On the back, the thoracodorsal nerve innervates the large back muscle and the large round muscle. Both play an important role in arm movements. Lesions occur, for example, in neuralgic shoulder amyotrophy and arm plexus palsy.
What is the thoracodorsal nerve?
The thoracodorsal nerve belongs to the peripheral nervous system and is one of the fibers of the brachial plexus. Mainly, the nerve participates in the control of certain arm movements by innervating two muscles that are located on the back in humans. These are the teres major muscle and the latissimus dorsi muscle. The name of the thoracodorsal nerve is derived from its characteristic course: its path first leads it across the thorax before it ends at the back (dorsal) at the innervated muscles. An arbitrary movement of the arm begins in the human brain. An electrical signal originates in the motor center and travels to the spinal cord through neural fibers that pass through the spinal canal between two vertebrae. The origin of the thoracodorsal nerve is in the spinal cord between cervical segments C6 and C8. Its pathway already divides at the spinal cord and extends symmetrically into both halves of the body.
Anatomy and structure
The thoracodorsal nerve is part of the brachial plexus, which physiology refers to as the brachial plexus. It represents a collection of different nerves that neuronally supply different shoulder, back, and arm muscles. They do not form a tightly enclosed uniform tissue, but a loose collection of nerve fibers belonging to different pathways. The thoracodorsal nerve constitutes a fasciculus posterior of the brachial plexus, as it belongs to the posterior branches. The posterior fibers, in turn, form a subunit of the infraclavicular branches of the brachial plexus: these branches are all located below the clavicle. In addition to the thoracodorsal nerve, they also include the subscapular nerve, the radial nerve, the axillary nerve, and six other nerves. The thoracodorsal nerve sends its motor commands to the large back muscle (Musculus latissimus dorsi), which attaches anteriorly to the humerus; its origin is at some thoracic and lumbar vertebrae as well as at the ilium, the fascia thoracolumbalis, some ribs, the scapula and sacrum (Os sacrum). Other fibers of the thoracodorsal nerve lead to the teres major muscle, which is also located on the back, begins at the scapula and attaches to the humerus. Before reaching the muscles, the thoracodorsal nerve accompanies the subscapular artery in its course.
Function and Tasks
Transmitting nerve signals is the main function of the thoracodorsal nerve. In this process, the electrical charge of the action potential propagates along the nerve fiber (axon) that originates from the associated nerve cell. Most nerve fibers in the human body are surrounded by Schwann cells, which form a natural insulating layer. However, the Schwann cells do not adjoin each other without gaps. These interruptions are Ranvier rings, at which the cell depolarizes along the axon each time. When an action potential reaches such a section, it excites sodium ion channels located in the membrane. The sodium particles are positively charged: Therefore, when they flow inside after opening the channels, they cause a change in the electrical charge in this axon section. At the same time, the shift already excites the next segment. To restore the cell to its original state, the interior of the axon first actively releases potassium ions. They are also positively charged and thus balance out so that the electrical charge corresponds to the original one. Only then do transport molecules in the membrane move the correct particles back in and out until they also reach the correct ion composition. In the meantime, the axon cannot form a new action potential in this segment, which is why the duration is also known as the refractory period. It lasts for about two milliseconds. For this reason, a single nerve fiber – in the thoracodorsal nerve and all other nerves – can only ever function as a one-way pathway for signals. Different nerve fibers that are close together, however, can cover both directions.
Diseases
As a result of damage to the thoracodorsal nerve, motor and sensory disturbances may manifest. Such a lesion is possible, for example, in neuralgic shoulder amyotrophy. It represents an inflammation of the brachial plexus, which includes the thoracodorsal nerve. The inflammation manifests as sudden severe pain in the shoulders and upper arm (unilateral or bilateral) before partial or complete paralysis (paresis) occurs about a week later and the muscle tissue eventually disappears (atrophy). The deltoid muscle suffers the most from the symptoms of the disease, but symptoms may also extend to shoulder and arm muscles. Less commonly, the diaphragm is affected. Examinations can usually detect antigen-antibody complexes (immune complexes), which indicate the presence of an infection. Although the exact causes of neuralgic shoulder amyotrophy have not yet been determined, it appears to be associated with viral infections, vaccination reactions, overuse, and heroin use. Another example of thoracodorsal nerve damage is brachial plexus palsy, which results from injury to the nerve roots. Fibers of the brachial plexus tear off in this case and, as a result, can no longer transmit signals. Birth trauma or external force is usually responsible for the lesion. Depending on which fibers break off, failure of the corresponding neurons occurs.
