PARTS OF A SKELETAL MUSCLE : Grossly each skeletal muscle is composed of following parts:
Origin (Proximal attachment):
It is the proximal attachment, which usually remains fixed during contraction of the muscles.
Skeletal muscles usually take origin from the following structures :- Bones (most important).
1.Fasciae (e.g., Muscles of facial expression, serratus anterior).
2.Ligaments (e.g., Trapezius from ligamentum nuchae and supraspinous ligaments).
3.Tendons (e.g., Lumbrical muscles of the hand).
4.Articular capsules (e.g., Gastrocnemius from capsule of the knee joint).
5.Intermuscular septa (e.g., Brachioradialis, extensor Capri radialis longus).
When the structures providing origin to a skeletal muscle are separated from each other by a definite interval, the muscle is said to have two or more heads of origin.
Fleshy part or Fleshy belly :
A muscle may be completely or partly fleshy throughout its entire length. It consists of skeletal muscle cells which show following properties:
1.They are highly specialized, excitable and contractile.
2.They are highly vascular.
3.They are expensive in upkeep.
4.They are resistant to infection.
5.They are not able to withstand pressure or friction.
Tendon :
It is the rounded fibrous end of a skeletal muscle. The tendons show following properties:-
1.They are unspecialized, non-excitable and inelastic.
2.They are relatively less Vascular.
3.They are inexpensive in up-keep.
4.They are more vulnerable to infection due to poor blood supply.
5.The tendon fibres are actually not parallel but wind (or twine) about each other to make tendon strong.
All tendon fibres starting from fleshy belly reach the site of insertion of muscle. Thus the pull of the whole muscle could be transmitted to any part of the insertion during movements. In other words, we can say that during different position of a joint, different tendon fibres bear the strain.
The fibres of tendons pass across a pad of white fibro- cartilage before getting insertion into a bone. This pad of cartilage prevent fraying from frequent flexing during movements.
The cross-sectional area of a particular tendon is much as compared with corresponding fleshy belly. It is therefore, clear that whenever a muscle has fleshy origin and tendinous insertion, then the fleshy origin is much more extensive. It should be kept in mind that the tendinous insertion is more important than fleshy origin because it forms a point over which the force of muscle pull is concentrated.
The tendons may produce following impressions on the bones to which they are attached :-
i)Ridges, e.g., linea aspera of femur provides attachment to many muscles.
ii)Tubercles, e.g., Adductor tubercle of the femur provides attachment to the adductor magnus muscle:
iii)Greater and lesser tuberosities of humerus which provides insertion to supraspinatus, Infraspinatus, Teres minor and subscepularis muscles.
Lesser trochanter of femur which provides attachment to the tendon of psoas major muscle.
A bursa is a purse-like closed sac containing a fluid and intervening between the tendons and other hard structures like bones, cartilages, etc.
It reduces the friction, provides lubrication device and allows free movements of tendons over hard objects. It is much more effective than the areolar tissue. These bursae are called sub-tendinous bursae. They may be constantly communicating with the synovial cavities, e.g., the subscapularis bursa at the shoulder joint.
At the site of friction or pressure Hand, Foot, etc. the tendons are provided with tubular bursae called synovial sheaths of tendons. In hand and foot the tendons are passing between bones and retinacula which are offering considerable friction. In order to allow a good range of mobility, the synovial sheaths extends 1 cm proximally and distally from the site of friction.
Each synovial sheath is a double tube consisting of:
(a)an inner visceral tube; and an outer parietal tube.
The inner and outer tubes are united longitudinally along that surface of the tendon where degree of friction in less. By this union a longitudinal fold is formed called mesotendon.
Through mesotendon the blood vessels enter the tendon. It may disappear to reduce to thread like structures called vincula, e.g., long flexors of digitis. This adaptation increases the range of movements.
Aponeurosis:
It is a fibrous and membranous part of a muscle by which it is attached to distant parts, e.g., Bicipital aponeurosis of Biceps brachii muscle. It is a device for getting additional extensive attachment.
Insertion:
It is the distal moveable end of a skeletal muscle, which moves during contraction.
The nature of distal attachment may be :-
a)Muscular :Muscle fasciculi of one muscle interdigitate with the other.
Example:
- Levator ani muscle.
- Mylohyoid muscle.
Tendinous :
The distal end of the muscle is rounded and fibrous and is inserted into the bone.
Example:
Most of the muscles have tendinous insertions.
Aponeurotic :
The distal end of certain muscles become fibro-membranous – Aponeurosis.
Example:
- Palmaris longus muscle at its distal end forms palmar aponeurosis.
- Partly tendinous and partly aponeurotic :The distal ends of certain muscles may be tendinous and aponeurotic.
Example:
In Biceps brachii muscle the insertion is partly tendinous and partly aponeurotic.
Site of insertion in relationship with bones and joints are of three types:
1. Near the proximal end of bone close to a joint :
Example:
i)Biceps brachii is inserted into the proximal end of the radius near the Elbow Joint.
ii)Psoas major is inserted into the Lesser trochanter of femur close to the Hip Joint.
iii)The sartorius, gracilis and semitendinosus are inserted on the tibia bone near the Knee Joint.
Advantages:
The type of insertion near the proximal end of a bone and close to a joint is helpful in following ways:-
i)It makes the joint stable by keeping the bones in close apposition during movements.
ii)It brings about rapid movement of the joint close to which they are inserted.
Near the middle of the shaft of a bone:
Examples:
i)Adductors of the thigh.
ii)Pronator teres
iii)Deltoid
iv)Coracobrachialis.
v)Near distal end of a bone:
Examples:
i)A portion of adductor magnus; Distal end of femur (Adductor tubercle).
ii)Brachioradialis – Distal end of the radius bone.