B: Nerve Impulse Transmission within a Neuron: Action Potential - Biology LibreTexts
Each neuron receives an impulse and must pass it on to the next neuron and This causes complete depolarization of the neuron and an action potential is. In physiology, an action potential occurs when the membrane potential of a specific axon A neuron that emits an action potential, or nerve impulse, is often said to "fire" difference between the exterior and interior of the cell, called the membrane The diagram shows how sodium ions and potassium ions interact to show. Discusses how nerve impulses are transmitted. the cell compared to inside the cell. This difference in electrical charge is called the resting potential. The reversal of charge is called an action potential. It begins when the.
Voltage-gated sodium channels have two gates gate m and gate hwhile the potassium channel only has one gate n. Gate m the activation gate is normally closed, and opens when the cell starts to get more positive. Gate h the deactivation gate is normally open, and swings shut when the cells gets too positive. Gate n is normally closed, but slowly opens when the cell is depolarized very positive.
Voltage-gated sodium channels exist in one of three states: Deactivated closed - at rest, channels are deactivated.
The m gate is closed, and does not let sodium ions through. Activated open - when a current passes through and changes the voltage difference across a membrane, the channel will activate and the m gate will open. Inactivated closed - as the neuron depolarizes, the h gate swings shut and blocks sodium ions from entering the cell. Voltage-gated potassium channels are either open or closed. There are three main events that take place during an action potential: A triggering event occurs that depolarizes the cell body.
This signal comes from other cells connecting to the neuron, and it causes positively charged ions to flow into the cell body. Positive ions still flow into the cell to depolarize it, but these ions pass through channels that open when a specific chemical, known as a neurotransmitter, binds to the channel and tells it to open.
Neurotransmitters are released by cells near the dendrites, often as the end result of their own action potential! These incoming ions bring the membrane potential closer to 0, which is known as depolarization.
An object is polar if there is some difference between more negative and more positive areas. If the cell body gets positive enough that it can trigger the voltage-gated sodium channels found in the axon, then the action potential will be sent. Depolarization - makes the cell less polar membrane potential gets smaller as ions quickly begin to equalize the concentration gradients.
In most nerve cells, such a steady state is reached at about mV, mainly through potassium and a small contribution of sodium current. This potential is referred to as the resting potential, a phenomenon that occurs in all cells, but the height of the voltage is cell-type specific.
Excitable cells can make use of the resting potential through a specialised class of proteins, called voltage gated ion channels. These channels are ion-selective and can be opened and closed. The trigger that opens them is a reduction in membrane potential.
For example, sticking a fine glass needle into a cell does not only allow to measure the potential across its membranes Fig. This refractory period is important to prevent that the action potential can revert its direction and flow backwards. As will become clear in step 5 it is important that sodium and potassium channels open at different thresholds. Step 5 is the actual action potential A which is achieved through integrating the steps B-D in the right sequence; compare Fig.
B When an external stimulus shifts the membrane potential above threshold red curve segmentvoltage-gated sodium channels open and quickly close again blue zonedriving the membrane potential to positive values above red line. C The increasing membrane potential triggers the opening of voltage-gated potassium channels which only close gradually, driving the membrane potential back to values below resting potential. D Pumps and resting channels are contributing continuously; when the membrane potential changes, the rate of influx red arrow and efflux orange arrow of potassium ions through resting channels automatically shifts thus gradually bringing the resting potential back to normal values.
When all the different properties explained in steps are put together in the right sequence, an action potential can occur Fig. When a cell is being activated e. This depolarisation passively propagates along the membrane, quickly reaching further sodium channels which open and reinforce the depolarisation, thus actively driving a wave of depolarisation along the axon like a Mexican wave in a football stadium.
Nerve Impulse ( Read ) | Biology | CK Foundation
The depolarisation is immediately followed by the opening of the voltage-gated potassium channels which quickly repolarise the membrane. You may wonder why the action potential flows only forwards and not in both directions. On the one hand, this is due to the quick potassium-driven repolarisation below threshold levels. On the other, it is due to the refractory period of sodium channels who stay inactivatable for long enough to let the action potential pass by.
This is comparable to spectators who have just played their part in a Mexican wave and know that they do not have to stand up again.
The flow of an action potential within an axon; compare Fig. Epilepsy You may already have done a simple experiment where you vigorously shake vials with mutant fruit flies that are prone to epileptic attacks: Under the microscope you will see that they display leg tremor and convulsions of the whole body see film belowvery similar to the epileptic lion shown further up. One of these mutations first discovered in affects the Shaker gene, which turned out in the late 70s to encode a potassium channel, and became then the first cloned potassium gene ini.
But why do these flies undergo seizure when heavily shaken? As explained in step 4 of the action potential model see abovethis channel is required for re-polarisation of the nerve cell i. If the Shaker channel does not work properly, the nerve cell stays longer depolarised, i.
If voltage-gated potassium channels are defect, action potentials re-polarise much slower thick black line than under normal condition dashed red linepotentially staying above threshold when sodium channels exit from their refraction period.
For further explanations see Fig. When shaking flies, many of their sensory neurons get stimulated and send action potentials into the CNS, thus flooding it with activity, and this cannot be toned down sufficiently in Shaker mutant flies, leading to contraction of all muscles, hence spasm. When watching TV we occasionally get warned of flash photography, and this is because viewers may suffer from a photo-sensitive form of epilepsy — and this follows a similar logic as the shaking of flies. This analogy, together with the striking similarity of epileptic attack symtoms in a fly and a lion see films abovemight suggest that there are common traits between flies and higher animals and humans.
Given the fact that flies are easy to keep in high numbers Fig. Large scale drug testing is easy and cost-effective in flies. They are kept on feel containing different drugs or concentrations of drugs for a while and then tested: Effective drugs will reduce the recovery time. Opto- and Thermogenetics In this resource we mainly discussed rather simple neuronal networks.
However, neuronal networks can be of far higher complexity. Just think of reflex circuits: This is still rather simple. However, when testing the reflex, the doctor advises the patient to relax or even look away, and this is necessary because we can use our brain to switch the reflex off and this allows us to bend our leg voluntarily without the reflex counteracting the movement and generating antagonising or even harmful tension.
Neuroscience For Kids - action potential
To learn how these reflex networks are wired have a look here. Neuronal networks become even more complicated when thinking of complex behaviours, such as learning. For example, how do we store memory? We have known for a long time that loss of the hippocampus through injury or operation causes loss of spatial memory and other learning modesand for taxi drivers it has been demonstrated that their hippocampus is especially active when they navigate their way, and that their hippocampus even grows in size when they are learning their trade [ LINK ] [ LINK ].
As another example, fruit flies can learn to associate a certain odour with bad experiences e. In order to understand these complex learning behaviours, identifying the required specific brain regions is the first step. However, to proof and fully understand their functions and contributions will require more complex analyses.
We need to know 1 the patterns in which nerve cells are active in those networks Can we identify underlying rules or principles? To perform these experimental tasks, researchers have come up with clever strategies, referred to as opto- and thermogenetics Greek: The concept for optogenetic tools of the latter category is explained in Fig. This movie shows a central nervous system of a Drosophila maggot where neurons express a calcium-sensing protein which shines up under the fluoresent microscope when the calcium concentration in nerve cells increases upon activation.
This maggot is crawling which requires a wave of muscle contractions which is driven by a wave of motorneuron activity in the nervous system. Using optogenetics to activate a nerve cell. The algae Chlamydomonas has a light-sensitive eye spot helping it to move towards light required for its photosynthesis. The light spot carries a sodium-selective channel protein channel-rhodopsin, ChR that opens up upon blue light this is triggered by a conformational change of retinal, the same substance that photorecptors in our eyes use.
The gene for ChR has been cloned and can be brought into specific nerve cells who then carry it on their cell membrane.
35.2B: Nerve Impulse Transmission within a Neuron: Action Potential
Shining blue light on this nerve cells will open the ChR and let in sodium. As explained in step 3 of the action potential model see abovethis will trigger a nerve impulse. Therefore, shining blue light on these neurons triggers them to become active. The power of the optogenetic approach is explained with a simple example: The key role of a specific neuron giant fibre neuron within this flight response can be clearly demonstrated using optogenetic tools in head-less flies Fig.
The giant fibre neuron is required for a flight response. Visual information from they eye is quickyl transmitted via the gian fibre to motorneurons in the lower CNS to activate muscles that make the fly jump and and fly away. This whole program can be reproduced when stimulating these neurons via channelrhodopsin, even when the flies were decapitated see movie below.
Movie explaining the use of optogenetic tools for further explanations see the TED talk by Gero Miesenboeck Scientists discovered another nerve cell reaching from the brain into the lower CNS Figure The surprising outcome is shown in the movie below: Using optogenetic tools to activate a small group of dopaminergic neurons adjacent to the mushroom body whilst providing a certain odor in the absence of electroshock! Another group of nerve cells, likely acting after the dopaminergic ones, was enough to stimulate in the absence of any training!
Therefore, optogenetic tools can indeed be used to study complex neuronal networks! Synapses Synapses are close point contacts between neurons and other neurons or other types of excitable cells e. Synapses are highly specialised to pass on nerve impulses Fig.
The neuron which passes the nerve impulse on is referred to as the presynaptic cell, the receiving cell as the postsynaptic cell. Once the signal reaches the axon terminal, it stimulates other neurons. Formation of an action potential: The formation of an action potential can be divided into five steps. The hyperpolarized membrane is in a refractory period and cannot fire. At excitatory synapses, positive ions flood the interior of the neuron and depolarize the membrane, decreasing the difference in voltage between the inside and outside of the neuron.
Once the threshold potential is reached, the neuron completely depolarizes. At this point, the sodium channels return to their resting state, ready to open again if the membrane potential again exceeds the threshold potential.
Myelin and Propagation of the Action Potential For an action potential to communicate information to another neuron, it must travel along the axon and reach the axon terminals where it can initiate neurotransmitter release.