Muscle is composed of fibers, nerves and connective tissues and account for over 40% of the body weight. The fibers contract to produce tension on the associated tissues or tendons. Muscle tissue is enclosed in facia, which in turn is attached to other structures including ligament. There are three types of muscle tissues : skeletal, cardiac and smooth muscles. Muscle tissue provides a) posture and body support b) locomotion and c) heat production. Muscle tissue is examined with respect to function and types of muscle, structure and mechanism of contraction and attachments.
I. FUNCTION AND TYPES OF MUSCLE
· Locomotion: One of the most obvious functions of muscle tissue is locomotion. When muscles contract, the fibers pull or relax bones to which they are attached, thus causing locomotion.
· Muscle contraction is also the primary means of moving lymph in the lymphatic vessel. The circulation and return of venous blood is also partly dependent on muscle contraction.
· Heat production: The contraction of muscle produce heat, which keeps the body warm during the winter cold months. The production of heat is also an indication of active metabolic activity in the muscle tissue.
· The muscle provides the framework for the body in addition to maintaining posture and flexible joints.
· There are three types of muscle tissues: skeletal, cardiac and smooth muscles
a. skeletal muscle: multinucleated, striated; attached to bones producing locomotion, general support and posture; producing heat. Contraction is voluntary, and not long-sustained.
b. Cardiac muscle: found only in the heart and responsible for contraction of cardiac tissue and distribution of blood. It is striated, contains individual units called intercalated discs, and functions spontaneously without tiring.
c. Smooth muscle: tissue is anucleated, non-striated but similar to cardiac tissue relative to functional spontaneity and long sustained contraction. It is found in the lining of blood vessels, urinary bladder, kidneys, esophagus and small intestines.
II. STRUCTURE OF MUSCLE TISSUE
· All muscle tissues have a superficial covering of vary thickness called fascia, made of connective tissue and laced with adipose tissue.
· Inside the facia, the muscle tissue is surrounded by epimysium and individual muscle bundles or faciculus are surrounded by perimysium.
· Endomycium is the connective tissue that separates muscle fibers within a faciculus.
· The unit of a faciculus is a muscle fiber (or cell) called myofibril.
· Muscel cells are similar to other tissues. They contain mitochondria (lots of them), Golgi apparatus, ER, SER and other organelles.
Structure of Muscle fiber
· Unlike other cells, muscle fibers are the functional units of the tissue.
· A typical fiber is multinucleated, striated (striped) and surrounded by cell membrane called sarcolemma and cytoplasm called sarcoplasm.
· A network of membranous channels (sarcoplasmic reticulum) are dispersed throughout the sarcoplasm. Transverse tubes or T-tubes runs perpendicular (longitudinal) throughout the sarcoplasm.
· Myofibrils, the functional unit of a muscle tissue consist of protein filaments called myofilaments which consists of thin filaments, actin and thick filaments, myosin. The characteristic light and dark bands on a muscle are due to the arrangement of these myofilaments.
· The dark bands are called A-band and the light band is I- band. The I-band also contain thin lines called Z lines. The A-band and I-band overlap in arrangement. An H zone is an area of light filament where the A and I-bands do not overlap.
· Sacomeres is a subunit myofibril that forms a repeating band of Z-line to Z-line.
Muscle Twitch, Summation and Tetanus
· A physiograph is the equipment used in recording muscle contraction.
· Myogram is the tracing or chart showing muscle contractions. A typical myograph consist of a latent period (initiation), period of contraction and period of relaxation.
· The amount of force needed to produce a stimulus is called threshold stimulus
· Twitch is a response produced when muscle is sufficiently stimulated and produces a series of responses (provide sufficient time is allowed between stimulations and sufficient force is applied.). The response is directly related the strength of the voltage
· When the strength of stimulation and the frequency is stronger and faster, the muscle tissue does not recover sufficiently between stimulations, however the twitches summate or accumulate. The response produces summation due to incomplete tetanus (summation or fusion of series of twitches).
· When contraction is smooth, sustained and normal, this is called complete tetanus.
· Thus a summation is a series of tetani in response to increased frequency and voltage of stimulation.
· In vitro stimulation of muscle obeys the all-or-none law; This means that upon application of greater stimulus, each muscle fiber is individually recruited to contract maximally or not at all. The stronger the stimulus, greater the number of fibers recruited and the greater the response.
· Because of the individual recruitment, muscle contraction can result in a graded contraction.
· Treppe: A stircase-effect of muscle contraction is produced when series of strong electrical stimulus is administered and there is no relaxation between tetanus. It may represent a gradual increament in the response to the stimulus and may be due to an increase in intracellular calcium ion (Ca++) build up.
· A motor unit (a neuron and groups of muscle fibers) innervate and control muscle contraction. The unit contains a muscle endplate or organ, nerve(s), neuromuscular junction. Acetylcholine is the neurotransmitter in muscle that produce muscle contraction.
Isotonic and Isomeric Contractions
When muscle contract, the stimulatory force causing contraction must be stronger than forces opposing contraction. For example, in order to lift the arm alone or with weight (book, etc), the contractile force (tension) necessary to lift the arm and the weight must be greater than the forces of gravity.
· Contraction that results in relatively constant shortening in length of the muscle fibers is called isotonic contraction.
· Isometric contraction is produced, if the motor units recruited are too few or the opposing forces are too weak to shorten the muscle (contraction) and movement does not occur.
III. MECHANISM OF MUSCLE CONTRACTION
The sliding of thin filaments (actin) over and between thick filaments (myosin) produce contraction. ATP is required for the cross-bridge cycle.
· During muscle contraction the sacomeres and the myofibril elements shorten (decrease in length). However the A-bands (myosin) do not short but move close together.
· I-band which represent the distance between A band of successive sacomere also decrease in length (shortens). However, the actin elements within the I-band does not shorten.
Mechanism of contraction
· When activated a motor neuron releases acetylcholine into the synaptic cleft of the neuromuscular junction. Acetylcholine binds to receptors on the sarcolemma which triggers series of events leading to release of Ca++ ions from storage areas within the muscle fiber
· Ca++ ions activate (acts as a control switch) actin filaments (consist of two proteins: troponin and tropomyosin) and enable cross-bridges from myosin filaments to attach to the exposed sites on actin.
· One of the many roles of Ca ions in contraction is cause conformational changes in the shape of troponin and tropomysin that allows the cross-bridges to attach to actin and produce muscle contraction.
· Energy from ATP causes the cross-bridges to bend to pulling actin filaments towards the center of the sacomere. The actin and myosin filaments attach and detach as the molecules proceeds towards the center provided there is enough ATP, until maximum contraction is achieved.
· As soon as contraction occurs, the sarcolemma secretes an enzyme acetylcholinersterase, which decomposes acetylcholine. Decomposition of acetylcholine is necessary to prevent continued stimulation of the muscle fiber and to prepare the fiber for the next stimulus.
· Energy for contraction comes from ATP molecules in the cell. When the body harvest ATP energy in excess (energy in phosphate bones), some energy is temporarily stored as creatine phosphate (CP). When ATP energy is reduced (ADP), a phosphate bond is broken and phosphate energy is released to ADP to form ATP
Anaerobic Respiration and Oxygen debt.
ATP is formed during cellular respiration (36-38 moles) and thru anaerobic respiration (2-4 moles of ATP).
· The amt of ATP formed during anaerobic respiration is not enough to support life
· The ultimate purpose of Oxygen during cellular respiration is to accept the electrons (H+) produced from the oxidation of food molecules for the production of ATP (36-38 moles)
· During aerobic respiration (muscle inactivity), the muscle received it blood supply in the form of myoglobin, which supports the oxidation of glucose to pyruvic acid and CO2.
· Under anaerobic respiration (when there is not enough O2) Pyruvic acid is converted to lactic acid and small amount of ATP formed
· Lactic acid accumulation causes muscle irritation and cramps and fatigue develops. During a period of exercise break when the person is taking in sufficient debt is paid, lactic acid is converted in the liver back to glucose. Hence, after the break, the person feels reenergized.
Slow- and fast- twitch fibers
Muscles are classified on the basis of their contraction speed (time required to reach maximum tension). Base on this classification, there are three types of muscle fibers in humans: Slow-twitch (Type I), Intermediate Type IIA and fast (Type IIB) fibers.
· Classification is associated with different myosin APPase isozymes designated as “slow” or “fast” by which the tissues can be identified.
· Fast-twitch (type II) fibers are thick and contracts fast. They are also called white fibers, and are adapted to anaerobic respiration because of large storage of glycogen and glycolytic enzymes. For example the ocular muscles that position the eye have a high speed contraction reaching a maximum tension in 7.3 msecs.
· Slow-twitch: The soleus muscle in the leg by contrast has a high proportion of slow-twitch fibers and requires about 100 msec to reach max tension. Slow twitch fibers have a rich supply of capillaries, mitochondria and aerobic respiratory enzymes and high conc. of myoglobin. Because of the rich supply of myoglobin, they are also called red fibers.
· Intermediate fibers: are fast-twitch fibers with high concentration for aerobic capacity.
IV. MUSCLE DISORDERS
Cramps: involuntary painful, sustained titanic contractions of a muscle. Precise cause is unknown but a cramp may result from chemical changes in the muscle such as lactic acid accumulation or calcium deficiencies. Sometimes a severe blow to a muscle can produce a cramp
Fibrosis: an abnormal increase of fibrous connective tissue in muscle. Usually it results from connective tissue replacing dead muscle fibers following an injury.
Fibrositis: an inflammation of the connective tissue especially muscle sheaths and fascia associated with muscles, producing soreness and stiffness that is commonly called muscular rheumatism.
Muscular dystrophy: an inherited disease characterized by progressive degeneration of muscle tissue, resulting in atrophy(shrink) of affected muscle and general crippling of the patient.
Myositis: inflammation of the muscle tissue producing soreness and stiffness similar to fibrositis of the muscles
Strains: pulled muscles result when a muscle is stretched extensively; severe pain are common and function may be impaired.
Neurological Disorders Affecting Muscle
Botulism: poisoning caused eating contaminated food (e.g improperly canned vegetables, or meat that contain C. botulinium). Botulinium is a neurotoxin produced by the bacterium Clostridium botulinium. Toxin prevents releaase of acetylcholine necessary for muscle contraction. Condition may result in paralysis if not properly treated and death may occur.
Myasthenia gravis: is an autoimmune disease in which antibodies are produced that attached to the acetylcholine receptors on the sarcolemma, thus blocking or reducing the stimulatory effect of the neurotransmitter. The disease is characterized by extreme muscle weakness.
Poliomyelitis: viral disease of motor neurons in the spinal cord. Destruction of the neurons leads to paralysis of the skeletal muscle. The disease is rare in industrialized countries due to the availability of a polio vaccine. All
children in the US receive this vaccine which protects them from polio.
Spasms: sudden involuntary contraction of a muscle or a group of muscles. They may vary from simple twitches to severe convulsion and may be accompanied by pain. Spasms may be caused by irritations of the motor neurons to the muscle, emotional stress or neurological disorders. Spasm of the smooth muscle in the wall of the small intestines and respiratory passages or blood vessels can be harzadous. Hiccupping is one type of spasm.
Tetanus: disease is caused by sthe anaerobic bacterium Clostridium tetani (common in soil). Infection results from puncture wound. C. tetani produces a neutotoxin that affects motor neuron in the spinal cord resulting in continuous stimulation and titanic contraction of muscles.
V. ORIGINS AND INSERTION
Origin: immovable attachment and insertion is the movable attachment. Muscles are arranged in groups with opposing actions: agonist and antagonists
Muscles are named based on several criteria including shape, location, attachment and size. These criteria are only useful in remembering the names of muscles
Shape: rhomboideus (like in rhomboid); trapezius (as in trapezoid); Triceps or biceps (indicating # of heads of insertion).
Location: pectoralis (in the chest or pectus); intercostals (between ribs; brachii (upper arm)
Attachment: many facial muscles (zygomaticus, nasalis, temporalis); sternocleidomastoid (sternum, clavicle and mastoid process of the skull).
Size: maximus (larger, largest); minimus (smaller, smallest); longus (long); brevis (short)
Orientation of fibers: rectus (straight); transverse (across); oblique (in a slanting or slopping direction)
A. Facial muscle attachments
There are a wide variety of attachment of facial muscles. The are used for a variety of functions such as smilling, chewing, eye movement and movement of the tongue.
Facial expression and mastication
Located on the scalp, face and neck. They originate from the skull and insert on connective tissue of the skin.
a. Epicranius: consists of frontalis over the frontal bone and occipitalis over the occipital bone. The masseter and the temporalis are two major pairs of muscle that raise the mandible in the process of mastication.
b. Others include orbicularis, oculi, orbicularis oris, buccinatior, zygomaticus and platysma.
Muscles that move the Head
Several pairs of neck muscles are responsible for flexing, extending and turning the head. The sternocleidomastoid (origin is clavicle and sternum and insertion is mastoid process and temporal bone) and splenius capitis (orig: cervix and upper thorax vertebrae; insertion is mastoid process and temporalis bone) lower are two major muscles. Trapezoid can also extend the head althought that’s not one of its functions.
Muscles of the Abdominopelvic wall
These pair muscles provide support for the anterior and lateral portions of the abdominal and pelvic