Skill objectives:
• Define the term Myology.
• Identify various muscle tissues.
• State and briefly explain the physiological function of muscles.
• Explain the mechanism of contraction ad relaxation.
What is Myology?
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Myology is the specialized study of muscles and muscle tissue.
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Muscle (from Latin musculus "little mouse") is contractile tissue of the body and is derived from the mesodermal layer of embryonic germ cells.
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It makes up to 40-50% of the body weight.
Mention the various functions of muscles.
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Skeletal movement and locomotion.
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Vaso-dilation and vaso-constriction.
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GI tract peristalsis (which pushes food through the digestive system).
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Generation of body heat.
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Cardiac pump-circulation of blood.
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Eye movement or adjustment
Identify various type of muscle.
a) Depending upon its structure- striated and un-striated muscle. b) Depending upon its function- voluntary and involuntary muscle.
Classification of muscle -
Muscle
Smooth or un-striated or involuntary muscle.
Striated or voluntary muscle.
Cardiac
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State the functions of Smooth muscle.
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Regulates the flow of blood in the arteries.
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Moves the feed along gastrointestinal tract.
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Expels urine from the urinary bladder.
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Sends foetus out into the world from the uterus.
 Regulates the flow of air through the lungs.
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Adjust the diameter of pupil and thickness of the lens.
Discuss Smooth muscle on its occurrence, its structure.
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Smooth muscle also called involuntary or un-striated muscle. The term are used synonymous.
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Occurs in visceral structures associated with digestive, urogenital, respiratory, and vascular system.
 Smooth muscle is found in systems of the body that are autonomic in their function
Single-unit/ unitary smooth muscle.
Multi-unit smooth muscle.
99%. Has generalized action on entire organ. (Intestine, stomach).
1%. Have more specific functions. (Hair in the skin).
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Smooth muscle is made of single, fusiform (spindle-shaped) cells.
 Centrally located prominent nucleus.
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It gets its name because no striations are visible in them.
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Each smooth muscle cell contains thick (myosin) and thin (actin) filaments that slide against each other to produce contraction of the cell. The thick and thin filaments are anchored near the plasma membrane (with the help of intermediate filaments), not orderly arranged to form striations.
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They are regulated by ANS as they occur in associated with ANS, therefore they are influence by chemicals (i.e. drugs).
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Sizes vary from 50-250 micro meter in length, 5-10 micro meter in diameter.
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Since 99% is in visceral structures, therefore it is called as visceral muscle. As 1% occurs in iris and ciliary body of eye.
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Generally stimulation by a nerve is not required.
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Its ability to adjust to being stretched without increasing the final tension or the pressure exerted on the contents within a hollow viscus surrounded by smooth muscle. As it is stretched, the tension increases at first, but then in a few secs or mins the smooth muscle relaxes again to its original tension, even though it is still elongated.
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Smooth muscle cells in the walls of the hollow organs are arranged so that only a fraction of cells is supplied by autonomic nerves. These can bring about contraction, but nerve stimulation is not required for normal smooth muscle as stated earlier, since it has various pace-maker –cell points for its own depolarization and contraction resulting from stimuli such as distention, chemical ad hormonal influences, or myogenic self-excitation without any extrinsic stimulus.
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State the function of cardiac muscle .
The rhythmic contraction is responsible for the circulation of blood
Discuss cardiac muscle on its occurrence, its structure .
 It is restricted only in heart.
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It is also considered a type of striated muscle as it is having striations- how-everlike smooth muscle-contracts intrinsically and it is not under voluntary control.
Therefore it is also called as involuntary striated muscle.
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Usually one sometimes two prominent nucleus which is centrally located.
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Striated muscle, but cells are branched forming network.
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Regulated by ANS.
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Intercalated disk = where adjacent cells anastomose
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Rhythmic contractility (no nerve stimulus). As it has got pace-maker for the purposes.
 Drugs can increase or decrease heart rate &/or contractility
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Intercalated disks are located between cardiac muscles cells. These contain gap junctions which provide communicating channels between cells.
State the Function of skeletal muscle .
 For locomotion- as they are attached to the bone, they change the relative positions of bones i.e. angle change.
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Respiration- by contracting to change the volume of the thoracic cavity.
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Production of heat through shivering (which is the result of brief repetitive contractions of skeletal muscle throughout the body.
Discuss striated muscle on its occurrence, its structure and types of skeletal muscle
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Skeletal muscle is the muscle tissue we are most familiar with as the flesh or meat of our farm animals.
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These muscle are usually attached to the bones of skeleton thereby locomotion- hence the term skeletal muscle.
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Since it is under conscious therefore the name – voluntary muscle.
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Characterized by fibers with visible striations but not branched.
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Do not anastomose unlike cardiac muscle.
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Multiple nuclei peripherally arranged.
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Is under voluntary control- i.e. central nervous system controls its movement.
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Drugs can increase or decrease muscle Contraction.
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Require a nerve impulse from PNS (motor innervations) to each muscle fiber.
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Skeletal muscles contain two main types of fibers, which differ in the mechanism they use to produce ATP; the amount of each type of fiber varies from muscle to muscle and from animal to animal.
 There are two types of skeletal muscle fibres; Red and white fibers.
Differentiate between Red and White fibers.
Red-fibers White-fibers thickness
Mitochondria
Colour
Respiration
Contraction
Are thin
Rich
Dark red
Aerobic
Slow, but long.
Are much thicker. poor
Light
Anaerobic
Fast, but short. occurs flight or running Start flight or run.
Discuss the Structural skeletal musculature
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Individual muscle cell is called as fiber.
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Cell membrane covering a fiber or sheath = sarcolemma.
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Muscle is encased in CT elements which are continuous from the individual muscle fibers to the CT of the structure to which the muscle attaches & on which it exerts its pull when it contracts.
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Components of muscle fibers: 200 - 2000 myofibrils /muscle fiber - depending on the diameter of the muscle.
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Protein myofilaments- 2: 1 ratio of actin: myosin.
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Actin - protein myofilament that projects from the end of each sarcomere toward its center.
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Myosin: thick protein myofilament that is centrally located within a sarcomere
Explain the term “Neuromuscular Junction” (NMJ)
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Intimate association of the terminal branch of a nerve fiber with the muscle fiber is called …
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Not continuous with the nerve fiber.
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Space between the NMJ & muscle fiber centrally located on the surface of the muscle fiber.
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A motor unit is a single motor neuron and all of the corresponding muscle fibers it innervates .
Discuss briefly how the Motor Unit functions
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The number of muscle fibers within each unit can vary as mentioned before: thigh muscles can have a thousand fibers in each unit, eye muscles might have ten.
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In general, the number of muscle fibers innervated by a motor unit is a function of a muscle's need for refined motion. The smaller the motor unit, the more precise the action of the muscle. Muscles requiring more refined motion are innervated by motor units that synapse with fewer muscle fibers.
Describe the Smooth Muscle Innervation
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ANS fibers travel between the smooth muscle cells in a branching network of terminal fibrils with varicosities at intervals along their axons.
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When APs depolarize them the transmitter substance is released, diffuses to the smooth muscle cell membranes, & stimulation occurs.
Describe the mechanism of Smooth Muscle contraction
 As motioned earlier that the smooth muscle don’t need nerves for their contractility as it is not under conscious control.
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PM touch between adjacent cells --> syncytial connection between fibers.
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Contraction impulse (AP) --> syncytia --> spreads across the tissue
 Cells linked electrically but independent chemically (don’t require a chemical neurotransmitter to be secreted for AP to occur throughout the tissue) unlike skeletal muscle.
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Smooth muscle is sensitive all over NOT just at specific neural plates i.e., skeletal muscular NMJ.
 Contraction can be initiated by stretch, hormones, neural, chemical or mechanical stimuli.
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Slow sustained contraction often rhythmic i.e., peristalsis, permitting stretching of smooth muscle without much change in tension unless stretching is sudden --> contraction.
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Initial excitation --> influx of Ca++ & diffusion through the cytoplasm
(slow latent period) --> beginning of contraction with longer duration -->
Ca++ pumped out of cytoplasm into ER, mitochondria, & ECF --> relaxation
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Single unit or unitary smooth muscle - cells are closely packed together forming the network or syncytia, which has generalized action on an entire organ.
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Multiunit smooth muscle - groups of smooth muscle fibers (1% of smooth muscle) that are distinctly separated & require more specific ANS nerve stimuli to contract i.e., pilomotor fibers in skin, smooth muscle fibers in iris & ciliary body.
Describe briefly the Cardiac Muscle contraction.
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Cardiac muscle fibers = usually one nucleus, sarcolemma, sarcoplasm,
+myofibrils + SR + T –tubules.
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Forms a network.
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Intercalated disks = apposed cell membranes where gap junctions occur permitting electrical transmission from one cardiac muscle cell to the next - very low electrical resistance here.
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Intercalated disks: ions move easily along the axes of the cardiac muscle fibers so AP can travel from one cardiac muscle cell to all others.
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Interposed disks between segments of muscle & may cross the fiber in an irregular manner.
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The intercalated disks allow waves of depolarisations to sweep across the cells thus synchronizing muscle contraction.
What is Cardiac Pacemaker?
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Some of the cardiac muscle cells are auto-rhythmic, i.e. they contract even in the absence of neuronal innervation (known as pacemaker cells).
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S-A (sinoatrial) node --> generates APs rhythmically --> inherent contractility
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S-A node depolarizes faster than rest of cardiac muscle mass --> sets normal resting HR.
 Innervation from ANS (SNS + PNS) alters or regulates the HR which the pacemaker normally sets.
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Cardiac AP is much slower than skeletal muscle, lasting for 150 msec (.15 sec) in the atria & 300 msec (.30 sec) in the ventricles.
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Cardiac RMP -85 to -95; AP at + 105 mv
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Contraction time lasts as long as the AP does since cardiac AP has a long plateau (large Ca++ influx) instead of a sharp spike-potential
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--> provides time for pumping blood out of the ventricles and filling them again before the next beat & allows for repolarization & relaxation.
Explain the mechanism on Skeletal Muscle Contraction
Depolarization of muscle fibers:
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Arrival of a spinal or motor neuron impulse to a NMJ --> results to an AP on the sarcolemma --> results in release of ACh from synaptic vesicles in
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the end feet of the nerve fiber into synaptic cleft (space between nerve fiber terminal branch & muscle fiber) near the midpoint of the muscle fiber.
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ACh release is accelerated because Ca++ ions (in the sarcoplasmic reticulum terminal cisternae) in the sarcoplasm & from ECF enter the prejunction membrane when the nerve impulse arrives.
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ACh is the neurotransmitter that increases muscle fiber permeability to Na+ ions, after which depolarization begins.
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Depolarization proceeds in all directions from NMJ --> impulse generation
& conduction into all parts of muscle fiber via sarcotubular system --> initiation of muscle contraction.
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More synchronized contraction that is depolarization of all parts of muscle fiber ~ simultaneously by sarcotubular transmission.
Contraction process . Nerve impulse arrives at NMJ; ACh releases from synaptic vesicles and diffuses across synaptic cleft to bind to receptors on muscle cell membrane.-----

Binding of ACh leads to initiation of membrane action potential, which spreads over entire muscle cell surface membrane and, via T-tubules, into the interior of cell.-----

•
Action potential in T-tubule membrane inhibits the calcium ion pump of the adjacent terminal cisterns of SR; calcium ion rushes out of the terminal cisterns into the sarcoplasm ---
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Calcium ion binds to the troponin component of the troponin-tropomysin complex, moving the complex away from its position blocking sites on the actin chains.-

Myosin heads with ATP molecules bind to
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the now-exposed actin chains.--

The ATP molecules are hydrolyzed to yield energy which drives the change in angle of the myosin heads and pulls the attached actin chains toward the middle of the sarcomere.--

New ATP molecules bind to the vacant ATPase sites on the myosin heads; the myosin heads detach from the actin chains and return to their original angle and are attached to new binding sites on still –exposed actin chains--- The calcium ion pump of the SR recover from the period of inhibition by the muscle cell membrane actin potential; calcium ion is removed from the troponin molecules and is pumped back into the SR---

The troponin-tropomyosin complex resumes its blocking position on the actin chain; further binding of myosin heads to actin chain is prevented and the muscle cell relaxes.
Functional classification – skeletal muscle

Flexor – muscle that tend to reduce angle at the joint or cause bending.
 Extenso r – muscle that increase the angle at joint or extent.
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Adductor – muscle that pull limb towards median plane
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Abductor
– muscle that tend to pull limb away from median plane.
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Sphincte r – circular muscle around natural opening e.g. anal sphincter
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Cutaneous muscle – muscles present in skin of animal, involved in movement of skin to drive flies.
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Ligaments – chord that connect bone to bone.
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Tendon – Chord that connect muscle to bone
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