Neurons, Brain and Neurotransmission


   Map of Addiction  

      Glial Cells   

This dynamic change in neuronal connections is the basis of learning.The brain contains another class of cells called glia. There are as many as ten to fifty times more glial cells than neurons in the central nervous system. Glial cells are categorized as microglia or macroglia.

 Microglia are phagocytic cells that are mobilized after injury, infection or disease. They are derived from macrophages and are unrelated to other cell types in the nervous system. The three types of macroglia are oligodendrocytes, astrocytes, and Schwann cells. The oligodendrocytes and Schwann cells form the myelin sheaths that insulate axons and enhance conduction of electrical signals along the axons.

Scientists know less about the functions of glial cells than they do about the functions of neurons. Glial cells fulfill a variety of functions including:

• Glial cells function as supporting elements in the nervous system to provide structure and to separate and insulate groups of neurons.

• Oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system form myelin, the sheath that wraps around certain axons.

• Some glial cells are scavengers that remove debris after injury or neuronal death.

• Some glial cells buffer the potassium ion (K+) concentration in the extracellular space, and some glial cells take up and remove chemical neurotransmitters from the extracellular space after synaptic transmission.

• Some glial cells guide the migration of neurons and direct the outgrowth  of axons during development.

• Some glial cells induce formation of impermeable tight junctions in endothelial cells that line the capillaries and venules of the brain to form the blood-brain barrier.

• Glial cells may serve nutritive functions for nerve cells.

Neurons Use Electrical and Chemical Signals to Transmit Information

The billions of neurons that make up the brain coordinate thought, behavior, homeostasis, and more. How do all these neurons pass and receive information?

Neurons convey information by transmitting messages to other neurons or other types of cells, such as muscles. The following discussion focuses on how one neuron communicates with another neuron. Neurons employ electrical signals to relay information from one part of the neuron to another. The neuron converts the electrical signal to a chemical signal in order to pass the information to another neuron. The target neuron then converts the message back to an electrical impulse to continue the process.

In some ways, neurons act like computers.  That is, they receive messages, process their message, and send out the results as new messages to other cells. In the case of neurons, the message consists of chemicals that interact with the outer surface of the cell membrane. This chemical  interaction  with the cell membrane  causes  chemical  changes
within the receiving neuron.      

Close -up of The Synaptic Gap

Within a single neuron, information is conducted via electrical signaling. When a neuron is stimulated, an electrical impulse, called an action potential, moves along the neuron axon or dendrite. Action potentials enable signals to travel very rapidly along the neuron fiber. Action potentials last less than 2 milliseconds (1 millisecond = 0.001 second) and the fastest action potentials can travel the length of a football field in one second. Action potentials result from the flow of ions across the neuronal cell membrane.

 Neurons, like all cells, maintain a balance of ions inside the cell that differs from the balance outside of the cell. This uneven distribution of ions creates an electrical potential across the cell membrane. This is called the resting membrane potential. In humans, the resting membrane potential ranges from -40 millivolts (mV) to -80 mV with –65 mV as an average resting membrane potential. The resting membrane potential is, by convention, assigned a negative number because the inside of the neuron is more negatively charged than the outside environment of the neuron.

This negative charge results from the unequal distribution of sodium ions (Na+), potassium ions (K+), chloride ions (Cl-), and other organic ions. The resting membrane potential is maintained by an energy-dependent Na+-K+ pump that keeps Na+ levels low inside the neuron and K+ levels high inside the neuron. In addition, the neuronal membrane is more permeable to K+ than it is to Na+, so that K+ tends to leak out of the cell more readily than Na+ diffuses into the cell.

A stimulus occurring at the end of a nerve fiber starts an electrical change that travels like a wave over the length of the neuron. This electrical change, the action potential, results from a change in the permeability of the neuronal membrane. Sodium ions rush into the neuron, and the inside of the cell becomes more positive. The Na+-K+ pump then restores the balance of sodium and potassium to resting levels.

 However, the influx of Na+ ions in one area of the neuron fiber starts a similar change in the adjoining segment and the impulse moves from one end of the neuronal fiber to the other. Action potentials are an all-or-none phenomenon. Regardless of the stimuli, the amplitude and duration of an action potential are the same. The action potential either occurs or it doesn’t. The response of the neuron to an action potential depends on how many action potentials it transmits and the time interval between them.

Electrical signals carry information within a single neuron. communication between neurons (with a few exceptions in mammals) is a chemical process. When the neuron is stimulated, the electrical signal (action potential) travels down the axon to the axon terminals. When the electrical signal reaches the end of the axon, it triggers a series of chemical changes in the neuron. Calcium ions (Ca++) flow into the neuron. The increased Ca++ in the axon terminal then initiates the release of neurotransmitter. A neurotransmitter is a molecule that is released from a neuron to relay information to another cell. Neurotransmitter molecules are stored in membranous sacs called vesicles in the axon terminal. Each vesicle contains thousands of molecules of a neurotransmitter.

Neurons release neurotransmitters.

For neurons to release their neurotransmitter, the vesicles fuse with the neuronal membrane and then release their contents, the neurotransmitter, via exocytosis. The neurotransmitter molecules are released into the synaptic space and diffuse across the synaptic space to the postsynaptic neuron. A neurotransmitter molecule can then bind to a special receptor on the membrane of the postsynaptic neuron. Receptors are membrane proteins that are able to bind a specific chemical substance, such as a neurotransmitter. For example, the dopamine receptor binds the neurotransmitter dopamine, but does not bind other neurotransmitters such as serotonin.

Neurotransmitters bind to receptors.

The interaction of a receptor and neurotransmitter can be thought of as a lock-and-key for regulating neuronal function. Just as a key fits only a specific lock, a neurotransmitter binds only to a specific receptor. The chemical binding of neurotransmitter and receptor initiates changes in the postsynaptic neuron that may generate an action potential in the postsynaptic neuron. If it does trigger an action potential, the communication process continues.

 After a neurotransmitter molecule binds to its receptor on the postsynaptic neuron, it comes off of (releases from) the receptor and diffuses back into the synaptic space. The released neurotransmitter, as well as any neurotransmitter that did not bind to a receptor, is either degraded by enzymes in the synaptic cleft or it may be taken back up into the presynaptic axon terminal by active transport through a transporter or reuptake pump. Once the neurotransmitter is back inside the axon terminal, it is either destroyed or repackaged into new vesicles that may be released the next time the neuron is stimulated. Different neurotransmitters are inactivated in different ways.

Binding causes a set of chemical reactions within the receiving neuron. Those reactions start up the same kind of impulse that was fired in the sending neuron. In this way, the original impulse is conducted through the sending neuron -and through the rest of the neurons in a nerve pathway.Eventually, the impulse reaches its final destination, such as muscle, gland or organ.  The result is a change in the way we think, feel or behave.

Binding passes on the message.

The chemical reactions inside the receiving neuron are called second messengers.  Second messengers pass along the original message from the neurotransmitter.  In fact, neurotransmitters are sometimes called first messengers. 

References: (1) The Brain:  Understanding Neurobiology Through The   Study of Addiction: Neurons, Brain Chemistry and Neurotransmission; Lesson 2  (NIDA 2004)
                   2) (Neurotransmitters Send Chemical Messages)
Understanding Addiction: Permission Granted.                   

Compiled & Edited: D. Shrira     Updated: 8 January 2007