Muscle Dental
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Physiology of skeletal muscle for dental student By Dr Yaser Mohamed Ashour Professor of physiology Al Azhar Faculty of Medicine (Assuit)
Skeletal muscle • The muscle is an excitable tissue. • It is characterized by its ability to contract, which allows it to shorten and generate force following an action potential. • The human body contains three types of muscles: • (1) Skeletal • (2) cardiac • (3) smooth
Skeletal
Cardiac
Smooth
Control
voluntary
involuntary
involuntary
Cross striations
striated
striated
plane
Nerve supply
somatic
autonomic
autonomic
Location
Attached to skeleton
In the wall In the wall of the heart of viscera
Function
Movement of joints
Movement of blood
Movements of contents
Skeletal Muscle • It constitutes about 40 per cent of the body weight. • Most skeletal muscles begin and end in fibrous tendons, by which they are attached to the bone. • It is made up of very individual multinuclear muscle cells or fibers.
The Muscle Fiber • Skeletal muscle is made up of thousands of cylindrical muscle fibers often running all the way from origin to insertion. • The fibers are bound together by connective tissue through which run blood vessels and nerves.
Each muscle fibers contains:
– an array of myofibrils that are stacked lengthwise and run the entire length of the fiber. – mitochondria – an extensive endoplasmic reticulum – many nuclei. – Two types of tubules (longitudinal and transverse)
• The number of fibers is probably fixed early in life. • Increased strength and muscle mass comes about through an increase in the thickness of the individual fibers and increase in the amount of connective tissue.
• Because a muscle fiber is not a single cell, its parts are often given special names such as – sarcolemma for plasma membrane – sarcoplasmic reticulum for endoplasmic reticulum – sarcosome for mitochondrion – sarcoplasm for cytoplasm
Myofibril • Each myofibril along its length shows alternating light (I) and dark (A) bands • The A bands are bisected by the H zone • The I bands are bisected by the Z line.
Each myofibril is composed of many subunits lined up end-to-end. The portion between the two successive Z lines called sarcomere. These subunits are, of course, composed of myofilaments. These myofilaments are thick and thin proteins. Sarcomere is the functional and structural unit of the muscle fibre.
Sarcomere In each sarcomere: Thin myofilaments attached to Z line Thick myofilaments are found in the middle of the sarcomere and do not extend to the ends. Thus, a myofibril has alternating light and dark areas because each consists of many sarcomeres lined up end-to-end.
• Each myofibril contains alternate dark (A) and light (I) bands. • At the center of light (I) band there is a dark line called the Z line. • The thin proteins is bisected by Z line. • At the center of dark (A) band there is a lighter zone called the H zone. • In the middle of the H zone there is a dark line called M line to which myosin is kept in its place by attaching
Myofilaments • Each myofibril is formed of two types myofilaments (muscle proteins):-
–Thick protein (Myosin) –Thin proteins:- Actin - troponin - tropomyosin
Thick myofilaments • Are composed of a protein called myosin. • Each myosin molecule has a tail, which forms the core of the thick myofilament, plus a head that projects out from the core of the filament. • These myosin heads are also commonly referred to as cross- bridges
• The myosin head has several important characteristics: 1- It has ATP-binding sites into which fit molecules of ATP. ATP represents potential energy. 2 - It has actin-binding sites into which fit molecules of actin. Actin is part of the thin myofilament. 3 - It has a "hinge" at the point where it leaves the core of the thick myofilament. 4 - This allows the head to swivel back and forth, and the "swivelling" is, what actually causes muscle contraction.
Thin myofilaments It composed of 3 types of protein: ACTIN, TROPONIN, and TROPOMYOSIN. The actin molecules are spherical and form long chains. Each thin myofilament contains two such chains that coil around each other. TROPOMYOSIN molecules are thin molecules that wrap around the chain of actin. At the end of each tropomyosin is a troponin molecule. The tropomyosin and troponin molecules are connected to each other. Troponin molecules have binding sites for calcium ions.
Tubular system of skeletal muscle • There are two types of the muscle tubules:1- Longitudinal tubules. 2- Transverse tubules
Longitudinal tubules Longitudinal tubules (well developed sarcoplasmic reticulum):- They are tubules lying parallel to the myofibril and part of sarcoplasmic reticulum. - At their ends near the T- tubules, they enlarge forming large chambers called cisternae. Functions:- Store Ca++ in very high concentration in its cisternae. - It has Ca++ pump in its wall. - It contains a protein called cal-sequestrin that can bind 40 times as much as calcium.
Transverse tubules
These are infolds of the sarcolemma into the interior of the muscle fibres that penetrate all the way from one side of the muscle fibre to the opposite side. They contain extracellular fluid and are parallel to Z line. Function: T- tubules transmit the action potential to the interior of the ms fibres.
Neuromuscular junction • It is an area of contact between motor nerve and muscle fibres.
Neuromuscular junction • When the nerve fibre reach near to the muscle it loses its myelin sheath, and the neurilemma continue with the sarcolemma. • The axon breaks into several branches and each branch divide into multiple synaptic knob. • Neuromuscular junction = motor end plate composed of:1- Synaptic knob 2- synaptic cleft and 3- synaptic gutter,
Synaptic knob • Its cytoplasm contain:- Multiple synaptic vesicles that contain acetylcholine which is the chemical transmitter at motor end plate. - Multiple mitochondria that supply energy for formation of acetylcholine. • Its membrane (axolemma):– Voltage gated Ca2+ channels. – Multiple release sites on the inner surface of the surface of the membrane.
Synaptic cleft • It is the space just below the synaptic knob. • It contains acetylcholinesterase enzyme, which has the ability to break down the acetylcholine to choline and acetate.
Excitation-Contraction Coupling 1- On arrival of an excitation wave through the axon to M.E.P., it leads to increase the permeability of the membrane to the Ca++. 2. The entrance of the Ca++ to end of the axon it leads to phosphorlytion of the synapsin and release of the vesicles and sweeps of the vesicles to adhere to the presynaptic membrane. 3. The adherence of the vesicles to the membrane leads to the rupture of the vesicles and release of A.Ch. 4. The released A.Ch pass through synaptic cleft and it passes to the A.Ch. receptor on the postsynaptic membrane. 5. This process leads to increase in Na+ influx which leads to the depolarization of the postsynaptic membrane (sarcolemma).
Excitation-Contraction Coupling • Na+ diffusion produces end-plate potential (depolarization). • + ions are attracted to negative plasma membrane. • If depolarization sufficient, threshold occurs, producing APs.
Excitation-Contraction Coupling • APs travel down sarcolema and T tubules. • SR terminal cisternae releases Ca2+ from chemical release channels: – Electromechanical release mechanism.
• Ca2+ is also released through a Ca2+-induced Ca2+ release.
(continued)
Excitation-Contraction Coupling • Ca2+ attaches to troponin. • Tropomyosintroponin complex configuration change occurs. • Cross bridges attach to actin.
(continued)
- Movement of tropomyosin permits the myosin head to contact actin. - Contact with actin causes the myosin head to swivel. - During the swivel, the myosin head is firmly attached to actin. So, when the head swivels it pulls the actin (and, therefore, the entire thin myofilament) forward. (Obviously, one myosin head cannot pull the entire thin myofilament. Many myosin head are swiveling simultaneously, or nearly so, and their collective efforts are enough to pull the entire thin myofilament). .
- At the end of the swivel, ATP fits into the binding site on the cross-bridge & this breaks the bond between the cross-bridge (myosin) and actin. - The myosin head then swivels back. - As it swivels back, the ATP breaks down to ADP & P and the cross-bridge again binds to an actin molecule - As a result, the head is once again bound firmly to actin. However, because the head was not attached to actin when it swiveled back, the head will bind to a different actin molecule (i.e., one further back on the thin myofilament). Once the head is attached to actin, the cross-bridge again swivels,
As long as calcium is present (attached to troponin), contraction is continue. And, as they do, the thin myofilament is being "pulled" by the myosin heads of the thick myofilament. Thus, the thick and thin myofilaments are actually sliding past each other. As this occurs, the distance between the Z-lines of the sarcomere decreases. As sarcomeres get shorter, the myofibril, of course, gets shorter. And, obviously, the muscle fibers (and entire muscle) get shorter.
Mechanism of relaxation 1. After the contraction if completed, Ca++ is reuptake from around the muscle proteins back to the sarcoplasmic reticulum to be stored in the cistern. 2. Ca++ concentration in the sarcoplasm falls down. The Ca++ combined with troponin C is detached and recollected back into the sacroplasmic reticulum. 3. Tropomyosin is replaced back to its resting position to cover the active sites on actin . 4. The head of myosin get charged with ATP. The charging with ATP disconnects them from the active sites on actin. 5. The myosin head can not reconnect with the next active sites on actin because there site are now covered with tropomyosin. 6. The actin filaments are disengaged from the myosin filaments with elongation of the sarcomere and accordingly of the whole muscle fibres.
Changes associated with muscle contraction 1- Electrical changes Each muscle contraction is preceded by an action potential (A.P.). (A.P.): duration is 5 milliseconds. Its refractory period is short.
2- Mechanical changes On arrival of threshold impulse (action potential) to the muscle, the muscle response to this action potential. The response may be either increase in tension or contraction (shortening). Increase in tension: occurs when the load is too heavy to move, and the contraction is called isometric. Contraction (shortening): occurs when the load is small, and the contraction is called isotonic.
Isotonic
Isometric
Muscle length
Decreased
Remain Constant
Muscle tension
Remain constant
Increase
Energy of contraction
Converted to external work and waste heat
Converted to waste heat
Sliding of myosin and actin
Occur to a muscle extent
-
Duration of contraction
Long
short
O2 and nutrient requirement
Great
less
Heat production
Less
Great
simple muscle twitch
simple muscle twitch
2- Temperature • Warming: leads to stronger and faster contraction this is because warming decrease the viscosity and stimulate the chemical. • Cooling: has the opposite effect.
3- Fatigue.
• This is a decline in response due to previous activity. • Fatigue occur due to depletion of energy stores (ATP, CP, Glycogen and fatty acids), and due to accumulation of metabolic waste products in muscle fibres.
3- Effect of Fatigue.
4- Frequency of stimulation. • When a series of constant stimuli are applied to skeletal muscle at high frequency to fall at the end of relaxation phase of the preceding one. • This produces contraction with stronger force than the previous one up to a certain limit. • There is stepwise increase in the force of contractions at the beginning of multiple successive stimulation (This phenomena called staircase phenomena). • This due to release of Ca++ intracellulary with inability of reticulum to recalled the Ca++ again.
• Slow rate of stimulation: incomplete tetanus (repetitive contractions). • High frequency of stimulation: complete tetanus (fusion of contraction)
4- Frequency of stimulation.
4- Frequency of stimulation.
Skeletal muscle • The muscle is an excitable tissue. • It is characterized by its ability to contract, which allows it to shorten and generate force following an action potential. • The human body contains three types of muscles: • (1) Skeletal • (2) cardiac • (3) smooth
Skeletal
Cardiac
Smooth
Control
voluntary
involuntary
involuntary
Cross striations
striated
striated
plane
Nerve supply
somatic
autonomic
autonomic
Location
Attached to skeleton
In the wall In the wall of the heart of viscera
Function
Movement of joints
Movement of blood
Movements of contents
Skeletal Muscle • It constitutes about 40 per cent of the body weight. • Most skeletal muscles begin and end in fibrous tendons, by which they are attached to the bone. • It is made up of very individual multinuclear muscle cells or fibers.
The Muscle Fiber • Skeletal muscle is made up of thousands of cylindrical muscle fibers often running all the way from origin to insertion. • The fibers are bound together by connective tissue through which run blood vessels and nerves.
Each muscle fibers contains:
– an array of myofibrils that are stacked lengthwise and run the entire length of the fiber. – mitochondria – an extensive endoplasmic reticulum – many nuclei. – Two types of tubules (longitudinal and transverse)
• The number of fibers is probably fixed early in life. • Increased strength and muscle mass comes about through an increase in the thickness of the individual fibers and increase in the amount of connective tissue.
• Because a muscle fiber is not a single cell, its parts are often given special names such as – sarcolemma for plasma membrane – sarcoplasmic reticulum for endoplasmic reticulum – sarcosome for mitochondrion – sarcoplasm for cytoplasm
Myofibril • Each myofibril along its length shows alternating light (I) and dark (A) bands • The A bands are bisected by the H zone • The I bands are bisected by the Z line.
Each myofibril is composed of many subunits lined up end-to-end. The portion between the two successive Z lines called sarcomere. These subunits are, of course, composed of myofilaments. These myofilaments are thick and thin proteins. Sarcomere is the functional and structural unit of the muscle fibre.
Sarcomere In each sarcomere: Thin myofilaments attached to Z line Thick myofilaments are found in the middle of the sarcomere and do not extend to the ends. Thus, a myofibril has alternating light and dark areas because each consists of many sarcomeres lined up end-to-end.
• Each myofibril contains alternate dark (A) and light (I) bands. • At the center of light (I) band there is a dark line called the Z line. • The thin proteins is bisected by Z line. • At the center of dark (A) band there is a lighter zone called the H zone. • In the middle of the H zone there is a dark line called M line to which myosin is kept in its place by attaching
Myofilaments • Each myofibril is formed of two types myofilaments (muscle proteins):-
–Thick protein (Myosin) –Thin proteins:- Actin - troponin - tropomyosin
Thick myofilaments • Are composed of a protein called myosin. • Each myosin molecule has a tail, which forms the core of the thick myofilament, plus a head that projects out from the core of the filament. • These myosin heads are also commonly referred to as cross- bridges
• The myosin head has several important characteristics: 1- It has ATP-binding sites into which fit molecules of ATP. ATP represents potential energy. 2 - It has actin-binding sites into which fit molecules of actin. Actin is part of the thin myofilament. 3 - It has a "hinge" at the point where it leaves the core of the thick myofilament. 4 - This allows the head to swivel back and forth, and the "swivelling" is, what actually causes muscle contraction.
Thin myofilaments It composed of 3 types of protein: ACTIN, TROPONIN, and TROPOMYOSIN. The actin molecules are spherical and form long chains. Each thin myofilament contains two such chains that coil around each other. TROPOMYOSIN molecules are thin molecules that wrap around the chain of actin. At the end of each tropomyosin is a troponin molecule. The tropomyosin and troponin molecules are connected to each other. Troponin molecules have binding sites for calcium ions.
Tubular system of skeletal muscle • There are two types of the muscle tubules:1- Longitudinal tubules. 2- Transverse tubules
Longitudinal tubules Longitudinal tubules (well developed sarcoplasmic reticulum):- They are tubules lying parallel to the myofibril and part of sarcoplasmic reticulum. - At their ends near the T- tubules, they enlarge forming large chambers called cisternae. Functions:- Store Ca++ in very high concentration in its cisternae. - It has Ca++ pump in its wall. - It contains a protein called cal-sequestrin that can bind 40 times as much as calcium.
Transverse tubules
These are infolds of the sarcolemma into the interior of the muscle fibres that penetrate all the way from one side of the muscle fibre to the opposite side. They contain extracellular fluid and are parallel to Z line. Function: T- tubules transmit the action potential to the interior of the ms fibres.
Neuromuscular junction • It is an area of contact between motor nerve and muscle fibres.
Neuromuscular junction • When the nerve fibre reach near to the muscle it loses its myelin sheath, and the neurilemma continue with the sarcolemma. • The axon breaks into several branches and each branch divide into multiple synaptic knob. • Neuromuscular junction = motor end plate composed of:1- Synaptic knob 2- synaptic cleft and 3- synaptic gutter,
Synaptic knob • Its cytoplasm contain:- Multiple synaptic vesicles that contain acetylcholine which is the chemical transmitter at motor end plate. - Multiple mitochondria that supply energy for formation of acetylcholine. • Its membrane (axolemma):– Voltage gated Ca2+ channels. – Multiple release sites on the inner surface of the surface of the membrane.
Synaptic cleft • It is the space just below the synaptic knob. • It contains acetylcholinesterase enzyme, which has the ability to break down the acetylcholine to choline and acetate.
Excitation-Contraction Coupling 1- On arrival of an excitation wave through the axon to M.E.P., it leads to increase the permeability of the membrane to the Ca++. 2. The entrance of the Ca++ to end of the axon it leads to phosphorlytion of the synapsin and release of the vesicles and sweeps of the vesicles to adhere to the presynaptic membrane. 3. The adherence of the vesicles to the membrane leads to the rupture of the vesicles and release of A.Ch. 4. The released A.Ch pass through synaptic cleft and it passes to the A.Ch. receptor on the postsynaptic membrane. 5. This process leads to increase in Na+ influx which leads to the depolarization of the postsynaptic membrane (sarcolemma).
Excitation-Contraction Coupling • Na+ diffusion produces end-plate potential (depolarization). • + ions are attracted to negative plasma membrane. • If depolarization sufficient, threshold occurs, producing APs.
Excitation-Contraction Coupling • APs travel down sarcolema and T tubules. • SR terminal cisternae releases Ca2+ from chemical release channels: – Electromechanical release mechanism.
• Ca2+ is also released through a Ca2+-induced Ca2+ release.
(continued)
Excitation-Contraction Coupling • Ca2+ attaches to troponin. • Tropomyosintroponin complex configuration change occurs. • Cross bridges attach to actin.
(continued)
- Movement of tropomyosin permits the myosin head to contact actin. - Contact with actin causes the myosin head to swivel. - During the swivel, the myosin head is firmly attached to actin. So, when the head swivels it pulls the actin (and, therefore, the entire thin myofilament) forward. (Obviously, one myosin head cannot pull the entire thin myofilament. Many myosin head are swiveling simultaneously, or nearly so, and their collective efforts are enough to pull the entire thin myofilament). .
- At the end of the swivel, ATP fits into the binding site on the cross-bridge & this breaks the bond between the cross-bridge (myosin) and actin. - The myosin head then swivels back. - As it swivels back, the ATP breaks down to ADP & P and the cross-bridge again binds to an actin molecule - As a result, the head is once again bound firmly to actin. However, because the head was not attached to actin when it swiveled back, the head will bind to a different actin molecule (i.e., one further back on the thin myofilament). Once the head is attached to actin, the cross-bridge again swivels,
As long as calcium is present (attached to troponin), contraction is continue. And, as they do, the thin myofilament is being "pulled" by the myosin heads of the thick myofilament. Thus, the thick and thin myofilaments are actually sliding past each other. As this occurs, the distance between the Z-lines of the sarcomere decreases. As sarcomeres get shorter, the myofibril, of course, gets shorter. And, obviously, the muscle fibers (and entire muscle) get shorter.
Mechanism of relaxation 1. After the contraction if completed, Ca++ is reuptake from around the muscle proteins back to the sarcoplasmic reticulum to be stored in the cistern. 2. Ca++ concentration in the sarcoplasm falls down. The Ca++ combined with troponin C is detached and recollected back into the sacroplasmic reticulum. 3. Tropomyosin is replaced back to its resting position to cover the active sites on actin . 4. The head of myosin get charged with ATP. The charging with ATP disconnects them from the active sites on actin. 5. The myosin head can not reconnect with the next active sites on actin because there site are now covered with tropomyosin. 6. The actin filaments are disengaged from the myosin filaments with elongation of the sarcomere and accordingly of the whole muscle fibres.
Changes associated with muscle contraction 1- Electrical changes Each muscle contraction is preceded by an action potential (A.P.). (A.P.): duration is 5 milliseconds. Its refractory period is short.
2- Mechanical changes On arrival of threshold impulse (action potential) to the muscle, the muscle response to this action potential. The response may be either increase in tension or contraction (shortening). Increase in tension: occurs when the load is too heavy to move, and the contraction is called isometric. Contraction (shortening): occurs when the load is small, and the contraction is called isotonic.
Isotonic
Isometric
Muscle length
Decreased
Remain Constant
Muscle tension
Remain constant
Increase
Energy of contraction
Converted to external work and waste heat
Converted to waste heat
Sliding of myosin and actin
Occur to a muscle extent
-
Duration of contraction
Long
short
O2 and nutrient requirement
Great
less
Heat production
Less
Great
simple muscle twitch
simple muscle twitch
2- Temperature • Warming: leads to stronger and faster contraction this is because warming decrease the viscosity and stimulate the chemical. • Cooling: has the opposite effect.
3- Fatigue.
• This is a decline in response due to previous activity. • Fatigue occur due to depletion of energy stores (ATP, CP, Glycogen and fatty acids), and due to accumulation of metabolic waste products in muscle fibres.
3- Effect of Fatigue.
4- Frequency of stimulation. • When a series of constant stimuli are applied to skeletal muscle at high frequency to fall at the end of relaxation phase of the preceding one. • This produces contraction with stronger force than the previous one up to a certain limit. • There is stepwise increase in the force of contractions at the beginning of multiple successive stimulation (This phenomena called staircase phenomena). • This due to release of Ca++ intracellulary with inability of reticulum to recalled the Ca++ again.
• Slow rate of stimulation: incomplete tetanus (repetitive contractions). • High frequency of stimulation: complete tetanus (fusion of contraction)
4- Frequency of stimulation.
4- Frequency of stimulation.
