Download Chapter 2 Physical structure of a Neuron - Dendrites

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Transcript
Chapter 2
Physical structure of a Neuron
-
Dendrites
Soma or Cell body
Action hillock
Axon
Mylein Sheath
Presynaptic terminal
Types of Neurons
- Receptor or sensory neuron (sends inpulses towards CNS)
- Motor Neuron (sends signals from CNS)
- Interneuron (neurons connecting only to other neurons within a gland
or region i.e. thymus gland or cerebral cortex)
- Local Neurons (communicate with eachother through graded potentials,
not all ot none impulses)
Types of axons
- Efferent axon (carries signals away from structure)
- Afferent axon (brings information into a structure)
Types of Glia
- Glia are the "nannies" of neurons. The feed them, clean up after
them, and clothe them in mylein.
- Oligodendrocytes (create mylein sheaths on multiple axons)
- Schwann cells (create one section of mylein sheath on an axon. One
axon can have many Schwann cells)
- Microglia (Remove waste material)
- Astrocyte (star shaped glia that also remove wastes)
- Radial Glia (guide path of developing axons and dendrites during
neuron maturation)
Blood-Brain Barrier
- Protect brain from barages of chemicals that may be present in the
blood
- Access to the brain is therfore regulated
- Fat soluble molecules are free to diffuse through the barrier at
will, that's how THC affects you so quickly
- Small uncharged molecules are free to diffuse
- Charged molecules like amino acids and sugars are actively
transported via ports made from proteins
Nerve Impulses
- Polarization (The difference in charge between the inside and ouside
of the membrane. Caused by actively pumping + charged sodium out of the
cell)
- Resting Potential ( The voltage difference between the inside and
outside of the membrane when the neurn is ready to fire. 70mV.)
Note: Resting potential is achieved by actively pumping 3 Na+ out of
the cell while simeltaneously pumping 2 Ka+ into the cell. This results
in a net of 1+ charge being pumped outside of the cell, thus creating a
+ charge buildup outside the membrane and a - charge buildup inside of
the membrane
The resting potential is analagous to a compressed spring. It is ready
to spring into action at a moments notice and fire off an impulse.
Beginning of a nerve impulse occurs at the action hillock. Once a
sufficient depolarization occurs, i.e. when enough Na+ rushes into a
cell causing the neuron to pass its threshold, the voltage gated Na+
channels will open and allow a flood of Na+ to enter the cell.
Meanwhile Ka+ is leaking out of the cell as always. This changes the
inside of the membrane to a + charge. When the voltage at the membrane
reaches +30mV, the Na+ voltage gated channels slam shut and prevent
further depolarization of the membrane at that gate. At this point, the
localized +30 mV charge opens the next voltage gated Na+ channel and
the process repeats like a domino effect called propagation. The
resting potential is restored by actively pumping Na+ out of the cell 3
at a time while Ka+ is brought into the cell 2 at a time. The cell
actually pumps out too much Na+ and a condition called
hyperpolarization occurs. The time that it takes to go from the
hyperpolarization back to resting potential is called the refractory
period.
"all or none"
Refractory period
- Absolute (the cell is still hyperpolarizing and is incapable of
firing off another nerve impulse)
- Relative (the cell is in the process of correcting the
hyperpolarization and can fire again due to intense stimulus)
Note: Myleinated axons cause nerve impulses to travel faster. This is
because the electrical impulses can jump to the 'Nodes of Ranvier' at
the gaps between myleinated segments of the axon, instead of having to
stop at every Na+ gate and wait for it to open.