Biomedical Engineering Reference
In-Depth Information
across the membrane of the postsynaptic neuron. Postsynaptic currents may flow
inward or outward, depending upon the type of neurotransmitter involved (gluta-
mate or GABA), and the type of ion that flows in response. Excitatory synapses
have the effect of increasing the membrane potential of the postsynaptic cell, i.e.,
making it more positive, while inhibitory synaptic connections decrease it. Neurons
are exclusively inhibitory or excitatory.
A.1.2
Cortical Anatomy
The mammalian cerebral cortex is the outer mantle of cells surrounding the central
structures, e.g., brainstem and thalamus. It is unique to mammals, and is believed
to be necessary for most higher-level brain functions. Topologically the cerebral
cortex is comprised of two spherical shells, corresponding to the two hemispheres
and connected by the corpus callosum. Cortical thickness varies between 2 and 3
mm in the human, and is folded around the subcortical structures so as to appear
wrinkled. Its average surface area is about 3000 cm
2
. It is estimated that there are
roughly 10
11
neurons in the human brain, and 10
10
of these in the cortex.
The diversity of structures of neurons is extraordinary. Approximately 85 % are
pyramidal cells whose dendritic trees have a distinctive, elongated geometry that
makes possible the generation of extracellular fields at large distances. The remaining
15 % may be broadly classified as stellate cells, whose dendritic trees are approxi-
mately spherical, and make little or no contribution to distant field. Both cell types
are interconnected and together form a single dynamical network, but it is believed
that the fields at large distances are dominated by pyramidal cells because of their
size and number.
Synaptic connections in the cortex are dense. Each cortical neuron receives 10
4
-
10
5
synaptic connections, with most inputs coming from distinct neurons. Pyrami-
dal cells make excitatory connections to both cell types. They make intracortical
connections over lengths ranging 0.5-3 mm, and cortico-cortical connections over
lengths ranging 1-20 cm. Stellate cellsmake inhibitory connections to both cell types.
They make intracortical connections over lengths ranging only 0.02-0.03 mm, much
shorter than pyramidal cells. Thus connections in the cerebral cortex are said to
exhibit long-range excitation and short-range inhibition.
An interesting distinction exists between intracortical and cortico-cortical con-
nections. Intracortical connections are made locally between neighboring neurons,
such that the probability of a connection between two neurons falls off smoothly
as a function of distance. In contrast, corticocortical connections are made via sub-
cortical white-matter fibers, and behave non-locally in the sense that connection to
a distant neuron does not imply connections with intermediate neurons. Because
of the diversity of scales of these synaptic connections, and the nonlocal nature of
the cortico-cortical connections, the cerebral cortex exhibits rich spatio-temporal
dynamics spanning a wide range of length and time scales.
