Neurobiology, Part 1

Overview of the nervous system

Structure of neurons and associated cells

Examples of neurons

Glial cells  10:1  50:1 glial cell:neuron ratio  Known functions   Creates myelin sheath (in the vertebrates only)   Why important?   Creates a matrix that connects neurons   Helps guide development of neural pathways   Blood-brain-barrier (tight junctions)   Provides metabolic support for neurons   NEW: appear to communicate chemically with other glial cells and neurons

Membrane potentials: the key to electrical signals   What is meant by the membrane potential?   Charge difference between the inside and ouside of the membrane   An “electrical signal” of the nervous system is a change in the memrane potential.   Example: Action potential…   What determines the value of the membrane potential?   Chemical gradients   Electrical gradients   Selective permeability of the membrane

Chemical and electrical gradients  Chemical gradients (“chalk talk”)

Na+-K+-pump: maintains the gradients  Think of it as operating in the background. It is no way involved in individual action potentials!

Chemical and electrical gradients  Electrical gradients (“chalk talk”)

Selective permeability   The membrane potential at any time is based on the permeability of the membrane to particular ions.   Impermeable ions cannot move and thus cannot influence the membrane potential.

Neuron at rest: ~ -70 mV  Why is the resting membrane potential negative?

Resting potential   At rest, the membrane is ~25 times more permeable to K+ than Na+, thus K+ is nearly solely responsible for the RP of neurons.   Relatively large number of K+ channels open at rest   These are “resting” or “passive” channels (always open)   Very few Na+ channels open at rest.

Action potentials  A rapid, transient change in the membrane potential from negative to positive and back again!  This is the nerve impulse!  What allows the changes in membrane potential to occur during an AP?  Changes in membrane permeability due to opening and closing of voltage-gated channels  Resultant movement of ions.

Action potentials  Follow the bouncing professor and make your own custom drawing!

Action potentials (cont.)  Know what is meant by the threshold  For each stage of the AP: Ask yourself:  What change in permeability occurred?  What type of channel opened or closed and why?  Which ion moved, and in which direction (in or out of cell?)  Understand why you see the direction you do.  What change in membrane potential occurred as a result of the ion movement?  What stopped the movement of the ion?

Propagation of the action potential  Passive spread of positive charge   Depolarization of next segment of the axon   Threshold reached  AP in next section

Myelin sheath  Cell type: Schwann cell

Saltatory conduction  Ions only able to move at the Nodes of Ranvier  AP “jumps” from node to node

The synapse: Write out steps in your own words!  NOTE: See diagram in text: Newer diagram clearly shows the voltage-gated calcium channels!

How is neurotransmitter activity stopped?  Three different ways… Know them!

Neural integration

 Summation: graded potentials (EPSPs and IPSPs) are summed to either depolarize or hyperpolarize a post-synaptic neuron