The study of the muscles -Myology.

Various movements of the body are made possible due to the contraction of the muscles.

40 to 50% of body weight is made up of muscles.

1. Skeletal movement and locomotion. 2. Vasodilatation / vasoconstriction. 3. GI tract peristalsis for movement of food

and undigested residue. 4. Generation of body heat through muscle

contraction in movement. 5. Cardiac pump - circulation of blood.

muscle

Smooth Cardiac Skeletal

Skeletal muscles may be arranged in any one of the forms mentioned below:

Sheet: muscle arranged into a large flat sheet spread over an area. E.g. muscles making the ventral abdominal walls- external abdominal obliquous

Sheet rolled into bundles: sheet of muscle rolled around limbs-Semi-membranosus

Bundles: muscle arranged into thick bundles either as a cone or spindle form-mainly involved in movement of bones for locomotion. E.g. biceps, triceps

Rings: muscles arranged into circular form around the natural orifices like eyes, mouth, anus, vagina- sphincter anii

Discrete cell or cluster: are muscles either occurring as single strand or arranged into cluster or group.

1. Flexors: -muscles located on the side of the limb towards which the limb bends. Bicep -elbow joint.

2. Extensors: -muscles located on the sides of the limb towards which the bending is restricted or not possible. e.g. triceps.

3. Adductors: -muscles that tend to pull a limb towards the median plane. e.g. pectoral muscles.

4. Abductor: -muscles that tend to move the limb away from the median plane. Eg. deltoideus

5. Sphincter: -muscle that surround the natural opening in the body. e.g. anal sphincter

 

6. Cutaneous muscles:-muscle seen beneath the skin and are responsible for the movement of the skin.

Ligaments: are the chords or bands that attach one end of a bone to another and is made up of tough connective tissues.

Tendon: chord that attach the muscle to the bones

Entire muscle bundle covered with fibrous connective tissue-Epimysium (fascia)

The connective tissue sheath covers bundles of muscle fibres (fasculi)-Perimysium

Connective tissue between individual muscle fibers-endomysium.

• Individual m. cell = fiber • Cell membrane covering a fiber = sarcolemma• T Tubules (transverse)-inward continuation of

sarcolemma. Impulses travels deep into the muscles through these tubules.

• Sarcoplasmic reticulum-network of membrane within sarcolemma.

• Sarcoplasm-equivalent to cytoplasm.Sorrounds muscle cells. Within SR.

• Components of muscle fibers: 200 - 2000 myofibrils - depending on the diameter of muscle; each myofibril has striations or banding

• Each myofibrils contains approximately 1500 thick and 3000 thin myofilaments

Thick filaments-bundles of myosin molecules Two parts- The tail lies parallel to one another

forming the length. Head-projects outwards like arms with

flexible attachment to tail

Thin filaments Made up of three types of proteins-Actin,

Tropomyosin, troponin. Troponin is attached to tropomyosin at

specific sites-Troponin-Tropomysin complex. Troponin-Tropomysin complex is winded on

Actin chain.

A bands-Anisotropic, dark, high density, thick filaments and overlapping thick and thin filaments

I bands- Isotropic, low density, light zones. Composed of thin filaments only.

Z line- dense line. Bisects each I band. One end of each thin filament is attached to Z line.

Sarcomere- the segment of myofribrils between adjacent Z lines (the fundamental unit of contraction)

• Arrival of a impulse to a NMJ releases

acetylcholine (neurotransmitter) there.• ACh is the neurotransmitter that increases

m. fiber permeability to Na+ ions, after which depolarization begins.

• Produces action potential on the sarcolemma.

• Propagation of action potential over the entire sarcolemma.

When action potential reaches specific receptors (dihydropyridine) on the sarcolemma-becomes permeable to Ca ions.

Ca ions diffuses from sarcoplasmic reticulum into sarcoplasm (sorroundings of muscle fibers).

Ca ions binds to the troponin-molecular changes- exposes the myosin binding sites on the actin.

Myosin head binds to actin. ADP and P on myosin head is released upon

binding.

Myosin head rotates from resting stage towards the centre of sarcomere.

Sliding action pulls the thin filament past the thick filament.

Whole muscle is shortened-contraction. Another ATP is attached to the myosin head-

myosin head detaches. Repeats the process of attachment, rotation,

detachment-shortening of whole muscle-contraction.

Contraction continues as long as excess Ca in the sarcoplasm.

No action potential in the sarcolemma- Ca is pumped back out of sarcoplasm with the help of energy from ATP.

No exposure of binding site for myosin head-Detached-Relaxation.

• Plasticity: Stress – relaxation property- stretching

without increasing the final tension or pressure exerted within the content. Without pain

• Smooth muscle does not lose its contractile ability if the expansion is within physiologic limits.

• Returns back to normal tension upon emptying.

• Contraction can be initiated by stretch, hormones, neural, chemical or mechanical stimuli.

• Slow sustained contraction often rhythmic i.e., peristalsis, permitting stretching of smooth m. w/out much change in tension unless stretching is sudden --> contraction

• Cardiac muscle fibers join, form a network • Intercalated disks = apposed cell

membranes where gap junctions occur permitting electrical transmission from one cardiac m. cell to the next - very low electrical resistance here

Individual cardiac muscle cells do not require nerve stimulation to contract

Action potentials first occur spontaneously within specialized myocardial pacemaker cells within the heart, and these are propagated throughout the heart by a specialized conduction system and from cell to cell via the intercalated disks.

Autonomic nerves innervate the pacemaker cells-modify the rate of spontaneous action potentials-determines contraction rate of the entire heart.

Muscle contraction signaling

Muscle contraction mechanism