Action potential transfer
Neuron anatomy: neuron consists of body and axon, body of neuron has branched tree called (dendrites), and axon then extend from the body end with branches also, called (nerve terminals).
Always remember that the signal (action potential) is transmitted from neuron body toward nerve terminals
· Transfer of signal (action potential) a long the axon is called propagation (of action potential).
· Action potential starts in a point in the neuron body called (axon hillock), it is the point where the axon and cell body meet and has lowest threshold (easy to generate action potential).
Nerves are tow types:
2) Non myelinated
· Myeline sheath is a multiple folds of plasma membrane of (shwann cell) [cells which are associated with nerves], this sheath contain lipid substance called (sphingomyelin) this substance is excellent (electrical insulator).
· Myelinated nerves (usually large nerves) has Myeline sheaths surrounding their axons, this sheath act as electrical insulator, and it covers the axons in intermittent pattern, as to say, each shwann cell gives folds of membrane (sheaths) for a distance of 1 – 3 millimeter on axon and the area between junction of tow shwann cell sheaths is un myelinated (has just axon) this area about 1 – 2 micrometers is called (node of Ranvier).
· What is important to know is: areas of axon that are myelinated can not have ion movement from extracellular to intracellular (axoplasm) because as we said Myeline sheath is excellent insulator, but, how could action potential transmitted through these types of nerves?…. it will be explained below. (See salutatory conduction).
· Areas of axon that are not myelinated (node of Ranvier) could have ionic movement in and out.
How action potential propagate (transfer)??
When the depolarization start in cell body (in axon hillock) this region excites the neighboring region of axon (and work as just like anew stimulus for them) and by this, next region will be depolarized too, and also this new depolarized region excites its neighboring region [work as anew stimulus (electrical)] for them and so on until the depolarization spread along the axon to the nerve terminals and this is what is called propagation or (signal conduction).
Types of Nerve conduction:
a. Serial conduction : in this type, action potential is conduction along the (unmyelinated axons) by above mentioned method, each area in the axon show basical events of ionic movement [Na+ in & K+ out] until the action potential reach nerve terminals.
Don’t forget, after action potential, Na+ – K+ Pump reestablish ionic distribution and along the axon, and imagine if the axon is about one or more meter!! How much energy we need for this pump. It obviously cost a lot…..
This type of conduction is slow and cost a lot of energy, it is slow because action potential needs continuous current of ions across each region of axon membrane along whole length of axon in order to reach end.
b. Saltatory conduction : this type is conducted in myelinated nerves (usually large nerves), and it jumps along nerve axon in nodes of Ranvier (where there is no Myeline sheath) and don’t flow through the parts contain Myeline. That’s why name salutatory .
Explanation: Basic events in action potential (ions movement in and out) could not happen in myelinated segments of axon but only in the nodes of Ranvier.
When action potential occur in first node, as to say movement of Na+ in ward and depolarization, this depolarization or (electrical changes) transmitted through intracellular fluid (axoplasm) until it reach the next node, and again, in next node action potential occur in ordinary way (movement of ions across axon membrane) and this will result in electrical changes in the intracellular fluid, again, this change transmitted through fluid to next node and so on…..
Features of myelinated nerves (Saltatory conduction): have fast conduction, and uses less energy than serial conduction [because we need Na+ – K+ Pump only in the Ranvier nodes and not along whole axon.
Functions of Myeline:
1. Increases velocity of nerve transmission.
2. Saves energy.
3. Allow repolarization to occur faster (with less ion transfer) because it happens in limited regions (nodes).
4. Isolate and protect nerves.
* Example on salutatory conduction: nerves that need fast transmission of signals like…. Fast reflexes
* Note: velocity of nerve conduction varies as little as 0.25 m / second in very small unmyelinated nerves to 100 m / second in vary large myelinated fibers.
There are tow questions (important) regarding propagation of impulse:
First, if stimulus is in the middle of axon… then, in which direction the impulse will propagate (conducted)?
Answer: in both directions, toward nerve terminals and toward cell body.
Second question: as we said that propagation start from one region to its neighbor in the direction of terminals and as to say from region 1 to region 2 to region 3…. Until nerve terminals… but, could the action potential (which is electrical change) in region 2 act as a stimulus again to region 1…? In other word, could for action potential, (after it transmitted forward) return back in opposite direction?
Answer: No … and you know why!! … I think you do… because region 2 when it is in action potential, region 1 is in (refractory period) so this action potential (which maybe considered as a stimulus) couldnot stimulate previous region because they are in refractory period).
1. In the end of neuron axon what is next? It is either second neuron or this meeting is called (synapse) or it is a muscle and in this case called (neuromuscular junction).
2. There is no contact between neuron – neuron (synapse) or neuron – muscles (neuro muscular junction) but there is a small gap filled with ECF. Called synaptic cleft or junctional cleft.
3. These synapses or junctions represent the stations in which a signal transmitted from one place to another (indirectly through neurotransmitter)
For example: from motor nerve – to muscles, to produce muscle contraction, and from one nerve to another like in CNS when processing information’s (signals).
1. before we discuss it, remember that nerve terminals is branching (nerve terminals) and each single branch will attach to single muscle cell (which contains contractile myofibrils) and this attachment is called motor end plate (neuromuscular junction).
2. by this motor end plate, the signals are transmitted from nerves to muscle to contract, as follow:
3. nerve impulse is conducted down axon to nerve terminals, where each terminal has slight enlarged end (to increase surface area) called terminal button, and each terminal (branch) attach to one muscle cell (muscle fiber), the membrane of button is called pre-junctional
membrane, and membrane of muscle is called post-junctional membrane, these membranes are separated by junctional gap.
4. arrival of action potential to pre-junctional membrane will increase the permeability of membrane to Ca++, so that Ca++ will move from out to in.
5. When Ca++ enters, it makes vesicles to migrate to the pre-junctional membrane and fuse with it; these vesicles contain chemical substance called Acetylcholine [which is neurotransmitter].
6. After fusion, the vesicles suddenly open to out side and release their content (acetyl chloline) to outside (junctional gap).
7. Acetyl chloline released in the gap, will combine specific receptors for it on post- junctional membrane (membrane of the muscle) these receptors called Acetyl chloline receptors.
8. These receptors are complex protein which has binding site (portion) to acetyl chloline and other portion act as a channel to Na+ & K+.
9. When this combination occurs, permeability of membrane (post-junctional) will increase to Na+ & K+, through these channels that opened by acetyl chloline (so, these channels are called chemical or ligand gated channels… remember?)
10. When these channels open (increase permeability), Na+ will rush to inside (because inside has large negativity), and by this, depolarization occurs in post-junctional membrane and….. Action potential (the goal), which is followed by muscle contraction.
1. When the acetyl chloline gated channels open and Na+ go in, Na+ (positive ion) will change the resting (post-junctional )membrane potential to ward threshold, so, we need enough number of channels opened so that enough amount of Na+ go in, so that we can reach the threshold. As to say we need enough number of acetyl chloline molecules, and if there is no enough number present, there will be only change in membrane (post-junctional) potential, without action potential or (muscle contraction) and this change in membrane potential could be graded (do you remember local response?).
2. Acetyl chloline is attached to it’s receptor for few milliseconds and couldn’t stay in it, because if do so, it will cause continuous muscle contraction, so, it has to be removed very quickly by enzyme (Acetylcholine esterase) which destroy acetyl chloline to acetate group and chloline group, which are reabsorbed by pre-junctional membrane to resynthesize acetyl chloline and store it in vesicles (this reabsorption is active process).
*Some acetyl group is not reabsorbed and escape through extra cellular fluid.
3. Acetyl chloline esterase present near acetyl chloline receptor in the post-junctional membrane.
4. Fuse of vesicles and then opening to outside is called exocytosis.
Transmission through synapses (meeting of tow neurons) has similar steps mentioned above. so they have similarities and differences with neuromuscular junction.