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PHYSIOLOGY OF THE MUSCLE

PHYSIOLOGY OF THE MUSCLE. muscles constitute 45 to 50% of the body weight and are of 3 types: Skeletal muscles Striated or voluntary muscles Smooth muscles Plain or involuntary muscles Cardiac muscle: located only in the heart. myogenic in action and regulated by aut. n.s.

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PHYSIOLOGY OF THE MUSCLE

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  1. PHYSIOLOGY OF THE MUSCLE muscles constitute 45 to 50%of the body weight and are of 3 types: • Skeletal muscles Striated or voluntary muscles • Smooth musclesPlain or involuntary muscles • Cardiac muscle: located only in the heart. myogenic in action and regulated by aut. n.s.

  2. THE SKELETAL MUSCLE • Each muscle fiber is surrounded with an elastic membrane called the sarcolemma. • Beneath the sarcolemma, several nuclei are found surrounded by the cytoplasm of the fiber called the sarcoplasm. • In the sarcoplasm longitudinal parallel fibrils called the myofibrils.

  3. THE NEURO-MUSCULAR JUNCTION • Is the site of junction between the nerve and the skeletal muscle fiber. • Structure of the neuro-muscular junction: 1. The axon terminals of the motor nerve fiber. 2. Motor end plate: contains: a. receptor sites for acetylcholine. b. an enzyme called cholinesterase that rapidly hydrolyses A-ch after performing its action. 3. the synaptic cleft.

  4. Mechanism of neuro-muscular transmission: It is carried out chemically by A.Ch. as follows: 1- by arrival of action potential Ca+ enter the nerve terminal. 2- fusion of the vesicles to the nerve cell membrane.

  5. 3. release of acetylcholine from the axon terminals. 4. depolarization of the motor end plate (end plate potential). 5. destruction of acetylcholine: after performing its action, to prevent the re-excitation of the muscle fiber.

  6. Changes that occur in the skeletal muscle after its stimulation • Electrical changes. • Excitability changes. • Chemical changes. • Mechanical changes.

  7. I. Electrical changes generally similar to those occurring in the nerve fiber with some differences which include: • The R.M.P in skeletal muscle fiber is about - 85 mv. • The magnitude of the action potential is about 125 mv (from -85 to + 40 mv) • The duration of the action potential is longer. • The action potential in skeletal muscle fiber is followed by its contraction.

  8. II. Excitability changes • Similar to that of the nerve. • The skeletal muscle fiber recovers its excitability completely at the beginning of its contraction and can therefore respond to other stimuli while it is in the phase of contraction by a preceding stimulus (tetanus).

  9. III. Chemical changes Energy is needed for: a. At rest • Maintenance of the polarized state of the cell membrane. • Synthesis of muscle proteins, glycogen and other organic compounds.

  10. b. During activity: For performance of muscle contraction 1- Breakdown of ATP ATP ADP + Pi + E (energy) 2- Breakdown of creatine phosphate (CP) CP creatine + Pi + E 3- Breakdown of glycogen Glycogen Pyruvic acid + E

  11. a. In presence of O2 (aerobic) Pyruvic acid CO2 + H2O + E b. In absence of O2 (anaerobic) Pyruvic acid Lactic acid The lactic acid accumulates in the muscle fiber, then gradually diffuses to the blood.

  12. IV. Mechanical changes Motor Unit: A motor unit consists of a single motor neuron, its axon and the number of muscle fibers supplied by this axon. (Number of muscle fiber in motor unite)

  13. Muscle contraction: It is the ability of muscle to convert chemical form of energy to a mechanical form. Types of contraction: 1. Isotonic contraction It has the following characteristics: • The muscle shortens in length but the tension remains constant. • work is done. • About 30% of the chemical energy converted into work and 70% is lost as heat.

  14. 2.Isometric contraction It has the following characteristics: 1- The muscle does not shorten in length, muscle tension increases markedly. 2- No work is performed by the muscle. 3- Most of the energy released during contraction is lost as heat.

  15. Starling’s Law: This law states that “Within limits, the force of contraction of a muscle is directly proportional to the initial length of the muscle fibers” Thus, if a muscle is slightly stretched before its contraction, this leads to an increase in its force of contraction. However, if the muscle is overstretched, the muscle contraction becomes weaker.

  16. The all or none rule: Single muscle fiber contracts maximally when excited by athreshold stimulus or over but it does not contract at all if excited by a subthresholdstimulus. The whole muscle does not obey this rule.

  17. Thank You

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