Question

Explain the origin, insertion, and action of the following 10 muscles: Sternocleidomastoid, Occipitofrontalis, Zygomaticus major and minor, Masseter, Lateral rectus, Diaphragm, Pectoralis major, Biceps brachii, Rectus femoris, and Gastrocnemius

Answer 1

1. Sternocleidomastoid Muscle:

A. Origin: Sternocleidomastoid muscle is the large superficial muscle of the neck. It consist of two parts:

a. Sternal Head- It arises from the superolateral part of the manubrium sterni.

b. Clavicular Head- Arises from the medial one third of the clavicle. Lesser supraclavicular fossa is the triangular depression between these two heads. The internal jugular vein overlies in this depression.

B. Insertion: It is inserted into the

a.Lateral Surface of Mastoid process- by a thick tendon.

b.Occipital Bone- by the thin aponeurosis into the lateral half of the superior nuchal line.

C. Action: The action depends on the contraction of the Sternocleidomastoid muscle of the corresponding side.

If the muscle of one side contracts, it tilts the head towards the shoulder of the same side and turns the chin to the opposite side.

If the muscles of both the sides contract together, it helps in forced insipration, brings the head forward as in eating and flex the neck against resistance with the longus colli.

2. Occipitofrontalis Muscle:  Occipitofrontalis muscle is the muscle of the scalp,extending from the eyebrows to the occipital bones.

Origin: Occipitofrontalis muscle is made up of two bellies, Frontal and Occipital Bellies.

Epicranial Aponeurosis- thick fibrous sheath which connects these two bellies .

Frontal belly- also known as Frontalis, originates from the skin of the eyebrows and the muscles of forehead.

Occipital Belly- also known as Occipitalis,originates from lateral 2/3rd of the superior nuchal lines. The superior nuchal lines are the curved bony ridges marking the junction of the head and the neck.

Insertion: Both (frontal belly and occipital belly) are inserted into the Epicranial Aponeurosis.

Action: Both of these bellies perform different actions.

The frontal belly helps in elevating the eyebrows and wrinkles the skin of the forehead whereas the occipital belly helps in retracting the scalp.

3. Zygomaticus major and minor : Zygomaticus major and minor are the muscles around the mouth.

Origin:

Zygomaticus major: it is the muscle of the face and originates from the lateral surface of the zygomatic bone ( posterior aspect) .

Zygomaticus minor: it is also the facial muscle and originates from the lateral surface of zygomatic bone. ( Anterior aspect).

Insertion: Zygomaticus major inserts into the skin at the angle of the mouth while Zygomaticus minor inserts into the upper lip.

Action: Zygomaticus major: helps in smiling ( pulls the angle of the mouth upwards and laterally).

Zygomaticus minor helps in elevating the upper lip.

4. Masseter Muscle: Masseter is the muscle of Mastication. It helps in the movement of mandible.

Origin: Masseter muscle covers the ramus of the mandible (lateral surface). This muscle consist of three layers:

a. Superficial layer: Being the largest one,it originates from the anterior 2/3rd of the lower border of the zygomatic arch.

b. Middle layer: originates from the lower border of posterior 1/3rd of zygomatic arch.

c. Deep layer: it originates from the deep surface of zygomatic arch.

Insertion: 1. Superficial layer inserts into the lateral surface of the ramus of mandible ( lower part)

2. Middle layer inserts into the central part of ramus of mandible.

3. Deep layer inserts into the rest of the ramus of mandible.

Actions: Masseter muscle helps in mastication and speech. It elevates the mandible during biting and also causes protrusion of the mandible.

Note: Due to shortage of time and according to the guidelines, i can solve only the first four sub parts of this question. Kindly ask the remaining in the form of a new question.