The Nerve Impulse.

The Neuron at Rest The plasma membrane of neurons contains many active Na-K-ATPase pumps. These pumps shuttle Na+ out of the neuron and K+ into the neuron when ATP is hydrolyzed. Three Na+ are pumped out of the neuron at a time and two K+ ions are pumped in

This creates a concentration gradient for Na+. As Na+ accumulates on the outside of the neuron, it tends to leak back in. Na+ must pass through proteins channels to leak back through the hydrophobic plasma membrane. These channels restrict the amount of Na+ that can leak back in. This maintains a strong positive charge on the outside of the neuron

The K+ inside the neuron also tends to follow its concentration gradient and leak out of the cell. The protein channels allow K+ to leak out of the cell more easily. As a result of this movement in Na+ and K+ ions, a net positive charge builds up outside the neuron and a net negative charge builds up inside.

This difference in charge between the outside and the inside of the neuron is called the Resting Potential. The resting potential in most neurons is –70 mV. When the neuron is at rest, it is polarized

Initiation of the Action Potential A change in the environment ( pressure, heat,sound, light) is detected by the receptor and changes the shape of the channel proteins in part of the neuron –usually the dendrites. The Na+ channels open completely and Na+ ions flood into the neuron. The K+ channel close completely at the same time and K+ ions can no longer leak out of the neuron in that particular area.

The interior of the neuron in that area becomes positive relative to the outside of the neuron. This depolarization causes the electrical potential to change from –70 mV to + 40 mV The Na+ channels remain open for about 0.5 milliseconds then they close as the proteins enter an inactive state. The total change between the resting state (-70 mV) and the peak positive voltage ( +40mV) is the action potential ( about 110 mV)

The spike in voltage causes the K+ pumps to open completely and K+ ions rush out of the neuron. The inside becomes negative again. This is repolarization. So many K+ ions get out that the charge goes below the resting potential. While the neuron is in this state it cannot react to additional stimuli. The Refractory period lasts from 0.5 to 2 milliseconds. During this time, the Na-K-ATPase pump reestablishes the resting potential.

Transmission of the impulse The stimulus induces depolarization in a very small part of the neuron, at the dendrites. The sequence of depolarization and repolarization generates a small electrical current in this localized area. The current affects the nearby protein channels for Na+ and causes them to open.

When the adjacent channels open, Na+ions flood into that area of the neuron and an action potential occurs. This in turn will affect the areas next to it and the impulse passes along the entire neuron. The electric current passes outward over the membrane in all directions BUT the area to one side is still in the refractory period and is not sensitive to the current. Therefore the impulse moves from the dendrites toward the axon.

Threshold stimulus Action potentials occur only when the membrane in stimulated (depolarized) enough so that sodium channels open completely. The minimum stimulus needed to achieve an action potential is called the threshold stimulus. If the membrane potential reaches the threshold potential (generally mV less negative than the resting potential), the voltage-regulated sodium channels all open. Sodium ions rapidly diffuse inward, & depolarization occurs.

All-or-None Law Action Potentials occur maximally or not at all. In other words, there's no such thing as a partial or weak action potential. Either the threshold potential is reached and an action potential occurs, or it isn't reached and no action potential occurs. However, different neurons have different densities of Na+ channels and therefore have different APs

The AP remains constant as it travels down the neuron. Its amplitude is always the same because it corresponds to wide open Na+ channels. The frequency of the AP can change.

Conduction Velocity impulses typically travel along neurons at a speed of anywhere from 1 to 120 meters per second the speed of conduction can be influenced by: –The diameter of a fiber. Velocity increases as diameter increases. –Temperature. As temperature increases, the velocity increases. Axons of birds and mammals can be very small because of the high body temperature. –the presence or absence of myelin.

Neurons with myelin (or myelinated neurons) conduct impulses much faster than those without myelin. Because fat (myelin) acts as an insulator, membrane coated with myelin will not conduct an impulse. So, in a myelinated neuron, action potentials only occur along the nodes and, therefore, impulses 'jump' over the areas of myelin - going from node to node in a process called saltatory conduction (the word saltatory means 'jumping')

Summary The Action Potential, or nerve impulse is an electrochemical event involving the rapid depolarization and repolarization of the nerve cell membrane. The axon terminals of one neuron do not touch the dendrites of other neurons. What happens when the impulse reaches the axon terminal?