The ansa cervicalis (profunda) or cervical nerve loop lies beneath the sternocleidomastoid muscle and contains fibers from cervical spinal cord segments C1 to C3. It is responsible for controlling the lower hyoid (infrahyoid) muscles and can lead to the development of dysphagia when lesioned.
What is the ansa cervicalis?
The ansa cervicalis is a loop of nerves located under the sternocleidomastoid muscle in the neck. Without addition, medicine usually refers to the ansa cervicalis as the ansa cervicalis profunda, which is the original name of the loop. Anatomists used to distinguish between the superficial cervical loop (ansa cervicalis superficialis) and the deep cervical loop (ansa cervicalis profunda). The ansa cervicalis superficialis represents a junction of two nerves: It is located not below but on the sternocleidomastoid muscle and connects the transverse collateral nerve to the ramus colli nervi facialis. The latter represents a branch of the facial nerve. This corresponds to the seventh cranial nerve. The term ansa cervicalis superficialis no longer occurs in the new nomenclature. More rarely, anatomists also refer to the cervical ansa as the hypoglossal ansa because it runs in the carotid triangle (trigonum caroticum) next to the hypoglossal nerve.
Anatomy and structure
On the basis of its fibers, two roots of the ansa cervicalis can be anatomically distinguished: the radix inferior and the radix superior. The nerves that are part of the radix inferior originate in the cervical spinal cord segments C2 and C3. In contrast, the radix superior consists of fibers associated with spinal cord segments C1 and C2. Both roots of the cervical ansa contain nerve cords that originate from the cervical plexus and include both motor and sensory fibers. The cervical plexus is a plexus of nerves in the neck of humans and includes axons not only from segments C1 to C3, but also from C4 and (to a lesser extent) C5. The ansa cervicalis is located under the sternocleidomastoid muscle, which participates in certain head movements and assists in breathing as an accessory muscle. In its course in the neck, the cervical ansa first passes the internal jugular vein and then the scalenus anterior muscle before reaching the carotid triangle (trigonum caroticum). There it meets the hypoglossal nerve (12th cranial nerve), to which, however, the cervical ansa maintains no anatomic or functional connection.
Function and Tasks
Nerve fibers from the ansa cervicalis control the movements of the inferior hyoid muscles. This is also known as the infrahyoid musculature and consists of the omohyoid muscle, the sternohyoid muscle, the sternothyroid muscle, and the thyrohyoid muscle. As a group, the lower hyoid muscles participate with other muscles (for example, the upper hyoid or suprahyoid muscles) in the swallowing process, which requires precise coordination of movements. The complex interplay of muscles succeeds in healthy people thanks to the swallowing centers in the brainstem and cerebrum and the interconnection of the innervating peripheral nerves. These motor nerve fibers are efferent pathways that descend from the brain through the spinal cord and eventually enter the peripheral nervous system via the spinal nerves. In this process, the neuronal signal switches from one nerve cell to the next by crossing biochemical synapses. At such a switch point, nerves can compute information that arrives at their membrane. Activating (excitatory) and inhibiting (inhibitory) action potentials enter into this calculation according to the summation principle, which also takes into account their respective strengths. On muscle cells, the motor end plate forms the connection to the supplying nerve. The common innervation of the infrahyoid muscles by the ansa cervicalis helps to coordinate their movements during the swallowing process: Muscles that contract at the same time can receive electrical signals from the same nerve pathway, which only divides into individual fibers at a later time and thus addresses different muscle cells. In addition, the interaction helps to automatically inhibit certain muscles while others are active. Such inhibition avoids muscles interfering with each other.
Diseases
Damage to the ansa cervicalis can impair the function of the infrahyoid muscles and lead to the development of dysphagia. Space-occupying tumors, injuries, and tissue infections can directly damage the ansa cervicalis. Because its nerve fibers originate from the cervical plexus, a lesion of the nerve plexus also affects the ansa cervicalis. Radiation therapy for the treatment of breast carcinoma can damage the cervical plexus in some cases, leading to the loss of nerve fibers that also pass through the ansa cervicalis. However, absent or defective transmission of information in the cervical nerve loop may also be due to neuromuscular diseases such as myasthenia gravis. Blocked acetylcholine receptors impair signal transmission at the motor endplate in this disease. Myasthenia gravis typically affects the eye muscles first, before the muscles that hold up the head and the facial muscles also suffer from paralysis. The neuromuscular disorder may spread to other muscles, including the respiratory muscles. Possible neuromuscular causes of dysphagia also include Guillain-Barré syndrome (which is due to inflammation of the nerves) and myotonic dystonia (which causes tone disturbances). Even higher up in the hierarchy of information processing, diseases of the brain can cause the ansa cervicalis to receive inadequate nerve signals, resulting in dysphagia. Neurodegenerative diseases such as Alzheimer’s, Parkinson’s, ALS, and Huntington’s disease may be considered for this purpose, as well as tumors, strokes, and hemorrhages in the brain.
