Presentation on theme: "Motor Unit: The Nerve-Muscle Functional Unit"— Presentation transcript:
1 Motor Unit: The Nerve-Muscle Functional Unit Motor unit = a motor neuron and all (four to several hundred) muscle fibers it supplies
2 neuromuscular junctions Spinal cordMotor neuroncell bodyMuscleNerveMotorunit 1unit 2fibersneuronaxonAxon terminals atneuromuscular junctionsAxons of motor neurons extend from the spinal cord to themuscle. There each axon divides into a number of axonterminals that form neuromuscular junctions with musclefibers scattered throughout the muscle.Figure 9.13a
3 Motor UnitsSmall motor units in muscles that control fine movements (fingers, eyes)Large motor units in large weight-bearing muscles (thighs, hips)
4 Motor UnitMuscle fibers from a motor unit are spread throughout the muscle so that a single motor unit causes weak contraction of entire muscleIf a muscle has more than one motor unit…Motor units in a muscle usually contract asynchronously; helps prevent fatigue
5 Muscle Tone Constant, slightly contracted state of all muscles Due to spinal reflexes that activate groups of motor units alternately in response to input from stretch receptors in musclesDoes not produce movement, but keeps muscles firm, healthy, and ready to respondHelps maintain posture and stabilize joints
6 Muscle changes in length and moves the load 2 Main Categories of ContractionIsotonicMuscle changes in length and moves the loadIsometricThe load is greater than the tension the muscle is able to developThe muscle neither shortens nor lengthens
7 Isotonic Contractions Isotonic contractions are either concentric or eccentric:Concentric contractions—the muscle shortens and does workEccentric contractions—the muscle generates force as it lengthens
11 Check Point!!!!!! What is a motor unit? Betty White just signed up for a chin up competition to raise money for a local animal shelter. What type of muscle contractions are occurring in her biceps muscles immediately after she grabs the bar? As her body begins to move upward toward the bar? When her body begins to approach the mat?
12 Where does the body get the energy needed for contraction?
13 Muscle Metabolism: Energy for Contraction ATP is the only source used for muscle contractionMuscles store only 4–6 seconds worth of ATP at any given time.ATP must therefore be created as quickly as it is broken down for a contraction to continue
14 Muscle Metabolism: Energy for Contraction ATP is regenerated by:Direct phosphorylation of ADP by creatine phosphate (CP)2. Anaerobic pathway (glycolysis)3. Aerobic respiration
15 Direct Phosphorylation Fastest form of ATP regenerationCreatine phosphate (CP) is stored in the musclesAfter stored ATP is used, CP combines with ADP to make more ATPCreatine phospate + ADP Creatine + ATPCan provide enough energy for 15 seconds of vigorous activity
16 Coupled reaction of creatine phosphate (CP) and ADP Energy source: CP(a) Direct phosphorylationOxygen use: NoneProducts: 1 ATP per CP, creatineDuration of energy provision:15 secondsCreatinekinaseADPCPATPFigure 9.19a
17 Anaerobic Pathway Glycolysis & Lactic Acid Formation Makes ATP by breaking down glucoseGlucose breaks down into 2 pyruvic acid molecules, releasing energy to make ATP2 ATP are produced per glucose moleculePyruvic acid is then converted into lactic acidProduces less ATP than aerobic but it’s FAST!Can support strenuous activity for about a minute
18 Glucose (from glycogen breakdown or delivered from blood) Energy source: glucoseGlycolysis and lactic acid formation(b) Anaerobic pathwayOxygen use: NoneProducts: 2 ATP per glucose, lactic acidDuration of energy provision:60 seconds, or slightly moreGlucose (fromglycogen breakdown ordelivered from blood)Glycolysisin cytosolPyruvic acidReleasedto bloodnet gain2Lactic acidO2ATPFigure 9.19b
19 Aerobic PathwayProduces 95% of ATP during rest and light to moderate exerciseOccurs in the mitochondria and requires OxygenTurns fuels (stored glycogen, bloodborne glucose, pyruvic acid from glycolysis, and free fatty acids) into ATPProduces 32 ATP per glucose, CO2, & H20
20 Glucose (from glycogen breakdown or delivered from blood) Energy source: glucose; pyruvic acid;free fatty acids from adipose tissue;amino acids from protein catabolism(c) Aerobic pathwayAerobic cellular respirationOxygen use: RequiredProducts: 32 ATP per glucose, CO2, H2ODuration of energy provision: HoursGlucose (fromglycogen breakdown ordelivered from blood)32O2H2OCO2Pyruvic acidFattyacidsAminoAerobic respirationin mitochondriaATPnet gain perglucoseFigure 9.19c
21 Short-duration exercise Prolonged-durationexerciseATP stored inmuscles isused first.ATP is formedfrom creatinePhosphateand ADP.Glycogen stored in muscles is brokendown to glucose, which is oxidized togenerate ATP.ATP is generated bybreakdown of severalnutrient energy fuels byaerobic pathway. Thispathway uses oxygenreleased from myoglobinor delivered in the bloodby hemoglobin. When itends, the oxygen deficit ispaid back.Figure 9.20
22 Muscle FatiguePhysiological inability to contract even with a neural impulseOccurs when:ATP production fails to keep pace with ATP usageWhen ATP use exceeds its productionTotal lack of ATP occurs rarely, results in states of continuous contraction (contracture) Example: Writer’s Cramp
23 Oxygen Deficit Exercise uses oxygen stores in the muscle Oxygen demand rises so extra Oxygen must be taken in by the body to restore normal levels of OxygenVigorous exercise heavy breathing restores balance
24 Heat Production During Muscle Activity Only about 40% of the energy released in muscle activity is useful as workRemaining energy (60%) given off as heatDangerous heat levels are prevented by radiation of heat from the skin and sweatingConversely, shivering will increase body temperature when cold
25 Critical Thinking!!!When Eric returned from jogging, he was breathing heavily, sweating profusely, and complained that his legs ached and felt weak. His wife poured him a sports drink and urged him to take it easy until he could “catch his breath.” On the basis of what you have learned about muscle energy metabolism, respond to the following questions: Why is Eric breathing heavily? What ATP-generating pathway have his working muscles been using that leads to such breathlessness? What metabolic pathway might account for his sore muscles and his feeling of muscle weakness?