Anatomy and Function of Skeletal Muscles

Muscle Pairs: Agonists and Antagonists

Muscles rarely work in isolation. To produce controlled movement, they function in antagonistic pairs.

• Agonist: The primary muscle responsible for a specific movement (the "prime mover").
• Antagonist: The muscle that opposes the movement of the agonist. To allow the agonist to contract, the antagonist must relax or stretch.

For example, in the arm, the biceps brachii and triceps brachii act as opposing forces to control elbow flexion and extension.

Regional Anatomy

The Rotator Cuff

The shoulder is a highly mobile but inherently unstable joint. To keep the humerus seated within the glenoid cavity, a group of four muscles known as the rotator cuff provides essential stability.

A common mnemonic used to remember these muscles is SITS:

1. Supraspinatus
2. Infraspinatus
3. Teres minor
4. Subscapularis

Abdominal Muscles and the Trunk

The abdominal wall is composed of several layers of muscle that facilitate trunk movement and protect internal organs. Key muscles include the external oblique and the internal oblique.

The integrity of these muscle layers and the associated inguinal ligament is clinically significant; a weakness in this anatomical region can lead to a hernia, where internal tissue protrudes through the abdominal wall.

Neuromuscular Anatomy

Muscle contraction is not just a mechanical event; it is a neurological one. The connection between the nervous system and the muscular system is known as neuromuscular anatomy.

Cranial Nerves and Muscle Control

In the head and neck region, muscles are controlled by specific cranial nerves. These nerves transmit electrical impulses from the brain to the muscles of the face, jaw, and throat, allowing for complex expressions and functions like swallowing.

The Cross-Bridge Cycle

At the microscopic level, the excitation of a muscle leads to the "cross-bridge cycle," where molecular motors within the muscle fibers pull on actin filaments to shorten the muscle fiber.

Exam Note: When studying muscle excitation, ensure you can differentiate between the electrical signal (action potential) and the mechanical result (the cross-bridge cycle).