Axial Muscle Anatomy and Functional Organization
General Terminology and Functional Groups
Attachment Terminology:
Muscles are often described using the terms origin and redemption (likely a phonetic error in the transcript for insertion) to denote their connection points.
Alternatively, the terms proximal versus distal attachments may be used to describe where a muscle connects to the skeleton.
In the current axial skeleton curriculum, students are not required to memorize specific origins and insertions, though this is a major focus in advanced Occupational Therapy (OT) and Physical Therapy (PT) programs.
Functions of the Axial Skeletal Muscles:
Support of the head and spinal column.
Facial expressions and communicative gestures.
Vital processes such as chewing (mastication), swallowing, and breathing.
Excretory and reproductive functions including defecation, urination, coughing, and supporting the pelvic floor.
Pelvic floor muscle tone is directly correlated with the ability to control the bowels and urination.
Organizational Classification: Axial muscles are organized into five primary groups:
Head and neck muscles.
Vertebral column muscles. . Respiratory muscles.
Abdominal wall muscles.
Pelvic floor muscles.
Axio-appendicular Muscles: These muscles serve as a bridge or crossover, connecting the appendicular skeleton to the axial skeleton. Their origin sites are typically along the axial skeleton (such as the skull or spinal processes) while they insert on the appendages.
Anterior Axio-appendicular Muscles: Includes the pectoralis major, pectoralis minor, serratus anterior, and the subclavius (located beneath the clavicle).
Posterior Axio-appendicular Muscles: Includes the trapezius, latissimus dorsi, rhomboid major, rhomboid minor, and levator scapulae.
Head and Neck Muscles: Facial Expression
Control and Innervation: All facial muscles are controlled by the facial nerve (Cranial Nerve VII). This nerve has five distinct branches (including the mandibular and maxillary branches) which innervate different regions of the face.
Characteristics of Facial Muscles: Unique among skeletal muscles, these attach to the skull bones and then directly to the skin, allowing for the manipulation of skin to create expressions.
Specific Facial Muscles and Functions:
Frontalis: Located on the frontal bone. Its primary action is wrinkling the forehead. It is part of a larger structure called the occipitofrontalis, which consists of two muscles connected by an aponeurosis.
Orbicularis Oculi: A circular (orbicularis) muscle located in the ocular region. Functions include closing the eyes, blinking, and winking.
Orbicularis Oris: A circular muscle in the oral region. Functions include closing the mouth and puckering the lips for kissing.
Zygomaticus Major: Known as the "smiling muscle."
Zygomaticus Minor: Works with the major; the space between these two muscles can lead to the formation of dimples if a hollowed portion exists.
Platysma: A very thin, superficial muscle of the neck. It becomes weak with age, leading to the "banding" appearance of the neck. It tenses when the neck muscles are strained.
Depressor Anguli Oris: Depresses the sides of the mouth at an angle (frowning).
Levator Labii Superioris and Levator Anguli Oris: Muscles that elevate the mouth or upper lip.
Rhizorius: Mentioned as a muscle of the face.
Muscles of Mastication (Chewing)
All four major muscles of mastication are attached to the mandible.
Masseter: Attached to the mandible; its primary role is to raise and lower the jaw. It is often visible in the jawline when someone is chewing or clenching their teeth in anger.
Temporalis: Located in the temple region; also aids in elevating the mandible.
Pterygoid Muscles (Medial and Lateral): These are deep muscles located beneath the temporalis and masseter.
Pronunciation: The "p" is silent ( is pronounced like ).
Action: These muscles move the mandible from side to side and are involved in protraction.
Intrinsic and Extraocular Muscles of the Eye
Total Count: There are six muscles for each eye, totaling muscles for both eyes.
The Recti Muscles (Straight Fascicles):
Medial Rectus: Closer to the nasal cavity.
Lateral Rectus: Closer to the temporal bone.
Superior Rectus: Located on the top of the eye, closer to the brain.
Inferior Rectus: Located on the bottom of the eye.
The Oblique Muscles (Angled/Twisting):
Superior Oblique: This muscle passes through a structural pulley called the trochlea. Even though its fascicles are in a straight arrangement, the mechanics of the trochlea cause the muscle to pull the eye at an angle/rotate it upon contraction.
Inferior Oblique: Causes a twisting/rotating motion of the eye without a trochlea.
Anterior Neck Muscles and the Sternocleidomastoid
Hyoid Bone Attachments: The hyoid bone serves as an anchor for eight muscles.
Suprahyoid Muscles: Four muscles located superior to the hyoid (including the mylohyoid, digastric—which has anterior and posterior bellies, and the geniohyoid). These assist in swallowing and speaking.
Genioglossus: Identified as the muscle of the tongue. It is considered both a muscle and an organ due to its complex innervation and blood supply.
Infrahyoid Muscles: Muscles located inferior to the hyoid, lying on top of the larynx.
Sternocleidomastoid (SCM):
Composition: Has two "heads" or ends: the sternal end and the clavicular end.
Attachments: It attaches to the mastoid process of the temporal bone, the sternum, and the clavicle.
Action: It is referred to as the "selfie muscle." When the left SCM contracts, it turns the head to the right. It acts as the primary muscle for turning the neck, though it works alongside the trapezius and scalene muscles.
Muscles of the Vertebral Column
Erector Spinae Group: These muscles are responsible for keeping the body standing erect and maintaining posture.
Mnemonic for the Group: "I Love Spinach" (Iliocostalis, Longissimus, Spinalis).
Iliocostalis: The most lateral group. It consists of the lumborum, thoracis, and cervical parts.
Longissimus: The middle group. It consists of the capitis (head), cervical, and thoracic parts.
Spinalis: The most medial group (closest to the spine). It and its related semispinalis consist of the capitis, cervical, and thoracic parts.
Transversospinalis: A group of deep, intricate muscles located beneath the erector spinae, including the multifidus and rotatores.
Quadratus Lumborum: A muscle of the lower back that also acts as a hip flexor alongside the iliopsoas (composed of the iliacus and psoas major). If this muscle is chronically tight (often due to prolonged sitting), it causes an anterior pelvic tilt, leading to an exaggerated lumbar curvature and potential displacement of the intervertebral disc.
Muscles of Respiration
The Diaphragm: The primary muscle of breathing.
Anatomical Role: It divides the thoracic cavity from the abdominopelvic cavity.
Movement Mechanics:
Relaxed State: The diaphragm is in a dome shape, indicating exhalation.
Contracted State: The diaphragm flattens and becomes more horizontal, which expands the thoracic cavity and causes inhalation.
Openings: The central tendon has three major openings for the inferior vena cava, the esophagus, and the aorta.
Non-respiratory Roles: Participation in defecation, urination, childbirth, and the "respiratory pump" which moves venous blood back to the heart.
Intercostal Muscles:
External Intercostals: Start at the vertebral column and wrap approximately of the way around the ribs toward the front. Used for inhalation.
Internal Intercostals: Start at the sternum and wrap approximately of the way around toward the back. Used for exhalation.
Other involved muscles include the scalenes and the transversus thoracis.
Muscles of the Abdominal Wall
Rectus Abdominis: The medial-most muscle; essentially the "six-pack." It is covered by a thick connective tissue called the rectus sheath (an aponeurosis).
External Oblique: The superficial lateral muscle. Its fibers run inferomedially (down and toward the midline).
Internal Oblique: Deep to the external oblique. Its fibers run superomedially (up and toward the midline).
Transversus Abdominis: The deepest layer of the abdominal wall. It serves as a "built-in safety belt" or weight belt. Bracing the core involves contracting this muscle to protect the lumbar vertebrae during heavy lifting.
Pelvic Floor Muscles
Functions include controlling defecation, urination, and reproductive processes.
Key Muscles: Includes the levator ani (composed of multiple muscles) and the obturator internus.
Clinical Connection: Gluteal strength (gluteus maximus, medius, and minimus) indirectly supports and stabilizes the pelvic floor.
Questions & Discussion
Q: Can we review eye contraction mechanics one more time?
A: To look to the right: The lateral rectus of the right eye contracts (shortens), while the medial rectus of the right eye stretches. Simultaneously, the medial rectus of the left eye contracts (shortens) to pull that eye toward the right.
Q: Does "contract" mean the muscle shortens or expands?
A: Contraction means the muscle shortens. The opposing muscle stretches.
Q: Is the tongue an organ or a muscle?
A: It is both. It is a muscle with its own blood supply and complex innervation.
Q: What is the most deepest muscle of the abdominal wall?
A: The transversus abdominis.
Q: What is the medial most muscle of the abdominal wall?
A: The rectus abdominis.
Q: Are there any tricky spelling issues to watch out for like the tibia/fibula?
A: Not specifically tricky, but students often forget to include "brachii" when naming the biceps or triceps. It is essential to include the full name: biceps brachii or triceps brachii.
Exam Laboratory Preparations
Exam Date: Wednesday (following July 4th weekend).
Required Study Materials:
Chapter 10: Slide 8, Image A (Muscle organization model).
Chapter 11: Slides 5, 6, 7, and 8. These include figures 11.1 a, b, c, d, covering both axial and appendicular muscles.
Slide 11 (facial muscles) has been explicitly removed from the lab exam requirements to lessen the load.
Study Recommendations: Print the slides, use Post-it notes to cover labels, and practice identifying the muscles. Focus on distinguishing anterior versus posterior views and identifying duplicates like the deltoid, which appears on both sides.