Muscles And Its Classifications

What is a Muscle?

Muscle is the basic structure for the movements. The cells of origin of all muscles are called myocytes and all muscle fibers contain contractile proteins i.e. myosin and actin.

Classification of Muscle

Divided in 3 groups on functional and histological bases:
A. SKELETAL, Striated, Voluntary
B. CARDIAC, Striated, Involuntary
C. SMOOTH, Non-striated, Involuntary

Skeletal Muscles
MAIN FEATURES:

Attached to bones
Voluntary i.e. these are under
our conscious control
Supplied by somatic nerves
Fatigue easily
Individual cells are large i.e. from few mm to many cm.

Main histological features:

Each cell is long tublar fiber
Longitudinal & cross striations
Multi nucleated
Nuclei arranged at the periphery
Physiology

MOTOR UNIT: Number of muscle fibers supplied by one motor neuron.
Ratio of fibers supplied depends upon functional demands.


For muscle performing precise & delicate movements ratio is 1:10
For large limb muscles meant for gross movements ratio is 1:1000
INTERNAL STRUCTURE:
Association with C.T:
1. Endomysium
2. Perimysium
3. Epimysium
PARTS:

Origin: Least mobile
Insertion: Most mobile
Belly: Fleshy part
Tendon: Non-cartilaginous part attached to bone
Aponeurosis: Tendons of some muscles form flat sheets
MORPHOLOGICAL CLASSIFICATION:

PARALLEL:
Quadrilateral e.g. Quadratus femoris, thyro-hyoid
Strap e.g. Sartorius, sternohyoid
Strap with tendinous intersections e.g Rectus abdominus
Fusiform- e.g Long head of biceps
B. OBLIQUE:
Triangular e.g. Adductor longus
Spiral e.g. Pectoralis major
Cruciate-two layers e.g. Sternocleidomastoid
Digastric-two bellies & intermediate tendon e.g. Omohyoid

PENNATE: tendon in center & direction of fibers at some angle
1. Unipennate e.g. Flexor pollicis longus
2. Bipennate e.g. Dorsal interossei
3. Multipennate e.g. Deltoid
4. Circumpennate e.g. Tibialis anterior

C. More than One Tendon of Origin:

Bicipital e.g Biceps femoris
Tricipital e.g Triceps
Quadricipital e.g Quadriceps femoris

FUNTIONAL CLASSIFICATION:


PRIME MOVER OR AGONIST: Muscle which initiates, maintains and completes a movement is prime mover for that particular movement e.g. Brachialis is agonist for flexion at elbow joint.

ANTAGONIST: Muscle that work opposite to agonist is called antagonist e.g. Brachialis is antagonist to triceps.

SYNERGISTS: Muscles of opposite group which prevent undesired movements.

FIXATORS: Simultaneous contraction of agonists and antagonists fixes a joint or a bone, so that movement can be carried out more distally e.g. Flexors and extensors of wrist contract simultaneously to fix the wrist thus permitting smooth movements at fingers.

ANTI GRAVITY: Muscles acting against gravity. Muscles which maintain the anatomical position, work against gravity. Maintenance of anatomical position is mainly function of extensors

Types of Muscle Contraction:

1. Isometric Contraction: Length not changed, tension increased & no movement produced. (Load is more than muscle power)

2. Isotonic Contraction: Length decreases, tone remains constant & movement take place. (Load less than muscle power)

3. Eccentric Action or Paradoxical Action: Initial isotonic contraction, followed by isotonic relaxation of same muscle & gravitational pull adds to the action.

Cardiac Muscle

Restricted to heart.

MAIN FEATURES:

No bony attachment
Involuntary
Supplied by autonomic nerves
Does not get fatigued

MAIN HISTOLOGICAL FEATURES:

Size: Individual cells are up to 100 um long
Longitudinal and cross striations
Each cell has single or at most
two nuclei located centrally
All cells branch and make a network
All cells connected to each other
by intercalated discs

Smooth Muscle

MAIN FEATURES:
No bony or tendinous attachment
Involuntary
Supplied by autonomic nerves
Slow but prolonged contraction
Can under go mitotic division throughout life (skeletal muscle fibers cannot divide after differentiation, cardiac muscle fibers can only divide till birth)

MAIN HISTOLOGICAL FEATURES:

Spindle shaped cell, thick in the middle and tapering ends
Single nucleus
No striations

FUNCTIONAL LOCATIONS:

To regulate diameter e.g in blood vessel walls
Propulsion of liquids or solids e.g in GIT or ureter
Expulsion of contents e.g in urinary bladder or uterus