· Definition: A group of specialized cells similar in embryonic origin, form and function is called tissue.
· But the term histology was given by a German histologist Mayer 1819.
· Study of tissue is called histology.
· Marcello Malpighi, an Italian scientist established a separate branch for the study of tissues so he is the founder of histology.
· Marcello Malpighi, an Italian scientist established a separate branch for the study of tissues so he is the founder of histology.
· Types of Animal Tissues:
· Depending upon the location and function, animal tissue is divided into the following 4 types:
A. EPITHELIAL TISSUE
· Depending upon the location and function, animal tissue is divided into the following 4 types:
A. EPITHELIAL TISSUE
B. MUSCULAR TISSUE
C. CONNECTIVE TISSUE
D. NERVOUS TISSUE
MUSCULAR TISSUE
· Muscular tissues are mesodermal in origin.· These tissues are made up of highly specialized, thin and elongated cells called muscle fibres which are surrounded and held together by the connective tissue.
· They exhibit a high degree of contractility, irritability and conductivity.
· They also possess the properties of extensibility and elasticity.
· These muscles fibres are formed from myoblasts or muscle producing cells.
· These muscles fibres are formed from myoblasts or muscle producing cells.
· Each muscle fibre consists of numerous fine fibrils called myofibrils and its cytoplasm is called sarcoplasm which contains a network of endoplasmic reticulum is called sarcoplasmic reticulum.
· Muscles make up about 40% of the body weight in mammals.
· Muscles make up about 40% of the body weight in mammals.
· They are supplied with blood vessels.
· These blood vessels always provide the nutrition and take away the metabolic wastes.
· Many mitochondria or sarcosomes, glycogen granules are present between the myofibrils to provide the energy for contraction from the oxidation of the glucose.
· Many mitochondria or sarcosomes, glycogen granules are present between the myofibrils to provide the energy for contraction from the oxidation of the glucose.
· General functions of muscle tissues are:
· Brings about the movement of body parts and helps the organism in locomotion.
· They support the bones and other structures of the body.
· They are also responsible for the peristalsis in tubules, viscera, heartbeat, and production of sound.
· They play important role in facial expression and gestures.
· Muscles fibres perform some other important functions like in ingesting the food, in passing urine and faeces, in mating, in egg-laying, delivering the baby, in feeding the young ones, in moving the senses organs and in the posture of the individual.
· Brings about the movement of body parts and helps the organism in locomotion.
· They support the bones and other structures of the body.
· They are also responsible for the peristalsis in tubules, viscera, heartbeat, and production of sound.
· They play important role in facial expression and gestures.
· Muscles fibres perform some other important functions like in ingesting the food, in passing urine and faeces, in mating, in egg-laying, delivering the baby, in feeding the young ones, in moving the senses organs and in the posture of the individual.
Types of Muscular Tissues:
· Depending upon the location, structure and function muscles are divided into three types:
A. STRIATED OR VOLUNTARY MUSCLE OR STRIPED OR SKELETAL MUSCLE
B. UNSTRAITED OR INVOLUNTARY MUSCLE OR SMOOTH OR VISCERAL MUSCLE
C. CARDIAC MUSCLE
· Depending upon the location, structure and function muscles are divided into three types:
A. STRIATED OR VOLUNTARY MUSCLE OR STRIPED OR SKELETAL MUSCLE
B. UNSTRAITED OR INVOLUNTARY MUSCLE OR SMOOTH OR VISCERAL MUSCLE
C. CARDIAC MUSCLE
A. STRIATED MUSCLE
1. STRUCTURE· Skeletal muscles of the body are made of numerous cylindrical cells with several nuclei (coenocytic), arranged in parallel, longitudinally over the entire length of the muscle.
· The muscle cells measure 1 to 40 mm in length and 10 to 100 mm in diameter.
· The muscle cells are termed muscle fibres or myofibres.
· A plasma membrane encloses each muscle fibre is called sarcolemma.
· Cytoplasm of muscle fibres, known as sarcoplasm, which contains numerous mitochondria and nuclei.
· Endoplasmic reticulum is extensive, modified to form the sarcoplasmic reticulum (SR).
· Muscle fibres, in turn, are made up of numerous myofibrils. They are about 1 um in diameter, arranged in parallel to the long axis of the muscle fibre, separated from each other by sarcoplasm. Several hundred to thousands of myofibrils are present in each muscle fibre.
· Myofibrils contain the myofilaments. There are two types - thick filament and thin filament.
· Each thick filament is surrounded by six thin filaments on all sides.
· The myofilaments are made of polymers of the contractile proteins – actin and myosin.
· The myofilaments are made of polymers of the contractile proteins – actin and myosin.
· Each myofilament contains 1500 thick filaments and twice that number of thin filaments.
· The thick and thin filaments interdigitate in an alternate pattern giving rise to alternate light and dark bands.
· Light and dark bands are the results of refractive properties of the alternating interdigitating arrangement of the filaments. This is responsible for the striated appearance of the skeletal muscle.
· The light band is termed I-band (isotropic band) and the dark band is called A-band (anisotropic band).
· A line of highly refractile material present in the centre of the I-band is the Z-line or Krause's membrane.
· Light and dark bands are the results of refractive properties of the alternating interdigitating arrangement of the filaments. This is responsible for the striated appearance of the skeletal muscle.
· The light band is termed I-band (isotropic band) and the dark band is called A-band (anisotropic band).
· A line of highly refractile material present in the centre of the I-band is the Z-line or Krause's membrane.
· A less refractile region in the A-band is the H-line (zone) or Hensen's disc.
· Z-line runs across the entire muscle and is the region of the junction of the myofibrils.
· Z-line runs across the entire muscle and is the region of the junction of the myofibrils.
2. SARCOMERE
· Sarcomere is the part of the myofibril between two successive Z-lines.
· Sarcomere is the part of the myofibril between two successive Z-lines.
· It is the fundamental unit of muscle contraction.
· A sarcomere measures between 1.5 to 3.9 um (average-2.2 um) in length depending on the state of the muscle fibres.
· Each sarcomere consists of one A-band, and one-half of two I-bands on either side of the A-band.
· Thin filaments extend from two adjacent Z-lines and interdigitate with the thick filaments in the A-band.
· H-zone is the region where there is no overlap of thin filaments.
· M-line is a dark staining region in the centre of the H-zones which contains proteins that link the thick filaments together.
· I-bands contain only thin filaments, whereas the A-bands contain thick and thin filaments.
· A sarcomere measures between 1.5 to 3.9 um (average-2.2 um) in length depending on the state of the muscle fibres.
· Each sarcomere consists of one A-band, and one-half of two I-bands on either side of the A-band.
· Thin filaments extend from two adjacent Z-lines and interdigitate with the thick filaments in the A-band.
· H-zone is the region where there is no overlap of thin filaments.
· M-line is a dark staining region in the centre of the H-zones which contains proteins that link the thick filaments together.
· I-bands contain only thin filaments, whereas the A-bands contain thick and thin filaments.
3. CONTRACTILE PROTEINS
· The thick and thin filaments are made up of contractile proteins.
· The thin filaments measure about 1 um in length and 9 nm in diameter whereas the thick filaments are about 1.6 um long and 15 nm wide.
· Thin filaments contain - actin, tropomyosin and troponin in the ratio of 7:1:1.
· Thin filaments contain - actin, tropomyosin and troponin in the ratio of 7:1:1.
· Thick filaments contain myosin.
· Actin is a monomeric globular protein (G-actin), present as a polymerized double-stranded helix (F-actin) and is the backbone of thin filaments.
· Tropomyosin is a regulatory protein loosely folded around the actin double-strand to form a coiled-coil, each molecule of tropomyosin extends over seven actin monomers.
· Troponin is a protein complex bound to a tropomyosin molecule. It consists of three sub-units - Troponin-I, Troponin-C and Troponin-T.
· Tropomyosin and troponin molecules are linked together and regulate actin-myosin interaction.
· Myosin, a complex-binding protein, is the main component of the thick filaments.
· Tropomyosin is a regulatory protein loosely folded around the actin double-strand to form a coiled-coil, each molecule of tropomyosin extends over seven actin monomers.
· Troponin is a protein complex bound to a tropomyosin molecule. It consists of three sub-units - Troponin-I, Troponin-C and Troponin-T.
· Tropomyosin and troponin molecules are linked together and regulate actin-myosin interaction.
· Myosin, a complex-binding protein, is the main component of the thick filaments.
· Each myosin molecule contains six polypeptides chains-2 heavy chains and 4 light chains.
· The two heavy chains intertwine forming a double helix with their ends forming globular structures, termed the myosin heads that lie side by side at one end of a double helix.
· The 4 light chains form part of the myosin head, two for each side.
· The part of the myosin molecule is named the tail.
· The thick filament is made of several of these sin molecules, the tails of each myosin molecule binding to form the body of the filament.
· The thick filament is made of several of these sin molecules, the tails of each myosin molecule binding to form the body of the filament.
· The myosin heads project from all sides of the body and are the cross-bridges.
· The cross-bridges are flexible at two places in the myosin molecule, referred to as hinges.
· One of them is where it leaves the body - the arm, and the other is where the two myosin heads are linked to the arm.
· The hinges have a key role in the contractile mechanism.
· Thick filaments are bipolar, with the myosin molecules facing outwards from the central region.
· Cross-bridges are absent in the H-zone.
· Myofibrils contain structural proteins that are responsible for their structural integrity.
· Cross-bridges are absent in the H-zone.
· Myofibrils contain structural proteins that are responsible for their structural integrity.
· Thin filaments are anchored to the sarcolemma by a large protein, dystrophin.
· Dystrophin connects the actin molecules to a glycoprotein complex in the sarcolemma.
· The glycoprotein complex, in turn, is linked to laminin in the extracellular matrix.
· Actinin, which is a bridging protein, binds actin to the Z-line.
· Titin, a very large protein, connects the ends of the thick filaments.
4. SARCOTUBULAR SYSTEM
· Sarcotubular system is a network of membranous tubules in the myofibres.
· Sarcotubular system is a network of membranous tubules in the myofibres.
· It consists of:
- Transverse tubule (T-tubules)
- Sarcoplasmic reticulum (SR)
· Sarcolemma contains many pores.
· These pores project as tubes into the substance of the muscle fibres forming a grid perforated by the myofibrils.
· This grid of membranous tubes surrounding the myofibrils in the transverse tubules or T-tubules.
· T-tubules do not open into the substance of the myofibrils but pass through completely and open somewhere else in the sarcolemma.
· The T-tubules contain ECF and communicate with the extracellular space via the pores on the sarcolemma.
· The main function of the T-tubules is to conduct action potentials from the sarcolemma to the interior of the muscle fibre, specifically to the sarcoplasmic reticulum.
· At the junction between the A- and I-band, the T-tubules are in close contact with the terminal cisternae of the sarcoplasmic reticulum.
· At these points, the terminal cisternae lie on either side of T-tubules forming the triads.
· Voltage-sensitive receptors, called dihydropyridine receptors, are present in the T-tubules at the triads.
· This grid of membranous tubes surrounding the myofibrils in the transverse tubules or T-tubules.
· T-tubules do not open into the substance of the myofibrils but pass through completely and open somewhere else in the sarcolemma.
· The T-tubules contain ECF and communicate with the extracellular space via the pores on the sarcolemma.
· The main function of the T-tubules is to conduct action potentials from the sarcolemma to the interior of the muscle fibre, specifically to the sarcoplasmic reticulum.
· At the junction between the A- and I-band, the T-tubules are in close contact with the terminal cisternae of the sarcoplasmic reticulum.
· At these points, the terminal cisternae lie on either side of T-tubules forming the triads.
· Voltage-sensitive receptors, called dihydropyridine receptors, are present in the T-tubules at the triads.
· These receptors function as voltage sensors and stimulate the release of calcium ions from the adjacent terminal cisternae during excitation-contraction coupling.
· Sarcoplasmic reticulum :
· Sarcoplasmic reticulum :
· Sarcoplasmic reticulum is the modified endoplasmic reticulum of muscle cells.
· SR is abundant in muscle cells, forming a meshwork around each of the myofibrils.
· SR stores calcium ions and releases them during excitation-contraction coupling.
· SR is abundant in muscle cells, forming a meshwork around each of the myofibrils.
· SR stores calcium ions and releases them during excitation-contraction coupling.
· SR contains Ca2+-ATPase or calcium pump, which actively transports calcium ions from the sarcoplasm into the SR.
· The release of calcium ions from the SR. during excitation-contraction coupling is via the ligand-gated calcium ion channels, called ryanodine receptors present in the SR.
· The release of calcium ions from the SR. during excitation-contraction coupling is via the ligand-gated calcium ion channels, called ryanodine receptors present in the SR.
5. NEUROMUSCULAR JUNCTION
· The skeletal muscles are innervated by the somatic nerves.
· The nerve fibres are axons of large alpha-motor neurons present in the ventral grey of the spinal cord.
· Motor neurons carry impulses to skeletal.
· Motor neurons carry impulses to skeletal.
· The terminal endings of the axons lose their myelin sheath near the middle of the muscle fibres and cluster in a narrow patch in the centre of muscle fibre, referred to as the motor point.
· A specialized synapse, called the neuromuscular junction, is formed at the point where the terminal branch of the axon comes into contact with the muscle fibre.
· The axon terminal divides into short curling branches and is spread over an elliptical area called the motor endplate.
· The nerve terminals invaginate into the muscle fibre but lie outside the sarcolemma.
· A specialized synapse, called the neuromuscular junction, is formed at the point where the terminal branch of the axon comes into contact with the muscle fibre.
· The axon terminal divides into short curling branches and is spread over an elliptical area called the motor endplate.
· The nerve terminals invaginate into the muscle fibre but lie outside the sarcolemma.
· Motor endplate is covered by one or more Schwann cells and is insulated from the surrounding fluid.
· At the motor endplate, the nerve ending and the muscle fibre are separated by space, called synaptic cleft, about 50 to 100 nm wide.
· At the synaptic cleft, the sarcolemma is thrown into numerous folds called junctional folds.
· At the motor endplate, the nerve ending and the muscle fibre are separated by space, called synaptic cleft, about 50 to 100 nm wide.
· At the synaptic cleft, the sarcolemma is thrown into numerous folds called junctional folds.
· The junctional folds contain mitochondria, SR and Golgi complex.
· The nerve ending contains mitochondria and numerous vesicles containing the neurotransmitter, acetylcholine (Ach).
· The nerve ending contains mitochondria and numerous vesicles containing the neurotransmitter, acetylcholine (Ach).
· Synaptic cleft contains reticular fibres through which ECF diffuses into the neuromuscular junction.
6. Muscle contraction follows an action potential:
· A stronger stimulus applied during the action potential will not excite the muscle. The muscle is inexcitable to any magnitude of the stimulus. This is termed the refractory period of the muscle.
· Refractory period of skeletal muscle lasts about 1 to 3 C.
· The skeletal muscles are refractory during the period of action potential but the contraction that follows lasts longer around 100 milliseconds.
· On the other hand, in the case of cardiac muscle, the duration of the action potential is longer (200 msec) and lasts throughout its contraction.
· The skeletal muscles are refractory during the period of action potential but the contraction that follows lasts longer around 100 milliseconds.
· On the other hand, in the case of cardiac muscle, the duration of the action potential is longer (200 msec) and lasts throughout its contraction.
· The function of the skeletal muscle is to develop tension and perform mechanical work.
· The group of muscle fibres and the motor nerve fibre innervating them is termed a motor unit.
· The number of muscle fibres in a motor unit varies depending on the nature of the muscle function.
· The number of muscle fibres in a motor unit varies depending on the nature of the muscle function.
· In the leg muscles, a motor unit contains about 2000 fibres per unit.
· In the extrinsic muscles of the eyeball, it is less than 5 muscle fibres per unit.
· The somatic nerve fibres to the motor unit are excitatory.
· In the resting state, the skeletal muscles are maintained in a state of partial contraction. This is referred to as muscle tone.
· The skeletal muscles exhibit two types of contractions. These are termed:
· In the resting state, the skeletal muscles are maintained in a state of partial contraction. This is referred to as muscle tone.
· The skeletal muscles exhibit two types of contractions. These are termed:
(1) Isometric contraction and
(2) Isotonic contraction.
· In isometric contraction, there is no change in the length of the muscle. The load is greater than the tension or force generated by the muscle, and the muscle does not shorten.
· For example, maintenance of posture against gravity is an isometric type of contraction.
· In isotonic contraction, shortening of the muscle occurs. Tension or force generated by the muscle is greater than the load and the muscle shortens. Walking, lifting of a load involve isotonic contraction.
· In isometric contraction, there is no change in the length of the muscle. The load is greater than the tension or force generated by the muscle, and the muscle does not shorten.
· For example, maintenance of posture against gravity is an isometric type of contraction.
· In isotonic contraction, shortening of the muscle occurs. Tension or force generated by the muscle is greater than the load and the muscle shortens. Walking, lifting of a load involve isotonic contraction.
Twitch
· Twitch is the mechanical response of a skeletal muscle to a single stimulus. The duration of a twitch varies in different muscles, ranging from -20 to 200 msec.
· The mechanical response lasts longer than the action potential (2 to 4 msec).
· Twitch is the mechanical response of a skeletal muscle to a single stimulus. The duration of a twitch varies in different muscles, ranging from -20 to 200 msec.
· The mechanical response lasts longer than the action potential (2 to 4 msec).
Tetanus
· If a muscle is stimulated very rapidly, it does not relax at all between stimuli. The summation produces a smooth sustained contraction of greater magnitude, called tetanus.
· Maximum shortening will cause the ends of the thick filaments to compress against the Z-line and tension is reduced considerably.
· Tension developed during isometric tension generation is a function of the magnitude of overlap between actin and myosin filaments.
· If a muscle is stimulated very rapidly, it does not relax at all between stimuli. The summation produces a smooth sustained contraction of greater magnitude, called tetanus.
· Maximum shortening will cause the ends of the thick filaments to compress against the Z-line and tension is reduced considerably.
· Tension developed during isometric tension generation is a function of the magnitude of overlap between actin and myosin filaments.
Force-Velocity relationship
· Tension developed by a muscle is related to the velocity of contraction.
· Faster the contraction, the lesser is the tension developed. A lighter load can be lifted quickly compared to a heavy load.
· Velocity of contraction is maximal with no load, and tension is maximal with no shortening.
· Maximum tension can develop when the load and velocity of contraction are both approximately one-third of the maximal values.
· Tension developed by a muscle is related to the velocity of contraction.
· Faster the contraction, the lesser is the tension developed. A lighter load can be lifted quickly compared to a heavy load.
· Velocity of contraction is maximal with no load, and tension is maximal with no shortening.
· Maximum tension can develop when the load and velocity of contraction are both approximately one-third of the maximal values.
Muscle fatigue
· Repeated and prolonged contractions of skeletal muscles resulting in a state of loss of muscle power, called muscle fatigue.
· Fatigue may develop due to several causes such as psychic factors, depletion of energy stores in the muscle, since one set of fatigue is related to the rate of utilization of muscle glycogen.
· Fatigue can arise owing to failure at a number of different sites - central synapses, motor endplate or the contractile machinery.
· Muscle fatigue is hastened by interruption of blood flow to the contracting muscles.
· Repeated and prolonged contractions of skeletal muscles resulting in a state of loss of muscle power, called muscle fatigue.
· Fatigue may develop due to several causes such as psychic factors, depletion of energy stores in the muscle, since one set of fatigue is related to the rate of utilization of muscle glycogen.
· Fatigue can arise owing to failure at a number of different sites - central synapses, motor endplate or the contractile machinery.
· Muscle fatigue is hastened by interruption of blood flow to the contracting muscles.
Rigour Mortis
· Rigour Mortis is a condition that occurs several hours after death, with the muscles of the body in a state of contracture.
· The muscles contract and become rigid in absence of action potentials.
· The rigidity is the result of loss of ATP, which normally uncouples the cross-bridge links to actin filaments during relaxation.
· In Rigour Mortis, myosin heads attach to actin filaments in an abnormal fixed manner.
· The muscles remain in rigour until the muscle proteins are degraded by autolysis. This occurs about 15 to 25 hours after death.
Energy supply for muscle contraction
· Energy is required for active muscle contraction. The immediate source of energy is ATP.
· Energy is utilized for:
· uncoupling of the cross-bridge links from the active sites on the actin filaments, to produce the 'power stroke' for the sliding movements of the filaments
· active transport of calcium ions from sarcoplasm, after contraction, into the SR
· maintenance of Na+ and K+ ion concentrations across muscle membrane for the propagation of action potentials.
· The concentration of ATP in the muscles, at any instant, is very low.
· This is sufficient only for a few contractions. If the muscle is to contract for a longer duration, an adequate supply of ATP must be maintained.
· Muscles contain another energy-rich compound - phosphocreatine (creatine - phosphate - CP).
· Phosphocreatine carries an energy-rich phosphate bond similar to that of ATP but has higher energy. Hydrolysis of phosphate from CP releases 10,300 calories per mole, against 7,300 calories per mole released by hydrolysis of ATP.
· Hence, phosphocreatine can phosphorylate ADP and generate ATP, by the following reaction: ADP + CP → ATP + C
· Energy is required for active muscle contraction. The immediate source of energy is ATP.
· Energy is utilized for:
· uncoupling of the cross-bridge links from the active sites on the actin filaments, to produce the 'power stroke' for the sliding movements of the filaments
· active transport of calcium ions from sarcoplasm, after contraction, into the SR
· maintenance of Na+ and K+ ion concentrations across muscle membrane for the propagation of action potentials.
· The concentration of ATP in the muscles, at any instant, is very low.
· This is sufficient only for a few contractions. If the muscle is to contract for a longer duration, an adequate supply of ATP must be maintained.
· Muscles contain another energy-rich compound - phosphocreatine (creatine - phosphate - CP).
· Phosphocreatine carries an energy-rich phosphate bond similar to that of ATP but has higher energy. Hydrolysis of phosphate from CP releases 10,300 calories per mole, against 7,300 calories per mole released by hydrolysis of ATP.
· Hence, phosphocreatine can phosphorylate ADP and generate ATP, by the following reaction: ADP + CP → ATP + C
B. NON-STRIATED MUSCLES
· They are known by several terms- visceral muscle, plain muscle, non-striated muscle or involuntary muscles.· Smooth muscles are involuntary in action or can not be moved by one's own will. So they are also called involuntary muscles. They show prolonged and slow contraction and relaxation even frequency and intensity of these may vary.
· Smooth muscles are present in the viscera, the blood vessels and other structures like the skin, eyeball. Smooth muscles provide the means to alter the position or dimensions of the tissues.
· Smooth muscle cells are spindle-shaped, with a central nucleus and are smaller than skeletal muscle fibres (length: 100 um diameter: 5 um).
· The cells are grouped in bundles (fascicles) by connective tissues and the orientation of the muscle bundles is variable. To achieve the tightest packing, the narrower part of one cell lies adjacent to the broad parts of neighbouring cells.
· In the blood vessels they are circumferential, in the intestines, the orientation is both longitudinal and circular.
· The cells are separated by a gap of 40 to 80 nm. A reticular network around the cell bridges the gap between the cells.
· Gap- junctions are abundant and provide a low resistance path for the passage of electrical activity.
· Sarcoplasmic reticulum is less well developed and T-tubules are absent.
· Actin and myosin filaments are present but are not organized or randomly distributed into sarcomeres.
· The cytoplasm contains a network of ‘intermediate filaments anchored by ovoid structures, called dense bodies.
· Dense bodies function like Z-lines in skeletal muscle sarcomeres, and the actin fibres are connected to them.
· Actin filaments occupy a greater volume, and myosin filaments are less in number. Groups of 3 to 5 myosin filaments are surrounded by many actin filaments.
· The arrangement of the myofilaments enables the smooth muscle to undergo much greater changes in length.
· Sarcoplasmic reticulum is less well developed and T-tubules are absent.
· Actin and myosin filaments are present but are not organized or randomly distributed into sarcomeres.
· The cytoplasm contains a network of ‘intermediate filaments anchored by ovoid structures, called dense bodies.
· Dense bodies function like Z-lines in skeletal muscle sarcomeres, and the actin fibres are connected to them.
· Actin filaments occupy a greater volume, and myosin filaments are less in number. Groups of 3 to 5 myosin filaments are surrounded by many actin filaments.
· The arrangement of the myofilaments enables the smooth muscle to undergo much greater changes in length.
· It has fewer mitochondria, glycogen granules and sarcoplasmic reticulum or cell organelles.
· These muscles have good regenerative capacity because after injury viable smooth muscle cells and pericytes from blood vessels undergo mitosis and provide for the replacement of the damaged tissue.
· These muscles have good regenerative capacity because after injury viable smooth muscle cells and pericytes from blood vessels undergo mitosis and provide for the replacement of the damaged tissue.
1. INNERVATION
· Smooth muscles are innervated by autonomic fibres. In most of the viscera, both parasympathetic and sympathetic innervate the smooth muscles.
· In such structures their actions are antagonistic. In some tissues, only one type of autonomic nerve is present.
· The smooth muscles in the blood vessels receive only sympathetic fibres. The innervation of the alimentary tract is very complex since the gut wall has its own intrinsic innervation.
· Smooth muscles are innervated by autonomic fibres. In most of the viscera, both parasympathetic and sympathetic innervate the smooth muscles.
· In such structures their actions are antagonistic. In some tissues, only one type of autonomic nerve is present.
· The smooth muscles in the blood vessels receive only sympathetic fibres. The innervation of the alimentary tract is very complex since the gut wall has its own intrinsic innervation.
1. single-unit smooth muscle
2. multi-unit smooth muscle
1. Single unit (unitary) smooth muscle
· They have a number of muscle fibres closely joined together.
· All the muscle fibres of this muscle contract together as a single unit automatically and rhythmically.
· They are located in the wall of hollow visceral organs such as the stomach intestine and urinary bladder and blood vessels.
· This type of smooth muscle is a mass of fibres that function as a single unit—functional syncytium.
· The muscle fibres are arranged in bundles or sheets and their cell membrane is structurally linked together at adherent junctions.
· The gap junctions between the individual fibres permit rapid spread o electrical activity.
· The single-unit muscle resembles the cardiac muscle in its function. They are innervated by autonomic nerves.
2. Multi-unit smooth muscle
· Their muscle fibres are not closely joined together.
· The individual muscle fibre contracts as a separate unit more or less as independent muscle fibres.
· They are present at the hair root, on the wall of large blood vessels, ciliary and iris muscle, dilator pupillae, sphincter pupillae and the arrector pili of the hair follicles in the skin belong to the multi-unit type.
· These smooth muscles resemble skeletal muscle and are made up of discrete muscle fibres.
· The muscle fibres are innervated by a single autonomic nerve fibre and are insulated from each other by a basement membrane.
· They seldom exhibit spontaneous contraction and are controlled by the nerve impulses
· Their muscle fibres are not closely joined together.
· The individual muscle fibre contracts as a separate unit more or less as independent muscle fibres.
· They are present at the hair root, on the wall of large blood vessels, ciliary and iris muscle, dilator pupillae, sphincter pupillae and the arrector pili of the hair follicles in the skin belong to the multi-unit type.
· These smooth muscles resemble skeletal muscle and are made up of discrete muscle fibres.
· The muscle fibres are innervated by a single autonomic nerve fibre and are insulated from each other by a basement membrane.
· They seldom exhibit spontaneous contraction and are controlled by the nerve impulses
Some related terms with smooth muscles
a. VARICOSITIES
· Smooth muscles do not have discrete neuromuscular junctions, instead varicosities are present.
a. VARICOSITIES
· Smooth muscles do not have discrete neuromuscular junctions, instead varicosities are present.
· Varicosities are beaded swellings in the postganglionic autonomic nerve endings that contain vesicles filled with neurotransmitters.
· Neurotransmitters combine with specific receptors on the syncytial-linked muscle cells and produce their effects.
· Neurotransmitters combine with specific receptors on the syncytial-linked muscle cells and produce their effects.
ELECTRICAL PROPERTIES
a. Resting membrane potential
· Resting membrane potential of smooth muscle is variable and not stable.
· Resting membrane potential of smooth muscle is variable and not stable.
· In general, resting membrane potential ranges between -50 to -55 mv.
· As in other excitable cells, the membrane potential is due to diffusion of K+ ions and the permeability properties of the cell membrane
· As in other excitable cells, the membrane potential is due to diffusion of K+ ions and the permeability properties of the cell membrane
b. Action potential
· Action potentials are generated and conducted in a single-unit type of smooth muscle.
· Action potentials are generated and conducted in a single-unit type of smooth muscle.
· Two kinds of action potentials can be differentiated in these muscles.
· The duration ranges from 10 to 50 msec and can be evoked by various mechanisms. Nerve impulse, hormones, stretch or spontaneous electrical activity of muscle cells— all these factors can initiate an action potential.
· Action potentials of the type seen in ventricular muscle occur in some smooth muscles. The initial spike is followed by prolonged repolarization (plateau), the duration may be as long as 1 sec. Uterine smooth muscle and vascular smooth muscle generate this type of action potential. These muscles undergo prolonged contraction. Slow calcium ion channels are said to be the cause of prolonged repolarization.
· Smooth muscles of the intestinal wall undergo spontaneous depolarization and repolarization and give rise to action potentials. The spontaneous fluctuation in membrane potentials is termed basal electrical rhythm (BER) or slow waves.
· In the multi-unit type of smooth muscle, contraction occurs without action potential. The muscles are stimulated by transmitters released at the autonomic nerve endings. The transmitters depolarize the sarcolemma and induce a localized potential, termed junctional potential. The junctional potential spreads electrotonically over the muscle and initiates contraction.
· The duration ranges from 10 to 50 msec and can be evoked by various mechanisms. Nerve impulse, hormones, stretch or spontaneous electrical activity of muscle cells— all these factors can initiate an action potential.
· Action potentials of the type seen in ventricular muscle occur in some smooth muscles. The initial spike is followed by prolonged repolarization (plateau), the duration may be as long as 1 sec. Uterine smooth muscle and vascular smooth muscle generate this type of action potential. These muscles undergo prolonged contraction. Slow calcium ion channels are said to be the cause of prolonged repolarization.
· Smooth muscles of the intestinal wall undergo spontaneous depolarization and repolarization and give rise to action potentials. The spontaneous fluctuation in membrane potentials is termed basal electrical rhythm (BER) or slow waves.
· In the multi-unit type of smooth muscle, contraction occurs without action potential. The muscles are stimulated by transmitters released at the autonomic nerve endings. The transmitters depolarize the sarcolemma and induce a localized potential, termed junctional potential. The junctional potential spreads electrotonically over the muscle and initiates contraction.
Types of stimuli
· Unlike skeletal muscles, smooth muscles are either excited or inhibited by a variety of factors—neural, humoral, mechanical or metabolic.
a. Neural
· Transmitters (Ach, Norepinephrine- NE) released at the postganglionic nerve endings either excite or inhibit smooth muscles.
· Unlike skeletal muscles, smooth muscles are either excited or inhibited by a variety of factors—neural, humoral, mechanical or metabolic.
a. Neural
· Transmitters (Ach, Norepinephrine- NE) released at the postganglionic nerve endings either excite or inhibit smooth muscles.
· The response depends on the receptor and tissue type.
· If the smooth muscle of an organ is supplied by both autonomic divisions, Ach and NE have opposite effects, eg; GI tract-Ach is excitatory, NE is inhibitory.
· If the smooth muscle of an organ is supplied by both autonomic divisions, Ach and NE have opposite effects, eg; GI tract-Ach is excitatory, NE is inhibitory.
b. Humoral
· Several hormones induce relaxation or constriction of smooth muscle types, through specific receptors on syncytial-linked muscle cells.
· Several hormones induce relaxation or constriction of smooth muscle types, through specific receptors on syncytial-linked muscle cells.
· Oxytocin specifically stimulates uterine muscle contraction.
· Histamine, cytokines, inflammatory factors are some of the substances that act on smooth muscles.
· Histamine, cytokines, inflammatory factors are some of the substances that act on smooth muscles.
c. Mechanical
· Stretch of smooth muscles also depend on cation ion channels of the cytoskeleton.
· Stretch of smooth muscles also depend on cation ion channels of the cytoskeleton.
· These channels conduct calcium and sodium ions, depolarize the cell membrane and produce a contraction.
· This is called myogenic contraction or myogenic tone.
d. Metabolic
· Metabolic changes in the chemical composition of ECF affect smooth muscle activity.
· Metabolic changes in the chemical composition of ECF affect smooth muscle activity.
· Vascular smooth muscle is decreased by a rise in H+ ion concentration or hypoxia.
Contraction
· Actin and myosin filaments bring about contraction of smooth muscles, by sliding movement of thin and thick filaments.
· Rise in calcium ion concentration in the sarcoplasm initiates the contraction.
· Smooth muscles lack T-tubules, troponin and tropomyosin are absent.
· Calmodulin, a calcium-binding sarcoplasmic protein binds with calcium ions.
· Myosin phosphorylation is an important step in actin-myosin interaction. The sarcoplasmic enzyme, myosin light-chain kinase, phosphorylates myosin.
· Actin and myosin filaments bring about contraction of smooth muscles, by sliding movement of thin and thick filaments.
· Rise in calcium ion concentration in the sarcoplasm initiates the contraction.
· Smooth muscles lack T-tubules, troponin and tropomyosin are absent.
· Calmodulin, a calcium-binding sarcoplasmic protein binds with calcium ions.
· Myosin phosphorylation is an important step in actin-myosin interaction. The sarcoplasmic enzyme, myosin light-chain kinase, phosphorylates myosin.
· The different types of stimuli that excite the smooth muscle increase the sarcoplasmic Ca2+ level, by various mechanisms.
· Calcium ions bind to calmodulin, forming Ca—Calmodulin complex. This complex activates the myosin light-chain kinase which results in phosphorylation of myosin.
· Phosphorylated myosin binds to actin to form cross-bridges.
· The myosin head swivels and slides the actin filament along myosin and in the process pulls on the dense bodies.
· ATP binds to cross-bridge and the actin-myosin link is broken.
· Hydrolysis of ATP releases energy that resets myosin head to bind to a new actin site.
· Formation and dissociation of crossbridges—cross-bridge cycling generates tension and proceed as long as calcium ion concentration is elevated and myosin remains phosphorylated. For example, uterine contractions during labour and maintenance of blood pressure during exercise.
· Calcium ions bind to calmodulin, forming Ca—Calmodulin complex. This complex activates the myosin light-chain kinase which results in phosphorylation of myosin.
· Phosphorylated myosin binds to actin to form cross-bridges.
· The myosin head swivels and slides the actin filament along myosin and in the process pulls on the dense bodies.
· ATP binds to cross-bridge and the actin-myosin link is broken.
· Hydrolysis of ATP releases energy that resets myosin head to bind to a new actin site.
· Formation and dissociation of crossbridges—cross-bridge cycling generates tension and proceed as long as calcium ion concentration is elevated and myosin remains phosphorylated. For example, uterine contractions during labour and maintenance of blood pressure during exercise.
Relaxation
· Relaxation of smooth muscles is dependent on several calcium ion lowering mechanisms:
· Calcium pump in the SR, pump back calcium ions in absence of the stimulus.
· Relaxation of smooth muscles is dependent on several calcium ion lowering mechanisms:
· Calcium pump in the SR, pump back calcium ions in absence of the stimulus.
· Calcium reuptake by SR is called ‘sequestration'.
· Hyperpolarization produces closure of voltage-gated calcium ion channels.
· Reversal of myosin phosphorylation by the enzyme, myosin light chain phosphatase.
· Hyperpolarization produces closure of voltage-gated calcium ion channels.
· Reversal of myosin phosphorylation by the enzyme, myosin light chain phosphatase.
C. CARDIAC MUSCLE
· This muscle consists of a network of interconnected muscle fibres.· Each muscle fibre is a long, cylindrical, uni-nucleated and has no definite sarcolemma.
· Mature cardiac muscle cell is approx 15 microns in diameter and about 85 to 100 microns in length.
· They show faint but regular cross striations due to the presence of myofibrils that contain the myosin and actin filaments in the alternate bands.
· Each muscle cell are connected to each other by special zigzag junctional complexes called intercalated discs.
· Intercalated disc is actually the cell membrane present in the dark staining transverse lines that cross the chains of cardiac cells at irregular intervals.
· This membrane functions as the booster of contraction waves and allows spreading the wave of muscle contraction from cell to cell as well as throughout the network.
· Each fibre possesses abundant cytoplasm and numerous mitochondria (about 40 per cent of the cytoplasmic volume), fatty acids in the form of triglycerides is the main source of energy production and glycogen granules.
· It is because they need a large amount of energy to contract throughout their life. Cardiac muscle is supplied by the nerve fibres of the autonomic nervous system.
· They are not under the control of the will of animals. They never get fatigued or they are immune to fatigue. They have a very rich blood supply.
· They are myogenic in nature. They are found on the wall of the heart and wall of large veins such as pulmonary and Precaval veins.
· Cardiac muscle has virtually has no regenerative power beyond childhood defects or damage in the heart muscle are generally replaced by proliferation of connective tissue forming myocardial scars.
Also, Read our Other Notes Related to Animal Tissues: