The muscles of the chest and back

Introduction

The human muscular system is a complex network that enables movement, maintains posture, and regulates bodily functions. This comprehensive study will delve into the essential aspects of the muscles located in the chest (pectoral muscles) and back (dorsal muscles), providing an in-depth understanding of their structure, function, and interconnections.

Overview

Understanding the muscular system requires a thorough grasp of its various components, including origins, insertions, actions, innervations, and vascular supplies. This course will elucidate these aspects for the muscles in question while highlighting their role in overall human movement and posture.

Learning Objectives

Upon completion of this course, students should be able to:

1. Describe the anatomy, histology, and physiology of the pectoral and dorsal muscles.
2. Identify the origins, insertions, actions, innervations, and vascular supplies of these muscles.
3. Explain the role of these muscles in human movement, posture, and overall functionality.
4. Understand the functional relationships between the pectoral and dorsal muscles and other muscular systems.
5. Recognize the clinical implications of dysfunctions or injuries affecting these muscle groups.

Anatomy of the Chest Muscles (Pectoralis Major and Minor)

Pectoralis Major

Origins, Insertions, and Actions

Origin

The pectoralis major originates from:

• The sternum (costal and manubrium)
• The clavicle (anterior surface)
• The anterior surface of the 3rd to 5th ribs

Insertion

The pectoralis major inserts onto the greater tubercle of the humerus.

Actions

• Flexion of the arm at the shoulder joint (flexion adduction)
• Medial rotation of the forearm (internal rotation)

Innervation and Vascular Supply

Innervation

The pectoralis major is innervated by the medial (or upper) pectoral nerve, a branch of the anterior division of the brachial plexus.

Vascular Supply

The pectoralis major is supplied by:

• The thoracic acromial vessels (branch of the subscapular artery)
• The pectoral branch of the 2nd to 4th intercostal arteries
• The musculophrenic artery (branches from the axillary artery)

Pectoralis Minor

Origins, Insertions, and Actions

Origin

The pectoralis minor originates from:

• The 3rd to 5th ribs (costal cartilages)
• The coracoid process of the scapula

Insertion

The pectoralis minor inserts onto the medial (or anterior) surface of the 2nd and 3rd costal cartilages, as well as the suprasternum.

Actions

• Assists in the elevation of the ribs during inspiration (inspiration)
• Helps to depress the scapula (scapular depression)

Innervation and Vascular Supply

Innervation

The pectoralis minor is innervated by the medial (or upper) pectoral nerve, a branch of the anterior division of the brachial plexus.

Vascular Supply

The pectoralis minor is supplied by:

• The thoracic acromial vessels (branch of the subscapular artery)
• The pectoral branches of the 2nd to 4th intercostal arteries

Anatomy of the Back Muscles (Latissimus Dorsi, Rhomboids, Trapezius, Erector Spinae)

Latissimus Dorsi

Origins, Insertions, and Actions

Origin

The latissimus dorsi originates from:

• The thoracic vertebrae (T3 to T12)
• The lower 4 or 5 ribs
• The lumbar vertebrae (L1 to L5)
• The spinal fascia
• The sacrotuberous ligament

Insertion

The latissimus dorsi inserts onto the intertubercular groove of the humerus.

Actions

• Extension of the arm at the shoulder joint (extension adduction)
• Adduction of the arm at the shoulder joint
• Medial rotation of the forearm (internal rotation)

Innervation and Vascular Supply

Innervation

The latissimus dorsi is innervated by the thoracodorsal nerve, a branch of the posterior division of the brachial plexus.

Vascular Supply

The latissimus dorsi is supplied by:

• The subscapular artery (branch of the axillary artery)
• The thoracolumbar arteries (arteries from the intercostal and lumbar systems)

Rhomboid Muscles

Origins, Insertions, and Actions

Origin

The rhomboids originate from:

• The 5th and 8th vertebrae
• The spinous processes of the 2nd to 7th thoracic vertebrae

Insertion

The rhomboids insert onto:

• The medial (or inner) borders of the scapula (scapular border)

Actions

• Assist in the depression and retraction of the scapula (scapular depression, retraction)
• Help to elevate the ribs during inspiration (inspiration)

Innervation and Vascular Supply

Innervation

The rhomboids are innervated by:

• The dorsal primary rami of the cervical nerves (C5, C4)
• The dorsal primary rami of the thoracic nerves (T1 to T3)

Vascular Supply

The rhomboids are supplied by:

• The branches from the dorsal scapular artery (a branch of the subclavian artery)
• The intercostal and posterior intercostal arteries (arteries from the thoracic system)

Trapezius

Origins, Insertions, and Actions

Origin

The trapezius originates from:

• The occipital bone
• The spinous processes of the cervical vertebrae (C7 to T12)
• The lateral third of the acromion process of the scapula

Insertion

The trapezius inserts onto:

• The medial (or inner) border of the scapula (scapular border)
• The spinous processes of the cervical vertebrae (C7 to T12)

Actions

• Assist in the elevation, depression, and retraction of the scapula (elevation, depression, retraction)
• Help rotate the neck (rotation of the head)

Innervation and Vascular Supply

Innervation

The trapezius is innervated by:

• The accessory nerve (cranial nerve XI)
• The cervical plexus (C3, C4 nerves)

Vascular Supply

The trapezius is supplied by:

• The transverse cervical artery (a branch of the external carotid artery)
• The dorsal scapular artery (a branch of the subclavian artery)

Erector Spinae

Origins, Insertions, and Actions

Origin

The erector spinae originate from:

• The spinous processes of the vertebrae (cervical to lumbar regions)
• The transverse processes of the thoracic and lumbar vertebrae

Insertion

The erector spinae insert onto:

• The sacrum and coccyx (iliolumbar ligament, sacrotuberous ligament)

Actions

• Assist in the extension of the spine (extension)
• Help maintain posture by providing stability to the vertebral column (postural support)

Innervation and Vascular Supply

Innervation

The erector spinae are innervated by:

• The dorsal primary rami of the cervical, thoracic, and lumbar nerves (C1 to L5)

Vascular Supply

The erector spinae are supplied by:

• Branches from the vertebral arteries (cervical region)
• Branches from the costocervical trunk (thoracic region)
• Branches from the lumbar arteries (lumbar region)

Functional Relationships and Clinical Implications

Functional Relationships

Understanding the functional relationships between these muscles is crucial for comprehending their role in overall human movement and posture. For example, the pectoralis major works in concert with the latissimus dorsi during pulling movements such as rowing or climbing. Similarly, the trapezius coordinates with the rhomboids to stabilize the scapula during arm movements.

Clinical Implications

Dysfunctions or injuries affecting these muscle groups can lead to various impairments and discomforts. For instance, an imbalance between the pectoralis major and latissimus dorsi may result in postural problems and limited range of motion in the shoulders. Similarly, issues with the rhomboids can cause shoulder pain or even referred pain up the neck. Understanding these potential complications is essential for identifying and addressing them promptly to ensure proper recovery and overall muscular health.