissner corpuscle: not as accurate as Merkel cells. They are more
sensitive as they show more action potentials when sensing touch
- Ruffini corpuscle: less sensitive
- Pacinian corpuscle: do not react in same way as Merkel cells. They
react very well to stimulus however. But they cannot understand the
shape or the details of the touch
Only the Merkel cell here is a cell. The rest are axons that are wrapped
around structures that can sense the displacement of the skin.
These receptors have different functions.
,Proprioception: sense of the position of muscles and joints of the body
These axons are wrapped around certain structures (muscles for example). If the muscle is stretched
than the axon can sense this and sends this signal further to the brain. Similar structures are located
in tendons that can sense the position of the joints of the body.
→ 2. Location of receptors
What is the position of the stimulus relative to the body?
Our skin has dermatomes, which are certain areas that are
perceived by different parts of the brain.
- Topographical arrangement of neuronal receptive
fields (retinotopy in retina, tonotopy in cochlea,
gustotopy in gustatory cortex)
- Spatial resolution is determined by the size of
receptive field and density of receptors → fingertips
very sensitive to small changes in shape of object.
Wrist has bigger receptive field → back or stomach
even bigger.
o Examples: fovea in retina, fingertips for touch
have smaller receptive fields
→ 3. Intensity of the stimulus
How strong is stimulus?
- Sensory threshold (determined by the sensitivity of the receptors). How much stimulation is
needed to trigger action potentials?
- Change in energy of the stimulus:
o → change in membrane potential
o → translation into digital code of action potentials
→ 4. Timing of the stimulus
How does the stimulus change in time?
- Slowly adapting responses: changes in stimulus are coded in
frequency
- Rapidly adapting responses: start and end of stimulus are most
important. So, they fire action potentials at the beginning of the
stimulus and then the stimulus continues but there is no reaction
from these neurons. Only when this stimulus ends there is a reaction
again
- Adaptation: a constant stimulus fades from consciousness → clothes
that you wear for example. However, if something changes, we have
our rapidly adapting neurons to detect those changes
- Sensory systems are able to detect contrasts and motion
Sensory system
1. Reception: sensory receptors translate the energy of the stimulus into electrical signals
(modality, location, intensity and timing).
2. Transport: axons transport the signal to the series of relay nuclei (parallel processing,
topographical representation, cross-over, feedback connections)
3. Processing: interneurons and local circuitry in nuclei process the signal
,Transport of sensory information: parallel processing
in the picture it starts from the dorsal root ganglion to the horn to the
caudal and rostral medulla (upper part) and the info goes on to the
thalamus and cortex.
- multiple parallel pathways increase the speed of processing
- Topographical representation is maintained (upper limbs lateral
position in dorsal column, lower limbs are positioned more
medial) → So, upper limbs are positioned more lateral (to the
side) from the spinal cord and the lower limbs are positioned
more medial towards the spinal cord
- Cross over of sensory information to the opposite hemisphere
(decussation) (left goes to right part of brain and vice versa)
- Feedback connections/descending projections
Sensory system
1. Reception: sensory receptors translate the energy of the
stimulus into electrical signals (modality, location, intensity and
timing).
2. Transport: axons transport the signal to the series of relay nuclei
(parallel processing, topographical representation, cross-over,
feedback connections)
3. Processing: interneurons and local circuitry in nuclei process
the signal
all sensory information passes through thalamus before reaching the
cortex
- Major relay station for sensory and motor information
- Y-shaped division into anterior, lateral and medial thalamus (see
picture)
- Projects to the middle layers of cortex
Thalamus has different nuclei. Information of touch will go through VPM
nucleus and VPL. Information of vision → LGN. Hearing → MGN.
, Eventually information will reach the cortex.
First, it reaches the primary sensory cortices
(yellow).
The cortex has a stereotypical structure. It
consists of different columns. 1 column of
the cortex will be dedicated processing one
feature of each stimulus. All those cells will
process information from one area of your
skin. Different parts of information will come
to different cells within this one functional
column. Each column also has 6 layers. Each layer again
has different functions.
So, for example sensation in your thumb → will arrive
in layer IV. This is the input that comes from the
thalamus. Then, this information will spread to layer II
and III, which are important for connecting different
cortical regions. Also, layer 5&6 will process this
information and will send this to the basal ganglia and
back to the thalamus.
functionally distinct columns in the sensory
cortex
Here you see an example from the monkey
cortex (sensory). You see the topographical
orientation. If you zoom in on the blue part of
the cortex, then you see that each digit of the
hand (each finger of the monkey) has its own
representation. For D4, you see that for one
digit two types of information are kept
separate → slowly adapting and rapidly
adapting neuron receptors. These two
different receptors will send information to
different parts of cortex.
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