Algonquin College
7.3 - Nervous Tissue Physiology
Physiology of Nervous Tissue

Nervous tissue is specialized in sensing and responding to stimuli. Neurons must be able to detect stimuli and convert sensory information into meaningful messages in the form of nerve impulses. These electrical nerve impulses (action potentials) are the means by which different parts of the body communicate; from sensory input to integration to motor output.

The Neuron at Rest

At rest, the inner surface of the neuron cell membrane is strongly negatively charged relative to the outside surface. The charge of the inside of the membrane relative to the outside is called the membrane potential. This negative membrane potential is caused by an imbalance in the number of positive ions concentrated near the inner and outer membrane. Neurons do not send action potentials as long as their membrane potential remains negative. When the cell membrane of the neuron is in its negatively charged inactive state, it is said to be polarized.

Inside the cell the common positive ion is the potassium ion (K+), whereas on the outer surface the common positive ion is the sodium ion (Na+). At rest, a much higher concentration of sodium ions (outside) causes the inside of the cell to be more negative than the outside. K+ and Na+ ions cannot cross the membrane readily; they must both cross through membrane channels. In the neural membrane these ion channels are voltage-gated and each only opens when the membrane potential reaches a certain level of negative or positive charge. Voltage-gated Na+ channels are open only when the membrane potential is slightly above the resting potential, whereas voltage-gated K+ channels only open when the membrane potential is near its peak.

Action potential generation

Action potential generation consists of four phases: initiation, rising phase, falling phase and recovery phase.

Initiation

In order to initiate a nerve impulse (action potential), a neuron must be stimulated to change its membrane potential. Stimuli for neurons depend on the type of neuron (ex. Light is the stimulus in neurons of the eye), but most commonly neurons are stimulated by chemical signals (neurotransmitters) from other nearby neurons. Once the neuron has been activated by a stimulus, a small section of its membrane will begin to depolarize (become less negatively charged on the inside) due to a small amount of positive ions entering the cell. This initial local depolarization will trigger the rising phase to begin in the area of the initial depolarization.

Rising phase

Once the inside of the neuron membrane has been slightly depolarized, the membrane potential has risen enough to cause voltage-gated Na+ channels to open. Since Na+ ions are much more concentrated on the outside of the membrane than the inside, Na+ ions rush into the cell through the newly opened Na+ channels following their concentration gradient. This influx of Na+ ions causes a rapid and large depolarization of the membrane. The membrane switches from being negatively charged to being positively charged almost instantly. At the peak of the rising phase, when the membrane has been completely depolarized, voltage-gated K+ channels open and voltage-gated Na+ channels close, initiating the falling phase.

Falling phase

Once the voltage-gated K+ channels are open at the positive peak of the action potential, K+ ions flood down their concentration gradient and rapidly diffuse out of the cell through the K+ channels. The rapid diffusion of the potassium ions is called the repolarization of the membrane, since it causes the membrane potential to quickly decline back to resting levels. Once the membrane potential has reached its resting levels, the membrane potential has reached its minimum value (the trough of the action potential) and voltage-gated K+ channels close.

Recovery phase

Although the membrane potential has now returned to near its resting value, the concentration gradients of Na+ and K+ ions across the membrane have been reversed. Na+/K+ ATP-ase pumps restore the sodium and potassium ion gradients by pumping Na+ ions out of the cell and K+ ions into the cell. For each ATP used by a Na+/K+ ATP-ase pump, 3 sodium ions are pumped out of the neuron and 2 potassium ions are pumped into the neuron. Once the concentration gradients of Na+ ions and K+ ions have been restored, the recovery phase is complete. Until the recovery phase is complete, the neuron is incapable of performing another action potential. The period of time during which a neuron cannot be excited by a stimulus is called the refractory period. During the refractory period, sodium and potassium channels will not open even if stimulated, since that region of the neuron is “busy” already performing an action potential or recovering from a recent action potential.

Click here for a tutorial about action potential generation



Action Potential Propagation

Once an action potential has been initiated in one region of the neuron membrane, it must spread throughout the length of the dendrites, cell body and axon in order to reach the axon terminal(s). The process by which an action potential spreads is called action potential propagation.

During the rising phase of an action potential a large number of Na+ ions flood into the cell where voltage-gated Na+ channels open. This causes the region around the open Na+ channels to become positively charged. Even though the membrane potential only becomes positively charged immediately around the area of the open Na+ channels, some of the Na+ ions diffuse into the nearby cytoplasm. This increases the membrane potential of nearby regions to reach their threshold, initiating an action potential in the next section of the membrane.

Action potential propagation only continues in one direction because the ion channels in the previous section of the neuron membrane are still in their refractory period and cannot yet open again. In myelinated neurons the propagation of an action potential is called saltatory conduction because it “jumps” from one node of Ranvier to the next. It “jumps” instead of following a continuous path, since ions cannot flow across regions of the neuron membrane covered in myelin sheath. Saltatory conduction is much faster than the continuous path of action potential propagation in an unmyelinated neuron.