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Hearing the call of Neurons
Winter 2017
Peter Woodruff
Shining a light on the retina
Problems:
1. Tissues are
.
How do we see the details inside?
2. Most cells are more or less
How do we tell one from another?
3. Neurons are often long and thin.
How do we follow the branches?
Left: schematic drawing by Santiago Ramon y Cajal
(circa 1900).
Right: section through a rat retina.
Neurons and Synapses
Microscopes first used in 17th century
For 200 years, nervous system seen largely as fatty globules in tissue
Then:
• Discovery of neurons with their dendrites and axons
• Neuron doctrine
• Discovery of synapses and chemical transmission
What led to these changes?
Neurons and Synapses
Discoveries depended on improvements in instruments, experimental
technique and conceptualization
Microscope
2. Thin slicing
3. Staining to distinguish cell / tissue parts
1.
Neurons and Synapses
Discoveries depended on improvements in instruments, experimental
technique and conceptualization
1.
Microscope
~1670
Neurons and Synapses
Discoveries depended on improvements in instruments, experimental
technique and conceptualization
1.
Microscope
x-section of optic nerve:
Neurons and Synapses
Discoveries depended on improvements in instruments, experimental
technique and conceptualization
1.
Microscope
Cork “cells” through
microscope
Neurons and Synapses
Discoveries depended on improvements in instruments, experimental
technique and conceptualization
1.
2.
Microscope
Thin slicing
How to cut soft, squishy tissue? Why cut?
Neurons and Synapses
Discoveries depended on improvements in instruments, experimental
technique and conceptualization
1.
2.
Microscope
Thin slicing
How to cut soft, squishy tissue? Why cut?
Fresh tissue sample
Neurons and Synapses
Discoveries depended on improvements in instruments, experimental
technique and conceptualization
1.
2.
Microscope
Thin slicing
How to cut soft, squishy tissue? Why cut?
Ideally: use a sharp knife, hard material
Neurons and Synapses
Discoveries depended on improvements in instruments, experimental
technique and conceptualization
1.
2.
Microscope
Thin slicing
How to cut soft, squishy tissue? Why cut?
Ideally: use a sharp knife, hard material
So: harden tissue! How?
Neurons and Synapses
Discoveries depended on improvements in instruments, experimental
technique and conceptualization
1.
2.
Microscope
Thin slicing
How to cut soft, squishy tissue? Why cut?
Ideally: sharp knife, hard material
So: harden tissue! How?
Neurons and Synapses
Discoveries depended on improvements in instruments, experimental
technique and conceptualization
1.
2.
Microscope
Thin slicing
How to cut soft, squishy tissue? Why cut?
Ideally: sharp knife, hard material
So: harden tissue! How?
? Or replace water with wax or plastic.
Take sample, stop enzymes, remove water, replace with
wax or plastic
Take sample, stop enzymes, remove water, replace with
wax or plastic
Now we need to cut! Very carefully!
Cutting the sample: use a microtome
Cutting the sample
Cutting the sample
A ribbon of 4 mm sections being cut
from a paraffin block using a rotary
microtome.
Next: flatten and put onto
microscope slide.
The “Big Brain” project cutting it close.
A special microtome cuts a human
brain preserved in paraffin wax into
20-micrometre thick slivers to help
map its anatomical structure with
high resolution
Neurons and Synapses
Discoveries depended on improvements in instruments, experimental
technique and conceptualization
1.
2.
3.
Microscope
Thin slicing
Staining to distinguish cell / tissue parts
Colored dyes stick to some parts more than others
Stained slides
cerebellum
Neurons and Synapses
Discoveries depended on improvements in instruments, experimental
technique and conceptualization
4. Cell Theory: Schleiden and Schwann + ~1839:
• All living organisms are composed of one or more cells. BUT viruses?
• The cell is the basic unit of structure and organization in organisms.
• Cells arise from pre-existing cells.
Neurons and Synapses
Discoveries depended on improvements in instruments, experimental
technique and conceptualization
5. Neurons are cells (1890s) with dendrites, cell bodies and axons
Retinal neurons have
many different shapes and
sizes. A midget bipolar and
a parasol-type ganglion
cell are shown.
Neurons and Synapses
Discoveries depended on improvements in instruments, experimental
technique and conceptualization
5. Neurons are cells (1890s) with dendrites, cell bodies and axons
Neurons and Synapses
Discoveries depended on improvements in instruments, experimental
technique and conceptualization
6. Camillo Golgi developed silver staining technique which only
stained some neurons (defective?)
7. Santiago Ramon Y Cajal improved Golgi’s technique
Santiago Ramón y Cajal and
the Neuron Doctrine
Santiago Ramón y Cajal, from the book “The Beautiful Brain.”
A diagram suggesting how the eyes might transmit a unified picture of the world to the brain; a
purkinje neuron from the human cerebellum, circa 1900; and a diagram showing the flow of
information through the hippocampus in the brain
Ramón y Cajal
A self-portrait of Ramón y Cajal
in his laboratory in Valencia,
Spain, about 1885 artist, photographer, doctor,
bodybuilder, scientist, chess
player and publisher. He was
also the father of modern
neuroscience.
What he saw
Pyramidal cells stained with the Golgi method
improved by Ramón
y Cajal.
This method, developed by the Italian
scientist Camillo Golgi, made it possible to
see the details of a whole neuron without the
interference of its neighbors.
In 1906 he and Golgi shared a Nobel
Prize.
Large pyramidal neuron in the
cerebral cortex, the outside part of
the brain,
visible by naked eye (as a dot)
Purkinje neuron from
cerebellum, illustrating its
treelike structure in great detail
Neuron Types
The nervous system
has many
types of cells,
not just neurons
Neuron Doctrine
1. The neuron is the structural and functional unit of the nervous
system
2. Neurons are individual cells not continuous with other cells
3. Neurons have three functional parts: dendrites, soma and axon.
4. Conduction preferentially flows from dendrites through soma to
ends of the axon (axon terminals, with their synapses)
Nerve impulse passing along axon
dendrites
axon
• Unidirectional
• All-or-nothing
• Delay after firing
Message flows down axon to synapse.
Transmission to post-synaptic cell affects that cell’s behavior:
may become more easily excited (excitatory synapse) or
may become less easily excited (inhibitory synapse)
Neuron Doctrine
5. Synapse identified (1846), could be electrical or chemical
transmission
Unidirectional (generally axon to dendrite)
Excitatory and inhibitory action
Delay in transmission
Neuron Doctrine
Santiago Ramón y Cajal and the Neuron Doctrine
Hunched Over a Microscope, He Sketched the Secrets of How the
Brain Works
By JoAnna Klein, New York Times
https://www.nytimes.com/2017/02/17/science/santiago-ramon-y-cajal-beautiful-brain.html?_r=0
The Beautiful Brain: The Drawings of Santiago Ramón y Cajal
Saturday, Jan 28 2017 - Sunday, May 21 2017
Minneapolis; Vancouver, British Columbia; New York; Cambridge, Mass.; and Chapel Hill, N.C.
How can one sense movement of a fluid?
How to sense movement of a fluid?
Hair versus hair cells
How to sense movement of a fluid?
How to sense movement of a fluid?
Sensitive Sharks
Sensitive Sharks
Linear Acceleration Coding by Maculae
Tilt causes shearing
forces on some hair cells
which depolarize as
others hyperpolarize.
Rotational Coding by Semicircular Canals
As one of the canals moves
in an arc with the head, the
internal fluid moves in the
opposite direction, causing
the cupula and stereocilia
to bend. Brain interprets
relative activation of all six
canals to give precise
indication of head
movement.
Sound attributes
• Frequency: vibrations per second, in hertz, Hz
• Amplitude: sound pressure a sound intensity, as in decibels, dB
• Temporal variation: eg: duration
• Tone: simple sound
• Complex sound: contains >1 frequency
Fun Facts about Hair Cells
• Hair cells can detect deflections of 0.3 nanometers (< size of atom!)
•
Threshold of hearing: 1 billionth of atmospheric pressure
• Can convert stimulation into nerve impulse in 10 microseconds
• Threshold of pain at 130 dB is 1013 times more intense than
threshold of hearing, at 0 dB!
When the stereocilia
are bent in response
to a sound wave, an
electromotile
response occurs.
With every sound
wave, the cell
shortens and then
elongates.
This pushes against
the tectoral
membrane,
selectively amplifying
the vibration of the
basilar membrane.