6 HUMAN HEALTH AND PHYSIOLOGY 1 151 neurons contain sodium (Na+) and potassium (K+ ) ions. Impulses occur
as these important ions move in and out through the plasma membrane.
When a neuron is not transmitting an impulse, it is said to be at its resting potential. The resting potential is the potential diff erence across the plasma membrane when it is not being stimulated – for most neurons, this potential is −70 mV. The inside of the axon is negatively charged with respect to the outside (Figure 6.22).
As a nerve impulse occurs, the distribution of charge across the membrane is reversed. For a millisecond, the membrane is said to be depolarised. As charge is reversed in one area of the axon, local currents depolarise the next region so that the impulse spreads along the axon (Figure 6.23). An impulse that travels in this way is known as an action potential.
Figure 6.24 (overleaf ) explains what is happening at the plasma membrane of the neuron as an action potential is generated.
1 When a neuron is stimulated, gated sodium channels in the membrane open and sodium ions (Na+) from the outside fl ow in. They follow both the electrical gradient and the concentration gradient, together known as the electrochemical gradient, to move into the cell. The neuron is now said to be depolarised.
2 For a very brief period of time, the inside of the axon becomes positively charged with respect to the outside as sodium ions enter. At this point, the sodium channels close.
3 Now, gated potassium channels open and potassium ions (K+) begin to leave the axon, moving down their electrochemical gradient to re-establish the resting potential, a process known as repolarisation.
4 Because so many potassium ions start to move, the potential diff erence falls below the resting potential. At this point, both sodium and potassium channels close. The resting potential is re-established by the action of sodium–potassium pumps, which move ions back to restore the resting potential.
An action potential in one part of an axon causes the depolarisation of the adjacent section of the axon. This occurs because local currents are set up between adjacent regions and these cause ion channels to open, allowing sodium ions in and potassium ions out of the axon.
Figure 6.22 At rest, sodium ions are pumped out of the neuron and potassium ions are pumped in, to establish the resting potential.
Inside the neuron is negatively charged because of the presence of chloride and other negative ions.
Figure 6.23 When an impulse passes along the neuron, sodium ions diffuse via ion channels and the potential is reversed. This process is called an action potential.
sodium ions are constantly pumped out and potassium ions are pumped in
Na+
K+ K+
K+
protein molecules making up the sodium–potassium pump
Na+
K+
Na+
K+
axon
+ + + + + + + + + + + + + + + + + + + + + + + +
– –
– – –
– – – – – – –
– –
– – –
– – – – – – –
impulse travels in this direction Na+Na+Na+Na+
area of impulse next area to be
stimulated area returning
to rested state
+ + + + – – – – – + + +
+ + + + – – – – – + + + +
+
+ + +
– – – – – – –
+ +
+ + +
– – – – – – –
Resting potential the electrical potential across the plasma membrane of a neuron that is not conducting an impulse
Action potential the reversal and restoration of the resting potential across the plasma membrane of a neuron as an electrical impulse passes along it
The action potential travels along the neuron rather like a ‘Mexican wave’. The impulse can only pass in one direction because the region behind it is still in the recovery phase of the action potential and is temporarily unable to generate a new action potential. The recovery phase is known as the refractory period.
The synapse
A synapse is the place where two neurons meet. Two neurons do not touch one another and the tiny gap of about 20 nm between them is
Figure 6.24 The action potential.
CI–
CI–
CI–
CI– CI–
CI– K+
K+ K+
K+ K+ K+
K+
Na+
Na+ Na+ Na+
Na+
Na+ Na+
Na+
CI–
CI–
CI–
CI– CI–
CI– K+
K+
K+ K+
K+ K+
K+ Na+
Na+
Na+ Na+
Na+
Na+ Na+
CI–
CI–
CI–
CI– CI–
CI– K+
K+
K+ K+
K+ K+
K+ Na+ Na+
Na+
Na+ Na+ Na+
Na+ Na+
Inside of neuron
Outside of neuron
When stimulated Na+ channels open and Na+ ions flow into the neuron.
1
Na+ flow causes rapid depolarisation of the membrane.
2
Gated K+ channels open and K+ flows out, causing repolarisation.
3
direction of impulse
refractory period resting potential
Elapsed time/ms
0 1 2 3 4 5
+40
0
Membrane potential/mV –70
closed K+ channel
Na+/K+ pump openNa+
channel
open K+ selective channel
Inside of neuron
Outside of neuron open K+
channel
Na+/K+ pump closed
Na+ channel
open K+ selective channel
Inside of neuron
Outside of neuron closed K+
channel
Na+/K+ pump closed
Na+ channel
open K+ selective channel
At rest, most K+ ions are inside the axon, but it remains negatively charged due to the presence of negative ions. As K+ ions are pumped into the axon by the Na+/K+ pump, they need to diffuse back out in order to maintain this negative resting potential.
6 HUMAN HEALTH AND PHYSIOLOGY 1 153 known as the synaptic cleft. Action potentials must be transmitted across
this gap for the impulse to pass on its way and this is achieved by the presence of chemicals known as neurotransmitters. Neurotransmitters are held in vesicles in the pre-synaptic cell until an action potential arrives.
They are then released into the synaptic cleft, and diff use across to the post-synaptic membrane. There they can cause another action potential to be produced.
The synapse shown in Figure 6.25 uses the neurotransmitter
acetylcholine (ACh) and is a cholinergic synapse. ACh binds to receptors and causes depolarisation of the post-synaptic membrane and the initiation of an action potential. Once an action potential is generated in the post-synaptic membrane, ACh in the synaptic cleft is deactivated by acetylcholinesterase enzymes and the products are reabsorbed by the pre-synaptic membrane to be remade and repackaged in vesicles.