BY4: The nerve impulse
The electrical change associated with a typical nerve impulse is very small (50 millivolts). Nevertheless nerve impulses can be recorded and measured using an apparatus which is sensitive to small electrical changes. Impulses can be picked up from the nerve through a pair of microelectrodes and fed into a cathode ray oscilloscope. This can measure the magnitude and speed of transmission of impulses and analyse the pattern of impulses generated in different parts of the nervous system.
The electrical change associated with a typical nerve impulse is very small (50 millivolts). Nevertheless nerve impulses can be recorded and measured using an apparatus which is sensitive to small electrical changes. Impulses can be picked up from the nerve through a pair of microelectrodes and fed into a cathode ray oscilloscope. This can measure the magnitude and speed of transmission of impulses and analyse the pattern of impulses generated in different parts of the nervous system.
Neurones transmit electrical impulses along the cell surface membrane surrounding the axon. Experiments involving inserting microelectrodes into axons and measuring the changes in electrical charge have shown that in a resting axon, the inside of the membrane has a negative electrical charge compared to the outside.
This resting potential is the potential difference between the inside and the outside of a membrane when a nerve impulse is not being conducted. In human nerve cells this value is -70mV (inside the axon relative to outside)
Resting potentials are typically minus values, the minus indicating the inside is negative with respect to the outside. The membrane is said to be polarised
Sodium ions and potassium ions are transported across the membrane against a concentration gradient by active transport
This involves sodium-potassium exchange pumps (these are intrinsic carrier proteins) which maintain the concentration and an uneven distribution of sodium ions and potassium ions across the membrane. However, the Na+ ions are passed out faster than the K+ ions are brought in (3Na+ ions are transported out but only 2K+ ions are transported in). Also K+ ions are able to diffuse back out as the K+ ion channels are "leaky" . The net result is that the outside of the membrane is positive compared to the inside. The outward movement of positive ions means that the inside becomes slightly negative.
The action potential
"An action potential is when a nerve impulse is initiated and there is a temporary reversal in the membrane potential away from -70mV to +40mV inside the axon in relation to outside"
• Nerve impulses are due to changes in the permeability of nerve cell membrane to K+ ions and Na+ ions which leads to changes in the potential difference across the membrane and the formation of action potential
• Suitable stimulation of an axon results in change of potential across the membrane from a negative inside value of about –70mV to a positive inside value of +40mV. This change is called an action potential and lasts about three milliseconds. The membrane is said to be depolarised. When the resting potential is re-established, the axon membrane is said to be repolarised
• The action potential is the result of a sudden increase in the permeability of the membrane to Na+ ions. This allows a sudden influx of Na+ ions which depolarises the membrane
• A fraction of a second after this depolarisation the K+ diffuse out and repolarises the membrane
• There is an overshoot of K+ leaving as the K+/Na+ pump restores the ionic balance. This is called the refractory period during which another action potential cannot be generated. This time delay ensures a unidirectional impulse and limits frequency.
Action Draw and interpret an oscilloscope trace (showing an action and resting potential).
This resting potential is the potential difference between the inside and the outside of a membrane when a nerve impulse is not being conducted. In human nerve cells this value is -70mV (inside the axon relative to outside)
Resting potentials are typically minus values, the minus indicating the inside is negative with respect to the outside. The membrane is said to be polarised
Sodium ions and potassium ions are transported across the membrane against a concentration gradient by active transport
This involves sodium-potassium exchange pumps (these are intrinsic carrier proteins) which maintain the concentration and an uneven distribution of sodium ions and potassium ions across the membrane. However, the Na+ ions are passed out faster than the K+ ions are brought in (3Na+ ions are transported out but only 2K+ ions are transported in). Also K+ ions are able to diffuse back out as the K+ ion channels are "leaky" . The net result is that the outside of the membrane is positive compared to the inside. The outward movement of positive ions means that the inside becomes slightly negative.
The action potential
"An action potential is when a nerve impulse is initiated and there is a temporary reversal in the membrane potential away from -70mV to +40mV inside the axon in relation to outside"
• Nerve impulses are due to changes in the permeability of nerve cell membrane to K+ ions and Na+ ions which leads to changes in the potential difference across the membrane and the formation of action potential
• Suitable stimulation of an axon results in change of potential across the membrane from a negative inside value of about –70mV to a positive inside value of +40mV. This change is called an action potential and lasts about three milliseconds. The membrane is said to be depolarised. When the resting potential is re-established, the axon membrane is said to be repolarised
• The action potential is the result of a sudden increase in the permeability of the membrane to Na+ ions. This allows a sudden influx of Na+ ions which depolarises the membrane
• A fraction of a second after this depolarisation the K+ diffuse out and repolarises the membrane
• There is an overshoot of K+ leaving as the K+/Na+ pump restores the ionic balance. This is called the refractory period during which another action potential cannot be generated. This time delay ensures a unidirectional impulse and limits frequency.
Action Draw and interpret an oscilloscope trace (showing an action and resting potential).