Neuroscience A few highlights of the nervous system.
Golgi (1843-1926) found a way to stain nerve cells so that
they could be seen. As luck would have it only a very few would be stained
and seen. Ramon y Cajal (1852-1934) discovered that the nerve network consisted
of individual neural cells which were already called neurons. Sherrington
(1857-1952) discovered the synapse, the gap between two neurons which the
neural impulse has to cross. We learned that Du Bois-Reymond discovered
the action potential in the middle of the 19th century.
Karl Lashley (1890-1958) tried to find the "engram" and could
not. He proposed the concepts of equipotentiality and mass action as he
argued against detailed localization of function for complex learning.
Richard Thompson (1930- ) may have found the location of a part of a conditioned
eyeblink response in a location in the cerebellum in the 1980s. Currently
the biases are more toward localization of function but the final report
is not in.
Other aspects of broader localization, is the possible location
of different modes of responding in the different hemispheres of the cerebral
cortex. Sperry and Gazzaniga have studies "split-brain" subjects; patients
who for medical reasons have had their corpus callosums severed. The left
being more analytic and the right being more holistic. Some functions seem
to change location as the individual gains expertise. Most people discriminate
tones in their left hemisphere whereas professional musicians seem to do
so in their right hemisphere. ) Sperry is one of the people to redirect
attention to consciousness and the mind-body problem, which currently has
many people involved.
Brain localization is quite interesting. Visual form identification
seems to be in one location and hand-eye coordination in another. There
are single cells that respond to both visual stimuli and touch stimuli
from similar locations on space. There are cells, discovered by Hubel &
Wiesel that respond to particular size and orientation of visual stimuli.
There are some that respond to a stimulus moving to the left alone, or
staying still while the background moves to the right. There are cells
(Olds) which when stimulated are extremely reinforcing; i.e. The animal
will press a bar to be stimulated there.
Neurons---more than 1010
. There are 1014 to
1015 synapses. There
are currently thought to be at least 200 different neurotransmitters in
humans. However, we have the same neurotransmitters found in invertebrates
and lower mammals.
Neurons have action potentials when they depolarize, followed
by a period of no response, so they have a maximum speed of firing. There
are also local potentials, which may interact across neurons, but definitely
make the neuron more or less likely to fire.
Cortex in human about 400 times the area of a mouse and about
3 to 4 times as thick. About 4 to 6 layers in cortical cover, there are
about 600 million synapses per cubic millimeter. Crick claims that one
can identify more than 100 regions anatomically that are duplicated from
one human to another.
Local structure of the relations between neurons is very
variable. Seems to be essentially random.
Many neurons die very early. There is selectivity, but the
nature of the selectivity is not entirely clear.
Some neurons are excitatory, some inhibitory. No neuron has
been found that does both. Many are local, some travel quite a distance.
Sensory neurons have receptive fields, local areas to which
they respond.
There is a fair amount of localization of function identified.
One question is how localized is it?
Brain is a huge energy user. 25% of the blood flow supplying
oxygen goes to the brain. It is always active, at rest one can record alpha
waves. When alert there are faster patterns. Evoked response potential
suggests that there are carrier waves on which specific reactions occur.
Brain is very complex. It is estimated that as many as 50%
of all the genes in the human genome are primarily concerned with the brain.
PET scans show that different parts of the brain become more
active for particular activities.
Single cell recordings show particular responses, Hubel &
Wiesel, orientation cells, directional cells
color cells, small moving object cells in frog (bug detectors)
localization in space, isolation (Moscowitz)
Pathways can be found from many parts of the brain to many
other parts, both between cortical areas and between cortex and lower centers.
Neuron pathways are not entirely determined by the genome.
Studies of nerve structure in cloned animals show different numbers of
dendrites and different paths taken by the nerves. Currently neuroscience
is one of the hottest areas in all science.