Macroscopic organization and sensitive innervation of the tendinous intersection and the lacertus fibrosus of the biceps brachii muscle in the ass and horse.

This research reveals the unique structures and nerve supply of the biceps brachii muscle in donkeys and horses and discusses how receptors in these parts may play a role in coordinated movement of the limb joints.

Muscle Structures

• The study identifies a well-developed and organized lamina, or flat structure, running through the biceps brachii muscle belly in donkeys and horses. This structure forms a major tendinous intersection that connects the proximal (closer to the center of the body) and distal (further from the center) tendons of the muscle.
• The lacertus fibrosus, another structure significant to the muscle, arises from this lamina near the distal tendon. This fibrous band merges with the fascia, a sheet of connective tissue, of the forearm and joins the extensor carpi radialis muscle, which aids in extending and abducting the wrist.

Nerve Supply and Receptors

• The nerves that supply the biceps tendinous intersection originate from the intramuscular branches of the musculo-cutaneous nerve, which innervates certain muscles of the upper limb and sends sensory information from the skin of the forearm to the brain.
• The lacertus fibrosus receives its nerve supply from the cutaneous branches of the same nerve. Cutaneous nerves are responsible for detecting sensations such as touch, pain, and temperature on the skin’s surface.
• Both the biceps brachii tendinous intersection and lacertus fibrosus are supplied with free and encapsulated nerve endings, with the encapsulated corpuscles represented by several types of receptors sensitive to mechanical pressure and vibration, including rare Pacinian and Ruffini receptors and numerous Golgi tendon organs.

Proposed Function of Proprioceptors

• The researchers hypothesize about the role these proprioceptive parts play. Proprioceptors are sensors in our body that provide information about joint angle, muscle length, and tension, all of which are crucial to our sense of body position and movement.
• The study suggests these proprioceptors in the biceps brachii may coordinate shoulder, elbow, and wrist movements during both flexion (bending) and extension (straightening) of the limb in these animals. This coordination is mediated by the continuous relay of sensory information from these receptors to the brain.