Nerve impulse

A nerve impulse – an electrical charge – is the signal that passes from one neuron to the next and finally to an end organ, such as a group of muscle fibers, or back to the CNS. For simplicity, think of the nerve impulse traveling through a neuron much as electricity travels through the electrical impulse is generated and how it travels through a neuron.

Resting membrane potential

The cell membrane of a neuron at rest has a negative electrical potential of about -70mV. This means that if one were to insert a voltmeter probe inside the cell, the electrical charges found there and the charges found outside the cell would differ by 70mV, and the inside would be negative relative to the outside. This electrical potential difference is known as the resting membrane potential(RMP). It is caused by a separation of charges across the membrane differ, the membrane is said to be polarized.

The neuron has a high concentration of potassium ions(K⁺) on the inside of the membrane and a high concentration of sodium ions(Na⁺) on the outside. The imbalance in the number of ions inside and outside the cell causes the RMP. The imbalance in the number of ions inside and outside the cell causes the RMP. This imbalance is maintained in two ways. First, the cell membrane is much more permeable to K⁺ than to Na⁺, so the K⁺ can move more freely. Because ions tend to move to establish equilibrium, some of the K⁺ will move to an area where it is less concentrated, outside the cell. The Na⁺ cannot move to the inside as easily. Second, sodium-potassium pumps in the neuron membrane, which contain Na⁺-K⁺ adenosine triphosphatase(ATPase), maintain the imbalance on each side of the membrane by actively transporting potassium ions in the sodium ions out. The sodium-potassium pump moves three Na⁺ out of the cell for each two K⁺ it brings in. The end result is that more positively charged ions are outside the cell than inside, creating the potential difference across the membrane. Maintenance of a constant RMP of about -70mV is primarily a function of the sodium-potassium pump.

Depolarization and hyperpolarization

If the inside of the cell becomes less negative relative to the outside, the potential difference across the membrane decreases. The membrane will be less polarized. When this happens, the membrane is said to be depolarized. Thus, depolarization occurs any time the charge difference becomes less than the RMP of -70mV, moving closer to zero. This typically results from a change in the membrane’s Na⁺ permeability. 

The opposite can also occur. If the charge difference across the membrane increases, moving from the RMP to an even more negative value, then the membrane becomes more polarized. This is known as hyperpolarization. Changes in the membrane potential are actually signals used to receive, transmit, and integrate information within and between cells. These signals are of two types, graded potentials and action potentials. Both are electrical currents created by the movement of ions.

Graded potentials

Graded potentials are localized changes in the membrane potential. These changes can be either depolarizations or hyperpolarizations. The membrane contains ion channels with ion gates that act as doorways into and out of the neuron. These gates are usually closed, preventing ion flow; but they open with stimulation; allowing ions to move from the outside to the inside or vice versa. This ion flow alters the charge separation, changing the polarization of the membrane.

Graded potentials are triggered by a change in the neuron’s local environment. Depending on the location and type of neuron involved, the ion gates may open in response to the transmission of an impulse from another neuron or in response to sensory stimuli such as changes in chemical concentrations, temperature, or pressure.

Recall that most neuron receptors are located on the dendrites( although some are on the cell body), yet the impulse is always transmitted from the axon terminals at the opposite end of the cell. For a neuron to transmit an impulse, the impulse must travel almost the entire length of the neuron. Although a graded potential may result in depolarization of the entire cell membrane, it is usually just a local event such that the depolarization does not spread very far along the neuron. To travel the full distance, an impulse must generate an action potential.

Action potentials

An action potential is a rapid and substantial depolarization of the neuron’s membrane. It usually lasts about 1ms. Typically, the membrane potential changes from the RMP of about -70mV to a value of about +30mV and then rapidly returns to its resting value. This is illustrated in the picture below. How does this marked change in membrane potential occur?

All action potentials begin as graded potentials. When enough stimulation occurs to cause a depolarization of at least 15 to 20mV, an action potential results. In other words, if the membrane depolarizes from the RMP of -70mV to a value of -50 to -55mV, the cell will experience an action potential. The membrane voltage at which a graded potential becomes an action potential is called the depolarization threshold. Any depolarization that does not attain the threshold will not result in an action potential. For example, if the membrane potential changes from the RMP of -70mV to -60mV, the change is only 10mV and does not reach the threshold; thus, no action potential occurs. But any time depolarization reaches or exceeds the threshold, an action potential will result. This is the all-or-none principle.

When a given segment of an axon is generating an action potential and its sodium gates are open, it is unable to respond to another stimulus. This is reffered to as the absolute refractory period. When the sodium gates are closed, the potassium gates are open, and repolarizing is occurring, the segment of the axon can then respond to a new stimulus, but the stimulus must be of substantially greater magnitude to evoke an action potential. This is reffered to as the relative refractory period.

Propagation of the action potential

Now that we understand how a neural impulse, in the form of an action potential, is generated, we can look at how the impulse is propagated, or how it travels through the neuron. Two characteristics of the neuron become particularly important when we consider how quickly an impulse can pass through the axon: myelination and diameter.

Myelin sheath

The axons of many neurons, especially large neurons, are myelinated, meaning they are covered with a sheath formed by myelin, a fatty substance that insulates the cell membrane. This myelin sheath is formed by specialized cells called Schwann cells.

The sheath is not continuous. As it spans the length of the axon, the myelin sheath exhibits gaps between adjacent Schwann cells, leaving the axon uninsulated at those points. These gaps are reffered to as nodes of Ranvier. The action potential appears to jump from one node to the next as it transverses a myelinated fiber. This is reffered to as salutatory conduction, a much faster type of conduction than occurs in unmyelinated fibers.

Myelination of peripheral motor neurons occurs over the first several years of life, partly explaning why children need time to develop coordinated movement. Individuals affected by certain neurological diseases, such as MS as discussed in our chapter opening, experience degeneration of the myelin sheath and a subsequent loss of coordination.

Diameter of the neuron

The velocity of nerve impulse transmission is also determined by the neuron’s size. Neurons of larger diameter conduct nerve impulses faster than neurons of smaller diameter because larger neurons present less resistance to local current flow.