A nerve impulse travels along the axon to the axon terminal. Axons usually have several rounded synaptic knobs at their terminals, which dendrites lack. These knobs contain arrays of membranous sacs, called synaptic vesicles, that contain neurotransmitter molecules. When a nerve impulse reaches a synaptic knob, voltage-sensitive calcium channels open, calcium diffuses inward from the extracellular fluid. The increased calcium concentration inside the cell initiates a series of events that causes the synaptic vesicles to fuse with the cell membrane, releasing their neurotransmitter by exocytosis.
Synapses provide the informational potential of the nervous system, as billions of neurons make many trillions of connections. The human brain at birth contains 60 to 100 billion neurons. If that number is equated to the number of trees in the Amazon rain forest, then the number of synapses can be compared to the number of leaves on those 60 to 100 billion trees.
Factors Affecting Impulse Conduction
A number of substances alter axon membrane permeability to ions. For example, calcium ions are required to close sodium channels in axon membranes during an action potential. Consequently, if calcium is deficient, sodium channels remain open, and sodium ions diffuse through the membrane again and again so that impulses are transmitted repeatedly. If these spontaneous impulses travel along axons to skeletal muscle fibers, the muscles continuously spasm (tetanus or tetany). This can occur in women during pregnancy as the developing fetus uses maternal calcium. Tetanic contraction may also occur when the diet lacks calcium or vitamin D or when prolonged diarrhea depletes the body of calcium.
A small increase in the concentration of extracellular potassium ions causes the resting potential of nerve fibers to be less negative (partially depolarized). As a result, the threshold potential is reached with a less intense stimulus than usual. The affected fibers are very excitable, and the person may experience convulsions.
If the extracellular potassium ion concentration is greatly decreased, the resting potentials of the nerve fibers may become so negative that action potentials cannot occur. In this case, impulses are not triggered, and muscles become paralyzed.
Certain anesthetic drugs, such as procaine, decrease membrane permeability to sodium ions. In the tissue fluids surrounding an axon, these drugs prevent impulses from passing through the affected region. Consequently, the drugs keep impulses from reaching the brain, preventing perception of touch and pain. ■
Released neurotransmitter molecules diffuse across the synaptic cleft and react with specific receptor molecules in or on the postsynaptic neuron membrane. Effects of neurotransmitters may vary. Some open ion channels, and others close them. Because these ion channels respond to neurotransmitter molecules, they are called chemically-sensitive, in contrast to the voltage-sensitive ion channels involved in action potentials. Changes in chemically-sensitive ion channels create local potentials, called synaptic potentials, which enable one neuron to influence another.
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