Chapter 10: Muscular System: Axial Musculature

Muscular System: Axial Musculature muscles are logically organized into four primary groups based on location and function. The first group, the muscles of the head and neck, are crucial for verbal and nonverbal communication, as well as feeding movements. This complex region includes the muscles of facial expression (such as the orbicularis oris, buccinator, and platysma), which originate on the surface of the skull and insert into the dermis of the skin, all innervated primarily by the facial nerve (N VII). Movements related to sight are controlled by the six extra-ocular muscles (rectus and oblique groups) that position the eye, innervated by cranial nerves III, IV, and VI. Mastication (chewing) relies on powerful muscles like the masseter and temporalis, along with the pterygoid muscles, all controlled by the trigeminal nerve (N V). The muscles of the tongue (genioglossus, hyoglossus, etc.) and the pharyngeal muscles (constrictors and laryngeal elevators) coordinate the delicate and complex actions required for speech and swallowing, largely innervated by the hypoglossal (N XII), glossopharyngeal (N IX), and vagus (N X) nerves. The anterior neck muscles (e.g., sternocleidomastoid and digastric) adjust the position of the larynx and mandible. The second major group, the muscles of the vertebral column, consists of the intrinsic back muscles that interconnect and stabilize the vertebrae, innervated by the dorsal rami of spinal nerves. These intrinsic muscles are arranged in three layers: the superficial splenius muscles; the intermediate layer composed of the large spinal extensors known as the erector spinae (iliocostalis, longissimus, and spinalis groups); and the deep transversospinalis muscles (semispinalis, multifidus, rotatores) which make delicate positional adjustments. Because gravity assists in flexion, there are relatively few primary spinal flexors, such as the longus capitis and quadratus lumborum. The third group is the oblique and rectus muscles, forming the muscular walls of the thoracic and abdominopelvic cavities. The oblique muscles (including the scalenes, intercostals, and external/internal abdominal obliques) compress structures and rotate the vertebral column. The rectus abdominis, divided by tendinous inscriptions and the midline linea alba, is the key flexor of the vertebral column. Included in this group is the diaphragm, the essential muscle of respiration, which separates the thoracic and abdominopelvic cavities. Finally, the fourth group encompasses the muscles of the perineum and the pelvic diaphragm, which span the pelvic outlet to support organs, flex joints, and control sphincters in the urogenital and anal triangles, providing voluntary control over urination and defecation. Clinically, excessive pressure during activities like heavy lifting can lead to hernias, such as inguinal or hiatal hernias, where an internal organ protrudes through a weakened muscular wall or partition.