Inside the cytoplasm of a neurone, there are large negatively charged organic particles, which are too big to cross the membrane. Ordinarily there are a large number of potassium ions (K+) inside the cell, which diffuse across the selectively permeable membrane. Eventually equilibrium is reached where the net movement of K+ ions into and out of the cell is equal. This is due to the attractive forces of the negatively charge organic molecules. This point is known as the resting potential and is generally said to be around -70mv. When a stimulus stimulates a receptor, a potential difference is created across the cell. This is detected by the electrosensitive Sodium (Na +) channels closest to the stimuli, which consequently open allowing sodium ions to diffuse in as a result. A local current is produced that causes the next sodium channel to open and so on, consequently the impulse travels along the nerve. This is known as an action potential. The inside gradually becomes more positive compared to the outside and generally the potential difference is said to climax to around +30mv. This process is known as depolarisation and only last a few milliseconds. As the cell is now highly positive it is very desirable for it to return back to the resting potential. Sodium channels then close and will not reopen for a short period of time known as the absolute refractory period. At this time Na+ ions are actively pumped out of the cell and the membrane becomes more permeable to K+. Additional channels open and potassium diffuses down a concentration gradient into the cell. This restores the negative charge of the axon.
Myelinated Neurone.
In a myelinated neurones a nervous impulse travels down the neurone by a mechanism known as saltatory conduction. But how does this work? As before when a stimulus stimulates a receptor, a potential difference is created across the cell. Electrosensitive Sodium (Na +) channels closest to the stimuli, open and sodium ions diffuse in.
