Connective Tissue Sheaths

Connective Tissue Sheaths

• Skeletal muscle fibers are held together by connective tissue sheaths.
• These sheaths include the epimysium, perimysium, and endomysium.
• The epimysium is the outermost layer of dense, irregular connective tissue that surrounds the entire muscle.
• The perimysium surrounds each fascicle, which is a group of muscle fibers bundled together.
• The endomysium is a fine sheath of loose connective tissue that surrounds each muscle fiber within a fascicle.

Function of Connective Tissue

• The fibrous connective tissues bind muscle fibers, holding them in parallel alignment, enabling them to work together to produce force.
• The endomysium merges with the perimysium, which is continuous with the epimysium.
• The three sheaths converge to form the tendon, which connects skeletal muscles to bones.
• Muscle fiber contraction pulls on the endomysium, which in turn pulls on the perimysium, epimysium, and tendon, transmitting the force to the bone being moved.
• The sheaths provide elasticity to the muscle and carry blood vessels and nerves.

Nerves and Blood Vessels

• Each skeletal muscle is supplied by one nerve, one artery, and one or more veins, entering/exiting near the muscle's midpoint.
• These nerves and vessels branch in the intramuscular connective tissue, serving individual muscle fibers.
• The rich blood supply to muscles supports the nutrient and oxygen demands of contracting muscle fibers.
• Capillaries in the endomysium form a network that is wavy during muscle fiber contraction and straight when the muscle extends.

Muscle Attachments

• A muscle attachment is where a muscle connects to a bone.
• Skeletal muscles extend from one bone to another, crossing at least one movable joint.
• Muscle contraction causes one bone to move while the other typically remains fixed.
• The origin is the muscle attachment on the less movable bone.
• The insertion is the attachment on the more movable bone; it is pulled toward the origin during contraction.
• In limb muscles, the origin is the more proximal attachment, and the insertion is the more distal attachment.
• The functions of the origin and insertion can switch, depending on body position and movement.

Types of Muscle Attachments

• Muscles attach to origins and insertions via fibrous connective tissues that extend into the bone's periosteum.
• Direct (fleshy) attachments involve short connective tissue strands, making it appear as if muscle fascicles attach directly to the bone.
• Indirect attachments involve connective tissue extending beyond the muscle fibers to form a cord-like tendon or a flat sheet called an aponeurosis.
• Indirect attachments are more common, and most muscles have tendons.
• Raised bone markings, like tubercles, trochanters, and crests, are often present where tendons meet bones.
• Some tendons and aponeuroses attach to skin, cartilage, fascia sheets, or a seam of fibrous tissue called a raphe.

Multi-Joint Muscles

• Biarticular or multijoint muscles span two or more joints.
• Contraction of a muscle spanning multiple joints causes movements at those joints.