Structure of the Neuron

• Dendrites

Filaments that receive information from other neurons, receptors, etc.

• Soma (cell body)

Contains nucleus of the cell.
Manufactures enzymes and molecules to maintain cell life.
NOTE: In the PNS, a group of cell bodies and dendrites are called ganglion and in the CNS, it would be called a nucleus.

• Axon

Filament that transmits information to the dendrites or soma of other neurons.
NOTE: In the PNS, a group of axons is called a nerve and in the CNS it is called a tract.

• Myelin Sheath

Insulation on axon to improve speed of transmission.
Also insulates axon from activity of other neurons.
Other manufacturing, nurturing, and clean-up tasks

• Axon Terminal

Where neurotransmitters are stored.

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How Neurons Fire

• Electrochemical process

Propagation of impulse within neuron (electrical).
Transmission of impulse between neurons (chemical).

• Resting Potential

Neuron at rest.
Not firing.
Stable, negative charge inside neuron relative to outside (- 70 mV)
Sodium (Na+) outside the neuron.

• Action Potential

Neuron receives sufficient stimulation from receptor, other neurons, etc.
Na+ ions rush in.
Interior of axon becomes positive relative to the outside (+ 40 mV)
This release of energy passes down the axon and is the firing of the neuron.
Once neuron fires, resting potential returns.

• All or None Law

Once neuron fires, the impulse moves down the axon at the same intensity.
No stopping it.

• Impulse frequency

While there are differences in speed of transmission between neurons, increase in neuron stimulation doesn’t cause stronger impulse. Speed and electrical change remains the same regardless of intensity.
Stronger stimulation leads to more frequent firing of the neuron or more neurons activated.

• Absolute Refractory Period

Brief period of time after firing when neuron won’t fire no matter how much stimulation given.
As brief as 1/1000 of a second. HOWEVER, means a limit as to how many times neuron can fire per
second (the maximum is 500-600 times/second).

• Think of neuron like a toilet that works well

Information flows in 1 direction (out to the sewage!)
It is all or none - the toilet doesn't have "half flush" days
There is a period of time that, no matter how much you jiggle the handle, no flushing will occur

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How Neurons Send Messages

• Synapse

Small gap or cleft between the axon of one neuron and the dendrite of another.

• Axon terminal

Contains synaptic vesicles

• Neurotransmitters

Chemical substances released by synaptic vesicles.
Taken up by the axon of the receiving neuron
Work in Lock and Key fashion
Inhibitory (IPSP): tells the neuron NOT to fire)
Excitatory (EPSP): tells the neuron to fire)
If gets more excitatory information than inhibitory, neuron will fire, if more inhibitory than excitatory, will not fire.
A neuron AT REST will occasionally spontaneously fire
           Excitatory information - causes depolarization (an increase in neural firing)
           Inhibitory information - causes hyperpolarization (a decrease in neural firing)

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Neurochemicals
    * Selected neurotransmitters
          o Acetylcholine
                + motor, learning memory, contract muscles, REM sleep
                + Alzheimers, black widow spider bite
          o Dopamine
                + motor, attention, emotions
                + Parkinson’s, schizophrenia, drug abuse
          o Norepinephrine
                + regulation of arousal, eating and sleeping
               
         o Serotonin
                + arousal, sleep, learning, mood, aggression, appetite
                + depression
          o Endorphins
                + Pain reduction, pleasure
                + possibly related to "runner’s high"

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