Nerve impulse long note

 The resting membrane is charged, and is called a polarized membrane (negative inside compared to outside), and is more permeable to potassium than sodium  when an electrical current stimulates the nerve, voltage-gated ion channels open allowing sodium ions into the cell  the flood of sodium ions wipes out the voltage difference, called depolarization  the inside begins to develop a positive charge with respect to the outside  following depolarization the sodium channels begin to close, called channel inactivation  Potassium gated channels open and the movement of potassium ions outwards helps to reestablish the voltage difference and return to resting potential  a sodium-potassium pump in the cell membrane helps restore the original condition of the resting membrane by pumping 3 Na + ions out of the neuron for every 2 K + ions into the neuron (uses ATP) and is called repolarization  depolarization and return to resting potential is called an action potential  action potential is all or nothing, the amount of stimulation required is the threshold value  one action potential takes 5 milliseconds  the electrical potential or voltage change is the same for all neurons  the recovery time is also the same  the accumulation of sodium within the cell is reduced by the Na-K pump

 during the few milliseconds of depolarization, when sodium ions move in and before the gates are closed, the site of stimulation has less charge than the membrane surrounding it  the potential difference creates a small, localized current which influences nearby voltage-gated sodium channels to open, permitting sodium ions to enter the cell and depolarizing these sites  during the time required for the voltage-gated sodium channels to recover from their inactive state another action potential cannot occur, this is called the refractory period  this allows for the movement of the nerve impulse away from the stimulus site  the amplitude of depolarization is always the same, the number or frequency of firings can change  the myelin sheath of the Schwann cells prevent the movement of ions across the neuron membrane  voltage-gated channels are concentrated in the Nodes of Ranvier  the electrical current jumps from node to node causing depolarization  allows the signal to move faster and conserves energy

 the action potential passing along the axon eventually reaches the end, often branched, called the terminus or presynaptic membrane  could be associated with dendrites of another neuron, a muscle cell or a secretory gland  passage across the gap is chemical  wave of depolarization stimulates the release of neurotransmitters into the synaptic cleft  chemicals rapidly pass to the other side where they combine with receptor molecules in the membrane of the target cell (postsynaptic membrane)  the binding of the neurotransmitters to the receptor causes chemically-gated ion channels to open  over 60 neurotransmitters are used for a variety of messages, also the chemically gated channels can be different

Neuromuscular Junctions  target cell is muscle  neurotransmitter is acetylcholine  acetylcholine binds to receptors opening chemically-gated sodium ion channels  ion flow into the cell depolarizes the postsynaptic muscle cell membrane which contains voltage-gated sodium ion channels  a wave of depolarization passes down the muscle cell permitting calcium ions into the muscle cell which triggers contraction  necessary to destroy residual(old) neurotransmitter in the synaptic cleft otherwise the postsynaptic membrane would remain depolarized  enzyme called acetylcholinesterase is present in the cleft  it inactivates one acetylcholine molecule every 40 microseconds, permitting as many as 1000 impulses per second to be transmitted across the neuromuscular junction

Neural Synapses  target cell is another neuron  postsynaptic cell could be depolarized or hyperpolarized  different neurotransmitters each have a specific receptor  these neurons can be excitatory neurons or inhibitory neurons

 excitatory neurons are neurons that release neurotransmitters that target receptors connected to Na+ channels to depolarize the postsynaptic membrane to cause an action potential  inhibitory neurons are neurons that release neurotransmitters that target receptors connected to K+ channels to hyperpolarize the postsynaptic membrane to prevent an action potential

It inhibits action potentials by increasing the stimulus required to move the membrane potential to the action potential threshold.

 more than one neuron can synapse with the same post-synaptic neuron  nerve summation involves combining the effects of excitatory and inhibitory neurotransmitters to determine whether the threshold of excitation is reached in the postsynaptic neuron.  Summation allows for prioritizing of information and coordinating of muscle movements.

 Some nerve poisons (e.g., scorpion venom) open Na+ channels and shut K+ channels & disrupts any action potentials.  Local anesthetic drugs (Novocain, Xylocaine) block the Na+ channels and prevent action potentials along sensory neurons.  General anesthetics used in hospitals (ether, chloroform) open some K+ channels in the brain a bit wider than usual. This counter-acts the effects of Na+ channels being opened and prevents action potentials from propagating, too.