There are approximately 100 billion neurons in the human brain (Williams & Herrup, 1988). Each neuron has three main components: dendrites, the soma, and the axon (see Figure 2). Dendrites are processes that extend outward from the soma, or cell body, of a neuron and typically branch several times. Dendrites receive information from thousands of other neurons and are the main source of input of the neuron. The nucleus, which is located within the soma, contains genetic information, directs protein synthesis, and supplies the energy and the resources the neuron needs to function. The main source of output of the neuron is the axon. The axon is a process that extends far away from the soma and carries an important signal called an action potential to another neuron. The place at which the axon of one neuron comes in close contact to the dendrite of another neuron is a synapse (see Figures 2–3). Typically, the axon of a neuron is covered with an insulating substance called a myelin sheath that allows the signal and communication of one neuron to travel rapidly to another neuron.
Figure 2. Basic structure of a neuron.
The axon splits many times, so that it can communicate, or synapse, with several other neurons (see Figure 2). At the end of the axon is a terminal button, which forms synapses with spines, or protrusions, on the dendrites of neurons. Synapsesform between the presynaptic terminal button (neuron sending the signal) and the postsynaptic membrane (neuron receiving the signal; see Figure 3). Here we will focus specifically on synapses between the terminal button of an axon and a dendritic spine; however, synapses can also form between the terminal button of an axon and the soma or the axon of another neuron.
A very small space called a synaptic gap or a synaptic cleft, approximately 5 nm (nanometers), exists between the presynaptic terminal button and the postsynaptic dendritic spine. To give you a better idea of the size, a dime is 1.35 mm (millimeter) thick. There are 1,350,000 nm in the thickness of a dime. In the presynaptic terminal button, there are synaptic vesicles that package together groups of chemicals called neurotransmitters (see Figure 3). Neurotransmitters are released from the presynaptic terminal button, travel across the synaptic gap, and activate ion channels on the postsynaptic spine by binding to receptor sites. We will discuss the role of receptors in more detail later in the module.
Types of Cells in the Brain
Not all neurons are created equal! There are neurons that help us receive information about the world around us, sensory neurons. There are motor neurons that allow us to initiate movement and behavior, ultimately allowing us to interact with the world around us. Finally, there are interneurons, which process the sensory input from our environment into meaningful representations, plan the appropriate behavioral response, and connect to the motor neurons to execute these behavioral plans.
There are three main categories of neurons, each defined by its specific structure. The structures of these three different types of neurons support their unique functions. Unipolar neurons are structured in such a way that is ideal for relaying information forward, so they have one neurite (axon) and no dendrites. They are involved in transmission of physiological information from the body’s periphery such as communicating body temperature through the spinal cord up to the brain. Bipolar neurons are involved in sensory perception such as perception of light in the retina of the eye. They have one axon and one dendrite which help acquire and pass sensory information to various centers in the brain. Finally, multipolarneurons are the most common and they communicate sensory and motor information in the brain. For example, their firing causes muscles in the body to contract. Multipolar neurons have one axon and many dendrites which allows them to communicate with other neurons. One of the most prominent neurons is a pyramidal neuron, which falls under the multipolar category.
It gets its name from the triangular or pyramidal shape of its soma (for examples see, Furtak, Moyer, & Brown, 2007). In addition to neurons, there is a second type of cell in the brain called glia cells. Glia cells have several functions, just a few of which we will discuss here. One type of glia cell, called oligodendroglia, forms the Figure 3. Characteristics of a synapse.
myelin sheaths mentioned above (Simons & Trotter, 2007; see Fig. 2). Oligodendroglia wrap their dendritic processes around the axons of neurons many times to form the myelin sheath. One cell will form the myelin sheath on several axons. Other types of glia cells, such as microglia and astrocytes, digest debris of dead neurons, carry nutritional support from blood vessels to the neurons, and help to regulate the ionic composition of the extracellular fluid. While glial cells play a vital role in neuronal support, they do not participate in the communication between cells in the same fashion as neurons do.