Contrary to popular belief, nerve signals are transmitted through chemical changes and not by electricity. The transmission of nerve signals happens in two very important ways.  First of all nerve signals must travel through long nerve cells.  Secondly, nerve signals must pass from one nerve to another.  Special chemicals called neurotransmitters help near signals pass from one cell to the next. The myelin sheath is important for protecting nerve signals (sometimes called a nerve impulse) and also helps speed up signals by a process called saltatory conduction.

What is a nerve impulse?

A nerve impulse is a nerve signal that passes from one end of the nerve cell to the other.  Nerve impulses can also pass from one cell to another at junctions called synapses. 

How does a nurse signal pass through a nerve cell? 

Prior to stimulation, a nerve cell is an a state of polarization.  This basically means that there is an abundance of sodium ions outside of the cell and potassium ions (along with negative ions) within the cell.   This leads to a positive charge on the outside of the cell membrane and a relatively negative charge of the inside. When the nerve is activated by a stimulus, such as a neurotransmitter, the cell membrane becomes very permeable to sodium ions which rush into the cell. This movement of ions causes a reversal of charges called depolarization.  The outside of the cell now has a negative charge in the inside has a positive charge.  This process once again is quickly reversed in a process called repolarization.  Repolarization basically recharges the cell preparing it for another nerve signal. 

Interestingly, this process of the polarization and repolarization occurs in different parts of the cell at different times.  In fact, this process progresses down the length of the cell similar to a row of falling domino's.

The process of the polarization followed by repolarization is sometimes called an action potential. 

How does the myelin sheath help speed up nerve signals? 

The myelin sheath contains gaps called nodes of Ranvier.  The transmission of ions in and out of the cell can only occur at these gaps.  Nerve signals jump from one node of Ranvier to another in a process called saltatory conduction.  A nerve signal can pass much more quickly from one node of Ranvier to another through a myelin sheath. The term saltatory comes from a Latin term meaning dance (and now is used to indicate jump).  Imagine if the nurse signal passing from one node of Ranvier to the next like a frog jumping from one lily pad to the next.

How does a nervous signal pass from one nerve to the next? 

Nerve cells do not touch one another directly. Nerve signals pass between nerve cells by way of gaps called synapses.  At the synapse, the electrical signal from the pre-synaptic nerve stimulates the release of a chemical called a neurotransmitter.  This chemical diffuses across the gap to the surface of the post-synaptic nerve.  Because chemicals travel more slowly than electrical nerve impulses, these gaps must be very small. Chemical receptors on the post-synaptic nerve cause depolarization of that nerve and the  signal is transmitted. Another chemical called an inactivator, quickly inactivates the chemical neurotransmitter such that only one nerve impulse is transmitted. 

An example of a neurotransmitter is acetylcholine.  The inactivator chemical for acetylcholine is cholinesterase.  Some other neurotransmitters include: dopamine, GABA, norepinephrine, glutamate, and seratonin.  Each of these neurotransmitters has its own deactivator chemical.  Many neurotransmitters are recycled by nerves in a process called reuptake.  The reuptake of neurotransmitters is a frequent target of medications that affect the nervous system.  Serotonin reuptake inhibitors (like Prozac) work in this way.

What is the difference between an excitatory nerve and an inhibitory nerve? 

When we think of a nerve signal passing from a one nerve to the next, we are typically thinking about an excitatory nerve.  This means that one nerve excites or stimulates the following nerve to continue passing the nerve signal. 

Many nerves are of the inhibitory type.  These nerves pass a signal to the next nerve that actually decreases the likelihood that the next a nerve will continue passing the signal.  Inhibitory nerves cause hyper-polarization of the post-synaptic nerves.  This process is important when a single nerve receives signals from multiple other nerves.  In a sense, majority rules.  If most of the nerve signals transmitted to a post-synaptic nerve are inhibitory then the smaller number of excitatory nerves will be unable to send the signal. This applies a system of checks and balances that prevents stray impulses from being passed inappropriately.

Can a nerve impulses travel backwards? 

Nope.  However, there are nerves that travel in both directions. For example, if your finger touches a hot stove a nerve signal will transmit from your finger to the central nervous system through afferent nerves.  Afferent nerves travel from the body parts to the central nervous system. The central nervous system will then send a signal through the efferrent nerves to the muscles out of your hand and arm telling them to pull away.  Nerve impulses are prevented from traveling backwards at the synapses because there are no narrow transmitters released from the downstream nerve.

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