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Neuronal networks and behaviour summary somatosensory systems
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This document contains a summary of the lecture, and chapter 9 of the book neuroscience by purves about somatosensory systems.
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The somatosensory system chapter 9The human brain is built of neurons. The human cortex consists of Sensory receptors are activated by the energy of the stimulus
about 86 billion neurons that each have around 1000-10.000 Non-sensory neurons (brain) receive information from the axons of
synapses. Neurons communicate with each other via synaptic other neurons. Sensory neurons do not have these axons, so they
transmission via the synaptic vesicle cycle. There are two kinds of need cells that react to the stimulus (for example rods and cones
potentials, the excitatory postsynaptic potential (EPSP) which is in the retina that react to light).
regulated by the neurotransmitter glutamate that binds to the
AMPA and NMDA receptors, and the inhibitory postsynaptic
potential (IPSP) which is regulated by the neurotransmitter GABA
that binds to GABAergic neurons.
Synaptic transmission
Axons release neurotransmitters that bind to postsynaptic
receptors and determine whether the postsynaptic cell membrane Types of sensory receptors
will depolarise and generate an EPSP or hyperpolarise and • Mechanical: touch and proprioception, hearing, balance. A
generate an IPSP. EPSPs increase the excitability of the cell and physical stretch or tension on the receptor deforms the membrane
increase the probability of action potential generation. IPSPs and opens the channels.
decrease excitability and probability of action potential • Chemical: pain, itch, smell, taste. Regulated by binding to a
generation. In the neuronal network each neuron will integrate chemical receptor.
multiple EPSPs and IPSPs, the sum will determine whether the • Photoreceptors: vision, photoreceptors in the retina. There is a
cell fires an action potential. The frequency of the action change in the conformation of photosensitive proteins.
potential conveys the signal. Drugs can also bind to the receptors • Thermal: temperature
and stimulate the action of neurotransmitters. Receptor activation results in the change of postsynaptic potential
Perception of the outside world Skin mechanoreceptors
The sensory system receives and translates information into the There are four types of receptors in the skin:
language of the system: sensory receptors/axons translate the 1) Merkel cells
energy of the stimulus into electrical signals. After this the 2) Meissner corpuscle
information is transported for processing: axons transport the 3) Ruffini corpuscle
signal to the series of relay nuclei and processed information gets 4) Pacinian corpuscle
integrated: interneurons and local circuitry in nuclei process the
signal. An example of this is the receptors in the skin that can
sense a stretch when the skin is touched. This opens ion channels
and lets ions in and generates a depolarising current and an
action potential. Axons are used to transport the signal to the On the right, you can see the different responses of the different
dorsal root ganglion cells that lay close to the spinal cord and cells in the skin. The Merkel cells are the most accurate to
sent their axons to the skin. Finally, the information is determine the shape of the braille letters. The Pacinian corpuscle
transmitted to different circuits in the spinal cord and brain. reacts differently than the Merken cells and is not used to
determine the shape. These cells are mostly used when you for
example are playing an instrument.
Proprioception
Proprioception is the sense of the position of muscles and joints
> Axons that transmit touch sensory stimuli (mechanoreceptors) of the body. There are special proprioceptors, including muscle
are often encapsulated by special cells that help tune the axons spindles, Golgi tendon organs, and joint receptors that are located
to make them more sensitive to stimulation (lower thresholds for in the muscles and tendons that can sense this. Their axons are
action potential generation). Axons that lack these cells are wrapped around certain structures and when the muscle is
referred to as free nerve endings and are important in the stretched, the axons sense this and send a signal to the brain, so
sensation of pain. the brain knows where the muscles are and complex movements
can be accurately performed.
Sensory systems
1) Reception: sensory receptors translate the energy of the
stimulus into electrical signals (modality (type of the stimulus),
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
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