Sequence of Events on the Neuron for an Action Potential

1. Open K+ channels create the resting potential( -60mv)
2. The Na+ channels open depolarizing the cell membrane to threshold.
3. Additional gated Na+ channels open to create the action potential or wave of electricity( + 50 mv)  The action potential is due to the opening of the gated Na+ channels!!!!!!!
4. The Na+ gated channels are inactivated or closed.  This initiates the return to the resting potential
5. The K+ gated channels open, repolarizing and even hyperpolarizing the cell membrane( hyperpolarization is when the interior becomes more negative in comparison to the outside of the cell than the resting potential)
6. The voltage gated sodium channels cannot open for 1-2 milliseconds.  This may be explained by the fact that they have  an activation and an inactivation gate.  The period or time between nervous impulses is referred to as the refractory period.
7. The battery is charged by the sodium and potassium pump that helps to maintain the balance of ions in the extracellular and intracellular fluid of the cell
8. In myelinated axons he nervous impulse “ jumps” down the axon.   The openings in the myelin sheath are referred to as the Nodes of Ranvier.  The impulse jumps from node to node.  In reality the action potential is initiated at the node and spreads from node to node

The Neuromuscular Junction

1. The neuron ends in telondendria or end bulbs
2. There is a space between the neuron and the muscle.  The ends of the neuron branches and forms many synaptic junctions between the neuron and muscle fibers
3. When the action potential reaches the end bulbs it causes the release of chemicals called neurotransmitters from vesicles in the bulbs.  In the case of muscles, the neurotransmitters is Acetylcholine.( Ach).  When the Na+ gates open, Ca++ rushes in through Ca++ gated channels that are totally unique.
4. The neurotransmitter is released from the vesicles in the endbulbs  by the process of endocytosis due to the increase in the Ca++ concentration in the cell.
5. The membrane of the neuron is called the presynaptic membrane.  The neurotransmitter flows across the synaptic space or cleft to the postsynaptic membrane( 20 to 40 nm)
6. The postsynaptic membrane is referred to as the motor endplate
7. The acetylcholine binds to receptors on the postsynaptic membrane and activate the chemically gated channels there.  While there are few Na+ channels in the actual region of the motor end plate this perturbation results in the opening of the Na+ gated channels along the muscle fiber membrane.
8. This phenomenon results in the activation of the endoplasmic reticulum and the release of Ca++ ions in side of the muscle fibers.  These accompanying changes induce the skeletal muscle fiber to contract.
9. The enzyme acetylcholinesterase inactivates the acetylcholine.  This limits the activity of this neurotransmitter.  This causes the breakdoewn of acetylcholine, which is then resynthesized in the neuron.
10. Slow versus fast neurotransmitters.  Fast neurotranmitters like Ach bind to the postsynaptic membrane of muscle cells.
11. Slow neurotransmitters bind through other neurotransmitters like Norepinephrine to the membrane of endocrine glands.  They also bind to a receptor on the postsynaptic membrane.  Here they initiate a cascade of steps that is mediated by the second messenger system.  The second messenger system is a system of proteins called G proteins that integrates information coming from outside of the cell and delivers the messages to the nucleus.
12. At this point in time there are more than 25 neurotransmitters.  The mechanism of some is not fully understood.

In the brain the simple amino acids glutamic acid and aspartic acid are used as excitatory and glycine is inhibitory.  Another important group of neurotransmitters in the brain are dopamine and norepinephrine that are derivatives of tryptophan.  Peptides also function as neurotransmitte