The nervous system has three main functions: sensory input, integration of data, and motor output. The neurons (or excitable nerve cells) of the nervous system conduct electrical impulses, or signals, that serve as communication between sensory receptors, muscles and glands, and the brain and spinal cord. Neural impulses from sensory receptors are sent to the brain and spinal cord for data integration. After the brain has processed the information, neural impulses are then conducted from the brain and spinal cord to muscles and glands, which is called motor output.
A neuron affects other neurons by releasing a neurotransmitter that binds to chemical receptors. The effect upon the postsynaptic (receiving) neuron is determined not by the presynaptic (sending) neuron or by the neurotransmitter itself, but by the type of receptor that is activated. A neurotransmitter can be thought of as a key, and a receptor as a lock: the key unlocks a certain response in the postsynaptic neuron, communicating a particular signal.
These electrical signals must travel along the axon of the neuron and cross the synapse to effect change from neuron to neuron. When a nerve is first stimulated (for example by pressure, electricity, chemicals, etc.), its resting potential changes. Different neurons are sensitive to different stimuli, although most can register pain. The rapid change in polarity that moves along the nerve fiber is known as the action potential, and has several stages:
- Depolarization. The membrane of a neuron is normally at rest with established sodium ion (Na+) and potassium ion (K+) concentrations on either side (Figure 1). A stimulus will start the depolarization of the membrane. Depolarization, also referred to as the "upswing," is caused when positively charged sodium ions (Na+) suddenly rush through open sodium gates into a nerve cell. As additional sodium rushes in, the membrane of the stimulated cell actually reverses its polarity so that the outside of the membrane is negative relative to the inside. The change in voltage stimulates the opening of additional sodium channels (called a voltage-gated ion channel), providing what is known as a positive feedback loop.
- Repolarization. The "downswing" of repolarization is caused by the closing of sodium ion channels and the opening of potassium ion channels, resulting in the release of positively charged potassium ions (K+) from the nerve cell. This expulsion acts to restore the localized negative membrane potential of the cell.
- Refractory Phase. The refractory phase is a short period of time after the depolarization stage. Shortly after the sodium gates open, they close and go into an inactive conformation. The sodium gates cannot be opened again until the membrane is repolarized to its normal resting potential. The sodium-potassium pump returns sodium ions to the outside and potassium ions to the inside. During the refractory phase this particular area of the nerve cell membrane cannot be depolarized.
This action of depolarization/repolarization/recovery moves along a nerve fiber like a very fast wave. While an action potential is in progress, another cannot be generated under the same conditions. In unmyelinated axons (axons that are not covered by a myelin sheath), this happens in a continuous fashion because there are voltage-gated channels throughout the membrane. In myelinated axons (axons covered by a myelin sheath), this process is described as saltatory because voltage-gated channels are only found at the nodes of Ranvier, and the electrical events seem to “jump” from one node to the next. Saltatory conduction is faster than continuous conduction. The diameter of the axon also makes a difference as ions diffusing within the cell have less resistance in a wider space. Damage to the myelin sheath from disease can cause severe impairment of nerve cell function. In addition, some poisons and drugs interfere with nerve impulses by blocking sodium channels in nerves.
The amplitude of an action potential is independent of the amount of current that produced it. In other words, larger currents do not create larger action potentials. Therefore, action potentials are said to be all-or-none signals, since either they occur fully or they do not occur at all. The frequency of action potentials is correlated with the intensity of a stimulus. This is in contrast to receptor potentials, whose amplitudes are dependent on the intensity of a stimulus.
Reuptake refers to the reabsorption of a neurotransmitter by a presynaptic (sending) neuron after it has performed its function of transmitting a neural impulse. Reuptake is necessary for normal synaptic physiology because it allows for the recycling of neurotransmitters and regulates the level of neurotransmitter present in the synapse, thereby controlling how long a signal resulting from neurotransmitter release lasts.