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Sliding Filament Theory - Skeletal Muscle :



The sliding filament theory is the method by which muscles are thought to contract. It is recommended that you read the muscle structure article before continuing with the sliding filament theory.

The diagram below is a common one used to explain sliding filament theory but dont worry about trying to understand it all just yet.





At a very basic level each muscle fibre is made up of smaller fibres called myofibrils. These contain even smaller structures called actin and myosin filaments. These filaments slide in and out between each other to form a muscle contractions, hence called the sliding filament theory!

The diagram above shows part a myofibril called a sarcomere. This is the smallest unit of skeletal muscle that can contract. Sarcomeres repeat themselves over and over along the length of the myofibril.



Here is a quick reminder of all the structures involved:

Myofibril: A cylindrical organelle running the length of the muscle fibre, containing Actin and Myosin filaments.
Sarcomere: The functional unit of the Myofibril, divided into I, A and H bands.
Actin: A thin, contractile protein filament, containing 'active' or 'binding' sites.
Myosin: A thick, contractile protein filament, with protusions known as Myosin Heads.
Tropomyosin: An actin-binding protein which regulates muscle contraction.
Troponin: A complex of three proteins, attached to Tropomyosin.
Here is what happens in detail. The process of a muscle contracting can be divided into 5 sections:

A nervous impulse arrives at the neuromuscular junction, which causes a release of a chemical called Acetylcholine. The presence of Acetylcholine causes the depolarisation of the motor end plate which travels throughout the muscle by the transverse tubules, causing Calcium (Ca+) to be released from the sarcoplasmic reticulum.

In the presence of high concentrations of Ca+, the Ca+ binds to Troponin, changing its shape and so moving Tropomyosin from the active site of the Actin. The Myosin filaments can now attach to the Actin, forming a cross-bridge.

The breakdown of ATP releases energy which enables the Myosin to pull the Actin filaments inwards and so shortening the muscle. This occurs along the entire length of every myofibril in the muscle cell.

The Myosin detaches from the Actin and the cross-bridge is broken when an ATP molecule binds to the Myosin head. When the ATP is then broken down the Myosin head can again attach to an Actin binding site further along the Actin filament and repeat the 'power stroke'. This repeated pulling of the Actin over the myosin is often known as the ratchet mechanism.

This process of muscular contraction can last for as long as there is adequate ATP and Ca+ stores. Once the impulse stops the Ca+ is pumped back to the Sarcoplasmic Reticulum and the Actin returns to its resting position causing the muscle to lengthen and relax.
It is important to realise that a single power stroke results in only a shortening of approximately 1% of the entire muscle. Therefore to achieve an overall shortening of up to 35% the whole process must be repeated many times. It is thought that whilst half of the cross-bridges are active in pulling the Actin over the Myosin, the other half are looking for their next binding site.



Stretched Muscle





Looking at the diagram above again, shows a stretched muscle where the I - bands and the H - zone is elongated due to reduced overlapping of the myosin and actin filaments. There would be reduced muscle strength because few cross bridges can form between teh actin and myosin.



Partially Contracted Muscle





The diagram above shows a partially contracted muscle where there is more overlapping of the myosin and actin with lots of potential for cross bridges to form. The I - bands and H - zone are shortened.


Fully Contracted Muscle





The diagram above shows a fully contracted muscle with lots of overlap between the actin and myosin. Because the thin actin filaments have overlapped there is a reduced potential for cross bridges to form again. Therefore there will be low force production from the muscle.

Nerve Propagation & Motor Units :


Nerve propagation is the way in which a nerve transmits an electrical impulse. In order to understand this, it is important to understand the structure of a motor neurone (nerve)




Nerve Structure :

Each neurone contains a cell body and an axon. The cell body contains a nucleus which is the centre of operation for the neurone and dendrites or branched projections which act to conduct electrical impulses towards the nucleus.

The axon (long thin part of the neuron) carries the electrical impulses away from the cell body and towards the muscle. At the end the axon branches into axon terminals and end at synaptic knobs which have contact with the muscle. Surrounding the Axon is a fatty covering called the Myelin sheath which acts to insulate the nerve. The sheath is not continuous however and contains breaks, known as nodes of ranvier. The impulse jumps from one node to the next, allowing a more rapid conduction. The picture below shows the structure of a motor neurone.



Nerve Propagation :

Nerve propagation is the way in which an impulse is transmitted along the nerve. When not under impulse a nerve has a negative charge compared to its surroundings. This negative charge is called the resting membrane potential and in this state the neurone is polarised. In order for an impulse to travel along the neurone, the resting membrane potential must be changed and become depolarised.

This occurs because the stimulus allows a surge of Na+ ions (sodium) into the cell, which changes the charge, making the inside positive compared to its surroundings. This is depolarisation. When this reaches a threshold, an action potential is established and the impulse can travel along the neurone.

The 'all or none' law states that there must be a minimum level of depolarisation for an action potential to occur. Without reaching this level, no impulse will be propagated.

Prior to another action potential occuring the resting membrane potential must be restored. This ensures that each stimulus is kept separate. This repolarisation is achieved by the movement of K+ (Potassium) ions out of the cell, restoring the internal negative charge.


Motor Unit :

A motor unit is described as a single motor neurone and all of the muscle fibres it innervates. A motor unit can contain anywhere between 10 and thousands of muscle fibres. Muscles which produce large powerful movements contain motor units with large numbers of fibres, and those for small intricate movements contain only a few fibres per motor unit.

Where the synaptic knobs of the neurone meet the muscle fibres is known as the neuromuscular junction. When an impulse reaches the neuromuscular junction, a neurotransmitter called Acetylcholine is released which filters across the synaptic cleft (microscopic space between the synaptic knob and motor end plate). This causes depolarisation of the motor end plate and puts the sliding filament theory of muscular contraction into practice.

The 'all or none' law as mentioned above also applies to the contraction of fibres within a motor unit. When a motor unit activates, all of the fibres within the unit contract and at full force, there is no strong or weak contraction. The strength of the resultant whole muscular contraction depends upon the number of motor units recruited.

Another way of increasing the stength of a muscle contraction is by decreasing the time between impulses so that the muscle fibres do not have time to relax, resulting in a continuous wave of contractions known as wave summation. To produce a strong contraction all motor units in the muscle are recruited, but only for a short time. In order to increase the length of a contraction a kind of rotation system is implemented whereby some units contract while others rest and continuously alternate. This is known as spatial summation or tetanus.







A single contraction of one motor un






Spatial summation







Wave summation

Blood Flow :


The rate and distribution of blood flow through the circulatory system is variable and related to several factors. Physical activity, cardiac output and venus return.




Physical Activity :

With exercise, metabolism speeds up and because of this the muscles require more oxygen
So the heart beats faster to supply the muscles with more oxygen-rich blood
In turn the speed of blood flow increases.






Cardiac Output :

Due to an increase in heart rate (and stroke volume) to meet demands, cardiac output (the volume of blood pumped out of the heart in one minute) automatically increases
The faster and harder the heart pumps, the higher the rate of blood circulation.


Venous Return :

Venous return is the return of blood to the heart via venules and veins
If this is slow, the volume of blood pumped from the heart with each beat (stroke volume) is lower
This lowers cardiac output and reduces blood pressure and flow rate.


Blood Pressure :

Blood pressure is defined as the force exerted by the blood against the vessel wall. Blood pressure is highest in arteries and gradually decreases as it passes through arterioles, capillaries, venules and finally, veins.

Blood pressure is also variable and can increase due to exercise where the cardiac output increases thus forcing more blood through the arteries or by altering the peripheral resistance. This occurs by vasoconstriction, increases in blood viscosity (thickness) and changes in shape or size of the vessels. The regulation of blood pressure is the responsibility of the sympathetic and parasympathetic nervous systems.


Vasomotor Control :

When blood pressure falls sensors called baroreceptors are stimulated which cause a nervous impulse to be trasmitted to the arterioles, causing them to vasoconstrict, which results in an increased blood pressure due to the smaller cross sectional area through which the blood can pass.

In reverse, raised blood pressure also stimulates barorecptors which causes impulses directing the arterioles to vasodilate, increasing the area through which blood can pass and consequently reducing blood pressure.


Venomotor Control :

Veins too can alter their shape in response to stimuli recieved from the sympathetic and parasympathetic nervous systems. They do not have such a thick muscular wall as arteries although are capable of increasing the venomotor tone of their walls to alter their shape to increase or decrease blood pressure, although this is not as effective as the vasodilation/constriction which occurs in arteries. For this reason veins require some extra help to increase their pressure and return the blood to the heart.

Venous Return :

Venous return (blood returning to the heart) must constitute three fifths of the blood circulating the body at any time in order to maintain a steady blood flow. At rest this is not a problem, however, during exercise the blood pressure in the veins is not high enough to increase the level of venous return and so maintain the higher stroke volume and cardiac output which exercise requires. A number of mechanisms are used which help to increase venous return:

Pocket Valves: located within the veins prevent the backflow of blood and help it towards the heart
Muscle Pump: Many veins are situated between skeletal muscles, which when they contract and relax, squeeze on the veins and help push the blood back towards the heart.
Smooth Muscle: The wall of each vein contains smooth muscle which contracts to help push the blood back towards the heart
Respiratory Pump: The respiratory pump helps return blood in the thoracic cavity and abdomen back to the heart. Whilst exercising we breathe faster and deeper which rapidly changes the pressure within the thorax between high and low to help to squeeze the blood in the area back to the heart.
Gravity: Veins in the upper body are aided by gravity in order to return blood to the heart.

Arteries :


Arteries are blood vessels which carry blood away from the heart. All of which, with the exception of the pulmonary artery, carry oxygenated blood. The most widely known artery within the human body is the Aorta.

This is the largest of all blood vessels and transports blood away from the left ventricle of the heart where it then branches into smaller arteries.

As the arteries devide further they become smaller and smaller, until they are classed as arterioles. Arterioles continue to branch into smaller and smaller vessels which, once they have decreased in size below 10 micrometers in diameter are known as capillaries.

The pulmonary artery, is classed as an artery as it carries blood away from the heart, however it carries deoxygenated blood. The blood it carries has travelled around the body and back to the heart where it is pumped, via the pulmonary artery, to the lungs to release waste products and pick up more oxygen.



Structure :

The artery walls consist of three layers:

Tunica Adventitia: This is the strong outer covering of arteries and veins which consists of connective tissues, collagen and elastic fibres.
Tunica Media: This is the middle layer and consists of smooth muscle and elastic fibres. This layer is thicker in arteries than veins.
Tunica Intima: This is the inner layer which is in direct contact with the blood flowing through the artery. It consists of an elastic membrane and smooth endothelial cells. The hollow centre through which blood flows is called the lumen.





Smaller arteries and arterioles contain more smooth muscle tissue in order to control the changing pressure of the blood flow. This change in pressure is a direct effect of the pumping of the heart. During the diastolic phase blood pressure is low due to the rest period of the heart. In the systolic phase the heart contracts, forcing blood through the arteries and subsequently increasing the pressure. This change in pressure within an artery is what you can feel when you take a pulse.

Capillaries :


Capillaries are the smallest of all blood vessels and form the connection between veins and arteries. As arteries branch and divide into arterioles and continue to reduce in size as they reach the muscle they become capillaries. Here the capillaries form a capillary bed, which is a vast expanse of very small vessels forming a network throughout the muscle. However, unlike veins and arteries, their main function is not transporting blood. They are specially designed to allow the movement of substances, mainly gases Oxygen and Carbon Dioxide into and out of the capillary.


Gaseous Exchange :

The oxygen carried within the red blood cells as Oxyhaemoglobin, at this point dissociates from the Haemoglobin and passes through the capillary wall into the muscle cells where it is 'picked up' by Myoglobin, the muscle cells equivalent to Haemoglobin. The Oxygen can now be used in aerobic metabolism to provide the muscle with energy.

The waste product produced during aerobic metabolism is Carbon Dioxide. Due to the lower concentration of Carbon dioxide in the capillaries than the muscle tissue (especially during high levels of metabolism) there is a surge through the capillary wall. From here the blood continues into venules and then veins which return the deoxygenated and CO2 rich blood back to the heart and then on to the lungs where the CO2 is exhaled and more Oxygen is taken up.


Structure :

Capillaries have very thin walls comprised only of endothelial cells, which allows substances to move through the wall with ease. Capillaries are very small, measuring 5-10 micrometres in width. However, the cross-sectional area of capillaries within an average size muscle would be larger than that of the Aorta. This allows a fast and efficient transfer of oxygen-carrying red blood cells to the site where they are needed.

Veins :


Veins are blood vessels which carry deoxygenated (or very low levels of oxygen) blood back to the heart. The exception to this rule is the pulmonary vein, which carries oxygenated blood, from the lungs, back to the heart, ready to be pumped around the rest of the body.

At tissue level, capillaries drain blood into venules, which are very small veins, which as they return to the heart merge into larger veins before reaching either the Superior Vena Cava (if returning from tissues and organs above the heart) or the Inferior Vena Cava (if returning from tissues and organs below to the heart). The Inferior Vena Cava is larger than the Superior Vena Cava. These two large arteries merge and return blood to the right atrium of the heart.


Structure :

The structure of veins is similar to that of arteries, again consisiting of three layers:





Tunica Adventitia: This is the strong outer covering of arteries and veins which consists of connective tissues, collagen and elastic fibres.
Tunica Media: This is the middle layer and consists of smooth muscle and elastic fibres. This layer is thinner in veins.
Tunica Intima: This is the inner layer which is in direct contact with the blood flowing through the vein. It consists of smooth endothelial cells. The hollow centre through which blood flows is called the lumen. Veins also contain valves which prevent the back flow of blood and aid venous return.