Signalling and the Action Potential
Nerve Signalling
Synapses
Your body is able to pass information and signals across synapses. A synapse is the connection piece between two neurons that transmits signals between the neurons. There are two kinds of synapses, chemical and electrical.
Electrical synapses
Electrical synapses pass messages as electrical signals and occur when the membranes of the two neurons are in direct contact. Ions are able to directly flow between the two neurons through a gap junction. This type of synapse can transmit signals at a very fast rate.
Chemical synapses
Chemical synapses pass signals as chemical molecules and occur when there is a gap between the two neurons. This gap is known as the synaptic cleft. When the signal reaches the synaptic cleft, the axon terminal releases neurotransmitters, which then bind with the neuron and open the ion gates. By opening the gates, ions can diffuse into the next neuron and the signal continues on.
Conduction of Signals
In a cell there is a difference in charges which creates voltage. The inside of the cell is mainly positive charges while the outside is mainly negative. This is known as the membrane potential. The charge difference gives the cell the potential and ability to send an electrical signal. If the cell is triggered by something (ex. placing your hand on a hot stove) it will then send that electrical signal to remove your hand from the stove. This stimuli will cause a change in the membrane potential.
In the membrane of the cell there are also ion channels which control the flow of ions in and out of the cell, and therefore helps control the membrane potential. The sodium-potassium ion channel, pumps three sodium out for every two potassium that enters the cell. It is this pump that creates the imbalance of charges in the membrane.
Parts of a Neuron
In a neuron, the dendrite receives a signal which then travels through the axon and the myelin sheath to the axon terminal. The axon terminal then sends it across the synapse to another neuron. The message will be passed on from neuron to neuron until it reaches the cell or organ the message was intended for.
Action Potential
Dendrite: Receives signals and messages.
Axon: Passes signals along.
Myelin sheath: Acts as an insulator for the neuron. It helps pass signals along faster as the signal does not need to wait for all the ions to diffuse in and out of the membrane before the signal can carry on.
Nodes of Ranvier: Gaps in the myelin sheath. Their purpose is to conduct electricity.
Axon terminal: Sends neurotransmitters across the synapse to pass the signal along.
What is it? (General)
Action potential occurs when a nerve cell receives a signal and is then excited, causing there to be a difference in voltage. At your resting rate your neurons are at resting potential which is -70 mV. The neuron then receives a stimulus. If the signal is not strong enough the signal is not sent through and the neuron returns to resting potential. If the signal is strong enough then depolarization begins. After Depolarization, the cell tries to balance itself and enters repolarization. Hyper-polarization occurs after, when the neuron, in an effort to re-balance the voltage difference, releases too much potassium. Eventually the charge balance is evened out and the cell returns to its resting state.
What is it? (Specifics)
Resting potential: -70 mV.
Stimulus: Something happens that causes a signal to be sent to a specific organ or cell. Example: When you place your hand on a hot stove a signal is sent to remove your hand to prevent burning.
All or nothing principle (failed initiations): In order for the signal to be sent to the organ or cell it has to be a strong enough stimulus to reach the threshold potential. If it does not reach the threshold potential, the signal will not be sent and the cell will return to its resting potential. This is a failed initiation. If it does reach the threshold then the signal will be sent through and action potential can occur.
Threshold potential: -55 mV.
Depolarization: Sodium channels open allowing sodium (positive ion) to enter the cell causing the membrane voltage to become positive, rising to around 40 mV.
Repolarization: Potassium channels open allowing potassium (positive ion) to exit the cell causing the membrane voltage to become negative.
Hyper-polarization: Too much potassium exits the cell which causes the membrane potential to be more negative than its resting state. The cell quickly balances back to resting potential (-70 mV).
Refractory period: The neuron is not able to react to any other stimulus during this time. This prevents more than one signal passing down the axon at the same time. During the refractory period, no other action potential are able to begin or occur.
This video gives a clear and in-depth explanation about action potential.
References
Text:
Chudler, E. H. (n.d.). Neuroscience For Kids. Retrieved November 06, 2017, from https://faculty.washington.edu/chudler/synapse.html
S. (2017, February 12). Difference Between Chemical and Electrical Synapse. Retrieved November 07, 2017, from http://www.differencebetween.com/difference-between-chemical-and-vs-electrical-synapse/
Pictures:
Action potential. (2017, October 26). Retrieved November 06, 2017, from https://en.wikipedia.org/wiki/Action_potential
Axon. (n.d.). Retrieved November 06, 2017, from http://greymattersjournal.com/glossary/axon/
Axon terminal. (2017, August 30). Retrieved November 07, 2017, from https://en.wikipedia.org/wiki/Axon_terminal
Synapse? (n.d.). Retrieved November 07, 2017, from http://openneuronproject.org/synapse/
Video:
C. (2015, March 02). Retrieved November 07, 2017, from https://www.youtube.com/watch?v=OZG8M_ldA1M