Neurons carry messages using a chemical version of electricity. The outer surface of all types of cells is called the membrane. The axons of neurons have special membranes that maintain a difference in the balance of positive and negative charges across the membrane, like a battery. The charge of a neuron's battery is based on the number of sodium (Na+) and potassium (K+) ions inside and outside the membrane. Messages are carried, or conducted, along axons and dendrites by the change in electrical charge across the membrane.

An 'action potential' is the name for an electrical message carried along the axon or dendrite of a neuron, like an electrical impulse carried along a wire. These tiny electrical signals would not be able to travel very far if the axons were not insulated. Once this electrical charge or 'action potential' reaches the synapse, it triggers neurotransmitter release to enable the signal to reach the next neuron in the chain.

A wire carries electricity both faster and farther when it is. The longer the wire is, the more important the insulation is for efficient conduction. The same thing is true for axons and action potentials that travel down them. An action potential is triggered within the cell body of the neuron after the neuron receives signals from other neurons (see Neurotransmission). The insulation around axons is provided by a protein covering called myelin. In the brain and spinal cord, myelin is made by oligodendrocytes. The myelin wraps around each axon in many layers. Axon fibers insulated by myelin can carry action potentials at a speed of 100 meters per second, while axon fibers without myelin can only carry action potentials at a speed of 1 meter per second.

The Oligodendrocyte

Oligodendrocytes are one kind of glial cell (the other two glial cells"Tutorial6.html"astrocytes and microglia - are described elsewhere). Oligodendrocytes produce the myelin that insulates axons and makes synaptic transmission faster. Schwann cells, another kind of glial cell, are found in the peripheral system and make a slightly different kind of myelin.

An 'action potential' is the name for an electrical message carried along the axon or dendrite of a neuron, like an electrical impulse carried along a wire. A wire carries electricity both faster and farther when it is. The longer the wire is, the more important the insulation is for efficient conduction. Oligodendrocytes and Schwann cells produce the myelin tha insults axons.

The Astrocyte

Astrocytes are a specialized type of glial cell, and are the primary support cells of the brain and spinal cord. Astrocytes make and secrete neurotrophic factors necessary for neurons to survive and they provide an extra storehouse of energy for hard-working neurons.

Additionally, astrocytes break down and remove proteins or chemicals that could be harmful to neurons, like extra neurotransmitters, especially the neurotransmitter glutamate, which can cause neurons to become overexcited and die by a process called "excitotoxicity.

After an injury to the CNS, astrocytes divide (or proliferate) to make new cells and surround the injury site, making a barrier called a "glial scar.

The Neuron

Specialized cells, called neurons, make complex connections with one another to send and receive messages in the brain and spinal cord. There are approximately one hundred billion neurons in the brain and spinal cord combined. This means that your brain and spinal cord have seventeen neurons for every person on Earth. Obviously, this also makes the whole system very complex.

Each neuron is made up of a cell body, which houses the nucleus and much of the machinery of the cell, and axons and dendrites, which carry messages from one neuron to another. Different types of neurons (as many as ten thousand different subtypes) are further specialized to send and receive information (sensory, motor, visual, etc), or to integrate this information upon receipt.

If one imagines the brain and spinal cord as a computer, the neurons are like the switches and circuitry that make it work. Many neurons working together in networks are responsible for every decision made, every emotion or sensation felt, and every action taken.

The Synapse and Neurotransmission

Messages are passed from neuron to neuron through synapses, using chemicals called neurotransmitters. Synapses are very small gaps between neurons. To transmit a message across a synapse in response to an incoming action potential, neurotransmitter molecules are released from one neuron, the 'pre-synaptic' neuron, and diffuse across the gap to the next neuron, the 'post-synaptic' neuron. Once there, the neurotransmitter causes a new action potential to be formed, and the process begins again to carry the message to its destination. Amazingly, a single axon can form synapses with as many as 1,000 other neurons.

Each area of the body sends and receives specialized input and connections via the neuronal axons running to and from it. The action potential messages carried by these connections are the & on the spinal cord highway, running to and from the brain. There are millions and millions of connections between neurons within the spinal cord alone. The correct connections are hooked up during development using positive (neurotrophic factors) and negative signals (like NOGO, discovered by the Schwab lab) to fine-tune them. This process begins at birth and continues throughout infancy and beyond as the bones and muscles grow. When these connections are broken, there is a tremendous amount of reorganization to be done.

Microglia - The Brain's Immune Cell

Microglia are immune cells for the brain. In the normal brain, microglia are quiescent or inactive. But after an injury, these cells undergo a dramatic change, altering their appearance, migrating to the site of the damage to help clear away and clean up dead and dying cells and cell debris. Microglia can also produce small signaling molecules, called cytokines, to trigger astrocytes or additional cells of the immune system to respond to the injury site. This cleanup process is probably an important process for the recovery of function after CNS injury.

Microglia - The Brain's Immune Cell

Microglia are immune cells for the brain. In the normal brain, microglia are quiescent or inactive. But after an injury, these cells undergo a dramatic change, altering their appearance, migrating to the site of the damage to help clear away and clean up dead and dying cells and cell debris. Microglia can also produce small signaling molecules, called cytokines, to trigger astrocytes or additional cells of the immune system to respond to the injury site. This cleanup process is probably an important process for the recovery of function after CNS injury.