1. Nerve Impulse

2. A nerve impulse is an impulse from another nerve or a stimulus from a nerve receptor. A nerve impulse causes:  The permeability of the membrane to sodium ions suddenly increases.  Sodium ions diffuse rapidly from the outside to the inside of the membrane. This reverses the polarity of the cell membrane (inside positive and outside negative).

3.  This reversal occurs in a small area of the membrane and results in a flow of electrical current that affects the permeability of the adjacent areas of the membrane  The reversal of polarization is the nerve impulse and it travels the length of the axon.  High permeability of the membrane to sodium ions last only a fraction of a second and then returns to normal.  The opening of the potassium gates to allow potassium diffusion and the continual action of the sodium- potassium pump allow normal distribution of ions to be restored.

4. The period from the initiation of the action potential to immediately after the peak is referred to as the absolute refractory period (ARP).This is the time during which another stimulus given to the neuron (no matter how strong) will not lead to a second action potential because Na+ channels are inactivated during this time. The absolute refractory period takes about 1-2 ms. After the absolute refractory period, Na+ channels begin to recover from inactivation and if strong enough stimuli are given to the neuron, it may respond again by generating action potentials. However, during this time, the stimuli given must be stronger than was originally needed when the neuron was at rest. This situation will continue until all Na+ channels have come out of inactivation. The period during which a stronger than normal stimulus is needed in order to elicit an action potential is referred to as the relative refractory period (RRP).  A brief recovery period occurs during which the nerve cell membrane cannot be stimulated to carry impulses.  This refractory period lasts a few thousandths of a second.

5.  The rate at which an impulse travels depends on the size of the nerve and whether or not it is myelinated (unmyelinated = 2 m/s and myelinated = 100 m/s). In myelinated fibers the signal jumps from one node of Ranvier to the next. This is saltatory conduction and occurs because the membrane at the node is highly sensitive result this uses less energy due to polarization only at the nodes.

6.  For a nerve impulse to be transmitted, the stimulus must be at least a certain minimum strength or must reach a threshold.  The impulses transmitted by a given neuron are all alike, a neuron operates on a “all-or-none” basis.  The strength of the stimulus is measured by two effects:  1. A stronger stimulus causes more impulses to be transmitted each second.  2. Different neurons have different thresholds.  A large number of neurons fire when a stimulus is stronger.

7.  http://outreach.mcb.harvard.edu/animations/actionpotential_short.swf http://outreach.mcb.harvard.edu/animations/actionpotential_short.swf  http://www.mind.ilstu.edu/curriculum/neurons_intro/flash_action_potential.php?modGUI=232&compGUI=1827&itemGUI=3156 http://www.mind.ilstu.edu/curriculum/neurons_intro/flash_action_potential.php?modGUI=232&compGUI=1827&itemGUI=3156  http://hyperphysics.phy-astr.gsu.edu/hbase/biology/actpot.html http://hyperphysics.phy-astr.gsu.edu/hbase/biology/actpot.html  http://hyperphysics.phy-astr.gsu.edu/hbase/biology/nervecell.html#c2 http://hyperphysics.phy-astr.gsu.edu/hbase/biology/nervecell.html#c2