Review of the Sympathetic and Parasympathetic Divisions - Online Tutor, Practice Problems & Exam Prep

The heart receives dual innervation from the sympathetic and parasympathetic nervous systems. Sympathetic fibers, originating from T1 to L2, increase heart rate through short preganglionic fibers synapsing in trunk ganglia. In contrast, parasympathetic fibers arise from the cranial and sacral regions, decreasing heart rate via long preganglionic fibers synapsing near or on effectors. Both systems feature preganglionic and postganglionic fibers, but differ in origin and synapse locations, highlighting their opposing effects on organ function.

Review of the Sympathetic and Parasympathetic Divisions

In this video, we're going to be doing a quick review of our sympathetic and parasympathetic nervous systems by examining the dual innervation of the heart. As a reminder, dual innervation refers to the fact that most of our organs are innervated by both the sympathetic and parasympathetic nervous systems. So, to orient you to our image here, we've combined the two more abstract images that we've been working with up to this point. On the left with this yellowish background, we are depicting the parasympathetic innervation of the heart, and on the right with this blue background, we are depicting the sympathetic innervation of the heart.

We're going to start here on the sympathetic side and go through and label the anatomy and compare and contrast these two divisions. As a reminder, with our sympathetic fibers, they are going to be originating from spinal cord segments T₁ through L₂, those lethal tigers lurking that turn on your fight or flight response. And we're going to have these preganglionic fibers emerging from our spinal cord, and preganglionic just means before the ganglia or before the synapse. You can see that this is quite a short fiber because it only has to go from the spinal cord to that sympathetic trunk where it will synapse within a trunk ganglion. And it will, of course, be synapsing with these postganglionic fibers that just means after the ganglia or after the synapse, and then those postganglionic fibers will directly innervate the effector organ, in this case, the heart and that sympathetic activity is going to increase our heart rate.

Now, as a reminder, not all sympathetic innervation looks quite like this. We can also have those splanchnic nerves. Right? So we did depict that here for you. We have the innervation of the bladder here as an example. So we have this long splanchnic nerve now, and that just means that that nerve is going to bypass the sympathetic trunk. It's going to pass right through it without ever synapsing and instead it's going to be synapsing in one of these collateral ganglia. So it's synapsing within a collateral ganglion, and then we have that short little postganglionic fiber, and then we have the innervation of the bladder that way. So remember, from about T₅ down most of our preganglionic fibers are going to be these splanchnic nerves.

And then moving over here to the parasympathetic division, we are starting again with a preganglionic fiber. The big difference here is that these fibers will now be originating from the cranial region, mainly from the brainstem, as well as from the sacral region down here. So, we have our preganglionic fiber and you can see it is quite long, and that's because the parasympathetic division does not have any sort of trunk the way we see on the sympathetic side. Instead, these preganglionic fibers basically go out almost all the way or all the way to their effectors where they either synapse directly on the effector or in terminal ganglia very close to them. We have these long preganglionic fibers and then these short little postganglionic fibers. And, of course, this parasympathetic activity is going to decrease the heart rate, having that rest and digest effect.

In terms of similarities, we are always going to have a preganglionic fiber and a postganglionic fiber. They're going to be conveying a nerve impulse to an effector, and there's always going to be some type of synapse between those two fibers. The big differences are going to be where those fibers originate from. So on our sympathetic side, T₁ through L₂ and on our parasympathetic side, the cranial and sacral regions will be the origin point. And then the other big difference is really just where they synapse. So, again, in our sympathetic division, we're going to be synapsing either in the sympathetic trunk ganglia or in those collateral ganglia. And then on the parasympathetic side, we will be synapsing in terminal ganglia, which are very close to the effectors or directly on the effectors themselves. Those are the big similarities and differences of these two divisions. And, of course, functionally they have a lot of differences, having opposite effects on your organs for the most part. Alright. So, that is our review of the sympathetic and parasympathetic nervous systems and I will see you guys in our next video. Bye bye.

Review of the Sympathetic and Parasympathetic Divisions Example 1

Okay. So, this one asks us, the sympathetic division contains a ganglionic trunk where many of its preganglionic fibers synapse. The parasympathetic division does not contain a similar structure. Instead, its preganglionic fibers can synapse in a terminal ganglion. So, remember in our sympathetic division we have that structure, the sympathetic or ganglionic trunk, that runs parallel to our spinal cord where many of our synapses will take place. So, our first blank is going to be the word "trunk," so our answer is either going to be b or c based on that. And then in our parasympathetic division, we don't have that structure at all, right? And instead, we're going to have many of our synapses taking place in these terminal ganglia very close to the effector organ. So, our second blank is "terminal," and our answer here is going to be b. And there you go.

Here’s what students ask on this topic:

What is the difference between sympathetic and parasympathetic innervation of the heart?

The sympathetic and parasympathetic nervous systems both innervate the heart but have opposite effects. Sympathetic fibers originate from spinal cord segments T1 to L2 and increase heart rate. They have short preganglionic fibers that synapse in the sympathetic trunk ganglia. In contrast, parasympathetic fibers arise from the cranial and sacral regions and decrease heart rate. These fibers have long preganglionic fibers that synapse near or directly on the heart. Both systems feature preganglionic and postganglionic fibers, but differ in their origin and synapse locations.

How do preganglionic and postganglionic fibers differ in the sympathetic and parasympathetic systems?

In the sympathetic system, preganglionic fibers are short and originate from T1 to L2, synapsing in the sympathetic trunk ganglia. Postganglionic fibers are longer and innervate the effector organs. In the parasympathetic system, preganglionic fibers are long, originating from the cranial and sacral regions, and synapse near or directly on the effector organs. Postganglionic fibers in this system are short. Both systems have preganglionic and postganglionic fibers, but their lengths and synapse locations differ significantly.

What are the functional differences between the sympathetic and parasympathetic nervous systems?

The sympathetic nervous system prepares the body for 'fight or flight' responses, increasing heart rate, dilating pupils, and inhibiting digestion. It originates from T1 to L2 spinal segments. The parasympathetic nervous system promotes 'rest and digest' activities, decreasing heart rate, constricting pupils, and stimulating digestion. It originates from the cranial and sacral regions. These systems have opposing effects on organ function, ensuring the body can respond appropriately to different situations.

What is dual innervation and how does it apply to the heart?

Dual innervation refers to an organ receiving nerve fibers from both the sympathetic and parasympathetic nervous systems. In the heart, this means it is innervated by sympathetic fibers that increase heart rate and parasympathetic fibers that decrease heart rate. This dual control allows for precise regulation of heart function, enabling the body to adapt to varying demands, such as during exercise or rest.

Where do sympathetic and parasympathetic fibers originate?

Sympathetic fibers originate from the thoracolumbar region of the spinal cord, specifically from T1 to L2 segments. Parasympathetic fibers, on the other hand, originate from the craniosacral regions, which include the brainstem (cranial nerves) and the sacral spinal cord segments. These origins are key to understanding their different pathways and effects on the body.