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The trapezius is a large, flat, triangular muscle that extends over the back of the neck and upper thorax. It is the most superficial of the extrinsic muscles of the back. The trapezius is involved in movements of the shoulder girdle and head and neck, and is therefore considered a muscle of the upper limb. The trapezius is the only muscle of the upper limb that does not receive its innervation from the brachial plexus. Instead, it is supplied by the accessory nerve (CN XI) and the anterior rami of the C3 and C4 spinal nerves.
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What You'll Learn
The accessory nerve (CN XI)
The trapezius muscle is a large, triangular muscle located at the back of the neck and thorax. It is the only muscle of the upper limb that does not receive its innervation from the brachial plexus. Instead, the trapezius muscle is innervated by the accessory nerve, also known as cranial nerve XI, as well as the anterior rami of the C3 and C4 spinal nerves.
The accessory nerve then travels through the skull and exits through the jugular foramen, a large opening in the lateral skull, along with the vagus nerve (CN X) and the glossopharyngeal nerve (CN IX). From there, the nerve descends between the internal jugular vein and the internal carotid artery, passing deep to the sternocleidomastoid muscle in the neck.
As the accessory nerve descends into the neck, it gives off branches that supply the sternocleidomastoid muscle and the trapezius muscle. The branch that supplies the trapezius muscle is known as the spinal accessory nerve. This branch provides motor innervation to the muscle fibres of the trapezius, allowing for movements of the shoulder girdle and scapula.
Injuries to the accessory nerve (CN XI) can result in weakness in abducting the shoulder above 90 degrees and difficulty with arm adduction and abduction. This injury is known as trapezius palsy and is associated with a drooping shoulder and pain in the shoulder and neck. The accessory nerve also contributes to the sensory function of the trapezius muscle, transmitting proprioceptive information, including the sense of joint position and pain.
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Anterior rami of the C3 and C4 spinal nerves
The trapezius muscle is a large, triangular, paired muscle located on the posterior aspect of the neck and thorax. It is the only muscle of the upper limb that does not receive its innervation from the brachial plexus. Instead, it is innervated by the accessory nerve (CN XI) and the anterior rami of the C3 and C4 spinal nerves.
The C3 and C4 spinal nerves are part of the cervical plexus, which is a network of nerve fibres that supplies innervation to some of the structures in the neck and trunk. The cervical plexus is formed by the anterior rami of the cervical spinal nerves C1-C4, with the C5 nerve sometimes included due to its role in forming one of the plexus's motor branches. These nerves communicate with one another in a superior-inferior fashion close to their origins.
The C3 and C4 spinal nerves contribute to the formation of several branches of the cervical plexus, including the supraclavicular nerves, the phrenic nerve, and the ansa cervicalis. The supraclavicular nerves are formed by the C3 and C4 spinal nerves and provide sensation to the skin above and below the clavicle. The phrenic nerve arises from the anterior rami of C3-C5 and provides motor innervation to the diaphragm. The ansa cervicalis is a nerve loop formed by the C1-C3 spinal nerves that innervate the infrahyoid muscles in the anterior cervical triangle.
The anterior rami of the C3 and C4 spinal nerves play a crucial role in the innervation of the trapezius muscle, along with the accessory nerve. This unique innervation distinguishes the trapezius from other upper limb muscles, which typically receive their innervation from the brachial plexus.
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Proprioceptive/sensory fibres from the muscle
The trapezius muscle is a large, triangular muscle located on the posterior aspect of the neck and thorax. It is the only muscle of the upper limb that does not receive its innervation from the brachial plexus. Instead, motor innervation to the trapezius is conveyed by the accessory nerve (CN XI) or cranial nerve XI, as well as the anterior rami of the C3 and C4 spinal nerves. These nerves also contain proprioceptive/sensory fibres from the muscle.
The proprioceptive/sensory fibres from the trapezius muscle play an important role in providing sensation and proprioception to the muscle. Proprioception refers to the sense of joint position and movement, allowing the body to maintain balance and coordination during movement. These fibres travel via the ventral rami of the third (C3) and fourth (C4) cervical spinal nerves.
The trapezius muscle has a wide range of functions, including stabilising and moving the scapula, controlling movements of the shoulder and upper limb, and supporting the arm. It is involved in abduction of the upper limb at the shoulder by rotating the scapula. The upper fibres of the trapezius elevate the scapula, while the middle fibres retract it and the lower fibres depress it.
Injury to the cranial nerve XI, which supplies innervation to the trapezius muscle, can cause weakness in abducting the shoulder above 90 degrees. This injury can also lead to trapezius palsy, resulting in difficulty with arm adduction and abduction, drooping shoulders, and shoulder and neck pain.
The proprioceptive/sensory fibres from the trapezius muscle are crucial for its function and coordination during movement. They provide the necessary sensory input for the muscle to stabilise and move the scapula effectively, contributing to the overall function of the upper limb and shoulder.
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The descending, transverse and ascending parts of the trapezius
The trapezius muscle is a large, flat, triangular skeletal muscle that originates from the skull, nuchal ligament, and the spinous processes of C7-T12. It is composed of three functional parts: the descending (upper) part, the transverse (middle) part, and the ascending (lower) part.
The descending part of the trapezius muscle originates from the external occipital protuberance of the occipital bone, the superior nuchal line of the occipital bone, the nuchal ligament, and the spinous process of the seventh cervical vertebra. The fibres of this part travel inferolaterally along the posterior cervical region and converge to a tendon, which inserts onto the posterior aspect of the lateral one-third of the clavicle and the anterior half of the acromion of the scapula. The descending part contributes to the formation of the posterior cervical region and the lateral cervical region. It is involved in multiple actions, including elevating, retracting, and upwardly rotating the pectoral girdle at the acromioclavicular and sternoclavicular joints. During unilateral contraction, it laterally flexes the head and neck to the same side, while bilateral contraction results in an extension of the head and neck.
The transverse (middle) part of the trapezius muscle is supplied by the superficial cervical artery or a branch from the transverse cervical artery. The fibres of this part originate from the spinous processes of T1-T4 vertebrae and their intervening supraspinous ligaments. These fibres are directed horizontally, running laterally towards the shoulder. The transverse part acts together with the rhomboids to produce a retraction of the scapula by pulling it towards the midline.
The ascending part of the trapezius muscle is supplied by muscular branches of the dorsal scapular artery, which arises from the subclavian artery. The fibres of this part originate from the spinous processes of the T4-T12 vertebrae and their respective supraspinous ligaments. The fibres travel superolaterally along the upper back and converge to a tendon, which inserts onto the medial half of the spine of the scapula. The ascending part is involved in depressing the pectoral girdle, retracting the pectoral girdle, and upwardly rotating the pectoral girdle at the acromioclavicular and sternoclavicular joints. This part also contributes to the formation of the triangle of auscultation.
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The trapezius's role in scapular retraction/adduction
The trapezius is a large, paired trapezoid-shaped muscle that extends from the occipital bone to the lower thoracic vertebrae of the spine and laterally to the spine of the scapula. It is the only muscle of the upper limb that does not receive its innervation from the brachial plexus. Instead, it is innervated by the accessory nerve (CN XI) and the anterior rami of the C3 and C4 spinal nerves.
The main function of the trapezius is to stabilize the scapula and control its movement during movement of the shoulder and upper limb. The upper fibres of the trapezius elevate the scapulae, the middle fibres retract the scapulae, and the lower fibres depress the scapulae. The middle fibres of the trapezius are developed by pulling the shoulder blades together, which is known as scapular retraction.
Scapular retraction exercises can be used as an alternative to overhead exercises to activate the infraspinatus, upper, and lower trapezius muscles in people with and without shoulder pain. During scapular retraction, the middle fibres of the trapezius contract to draw the scapula medially. This movement can be performed under progressive adduction loads to increase the difficulty and further activate the trapezius muscle.
Dysfunction of the trapezius can result in winged scapula, abnormal mobility or function of the scapula (scapular dyskinesia), and neck pain. Therefore, it is important to include exercises that target the trapezius muscle to maintain proper scapular control and prevent pain and dysfunction.
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Frequently asked questions
The trapezius muscle is innervated by the accessory nerve (CN XI or cranial nerve XI). It also receives proprioceptive fibres from the third and fourth cervical ventral rami (C3 and C4).
The trapezius is a large, paired trapezoid-shaped surface muscle that extends longitudinally from the occipital bone to the lower thoracic vertebrae of the spine. It is the most superficial muscle on the posterior aspect of the neck and thorax.
The main function of the trapezius is to stabilize the scapula in its anatomical place and control it during movements of the shoulder and upper limb. It is also involved in the movement of the head and neck.
