• University of Toronto

• BIO 304

  BIO304 – Lecture 2

Understanding neuron shape and structure.
- We are going to think about historical observations. A researcher Ramon y Cajal,
the Neuron doctrine.
- Golgi- labelling neurons, advances in neuronal tracing.
1. Early Inreoads into Understanding the Nervous System
o We didn’t always know that neurons are cells
o The cell theory in 1838 was
 made possibly by technological advances in microscopy
 Based on combined observations by numerous scientists. Robert Hooke
was the first to coin the word cell
o Cell Theory
 All living organisms are composed of cells
 The cell is the basic unit of structure and organization in organisms
 All cells come from pre-existing cells. Living things are cellular
o Reticular Theory of the Nervous System ingot\red the cell theory (1861)
 A camp of researchers referred to the nervous system as a reticulum- a
nervous system is made up of a single contiguous network, not sperate
cell.s
 This shows that they wanted to show neural transmission. This
kind of violates cell theory.
 Camillo Golgi and von Gerlach were strong proponents of the reticular
theory
 Camillo Golgi (found the golgi apparatus) invented reazone near,
o “the black reaction’ aka Golgi Stain
o It randomly labels some neurons in the tissur but not all of
them.
o This permits single cell tracing with potassium dichromate
and silver nitrate.
o Neurons are densely packed, but since golgi staining only
labels one subset, we can see how the neurons are shaped.
It is not known why it is able to do this. The dye is toxic so
the tissue sample must be dead
o Santiago Ramon y Cajal
 studied tissues of vertebrates and invertebrates
 he was a histologist and looked at nervous system tissues.
 He was first to see the potential of golgi staining and then disproved
the reticular theory. “if they were connected, all of the sample would
be stained.”
 He disapproved reticular theory in favor of neuron theory
 The nervous system is made up of neurons
o The neuron doctrine murdered the reticular theory. It is a broad synthesis of
principles about the nervous system organization put forth by Heinrich
Waldeye
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o Although walder s credited for putting forward the word doctorine, However
it was Ramón y Cajaland his colleagues that generated the data used for
formulating the neuron doctrine...
o
Tenets of Neuron Doctrine
1. Neurons (cells) are the functional units of the nervous system
a. Nervous fibers from the functional units of the nervous system
 Nerve fibers project from the soma of single neurons
 The nucleus is the nutritive center of the neuron
2. Law of dynamic polarization
o Nerve cells have a single axon that serves as an effector
o Dendrites and call body serve s receptor surfaces of the neuron
o Information travels from dendrites to axons
3. Neurons communicate via regions of the cell-cell contact (synapses
o Charles Scott Sherrington put forward the concept of synaptic transmission,
but he didn’t have definitive proof came later with electron microscopy,
which gives higher resolution. They saw the vesicles on the edge of the
neuron. We look inside the cells in high detail.
o Dense arrangement is due to a lot of protein in the post synaptic region
There are problems with the neuron doctrine however. Our view of the
nervous system is now different from this theory, as there are always
exceptions.
- Extensive Gap junctions (electrical synapses) in the CNS encroach upon reticular
theoryactually back the reticular theory
- Axons can sometimes act as dendrites and dendrites can act as axons (reciprocal
synapse)
- Signals can travel against polarity (bacj
Nevertheless, our first inroads towards understanding the nervous system
came from detailed studies of neuron morphology, projection and connectivity
(represented by the connectome, showing all the connections in an animal’s
nervous system)
- Discoveries were made because there was new technology invented- that theme
recurs in science over and over, where technology comes and people are able to
make new insights.
However, recent findings indicate that neural "connectomics" is not enough
- analogy: if you merely map the entire system of roadways in a city, would that be
enough to understand all of the traffic patterns that flow through it at different
times of the day?
Major insights were made purely from tracing- understanding shapes and connections of
neurons was enough for Ramon Y Cajal to understand the neural system better. Needless
to say, these people created.a profound influence on many for years to come. The
baseline Is still popular- to understand the nervous system, we need to understand how
the information flows in the neurons. Mapping is still important today. Understanding the
 
connectome is not necessarily enough, however. If you were to map all the roads in the
city, you might understand traffic, but having that map will not give you a complete
picture- we need to understand all traffic at all times of the day- how does it move?
How has neuronal tracing technology improved?