In this video, we will learn about the structures of the nervous system and how those structures are adapted to their functions. We’ll also learn about how the structures of the nervous system work together to integrate sensory information and coordinate a response. So, let’s put some of our billions of neurons to work and get started.
The nervous system performs many varied and essential jobs within the human body. The nervous system allows us to interact with the outside world through our senses and our movements. It controls the actions of other organ systems as well as many of our involuntary responses. The nervous system monitors and responds to changing conditions inside and outside of our bodies. This allows us to maintain a constant, normal internal environment known as homeostasis. And the nervous system achieves all of this by being a system of rapid communication.
Our nervous system is constantly sending signals from one part of the body to another, integrating those signals into useful information and coordinating appropriate responses. Imagine for a moment that a friend offers you a nice hot slice of pizza. Initially, you’ll notice the pizza using your sense of sight and your sense of smell. The pizza may trigger memories of other times you had pizza. You may think to yourself, “Am I hungry right now?” and decide whether or not eating pizza is a good idea for you. At the same time, involuntary reactions begin. Your mouth may start to water, and your digestive system will prepare for the potential meal. Eventually, you’ll also take some voluntary actions, like reaching out to take the plate and beginning to eat.
The functions of the nervous system fall into three categories. The sensory functions or how information is gathered falls under the category of input. Interpretation functions, such as thinking and deciding, fall under the category of processing. And motor functions, such as controlling the body’s response, can also be described as output. And we know that in biology, structure and function are always directly related. So, we’ll see this pattern of input, processing, and output reflected in all the structures of the nervous system.
Our nervous system is often separated into two divisions: the central nervous system, or CNS, and the peripheral nervous system, or PNS. The central nervous system consists of the brain and the spinal cord. Within the nervous system, the central nervous system carries out the processing category of functions. The peripheral nervous system consists of the nerves that carry signals to and from the central nervous system. The afferent or sensory nerves carry information from the body to the central nervous system. These nerves fall under the input category. And the efferent or motor nerves carry information from the central nervous system to the rest of the body. Their function falls under the output category.
Now that we’ve got a general overview of the structure and function of the nervous system, let’s zoom in to the cellular level. The functional cell of the nervous system is the neuron, or nerve cell. The neuron consists of the cell body and one or more specialized extensions of the cell membrane. The cell body is the processing center for the neuron since it contains the nucleus, which directs all of the cells activities. The cell body carries out the basic life processes for the cell. It’s also worth mentioning that mature neurons are nonmitotic, meaning that they’re not able to reproduce. Damaged neurons are either repaired or replaced by stem cells.
Dendrites are extensions of the cell membrane that are specialized for the function of signal input. Dendrite comes from the Greek word for tree because these parts of the neuron often look like branches. Dendrites are able to receive signals from other cells or send stimulation directly from the environment. The extension of the neuron specialized for signal output is called the axon. The function of the axon is to transmit signals to other cells. Axons vary in length, and in humans, they can be up to one meter long.
In order to speed up transmission over longer distances, some axons are coated in what’s known as a myelin sheath. The myelin sheath is a layer of fatty material that serves as electrical insulation. The nerve impulse leaps between gaps in the myelin sheath known as the nodes of Ranvier. In this way, myelinated neurons are able to transmit signals much more quickly than unmyelinated neurons. The axon eventually branches and ends in several axon terminals. The axon terminal is responsible for passing the signal to a receiving cell across a space known as the synapse. The cell that receives the signal may be a muscle cell, a gland cell, or another neuron. All of the structures of the neuron work together to carry out the neurons function of transmitting signals.
When a dendrite is stimulated, it generates an electrical impulse. The impulse travels along the cell membrane and then along the axon. Finally, the signal reaches the synapse, which we know is a small gap. So, how does the signal actually get from one cell to the other? Well, some neuron-to-neuron synapses have special connections that allow the electrical signal to be transferred directly. But most of the time the signal crosses the synapse with the help of special chemicals known as neurotransmitters. And the name of this process is neurotransmission. Let’s take a closer look at the synapse and see how neurotransmission occurs.
Here we have a much closer view of the synapse between a neuron and another cell. The space between these two cells is often called the synaptic cleft. The synaptic cleft is a tiny gap, only about 20 nanometers across. The cell that sends the signal is called the presynaptic cell because it’s the cell that comes before the synaptic cleft. The cell that receives the signal is called the postsynaptic cell because it’s the cell that comes after the synaptic cleft.
Neurotransmission is unidirectional, meaning that it only goes in one direction from the presynaptic cell to the postsynaptic cell. And the process of neurotransmission occurs in four steps. First, the signal arrives from the axon to the axon terminal. This causes special structures within the axon terminal, called synaptic vesicles, to fuse with the membrane of the presynaptic cell. The synaptic vesicles are filled with neurotransmitters, and those neurotransmitters are released into the synaptic cleft. Then, the neurotransmitters bind with receptors on the surface of the postsynaptic cell, which activates them to trigger some sort of reaction.
Having done its job, the neurotransmitter must be cleared from the synaptic cleft, and this can happen in one of two ways. In some cases, the neurotransmitter is broken down by special enzymes. And in other cases, the neurotransmitter is recycled back into presynaptic vesicles to be used again. When we talk about the steps of neurotransmission, it can sound like it takes quite a while, but in some neurons, this process is occurring 50 times in a second.
Well, now that we’ve learned about the general structure of a neuron and how neurotransmission occurs, let’s take a look at how different types of neurons are adapted to their specific functions. Throughout our body, our neurons can be divided broadly into three categories. There are sensory neurons or neurons that are specialized to receive signals or input information. There are relay neurons, also sometimes called interneurons, which are specialized for processing information. And we have motor neurons, which are responsible for output. They affect change.
Different types of neurons will have a slightly different arrangement of dendrites and axon terminals based on their specific function. Sensory neurons typically have one long extension out of the cell body that splits into the dendrites at one end and an axon at the other. The dendrites of sensory neurons are adapted to receive specific types of sensory signals.
Relay neurons generally have a cell body with a dendrite attached to one end and an axon to the other. These types of neurons tend to be highly branched because they’re adapted to processing information and determining where it goes next. Motor neurons usually have a cell body with several dendrites directly attached and one long axon. Motor neurons are specially adapted to initiate a response in the muscle cell, gland cell, or neuron that they connect to.
Sensory neurons and motor neurons are often myelinated, since myelin speeds up the transmission of signals and these neurons function to carry information from place to place. Relay neurons are often not myelinated, since they’re adapted to processing information as opposed to transmitting it over long distances. Sensory neurons and motor neurons are found throughout the nervous system, especially in the peripheral nerves, which are responsible for information input and output.
Relay neurons are only located in the central nervous system, where all the processing and integration of information occurs. Different types of neurons have different adaptations based on their specific function, and they all work together within the nervous system to allow it to do its job.
Next, let’s wrap up our lesson by taking a moment to review what we’ve learned. In this video, we learned about the structure and the function of the parts of the nervous system. We mostly focused on the different parts of the main functional cell of the nervous system, the neuron. We learned about how the parts of the neuron are adapted to their function and how different types of neurons will have slightly different arrangements of these structures to allow them to carry out different functions.