VAN 121: Veterinary Gross Anatomy-II (2+2)

Skeletal muscle

SKELETAL / STRIATED MUSCLES

• They are both directly or indirectly attached to the skeleton and hence often named as skeletal muscles
• Striated muscle is composed of long, unbranched muscle fibres, which shows cross striations under a microscope; hence it is called as striated muscle
• Contraction of this striated muscle occurs as per the will of the animal. Hence, they are also named as voluntary muscles
• Morphologically, each muscle is considered as individual organs made up of several muscle fibres. There are about 200-250 paired and few unpaired muscles present in the domestic mammals.
• A delicate connective tissue sheath, the endomycium surrounds each muscle fibre. Several muscle fibres grouped together to form fasciculus, which is covered by perimycium.
• A muscle as a whole is composed of many fasciculi and is surrounded by epimycium, which is closely associated with the fascia and sometimes fused with it.
• Each muscle consists of a central portion called belly and two ends
• Each end of the muscle is attached to bone or cartilage or to skin by means of either tendon or ligament
• When a muscle contracts and shortens, one end of its attachments usually remains fixed and the other end alone moves
• The fixed attachment is called origin; the movable one is called insertion. In the limbs, the more distal parts are usually mobile. Therefore, the distal attachment is usually called the insertion

Tendon

• A narrow band of white fibrous tissue that attaches muscle to bone or cartilage or other tissues is known as tendon
• Instead, some muscles are connected to bone or other muscles by a broad sheet of fibrous connective tissue called aponeurosis, which is often glistening. E.g. Linea alba.

Blood and nerve supply

• The muscle tissue is richly supplied with blood. Large arteries are accompanied by the veins and lymphatics
• The lymph vessels are few
• The nerve fibre reaches the muscle fibres and terminates into ramified expansions and is termed as end plates
• The nerves supplying the muscles are sensory and vasomotor
• Voluntary muscles are named according to the physical characteristics as follows:

Colour

• Based on the colour the striated muscles are classified into two types; red and white muscles
• The difference in colour is due to the amount of myoglobin and cytochrome in the muscle cell
• The red muscle is found in areas, which perform sustained and continuous work and are generally situated at the deeper aspect of the body whereas white muscles are found in areas, which perform quick and intermittent movement and located superficially

Action 

• According to the force of action, the muscles are classified as spurt muscles which provide acceleration motion to the joints. E.g. brachialis and shunt muscles which provide stabilizing centripetal force on a joint. E.g. flexor carpi radialis
• A portion of muscle name is often related to its function. Muscles that flex the joints are named flexors. E.g. flexor carpi radialis
• Muscles that extend the joint are named extensors. E.g. extensor carpi radialis

Shape

• A muscle’s name can reflect its distinctive shape. E.g. deltoideus. The term ‘deltoid’ means triangular shaped structure

Location

• A muscle’s name can indicate its physical location in the body. E.g. biceps brachii, the muscle that is located in the brachial (arm) region

Direction of muscle fibres

According to the direction of fibres the muscles are divided into

• Parallel muscles where the fibres are long and are placed parallel to the line of pull. E.g. biceps brachii
• Pennate muscles the muscle fibres are short and attached to the tendon at acute angle. This arrangement is known as pennate arrangement. They may be unipennate, bipennate and multipennate etc., according to the direction of the fibres in relation to the tendons. E.g. deltoid
• Spiral muscle: the fibres are arranged in a twisted manner. E.g. brachialis-helps rotational movements
• Cruciate muscles: in these muscles the fibres are arranged in superficial and deep plane, crossing like ‘X’. E.g. masseter
• Rectus and oblique muscles: The muscle fibres that run straight in lengthwise are named as rectus.E.g. rectus abdominis. Some muscle fibres run in an obilique direction.E.g. obliquus abdominis

Number of heads / divisions

• This refers to the number of attachment sites that a muscle has at its origin.E.g. biceps brachii, a muscle having two heads of origin

Attachment Sites

• Origin and insertions are used to name some muscles E.g. sterno-cephalicus. The origin of this muscle is sternum and insertion towards the head (cephalic)