Soil - properties and contamination

Soil is a living system that has to remain in a state of dynamic balance to be healthy. It is a complex ecosystem in which the substances move in cycles from plants to animals, to soil bacteria and back again to plants. Solar energy is the natural fuel that drives the soil cycles and living organisms of all kinds are necessary to sustain the whole system and keep it in balance. Soil is an essential base of human life.It is a  site for agricultural and forest production, a place for storing raw materials and wastes, a constituent element of landscape and a mirror of the history of civilizations and cultures.

Soil is a biologically active, complex mixture of minerals, organic materials, living organisms, air and water. Soil is an interface between biosphere, lithosphere, hydrosphere and atmosphere. Land occupies 38% of the total space. Soil is the outer most layer of the earth’s crust (Earth’s living skin-1/3 of surface). About 95% of human food is derived from the earth. Only 10% of the world’s land area is suitable for growing crops. Soil quality is degrading worldwide. Soil biota depend on the soil environment for their energy and nutrient supply.

 Soil functions 

Soil is an essential natural resource. It is an integrator of all parts of ecosystems. It is a medium for plant growth, a home for organisms and a storehouse of water, heat and chemicals.It is a decomposing medium for wastes, a source material for construction of shelter and a buffer system  to neutralize harsh environmental changes. Soil is a dynamic universal ecological system and a complex heterogeneous medium. A vital resource that provides food, feed, fuel and fiber. Soil is a material which nourishes and supports growing plants. Soil is a mixture of mineral matter, organic matter, water and air. (Example: Loam soil = 45% mineral matter, 5% organic matter, 25% water, and 25% air). Soil is a three phase system of a solid phase (Mineral matter,organic matter), a liquid phase (soil water) and a  gaseous phase( soil air).

Major components of soil

 Soils have four major components: Mineral matter contains three fractions, sand, silt, and clay. Organic matter contains appreciable quantities of nitrogen, phosphorus and sulfur. Air and water  occupy the pore spaces in soils.

Soil volume -Soil consists of organic particles and inorganic matter with pore spaces between and within them. Pore spaces contain soil air, and soil solution.  In other words, soil volume consists of solid, liquid and gaseous phases.

Types of soil water (i.e. water in the soil) 

 Gravitational water:  Gravitational water fills all the pore-space, and leaves no room for oxygen and gaseous exchange.

Capillary water: This water which is held with the force of surface tension by the soil particles, and is resistant to the forces of gravity. 

Hygroscopic water: This water is held so tightly (by surface tension) to the soil particles that the plant roots can't take it up.

Physical Properties of Soils 

 Soil Texture –the relative proportions of sand, silt, and clay in the soil. e.g. sandy soil, silty soil,and clay soil.

Soil Structure – Structure refers to the arrangement of soil particles. E.g., Granular, Platy, Wedge, Blocky, Prismatic, and Columnar. Soil structure is of particular importance in the absorption of water and the circulation of air.

Soil Color - Color in soils is due primarily to two factors, humus content and the chemical nature of the iron compounds. Humus has a dark brown or black color. Iron is an important color material which stains mineral particles. Ferrous oxide gives gray color. Ferric oxide gives red color. Hydrated ferric oxide gives yellow color.

Soil pH is primarily controlled by the concentration of free hydrogen ions in the soil matrix. Acidic soils have a relatively large concentration of hydrogen ions. Alkaline soils have a relatively low concentration of hydrogen ions.

Soil Types - Depending on the size of the particles in the soil, it can be classified as:

sandy soil, silty soil, clay soil, loamy soil, peaty soil and chalky soil.

Soil contamination

It is the presence of man-made chemicals or other alteration to the natural soil environment. Contaminants bind tightly to the soil. Contaminants evaporate into the air and end up in soil/ground water. The common chemicals of soil contamination include petroleum hydrocarbons, pesticides, heavy metals and solvents.Sources of soil contaminants comprise industrial wastes, active mine wastes, solid wastes and waste waters.

 Petrochemicals- Consumption, transportation and extraction of fossil fuels. 
Agricultural chemicals- Pesticides, fertilizers.
Solid wastes - Wastes from agriculture – crop and farm residues, animal manure.
Wastes from mining – coal wastes, metal ore wastes.
Industrial wastes- solvents chemicals, paints.
Solids from sewage treatments -biomass sludge, settled solids.
Ashes – residues from solid fuels
Garbage – glass, metals, clothes, plastics, wood, papers.

The intensive use of agrochemicals reduces soil fertility, soil biodiversity, nitrogen fixation and crop yield. Pesticides may infiltrate soil, carried away by wind, spread by runoff, leach out into the groundwater and finally reach rivers, lakes and ocean.

Human health effects

Exposure to heavy metals in soil results in nervous system disorders, kidney damage and liver toxicity. Exposure to agricultural chemicals causes cancer and infertility. Chronic exposure to other industrial toxins may causes birth defects, nervous system disorders and kidney diseases.

Quote for reflection

"We abuse land because we regard it as a commodity belonging to us. When we see land as a community to which we belong, we may begin to use it with love and respect." 

-Aldo  Leopold, A sand county Almanac.

Endocrine system - characteristics and functions

Endocrinology is the science dealing with the chemical integration of the physiological functions of an organism.  The science of Endocrinology was born from the experiments of Bayliss and Starling(1902 to 1905). Pende introduced the term ‘Endocrinology’ (Greek, endon=within; krinein= to separate). There are two types of glands in human body: exocrine glands or glands with duct or glands of external secretion e.g. salivary glands and endocrine glands or ductless glands or glands of internal secretion e.g. pituitary gland, thyroid gland.

A hormone is a regulatory chemical that is secreted into the blood by an endocrine gland. The blood distributes the hormones throughout the body. The hormones produce specific physiological effect on certain target organs. The word hormone is derived from the Greek word ‘hormon’ which means ‘to excite’. Bayliss and Starling in 1905 coined the term ‘hormone’.

 Types of chemical messengers

·         Hormones- are products of endocrine glands.

·         Neuro-hormones – are products of nerve cells and released at neurohemal organ.

·         Neurohumors- are products of nerve cells and released at axons.

·         Phytohormones –plant hormones e.g.auxin.

·         Pheromones –ectohormones.

Properties of hormones

 Hormones are low molecular weight chemical messengers. They are secreted in trace amounts in response to specific secretary stimuli. They are soluble in water and act as catalysts.

A single hormone has multiple effects on a single target tissue or on several different tissues. Hormones have high degree of action specificity. They are not species specific and non-antigenic. They are inactivated as soon as their functions are over. Endocrine glands are under the control of nerves.

Chemical classes of hormones

1.     Amines - Secreted by gland cells of nervous origin. e.g.,Adrenaline, ADH.

2.     Steroids - Secreted by glands derived from coelomic mesothelium e.g., corticosteroids, estrogens, androgens.

3.     Proteins - Hormones made up of amino acids and polypeptides e.g., thyroxine, calcitonin, STH.

Steroid hormones are formed from cholesterol. They are lipid-soluble hydrophobic molecules. They are secreted by the gonads, adrenal cortex and placenta e.g., aldosterone, cotisol, estrogen, testosterone and progesterone.

Peptide hormones are short chains of amino acids. They are water-soluble. They are secreted by pituitary and parathyroid glands e.g., oxytocin, vasopressin, calcitonin, insulin

Amine hormones - They are derived from the amino acid tyrosine. They are secreted from the thyroid and the adrenal medulla.

Mechanism of hormone action

 Hormones activate or inhibit enzyme systems e.g., phosporylase. Hormones alter cell permeability. For example Insulin promotes transfer of glucose. Hormones directly activate or suppress particular genes. For example ecdysone causes puffing of certain genes.

Control of hormone secretion
 Humoral signal-secretion depends upon the level of blood ions and nutrients. E.g. insulin production controlled by blood glucose.
Neuronal signal- secretion depends on nerve impulses-stimulation of sympathetic nervous system release catecholamines.
Hormonal signal- some hormones control the release of hormones from another endocrine tissue-hypothalamus hormones.

Hormone secretion can be stimulated / inhibited by other hormones - e.g., trophic hormones, plasma concentrations of ions /nutrients, neurons and mental activity and environmental changes - e.g. light, temperature

Concept of feedback mechanism

The production of a hormone by an endocrine gland is controlled by its circulatory level. A reciprocal relationship exists between the blood level of a hormone and rate of its synthesis and secretion. Feedback mechanisms help in maintaining homeostasis within the endocrine system. This concept is proposed by Moore and Price in 1932. It is a two-way communication between the endocrine gland and the target gland. It is a self-balancing mechanism.
 In a negative feedback system, a gland is sensitive to the concentration of the substance it regulates. When the concentration of the regulated substance reaches a certain threshold level, it inhibits the gland from secreting more hormones until the concentration returns to normal. For example the neurons in the hypothalamus secrete thyroid releasing hormone (TRH), which stimulates the anterior pituitary to secrete thyroid-stimulating hormone (TSH). TSH stimulates the synthesis and secretion of thyroid hormones. When blood levels of thyroid hormones increase above a certain threshold, TRH-secreting neurons in the hypothalamus are inhibited and stop secreting TRH.
In a positive feedback, the increased activity of an endocrine gland is followed by stimulation. The positive feedback is not common. For instance oxytocin acts on uterine muscles during child birth. The secretion stops when baby leaves birth canal. The positive feedback is easily observed under experimental conditions e.g. muscular injection of estrogen in female induces ovulation.

Hormone transport 
Hormones are normally present in blood plasma. Steroid and thyroid hormones bind to transport proteins in the plasma. Amines and peptide hormones are hydrophilic and mix easily with blood plasma. Unbound hormones have shorter half life and bound hormones exhibit longer half life. Transport proteins protect circulating hormones.

Hormone receptors
Hormones activate only those cells that have receptors for them. Hormone receptors are located on plasma membrane, in the cytoplasm or in the nucleus. Hormone – receptor interactions exhibit specificity and saturation. Protein hormones react with receptors on the surface of the cell. Steroid hormones react with receptor sites inside a cell.

Functions of endocrine system

1.     Homeostatic function – endocrine system regulates body fluid composition, rate of gaseous exchange and cardiovascular functions.

2.     Integrative function – endocrine system supports the role of nervous system.

3.     Morphogenetic  function – It influences embryonic development

4.     Permissive function - Certain hormones require the presence of another hormone for the expression of their activity.

Endocrine disrupting chemicals (EDCs)

EDCs are both natural and man-made chemicals that may mimic or interfere with the function of hormones in the body. Environmental endocrine disrupting chemicals include persistent organic pollutants (POPs) such as pesticides (DDT), dioxins, polychlorinated biphenyls (PCBs) and plasticizers (biphenol A). Endocrine disruption is an important public health concern. EDCs produce adverse developmental, reproductive, neurological and immune effects in both humans and wildlife. In humans, EDCs may cause male infertility, reproductive abnormalities, reproductive diseases and neuro-developmental disorders.

Nerve impulse conduction -mechanism and characteristics

Nervous system consists of  interconnecting fibers of communication network. In the ‘hard-wiring’ of the nerves, the signals travel in the form of a flow of electrical current called nerve impulses. Irritability is the universal property of life which means the capacity of organisms to respond to changes in the environmental conditions. The specific environmental stimulus elicits a change in an organism is termed a response. The stimulus-response reactions afford internal constancy in the face of environmental changes.

Discovery of neuron

Camillo Golgi (1843-1926) invented a specific    staining technique for neurons. Cajal in 1888 identified the networks of nerve cells. Golgi and Cajal received the Nobel Prize in 1906 for Medicine and Physiology. Wilhelm His in 1886 showed that the dendrites and axons grow out progressively from the immature neurons in the brains of embryos. Henri Forel (1848-1931) in 1886,observed that when the  cell body dies or an axon is cut, degeneration  of the neuron stop at the junction to another neuron, thus giving evidence that they are separate.

Description of a neuron

A neuron consists of a cell body and two kinds of processes, the dendrites and the axon. The cell body has neuroplasm, a nucleus, nissl bodies, neurofibrils and a cell membrane. The dendrites carry impulses towards the cell body. The axon carries impulses away from the cell body. The axon originates from axon hillock of the cell body. The axon is surrounded by two coverings: myelin sheath and Schwann sheath. These two coverings are interrupted at intervals by nodes of Ranvier. The fine branches at the end of axon are called axon terminals.

Properties of neuron

1.    Excitability –a nerve can be stimulated by suitable stimulus-mechanical, thermal, chemical, electrical.

2.    Conductivity – impulse is conducted similar to cable conduction and digital in character.

3.    All or none law – the stimulus should be in adequate threshold strength.

4.    Refractory period – when the nerve fiber is once excited, it will not respond to a second stimulus for a brief period.

5.    Indefatigability – nerve is normally not fatigued.

6.    Adaptation –the nerve quickly adapts itself.

7.    Accommodation – slowly applied stimulus is accommodated.

Definition of nerve impulse

•         A nerve impulse is the sum total of physical and chemical events associated with the transmission of a signal along an axon.

•         A wave of physiological activity- primarily an electrical phenomenon.

Stimulus is defined as a sudden change in the environment which is strong enough to cause a response in the living organism. All or none law indicates the relation between the stimulus and response. A stimulus, if it is capable of causing a response, causes a maximum response. If it is below the capacity, it will not cause any response. The lowest strength of stimulus required to give rise to an action potential is the threshold stimulus. A stimulus which is less than the threshold fails to induce any response-sub-threshold stimulus. A stimulus which is greater than optimum is supra threshold stimulus.

Stimulation can be affected by strength and duration. The weaker the stimulus, the longer it will have to be applied to produce a response. The nerve takes lesser time to respond for a stronger stimulus.

Velocity of impulse conduction

Johannes Muller believed that nerve impulse travelled at a speed of light-186,000 miles per sec. Helmholtz showed that velocity of conduction was 100m per sec. (about 10 times faster than a man can run). The velocity varies from 100 m per sec in large fibers to 0.5 m per sec in small non-myelinated fibers.

Refractory period of impulse conduction

Once an impulse has passed over any part of the neuron, for a short time it is unable to conduct any other stimulus. This brief period of non-conductivity is called refractory period. Under good physiological conditions, the nerve fiber is indefatigable.

                   Mechanism of impulse conduction

Bioelectricity

L. Galvani in 1786 discovered the presence of electrical current in nerves and muscles.

Electrophysiology

Du Bois Raymond in 1848 concluded that impulse transmission was electrical by a wave of relative negativity.

Electrochemical basis of bioelectricity

Hodgkin and Huxley in 1939 demonstrated the electrical and chemical processes involved in bioelectricity.

Demonstration of electrogenesis – microelectrode studies

In order to confirm the generation of electricity, one microelectrode is placed on the outer surface of the nerve membrane and the other placed inside the nerve cell. When the terminals are connected to a galvanometer, the needle show a deflection indicating the flow of electrical current (-70mV). In a resting neuron the electrical potential ranges from 20 to 100 mV.

Resting potential

According to the membrane potential hypothesis of Bernstein (1902), the differential concentration of ions between the inside and outside of the nerve cell is the basis of resting potential. The inner side of the nerve contains large negatively charged non-diffusible protein ions and smaller diffusible K⁺ and Cl^- ions. Na⁺ ions are more concentrated on outside of the nerve cell. Radio-isotopic studies showed that potassium and sodium ions readily diffuse through the nerve cell membrane.

In a state of physiological rest, the inner side of a neuron is electrically negative to outside. This difference in electrical charge is called resting potential. The resting potential is maintained as long as the cell is alive and active. The resting potential depends upon the selectivity and variable permeability of cell membranes. The unequal distribution of ions maintains the resting potential.

Factors influencing ionic imbalance in a nerve cell

·       Active transport

·       Concentration gradient

·       Membrane permeability

·       Electrostatic attraction

Ion-exchange pump - This Na⁺ - K⁺ pump maintains unequal concentration of ions in the nerve fibers by the process of active transport. This pump changes the electrical character of the nerve fibers. ATP is used as energy for the process.

Concentration gradient - The active transport establishes the concentration gradient. The extrusion of sodium ions is linked with active uptake of potassium ions. The rates of diffusion of ions depend upon gradients and membrane permeability.

Donnan membrane equilibrium - “at equilibrium, the product of the concentration of the diffusible ions inside the membrane equals the product of the concentration of the diffusible ions outside.” (Donnan 1928).

Selective membrane permeability - All the plasma membranes are selectively permeable to ions. This membrane selectively allows inward diffusion of K⁺ and prevents inward diffusion of Na⁺ ions.

Electrical gradient - The inner side of the nerve cell is negative to the outside. There is a growing attraction between ions of the same charges. The parallel diffusion of ions restores the resting potential.

Action potential (Hodgkin 1949, Huxley 1952 and Katz 1959)

When a nerve is stimulated, Na+ ions suddenly move into the cell and causing a positive potential. The influx of Na + ions reaches its peak in 100msec.Sodium permeability causes depolarization. The inward diffusion of Na + ions halted near the peak of action potential. The diffusion K+ ions restore the membrane potential –called repolarization. By using radioactive sodium (Na ²⁴) and potassium (K⁴²) Hodgkin and Keynes (1955) showed that inward flux of sodium was increased 20 times and outward flux of potassium 3-4 times after stimulation of nerve.

             Properties of Impulse conduction

The conduction of nerve impulse causes two phases in the moving wave called biphasic action potential. The conduction of nerve impulse in a myelinated fiber is so rapid because the action potential skips from node to node and this phenomenon is called saltatory conduction. The magnitude of nerve impulse transmission occurs without decrement is termed Non-decremental conduction. The velocity of conduction in a myelinated nerve is directly proportional to its diameter. The property of myelinization speeded up the conduction velocity and consumption of energy.

Muscle contraction - biochemical mechanisms and theories

A muscle contraction is a key biological process by which animals move by contracting their muscles. Movement is the basic property of living systems. The vital activities like digestion, reproduction, excretion and circulation are all possible by the contraction of muscles.
Muscles are complex biological motors, which convert chemical energy into mechanical work and force. In human beings, muscles constitute 40% of the body. The energy for muscle contraction is obtained from the chemicals adenosine triphosphate (ATP) and creatine phosphate ( CP).

                                          Uses of muscles

Skeletal muscle – skeletal muscle gives shape and structure to the body. It enables animals to maintain erect posture. It brings about movement. It helps the animal to secure food and shelter and escape from danger.It helps to communicate its wishes.

Smooth muscle - Smooth muscle assists breathing movements. It aids in hearing and vision.It helps the processes of digestion, excretion, reproduction and circulation. It helps to propel the digested food, body fluids, glandular secretions and waste products. It pumps blood to all parts of the body.

Characteristics of muscles

•         Excitability - the ability to receive and respond to stimuli.

•         Conductivity - The ability  to receive a stimulus and transmit a wave of excitation (electrochemical activity)

•         Contractility - the ability to shorten forcibly when stimulated.

•         Extensibility - the ability to be stretched or extended.

•         Elasticity - The ability to bounce back to original length

                            Types of muscles

Unstriped or plain muscle – It has elongated spindle shaped muscle fibers with thickened central belly and two pointed terminals. The cytoplasm is granular without any cross striations. A rod like nucleus is placed in the center. Myofibrils are arranged in longitudinal axis. They are found in the alimentary canal, respiratory tract, uterus, urinary bladder, arteries and veins.

Cardiac or heart muscle – heart muscles are branched and form a net work. The two adjacent muscle cells form tight junctions in the form of intercalated discs.They exhibit faint transverse striations. They are innervated by autonomic nerve fibers. The nucleus is round or oval in shape. They have fewer myofibrils with greater amount of sarcoplasm for more storage of glycogen and the sarcolemma is indistinct.

Skeletal or striped or muscle – are complex, elongated, cylindrical and fast moving muscles. They vary considerably in size, shape and arrangements of fibers. The size ranges from the smallest stapedium muscle of the middle ear to larger thigh muscles of the human body. Each fiber is multinucleated with transverse and longitudinal striations. The cytoplasm is composed of myofibrils with many myofilaments. Large number tubules run through sarcoplasm and form sarcoplasmic reticulum. The sarcosomes or mitochondria supply ATP to the myofibrils.

        Light microscopic structure of skeletal muscle

Each muscle fiber  displays  dark anisotropic bands or  A bands and light  isotropic bands or I bands. Each A band has a less denser region called H band or Hensen’s line. In each I band, there is a dense cross line called Z band. The area between two adjacent Z lines is called a sarcomere.

Fibrillar system of skeletal muscle

The myofilaments consist of thick myosin filaments and thin actin filaments and are arranged in an overlapping manner. The myosin filaments bear thick knob like projections called cross bridges. The sarcoplasmic reticulum is made up of longitudinal system of canals between myofilaments. There is also T system of canals.

                          Chemical composition of muscle

Water - Muscle contains about 75-80% of water. Water provides a good medium for inorganic and organic compounds. Water reduces friction and dehydration of muscles during contraction.

Proteins - Muscle consists of 3 types of proteins namely structural proteins (e.g., collagen, elastin), contractile proteins (e.g., myosin, actin, and tropomyosin) and enzymatic proteins (e.g., adenosine triphosphatase, creatine phosphatase and lactic dehydrogenase).

Minerals - Calcium ions of sarcoplasm initiates muscle contraction. Magesium ions never initiate muscle contraction but important for muscle coordination. Sodium and Potassium ions set the action potential of impulse conduction.

Organic compounds - Muscle is a storehouse of glycogen and oxidation of glycogen provides energy.  The lipids found in the form of phospholipids. The activity of muscle is proportional to the amount of phospholipids. ATP is the primary source of energy for muscle contraction. ATP molecules found associated with G-actin.

Contractile proteins- Myosin is the  prime contractile element of muscle. It has a triple helical structure. Its molecular weight is 420,000. The hydrolysis of myosin with enzyme trypsin yields two fractions – heavy meromyosin (HMM) and light meromyosin (LMM). Hmm acts as an enzyme ATPase for splitting of ATP into ADP and Pi. The hydrolysis of HMM with papain yields sub- fragment 1 and sub-fragment 2.

Actin -  is non-contractile and elastic in nature. Actin is made up of spherical molecules (G-actin) with a molecular weight  of 60,000. G-actin polymerizes into double stranded helices called fibrous or F-actin.

G-actin +ATP -àF-actin +ADP +Pi.

           Association and disassociation of actomyosin

1 mole of actin +3 mole of myosin -à actomyosin (super- precipitation).

Actomyosin +ATP—^{ca++, mg++}àactin +myosin +ADP

Tropomyosin – is a non-contractile, fibrous protein. It plays important role in sensitizing actin and myosin molecules to calcium ions. This sensitivity is important in order to switch contraction on or off.
Troponin - Troponin occurs at intervals on the actin filament. Troponin takes up ca++ ions from the sarcoplasm to initiate muscle contraction. In muscle troponin and tropomyosin combines to form troponin- tropomyosin system.

Sources of chemical energy

Adenosine triphosphates (ATP) - ATP is the immediate source of energy for muscle contraction. The breakdown of phosphate bond  of ATP releases maximum energy.

Anaerobic glycolysis:

Glucose -à 2 moles of lactic acid +8ATPs.

Aerobic glycolysis coupled with Kreb‘s cycle:

Glucose --à6 CO2 + 6H2O +38 ATPs.

Creatine phosphate (CP) or phosphagen - It forms a reservoir of high energy phosphate in the muscle.It cannot be used as a direct source of energy. It can be used for regeneration of ATP from ADP.

Creatine phosphate----------àcreatine + phosphoric acid

Phosphoric acid +ADP -------à ATP

Glucose – glycolysis as a source of energy - Glucose is stored in the muscle in the form of glycogen. Muscle glycogen is converted into glucose by glycogenolysis. Glucose is oxidized by glycolysis.

C⁶H¹²O⁶   + 6O²--------à6CO² +6H²O +38 ATP

Cori’s Lactic acid cycle

The oxidation of lactic acid to carbon dioxide produces energy for the reconversion of ADP to ATP. The lactic acid produced in the muscle contraction passes into blood stream and is transported to the liver. Within the liver, lactic acid is converted to liver glycogen and then to blood glucose. The conversion of lactic acid to glycogen requires oxygen. Muscle glycogen comes only from the glucose of the blood.

Biochemical basis of muscle stimulation

The stimulation of nerve from the central nervous system initiates electrical changes in the muscle. The depolarization of sarcolemma is caused by the sudden influx of Na+ ions and efflux of k+ ions. The nerve impulse spreads in the muscle and releases Ca2+ ions from the sarcoplasmic reticulum. The flooding of Ca2+ ions starts the contractile machinery.

       Molecular changes during muscle contraction
The Ca2+ ions bind to the troponin molecules. Troponin – Ca2+ complex removes tropomyosin blockage of actin sites. The heads of myosin – ATP complex form Cross-bridges to actin filament. The hydrolysis of ATP induces conformational changes in the heads of myosin.
1 mole of Actin + 3 moles of myosin à Actomyosin

       Molecular changes during muscle relaxation –

The Ca2+ ions are sequestered from actin filament by sacroplasmic reticulum. The Ca2+ ions are released from troponin – Ca2+ complex. Troponin permits tropomyosin return to blocking position. Then there is a separation of myosin-actin cross-bridges. ATP – myosin Complex reformed in heads of thick filament.

Physical changes during muscle contraction
Heat production -liberation of heat is always associated with muscle contraction.

Electricity generation -small amount of electrical energy is released.

Volume changes –negligible changes in volume occur.

Change in optical properties -changes in the birefringence and transparency occur at the muscle fiber.

Sound production -muscle sound noted during contraction.

Theories/ models  of muscle contraction

Sliding filament theory 

This theory was evolved independently and more or less simultaneously by A.F Huxley and H.E. Huxley around 1950s. According to this theory, the force of contraction is developed by the cross bridges in the overlap region. The active shortening is caused by the movement of the cross bridges, which causes one filament to slide over the other. During muscle contraction, the actin filaments alone show movement. But the myosin filaments remain static. The mechanical movement utilizes the energy derived from the breakdown of ATP molecules.

 Electrochemical theory – Davies model (1963)
 In the resting state, the cross        bridges are in slanting position due to negative charges in both the basal and tips of cross bridges due to the concentration of magnesium ions. After the stimulation of muscle, the release of calcium ions change the electrical character which leads to mutual repulsion and shortening of the cross bridges. The attachment of cross bridges on the actin filaments cause sliding of actin filaments while myosin filaments remain static.

Rowboat model- Huxley-Simmons model – According to this scheme, there are flexible hinges on myosin: one between S1 heads and the long rods and the second between S2 and the LMM at the trypsin reaction site. When the head piece (S1) binds to the exposed site on actin it is thought to rotate. This type of rotation occurs simultaneously at numerous locations of actin – myosin filaments which cause shortening of the muscle. The energy for this process derived from the hydrolysis of ATP.