Identify and describe the order of the neural message, from beginning of stimulation to reception in the brain.

Neurons, Dendrites, Axons, Cell Body, Myelin Sheath

Before you can understand the biology of human behavior, you need an understanding of the human nervous system. Neurons, or nerve cells, are the cells that transmit and process the information in the entire animal world. The human nervous system is the most complex of all animals, however. Although there are many types of neurons, each type is just a simpler or more complex variation of the same structure and function. Each cell begins with bushy branch-like structures called dendrites. The function of the dendrite is to receive information. The sparser the dendrite’s branches, the less sensitive that dendrite is in receiving sensory input. The denser the dendrite’s branches, the more sensitive that dendrite is in receiving and transmitting sensory input. After receiving sensory input, the dendrite sends a sensory message through the cell body, the life-support center of the neuron, to the axon, which then passes the information to other neurons, muscles, or glands. Axons can be quite short or up to several feet long. Because the message sent along the axon is electrical by nature, there is a need to insulate some axons. Just like some electrical circuits do not need insulation at each and every point of the circuit, some axons do not need to be insulated very much or at all. Other circuits, however, need to be insulated, or the message gets “short circuited.” Insulated circuits are able to send and receive information much more quickly than those that are not insulated with a layer of fatty tissue. This insulation of tissue is made of glial cells and is called the myelin sheath. Some cells do not need the myelin sheath to function as designed. Those that do need this insulation do not function properly if the myelin sheath is compromised or lost. For example, people with multiple sclerosis lose muscle control when formerly myelinated axons lose the insulation of the myelin sheath.

Action Potential, Resting Potential

When an axon is in its resting state, it is polarized. The energy created by neural electricity is called the action potential. This electricity is generated by the movement of ions. This electrical charge is brief and lasts just long enough to take the message from the receptor dendrite to the end of the axon. Neurons fire in an “all or nothing” fashion. When the receptor dendrites receive stimulation, the neuron’s electrical charge changes. The neuron is surrounded by fluid that is electrically charged differently (mostly positive) than the particles inside of the neuron (mostly negative). When a charge comes from neural stimulation, the electrical charge is sent along the axon through pumping sodium and potassium in and out of the axon. The axon becomes imbalanced or depolarized as the positively charged atoms enter the neuron. This activity is frantic because an axon senses the need to get back to its balanced, comfortable resting state, or resting potential. In the resting state, the axon is polarized with mostly positively charged ions outside and negatively charged ions inside.

Synapse, Synaptic Gap, Synaptic Vesicles, Terminal Branches, Buttons and the Axon Terminal

The message carried by the axon is also chemical. At the end of the axon is a terminal branch or button. Terminal branches and buttons release chemicals called neurotransmitters containing the message carried by the axon. This message is contained in tiny bags or bubbles of membrane called synaptic vesicles. Just like tiny bubbles of gas in carbonated drinks don’t let that gas out until they float to the surface and pop, synaptic vesicles do not “pop” to release the neurotransmitters until the vesicles get to the surface of the terminal button or axon terminal. When the message chemicals are released, they would be lost forever unless they were caught or received by another axon. Axons do not touch each other, so the released message must be received somehow. After being released, the chemical message is sent across a gap to a new cell’s dendrites. This gap is called a synapse or synaptic gap. When this message is sent to the new receiving receptor sites on new dendrites, the original chemical message continues to be relayed and sent to new neurons until it is received by the brain, organ, tissue, or muscle needing the message. This process has led to the study of neurotransmitter abnormalities in depressed and violent patients. Biological psychologists have demonstrated that medications have assisted these patients in managing their emotions.

After a neural message reaches a synapse, it jumps that synaptic gap and gets picked up by a new dendrite and the relay continues until it reaches the spinal cord and goes to the brain. The brain then makes a decision and responds to the message. There are times, however, when the message needs to be sent and returned more quickly than the previous description allows. Several things help a message to move more quickly. The insulation of the myelin sheath described earlier helps to some degree. Another process also helps increase the speed described here: the simple reflex. The simple reflex takes a message directly from one location in the body to the spinal cord, then directly back to the body needing to have a response. Instead of going to the brain, the simple reflex takes the message from the sensory neuron to the interneuron then immediately back to the motor neuron so that quick responses can be made.