Faxon larger neurons harder to stimulate multiple

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Unformatted text preview: calized twitch of a small area twitch Nerve-Muscle Relationship (cont.) Nerve-Muscle Motor unit (cont.)~ • Small motor units common in areas needing Small fine control (eyes) fine • ~3-6 muscle fibers/axon & small neurons ~3-6 easily stimulated easily • Large units for strength (gastrocnemius) Large • ~1000 m.f./axon & larger neurons harder to stimulate Multiple motor units allows for muscle fibers to ‘work in shifts’ shifts’ • Long-term contraction (posture muscles, exercise) is Long-term possible because fatigued muscle fibers can rest possible Neuromuscular Junction Neuromuscular Neuromuscular junction= functional connection btw nerve fiber functional & target cell (muscle fiber) @ the synapse • • Synaptic knob= terminal end of each branching nerve fiber Motor end plate= depression on sarcolemma that s.knob depression resides w/in resides • Synaptic cleft= space btw s.knob & m.e.plate • Schwann cells= encloses neuromuscular junction & isolates it encloses from surrounding tissue fluid from Nerve impulses travel down axon to s.knob, neurotransmitters Nerve neurotransmitters released into s.cleft & stimulates 2nd cell (muscle fiber) released ACh (acetylcholine)= most common neurotransmitter; stored in most synaptic vessicles w/in nerve ending until time to release synaptic Neuromuscular Junction (cont.) Neuromuscular Sarcolemma has junctional folds (infoldings of membrane) & ~50 mil ACh receptors for bindings of Ach ~50 Basal lamina= surrounds entire muscle fiber & fills space of surrounds synaptic cleft (contains AChE) synaptic AChE (acetylcholinesterase)= enzyme, shuts down Ach enzyme, stimulation…allowing muscle to relax stimulation…allowing Table 11.2 Electrically Excitable Cells Electrically Muscle & nerve fibers are both electrically excitable cells Muscle due to ability to change voltage across plasma membrane membrane Resting, unstimulated cells are ‘more negative’ inside than Resting, outside plasma membrane outside • More anions inside than out= polarize or ‘charge’ More polarize membrane membrane • Large anions (proteins, nucleic acid, phosphates) remain Large inside cell while excess Na+ (outside) & K+ (inside) inside • Electrical potential (voltage)= difference in electrical charge difference from 1 pt to another from • -70mV @ resting membrane potential (RMP) is maintained -70mV resting by Na-K pumps of p.membrane by Electrically Excitable Cells (cont.) Electrically 1. When stimulated, Na-channels open & Na+ enter cell 1. (diffuse down it’s [gradient]) (diffuse • Causes ISF to briefly become + charged 2. Na-channels close, & K-channels open an instant later 2. • K+ flows out of cell (down it’s [gradient]) • Loss of K+ causes ISF to ‘return’ to – charge 3. Na-K movement across p.membrane results in a temporary 3. temporary voltage difference from – to + & back to - = action potential voltage action AP’s are perpetuated along p.membrane to form a ‘wave of AP’s’ that travel 1-i...
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