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neurons that stimulate skeletal muscle fibers to contract; threadlike ason that entends from the brain or spinal cord to a group of skeletal muscle fiber
somatic motor neurons
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synapse between a somatic motor neuron and a skeletal muscle fiber where muscle action potentials arise
neuromuscular junction
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a region where comunication occurs between two neurons or between a neuron and a target cell (between a somatic motor neron and a muscle fiber)
synapse
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small gap at most synapses that separates two cells which makes it necessary for the first cell to release a chemical called nerotransmitter to communicate with the second
synaptic cleft
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communicates with cells as it is released and travels to the next
neurotransmitter
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located at the NMJ, the end of the motor neuron, called the __________, divides into a cluster of synaptic end bulbs
axon terminal
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contains thousands of molecules of acetylcholine; 100's of membrane-enclosed sacs suspend in the cytsol within each synaptic end bulb
synaptic vesicles
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abbreviated ACh, the neurotransmitter released at the NMJ
actylcholine
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region of the sarcolemma opposite the synaptic end bulbs; muscle fiber part of the NMJ
motor end plate
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integral transmembrane proteins that bind specifically to ACh;30-40 million found within each motor end plate; abundant in junctional folds; ligand-gated ion channels
acetylcholine receptors
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Nerve impuls elcits a muscle action potential in the following way
- Release of acetylcholine
- Activation of ACh receptors
- Production of muscle action potential
- Termination of ACh activity
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first step of nerve impulse (nerve action potential)
Release of acetylcholine - Nerve impulse arrives at axon terminal of motor triggers a release of acetylcholine from synaptic vesicle of the synaptic end bulb. ACh diffuses across synaptic cleft (space) between motor neuron and motor end plate
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second step of nerve impulse eliciting a muscle action potential
Activation of ACh receptors - binding of two ACh molecules to the receptor on the motor end plate opening an ion channel in the ACh receptor; Na+ flow across the membrane
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third step of a nerve impulse eliciting a muscle action potential
Production of muscle action potential - inflow of Na+ down electrochimical gradient results in inside of muscle fiber more positively charged; triggers a muscle action potential; muscle action propagates along sarcolemma into the T tubule system; causes sarcoplasmic reticulum to release it stored Ca ++ into sarcoplasm resulting in contraction
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forth step of nerve impulse eliciting a muscle action potential in the following way
termination of ACh activity - acetylcholinesterase (AChE) enzyme attached to collagen fibers in the extracellular matrix of the synaptic cleft that breaks down into acetyl and choline to prevent activation of ACh receptor. Breakdown occurs when action potentials in the motor neuron cease and ACh is no longer released thus ending the production of muscle action potentials preventing further contraction of the muscle fiber.
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ATP is used for what purposes in muscle fibers
- contraction cycle
- pump Ca++ into the sarcoplasmic reticulum
- metabolic reactions involved in muscle contraction
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three ways the muscle fiber produce ATP
- creatine phosphate
- anaerobic cellular respiration
- aerobic cellular respiration
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excess ATP made during the resting stage of relaxed muscle is used to create _________, which is an energy rich molecule that is found only in muscle fibers
creatine phosphate
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catalyzes the transfer of one of the hight-energy phosphate groups from ATP to creatine forming creatine phosphate and ADP
creatine kinase
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small, amino acid-like molecule that is synthesized in the liver, kidneys, and pancreas and transported to muscle fibers
creatine
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ATP producing reactions that do not require oxygen; utilized when creatine phosphate is depleted; involves the catabolism of glucose to generate ATP; glycolysis
anaerobic cellular respiration
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10 reactions resulting inthe break down of glucose molecule into two molecules of pyruvic acid; use two ATP molecules to make four ATP
glycolysis
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in anaerobic respiration, pyruvic acid is converted into ________ in the cytosol. _________ then diffuses our of the skeletal system into the blood where liver cells can convert some of it into glucose. Helps to reduce the acidity of the blood.
lactid acid
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a series of oxygen-requiring reactions that produce ATP in mitochondria; pyruvic acid enters mitochondria where it is completely oxidized in reactions that generate ATP, CO2, and water and hear; yields 36 molecules of ATP
aerobic respiration
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sources of oxygen for aerobic respiration in muscle tissue
- oxygen that diffuses into mucsle fibers from the blood
- oxygen released by myoglobin within muscle fibers
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binds oxygen when it is plentiful and releases it when it is scarce
hemoglobin (in the RBC) and myoglobin (muscle)
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nutrients used for aerobic respiration
- pyruvic acid form glycolysis of glucose
- fatty acids from breakdown of triglycerides in adipose cells
- amino acids from the breakdown of proteins
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inability of a muscle to maintain force of contraction after prolonged activity
muscle fatigue
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feeeling of tiredness and the disre to cease activity that is cuased by changes in the central nervous system; perhaps protective
central fatigue
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factors thought to contribute to fatigue
- inadequate release of calcium ions from the SR
- depletion of creatine phosphate
- insuficient oxygen, depletion of glycogen and other nutrients
- failure of actin potentials
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refers to added oxygen, over and above the resting consupmtion of oxygen, that is taken into the body after exercise
oxygen debt
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three ways extra oxygen is used to pay back or restore metabolic conditions to the resting level
- convert lactic acid back into glycogen stores in the liver
- resynthesize creatine phosphate and ATP in muscle fibers
- replace the oxygen removed from myoglobin
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the force or tension that a single mucle fiber can produce depends....
on the rate at which nerve impulses arrive at the neuromuscular junction
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number of impulses per second
frequency of stimulation
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the total tension a whole muscle can produce depends on the ....
number of muscle fibers that are contractin in unison
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consists of a somatic motor neuron plus all the skeletal muscle fibers it stimulates
- motor unit
- *a single somatic motor neuron makes contact with an average of 150 skeletal muscle fibers which contract in unison
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the brief contaction of all the muscle fibers in a motor unit in response to a single action potential in its motor neuron
twitch contraction
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record of muscle contraction
myogram
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briedf delay occurs between application of the stimulus and the beginning of contraction wich last about two milliseconds; muscle action potential sweeps over the sarcolemma and calcium ions are released from the sacrcoplasmic reticulum
latent period
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last 10-100 msec; calcium binds to troponin, myosin-binding sites on actin are exposed and creossbridges form; peak tension occurs in the muscle fiber
contaction period
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lasting 10-100 msec, Ca++ is actively transported back into the sarcoplasmic reticulum, myosin binding sites are covered by tropomyosin, myosin head detach from actin, and tension in the muscle fiber decreases
relaxation period
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if two stimuli are appled, one right before the other, the muscle will respond to the first stimulus but not to the second. the muscle loses its excitability and cannot respond for a time
refractory period
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skeletal muscle has a _______ refractory period while carciac muscle has a _____ regractory period
shorter; longer
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the phenomenon where stimuli arriving at different times cause larger contraction is called ______.
occurs when additional Ca++ is released from the sarcoplasmic reticulum by subsequent stimul while the levels of Ca++ in the sarcoplasm are still elevated from the first stimulus.
wave summation
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sustained but wavering contraction
unfused tetanus
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80-100 stimulations per second where the muscle does not relax at all
fused tetanus
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