There are some interesting things being learned in Alzheimer’s disease research these days. I’d like to talk about them, but you need to understand a little bit of brain biology first. So this is my plan…
I’m going to write a few posts on the basics of brain biology. And maybe a glossary of terms, too. If you are interested, learning about the basics should make it easier to make sense of what you read or hear about this disorder. If you already know the basics, think of this post as a review, or jump to the post on AD brain. Once we get the basics out of the way, I’ll be able to talk about what research scientists are learning without making every post a tutorial.
So let’s start with the basics of what a normal brain looks like, and go from there:
The Human Brain
Under the microscope, the human brain presents a beautiful, yet silent landscape. Individual silver-stained neurons stand silhouetted against their surroundings like trees against a sunset. Their beauty, delicacy and orderliness have captured the minds and hearts of scientists for over a hundred years.
There are approximately 100 billion neurons in the brain, carrying and processing information. Several times that many other cells, called glia, feed and support the neurons. All these cells are serviced by a network of blood vessels ranging in size from large arteries to tiny capillaries. Organized into pathways, groups of neurons gather, process and respond to information from the outside world in ways that are similar for us all, yet unique to each.
Information comes to neurons through the senses, transmitted by specialized neurons, like the touch sensors in our skin or the rod and cone cells in our eyes. These sensory cells transform the input they receive into electrochemical messages.
The electrochemical messages are sent to the brain, where input from all the senses is combined. Some input is important and requires conscious attention. Some input is background, like the feel of clothing against your skin, and is not usually brought to our conscious attention. The brain decides automatically, based on experience, which input requires conscious attention and which does not.
We can change those automatic choices by focusing our attention on any part of the input we want. But to pay attention to all of it, all the time, would keep us too busy to get more important things done. The automatic choices made by the brain free us to think and create.
If sensory input requires a response—perhaps a verbal answer to a question or reflexively jerking your hand away from a too-hot frying pan—messages are sent back from the brain to the muscles that need to act. The number of neurons involved in this process can vary from as few as two (for the simplest reflex pathways) to whole networks of connected neurons numbering in the billions.
As a message travels through a network of connected neurons, it alternates between moving as an electrical signal and moving as a chemical signal. Within a neuron, messages are carried by changes in electrical charge that sweep from the receiving arms of the neuron (dendrites) through the cell body, to the transmitting arms (axons). Where one neuron’s transmitting axon meets another neuron’s receiving dendrite, special connection sites called synapses are formed. Messages are carried across synapses by neurotransmitters, the chemical messengers of the brain.
When a message is sent across a synapse, it is the amount of neurotransmitter released and received that determines the strength of the signal. In a normal, healthy brain, many synapses remodel themselves in response to the messages sent across. Synapses that are used frequently or that carry strong signals can actually become larger, while synapses that fall into disuse can shrink.
Completely new connections, new synapses, can be formed between two neurons in a pathway when frequent messages and strong signals pass between them. This would give you two synapses where there used to be only one, and would strengthen the message being carried by the pathway.
Interestingly, new synapses can also form when the loss of certain neurons in a well-used pathway has caused the neurons beyond them to become disconnected from the flow of information. In this case, neighbors of the lost neurons sometimes sprout new transmitting arms (axons) that reconnect the downstream neurons to the information flow.
Remodeling of synaptic contacts and the formation of new synapses form the basis for learning and memory. Pathways that are often used grow larger, stronger synapses and we remember the information that they carry more easily.
Neurons make new synapses most frequently when we are young, probably because we are learning so many things at that time. However, the ability to make new synapses and remodel old ones continues in the healthy brain throughout life.
Whew! Any questions?? Talk to me!