Phrenic Nerve – Thoracic Nerves

00:00 Welcome to this lecture on the nerves of the thoracic cavity.

00:06 This slide brings us to the learning objectives that you should be able to answer at the conclusion of this presentation.

00:13 First, describe the distributions of the phrenic nerve and the sympathetic and parasympathetic divisions of the autonomic nervous system.

00:21 Compare and contrast the general functions of the autonomic nervous system.

00:26 Describe the clinically applied anatomy of vocal cord paralysis and achalasia.

00:32 Describe the clinically applied anatomy of asthma, pancoast tumour and cardiac referred pain.

00:40 And then, we will summarize the key take-home messages from this presentation.

00:46 And then, lastly, provide attribution for the images that were used throughout this presentation.

00:55 This is the body map. And since we are looking at the nerves of the thoracic cavity, we will be focusing in on this area, in an anterior view and then, we will also be looking internally along the posterior thoracic wall. The nerves that we are going to be covering in this presentation are the phrenic nerves, the vagus nerves, the sympathetic trunks as well as autonomic nervous plexuses.

01:35 Our first nerve is that of the phrenic nerve. The phrenic nerve is formed from anterior rami of the third cervical spinal nerve, the fourth cervical spinal nerve and the fifth cervical spinal nerve and these anterior rami help to form, in part, the cervical plexus.

01:59 The phrenic nerve, once it’s formed, will have an anterior course to the anterior scalene muscle.

02:08 Here we see the right phrenic nerve on the right anterior scalene muscle, in the neck, and then we see the left phrenic nerve running on the anterior surface of the left anterior scalene.

02:19 All phrenic nerves are going to run on the lateral margins of the pericardium. They convey both motor and sensory fibers and the left is longer than the right and if you take a look at the diaphragm, here is the right dome. It is more elevated than the left dome. So, the left dome sits a little lower. Hence, the left phrenic has a slightly longer course to reach its destination.

02:49 This slide depicts the distribution of the phrenic nerve. The phrenic nerve does have pleural branches. These will supply the costal and mediastinal parietal pleura that’s associated with each lung apex. In addition, it has pericardial branches to help supply the pericardium. Those we can visualize in the illustration. Your right phrenic nerve is giving rise to pericardial branches in this area. Your left phrenic nerve is coursing along the pericardium. You can see some pericardium branches here as well as here.

03:25 The terminal branches of the phrenic nerve we see supplying the diaphragm, specifically the terminal branches to the diaphragm, convey motor fibers. There are also sensory fibers in these terminal branches and these are sensory to the diaphragmatic parietal pleura. In addition, there are sensory fibers that will innervate the parietal peritoneum that is located on the inferior surface of the diaphragm.

03:58 And then, our next slide will help us understand a phenomenon referred to as referred pain.

04:07 And to understand referred pain, we have to understand the dermatomes that are innervated by C3, C4, C5. And if we take a look here, here is the dermatome associated with C3.

04:24 It is bilateral, we are looking at the right side here.

04:28 C3 is also shown here on the left. So, it go up into the neck, over to the top of the shoulder; C4, little further medial on the shoulder, coming in to the midline and then to the opposite midline and over the top of the contralateral shoulder. C5 is not depicted here, but again, C5 is a level that helps to form the phrenic nerve. C5 would course out here more laterally on the shoulder and into the lateral upper... upper arm.

05:01 If there is irritation at the diaphragm, such as accumulation of blood in the peritoneal cavity, the pain gets referred to these dermatomal levels. Gall bladder can also cause referred pain to the same dermatomal level. So, the patient will complain of pain in these general dermatomal regions.