NSC - Fire Power: Tracing the Power of the Nerve Impulse Lesson
Fire Power: Tracing the Power of the Nerve Impulse
Electromechanical Signaling
In order to maintain homeostasis, it is important that messages are able to travel quickly between the central nervous system and the peripheral nervous system. Nerves convey information through the transfer of an electrochemical impulse known as an action potential. Action potentials occur at a very fast rate (sometimes reaching speeds of 120 m/s). These impulses travel from one neuron to another by crossing the synapse (the space between the dendrite of one neuron and the axon of its target neuron).
During this process, messages are converted from electrical to chemical and then back to electrical. You may be asking yourself, how can our bodies create an electrical message? In our bodies, we have electrically charged chemicals known as ions. There are four important ions in the nervous system that relate to this action potential: Na+, K+, Ca2+, and Cl-.
Resting Potential
When it is not signaling, a neuron is considered "at rest." Neurons at rest have an overall negative charge due to the nature of the selectively permeable membrane of the cell. Some ions like chloride ions (Cl-) and negative molecules such as proteins are unable to easily diffuse across the cell membrane, whereas potassium ions (K+) are able to easily move through a series of ion channels and pumps. The movement of potassium ions along with the concentration of ions inside and outside of the neuron creates an electrical potential. The resting potential of a neuron is approximately -70 mV.
Action Potential
Action potentials are created when a message is being sent by a neuron. They are generated by special ion pumps embedded in a neuron's cell membrane called voltage-gated ion channels. When a neuron is near its action potential, these channels remain closed. When a message is being sent, the electrical potential of the neuron will slowly increase until it reaches a threshold value. Once this value is met, the gated ion channels will open like floodgates and allow an inward flow of positive sodium ions (Na+). This is called depolarization. This means that while normally the inside of the cell is negative, during the depolarization it becomes positive due to the influx of sodium ions.
As more sodium ions enter into the cell, it causes a greater electrochemical gradient causing more gated ion channels to open and producing a greater electric current across the cell membrane. Depolarization proceeds explosively until all of the available ion channels are open.
Once the polarity of the cell membrane has been reversed, the ion channels rapidly inactivate. As the sodium channels close, sodium ions can no longer enter the neuron, and they are actively transported back out of the cell membrane. Potassium channels are then activated, and potassium ions are actively pumped out of the cell, returning the electrochemical gradient to the resting state.
Synapses
Synapses are essentially a minute gap where information is transmitted from one neuron to another. There are two basic types of synapses: chemical and electrical.
Chemical Synapses
Chemical synapses use chemicals called neurotransmitters to send signals from one neuron to another. Examples of neurotransmitters include acetylcholine and dopamine. Neurotransmitters are generated in the cell body, axon, or axon terminal and stored in small membrane-bound synaptic vesicles at the axon's terminal until needed.
During an action potential, voltage-gated calcium ion (Ca 2+) channels open in the neuron allowing Ca2+ to freely enter into the cell. This initiates a series of signals which causes the release of synaptic vesicles that fuse with the cell's membrane at the axon terminal. When a vesicle fuses, it releases its neurotransmitters into the extracellular space called the synaptic cleft between the presynaptic (signaling) neuron and the postsynaptic (receiving) neuron. Neurotransmitters then diffuse across the synaptic cleft and bind with receptor proteins on the cell membrane of the postsynaptic neuron.
When neurotransmitters bind to the postsynaptic receptor proteins, specific ion channels (depending upon the neurotransmitter) open. Neurotransmitters can have either an excitatory or inhibitory effect on the postsynaptic membrane. For example, acetylcholine acts as an excitatory neurotransmitter at the synapse between a nerve and muscle cell (called the neuromuscular junction). It causes an influx of positive ions into the membrane which depolarizes the neuron bringing it closer to the threshold and increasing the probability of firing an action potential. In contrast, the neurotransmitter GABA (gamma-aminobutyric acid) creates an inhibitory response by creating an influx of negative ions into a neuron. This hyperpolarizes the cell, reducing the likelihood that the postsynaptic neuron will fire an action potential.
Once neurotransmission has occurred, the neurotransmitter must be removed from the synaptic cleft to reset the postsynaptic membrane to receive another signal. This can be accomplished in three ways:
- The neurotransmitter can diffuse away from the synaptic cleft
- It can be degraded by enzymes in the synaptic cleft
- It can be recycled (sometimes called re-uptake) by the presynaptic neuron
Several drugs act at this step of neurotransmission. For example, some drugs that are given to Alzheimer's patients work by inhibiting acetylcholinesterase, the enzyme that degrades acetylcholine.
Electrical Synapse
Although there are far fewer electrical synapses within the nervous system, they still play unique and important roles in maintaining homeostasis. In an electrical synapse, the pre-and post-synaptic neurons are much closer together and physically connected by channel proteins called gap junctions. Ions are able to flow from one neuron to another through these gap junctions allowing current to pass directly from one cell to the next. In addition to the ions, other molecules, such as ATP, can diffuse through the large gap junction pores.
Compare the functionality of the chemical vs. electrical synapses in the table below:
Electrical | Chemical | |
Proximity | pre- and postsynaptic neurons are connected by gap junctions | pre- and postsynaptic neurons are separated by the synaptic cleft |
Timing | virtually instantaneous | approximately 1 ms for the message to go from cell to cell |
Directional | some are bidirectional | unidirectional |
Reliability | more reliable - less likely to be blocked | less reliable - more likely to be blocked because of the steps required for the message to go from an electrical to chemical and back to an electrical signal |
Number of Neurons Involved | can synchronize the electrical activity of a group of neurons (ex. the contraction of heart muscles) | communication only involving the presynaptic and postsynaptic neurons |
Control | less control - an action potential in one neuron will generate an action potential in all connected neurons. | more control - the target is more precise |
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