Lecture 6: Chapter 13 - Information Processing in Dendrites
This chapter deals with the information processing that occurs
in dendrites. This is where most of the information processing
occurs in neurons, because the soma is not the site of most inputs,
and because the axon is primarily an output device. The author
begins with some history about dendrites, and then continues with
neurons which have no axons -- these neurons provide nice examples
of what can be done without an axon.
Figure 13.1:
Originally it was understood by Ramón y Cajal that dendrites
were input devices, and that axons were output devices.
Figure 13.2:
Added to the notion of dendrites being input devices, was the
notion that the soma and the dendrites are also sites for local
integration. This includes local processing with nearby inputs
summing their effects, and the flow the combined effects of PSPs
from synapses into the soma and out as action potentials down
the axon.
Figure 13.3:
Added to the previous is "retrograde information flow"
from the soma back up into the dendrites. This is a signalling
of the output state back up into the dendrites. There are also
retrograde signals from the post-synaptic site back into the pre-synaptic
site through the action of either reciprocal synapses or autoreceptors.
Comment added after class: Simulations of a motoneuron
demostrated that a typical motoneuron will not send action potentials
back up the dendrites. A neuron will not do this without a significant
number of voltage gated Na and K ion channels in their membranes,
and motoneurons are not thought to have them. Thus, just which
neurons do propagate action potentials up their dendrites is not
yet clear. (Notice the example in figure 13.21)
Additionally, the question arose regarding whether or not the
inactivation of voltage gated channels would effect propagation
of PSPs. Voltage gated channels are not required for PSPs
except when specifically a part of the receptor, as in the NMDA
channel. However, the depolarization caused by the action potential
into the dendrite may cause "shunting" and therefore,
loss of impact of the PSP. Shunting is when ions flow out/in open
channels according to their equilibrium potentials in such a direction
so as to negate their potential effect. Typically, they would
flow in/out to generate a PSP. However, when the current is shunted
because of changes in the membrane potential from an event such
as an action potential or some other kind of event, the PSP current
flows in the opposite direction, and the impact is nullified.
Thus, the action potential propagating up the dendrite could impact
the PSPs in this manner. However, the changes of membrane potential
is quite brief, so it wouldn't be for too long.
Figure 13.4:
Finally, is added the concept of action potentials originating
in the dendrites, flowing out to the soma. This adds the ability
for greater local processing in the dendrites and more impact
of this processing. It also means that there can be local zones
of processing in dendrites.
Figure 13.6:
A horizontal cell in primates -- two local zones of input and
output. One is called a dendrite and one an axon, but both are
without action potentials. They each receive their own input and
produce their own local output. One receives rod input and the
other cone input. Their responses to the same amount of light
is very different. Thus, there is extreme compartmentalism within
the one neuron.
Figure 13.7:
Functional compartmentalism within a dendritic tree of an amacrine
cell - an axonless neuron. Distal dendrites are sites for both
input and output synapses. This form local input-output units.
Figure 13.8:
This is provided to demonstrate that there are calcium action
potentials in dendrites. Blockade of voltage dependent Na channels
and K channels does not prevent the generation of the long lasting
calcium action potential generated in the dendrite of a mitral
cell - a cell in the olfactory cortex.
Figure 13.10:
Local functional subunits in an invertebrate neuropil. These local
synaptic subunits form local processing units. These little units
are smaller than the neuron. The little circuit that this is a
part of can function without any action potentials, although action
potentials are important when allowed to occur.
Figure 13.11:
In another invertebrate, there are also local processing units
in the interneurons of locust leg where sensori-motor integration
occurs. Responses of non-spiking synapse can be graded depending
on the amplitude of the input. These types of neurons may be either
inhibitory or excitatory.
Figure 13.12:
The input to a dendrite with an axon (all neurons have only one
axon, but it may branch many times. However, there is only site
of action potential generation). The presence of an axon and its
output patterns, changes our concept of the dendrites' functions.
Sub-threshold inputs can only have impact insofar as they can
effect the production of an action potential.
Figure 13.13:
Mechanisms of enhancing the impact of distal dendritic synapses.
A) increased conductances at distal synapses; B) increased by
increased membrane resistance, but it is slower; C) voltage gated
channels boost the impact of inputs.
Figure 13.18:
Purkinje cell - demonstration of the differences between somatic
and dendritic action potentials. The somatic action potential
is Na ion based, while the dendritic is Ca ion based. The somatic
is short, and the dendritic is long lasting. Notice that the dendritic
spikes can be recorded in several places along the dendrites.
The dendritic spikes interupt the somatic spikes.
Figure 13.19:
Imaging calcium transients in spines. Blocking voltage sensitive
calcium channels abolishes them. The major point of this figure
for me is that one can image the activity in single spines!
Figure 13.21:
Stimulation to the distal dendrite first activates a somatic action potential. However, with higher levels of current, the site of action potential generation shifts to the dendrite. Thus, both sites can initiate action potentials, but the dendrite has a higher threshold. Both can then activate an action potential in the other site. These are both Na ion based action potentials due to voltage dependent channels in both dendrites and soma of this neuron.