Muscle

General Anatomy – Muscle

Learning Objectives:

  • After  reading this post you will be able to :
    • define muscle and list the characteristic features of muscles.
    • differentiate between skeletal, cardiac and smooth muscle.
    • List the connective tissue coverings of skeletal muscles and their function.
    • classify muscles based on arrangement of fascicles.
    • define motor unit and motor point.
    • Explain the terms atorphy and hypertrophy  and paralysis of muscle.

Q. What are the foure major properties of muscular tissue?
A. The three major characteristics of muscular tissue are:

Contractility: capacity of muscle cell/fiber to contract or shorten.
Excitability: ability of muscle fiber to respond to stimulation by nerves and hormones.
Extensibility: ability of muscle fibers to be stretched beyond their normal resting length to a limited degree.
Elasticity: Ability of muscle fibers to recoil to their original resting length after they are stretched.

Q. Enumerate the different types of muscles in the body. types of muscles
A. There are three types of muscles:

a. Skeletal/striated/voluntary muscle
b. Smooth/visceral/involuntary muscle
c. Cardiac muscle

 

Q. Compare the three types of muscles in a tabulated form.

 Comparison between the three types of muscles

Q. Describe the parts of skeletal muscle?parts of muscle
A. Skeletal muscle consists of two parts:
Fleshy part – is the contractile part called as muscle belly.
Fibrous part – is the non-contractile part. When it is cord-like, it is called tendon and when it is flattened, it is called aponeurosis.

Skeletal muscles have two ends:
Origin: is that end of the muscle which remains fixed during its contraction.
Insertion: is that end which moves during contraction. Usually in limbs, the origin of muscles is usually proximal to insertion.

However, origin and insertion, are at times interchangeable.

Q. Describe the connective tissue layers covering muscle/muscle fiber.
A. Each skeletal muscle is made of hundreds or thousands of muscle fibers/muscle cells which are arranged in fascicles (bundles of muscle fibers). Muscular tissue has three concentrically arranged layers of connective tissue. The connective tissue layers support and protect the muscle fibers and provide pathway for the passage of blood vessels and nerves to the muscle fibers. The three layers of connective tissue from within outside are:
Endomysium: each individual muscle fiber is surrounded by connective tissue layer called endomysium.
Perimysium: each bundle of muscle fiber (fasciculus) is surrounded by a layer of connective tissue called the perimysium.
Epimysium: the whole muscle is surrounded by a connective tissue sheath called epimysium.

Q. Classify the skeletal muscles according to the fascicular arrangement/ arrangement of muscle fibers.
A. Skeletal muscle is made up of fascicles (bundles of muscle fibers). The arrangement of fascicles vary, resulting in muscles with different shapes and functional capabilities. Classification of skeletal muscle according to the arrangement of muscle fibers is:

a. Parallel fasciculi: When muscle fibers are parallel to the line of pull. The range of movement is more, but the muscles with parallel muscular fasciculiforce of contraction is less. Following are the types of muscles with parallel fasciculi:
Strap like– e.g. Sartorius, sternohyoid, sternothyroid

Quadrilateral – e.g. Pronator quadrates, quadrates lumborum

Fusiform – e.g. Biceps brachii, diagastric

b. Oblique fasciculi: When muscle fibers are arranged obliquely to the line of pull. The range of movement is less, but the force of contraction is more. Following are the types of muscles with parallel fasciculi:
Unipennate – all muscle fibers are arranged obliquely on one side of the tendon e.g. Flexor pollicis longus, peroneus tertius

Bipennate – muscle fibers are arranged obliquely on both the sides of centrally placed tendon. e.g. Flexor hallucis longus, rectus femoris and dorsal interossei

Multipennate – a series of bipennate muscles e.g. deltoid, subscapularis

Circumpennate – muscle fibers converge into the central tendon from all sides e.g. tibialis anterior.

pennate muscles

c. Convergent fasciculi: muscle fibers  converge near the point of insertion (temporalis).

d. Circular fasciculi: the fibers surround a circular orifice/opening such as mouth ( orbicularis oris).

circular and convergent fasciculi of muscles

e. Spiral/twisted fasciculi: muscle fibers are twisted in arrangement close to their insertion.  e.g. pectoralis major and latissimus dorsi.

muscles with twisted fasciculi

f. Cruciate fasciculi: mucle fibers are arranged in superficial and deep planes and cross each other (sternocleidomastoid).

cruciate fasciculi of muscles

Q. Classify muscles according to their action.
A. a. Prime movers: Muscles that are responsible for the initiation and maintenance of desired movement are called prime movers. e.g. brachialis is the prime mover for flexion of elbow joint.

b. Antagonist: Muscles that opposes the action of prime mover. e.g. triceps which extends the elbow joint is antagonistic to the brachialis which is responsible for flexion of elbow joint. They help the prime mover by active relaxation to perform smooth action.

c. Fixators: They stabilize the proximal attachment of a muscle or joint so that the muscle can act more efficiently at the distal joint (it acts to eliminate the unwanted movement of prime mover’s origin). e.g. muscles attaching shoulder girdle to trunk contract as fixators to allow deltoid muscle to perform abduction at shoulder joint.

d. Synergists: they are special fixator muscles. Muscles that help in the action of prime movers (when prime mover is a long muscle and crosses more than one joint) by preventing the undesired movements at the intermediate joints. e.g. during flexion of fingers by the long flexors of digits, the wrist joint is fixed and extended by the extensors which act as synergist to the flexors (flexion of fingers is efficient when the wrist is fixed and extended).

Q. What are the different types of contractions a muscle can undergo?isometric and isotonic contraction
A. a. Isometric contraction: The muscle contracts and exerts force without alteration in the length or without any movement. e.g. flexors of the forearm trying to lift weight that is too heavy.

b. Isotonic contraction: The muscle contracts and shortens to produce movement. e.g. flexors of the forearm contract to lift a glass of water.

 

motor unit and motor pointQ. Describe briefly the motor unit.
A. Motor Unit: All the muscle fibers innervated by a single α motor neuron constitutes a ‘motor unit’. Trunk muscles have few large motor units and can perform only gross movements, whereas extraocular muscles of eye and muscles of thumb have smaller motor units (a single motor
neuron innervates 5-10 muscle fibers) and are capable of delicate and precise movements.

Q. What is motor point?
A. It is the point of entrance of the nerve trunk into the muscle. Usually a nerve enters the deep surface of the muscle Electrical stimulation of the muscle is most effective at the motor point.

 

Q. Describe the nerve supply of the skeletal muscle.
A. Skeletal muscle is supplied by somatic nerves. The nerve to a skeletal muscle is a mixed nerve containing 60% motor and 40% sensory fibers.

a. Motor supply: Extrafusal fibers of the muscle which produce movements are innervated by α motor neurons of the anterior horn of spinal cord or motor nuclei of cranial nerves( in case of head & neck muscles). Intrafusal fibers of the muscle spindle are innervated by the gamma motor  neurons.

nerve supply of muscle

b. Sensory supply: Sensory nerve fibers innervate muscle spindles and tendons, ligaments, joint capsules to carry  proprioceptive impulses to central nervous system.

Q. Define:
a. Muscle tone
b. Myotome
A. a. Muscle tone: Skeletal muscle fibres even in resting condition remain in a state of partial contraction. This is referred to as muscle tone.
b. Myotome: Muscle fibres supplied by a spinal nerve constitute a myotome.

APPLIED ASPECT

Atrophy of muscle: Muscle atrophy is a decrease in muscle mass.

Hypertrophy of muscle :Muscle hypertrophy is an increase in muscle mass due to an increase in muscle cell size.

Muscle atrophy and hypertrophy

Paralysis of muscle:Loss of motor power/power to produce movement. A muscle is unable to contract either due to injury to the motor nerve or spinal cord  motor areas of cerebrum is called paralysis.monoplegia, paraplegia , hemiplegia and quadriplegia

Monoplegia : muscles of one limb are paralyses

Hemiplegia. muscles of one side of the body are paralysed.

Paraplegia: muscles of both the lower limbs are paralysed.

Quadriplegia: muscles of all  the four limbs are paralysed.

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