Muscles of the Trunk

Learning Outcomes

Identify the muscles of the thoracic wall and state their function and nerve supply.
Identify the muscles of the abdominal wall and state their action and nerve supply.
Integrate the knowledge of the back and trunk muscles to describe the movements of the back.

Muscles of the Thoracic Wall

The thoracic wall is structured as an irregularly shaped cylinder with a superior thoracic aperture and a larger inferior thoracic aperture, which is closed by the musculotendinous diaphragm. The thoracic cavity contains three compartments: the left and right pleural cavities and the mediastinum.

Intercostal spaces are located between adjacent ribs and are populated by intercostal muscles. The intercostal nerves, arteries, and veins are found in the costal groove along the inferior margin of the superior rib, making their way through the planes of the two inner muscle layers.

Intercostal Muscles

These muscles assist with the movement of the ribs during respiration:

Subcostal Muscles: Located in the same plane as the innermost intercostal muscles, spanning multiple ribs, especially prevalent in the lower areas of the posterior thoracic wall. These muscles aid in elevating the ribs.
Transversus Thoracis: Positioned deeper on the anterior thoracic wall, it originates from the xiphoid process and the inferior body of the sternum. Its fibers run superiorly and laterally, attaching to the costal cartilages of ribs 3-6 and function to depress the ribs.

Respiration Mechanics

Respiration is governed by the pressure difference between the lungs and the atmosphere. During inhalation (inspiration), there is a reduction in alveolar pressure, which causes the lungs to expand. This expansion is facilitated by the contraction of the diaphragm and the elevation and outward movement of the rib cage, supported by intercostal muscles and accessory muscles of inspiration like the sternocleidomastoid, scalene muscles, and pectoralis minor. Conversely, during exhalation (expiration), the process reverses, leading to an increase in alveolar pressure.

Muscles of the Abdominal Wall

The abdominal wall comprises several components, including skin, superficial fascia, muscle layers, deep fascias, extraperitoneal fat, and the parietal peritoneum. It also provides structural support through five lumbar vertebrae and associated structures of the pelvic bones and thoracic cage.

Muscle Composition

The abdominal wall has three primary areas featuring various muscle groups:

Posterior Muscles:
- Psoas Major and Minor
- Quadratus Lumborum
- Iliacus
Lateral Muscles:
- External Oblique: A flat muscle that initiates from the posterior lateral aspect.
- Internal Oblique: Positioned deeper than the external oblique.
- Transversus Abdominis: The innermost layer that lies horizontally and is critical for core stability.
Anterior Muscles:
- Rectus Abdominis: Positioned centrally, creating the 'six-pack' appearance.
- Pyramidalis muscles: Small muscles located in front of the rectus abdominis.

Function of Abdominal Muscles

Muscles of the anterolateral abdominal wall perform several key functions:

Compress and support abdominal viscera.
Facilitates respiratory movements.
Flex and rotate the trunk (notably by the external and internal obliques).
Rectus abdominis primarily flexes the trunk.

Blood Supply and Nerve Supply

The abdominal wall is supplied by a neurovascular bundle positioned between the internal oblique and transversus abdominis muscle layers. The anterior abdominal wall's nerve supply comes from anterior rami of T7-T11 spinal nerves, the subcostal nerve (T12), iliohypogastric nerve, and the ilioinguinal nerve.

Dermatomes are categorized as follows:

T4 corresponds to the nipple level in males.
T10 is around the umbilicus.
T12 signifies the suprapubic region.
L1 covers the inguinal area.

Movements of the Back and Verterbral Column

The movements of the vertebral column greatly depend on the specific region and individual anatomical variations. The mobility is influenced by several factors:

The compressibility and elasticity of the intervertebral discs.
The thickness and orientation of articular processes.
The tension within the facet joint capsules.
The resistance from muscles and ligaments in the back.
Gravity and assistance from the abdominal wall.

Cervical Region Movements

Movement in the cervical region is quite free due to the following attributes:

Thick intervertebral discs relative to vertebral body size.
Articular processes are nearly horizontal and large.
Loose articular capsule of facet joints.
The slender nature of the neck with minimal surrounding tissue.
Flexion occurs most freely in this region.

Thoracic Region Movements

In the thoracic spine:

Primarily rotation occurs due to:

Thin intervertebral discs relative to body size.
Vertical orientation of articular processes inhibiting flexion and extension.
Ribs providing stability that also limits movement.

lumbar Region Movements

In the lumbar region, movements are also free, characterized by:

Thicker intervertebral discs compared to body size.
Vertical articular processes that allow substantial flexion and extension.
The unique configuration that limits rotation while maximizing extension.

Muscle Anatomy and Function of the Trunk

Muscle Anatomy and Function of the Trunk