Introduction to Neuroanatomy - Part 1 (The Nervous System)
The brain, the spinal cord, and everything else.
To learn more about neuroanatomy, please check out: "Neuroanatomy Text and Atlas," by John H. Martin.
- Anatomical Perspective: This refers to the structure of body parts
- Cytoarchitectonic Perspective:This refers to the study of the tissue in terms of cellular structure (1).
- Phylogenetic Perspective: This refers to the comparative study of living organisms in terms of their evolutionary history (2).
- Functional Perspective: This refers to the point of view that studies a specific body part, in this case, the brain, in terms of functions.
- Ontogenetic Perspective: This refers to the study of a part of the brain in terms of its development (3).
Using these perspectives, or other ones that are not mentioned here, we can look at the brain in terms of its different organizations. One of them would be a hierarchical approach, which refers to how the body parts are "connected" so they can interact with each. For example, when you see something, let's say a soccer ball, coming your way. The light reaches the receptors in your eyes. Then, a process known as transduction occurs. This is when a type of information, usually sensory info, gets "translated" or converted into another type of info, usually electric potentials for the brain (4). The signal then reaches the thalamus, which then sends a signal to the occipital lobe, which is the primary visual cortex (5). So as you can see this point of view follows how each part interacts with each other in a chronological sequence of events.
Cells of the Nervous System
There are two main cells in the nervous system. They are the neuron and the glia cell (6). I'm going to use the same material I used to explain these cells in my old psychopharmacology post (if you want to check that out, please check it out by clicking here: http://hbookreviews.blogspot.com/2016/07/introduction-to-psychopharmacology-part_17.html). The nervous system has two main divisions: The central nervous system and the peripheral nervous system. The former is composed of the brain and the spinal cord, the peripheral includes everything else. Each division has subsets, but we will cover each of them later. Neurons are located in the central nervous system (CNS) and their main function is to process information (7). As you can see some of the parts of the neurons are in the picture to the right, we will cover them later.
The physiology of neurons depends on their specialized function. An example of this would be the difference between sensory neurons and the motor ones. The former receives info from places such as the skin and sends it to the CNS (8), the latter sends information to the muscles so they can move. There are other types, but I don't want to cover them fully, an example of this would be interneurons, which are located between a sensory and a motor neuron (9).
Fun Fact: It is estimated that there are around 100 billion neurons in the brain (9)
Let us go back to neurons and talk about their parts. The structure of a neuron depends on its function, however, there are similitudes between all of them. These include the soma, which is the cell body of a neuron, the dendrites, which look like tree branches and receive information, an axon, which is the part of the neuron that sends info. There is another special structure called the synapse. It is composed of three parts: The synaptic cleft, which is the space between the terminal button and a dendrite of a neuron, the presynaptic cell, which is the neuron before the synaptic cleft, and the postsynaptic cell, which is the neuron after the cleft. Think of a neuron functioning as a telephone, the dendrites is where you hear the person speak to you (although there are some exceptions where the dendrites send signals, but this is uncommon) and the terminal button is where the microphone is for you to speak is located. They communicate with each other with chemicals known as neurotransmitters, we will learn more about them later.
Types of neurons
The most common type of neuron is called multipolar (10). This has one axon and many dendrites attached to its soma. A bipolar neuron has an axon from one side and a dendrite from the other.
Another type of cell is the unipolar neuron. This cell has only one stalk that divides itself into an axon and the dendrites. Bipolar cells are usually found in sensory systems such as vision. Unipolar neurons usually work with somatosensory functions such as feeling pain and temperature. Both of them receive information from the physical world and send it into the CNS.
Like it was stated before, neurons communicate with each other using neurotransmitters. These are released by the terminal buttons when an action potential happens (We will cover this later). A neurotransmitter is a chemical that has an effect on another neuron. Remember how the synaptic cleft is the space between a terminal button and a dendrite? Well, when the neurotransmitter is released it travels in this space in search of a receptor, which can usually be found on a dendrite of another cell either by inhibiting or exciting it. This is how neurons communicate.
Neurons are considered specialized cells for three reasons. The first one is that they are polarized. This refers to the fact that neurons are negatively charged on the inside. This allows them to communicate via action potentials (We will talk specifically about this later). The second reason is that they are excitable. This means that neurons are able to change their charge. (Our brain works on electrical activity generated by chemicals). The third reason is that they are modulatory. Each neuron is able to communicate with many other neurons. In other words, every thought we have, every movement we make, every behavior, can be observed in the brain in the form of electric activity across neurons.
10. Physiology of Behavior by Neil Carson
10. Physiology of Behavior by Neil Carson