Many important features of the spinal cord are visible in this cross section. This lab will be limited to the study of the basic features of neurons and glial cells - specific organs composed of neurons, including the retina of the eye and the organ of Corti of the inner ear, will be studied in the Sensory Systems lab, in conjunction with the Neuroanatomy course.
Once we can do that, we’ll be in a good position to delve even deeper into how the brain operates.The nervous system is extraordinarily complex, and it is therefore impossible to cover it in its entirety in a single laboratory. Researchers are still trying to agree on what these are, and how they should be classified. But part of what gives the brain its complexity is the huge number of specialised neuron types. So the answer to the question ‘What types of neurons are there?’ isn’t something we can fully answer yet. This is really the purpose of trying to classify neurons: in the same way as we can say that spinal cord sensory neurons bring sensory input from the periphery to the central nervous system, we would like to be able to say that the role of ‘neuron X’ in the hippocampus is to (for example) let you distinguish between similar but slightly different memories. Scientists would consider where the neuron projects to, what it connects with and what input it receives. The thought is that a single neuron type should perform the same function, or suite of functions, within the brain. In other words, a particular combination of features is one way of defining a neuron type. Furthermore, these different neurons have different electrical properties, different shapes, different genes expressed, different projection patterns and receive different inputs. Some GABA neurons, for example, send their axon mostly to the cell bodies of other neurons others prefer to target the dendrites. However, within categories we can find further distinctions. Looking at which neurotransmitter a neuron uses is one way that could be a useful for classifying neurons. In fact, researchers are still trying to devise a way to neatly classify the huge variety of neurons that exist in the brain. However, within any of these sensory or motor regions, there are tens or even hundreds of different types of neurons. Certainly, there are brain neurons involved in sensory processing – like those in visual or auditory cortex – and others involved in motor processing – like those in the cerebellum or motor cortex. Whereas in the spinal cord we could easily distinguish neurons based on their function, that isn’t the case in the brain. In the brain, the distinction between types of neurons is much more complex.
They are multipolar, just like motor neurons. As well as transferring signals between sensory and motor neurons, interneurons can also communicate with each other, forming circuits of various complexity.
InterneuronsĪs the name suggests, interneurons are the ones in between - they connect spinal motor and sensory neurons. Motor neurons have the most common type of ‘body plan’ for a nerve cell - they are multipolar, each with one axon and several dendrites. There are in fact two types of motor neurons: those that travel from spinal cord to muscle are called lower motor neurons, whereas those that travel between the brain and spinal cord are called upper motor neurons. These neurons transmit impulses from the spinal cord to skeletal and smooth muscles (such as those in your stomach), and so directly control all of our muscle movements. Motor neurons of the spinal cord are part of the central nervous system (CNS) and connect to muscles, glands and organs throughout the body. Most sensory neurons are pseudounipolar, which means they only have one axon which is split into two branches. A chemical input comes from taste or smell, which neurons then send to the brain. Thus, a physical input can be things like sound, touch, heat, or light.
The inputs that activate sensory neurons can be physical or chemical, corresponding to all five of our senses. Sensory neurons are the nerve cells that are activated by sensory input from the environment - for example, when you touch a hot surface with your fingertips, the sensory neurons will be the ones firing and sending off signals to the rest of the nervous system about the information they have received. For the spinal cord though, we can say that there are three types of neurons: sensory, motor, and interneurons. So just how many types of neurons are there? And how do scientists decide on the categories? For neurons in the brain, at least, this isn’t an easy question to answer. Just from looking down a microscope, however, it becomes very clear that not all neurons are the same. They are the fundamental units that send and receive signals which allow us to move our muscles, feel the external world, think, form memories and much more. Neurons are the cells that make up the brain and the nervous system.
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