Anatomy and physiology of muscle

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Anatomy and physiology of muscle

  1. 1. 1 King Edward medical university Muscle Anatomy and Physiology Rabia Mustafa
  2. 2. 2 Table of Muscle Definition ...............................................................................................................................3 Muscle Attachments............................................................................................................................3 Classification of Muscles :..................................................................................................................3 Muscle physiology..............................................................................................................................7 Types of muscle contraction:..............................................................................................................7 Physiology of smooth muscles............................................................................................................8 Physiology of skeletal muscles:..........................................................................................................9 Muscle Contraction...........................................................................................................................10 Excitation of Skeletal Muscle. ..........................................................................................................13 Mechanism of Contraction................................................................................................................14
  3. 3. 3 Muscle Definition Muscles are contractile unit , composed of bundle of muscle fibres forming flesh and tendons. Muscle Attachments Muscles are attached to bone via : Origin:  Proximal attachment  Least moveable  Closest to midline Insertion:  Distal attachment  Most moveable  Away from midline Classification of Muscles : Muscles are classified according to  Structure of muscle  Orientation of muscle fibers  Shape of muscle Classification according to shape of muscle  Quadrangular  Trapezoidal  Triangular  Rhomboidal  Fusiform  Digastric Classification according to orientation of muscle fibers:  Uni pennate ( extensor digitorum longus )  Bipennate(rectus femoris)  Multipennate(acromial fibers of deltoid)
  4. 4. 4  Circumpennate (orbecular oculi ) Classification according to structure of muscle  Skeletal muscles are the prime movers of the human body. The forces they produce under the control of the nervous system act on the bones to which they are attached to create the propulsive forces necessary for human movement.  Man has 640 skeletal muscles of many shapes and sizes 9 from the tiny stapedius muscle of the middle ear to the massive hip extensor, the gluteus maximus).  Muscles are situated across joints and are attached at two or more points to bony levers.  Each muscle is well adapted to provide an appropriate range, direction and force of contraction to meet the habitual requirements at the articulations over which it passes. Properties of Skeletal Muscles: 1. Irritability: is the ability of the muscle to respond to stimulus. 2. Contractility: is the capacity of the muscle to produce tension between it’s ends. 3. Relaxation: is the opposite of contraction and is the giving up of tension. Both contraction and relaxation progress from zero to maximal values over a finite time. 4. Dispensability: is the ability of the muscle to be stretched or lengthened up to a certain limit by an outside force; e.g. pull of an antagonist muscle, of gravity or by an opponent. The muscle suffers no harm so long as it is not stretched beyond its physiological limits. 5. Elasticity: is the ability of the muscle to recoil to its original length when an outside force is removed unless it has been overstretched.
  5. 5. 5 Smooth Muscles:  Composed of spindle shape contractile cells  Show slow and sustained contraction  Control by autonomic nervous system  Devoid of any type of striations Example  Arector pilli muscle  Muscles of viscera  Iris of pupil Cardiac muscles  Cylindrical, branched,  Anatomising Contractile cells  Show myogenic rhythematic contraction  Striated single nucleated muscles  Control by autonomic nervous system  Found in heart
  6. 6. 6 Comparison of skeletal,smooth and cardiac muscles:
  7. 7. 7 Muscle physiology Types of muscle contraction: There are two types of muscle contractions: Isotonic Contraction: The tension in the muscle remains constant despite of change in the muscle length maximal force of contraction exceeds the total load on muscle 1 Concentric contractions: Force generated is sufficient to overcome the resistance but muscle shortens 2Eccentric contractions: Force generated is insufficient to overcome the external load but muscle lengthens Isometric contractions: The muscle remains in the same length Example: holding an object up without moving it the muscle force precisely matches the load and no movement results : Muscle Contraction Isotonic Concentric Eccentric Isometric
  8. 8. 8 Physiology of smooth muscles: Muscle contraction:  Intracellular calcium concentration increase  calcium enters in cell and release from sarcoplasmic reticulum  calcium binds to calmodium  calcium calmodium activates myosin light chain kinase  MLCK phosphorylates light chain in myosin head and myosin ATPase activity  Active myosin cross bridges slide along actin and increase tension Muscle relaxation :  Free calcium decreases in cytosol when calcium is pumped out of cell or back into sarcoplasmic reticulum calcium unbinds from calmodium  Myosin phosphate remove phosphate from myosin which decreases myosin ATPase activity less myosin ATPase result in decrease of muscle tension
  9. 9. 9 Physiology of skeletal muscles: Physiological anatomy of skeletal muscles: When viewed under the microscope, the skeletal muscle fibre is seen to have regular striations. These striations are due to transverse alternating dark and light bands on the myofibrils, which are parallel, threadlike structures in the sarcoplasm (muscle cytoplasm) of a muscle fibre. Myofibrils, the smallest elements of the muscle fibre visible under the light microscope, are the contractile units of the fibre. With the electron microscope the striations on the myofibrils can be seen to arise from the arrangement of its subunits into thick and thin filaments. The thick filaments are composed largely of the protein myosin, the thin filaments of three proteins - actin (the principal one), tropomyosin, and troponin. The dark, or A band of the myofibril corresponds to the thick filaments, overlapped on either end with thin filaments; the light, or I band, corresponds to the region where there are only thin filaments. Two additional markings are of importance - the Z line, a narrow band in the central region of the I band representing a structure to which the thin filaments are attached on either side, and the H zone, a lighter region, located in the central portion of each A band, into which the thin filaments do not penetrate. The area between two adjacent Z lines, called a sarcomere, represents the repeating unit of a myofibril, each about 2.5 micrometers long in resting muscle.
  10. 10. 10 Muscle Contraction : The Motor Unit. As a result of terminal branching, a single nerve fibre innervates on average about 150 muscle fibres. All of these fibres and the single nerve fibre innervating them are called a motor unit because muscle fibres of the unit are always excited simultaneously and contract in unison. It is important to note that terminal divisions of a motor neuron are distributed throughout the muscle belly. Stimulation of a single motor unit, therefore, causes weak action in a broad area of muscle rather than a strong contraction at one specific point.
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  13. 13. 13 Excitation of Skeletal Muscle.
  14. 14. 14 Muscle fibres possess the property of being excitable. Any force affecting this excitability is called a stimulus, which, in muscle tissue, is usually conveyed by nerve fibres. The stimulus is an electrical impulse transmitted from a nerve fibre branch to a muscle fibre at a junctional region called the neuromuscular junction. At the junction a gap, the synaptic cleft, exists between a nerve branch terminal and a recess on the surface of the muscle fibre. A nerve impulse reaching the neuromuscular junction causes the release of acetylcholine, a neurotransmitter stored in synaptic vesicles within the nerve terminal. Acetylcholine crosses the gap and acts on the membrane of the muscle fibre, causing it to generate its own impulse, which travels along the muscle fibre in both directions at a rate of about 5 meters per second and is conducted to the sarcoplasmic reticulum via the T system. Mechanism of Contraction. The generally accepted conception of how muscle contacts is known as the ‘sliding filament model’. According to this model, the contraction is brought about by the sliding of the thin filaments at each end of a sarcomere toward each other between the stationary thick filaments. This draws the Z lines closer together, shortening the sarcomere. In sections of muscle, prepared at sequential stages of contraction, it can be seen that, as a sarcomere shortens, the I band of each myofibril (the region containing only thin filaments bisected by a Z line) narrows as the thin filaments move toward the centre of the sarcomere, while
  15. 15. 15 the A band (representing the length of the thick filaments) is unaltered. The H zone of the A band, the lighter, central region not penetrated by thin filaments in relaxed muscle, disappears as thin filaments come to completely overlap the thick filaments in the contracted state. When contraction is marked, a dense zone appears in the centre of the A band as a result of overlap of thin filaments from opposite ends of a sarcomere. In cross section this overlap is identified as a doubling (over the relaxed condition) of the ratio of thin to thick filaments. The sequence of events leading to the contraction of muscle may be summarised as follows: 1. The electrical impulse travelling along the membrane of a muscle fibre reaches the sarcoplasmic reticulum via the T tubules.
  16. 16. 16 2. This stimulates the release of calcium, which combines with the Tn-C subunits of troponin and induces a change in the conformation of the troponin molecules. 3. Tropomyosin moves away from the myosin binding sites on actin, and the myosin heads, charged with ATP, combine with actin (it has been suggested that the energized, force-generating state of the myosin heads is a complex of myosin with ADP and phosphate formed following the cleavage of ATP by myosin ATPase). 4. When actin and myosin interact, the energized myosin complex breaks down, providing the energy for the propulsive force (probably a swivelling of the myosin heads) for pulling the thin filaments toward the centre of the sarcomere. 5. Successive cycles, involving binding of ATP to myosin heads, detachment of myosin heads from actin, and reattachment in a new position on actin, followed by the power stroke that moves the thin filaments, result in the continued sliding of the thin filaments. 6. Contraction ends when calcium returns to the sarcoplasmic reticulum.

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