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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.