[Biology Class Notes] on Deltoid Muscle Pdf

What is a Deltoid Muscle?

It is the most superficial muscle of the shoulder joint, therefore, it defines the round contour of our shoulders. Deltoid has associated muscles that move and help in our overall movement of shoulders. Having the shape of the symbol delta, i.e. triangular, this muscle is termed as Musculus deltoideus in Latin. A deltoid muscle has three parts, namely-

1. Clavicular Part of Deltoid Muscle: It is also known as Musculus deltoideus pars clavicularis in Latin. It originates from the lateral third of the clavicle.

2. Acromial Part of Deltoid Muscle: Also known as Musculus deltoideus pars acromialis in Latin, it originates from the acromial of the scapula (posterior).

3. Spinal Part of Deltoid Muscle: Also known as the musculus deltoideus pars spinalis in Latin, it originates from the spine of the scapula.

All of these three points converge towards the insertion point called deltoid tuberosity, found on the lateral surface of the shaft of the humerus. 

Innervation: In order for the deltoid muscles to carry out all the movements, it requires a nerve to instruct when to do and when, this muscle is thus innervated by an axillary nerve called Nervus axillaris in Latin. This axillary nerve carries fibres from the C5 and C6 nerve roots of the brachial plexus. 

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What is the Function of Deltoid Muscle?

It’s an obvious question to arise and knowing its location and function of the deltoid muscles will help us understand its necessity. 

• Clavicular part of the deltoid muscle is an assistance for flexion and internal rotation in the arm of the shoulder joint.

• Acromial part of the deltoid muscle fibres perform abduction of the shoulder joint, i.e. movement of the arm away from the midline of the body.

• Spinal parts of the deltoid muscle assist in extension and external rotation of the shoulder joints.

• Thus, we can say that deltoid muscle plays a major role in the initial movement of shoulder abduction and also to prevent subluxation or dislocation of the upper arm bone head (while carrying a heavy load). 

• Deltoid muscle helps in the elevation of the arm during glenohumeral elevation and provides overall stability to the shoulder joints and upper arm. 

• Deltoid muscle traverses from the spine of the scapula to the lateral portion of the clavicle.

Interesting Facts about the Deltoid Muscle and Axillary Nerve

• Axillary Nerve: It runs posteriorly around the surgical neck of the humerus. Fractures in this region can damage the ancillary nerve and affect the functioning of the deltoid muscle. Other causes for the injury of the axillary nerve include  dislocation of the glenohumeral joint or compression during the incorrect use of crutches. 

• Symptoms of Injury: The axillary nerve injury symptoms include atrophy of the deltoid muscle resulting in the weakness and loss of muscle tone, making the shoulder look flattened rather than rounded. Also, loss of sensation in the skin lying over the deltoid muscle may take place.

• Axillary Nerve Injury Management: Depending upon the cause, the management of the injury is variable. Its management can be done through careful observation, physiotherapy and medication such as anti-inflammatories and painkillers. In extreme cases, surgery may be required.

• Clinical Significance of the Deltoid Muscle: As discussed above, the injury of the muscles and the common abnormalities related to it are fatty atrophy, tears and enthesopathy. Traumatic shoulder dislocation may cause deltoid muscle tears. Due to aging, muscular dystrophy and disuse, muscle atrophy may take place. 

• Presence in Other Animals: Deltoid muscle is also found in other animals besides humans, such as it is found in the great ape family as rotator cuff in Orangutan. It functions in brachiation and comprises the muscle mass required to support the body weight by the shoulders. Other apes like chimpanzees, deltoid muscle is much larger than humans that weighs an average of383.3 gm whereas in humans, it is 191.9 gm. Also, in bats it acts as an important component and it is present in reduced form in crown-group birds as they also have sternum attached muscles for support.