Skeletal Muscle Structure: Connective Tissue, Attachments, and Fascicle Orientation

Skeletal muscles are organs composed of muscle fibres (cells) contained within connective tissue.
The muscle also contains blood vessels, lymphatic vessels, and nerves.
Connective tissue supports, surrounds, protects, and provides pathways for vessels and nerves.

Three concentric layers surround skeletal muscle and work together to protect and strengthen it:
- Epimysium: dense, irregular connective tissue; completely surrounds the entire muscle.
- Perimysium: dense, irregular connective tissue; surrounds a small group of muscle fibres called a fascicle.
- Endomysium: composed predominantly of reticular fibres; separates each individual muscle fibre from one another.
All three layers are continuous with connective tissue that attaches muscle to other structures (bone or another muscle).
In some cases, these connective tissue layers extend beyond the muscle fibres to form a tendon.

Tendon: connective tissue extension that connects muscle to the periosteum of bone.
Example: the calcaneal (Achilles) tendon connects the gastrocnemius (calf muscle) to the calcaneus (heel bone).
Aponeurosis: a broad, flat sheet of connective tissue that can extend from muscle to attach to bone or other structures.
Example: the rectus sheath of the abdominal muscles is an aponeurosis.

Muscles can attach indirectly or directly to bones/structures:
- Indirect attachment:
- The muscle ends short of the bone and is connected to the bone via a tendon.
- Common examples: biceps brachii, palmaris longus, flexor carpi radialis.
- Broad connective tissue extensions contributing to indirect attachment include the palmar aponeurosis.
- Direct attachment:
- There is little separation between the muscle and the bone; attachment appears to be direct.
- Example: the lateral head of the triceps brachii.
- Microscopically, a very small gap exists and is filled with collagen fibres.
Note: The majority of skeletal muscles are attached to a bone at each end and cross at least one joint.

When a muscle moves, it moves one bone relative to another.
Origin: the bony site of attachment at the end that is relatively stationary.
Insertion: the attachment at the more mobile end.
Example: biceps brachii
- Origin: scapula (more stationary)
- Insertion: radius (moves during flexion of the elbow)

The strength and direction of pull are partly determined by fascicle orientation.
Five types of fascicle orientation:
- Fusiform: thick in the middle, tapered ends.
- Examples: biceps brachii, gastrocnemius.
- Parallel: relatively uniform width; fascicles run parallel.
- Example: sartorius (thigh).
- Features: can cover long distances; shorten less than other types; fewer fibers; produce less force.
- Triangular (convergent): broad origin, converges at insertion.
- Example: pectoralis major.
- Why strong: many fibres in the wider part.
- Pennate: feather-like fascicles that insert obliquely on a tendon running the length of the muscle.
- Subtypes:
  - Unipennate: fascicles approach tendon from one side.
  - Example: palmar interosseous muscle (hand).
  - Bipennate: fascicles approach tendon from both sides.
  - Example: rectus femoris.
  - Multipennate: multiple feather-like sections converging on a single tendon.
  - Example: deltoid (shoulder).
- Circular (sphincters): fibres arranged in a circular pattern forming rings around openings.
- Function: constrict or regulate passage through openings.
- Example: anal sphincter.

Some tendons pass under bands of connective tissue called retinacula.
Example: tendons of several forearm muscles pass under a retinaculum on their way to the hand.

Connective tissue layers (epimysium, perimysium, endomysium) transmit the force of muscle contraction to the tendon and ultimately to bone.
The continuity of connective tissue from muscle to tendon to bone helps distribute and regulate mechanical load.
Indirect attachments via tendons or aponeuroses allow muscles to span joints and apply force efficiently without requiring a direct bone-to-muscle contact.
Direct attachments are less common but provide a close, intimate connection between muscle and bone.
Fascicle arrangement influences both the range of motion and the force output of a muscle:
- Fusiform and parallel muscles favor range and speed with fewer fibres in some cases.
- Pennate and multipennate muscles pack more fibres into a given area, increasing force production at the expense of shortening distance.
The arrangement of tendons and aponeuroses, as well as the presence of retinacula, optimize force transfer and tendon routing to the appropriate bones and joints.