Excitable Tissue Muscles PDF 2025

 Objectives The students should be able to:  Define reflexes.  Enumerate components of reflexes.  Classify reflexes.  Define muscle tone.  Classify muscles.  Describe the mechanism of muscle contraction.  Define muscle twitch, summation, fatigue, hypertrophy, tetany and ri...

 Objectives The students should be able to:  Define reflexes.  Enumerate components of reflexes.  Classify reflexes.  Define muscle tone.  Classify muscles.  Describe the mechanism of muscle contraction.  Define muscle twitch, summation, fatigue, hypertrophy, tetany and rigor mortis.  introduction A reflex is an involuntary and nearly rapid movement in response to a stimulus.  Components : -receptor -afferent pathway (sensory neuron) -integrating center (spinal cord or brain ) -efferent pathway (motor or secretory neuron) -effector (muscle )  Classification of the reflexes  According to site of receptor 1.Superficial : withdrawal reflex 2.Deep : stretch reflex ,Golgi tendon reflex 3.Visceral : micturition ,defecation  According to number of synapses -monosynaptic ;stretch reflex -bi synaptic : Golgi tendon -poly synaptic ; withdrawal According to the site of integration: -simple (spinal cord ) -complex (brain stem ,cortex)  stretch reflex  Simple ,deep ,monosynaptic reflex  It is the contraction (response) of the skeletal muscle, preceded by stretch (stimulus)  Ex; knee jerk ,when patellar tendon is tapped by reflex hammer, it stretches the quadriceps muscle ,which is contracted reflexively Mechanism of stretch reflex: Muscle spindle is stretched (Receptor potential) all- or-non AP in afferent nerve. Number of the AP is proportionate to intense of stimulus. In the center (EPSPs) are formed at the synaptic junction All-or-non responses are generated in the efferent nerve. When the AP arrive to the skeletal muscle it triggers AP that bring about contraction Muscle tone: Is resistance of muscle to stretch due to continuous partial contraction. Present in all muscles of the body. Produced by γ efferent neurons. Function of muscle tone: 1.Maintain erect posture at rest & movement against gravity. 2.Tone of abdominal muscles hold the viscera back. 3.In the lower limbs, it is important for venous return. 4.Maintain normal body temperature.  The withdrawal reflex :  It is a spinal superficial polysynaptic reflex intended to protect the body from damaging stimuli.  When a noxious stimulus affect the receptors, APs will be triggered at receptors then transmitted to the sensory neurons which synapse with interneurons. The interneurons are connected to motor neurons causing contraction of flexors and relaxation of extensors. In the other limb the opposite occurs so as to keep posture control  PHYSIOLOGY OF MUSCLE  Muscle is classified in 2 different ways:  Striated or non-striated.  Voluntary or involuntary. 1-Skeletal Muscle: (striated voluntary ) Attached to bones its function is voluntary movement. 2-Smooth Muscle: (non-striated and involuntary) Components of hollow organs and tubes. Its function is propulsion of contents through various hollow internal organs. 3-Cardiac Muscle: (striated involuntary) Found only in the heart. Its function is to pump blood in the circulation. 11  PHYSIOLOGIC ANATOMY OF SKELETAL MUSCLE  Sarcolemma: Is the cell membrane of the muscle fiber.  Sarcoplasm: Is the intracellular fluid which occupy the space between myofibrils.( mitochondria)  Sarcoplasmic Reticulum Is an endoplasmic reticulum. It stores calcium ions necessary for muscle contraction. 12 Organization of skeletal muscle  Sarcomeres There can be thousands of sarcomeres within a single muscle cell. An individual sarcomere contains many parallel actin (thin) and myosin (thick) filaments. The sliding of myosin and actin proteins causes shortening of sarcomeres hence of the muscle.  Filaments  Myosin filament (thick) is composed of multiple myosin molecules (each molecule is composed of two heads and one tail). The myosin head functions as ATPase enzyme, which cleaves ATP and use the energy for contraction.  Actin filament (thin) is formed of 3 types of protein : actin, tropomyosin and troponin. 1. Actin is backbone of actin filament it is a double helix.  Myofilaments 2. Tropomyosin molecules are wrapped spirally around the actin helix. In resting state they lie on the active sites hiding them from myosin filament. 3. Troponin molecule is attached at one end of tropomyosin. It is composed of 3 subunits. troponin I, has strong affinity for actin, troponin T for tropomyosin and troponin C for calcium ions.  General Mechanism of Skeletal Muscle Contraction 1) An AP travels along a motor nerve to its endings on muscle fibers. 2) Secretion of a neurotransmitter (acetylcholine -Ach-). 3) Ach binds receptors to open "acetylcholine-gated" channels. 5) Na+ diffuses to the interior of the muscle fiber membrane. 6) This initiates an AP at the membrane. 7) The AP depolarizes the muscle membrane causing the sarcoplasmic reticulum to release large quantities of Ca2+. 17  General Mechanism of Skeletal Muscle Contraction 8. Ca2+ attach to troponin C causing conformational change in troponin tropomyosin complex which leads to uncovering of actin active sites causing binding of actin and myosin head and sliding of actin alongside the myosin. 9. After a fraction of a second, Ca2+ ions are pumped back. This removal of Ca 2+ from the myofibrils causes the muscle contraction to cease. 10. To detach myosin from actin it must bind a molecule of ATP again. 18  TYPES OF MUSCLE CONTRACTION A.Isometric contractions (equal distance) – tension increases, muscle same length. B. Isotonic contractions (equal tension) – tension is constant, muscle shortens. 20  Some definitions related to muscle contraction  A muscle contraction is called a muscle TWITCH.  Summation: A 2nd contraction before 1st subsides.  Tetany: Sustained contractions (requires energy - ATP)  Fatigue: Under repeated stimulation, contractions get feebler, lactate accumulates, pH changes lead to stoppage of contractions.  Hypertrophy: Increase of total mass of muscles  Rigor mortis: A state of extreme rigidity -contracture- seen after death, due to fixed interaction between actin & myosin ( no ATP to break actin-myosin bond).