In this video, we'll be going over some of the divisions of the motor or efferent division of the peripheral nervous system. We'll look first at differences between the somatic nervous system and the autonomic nervous system, and then take a deep dive into the two divisions of the autonomic nervous system. Firstly, the differences between autonomic and somatic nervous system. We're in the peripheral nervous system, and we're in the motor division, and we're looking at different pathways between that somatic system and the autonomic system. Down here at the bottom, illustrated in pink, is the somatic branch of the motor division. This is an example of a somatic motor neuron. It's myelinated, there's one single neuron, it's a simple pathway with one neuron having one target, that skeletal muscle. It exits the spinal cord at the ventral root (remember that the ventral root is the motor division), and it's efferent. Above, illustrated in yellow, is the autonomic pathway. This is located at the lateral horn of the spinal cord. The lateral horn is exclusively reserved for this autonomic motor pathway. It is located at the lateral horn, and then there's a two neuron chain. There's a pre-ganglionic neuron and a postganglionic neuron. They're named for their position relative to the ganglion. The first one has its cell body in the central nervous system, and it's called preganglionic. It's myelinated, it's a multi-polar neuron. The second one is called postganglionic even though its cell body is in a ganglion, and its axon is unmyelinated, and it stimulates smooth muscle or glands in a visceral organ, for instance the intestine. The somatic motor neuron, if it's severed, if this neuron is damaged, then that muscle would be paralyzed. Neurons can't heal from severe injury. In the autonomic nervous system, if there were to be damage to this chain, the effector, that smooth muscle or gland, would still work. The autonomic nervous system is more of a regulator than an instigator. So, somatic pathway, simple, straightforward, one single neuron controlling skeletal muscle. Autonomic system, a little bit more complicated, we have that two axon chain in order to reach an effector. Why is it that the autonomic motor nervous system uses two neurons in the chain to an effector? It's using two neurons in a chain, a pre-ganglionic and a postganglionic neuron, to affect smooth muscle, cardiac muscle, and glands, whereas the somatic nervous system only has one motor neuron that extends to an effector. This is a big difference between the somatic and the autonomic nervous system, but why is this? Why does the autonomic nervous system do this complicated procedure? It allows for increased communication and increased control. That is, it allows for neuronal convergence, where two neurons converge on the same target, or divergence. We can have axons from numerous pre-ganglionic neurons synapsing with a single ganglionic postganglionic cell, or you have axons from one preganglionic cell synapsing with and influencing numerous ganglionic cells. This is important because the autonomic nervous system acts quickly to affect effectors throughout the body all at once, so we can have stimulation of smooth muscle, of cardiac muscle, of glands, all at the same time. The autonomic nervous system gets divided up into two branches. The sympathetic autonomic nervous system has the fight or flight response, and the parasympathetic autonomic nervous system is the rest and digest response. The sympathetic autonomic nervous system gets your body ready for action, and the parasympathetic autonomic nervous system gets your body ready for resting. Both of them have a preganglionic and a postganglionic neuron, so they both have that two neuron set-up. In the sympathetic autonomic nervous system, the preganglionic neuron is short, it's myelinated, and it terminates just outside of the spinal cord right next to the vertebral column. That's where we'll find the autonomic ganglion of the sympathetic autonomic nervous system. And then the postganglionic fiber is unmyelinated. At the first synapse, acetylcholine is the neurotransmitter, and the sympathetic autonomic nervous system releases norepinephrine, also known as adrenaline. In the sympathetic autonomic nervous system, both the preganglionic neuron and the postganglionic neuron exhibit high levels of branching. This results in a diverged distribution, so the sympathetic autonomic nervous system can control multiple postganglionic neurons or effector organs. When you get scared, your whole body gets scared at once, because of all of this branching. For the parasympathetic pathway, there's still a preganglionic neuron and a postganglionic neuron. In the parasympathetic pathway, the preganglionic fiber is myelinated and it's long, whereas the postganglionic fiber is unmyelinated and short. The autonomic ganglia of the parasympathetic division would be located far away from the spinal cord, close to the organs that they control. At both synapses, acetylcholine is the neurotransmitter, and for both of the neurons, there is very little branching. This gives point to point control, and this means that the parasympathetic nervous system can give more regional activity, can relax just the brain, or adjust the digestive system, for instance. What are the effects of these, these branches of the autonomic nervous system on the body? I think it's best to illustrate it with some cat videos! In this first video, we have a cat watching the classic horror film Psycho, and its sympathetic division of the autonomic nervous system is being activated. The sympathetic autonomic nervous system accelerates heart rate, so you get heart pounding, widens bronchial passages, more oxygen coming in it, decreases motility of the digestive system, blood is being diverted to more important body systems like the muscles, you see pupillary dilation, hair standing on end, perspiration (that's sweating), and raised blood pressure. Any food that this cat has been eating, that meow mix, now it feels like it's sitting undigested in the stomach...and we're really seeing that sympathetic nervous system kicking in as this cat is getting really scared, getting into that fight or flight. So you can see that that cat was really ready for action, it was really ready for that fight or flight response, and in this case it was flight! This cat is one whose parasympathetic division of the autonomic nervous system is on full blast. So we'll take a look at this sleepy kitten. The parasympathetic division is called the rest and digest, it conserves energy, it slows the heart rate, it increases intestinal and gland activity, and it releases sphincter muscles in the gastrointestinal tract, causes you to relax, and allows you to do some of those background body processes like digestion. If you've ever been nervous about something and it feels like your mouth gets really dry, or like you've eaten something but it's just sitting undigested in your stomach, that's because your sympathetic nervous system is kicking in, it's diverting energy away from those digestive processes. Here's an illustration of this sympathetic nervous system in action. In the sympathetic nervous system, we have preganglionic neurons that have their cell bodies in the spinal cord, that's what this first segment is here, and then the axons only extend a short distance to the ganglia, which are illustrated here, and then the postganglionic neurons extend to the effectors. These nerves, these sympathetic nerves, only emerge from the thoracic and the lumbar region of the spinal cord, shown here, so this is sometimes called the thoracolumbar division of the nervous system. Only the thoracic and lumbar regions of the spinal cord will have that lateral horn of gray matter. The sympathetic division is responsible for accelerating the heart rate, for widening the bronchial passages, and then for decreasing some of the activity of the digestive system. It's responsible for causing pupillary dilation, it causes goosebumps in the skin, causes perspiration, and increases blood pressure; it's preparing you for activity. The sympathetic division is shown here, just re-emphasizing that we have a pre-ganglionic fiber, it's myelinated, it's short, and it shows high levels of branching. It extends to the autonomic ganglion, which is just outside of the spinal cord, and then the postganglionic fiber is long, extending to effectors and also branching, not myelinated, and releasing norepinephrine, that stress hormone. So where are these ganglia, these clusters of cell bodies that are just outside of the spinal cord for the sympathetic nervous system? Here we're seeing an illustration on the left of, there's the vertebrae, there's the spinal cord, and then we have spinal nerves from the spinal cord. This dorsal root ganglion right here, this would be the ventral root, and then they would be continuing out as spinal nerves. This string of yellow beans right here, that's the paravertebral ganglion chain. It runs next to the spinal column. This is where the cell bodies of the postganglionic neurons are, and then their axons extend out to the effectors. On the right here, you can see the same thing. We have the dorsal root emerging from the dorsal surface of the spinal cord, and then extending out of the ventral and the lateral side of the spinal cord is the sympathetic autonomic motor nerve. Illustrated in light blue is the preganglionic neuron with its cell body in the spinal cord, and then it's extending out through the ventral root, and merges with the dorsal root to form the spinal nerve, and it ends right here at the paravertebral ganglion. It enters into the paravertebral ganglion through a structure that's called the white ramus, that means white branch. This is because preganglionic neurons are myelinated, so they appear white. So the structure that they travel through is called the white ramus. The postganglionic neuron is illustrated in dark blue, it has its cell body in this paravertebral ganglion, and the axon exits through a structure that's called the gray ramus. It's called the gray ramus because postganglionic axons are unmyelinated. In this paravertebral ganglion chain, the cell bodies of the postganglionic neurons, with their unmyelinated axons, exit the ganglion to rejoin the spinal nerve by way of the gray ramus. Why do they overlap and do this loop-de-loop like this? We don't really know. There are three alternate paths that pre-ganglionic neurons can take to synapse with their postganglionic neurons. Remember that we said that in the sympathetic autonomic nervous system, the neurons can branch. What happens if they do branch? There are three different possibilities, and they're illustrated here. The first one is that they synapse in the paravertebral ganglion chain; they can branch up and down the paravertebral ganglion chain. This chain is all connected, so one pre-ganglionic neuron can synapse with multiple postganglionic neurons: it could affect respiratory, cardiac, digestive activity, all at once. One singular preganglionic neuron, illustrated in blue here, can branch to have divergent regulatory effects, and this is the prevailing pattern in the sympathetic autonomic nervous system. This is the most common. One another way that they can synapse is to go through the ventral root and enter into the spinal nerve and go through the white ramus but not stop in that paravertebral ganglion chain. They continue through to synapse somewhere else at a site that's called a collateral ganglion that's somewhere else in the sympathetic autonomic nervous system. This allows for more specific sympathetic regulation, for instance, for control on just one organ. There's a third option for the pathways that these pre-ganglionic neurons can take. The preganglionic neuron can have its cell body in the spinal cord, come out the ventral root, enter into the spinal nerve, go through the white ramus, and through the paravertebral ganglion chain, and then head all the way to the medulla of the adrenal gland, sitting on top of the kidney. This adrenal medulla is not an effector organ: it is the postganglionic neuron. It is made of neurons, and like other neurons, it secretes neurotransmitters. It secretes norepinephrine not into a synapse, but instead into the bloodstream, so everywhere in the body responds at once. This is what causes that fight or flight response globally within the body. The parasympathetic nervous system is, thank goodness, less complicated, so you can rest and digest while you think about it. Here we have the pre-ganglionic fiber, it's really long, it's myelinated, and then the postganglionic fiber, short, unmyelinated. The parasympathetic nerves are almost all pre-ganglionic fiber, and then only a short length is that postganglionic fiber. The ganglia are small, they're right next to organs, we don't really describe them other than that. This is called the craniosacral division, because the points of origin for the parasympathetic nerves are the brain stem or the sacral spinal cord. We'll only see them emerge from the spinal cord up here at the brain stem or down here at the sacral region. This one single nerve that has all of these branches affecting all of these organs, that's a cranial nerve, that's cranial nerve X, the vagus nerve. The vagus nerve has a really strong association with the parasympathetic division. It has inputs to multiple organs, including the heart. Stimulation of the vagus nerve could even temporarily stop the heartbeat, because it has such a strong influence on the heart. Of course, then the pacemaker cells of the heart would start the heartbeat again. The parasympathetic division is called the rest and digest system, it conserves energy, it slows the heart rate, it increases intestinal and gland activity, and it relaxes sphincter muscles in the gastrointestinal tract. And we'll look one more time at the division of the neurons here. Very long pre-ganglionic neurons, these are myelinated, these are multipolar neurons, they have their cell bodies in the spinal cord. The ganglia are small, and they're next to the organs that they control; they're anatomically indistinct. The postganglionic neurons are short, they're unmyelinated, and we see very little branching. They're releasing acetylcholine, a typical neurotransmitter, not that norepinephrine giving that adrenaline effect like the sympathetic division does.