So what does a nerve cell look like, and how does the transmission of signals play out in real-time?
Nerve Cells
There are many different types of nerve cells and they come in different shapes and sizes; however, they all have the same structure. Each neuron or nerve cell consists of a dendrite, cell body, axon, and presynaptic terminal. Unless you are a scientist, you probably don’t have access to a microscope. But you can get a feel for what a nerve cell looks like by looking at an old oak tree. Yes, nerve cells have a striking resemblance to trees. In fact the word dendrite literally means “tree,” and no doubt this is what scientists saw when they examined the nerve cells under the microscope.
Let’s say you are now looking at a big old oak tree. The dendrite would be the part that looks like the branches of the tree; its job is to receive signals from other neurons and pass them on to the cell body. The cell body is the powerhouse; it contains the DNA, proteins, and other chemicals. It’s where electrical impulses are initiated. In our analogy the cell body would be the junction where the branches—dendrites—converge into the tree. Then there’s the axon—the trunk of the tree. It receives electrical impulses from the cell body and carries them down into the roots of the tree—the axon terminal.
But it doesn’t stop here. In order for the nerve cell to carry important information about perception, thinking, learning, laying down and storing new memories, it has to get its message over to the next neuron. The electrical impulse from the axon must link up with chemicals in the axon terminal to get the message across the synapse and over to its neighboring nerve cell. This information occurs through chemical messengers.
Located in the axon terminal are small bags of chemicals, called neurotransmitters. Each bag contains 5,000 molecules of neurotransmitters that are released in response to strong electrical impulses from the cell body. As the impulse travels from the cell body down the axon to the end of the axon terminal, it causes an influx of calcium from outside of the cell to rush into the axon terminal, and this triggers the release of the chemicals. The chemicals are released out into the synaptic gaps where they float across to the appropriate receptors and then change back into electrical impulses for the next dendrite (Kandel, 2006).
A new area of discovery for nerve cell transmission through electrical impulses is the human heart. We have long known that the heart conducts electrical impulses during the contraction of the heart. However, new research documents that the heart, dubbed “the little brain,” contains 40,000 nerve cells and, like the brain, releases neurotransmitters and is also capable of storing memories. (Essene, n.d.)
Memory
So what do nerve cells have to do with my vivid memories of a childhood fire? Everything. This is where the initial communication takes place. It is here on the nerve cells that information is registered, encoded, and locked into permanent memory. This is not as simple as it sounds.
Psychiatrist and neuroscientist Dr. Eric Kandel won a Nobel Prize for his research in memory storage and formation. He demonstrated the molecular basis of memory and showed that long-term memory is a result of several factors: neurotransmitters, protein kinases, ion channels, and transcription factors called CREB, in which their proteins bind with DNA. These series of events cause certain genes within the nucleus of the cell to switch on and others to switch off. These chains of events initiate the growth of new synaptic connections and cause stabilization of memory formation (Kandel, 2006). Wow that’s a mouthful!
My memories of the fire during my childhood were encoded in what is called flashbulb memory. Flashbulb memories leave vivid pictures in the mind, just like a photo, a snapshot of who, what, when, and where. These memories are laid down during emotionally charged events. For instance, I remember exactly where I was on September 11, 2001, and can see the room in which I was standing and the vivid pictures on TV as the Twin Towers were coming down. Many older Americans can recall where they were when they heard about the assassinations of John F. Kennedy and Martin Luther King, Jr. These are flashbulb memories. But most long-term memories are not stored as highly charged, emotional events. In general, long-term memories require repeated exposure, spaced training with intervals of rest (Kandel, 2006). It is true, practice makes perfect when it comes to making memories.
There are many types of memories, but they fall into two stages. In his book In Search of Memory: The Emergence of a New Science of Mind, Kandel indicates the two stages of memory: short term, lasting from minutes to hours, and long term, lasting days to weeks to years. These memories have two forms: explicit and implicit, and they are stored in different areas of the brain. Kandel states:
In the short term, explicit memory for people, objects, places, facts, and events is stored in the prefrontal cortex. These memories are converted to long-term memories in the hippocampus and then stored in the parts of the cortex that correspond to the senses involved—that is, in the same areas that originally processed the information. Implicit memories of skills, habits and conditioning are stored in the cerebellum, striatum and amygdala. (Kandel, 2006)
Memories are stored all over the brain. My visual picture of the fire is stored in the occipital lobe of my brain, the site of vision. My words, “The house is on fire!” are stored in the area of hearing—the temporal lobe. The smell of the fire is recorded in the olfactory tract (the smell detectors of the nose), and then stored as an emotional memory in my limbic system. All this information is stored in memory on different nerve cell pathways, but it comes together to give me a complete recall.
Memories from that fire years ago are no doubt encoded in both my explicit and implicit memory: the people, place, and event. Not only do I recall what happened but also the emotions that accompanied the event. What good is a memory without an emotion? The event would be bland, especially if it was a positive emotion. Just think about it, what if you looked back at your wedding day and just remembered the event without the positive feelings of excitement and jubilation? What good is the memory?
Emotions give life and color to our experiences. But where do they come from? What caused the fear and terror I experienced during the fire? In order to answer these questions, we need to go a little deeper into another structure of the brain, the limbic system—the site of emotions.
Limbic System
The limbic system is the site from which positive and negative emotions emanate; it is also the site of learning and memory. It consists of several structures. The major ones include the hippocampus, thalamus, hypothalamus, septum, and amygdala.
The hippocampus is located in the temporal lobe of the brain and is involved with learning and memory.
The thalamus is located on top of the brainstem and acts like a relay station that receives, processes, and sends information to various parts of the brain, especially the higher levels of brain activity.
The hypothalamus is located under the thalamus; it has many functions, but for limbic purposes it involves the regulation of hormones and the autonomic nervous system.
The septum, located in the midline of the brain, is involved in mood, pleasure, sexual gratification, and rage.
These structures are involved in other roles as well as limbic functions; however, it is the amygdala, another limbic structure, whose key role is emotional reactions (Sapolsky, 2007).
The amygdala is a small, almond-sized structure—one in each hemisphere of the brain—located deep within the temporal lobes. It acts like a storage site for emotional responses, especially those related to fear. It is involved in the fight and flight response. This little structure reminds me of a fire truck speeding down the road, with lights flashing and alarms blaring. This visceral response can be so loud that it’s deafening, and all other controls in the brain appear to be on mute. The amygdala is a wonderful built-in feature to have if the house is on fire, as was the case when our house caught on fire. I was sounding the alarm at the top of my voice. My emotions were appropriately excessive. I