Part 4: Nerves and Ganglia

Identify the phrenic nerve on both left and right sides. The phrenic nerves run caudally through the mediastinum to the diaphragm (which they supply). (See DG. Figs. 3-14A, 3-16, and 3-20)

IMPORTANT DISSECTION NOTE: The following nervous system structures are found on both right and left sides; dissect only ONE side of the specimen up until the vagus nerve. Dissect both sides for the vagus nerve.

Within the thorax, identify the sympathetic trunk. The sympathetic trunk is found dorsally, inside the chest, running longitudinally along the ventral surface of the necks of the ribs. Follow the sympathetic trunk using blunt dissection, with the tip of an iris scissors, and trace it cranially. Note the ganglia as you go; ganglia are the small enlargements in the trunk at each intercostal space. (See DG. Fig. 3-20) As you reach the transition from thoracic region to neck region do the following:

• Near the first intercostal space, to the lateral side of the longus muscles, locate and identify the cervicothoracic ganglion. This ganglion is larger than those ganglia noted as you traced the path of the sympathetic trunk. The cervicothoracic ganglion will have several branches leaving it. (See DG Fig. 3-20)

  • • Comment: Note that you may see a small vertebral nerve leaving the cervicothoracic ganglion and coursing dorsally. This nerve will run alongside the vertebral a. within the transverse foramina of the cervical vertebrae. (See DG Fig. 3-20)

• Cranial to the cervicothoracic ganglion, the sympathetic trunk divides to form a loop around the subclavian a.; this loop is the ansa subclavia, which will be discussed in more detail in the Application section of the course. You need not identify the ansa subclavia specifically at this time, but you should be aware that it forms a ‘loop’ around the subclavian a.; the two sides of the ‘loop’ meet at the middle cervical ganglion. (See DG Fig. 3-20)

Re-identify the vagosympathetic trunk in the neck (previously identified in Chapter 3, alongside the carotid a. in the carotid sheath) and trace it caudally to the middle cervical ganglion. (See DG Fig. 3-20)

• Identify the middle cervical ganglion. As previously mentioned, the middle cervical ganglion is an enlargement found at the junction of the ansa subclavia and the vagosympathetic trunk.(See DG Fig. 3-20)

  • • Dissection Note: In cats, the middle cervical ganglion may be small/almost non-existent and difficult to distinguish, so be sure to look at an example in a dog as well.

      Related Learning Objective

      • D4.8

       

Identify the vagus nerve (on both left and right sides). The vagus nerve leaves the vagosympathetic trunk at the level of the middle cervical ganglion and continues caudally. More cardiac nerves may be seen leaving the vagus n. to innervate the heart. (See DG Fig. 3-20)

• Comment: While you need not dissect the recurrent laryngeal nerves, you should be aware that they are branches of the vagus and know their general path/location. On the LEFT side, the left recurrent laryngeal nerve leaves the left vagus nerve. It curves around the aortic arch and then continues cranially in the neck, alongside the trachea. On the RIGHT side, the right recurrent laryngeal nerve leaves the right vagus nerve and curves around the right subclavian artery and then continues cranially in the neck, alongside the trachea. These will be covered in more detail in the Application portion of this course.

  • • Dissection Note: If you wish to view the recurrent laryngeal nerves be sure to look at/ask about them at the demo table specimens. At least one cat and one dog will have them dissected out for you to see.

Return to the vagus nerves. Continue tracing both the left and right vagus nerves and identify where each divides into dorsal and ventral branches. (Each vagus nerve will split into dorsal and ventral branches, i.e., you will have a left dorsal and left ventral branch of the vagus AND a right dorsal and right ventral branch of the vagus.) (See DG Fig. 3-20)