Efferent Innervation of Skeletal Muscle

The efferent innervation of skeletal muscle is carried out by a lower motor neuron, its efferent nerve endings, and the motor end plate, which is the site of the neuromuscular junction. Individual lower motor neurons in the spinal cord supply a number of muscle fibers, widely distributed throughout a muscle, with axon terminal nerve endings. The transmission of efferent nerve impulses is done through the motor end plate at the neuromuscular junction.

In certain skeletal muscle, as few as three to five muscle fibers are supplied by each neuron. The regulation of contraction of such muscles, such as the extrinsic ocular muscles responsible for the delicate eye movement, is correspondingly precise. When efferent impulses stimulate skeletal muscle fibers to contract, these fibers always shorten to their fullest extent. Graded contractions of skeletal muscles depend on the number of muscle fibers stimulated to contract, and not on partial contraction of individual fibers.

The motor unit is the key to the efferent innervation of skeletal muscle. It consists of a lower motor neuron, the motor end plate, and skeletal muscle fibers that are innervated. Efferent impulses from the neuron cause all the muscle fibers to contract fully. When there are no impulses coming from the neuron, none of the fibers contracts at all. Therefore, the strength of a skeletal muscle contraction is the function of a number of motor units participating in the efferent innervation.

Below, schematic image of efferent innervation of skeletal muscle. You can see that upper motor neurons can also take part in such action in a more complex way. Upper motor neurons are located in the primary motor gyrus, or pre-central gyrus, in the frontal lobe of brain.

Muscles of Thenar Eminence

The muscles of thenar eminence are a group of four short muscles located on the lateral aspect of palm of human hand, at the base of the thumb (radial side of palm). They are four in number and they are innervated by the median (C6-C7) and ulnar (C8-T1) nerve. Their source of blood supply are the palmar branch of radial artery. Remember: the muscles of thumb are eight in number as four of them are located in forearm, while the other four lies at the base of thumb (thenar) but all of them move the short digit of hand.

1) Oppenens pollicis muscle.- Triangular in shape, it arises from the crest of the trapezium bone and the flexor retinaculum, and it is inserted into the first metacarpal bone. Action: it opposes the thumb in relation to the fingers, especially the little and ring finger.

2) Flexor pollicis brevis.- It is a superficial muscle which lies medially beside the abductor pollicis brevis. It arises from the flexor retinaculum, the trapezium and trapezoid bones. It is inserted into the base of proximal phalanx of thumb. Action: it flexes the proximal phalanx of thumb.

3) Adductor pollicis muscle.- Lying deep in the palm of hand, it emerges from the radiate carpal ligament, the capitate bone, and the third metacarpal bone. It is inserted into the base of proximal phalanx of thumb. Action: it adducts the thumb (it pulls it medially), assisting in flexing the proximal phalanx.

4) Abductor pollicis brevis.- It lies superficially, arising from the tendon of the abductor pollicis longus muscle and tubercle of trapezium. It is inserted into the lateral side of proximal phalanx of thumb. Action: it abducts thumb (it draws it away from the medial plane) and holds it in mild position.

Below, view of palm of right hand, showing the muscles of thenar eminence. Notice that the abductor pollicis muscle was cut off to expose the opponens pollicis.

Flexor Digitorum Sublimis

The flexor digitorum sublimis is one of the nineteen muscles of the forearm, and it belongs to the second layer of muscles. It lies under the palmaris longus and the flexor carpi radialis muscle, which leave grooves on it. It flexes the second phalanges of the four fingers of the hand.

The flexor digitorum sublimis muscle originates by two heads; the humero-ulnar head, which is long and narrow, arises from the medial epicondyle of the humerus and the coronoid process of ulna; while the radial head, which is wide and short, arises from the proximal palmar surface of radius. Then the two heads join to form a common belly which runs downwards, dividing into four long tendons. These tendons travel over to the hand, running through the carpal tunnel to be inserted into the bases of the middle phalanges of the fingers. At the level of the proximal phalanges, each tendon splits into two and, therefore, has two points of insertion at the base of the middle phalanges.

Action

The flexor digitorum sublimis flexes (bends) the middle phalanx of the index, middle, ring, and little finger.

Innervation

The flexor digitorum sublimis is innervated by the median nerve (C6-C7-T1), which arises from the brachial plexus.

Blood Supply

This long muscle is supplied by collateral branches of the radial and ulnar artery.

Below, image of muscles on palmar (anterior) aspect of forearm. You can see that the flexor digitorum sublimis is partially covered by the flexor carpi radialis and the palmaris longus muscle.

Opponens Pollicis

The opponens pollicis is a short muscle of the ball of thumb, on the radial side of palm. It is shaped like a thin triangular sheet, which lies under the abductor pollicis brevis muscle. It arises from the crest of the trapezium (carpal bone) and from the anterior annular ligament. Then it runs downwards and laterally to be inserted into the whole length of the radial border of the first metacarpal bone (of thumb).

Action

The opponens pollicis pulls the first metacarpal bone inwardly towards the center of palm, opposing the other four fingers.

Innervation

This small muscle is innervated by the median nerve, which emerges from C6 and C7 of brachial plexus.

Blood Supply

It is supplied by a palmar branch of radial artery, which forms the deep palmar arch.

Below, image of palm of right hand, showing the opponens pollicis muscle. The abductor pollicis brevis has been cut away to expose it.

Muscles of Thumb

The muscles of thumb are those which control the movement of this short digit of the human hand, such as flexing, adduction, and abduction. Some of them are located in the forearm, and some at the base of the thumb (thenar muscles) on palm of hand. Thus, some are long and powerful, and others are short.

The muscles of thumb are eight in number. There are three flexors, one adductor, two abductors, and two extensors. They are voluntarily controlled with precision by a large area in the precentral central gyrus (motor strip) of the brain frontal lobe. The great precision and strength of these muscles are the result of the human brain cortex evolution. Thus, the muscles that move the thumb are:

1) Flexor pollicis longus.- It flexes the thumb when you close it tightly, drawing it towards the palm of hand. It is supplied by a branch of the median nerve;

2) flexor pollicis brevis.- It also flexes the thumb to close it, pulling it inwardly towards the center of hand; it usually works together with the one mentioned above and the flexors of the four fingers when you grasp something. The first one is innervated by the anterior interosseous nerve, which is a branch of the median nerve, while the flexor pollicis brevis is supplied by a branch of the ulnar nerve;

3) oppenens pollicis muscle.- It arises from the crest of the trapezium bone and the flexor retinaculum, and it is inserted into the first metacarpal bone. It pulls the ball of thumb towards the center of palm. Hence, it is considered a flexor.

4) adductor pollicis.- It is a short broad muscle that adducts the thumb, drawing it sideways and inwardly towards the hand and base of the forefinger. It originates from by two heads from the capitate bone of wrist (the largest carpal bone) and the base of second metacarpal bone (of forefinger) and it is attached to the medial side of proximal phalanx of thumb. It is supplied by a deep branch of the ulnar nerve;

5) abductor pollicis brevis.- It opens up the thumb, pulling it away from the forefinger. It is innervated by a recurrent branch of the median nerve.

6) abductor pollicis longus.- This muscle works together with the brevis to open up the thumb, but it is innervated by a branch of the radial nerve.

7) extensor pollicis brevis.- It extends and abducts the thumb, pulling it away and backwards. It arises from dorsal aspect of distal radius and is inserted in the posterior side of base of thumb proximal phalanx. It is innervated by a branch of the posterior interosseous nerve.

8) extensor pollicis longus.- It extends the distal phalanx of thumb and is supplied also by a branch of the posterior interosseous nerve, which arises from the radial.

Below, view of right hand showing five muscles of thumbs.

Muscles of Tongue

Not only do the muscles of tongue play an important role in deglutition and tasting the food we eat but also in language production, since we use it to pronounce the letters of the alphabet. There are two types of muscles of tongue; extrinsic and intrinsic ones.

The extrinsic muscles are those located at the base of the tongue and they make it possible for us to swallow the food we eat; this is called deglutition. They are anchored to the hyoid bone, the mandible, and the styloid process and they are innervated by the hypoglossal nerve (CNXII) and receive oxygen-rich blood from the lingual artery, which originates from the external carotid artery. On the other hand, the intrinsic muscles constitute the body of the tongue and are involved in speech, whose center is located in the Broca’s area of the brain frontal lobe. They are also innervated by the hypoglossal nerve and supplied by branches of the lingual artery.

Extrinsic muscles

1) Genioglossus: it arises from the symphysis of mandible. Then it fans out as it runs up to insert at the base of tongue and at the hyoid bone. It depresses and protrudes the tongue.

2) Hyoglossus: it originates from the hyoid bone and goes up to insert between the intrinsic muscles of tongue. It draws back and down the tongue.

3) Stylohyoid: it emerges from the styloid process and is inserted into the sides of tongue. It pulls back this organ.

4) Chondroglossus: it originates from lesser horn of hyoid bone, inserting into the hyoglossus fibers.

Intrinsic muscles

1) Superior longitudinal

2) Transverse

3) Vertical

4) Inferior longitudinal

Below, two images of the muscles of tongue; the first one showing the extrinsic and the other the intrinsic muscles.

Intrinsic Muscles of Tongue

The intrinsic muscles of tongue are those muscular structures which form and constitute this organ of speech. They are inherent to the tongue proper, giving it shape. In other words, they give it the very well-known elongated, muscular consistency that we know, playing a key role in the articulation of human speech, especially in the pronunciation of consonant letters. The intrinsic muscles of tongue are four. From top to bottom, they are:

1) the superior longitudinal muscle of tongue, which travels from base to tip along the top of the organ with long fibers. Function: it bends the tongue, curling its tip backwards;

2) the inferior longitudinal muscle of tongue, which is long and narrow, lying in the tongue lateral of genioglussus muscle.

3) the transverse muscle of tongue (transversus linguae), which is made up of separate transverse muscle fibers, originating from the septum of tongue. Function: it raises the lateral aspects of tongue, making it convex, like a trough;

4) the vertical muscle, which is made up of short fibers lying between the dorsum and the inferior surface, with its function being flattening the tongue.

Action

The superior longitudinal muscle retracts the tongue and curls up its tip, while the transverse curls up the sides of this organ. The inferior longitudinal collaborates with the superior longitudinal muscle to draw back the tongue, making it thick. The vertical muscle flattens this organ sideways.

Nerve supply

They are innervated by a branch of the glossopharyngeal nerve (CN IX), which transmits sensory information, and by the hypoglossal nerve (CN XII), which supplies with motor innervation.

Blood supply

The intrinsic muscles of tongue are supplied by the deep lingual artery, which branches off the lingual artery, and this, from the external carotid artery.

Down below, inferior view of human tongue, showing the extrinsic muscles and only two of the intrinsic muscles of tongue: the inferior longitudinal and the transverse muscle.