1 Learn the key words and phrases:

disuse, enlarge, hypertrophy, atrophy, mitochondria, resist fatigue, fatigable fibers, filaments of actin and myosin, discontinued, reduction, capillary networks, motor neurons, reinnervate

2 Memorize the explanation to the terms:

increase becoming greater in size, amount, etc.

decrease becoming gradually less, smaller, etc.

Atrophy and Hypertrophy of Skeletal Muscles

Skeletal muscles are very responsive to use and disuse. For example, those that are forcefully exercised tend to enlarge. This phenomenon is called muscular hypertrophy. Conversely, a muscle that is not used undergoes atrophy – that is, it decreases in size and strength.

The way a muscle responds to use also depends on the type of exercise involved. For instance, when a muscle contracts relatively weakly, as during swimming and running, its slow, fatigue-resistant red fibers are most likely to be activated. As a result, these fibers develop more mitochondria, and more extensive capillary networks develop around the fibers. Such changes increase the fibers' abilities to resist fatigue during prolonged periods of exercise, although their sizes and strengths may remain unchanged.

Forceful exercise, such as weight lifting, in which a muscle exerts more than 75% of its maximum tension, involves the muscle's fast, fatigable white fibers. In response, existing muscle fibers develop new filaments of actin and myosin, and as their diameters increase, the whole muscle enlarges. However, no new muscle fibers are produced during hypertrophy.

Since the strength of a contraction is directly related to the diameter of the muscle fibers, an enlarged muscle is capable of producing stronger contractions than before. However, such a change does not increase the muscle's ability to resist fatigue during activities such as running or swimming.

If exercise is discontinued, there is a reduction in the capillary networks and in the number of mitochondria within the muscle fibers. Also, the size of the actin and myosin filaments decreases, and the entire muscle atrophies. Such atrophy commonly occurs when limbs are immobilized by casts or when accidents or diseases interfere with motor nerve impulses. A muscle that cannot be exercised may decrease to less than one-half its usual size within a few months.

The fibers of muscles whose motor neurons are severed not only decrease in size, but also may become fragmented and, in time, be replaced by fat or fibrous connective tissue. However, if such a muscle is reinnervated within the first few months following an injury, its function may be restored. Meanwhile, atrophy may be delayed by treatments in which electrical stimulation is used to cause muscular contractions against loads.

Post-text assignments

1 Respond to the following tasks:

1 Explain how muscles may become fatigued and how a person's physical condition may affect tolerance to fatigue.

2 Explain the differerence between atrophy and hyperthrophy of skeletal muscles

2 Choose the proper term to the identification:

1 The way (route) of administration of drugs. 2 The science dealing with the effect of drugs on living organisms. 3 Conditions which ban (forbid) the use of a drug. 4 The science that deals with medicinal products of plant, animal, or mineral origin in their crude or unprepared state. 5 The branch of pharmacology that deals with the effect and the reactions of drugs within the body. 6 Pertaining to against pain.

(a – contraindications; b – analgesic, analgetic; c – intramuscular; d – pharmacology; e – pharmacognosy; f – pharmacodynamics)

Key:

1 – c (intramuscular); 2 – d (pharmacology); 3 –

a (contraindications); 4 – e (pharmacognosy); 5 –

f (pharmacodynamics)

Unit 7

1 Match the muscles in column I with the descriptions and functions in column II:

I II

1) buccinator a) inserted on the coronoid process of the

mandible

2) epicranius b) draws the corner of the mouth

upward

3) medial pterygoid c) can raise and adduct the

scapula

4) platysma d) can pull the head into an

upright position

5) rhomboideus major e) consists of two parts – the

frontalis and the occipitalis.

6) splenius capitis f) compresses the cheeks

7) temporalis g) extends over the neck from

the chest to the face

8) zygomaticus h) pulls the jaw to the side in

grinding movements

I II

9) biceps brachii i) primary extensor of the elbow

10) brachialis j) pulls the shoulder back and

downward

11) deltoid k) abducts the arm

12) latissimus dorsi l) rotates the arm laterally

13) pectoralis major m) pulls the arm forward and

across the chest

14) pronator teres n) rotates the arm medially

15) teres minor o) strongest flexor of the elbow

16) triceps brachii p) strongest supinator of the

forearm

I II

17) biceps femoris q) inverts the foot

18) external oblique r) a member of the quadriceps

femoris group

19) gastrocnemius s) a plantar flexor of the foot

20) gluteus maximus t) compresses the contents of

the abdominal cavity

21) gluteus medius u) largest muscle in the body

22) gracilis v) a hamstring muscle

23) rectus femoris w) adducts the thigh

24) tibialis anterior x) abducts the thigh

2 Read the text and render its main idea:

ln the United States, approximately 3 million people present to emergency departments for treatment of traumatic facial injuries each year. Most of these injuries are relatively minor soft tissue injuries that simply require first aid care or primary closures. A small percentage of facial traumas (0.04-0.09%) require major repair with possible bony reconstruction. Motor vehicle accidents (MVAs) were the most frequent cause of facial injuries before 1970. In recent years, with the institution of state seat belt laws, the number of deaths from MVAs has declined along with the incidence of facial injuries, although the prevalence of facial trauma has remained fairly constant. This is due to the growing population and other human factors, such as on-the-job accidents, sports-related injuries, domestic interpersonal violence, self-inflicted wounds, and animal bites.

The mechanism of injury for facial trauma varies widely from one locality to the next, depending significantly upon the degree of urbanization, socioeconomic status of the population, and cultural background of each region. MVAs continue to be a primary contributor to significant facial injuries in rural areas. In contrast, in inner metropolitan areas, domestic violence is the leading cause of facial trauma despite a denser population, a difference that may be due to stricter enforcement of traffic laws.