Structure of Muscle (and associated connective tissues)

Periosteum is the outer layer of bone (as illustrated below).

It is to this layer that ligaments and tendons are attached.

Tendons attach muscle to bone.

They are tough pale coloured (whitish) cords formed from many parallel bundles of collagen fibres. Tendons are flexible (they bend around other tissues, changing position as they move), yet inelastic.

Some tendons are surrounded by tubular double-layered sacs that are lined with synovial membrane and contain synovial fluid. These structures are called 'tendon sheaths'. Their purpose is to minimise friction associated with movement at the join, and to facilitate movement of the joint.

The word 'fascia' means bandage, which fits as the tissue called fascia takes the form of sheets or broad bands of fibrous connective tissue that cover muscles or organs, forming an outer-wrapping.

There are two types of fascia:

1. Superficial Fascia, and
2. Deep Fascia.

Superficial fascia consists of areolar connective tissue and adipose tissue, and may also be referred to as the 'subcutaneous layer' of the skin.

Deep Fascia is more relevant to the study of muscle structures because it is deep fascia that holds the muscles together. It consists of dense fibrous connective tissue.

The type of muscle that causes movement of the skeletal system (especially limbs), and of skin in the cases of the muscles of facial expression in the head and neck area has many names. These include:

• 'Skeletal muscle' because it moves bones
• 'Voluntary muscle' because it is usually under conscious control
• 'Striated muscle' due to its striped appearance

Perimysium is a fibrous sheath that surrounds and protects bundles of muscle fibres.

(It is shown as thin pale grey lines in the cross-section of skeletal muscle illustrated above.)

Epimysium is fibrous elastic tissue that surrounds muscle.

There are usually many muscle fascicles that form a single muscle.

Epimysium surrounds the total bundle of many fascicles compared with perimysium (the fibrous sheath that surrounds and protects individual fascicles, filling the spaces between the fascicles within the bundle of fascicles that forms the muscle itself), and endomysium (the fine connective tissue that surrounds and protects each individual muscle fibre, also known as a 'muscle cell', hence filling the spaces between muscle fibres within each muscle fascicle).

The word fascicle (sometimes expressed as a 'fasciculus'), refers to a 'bundle', such as a bundle of muscle fibres e.g. as illustrated above, or alternatively a bundle of nerve fibres.

Endomysium is the fine connective tissue sheath that surrounds and encloses each individual muscle fibre.

Muscle fibres, also known as 'muscle fibers' (American spelling) and 'muscle cells', are special cells that can contract causing movement of other tissues / parts of the body.

There are three types of muscle:

1. Striated / skeletal muscle (causing the movement of bones / limbs)
2. Smooth muscle (surrounding organs and blood vessels), and
3. Cardiac muscle (forming the walls of the heart).

Myofibrils are small contractile filaments located within the cytoplasm of striated muscle cells. These filaments cause the distinctive appearance of skeletal = voluntary = striated muscle because they consist of bands of alternating high and low refractive index, which gives the muscles their striped appearance