University:
Glendale Community College
Course:
BIO160 | Introduction to Human Anatomy and Physiology
Academic year:

2019
Views:

297

Pages:

16
Author:

emaiftAddiniaf5

Neuromuscular Toxins and Paralysis • Many pesticides and “nerve gas” are chemicals that inhibit acetylcholinesterase by binding to it and preventing it from degrading ACh. – minor startle response can cause death through spastic paralysis and suffocation – atropine is an antidote that works by blocking ACh receptors • Tetanus or lockjaw is a spastic paralysis caused by a toxin of the soil bacterium Clostridium tetani. – the toxin blocks glycine, an inhibitor normally produced by the spinal cord that prevents overstimulation of muscles. • Botox - Clostridium botulinum is a related soil bacterium that produces botulinum toxin that blocks ACh release from motor neurons. • Curare causes flaccid paralysis with limp muscles that are unable to contract. – Curare is a South American plant toxin used by indigenous people to make poison darts. – Curare binds to ACh receptors without activating them. – Used as a muscle relaxant for surgery, but can cause respiratory arrest. Neuromuscular Toxins and Paralysis • Black Widow Spider Venom causes massive release of ACh that leads to uncontrolled muscle spasms. • Tetrodotoxin (TTX) blocks voltage-gated Na+ channels. – TTX blocks sodium movement into the neuron and the action potential along the nerve membrane ceases. – A single milligram or less of TTX - an amount that can be placed on the head of a pin, is enough to kill an adult. – 10-100 times more toxic than black widow spider venom Myasthenia Gravis • Autoimmune disease where antibodies bind to ACh receptors – Skeletal muscle cells become progressively less sensitive to ACh – Symptoms include: • drooping eyelids and double vision • difficulty swallowing • weakness of the limbs • respiratory failure • Disease affects mostly women between ages of 20 and 40 • Treated with acteylcholinesterase inhibitors, thymus removal or immunosuppressive agents Myasthenia Gravis Symptoms Drooping eyelids and weakness of muscles of eye movement Length-Tension Relationship • Amount of tension generated by a muscle depends on the length of the muscle before it is stimulated. • Overly contracted muscle cells develop a weak contraction because the thick filaments are already close to the Z discs and further contraction results in the ends of the thick filaments butting up against the Z disc. • Overly stretched muscle cells also develop a weak contraction because there is too little overlap of thin and thick filaments which does not allow for very many cross bridges too form. • Optimum resting length produces greatest force when muscle contracts. Length-Tension Relationship Muscle Twitch • Threshold is the minimum voltage necessary to produce an action potential in a muscle cell. – a single brief stimulus at threshold voltage produces a quick cycle of contraction and relaxation called a twitch that lasts less than 1/10 second. • A single twitch contraction is not strong enough to do any useful work. Stimulus Frequency and Muscle Tension Twitch Treppe • Twitch: at low frequency (up to 10 stimuli/sec) each stimulus produces an identical twitch with complete relaxation in between. • Treppe: at moderate frequency (between 10-20 stimuli/sec) the muscle relaxes completely between twitches, but the tension increases with each twitch because calcium is not completely reabsorbed into the SR. Also, the heat produced by muscle contraction increases myosin ATPase efficiency (as in warm-up exercises). Stimulus Frequency and Muscle Tension Incomplete Tetanus Complete Tetanus • Incomplete Tetanus results from higher frequency stimulation (20-40 stimuli/second) and gradually generates stronger, sustained contractions. – each stimulus arrives before the previous one recovers – this is called temporal summation because it results from the timing of the stimuli. • Complete Tetanus usually results from an artificially high stimulation frequency (40-50 stimuli/second) – muscle has no time to relax between stimulations – twitches fuse into a smooth, prolonged contraction – rarely occurs in the body under natural conditions Asynchronous Contraction • The reason for the smoothness of muscle contraction during muscle tetany is that the motor units function asynchronously. – throughout an actively contracting muscle, some motor units are contracting as others relax. – the muscle does not lose tension as motor units take turns developing tension within the muscle. Energy for Muscle Contraction: ATP • ATP is the ONLY source of energy for muscle contraction. • ATP is produced by: – aerobic respiration • produces about 36 ATP per glucose molecule • requires continuous oxygen supply, produces H2O and CO2 as waste – anaerobic fermentation • produces only 2 ATP per glucose molecule • occurs without oxygen and produces irritating lactic acid as waste Phosphagen Enzyme System Myokinase and Creatine Kinase generate ATP in the absence of O2. Myokinase uses Pi from ADP. Creatine Kinase uses Pi from creatine phosphate. Sources of ATP During Intense Exercise • Immediate Energy: About a 10 Second supply of ATP from: – Aerobic metabolism using O2 released from myoglobin in muscle cells. – Phosphagen System of enzymes that transfers Pi to ADP. • Short-Term Energy: About a 30-40 second supply of ATP from: – Glycogen-Lactic Acid System uses stored glycogen and anaerobic metabolism producing lactic acid. • Long-Term Energy: After about 40 seconds if the respiratory and cardiovascular systems are functioning well, they can catch up to O2 demand for aerobic respiration and supply more than 90% of ATP during sustained exercise. Oxygen Debt • The need to breath heavily after strenuous exercise is to provide maximal oxygen for: – replacing oxygen reserves in myoglobin (red respiratory pigment in muscle) and hemoglobin (red respiratory pigment in red blood cells) – replenishing the phosphagen system – converting lactic acid back into glucose in kidneys and liver – serving the elevated metabolic rate that occurs as long as the body temperature remains elevated from exercise Slow Twitch and Fast Twitch Fibers • Muscles contain a mixture of muscle fiber types based upon their metabolism and morphology • Skeletal Muscle Fiber Types: 1) Slow-twitch fibers (oxidative or red fibers) – abundnat mitochondria, myoglobin and capillaries – use aerobic respiration and are resistant to fatigue – examples: leg and thigh muscles (100msec/twitch) 2) Fast-twitch fibers (glycolytic or white fibers) – sarcoplasmic reticulum releases calcium quickly so contractions are quicker – uses available ATP quickly, then uses enzymes for phosphagen and glycogen-lactic acid systems, but can fatigue quickly – examples extraocular eye muscles (7.5 msec/twitch) 3) Intermediate fibers have combined characteristics of fast and slow twitch and are relatively rare in humans except for endurancetrained athletes. • Proportions of different muscle types in an individual appear to be determined genetically: “a born sprinter” or “ a born marathoner” but small changes in proportions can result from training. Muscle Fiber Histology FG = Fast Glycolytic white fibers SO = Slow Oxidative red fibers