Neuron’s, also known as nerve cell’s main function, is to transmit electrochemical impulses to one another in order to communicate.
The three types of neurons all have different functions which are:
Sensory – activated by sensory input. Ex. Touching a hot surface.
Interneuron – communicates between sensory and motor neurons and the central nervous system.
Motor neuron – allows us to move, speak, swallow, and breathe by sending commands from the brain to their respective muscles.
How a neuron actually works to send signals is through action potential.
Action potential – a brief electrical charge that travels down the length of the neuron; it is a result of the movement of positive ions into and out of the axon.
There are three parts to the action potential:
- Depolarization
- Repolarization
- Refractory period
Action potential starts with the resting potential. During resting potential, large negative ions inside the axon give the inside an overall negative charge, while Na+ ions hang around outside the axon. During this stage the voltage is -70mV.
Then, during depolarization incoming messages will stimulate certain sections of the axon which result in channels in the membrane opening allowing the Na+ that was outside to flow inside the axon. The voltage then increases to +30mV.
Next is repolarization. The new open channels allow K+ ions, which were inside the axon, to now be pumped out and exited out of the axon. The voltage then decreases back to the normal charge of -70mV. The repolarization step then causes the next section of the axon to start depolarization.
The refractory period is right after repolarization when the Na+ is inside the axon and the K+ is outside of it. But, in order to start up again the neuron must be at resting potential so, during the refractory period, pumps in the membrane push the Na+ back to the outside and the K+ back to the inside to resume their starting positions.
The flow of depolarization then works out because repolarization in one section of the neuron stimulates depolarization in the next section and the impulses can’t travel backwards because of the refractory period.
Action potentials are always the same size, either there is enough stimulation to start AP or there isn’t. Action potentials also travel at the same constant speed, and all have one ms refractory period. Action potentials can vary in frequency (a few pulses per second or 1000 pulses per second).
Even though all AP travel at the same speed, there are Schwann cells that are coated in myelin which wrap around the axon, so, those neuron fibers that are coated in the myelin have much faster action potentials. This is because the AP travels in the Nodes of Ranvier, not under the myelin so the effective length of the axon is shorter and therefore the impulses reach the end of the fibre much faster.
Once the action potential gets to the end of the axon that is when the message gets communicated to a different cell. However, this is done without actually touching the cell it communicates with. A chemical signal is sent from one end of the axon to the next cell.
Synapse – junction between two nerve cells where a signal is sent from axon to dendrite.
A synapse includes the axon terminal bulb (the end of the axon’s fingers), tiny space between the neurons and the end of the dendrite from the receiving neuron.
In the axon terminal bulb, a neurotransmitter is produced and then stored in synaptic vesicles. The neurotransmitter subunits are recycled.
The signal gets sent from one to the other when the action potential travels down the axon until it reaches the axon terminal bulb. The action potential causes the synaptic vesicles to move to the presynaptic membrane and release the neurotransmitters into synaptic gaps. The neurotransmitters then move through diffusion through the gap and binds to the receptors on the postsynaptic membrane of the receiving neuron.
Once the neurotransmitter binds to the receptor the neurotransmitter message is either received as excitatory or inhibitory:
- Excitatory: stimulates an AP on receiving neuron.
- Inhibitory: represses an AP on receiving neuron.
Based on how the message is received and the majority of signals is whether or not it will produce an action potential.
Then the left-over neurotransmitters move back to the synaptic gap where the neurotransmitter recycler is and they get bind to, broken down, and sent back to the axon terminal bulb for the next time.