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Muscular System

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1. Produce Movements- Muscle pulls tendons to move the skeleton2. Maintenance of posture – enables the body to remain sitting or standing3. Joint stabilization4. Guard entrances and exits: Encircle openings to digestive and urinary tracts. Control swallowing and urination5. Heat generation

Muscle contractions produce heatHelps maintain normal body temperature

– Contractility• Long cells shorten and generate pulling force

– Excitability• Electrical nerve impulse stimulates the muscle cell to

contract– Extensibility

• Can be stretched back to its original length by contraction of an opposing muscle

– Elasticity• Can recoil after being stretched

• Skeletal– Attached to bones– Makes up 40% of body weight– Responsible for locomotion, facial expressions, posture, respiratory

movements, other types of body movement– Voluntary in action; controlled by somatic motor neurons

• Smooth– In the walls of hollow organs, blood vessels, eye, glands, uterus, skin– Some functions: propel urine, mix food in digestive tract,

dilating/constricting pupils, regulating blood flow, – In some locations, autorhythmic– Controlled involuntarily by endocrine and autonomic nervous systems

• Cardiac– Heart: major source of movement of blood– Autorhythmic– Controlled involuntarily by endocrine and autonomic nervous systems

• Cells of muscles– Are known as fibers

• Plasma membrane is called a sarcolemma • Cytoplasm is called sarcoplasm• Muscle contraction

– Depends on two types of myofilaments (contractile proteins)

• One type contains actin• Another type contains myosin

– These two proteins generate contractile force

• Connective tissue and fascicles– Connective tissue sheaths

bind a skeletal muscle and its fibers together

• Epimysium – dense regular connective tissue surrounding entire muscle

• Perimysium – surrounds each fascicle (group of muscle fibers)

• Endomysium – a fine sheath of connective tissue wrapping each muscle cell

Connective tissue sheaths are continuous with tendons

Connective tissue sheaths are continuous with tendons

• Nerves and blood vessels– Each skeletal muscle supplied by branches

of• One nerve• One artery• One or more veins

– Nerves and vessels branch repeatedly– Smallest nerve branches serve

• Individual muscle fibers

• Muscle attachments– Most skeletal muscles

run from one bone to another

– One bone will move – other bone remains fixed

• Origin – less movable attachment

• Insertion – more movable attachment

Sarcolemma - cell membrane– Surrounds the sarcoplasm (cytoplasm of fiber)• Contains many of the same organelles seen in other cells• An abundance of the oxygen-binding protein myoglobin– Punctuated by openings called the transverse tubules (T-

tubules)• Narrow tubes that extend into the sarcoplasm at right angles

to the surface• Filled with extracellular fluid

Microscopic Anatomy of Skeletal Muscle

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• System of tubular sacs similar to smooth ER in nonmuscle cells

• Sarcoplasmic reticulum stores Ca+2 in a relaxed muscle• Release of Ca+2 triggers muscle contraction• Forms ‘triad’ with T-tubules

• A triad consists of a T-tubule and the segments of sarcoplasmic reticulum on either side.

• Myofibrils -cylindrical structures within muscle fiber– Are bundles of protein filaments (=myofilaments)

• Two types of myofilaments1. Actin filaments (thin filaments)2. Myosin filaments (thick filaments)

– At each end of the fiber, myofibrils are anchored to the inner surface of the sarcolemma

– When myofibril shortens, muscle shortens (contracts)

Microscopic Anatomy of Skeletal Muscle

• Basic unit of contraction of skeletal muscle– Z disc (Z line) – boundaries of each sarcomere – Thin (actin) filaments – extend from Z disc toward the center

of the sarcomere– Thick (myosin) filaments – located in the center of the

sarcomere • Overlap inner ends of the thin filaments• Contain ATPase enzymes

• A bands – full length of the thick filament– Includes inner end of thin filaments

• H zone – center part of A band where no thin filaments occur• M line – in center of H zone

– Contains tiny rods that hold thick filaments together• I band – region with only thin filaments

– Lies within two adjacent sarcomeres

• Myofibrils are built of 3 kinds of protein– contractile proteins

• myosin and actin– regulatory proteins which turn contraction on & off

• troponin and tropomyosin– structural proteins which provide proper alignment,

elasticity and extensibility• titin, myomesin, nebulin and dystrophin

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• Titin – a spring-like molecule in sarcomeres– Resists overstretching– Holds thick filaments in place– Unfolds when muscle is stretched

• Composed of one motor neuron and all the muscle fibers that it innervates

• There are many motor units in a muscle

• The number of fibers innervated by a single motor neuron varies (from a few to thousand)

• The fewer the number of fibers per neuron the finer the movement (more brain power)

Motor Units

• Motor neurons innervate skeletal muscle tissue– Neuromuscular junction is the point where nerve

ending and muscle fiber meet

Neurotransmitter – chemical released by nerve upon arrival of nerve impulse The neurotransmitter for skeletal muscle is

acetylcholine

Neurotransmitter attaches to receptors on the sarcolemma

Sarcolemma becomes permeable to sodium (Na+)

Sodium rushing into the cell generates an action potential

Once started, muscle contraction cannot be stopped

• When an action potential reaches the presynaptic terminal of the motor neuron acetylcholine is released in synaptic cleft

Mechanism of Contraction

Titin

• Ach is removed from the receptors by acetylcholinesterase

• Ligand-gated Na+ channels close• Na/K pumps reestablish the RMP• Ca++ ions leave troponin and are

brought back into the cisternae (this process needs energy)

• Tropomyosin moves back over the actin active site

• The myosin heads release their binding to actin

• The filaments passively move back into resting position

Muscle relaxation

It has the following steps:1.Before contraction begins, An ATP molecule binds to the myosin head of the cross-bridges.2.The ATPase activity of the myosin head immediately cleaves the ATP molecule but the products (ADP+P) remains bound to the head. Now the myosin head is in a high energy state and ready to bind to the actin molecule. 3.When the troponin-tropomyosin complex binds with calcium ions that come from the sarcoplasmic reticulum, it pulls the tropomyosin so that the active sites on the actin filaments for the attachment of the myosin molecule are uncovered.4.Myosin head binds to the active site on the actin molecule.

SLIDING FILAMENT THEORY

Figure 12.7

5. The bond between the head of the cross bridges(myosin) & the actin filaments causes a the bridge to change shape bending 45° inwards as if it was on a hinge, stroking towards the centre of the sarcomere, like the stroking of a boat oar. This is called a POWER STROKE.

6. This power stroke pulls the thin filament inward only a small distance. 7. Once the head tilts, this allows release of ADP & phosphate ions.8. At the site of release of ADP, a new ATP binds. This binding causes the

detachment of the myosin head from the actin.9. A new cycle of attachment-detachment-attachment begins.10. Repeated cycles of cross-bridge binding, bending and detachment

complete the shortening and contraction of the muscle.

SLIDING FILAMENT THEORY

Comparison of Skeletal, Cardiac and Smooth Muscle

Comparison of Skeletal, Cardiac and Smooth Muscle

• Skeletal muscle fibers are categorized according to– How they manufacture energy (ATP)– How quickly they contract

• Skeletal muscle fibers– Are divided into 3 classes

• Slow oxidative fibers (Type I)– Red Slow twitch

• Fast glycolytic fibers (Type IIx)– White fast-twitch

• Fast oxidative fibers (Type IIa)– Intermediate fibers

Isotonic contractions Myofilaments are able to slide

past each other during contractions

The muscle shortens

Isometric contractions Tension (forces) in the muscles

increases The muscle is unable to shorten

(not appear to be moving)

The presence of INTERCALATED DISKS allow cardiac muscle fibers to transmit impulses faster among themselves.

The INTERCALATED DISKS are special cell junctions between cardiac fibers that allow them to exchange chemicals and function as a group.

Smooth Muscle

• Does not contain sarcomeres.

• Contains > content of actin than myosin (ratio of 16:1).

• Myosin filaments attached at ends of the cell to dense bodies.

• Contains gap junctions.

Smooth Muscle Contraction

• Depends on rise in free intracellular Ca2+.• Ca2+ binds with calmodulin.

– Ca2+ calmodulin complex joins with and activates myosin light chain kinase.

• Myosin heads are phosphorylated.– Myosin heads binds with actin.

• Relaxation occurs when Ca2+ concentration decreases.

In smooth muscle, calcium ions combine with CALMODULIN to allow the actin and myosin cross-bridges to form.

CALMODULIN is found in smooth muscle and takes the place of troponin; a calcium-calmodulin complex initiates the sequence leading to the formation of the cross-bridges.

• Rigor mortis is a state of muscular stiffness that begins 3-4 hours after death and lasts about 24 hours

• After death there is Excessive release of calcium out of the sarcoplasmic reticulum in muscle that allow myosin heads to bind to actin

• Since ATP synthesis has stopped, crossbridges cannot detach from actin until proteolytic enzymes begin to digest the decomposing cells.

1. Write the functions of muscles? 2. How many types of muscles are found in human body?3. State the difference between the 3 types of muscle

present in human body 4. Write the names and locations of different connective

tissue wrappings of skeletal muscle? 5. Write about the microscopic anatomy of skeletal

muscle? 6. What is myofibrils?7. What is sarcomere? 8. Draw the picture of sarcomere? 9. Name the protein and its location found in sarcomere? 10. Write short notes on sliding filament theory? 11. What is the difference between isometric and isotonic

contraction?

12. Write the characteristics of muscles? 13. Define neuromuscular junction? 14. What is neurotransmitter? 15. Write the difference between the following?

supine and prone positionabduction and adductionFlexion and extension

16 . What is the reason for muscle fatigue?