Changes in Ionic Gate Activity
At the nerve-muscle junction and between many neurons in the brain and spinal cord the transmitter substance liberated from the terminals of the presynaptic cell is acetylcholine Molecules of acetylcholine are stored in the vesicles within the axon terminal When the nerve impulse arrives at the terminal calcium ions flow into the depolarized membrane causing several hundred synaptic vesicles to fuse with the presynaptic membrane Each vesicle load of acetylcholine causes some 2000 ionic channels to open up in the postsynaptic membrane Movement of sodium ions into the postsynaptic cell and outflow of potassium ions from the cell then occurs
The acetylcholine molecules in the synaptic gap are rapidly broken down by the enzyme acetylcholinesterase Most of the molecules resulting from this deactivation process will be reabsorbed back into the axon terminal of the presynaptic neuron to be resynthesized back into acetylcholine and re-stored in the terminal for future repeated use The relative simplicity of this process combined with the regular resynthesis of acetylcholine molecules means that this process is not affected to any degree by the aging process
At synapses in the central nervous system where acetylcholine is liberated a second group of presynaptic neurons terminate The transmitter substance liberated from the axon terminals of this second set of neurons is gamma-aminobutyric acid (GABA) GABA acts on a second set of receptors on the surface of the postsynaptic cell to open pores in that membrane that are selectively permeable to negatively charged chloride ions As the chloride ions pass through the open pores into the postsynaptic cell they increase the voltage across the membrane and temporarily inactivate the cell Approximately one-third of all cells in the central nervous system are thought to use GABA as their neurotransmitter No GABA -producing neurons terminate at the nerve-muscle interface
The actions of acetylcholine and GABA on the postsynaptic cell are complementary Whether the postsynaptic cell fires or fails to fire when both neurotransmitters are liberated simultaneously is determined by their summated or cumulative effect on the resting potential of the postsynaptic membrane If the membrane is depolarized because of the stronger acetylcholine the postsynaptic cell will not fire Like acetylcholine GABA is rapidly broken down and inactivated in the synaptic gap and reabsorbed into the presynaptic neuron
During aging there appears to be a 15 percent reduction in the GABA content of the brain A reduction in GABA secretion and liberation throughout the nervous system results in progressive deterioration of fine coordination of motor activities When the GABA neurons of the corpus striatum degenerate selectively Huntington's chorea is produced a hereditary condition in which uncontrolled movements are a predominant feature