Muscular Architecture of Head and Neck

What Is the Significance of Understanding the Muscular Architecture of Head and Neck?

Human muscles are mostly composed of large groups of muscle fibers. These fibers are innervated by neural cells (neurons) found in the spinal cord or in the brainstem (for the muscles of the neck and head). Let us know about the striated muscle fibers, especially in the anterior head and neck region that is formed at the earliest stages of muscle development, and that gives us an insight into the form, function, and physiology of muscle tissues of the head and neck responsible for all orofacial movements. Oral motor disorders can also be potentially diagnosed by understanding the principles behind head and neck muscle physiology.

What Is the Structure of Striated Muscle Fiber?

Each striated muscle is a composition of multiple muscle fibers. Each muscle is not only multinucleated in structure but can also be cylindrical. It may cross-orient with each fiber approximating anywhere from 1 to around 300 mm in length. It is made up of sarcolemma (cell membrane) enclosing sarcoplasm (cytoplasm). In the periphery beneath the sarcolemma is a number of evenly distributed longitudinal threads called myofibrils. Each myofibril shows alternate dark and light bands. Dark bands are known as A bands (isotropic) and the light bands are called I bands.

The sarcoplasm consists of the following:

• Supporting Tissue - It helps in the organization of the muscle.

• Endomysium - It surrounds each muscle fiber separately.

• Perimysium - It surrounds bundles (fasciculi) of muscle fibers of various sizes.

• Epimysium - It is in fact the entire layer that surrounds the muscle throughout.

• Connective Tissue of the Muscles- The connective tissue is usually continuous in association with the tendons.

What Is Muscle Architecture or Fascicular Architecture?

Muscle fibers are grouped into fascicles, which in turn, are grouped together to form a muscle. The size (length) and the number of fascicles determine the strength and range of movement of a muscle. Common fascicle patterns are as follows:

• Parallel Fascicles - Parallel fascicles have the typical long axis patterns placed parallelly with each fascicle possessing a flat shape of strap-like shape.

• Fusiform Fascicles - When the parallel fascicles bulge or appear spindle-shaped, they are called fusiform fascicles.

• Circular Fascicles - These fascicles have a concentric or rather circular ring within a ring pattern.

What Is the Action of Muscles?

Broadly, when a muscle contracts, it shortens around one-third (30 %) of its belly length and brings about movement. The range of movement depends on the length of fleshy fibers, and the power or force of movement on the number of fibers. However, the actual behavior of muscle contractions is more complex. During muscle contraction, when the length of the muscle tends to decrease, they are called isotonic contraction. When the length remains unchanged, it is called an isometric contraction. Several muscles hence may usually influence a particular body movement.

What Is the Henneman Principle?

The neurons that are involved in generating the action potentials to varying power-producing muscle fibers are also known as alpha-motoneurons. This alpha-motor neuron sends the long branch (the axon) to target muscles wherein the axon ends with further relatively short branches. Thus each of these terminal branches makes a connection innervating specific muscle fibers. In healthy muscles, each muscle fiber is only innervated by usually one terminal branch. The muscle fibers forming groups belong to a particular alpha-motor neuron innervating all the fibers from a group. When the alpha-motor neurons are involved in generating the impulse, then the muscle fibers get automatically innervated by receiving the excitatory signals arising from them. Hence, an alpha-motor neuron and all the muscle fibers it innervates are not different entities but are what can be viewed as a unit of singular muscle action called a "motor unit". These are the pyramidal and extrapyramidal systems that help in the stabilization of motor and neuromuscular control throughout the human body.

It is the pyramidal system that is responsible for initiating as well as and controlling voluntary movements as opposed to the extrapyramidal system that controls involuntary movements. During natural contractions thus, with increasing force production, the motor units get recruited in an organized manner, starting from the smallest of slower motor units, and then, with increasing force requirements, involving larger and larger or faster motor units. This rule is called or known as the size principle or the Henneman principle.

The total muscle force in this principle is a representation or superposition of the force produced by all the motor units within the muscle. Each motor unit can thus change its contribution to muscle force by changing the frequency of the action potentials which are generated by alpha-motoneuron causing the muscle force modulations or changes in the number of recruited motor units.

This gives us insight medically into two main methods of force modulation for the muscles:

1. By recruitment of motor units.

2. By changes in the frequency due to action potentials of these motor units.

Which Muscles Are Responsible for Head and Neck Movements?

1. Muscles of Facial Expression - A number of muscles are involved in creating facial expressions and body language. The occipitalis draws the scalp back. Frontalis raises eyebrows and wrinkles the skin of the forehead. The zygomaticus muscles are involved in smiling and laughing. The levator labii superioris raises the upper lip and the levator labii superioris alaeque nasi also raises the upper lip and dilates the nostrils. The orbicularis oris closes lips and the buccinator compresses the cheek.

2. Muscles of the Eye - The muscles that move the eyes are unique in that they do not insert into the bone. Instead, they insert it into the eyeball. The superior rectus raises the eye. The inferior rectus lowers the eye. The medial rectus rolls the eye medially. The lateral rectus rolls the eye laterally. The superior and inferior oblique muscles rotate the eyeball on an axis.

3. Muscles of Mastication - Mastication or chewing is caused by some very strong muscles. The masseter and the temporalis are the main muscles that close the jaw by bringing up the mandible in a bite grip. They are assisted by the medial and lateral pterygoid muscles.

4. Muscles Moving the Head - The main muscle that moves the head is the sternocleidomastoid muscle. Contraction of both sternocleidomastoid muscles causes flexion of the neck, and contraction of one at a time results in rotation to the left or right.

Conclusion

To conclude, understanding the musculature of the head and neck gives the physician or dentist maximum insight into the coordinated neurologic and subtle muscular mechanisms essential for orofacial expressions, and movements. It is also important in the diagnosis or identification of oral motor disorders of the head and neck.