Neurobiological Background - Hierarchical Neural Networks for Image Interpretation - page 27

a function of the relative contrast between the center stimulus and the surround stim-
ulus. These mechanisms are thought to mediate such perceptual effects as filling-in
[237] and pop-out [123].
Lateral connections may also be the substrate for the propagation of activity
waves that have been observed in the visual cortex [208] as well as in the retina.
These waves are believed to play a important role for the development of retinotopic
projections in the visual system [245].
2.4 Neurons
Individual nerve cells, neurons, are the basic units of the brain. There are about 10 11
neurons in the human brain that can be classified into at least a thousand different
types. All neurons specialize in electro-chemical information processing and trans-
mission. Furthermore, around the neurons many more glia cells exist, which are
believed to play only a supporting role.
All neurons have the same basic morphology, as illustrated in Figure 2.11. They
consist of a cell body and two types of specialized extensions (processes): dendrites
and axons. The cell body (soma) is the metabolic center of the cell. It contains the
nucleus as well as the endoplasmatic reticulum, where proteins are synthesized.
Dendrites collect input from other nerve cells. They branch out in trees contain-
ing many synapses, where postsynaptic potentials are generated when the presynap-
tic cell releases neurotransmitters in the synaptic cleft. These small potentials are
aggregated in space and time within the dendrite and conducted to the soma.
Most neurons communicate by sending action potentials down the axon. If the
membrane potential at the beginning of the axon, the axon hillock, exceeds a thresh-
Fig. 2.11. Structure of a neuron. The cell body contains the nucleus and gives rise to two
types of specialized extensions: axons and dendrites. The dendrites are the input elements
of a neuron. They collect postsynaptic potentials, integrate them and conduct the resulting
potential to the cell body. At the axon hillock an action potential is generated if the membrane
voltage exceeds a threshold. The axon transmits this spike over long distances. Some axons
are myelinated for fast transmission. The axon terminates in many synapses that make contact
with other cells (adapted from [117]).