2. Neurons, Brain Chemistry and Neurotransmission 

Communication between neurons is the foundation for brain function. Understanding how neurotransmission occurs is crucial to understanding how the brain processes and integrates information. Interruption of neural communication causes changes in cognitive processes and behavior.

The Brain Is Made Up of Nerve Cells and Glial Cells

The brain of an adult human weighs about three pounds and contains billions of cells. The two distinct classes of cells in the nervous system are neurons (nerve cells) and glia (glial cells).

An example of a cell.

The basic signaling unit of the nervous system is the neuron. The brain contains billions of neurons; the best estimates are that the adult human brain contains 1011 neurons. The interactions between neurons enable people to think, move, maintain homeostasis, and feel emotions. A neuron is a specialized cell that can produce different actions because of its precise connections with other neurons, sensory receptors, and muscle cells. A typical neuron has four morphologically defined regions: the cell body, dendrites, axons, and presynaptic terminals.

Figure 2:1 The neuron,or nerve cell, is the functional unit of the nervous system.  The neurons has proceses called dendrites that receive signals and an axon that transmits signals to another neuron.

The cell body, also called the soma, is the metabolic center of the neuron. The nucleus is located in the cell body and most of the cell’s protein synthesis occurs in the cell body.

A neuron usually has multiple processes, or fibers, called dendrites that extend from the cell body. These processes usually branch out somewhat like tree branches and serve as the main apparatus for receiving input into the neuron from other nerve cells.

The cell body also gives rise to the axon. Axons can be very long processes; in some cases, they may be up to one meter in length. The axon is the part of the neuron that is specialized to carry messages away from the cell body and to relay messages to other cells. Some large axons are surrounded by a fatty insulating material called myelin, which enables the electrical signals to travel down the axon at higher speeds.

The connection between neurons.

Figure2.2 Neurons transmit information to other neurons.  Information passes from the axon of the presynaptic neuron to the dendrites of the postsynaptic neuron.

Near its end, the axon divides into many fine branches that have specialized swellings called presynaptic terminals. These presynaptic terminals end in close proximity to the dendrites of another neuron. The dendrite of one neuron receives the message sent from the presynaptic terminal of another neuron.

The site where a presynaptic terminal ends in close proximity to a receiving dendrite is called the synapse. The cell that sends out information is called the presynaptic neuron, and the cell that receives the information is called the post-synaptic neuron. It is important to note that the synapse is not a physical connection between the two neurons; there is no cytoplasmic continuity between the two neurons. The intercellular space between the presynaptic and postsynaptic neurons is called the synaptic space or synaptic cleft.

Researchers originally thought that electrical impulses jumped these gaps, like electricity jumps across the gap in a spark plug.  Now we know this is not true.  Chemicals-not electrical impulses- travel across the gap

 An average neuron forms approximately 1,000 synapses with other neurons. It has been estimated that there are more synapses in the human brain than there are stars in our galaxy. Furthermore, synaptic connections are not static. Neurons form new synapses or strengthen synaptic connections in response to life experiences.

Figure 2.3:  The synapse is the site where chemicals pass between neurons.  Neurotransmitter is released from the presynaptic neuron terminals into the extracellular space, the synaptic cleft or synaptic space.  The released neurotransmitter molecules can then bind to specific recptors on the postsynaptic neuron membrane to elicit a response.   

References: (1) The Brain:  Understanding Neurobiology Through The  Study of Addiction: Neurons, Brain Chemistry and  Neurotransmission Lesson 2 (NIDA 2004)
                   (2) Neurons and Neurotransmitters: The "Brains" of The Nervous System (Permission Granted)  

Compiled & Edited By: D. Shrira        Dated:  6 January 2007