Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Neuromuscular Junction • Contraction of muscles is under the control of the nervous system: – via the motor end plate – which, together with the muscle cell, makes the neuromuscular junction • Structurally the neuromuscular junction is similar to, and operates in a similar manner to the synapse. • The light micrograph shows a motor end plate and the muscle fibre that it innervates. • Striations are visible in the muscle fibre at the top of the image. Motor end plate Nervous control Skeletal muscle is under the control of the voluntary nervous system. Each muscle is controlled by a motor neurone. motor neurone Motor neurones interact with muscles at a neuromuscular junction, sometimes called a motor endplate. This is a specialized form of synapse that forms between a neurone and muscle fibre. 3 of 36 neuromuscluar junction muscle fibre © Boardworks Ltd 2009 The neuromuscular junction 4 of 36 © Boardworks Ltd 2009 Summary – controlling movement 5 of 36 © Boardworks Ltd 2009 Motor end plate with vesicles containing neurotransmitter Neurotransmitter diffuses across the gap Depolarisation spreads to the T tube system Sarcolemma contains receptors for neurotransmitter Ca2+ Ca2+ Ca2+ ions released from the sarcoplasmic reticulum Ca2+ ions bind to the troponin and so exposes the myosin binding sites on the actin, allowing contraction (next lesson) Ca2+ Ca2+ Ca2+ • When an action potential arrives in the motor end plate – vesicles containing neurotransmitter migrate to and fuse with the membrane • release transmitter into the gap • The neurotransmitter – diffuses across the gap – binds to receptors in the sarcolemma • the membrane surrounding the muscle – causes depolarisation – depolarisation spreads to the T or tubule system which extends deep within the muscle cell • Depolarisation causes release of Ca2+ ions from the sarcoplasmic reticulum – bind to the muscle myofibrils and cause their contraction • Note: In common with synaptic transmission there is a minimum depolarisation needed to bring about an action beyond the nerve – there is a threshold level • However, at the neuromuscular junction, stimuli above threshold bring about an increased or graded response in a whole muscle. • Examine the following graphs which show the effect of stimulus intensity, summation and tetanus. Then, complete the description and interpretation of them. response Stimulus – larger stimuli are indicated by larger arrows response time Stimulus time Stimulus intensity If the stimulus is below threshold, there is no contraction of the muscle. If threshold is exceeded, contraction increases with increased stimulus intensity, i.e. there is a larger response. Note: Individual fibres have an ‘all or nothing response’, but the more fibres that are stimulated, the greater the overall contraction. Summation If an impulse arrives before the previous contraction has faded, a new contraction is started which is added to the effect of the previous one. Consequently, the overall contraction is greater than a single contraction would have been if the stimuli were further apart. This is summation. response Stimulus Tetanus Repeated large stimuli results in summation of the individual stimuli and produces a sustained and powerful contraction. This is tetanus. Clostridium tetani produces a toxin that causes sustained contraction of voluntary muscle, including those used in breathing. This can lead to death. time response Stimulus time Fatigue If stimulation is continued for a period of time, the contraction may fade as the muscle becomes fatigued. The muscle tires because respiration cannot keep pace with ATP demands, substrates are consumed and wastes like lactic acid accumulate. Neuromuscular junction Events in the nerve ending: Transmitter substance Cleft or gap Effect of action potential arriving Events in post synaptic membrane: Receptors in postjunction membrane Enzyme to remove transmitter Effect of depolarisation Key features Minimum depolarisation required for effect Effect of increasing stimulus intensity Summation Fatigue Acclimatisation Synapse