3.4.1 Nerve Impulse
- When the dendrite is stimulated by a strong enough signal, a nerve impulse is generated in an "all or nothing" response.
- The stimulus triggers chemical and electrical changes in the neuron. - The exterior of the cell has a large number of sodium ions present compared to the interior of the cell and is positively charged. - The interior of the cell contains more potassium ions than the exterior of the cell and is negatively charged. - The impulse causes sodium ions to flow into, and potassium ions to flow out of, the cell, causing a reversal in charges, with a positive charge now occurring on the interior of the cell and a negative charge on the exterior (the cell is said to be depolarized). - When the impulse “passes” along the axon, the nerve becomes polarized again as the potassium ions flow back into the cell and sodium ions move out of the cell. Until a nerve becomes repolarized it cannot respond to a new stimulus; the time for recovery is called the refractory period and takes about 0.0004 of a second. The more intense the stimulus, the more frequent the firing of the neuron. - When the impulse reaches the end of the axon, it causes the release of chemicals from small vesicles called neurotransmitters which diffuse across the synaptic gap, the small space between the axon and receptors in the dendrites.
