Skeletal muscles are classified into various groups according to following scheme :
I.According to the relative colour of skeletal muscle cells:
In lower vertebrates for example fish and chicken the skeletal muscle cells are red (leg of chicken) or pale (breast of chicken). The distribution between the red and white muscle cells is clearly visible in skeletal muscles of fore-mentioned animals.
However, in man these muscle cells are thoroughly combined into different ratios in various muscles.So depending upon the relative concentration of red and white skeletal muscle cells, the skeletal muscles is of following types:
A. Red Skeletal Muscles:
These muscles contain high proportion of red muscle cells. The red muscle cells show following characteristics:
- They contain large quantity of myoglobin which is responsible for red colour. The rich quantity of myoglobin provides them a store of O2 because this substance is a protein similar in structure to haemoglobin.
- The nuclei are more towards the centre.
- Amount of mitochondria is more.
- The Z-lines are thicker.
- They are more numerous is postural muscles.
- The contraction is slow and prolonged.
- The circulation is relatively well developed.
Examples:
(i) Soleus muscle
(ii)Vastus lateralis muscles.
White Skeletal Muscles :
These muscles contain high proportion of white muscle cells. The white muscle cells show following characteristics:
1.They contain very little quantity of myoglobin. Thus they appear pale and white in colour. Due to relatively low quantity of myoglobin the O2 store is very low and easily depleted during contraction.
2.They nuclei are more towards the periphery.
3.The amount of mitochondria is low.
4.The Z-lines are thinner.
5.The contraction is rapid and unsustained.
6.They are usually seen in segmental muscles (which perform rapid movements).
7.The circulation is relatively poor.
Examples:
i)Gastrocneminus muscle
ii)Vastus medialis muscle
iii)Muscles of wings
According to histochemical localization of enzyme activity within
the skeletal muscle cells:
A.Type A-Skeletal Muscles:
B.In these muscles the skeletal muscle cells are large, white in colour and fast-contracting (fast-twitch muscle cells).
Type B-Skeletal Muscles:
These muscle cells are characterized by richer content of low speed skeletal muscle cells.
Type C-Skeletal Muscles:
In these muscles the skeletal muscle cells are smaller in size, red in colour and fast-twitching.
The proportion of these three types of skeletal muscle cells vary from muscle to muscle and even within the various regions of a particular muscle.
III. According to architecture or form:
Disposition or arrangement of skeletal muscle cells in a particular muscle could be in two ways. Either the muscle cells are arranged parallel to the line of pull of whole muscle or oblique to the line of pull. Accordingly there are two main categories of skeletal muscles depending on the fascicular architecture.
A.Skeletal muscles with parallelly arranged muscle cells:
Here maximum range of mobility is assured because the contraction of muscle cells produces too much shortening.
These muscles are sub-grouped as follows:
1.Quadrilateral muscles:
The muscle cells are arranged in a parallel manner producing a flat quadrangular muscle mass.
Examples:
i)Hyoglossus muscle of tongue.
ii)Thyrohyoid muscle of larynx.
iii)Gluteus maximus muscle.
Fusiform or spindle shaped muscles:
The muscle cells are arranged in a parallel manner producing a spindle shaped muscle mass.
Examples:
i)Biceps brachii muscle.
ii)Flexor carpi radialis muscle
iii) Palmaris longus Strap or ribbon like muscle.
The muscle cells are arranged in a parallel manner a ribbon or strap like appearance.
Examples:
i)Sartorius
ii)Infrahyoid muscles
iii)Extrinsic muscles of eye ball producing
iv)Rectus abdominis Skeletal muscles with obliquely arranged muscle cells:
Here the range of mobility is reduced due to decreased shortening of muscle cells. The force of pull in these muscles is more as compared with muscles with parallel muscle cells.
These muscles are sub-grouped as follows:-
1.Pennate muscles:
These muscles show feature like pattern. The muscles cells are grouped in bundles or fascicles which lie diagonal to the long axis of muscle. This arrangement of muscle fascicles is responsible for achievement of greater strength of muscle contraction than parallel arrangement. The pennate muscles are of following sub-types :-
a)Semipennate muscles (Unipennate muscles) :
These muscles resemble a half feather. In these muscles the tendon forms a long axis of pull. The muscle fascicles are attached obliquely to one side of tendon.
Example:
i) Flexor pollicis longus muscle.
ii) Tibialis posterior muscle.
iii) Pennate muscles (bipennate muscles)
These muscles resemble a complete feather. The tendon of the muscle is placed centrally on which the muscle cells converge from both sides. The line of pull corresponds to the long axis of tendon.
Examples:
i)Rectus femoris muscle
ii)Dorsal introssei
Multipennate muscles:
These are composed of a series of pennate masses lying side by side or it may be a multiple of various pennate masses. These muscles are situated in those regions where limited range of mobility coupled with a considerable power is required.
Examples:
i)Deltoid muscle (A cromial fibres).
ii)Subscapularis muscle.
Cylindrical or radiate or circumpennate muscles:
In these muscles there is a central tendon over which the muscle fascicles are coverging from all sides.
Example:
i)Tibialis anterior muscle.
ii)Fan-shaped muscles:
In these muscles the fascicles are converging on the top of a tendon in such a manner that these resemble a foreign fan.
Examples:
- Gluteus minimus muscle
- Temporalis muscle
Triangular muscles:
Here the muscle fascicles are arranged similar to fan shape Muscles but external appearance of muscle resembles a triangle.
Examples:
- Subclavius muscle
- Pectoralis major muscle
- Pyramidalis muscle of rectus sheath
- Adductor magnus muscle
Circular muscles:
It is a modified form of muscles with oblique arrangement of skeletal muscle cells. Here the muscle cells are arranged in a circular pattern to form sphincters.
Examples:
i)Orbicularis oculi muscle surrounding the orbital opening.
ii)Orbicularis oris encircling the oral fissure (mouth).
Spiralized-muscles:
In these muscles the muscle fascicles are spirally arranged, and twisting may be seen.
Example:
The fibres of trapezeus which are arising from the pectoral girdle and vertebral spines show a twisting spiral arrangement in their superior part.
Cruciate muscle:
It is a modified form of spiralized muscles where the fascicles twist and spiral in such a manner that they give rise to cruciate or cris-cross appearance.
- Masseter
- Sterno-cleidomastoid
- Adductor magnus
Functions due to special form and architecture:
1.Muscles with parallel fasciculi (facicles), help in lifting more weight for a long distance.
2. Muscles with oblique fasciculi, help in lifting lighter weights.
3.Bipennate and multipennate artichitecture is responsible for much more powerful action.
4.Muscles with radially orier tated fasciculi can contract segmentally.
5.Spiralized and cruciate forms are helpful in rotatory movements.
According to the region :
The skeletal muscles are usually grouped for sake of study according to the area of the body in which they are located.
- Muscles of the upper limb
- Muscles of the lower limb
- Muscles of the trunk
According to the action:
The skeletal muscles are classified into following sub-groups depending upon their simple role during movements at a joint:
A)Prime movers:
These are those muscles which are constantly active in the initiation and maintenance of a particular movement.
Examples:
Brachialis is a prime mover of elbow joint during flexion.
Antagonists:
These are those muscles which undergo relaxation when the prime- mover start contracting; then they remain silent and finally again contract in order to restrict the range of mobility. Such muscles are called antagonists.
Examples:
- Triceps brachii is an antagonist during flexion at the elbow joint.
- The control and co-ordination between the activity of the prime mover and antagonist muscles are achieved by “reciprocal innervation” of the two muscles.
Fixators:
These are those muscles which increase the intra-articular compression and stabalize the position of a joint. They provide an immobile base over which other prime movers can act.
joint:
When the deltoid (Abductor up to 90°) is contracting the muscles of the rotator cuff also contract and stabalize the head of the humerus in the glenoid cavity. This stabalization of head enables the trapezius (prime-mover) to cause over head abductipn.
- Synergists: (Gk Syn = together; ergon = work)
Example:
When the deltoid muscle is producing abduction, the muscles inserted into greater and lesser tuberosities of the humerus contract and help in refined abduction up to 90° at the shoulder joint.