The pectoralis reflex is a stretch reflex of the pectoralis muscle that is one of the intrinsic reflexes. Stretching of the muscle tendon causes the muscle to contract in this process, inducing abduction of the upper arm at the shoulder joint. A pathologically altered pectoralis reflex is present in the setting of various nerve injuries.
What is the pectoralis reflex?
The pectoralis reflex is a stretch reflex of the chest muscle that is one of the intrinsic reflexes. The pectoralis reflex is a monosynaptic reflex of the pectoralis major muscle. The reflex movement belongs to the intrinsic reflexes. Thus, its efferents and afferents are in the same organ. The pectoralis reflex is also classified among the stretch reflexes. In this case, the stretching of the tendon on the muscle spindle triggers the contraction of the pectoralis muscle. The reflex movement corresponds to adduction of the arm in the shoulder joint. The reflex is interconnected with the spinal cord, where it is located in the nerve tracts of segments C5 to C8. The pectoralis lateral nerve from the fasciculus lateralis of the brachial plexus is connected to these segments and thus provides motor supply to the large and small pectoral muscles. Various nerve branches arise from the brachial plexus that play a role in the motor innervation of the arm, shoulder, and chest as well as the sensory innervation of these areas.
Function and purpose
As a stretch reflex, the pectoralis reflex is part of the length control system at the pectoralis major muscle. This muscle is a three-limb skeletal muscle on the upper part of the thorax, classified as the external pectoral muscles. These muscles connect the shoulder girdle to the trunk. The muscle causes adduction, internal rotation, and anteversion of the arm in the shoulder joint and also serves as an accessory respiratory muscle. The three portions of the pectoralis major muscle have different origins. The pars clavicularis originates medially at the clavicle. The pars sternocostalis begins at the ipsilateral sternal border and cartilages between the second and sixth ribs. The pars abdominalis originates at the anterior leaflet of the aponeurosis. The fibers of the pectoral muscle converge concentrically and meet in a flat tendon with attachment to the humeral crista tuberculi majoris. The muscle is innervated by the medial pectoral nerve and the lateral pectoral nerve. This nerve receives motor supply via spinal cord segments C5 to C7. Its origin is the fasciculus lateralis of the brachial plexus. In the trigonum deltoideopectorale, the motor nerve crosses the axillary vein and axillary artery, runs caudally from there, and breaks through the fascia clavipectoralis and the small pectoral muscle. At this point, the lateral pectoral nerve gives off smaller fibers to the medial pectoral nerve and enters the pectoralis major muscle. In the pectoralis major muscle, the motor nerve provides the efferent pathway of the pectoralis reflex. The afferent pathway of the reflex movement is at the contractile midpoint of the muscle spindle fibers, which are surrounded by sensory nerve fibers. These so-called Ia fibers carry stretch receptors. When the muscle stretches, the muscle spindle and its contractile midsection also stretch. The Ia fibers then generate an action potential that travels via spinal nerves to the posterior horn of the spinal cord, where it is transmitted via a single synapse to the anterior horn and switched to the α-motoneurons. This is how the contraction of skeletal muscle fibers is effected. Through negative feedback processes, a constant muscle length is maintained in this way despite any perturbations. The conduction velocity of the executing neurons plays an important role in the efficiency of reflex movement. The α-motoneurons conduct signals at 80 to 120 ms-1.
Diseases and disorders
The pectoralis reflex plays a role in neurologic diagnosis and the standardized reflex examination therein. During the examination, the physician stretches the tendon of the large pectoralis muscle by applying pressure to the chest and striking the applied fingers with the reflex hammer. The expected response is abduction of the upper arm, which occurs in the shoulder joint. If the patient’s response does not meet expectations, it may indicate certain conditions. A pathological pectoralis reflex is spoken of when the reflex response is extinguished, diminished or exaggerated.For intrinsic reflexes, the reflex response depends on both the excitation state of the responsible motoneuron and the functional integrity of the reflex arc. After stroke, the excitation state of the motoneurons may be abnormal. Intrinsic reflexes such as the pectoralis reflex are therefore often spastically increased after strokes. Thus, stretching of the large pectoralis muscle may also trigger a reflex twitch in surrounding muscle groups. On the other hand, an exaggerated pectoralis reflex may also be an indicator of an upper spinal cord lesion in segments C5 to C7 and thus appear as a so-called pyramidal tract sign. Such central nerve lesions may occur in the setting of spinal tumor disease or infarction. The inflammatory autoimmune disease multiple sclerosis or the degenerative nervous system disease ALS can also cause spinal cord lesions. To a certain degree, however, a slightly exaggerated pectoralis reflex can also be physiological and thus does not necessarily have to have disease value. This is especially true for patients with a generally lively reflex response. If the structures of the reflex arc damaged, the pectoralis reflex is not exaggerated, but weakened or extinguished. This may be the case, for example, due to peripheral nerve damage in the setting of neuritis or mechanical injury. In order to correctly assign and interpret abnormal reflex behavior, the neurologist must assess the patient’s overall picture. Further reflex testing and imaging of the spine and brain will help him or her classify the abnormal reflex response.