Neural tissue consists of two cell types: neurons and neuroglia. Neurons are the functional cell of the nervous system; they are nerve cells that are responsible for the transfer and the processing of information in the nervous system. They consist of a soma which is a cell body, an axon, an extension of the cell membrane that allows them to send information, and dendrites, extensions of the cell membrane that let them receive information. The neuron is capable of sending and receiving information. Because it's so specialized to this function of information sending and receiving, it loses some of the characteristics of a typical cell. It doesn't reproduce, so once a neuron is gone, it's gone for good. Neuroglia are supporting cells; they protect and nourish the neuron, they can bring in nutrients for neurons, provide protection, and they can help guide repair of neurons. Some of them may assist with signal transmission. Neuroglia vastly outnumber the number of neurons in the nervous system. If somebody has a brain tumor, it more likely originated in these neuroglial cells, because neuroglial cells are so numerous, and because mature neurons don't divide. We'll look first at the structure of a neuron. The neuron looks sort of like this squid structure. Here at the center is the cell body, that's the soma, that contains the nucleus, the mitochondria, ribosomes, and other organelles and inclusions. It has the nucleolus where the ribosomes are assembled, and we'll also find Nissl bodies, these are both fixed and free ribosomes, as well as other organelles, including the mitochondria, and the nucleus. At this end of the CELL we see these little branching structures. These are the dendrites. Dendrites are structures that are stimulated by environmental changes or by the activities of other cells. These dendrites are the receptors for information. They respond to a stimulus, that might be a touch, smell, or a neurotransmitter that's released by another neuron. You can think about these like the antenna on the TV, they're receiving the signal, and there can be many dendrites. This one long extension of the plasma membrane is the axon. The axon is one single long branch. It's longer than the dendrites. It conducts the nerve impulse, that is, the action potential, toward the synapse where it's giving that information to another cell or to an effector. The axon is the transmission line for nerve impulses. This nerve impulse is basically the same as an excitation event in a muscle fiber. The axon hillock is this thickened region here that tapers into the axon. The axon can extend from millimeters to feet. Long neurons that have their cell bodies at the end of the spinal cord have axons that reach all the way down to your toes. Axons branch into telodendria at their very end, which confusingly sounds like dendrites. Those telodendria end at axon terminals where the axon can affect another neuron or affect an effector organ, a muscle or a gland. At these axon terminals, we have vesicles that are filled with neurotransmitters and can be secreted across the synapse, same as we saw at that neuromuscular junction. What we're seeing here is a pre-synaptic cell in blue and a post-synaptic cell in purple. We'll always have just this one-way communication along a neuron, receiving signals at the dendrites and then transmitting signals down the axon, and conveying those signals from neurotransmitters released from those axon terminals at the synapse. There can be three different types of synapses. That is, where neurons reach out their axon to another cell. The first type is where a neuron synapses with another neuron, so a neuron can send a message to another neuron. We can have a neuromuscular synapse, also known as the neuromuscular junction, which we already saw when we looked at skeletal muscle, or we can have a neuroglandular synapse where a neuron is stimulating gland cells. So, a synapse is just anywhere that a neuron communicates with a receiving cell. A single neuron would either synapse with another neuron, synapse with muscle cells, or synapse with gland cells. A single neuron would not innervate all three types. We'll take another look at the synapse. Here up here at the top we're getting a bigger view, we can see a neuron, there's the soma (the cell body), these are all the bushy dendrites, those receptors, and then one single long axon, that transmission line that extends out and branches into telodendria which end in axon terminals at the synapse with a postsynaptic cell. So here we're seeing in the purple a presynaptic cell and then in the gray the postsynaptic cells. Here down at the bottom we're getting a close look at that synapse between two neurons. In the pink there is the presynaptic cell, and then down at the bottom would be the postsynaptic cell. The small space between them is the synaptic cleft, and what would happen is that it would have an action potential traveling across that presynaptic membrane and traveling down to telodendria, to the axon terminals. When that impulse or that action potential arrives at this presynaptic membrane down at the end of the axon terminal, this triggers a release of a neurotransmitter from the axon vesicles. Here that neurotransmitter is illustrated in green. A neurotransmitter is just a chemical that diffuses across the synaptic cleft and causes a response in the postsynaptic cell. So we have that green neurotransmitter, it diffuses across the synapse. It binds to the postsynaptic membrane. This binding action causes a change in the permeability of the postsynaptic membrane, and this change in permeability results in the action potential being conveyed down the next neuron, so we can get this process repeated. Now we'll get into a little bit of physiology, and talk a little bit about this nerve impulse or this action potential. This is just a preview for what you'll see if you continue in physio. So the nerve impulse, it's the action potential of a nerve. An action potential is an all-or-nothing, self-propagating, depolarization of the membrane followed by repolarization of the membrane. Remember when we talked about muscle cells, we talked about how all cells carry a charge, they're sort of like batteries, with a positive side and a negative side. So depolarization is switching that charge, repolarization bringing that charge back. So this is an electrical event, a cell acting like a battery with both positive and negative charge. Cells typically have a negative charge inside and a positive charge outside, and cells that are excitable, like muscle cells or like neurons, can temporarily change their charge, and that's an action potential. This is due to the exchange of charged ions across the membrane. The ability to conduct this impulse is known as excitability. A stimulus is anything that causes an action potential to occur. The stimulus has to overcome some threshold level for that particular neuron. That threshold level is the amount of stimulus that's required to create the action potential. Once it starts then it's propagated all along the length of the axon, it's almost like a wave of electricity traveling along the cell membrane down the dendrite to the cell body and down the axon all the way to those axon terminals. You don't need to know the physiology of the action potential, just know that it's happening at the cell membrane and that it's propagated along the length of the axon. I want to show a "two minute neuroscience" video about the action potential where they talk really fast and explain the action potential pretty clearly. "Two minute neuroscience, where I simplistically explain neuroscience topics in two minutes or less! In this installment I will discuss the action potential. The action potential is a momentary reversal of membrane potential that is the basis for electrical signaling within neurons. If you're unfamiliar with membrane potential, you may want to watch my video on membrane potential before watching this video. The resting membrane potential of a neuron is around negative 70 millivolts. When neurotransmitters bind to receptors on the dendrites of a neuron, they can have an effect on the neuron known as depolarization. This means that they make the membrane potential less polarized, or cause it to move closer to zero. This chart shows membrane potential on the y-axis and time on the x-axis. When neurotransmitters interacting with receptors causes repeated depolarization of the neuron, eventually the neuron reaches what is known as its threshold membrane potential. In a neuron with a membrane potential of negative 70 millivolts, this is generally around negative 55 millivolts. When threshold is reached, a large number of sodium channels open, allowing positively charged sodium ions into the cell. This causes massive depolarization of the neuron. As the membrane potential reaches zero and then becomes positive, this is known as the rising phase of the action potential. The influx of positive ions creates the electrical signal known as the action potential, which then travels down the neuron. Eventually the action potential reaches its peak, sodium channels close, and potassium channels open, which allows potassium to flow out of the cell. This loss of positive potassium ions promotes repolarization, which is known as the falling phase of the action potential. The neuron returns to resting membrane potential, but actually overshoots it, and the cell becomes hyperpolarized. During this phase, known as the refractory period, it is very difficult to cause the neuron to fire again. Eventually the potassium channels close and the membrane returns to resting membrane potential, ready to be activated again. The signal generated by the action potential travels down the neuron and can cause release of neurotransmitter at the axon terminals, to pass the signal to the next neuron." So, you're going to have a lot of fun in physiology talking about all of that, but because this is an anatomy class, what we're going to talk more about is the anatomy of neurons and how they're classified. Neurons can be classified according to their structure. This is the anatomical divisions of neurons. The first type is a bipolar neuron. With a bipolar neuron, the cell body, the soma is between a long dendrite and a long axon. With a pseudounipolar neuron, the cell body is off to one side of the axon, so it looks almost like it's just a straight shot down the axon, it's not interrupted by the cell body. In the multipolar neurons, there's typically a single axon and multiple dendrites. Multipolar neurons are the most common cell type in the central nervous system. In the central nervous system, a cell body and its associated dendrites could be getting signals from thousands of neurons, so multipolar, most common in the central nervous system. The pseudounipolar neurons are common in the sensory or afferent divisions of the central nervous system. This can send information from the peripheral nervous system to the central nervous system. So these are primarily sensory neurons, they're in the peripheral nervous system, and then bipolar neurons are rare, they're found in the retina and they're found in the olfactory epithelium. If they are classified according to their function, then they're divided up according to where they're sending information. There are neurons that are classified as afferent, this is the sensory division, it's sending information from the peripheral nervous system to the central nervous system. It's transmitting information from a sensor to the central nervous system. Efferent neurons would be considered the motor division, they're transmitting information from the central nervous system to the periphery. They're transmitting information to an effector. And an interneuron would be an association neuron, they transmit information within the central nervous system. They're the link between the sensory and motor divisions. They'd be located entirely within the central nervous system. So let's take a look at our functional components again. Here we have a stimulus, some change in the external environment or the internal environment, and that causes the action potential to occur at that receptor, causes some change, causes an excitability event at that receptor, and that causes information to be sent along a sensory neuron to the central nervous system. Up here, this sensory nervous, sensory neuron would be afferent (I'll try to write, I'll just write) a-f-f, so you can keep track that it's afferent, and this one would probably be pseudounipolar (I'll write; oh it's hard to write on the screen) so this sensory neuron is probably afferent and pseudounipolar, sending information to the central nervous system. The relay neuron is within the central nervous system; it would be considered an interneuron because it's entirely within the central nervous system, and it's probably multipolar and within the central nervous system. Then the central nervous system would cause some change in response through a motor neuron. This would be the efferent line, and most motor neurons are also multipolar. If you're wondering where we'd find bipolar neurons, that might also be up here in this sensory line. So now we can add a little bit more complexity to this functional classification of neurons. Up here at the top left, we have receptors, receptors or maybe free nerve endings in the skin. Receptors are essentially dendrites. Remember that dendrites are those antennas that are collecting the information, they're receiving parts of the cell. So we have dendrites that are receiving information, then this information is received by sensory neurons in the peripheral nervous system, and it's passed to the central nervous system, it's passed to the brain or the spinal cord. Information can be about internal conditions, which is shown in red here, or it can be about external conditions, which is shown in blue. Either way, we have information that's picked up by the peripheral nervous system, picked up by those afferent sensory neurons that are typically pseudounipolar or bipolar, and they pass information to the central nervous system. The central nervous system is where we'll find interneurons. Interneurons are multipolar, they're conveying information entirely within the central nervous system, helping to regulate the response. That information can then be passed to motor neurons, either controlling your somatic lines or your visceral lines. These motor neurons are also typically multipolar. They're now part of that efferent line. We either have somatic motor neurons that control skeletal muscles, and that's your voluntary response, or get this involuntary response, this visceral motor line, that controls visceral motor neurons in the peripheral nervous system, and can cause changes in visceral effectors, things like smooth muscles, glands, cardiac muscle, or adipose tissue.