Neuron

History:
10/26/12 - formatting updated
10/1/12 - original version

Stained Florescent Microscopy Source: http://phys.org/news175417796.html

Hand Drawing (Ramon y Cajal) Source: http://nobelprizewatch.wordpress.com/2011/12/06/

Computer Graphic Source: http://thetechjournal.com/science/neuron-implantation-can-rewire-brain-itself.xhtml

Background

Functional unit of nervous system

A cell specialized for the ___________________ and ___________________ of signals

____ billion in adult human brain (source)

Uses ____________________ and ______________________ systems to communicate

	In central nervous system	In peripheral nervous system
clusters of cell bodies
bundles of axons

Cell basics

Nucleus & DNA
Membrane
Organelles
Mitochondira

Energy metabolism: constant need for _____________ and _____________




Source: http://en.wikipedia.org/wiki/Neuron#Anatomy_and_histology


Source: Pinel

Structures for communication

Source: http://en.wikipedia.org/wiki/Neuron

Types of neurons

http://en.wikipedia.org/wiki/Neuron#Functional_classification

Glial Cells

Support cells
Generally outnumber neurons (as much as 10:1 in some parts of the brain)

Oligodendrocytes
    myelin extensions wrap around axons in central nervous system
    provides myelin to multiple neurons

Schwann cells
    provides myelin in peripheral nervous system
    1 Schwann cell per axon
    can aid in regeneration

Microglia
    aid in recovery, part of inflammation process

Astrocytes
    historically seen as "glue" or support cells
    increasingly recognized for communication abilities
    part of blood-brain-barrier and may regulate blood flow
    recycle neurotransmitters
  

Resting Potential

Where is this going:
    The neuron is going to rapidly move ions across its membrane
    It spends the energy in advance to setup for this action

Potential

    Energy that is available to do work
    Ball at top of slope, spring, laptop battery

Chemical gradients

Example of non-charged particles diffusing across barrier to reach equilibrium

Concentration gradient - the "downhill" change in concentration

Electrical gradients

Ions - molecules that carry a negative or positive charge

Electrostatic pressure - the force pushing molecules down the gradient (space below is intentionally blank for drawing)












Source: http://www.anselm.edu/homepage/jpitocch/genbio/nervousnot.html

Sodium-Potassium Pump

A pump that moves ___________________ out and _______________ in by using __________________

        (The brain uses about 20% of your daily calories, this is a major component)

Source: http://hyperphysics.phy-astr.gsu.edu/hbase/biology/nakpump.html


Not really a pump, but just a _________________

Source: http://en.wikipedia.org/wiki/Na%2B/K%2B-ATPase

Channels

Proteins embedded in the membrane that allow molecules to _____________ diffuse through

Voltage-gated channels - open and close in response to _________________________________

Ligand gated - open and close in response to _________________________________

Na+ channel - _________ at negative potentials, slower/faster to respond

K+ channels - _________ at negative potentials, slower/faster to respond

Neuron's Resting Potential

Typical resting potential is _______

Sodium (Na+)


Potassium (K+)


Cloride (Cl-)


Organic anion (A-)




Source: http://academic.uprm.edu/~ephoebus/id81.htm

Action Potential

Where we are going:
    How does a signal get passed down the neuron (along the axon)
    A sequence of events that disrupts the action potential

Post-synaptic potentials

    Axon terminals release neurotransmitters
    These neurotransmitters react with receptors on the next neuron
    Can cause the neuron to depolarize or hyperpolarize

Polarization

Source: http://www.polaris.iastate.edu/NorthStar/Unit1/activity1.htm

    Depolarization

    Hyperpolarization

Sub-threshold depolarization

Does NOT make membrane potential more positive than threshold (typically -65 mV)
A little Na+ comes in, making the potential more positive
K+ is pushed out by the incoming Na+ and the NaK pump is still working, so returns to resting potential

Above-threshold depolarization

DOES make membrane potential more positive than threshold (typically -65 mV)
As it depolarizes, more and more sodium channels open
Na+ starts coming in faster and faster, creating positive feedback
Full action potential occurs
Total number of ions flowing through membrane is relatively small, so concentrations do not change much

Stages

Source: http://www.dummies.com/how-to/content/understanding-the-transmission-of-nerve-impulses.html


Source: http://www.msdellasantina.com/Files%20AP/Ch%2048%20Neurons%2006_files/slide0078_image046.gif

    Depolarization / rising phase
       becoming more positive as Na+ channels open
       with all channels open, Na+ pushes potential up to +50 mV

    Repolarization
       at maximum positive voltage, Na+ channels close & no-longer voltage sensitive
       K+ channels eventually fully open
       K+ pushes out until voltage goes negative

    Undershoot
       With K+ fully open, potential goes more negative than resting
       Once K+ go back to mostly-closed-but-leaky, returns to resting potential

    Refractory Period
       Immediately after firing, another depolarization will not trigger an action potential
       absolute refractory period - no action potential possible
       relative refractory period - action potential requires stronger depolarization
       a few milliseconds long

Where in the neuron

Starts at axon hillock (typically)
Travels down axon
Triggers events at axon terminals
Does not automatically pass into next neuron

Source: http://www.anselm.edu/homepage/jpitocch/genbio/nervousnot.html

Saltatory conduction

Source: http://psych.hanover.edu/classes/sensation/WebNotes/Class04-2010.html

    Myelin covers most of neuron with a few gaps
    Ions only exchange across membrane at these gaps
    For reasons related to particle diffusion, this is faster than continuous conduction

    Myelinated neurons (e.g. motor neurons) - 100 m/s
    Unmyelinated neurons - 1 m/s

Principles

    All-or-none


    One directional


    Electrical


    Fast


    Active vs. Passive


Unanswered:
    What starts the action potential? How does a neuron get depolarized?