NATS 1860 Note 12
Neurons, Spikes, Synapses:
- Consider the following:
o Brain Maps
o Nerve cell components
o Membrane structure
o Ions and ion channels
o Action potentials (spikes)
o Spike propagation (how spikes get from one cell to another cell)
o Synapses (Cajal discovered that the ends of one cell don’t form a
junction on another cells, they meet at a “synapse”
o Excitation and inhibition
There are two types
One excites and makes it fire
One turns it off, and prevents from firing
o Mind altering drugs
o Small Neural Networks
Sensory and Motor Maps
- Map of body surface is two dimensional: the retina is a very two dimensional
o This is evidence as electrical brain stimulation
o If you do motor control in dogs (stimulate electrically on different
parts of the brain, different effects will play out)
Similar experiments were done on humans during epilepsy
- These maps are all distorted: they represent how much neural activity is
coming or going from rather than the physical area of the body.
o Based on the amount of information on muscles about how much
information has to come in
- Left half of the cortex receives information from the right half of our body,
whereas the left motor cortex controls the right side of the body
- Right half of the cortex senses the left half of the world and the right motor
cortex controls left side of body
- Corpus Callosum: axons connecting left and right cortex
Neurons (Nerve cells)
- A neuron is one nerve cell, and a nerve is typically around a million
connection of nerve cells
o A corpus callosum is a body of millions of nerve cells.
Axons are the main part of the nerve cell that are in nerve
Auditory nerves have 1.5 million axons
There are about 100 billion neurons in our brains and
thousands of synapses from each neuron
We have about 100 trillion synapse - A nerve cell has a cell-body (soma)
- It also has dendrites (dendritic tree/branches). They start at the soma and
then go to different directions
- The cell also has axons, another process that goes off.
o Dendrites and axons have different functions from the nerve cells
- The axons originates from a small thickening (protrusion) of the cell body
called the (Axon hillock)
- Finally, the axon terminates at all other axons in places called synapses
Parts of Neuron
- Dendrites and soma: they are site of synaptic contacts from other cells
o This is where it receives information with other cells
o This is the input site; the site of stimulation of the cell. The axon is a
long, thin cylinder that transmits electrical impulses from the cell to
other parts of the brain
It is the output of the cell.
This is how the nerves talk to another
- Synapse: contact between ends of an axon and dendrites of other neurons
- Axon hillock: beginning of axon where nerves impulses are first triggered.
Excitatory Neurons in Cortex:
- These are cells in cortexes in which the soma uses its dendrites
o The brain is very densely interconnected
- The cortex of our brain is roughly 3mm thick.
o If you take the entire surface area of our brain, it would be the size of
a large, thin crust pizza.
Neurons: Size and Time scale
- In order to record electrical activity from nerve cells, you stick an electrode
into the soma of a nerve cell, and you record the information
o This has been done on humans since 1985.
o From this discovery we know that we share the same cells with other
species, the only difference is the way we use the cell
- 1 micron = 1/1000 mm
- The Soma is about 10 microns in diameter
- Dendrites are about 3mm long. They’re much thinner
- Axon diameter is about 1-2 microns
o Axons can be as long as 1m. It’s still part of a single cell that is very
thin, but very long.
- 1 millisecond = 1/1000 second.
- Electrical pulses that the brain produces pulses that are about 1ms long
o This is very slow compared to a computer running on GHz
However, the amount of information our brain processes is
much greater Neuron Membrane
- It consists of two layers of lipid molecules
o The reason there are two is due to the structure of these molecules: