Muscle

Yahyea Baktiar Laskar
ZOOHCC-302: Physiology—Controlling & Coordinating System(U3)
MUSCLE
Compiled by:
Assistant Professor
Department of Zoology
Ramanuj Gupta Degree College
yahyea92@gmail.com
yahyeabaktiar.laskar@aus.ac.in

Introduction to Muscle
 Muscular tissue is a specialized tissue in animals which applies forces to different
parts of the body by contraction.
 The word muscle is derived from the Latin word “musculus” meaning little mouse.
 It is made up of thin and elongated cells called muscle fibers that controls the
movement of an organism.
 The human muscular system comprises more than 600 Muscles and makes up about
40-50% of our overall body weight.
 Muscles are essentially attached to blood vessels, bones and other internal organs.
 Muscles function by using up energy (ATP) by oxidation of carbohydrates and fats.
Properties of Muscular Tissue:
 Contractibility– ability of muscle cells to shorten forcefully.
 Extensibility– ability to be stretched.
 Elasticity– ability to recoil back to its original length after being stretched.
 Excitability– responds to a stimulus delivered from a motor neuron or hormone

Introduction to Muscle
Key Terms:
 The cytoplasm in the muscle fibers is called sarcoplasm.
 The membrane surrounding the muscle fibers is called sarcolemma.
 A voluntary muscle is a muscle that you choose to move, like those in the arms and
legs.
 Sarcoplasmic Reticulum: saclike membranes involved in the storage of calcium ions.

Types of muscle tissue
Skeletal Muscles
 Skeletal Muscles are voluntary muscles.
 They are attached to the bones and involved in different body parts
functioning.
 They come under the central nervous system control of the body.
 Skeletal Muscles are long and multinucleated.
 They are cylindrically shaped with branched cells which are attached to the
bones by collagen fibres and tendons, which are composed of connective
tissues.
Functions:
 The primary function of Skeletal muscle is contraction, which helps
produce heat in our body.
 Helps in maintaining the body posture and joint position.
 produce ATP and store glucose in the form of glycogen

Smooth Muscles
 They are involuntary muscles that are non-striated.
 Present in major organs whose movements are not controlled by the will,
such as the stomach, vessels, bladder, uterus, etc.
 Spindle-shaped with a single nucleus.
 Shorter than skeletal Muscles, length ranging between 20 to 200 μm and
thickness between 3-10 µm.
 These Muscles produce their connective tissue and lack actin, myosin and
filaments.
Functions:
 maintain the diameter of arteries and thereby maintain blood pressure.
 maintains the peristaltic movement and forces food through the digestive
tract.
 responsible for shrinking the size of the pupil.
 helps air go from the trachea to the lungs.

Cardiac Muscles
 Found only in the heart and are involuntary.
 Made up of cylindrical-shaped cells.
 Characterized by branched cylindrical fibres and a centrally located nucleus.
 These are striated Muscles responsible for keeping the heart moving by
circulating and pumping blood throughout the body.
 The interconnected Muscles provide flexibility and strength to the Cardiac
muscle tissue and are involved in rhythmic relaxation and contraction of the
heart Muscles.
Functions:
 Regulate rhythmic relaxation and contractions of the heart Muscles for
pumping blood.
 The specialized type of tissue called “pacemaker” cells that expand and
contract by responding to electrical impulses of the central nervous system.

Ultra structure of skeletal muscle
Ultrastructure of skeletal muscles
 It acquires its name because most of the muscles involved are attached to skeleton, and make it move.
 Also known as Striated muscle—because it cell (fibre) are composed of alternating light and dark band (stripe).
 Composed of muscle fibres. Each muscle fibre is long, cylindrical shaped with numerous nuclei.
 Each fibre is 1.2 inch long and 0.004 inch in diameter.
 Each fibre contains numerous myofibrils, which are made up of thick and thin threads called myofilament.
 Thick myofilament is composed of larger protein— myosin.
 Thin myofilament is composed of smaller protein— actin.
 When viewed with light microscope, skeletal tissue shows a pattern of alternating light and dark bands. The bands are caused
by the arrangement of actin and myosin myofilament in the muscle fibre.
 An overlapping of thick myosin and actin myofilaments produce- dark A band (anisotropic band); does not allow light to pass.
 Thin actin myofilament alone produce- light I band (isotropic band); allow light to pass.
 Cutting across each I band is a dark Z line.
 Within A band is a somewhat light H zone( Hensen’s disc), which consists only myosin myofilament.
 The area between two Z line is known as sarcomere, which is the fundamental contractile unit of myofibril.

Ultra structure of skeletal muscle
Ultrastructure of skeletal muscles
 They are supplied by voluntary nervous system (CNS
and PNS).
 They require large amount of energy, so are supplied
with blood vessels and numerous elongated
mitochondria and glycogen granules.

Mechanism of muscle contraction
Muscle Contraction Mechanisms
 The Sliding Filament Theory explains the process of muscle
contraction, proposed by two groups, Andrew Huxley and Ralph
Niedergerke, and Hugh Huxley and Jean Hanson (1954).
 According to this idea, muscle fibre contraction takes place when a
thin actin filament glides across a thick myosin filament.
 The stimuli supplied by the CNS via a motor neuron causes the muscle
to contract.
 When an impulse reaches an axon terminal or neuromuscular
junction, neurotransmitter-containing vesicles (acetylcholine) fuse
with the axon membrane.
 This generates action potentials in the sarcolemma.
 The action potential then extends from the sarcolemma to the T-
tubules and activates the sarcoplasmic reticulum, causing calcium ions
to be released into the sarcoplasm.

Muscle Contraction Mechanisms
 A rise in Ca2+ concentration in the sarcoplasm initiates filament sliding, whereas a decrease in concentration stops the process.
 At low Ca2+ concentration, the muscle fibre is relaxed due to the presence of Ca2+ active transport pumps in the sarcoplasmic
reticulum (SR) membrane, which transfers Ca2+ from the sarcoplasm into the SR.
 Troponin changes its form as a result of the Ca2+ released from the SR. This change in shape shifts the troponin-tropomyosin
complex away from actin myosin-binding sites.
 Myosin globular head functions as an ATPase, hydrolyzing the ATP molecule. Myosin uses the energy released by ATP
hydrolysis to bind to the exposed active site on the actin filament and form a cross bridge.
 This moves the connected actin filament towards the center of the A-band. The Z-line linked to these actions is likewise pulled
inwards, causing the sarcomere to shorten or contract.
 The H-zone narrows as the thin myofilaments move past the thick myofilaments. This shortens the I-band while keeping the A-
band the same length.
 Myosin then releases ADP + Pi and returns to its relaxed state. ATP attaches to myosin once again, and the cross bridge
between myosin and actin is broken. The ATP is hydrolyzed again, and the cycle of cross-bridge production and breakdown is
repeated, generating sliding. The procedure is repeated until the calcium ion is pumped back to the sarcoplasmic reticulum.

Mechanism of muscle relaxation
Muscle Relaxation Mechanisms
 Calcium returns to the sarcoplasmic reticulum following muscle
contraction.
 As the quantity of calcium in the sarcoplasm diminishes,
calcium does not bind to troponin C.
 Troponin changes form, and tropomyosin and troponin return
to their original positions and states.
 This prevents myosin from binding to actin by blocking its
active site.
 This induces muscular relaxation.
 An enzyme called acetylcholinesterase quickly degrades
acetylcholine (AChE).
 The formation of muscular action potentials is thereafter
terminated, and the Ca release channels in the sarcoplasmic
reticulum membrane shut.

Muscle twitch
Muscle twitch
 The motor neurons that innervate skeletal muscle fibers are
called alpha motor neurons.
 When an action potential travels down the motor neuron, it will
result in a contraction of all of the muscle fibers associated with
that motor neuron. The contraction generated by a single action
potential is called a muscle twitch.
 A single muscle twitch has three components. The latent
period, or lag phase, the contraction phase, and the relaxation
phase.
 The latent period is a short delay (1-2 msec) from the time when the action potential reaches the muscle until tension can be
observed in the muscle.
 The time required for Ca2+ to diffuse out of the SR, bind to troponin, the movement of tropomyosin off of the active sites,
formation of cross bridges, and taking up any slack that may be in the muscle.
 The contraction phase is when the muscle is generating tension and is associated with cycling of the cross bridges.
 The relaxation phase is the time for the muscle to return to its normal length.

Types of muscle contraction
Isotonic and isometric contraction
 Muscle contractions are described based on two variables: force (tension) and length (shortening).
 When the tension in a muscle increases without a corresponding change in length, the contraction is called an isometric
contraction (iso = same, metric=length).
 Isometric contractions are important in maintaining posture or stabilizing a joint.
 If the muscle length changes while muscle tension remains relatively constant, then the contraction is called an isotonic
contraction (tonic = tension).
 Isotonic contractions can be classified based on how the length changes. If the muscle generates tension and the entire
muscle shortens than it is a concentric contraction. An example would be curling a weight from your waist to your shoulder;
the bicep muscle used for this motion would undergo a concentric contraction.
 When lowering the weight from the shoulder to the waist the bicep would also be generating force but the muscle would be
lengthening, this is an eccentric contraction.
 Eccentric contractions work to decelerate the movement at the joint and generate more force than concentric contractions.

Factors influencing the force of muscle contraction
Wave summation and tetanus
 Muscle twitch can last up to 100 ms and that an action potential lasts only 1-2
ms.
 With the muscle twitch, there is not refractory period, so it can be re-stimulated
at any time.
 If we stimulate a single motor unit with progressively higher frequencies of
action potentials we’ll observe a gradual increase in the force generated by that
muscle. This phenomenon is called wave summation.
 Eventually the frequency of action potentials would be so high that there would
be no time for the muscle to relax between the successive stimuli and it would
remain totally contracted, a condition called tetanus.
 Essentially, with the high frequency of action potentials there isn't time to
remove calcium from the cytosol.

Factors influencing the force of muscle contraction
Muscle Treppe
 When a skeletal muscle has been dormant for an extended period and then
activated to contract, with all other things being equal, the initial contractions
generate about one-half the force of later contractions.
 The muscle tension increases in a graded manner that to some looks like a set of
stairs. This tension increase is called treppe, a condition where muscle
contractions become more efficient. It’s also known as the “staircase effect”.
 When muscle tension increases in a graded manner that looks like a set of stairs,
it is called treppe. The bottom of each wave represents the point of stimulus.
 It is believed that treppe results from a higher concentration of Ca2+ in the sarcoplasm resulting from the steady stream of
signals from the motor neuron. It can only be maintained with adequate ATP.

Disclaimer: The information/images in this study material has been adapted from various sources including but not limited to books, journals, websites, and
related materials openly available in the internet. This will be purely utilized for academic purpose only, by college students. The author has no intension of
using these information/images/other materials that may be copyrighted and included in this study material for commercial benefit.

Muscle

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