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Samenvatting "Introduction into Neuroscience" lecture 1 t/m 7 Sensory system
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Samenvatting "Introduction into Neuroscience" lecture 1 t/m 7 Sensory system
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Sensory system 1Sensory relay chain (suggest that the information is just passed on, not true. At
each transition the signal is processed:
Signal transduction: Convert the energy of the signal into:
• A change of membrane potential or
• An action potential
1st relay neuron:
• A pseudo-unipolar neuron in a peripheral ganglion (peripheral NS)
Sensory receptors are either specialized epithelia cells or free nerve endings of neurons (equipped
with receptor proteins). Each type of receptor is sensitive to a specific type of energy, and often to
only a narrow range of that type of energy:
• Stretch, pressure, heat, cold, light, etc.
Seemingly identical receptors may have different thresholds for the same stimulus, increasing the
dynamic range of the system.
All signal transduction results into either:
• A change of membrane potential which results in a charge of the rate of
release of a neurotransmitter, or
• An action potential or change in the rate of action potentials.
The 1st neuron in the sensory chain.
All peripheral sensors connect to or are part of a specialized relay neuron → the
pseudo-unipolary ganglion cell (neuron). In pseudo-unipolar neurons:
• The dendritic tree connects directly to an axon hollick and axon.
• Branches of this tree:
o May be synaptic contact with sensors (tactile
corpuscules), or
o May be directly stimulated as nude nerve endings.
• The action potentials are generated at the axon hillock,
then travel down the peripheral and central axon,
towards the Terminalia in the CNS.
Ganglionary neurons react to receptor signal with action
potentials. Two functional types:
• On/Off, silent when Off, action potentials when On ().
• More/Less, frequency modulation of continuously firing
ganglion cell (more precise) (→)
Area on the receptive surface which upon stimulation elicits a response in:
• Receptor
• Ganglionary neuron (1ste relay neuron)
• Spinal/bulbar neuron (2nd relay neuron)
• Thalamic neuron (3rd relay neuron)
• Primary cortical neuron.
The further, the more connections.
1
,Receptive field is shaped by:
• Convergence
o Many receptors connect to one neuron.
o There is the ratio between receptors and 1st relay neurons. A high ratio
indicates a sensitive system, a low ratio indicates a precise system.
• Divergence
o One receptor connects to several neurons.
o Exception: nude nerve endings
o At the same time as convergence
• Lateral inhibition
o Central neurons are excited
o Peripheral neurons are inhibited
o Between receptors and neurons in the visual system
o In other system only between relay neurons
Receptive field of receptor characterized by:
• Type of stimulus
• Size of receptive field
• Speed of adaptation (tonic versus phasic)
The complexity of the receptive field increases progressively.
• Orientation sensitive emerges
Influence of convergence.
Lateral inhibition increases contrast. Dendritic tree is
connected to pressure sensors. Because of overlap, more
neurons are stimulated at the same time. They are
connected with each other by inhibitory interneurons.
Complex receptive fields in cortical neurons.
Motion sensitive:
• Any direction
Direction sensitive:
• Much more responsive to motion in one direction
than in another.
Orientation sensitive = complex cell
• Respond best to motion along specific axis.
Motion sensitive neurons:
• Respond well to movement in all directions but not
selectively to movement in any one direction
Direction sensitive neurons:
• Respond much better to movement in one direction
than in another
Orientation sensitive neurons:
• Respond best to movement along a specific axis
2
, Receptor – ganglionary neuron – CNS.
Law of specific nerve energies:
The only thing a receptor reports is
the fact that it is stimulated, not the
cause of the stimulation:
• In principle receptors are only
sensitive to a specific
(bandwidth of) energy.
• But a forceful blow to the
head will also depolarize the
rods and cones, though in a
haphazard fashion, resulting
in flashes of light.
• Photoreceptors are also stimulated by a forceful blow to the head. This stimulation will result
in the perception of light flashes; photoreceptor stimulation will always be interpreted by the
CNS as photostimulation.
Law of specific nerve energies =
• The receptors reports ‘stimulation’
• The nerve reports ‘receptor stimulated’
The CNS can only interpret this message if it knows
where it comes from.
• The information of a specific type of receptor in a specific area ends up in a cortical area
dedicated to that specific type of information form that specific area.
The labeled line principle.
Different modalities of sensation (pain, touch, vision etc.) depend on the termination point in the
CNS.
• Nerve fibers transmit only one modality of sensation.
• Type of sensation experienced when a nerve fiber is stimulated is determined by the
termination point in the CNS.
Neural coding.
Qualitative information, what do we sense?
• Labeled line principle
Quantitative information, how intense is the sensation?
• Different receptors have different thresholds:
o Again, depends on labeled line principle.
• Per receptor:
o Duration of the stimulus determines length of pulse train
o Strength of the stimulus determines frequency of
pulse train.
Information about the type of stimulus is encoded in the area of the
CNS where the axon terminates. The action potentials only code for
presence and size of the stimulus.
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