Overview.
Action potential.
Synapse.
Signals.
Convergence and computation.
Communicating a signal, is one of the basic functions, of a neuron.
Typically a neuron, communicates with another neuron.
The neuron can receive signals from hundreds and thousands,
of other neurons.
The neuron sums up all these signals.
The incoming signals impact the membrane potential of the neuron.
A neuron receives electrical signals from hundreds of other neurons.
A typical incoming signal will be a charged ion, entering the dendrite of a neuron.
A neuron can receive signals from hundreds of other neurons.
All these signals are summed up, within the neuron.
This is called spatial summation.
An incoming signal can be strong or weak.
A strong signal will have, a high frequency signal.
The strength of the incoming signals is also summed up, by the neuron.
This is called temporal summation.
An incoming signal can be positive or negative.
These signals are called excitatory, or inhibitive signals.
These signals are also summed up by the neuron.
If the net result increases the membrane potential, to say, minus 55 millivolts,
It generates, a signal or action potential.
This is called the threshold level.
If the sum of the signals, does not reach threshold, no action potential is generated.
The site at which the decision is made, is the axon hillock.
If the threshold of minus 55 millivolts, is reached an action potential is generated.
This action potential is communicated to other neurons.
This is the fundamental principle, of the functioning of a neuron.
There is an interesting, and important property of a neuron.
A neuron takes a binary decision,
to fire an outgoing signal, or not to fire an outgoing signal.
This is like a true or false, or “all or none” response.
There is no in-between.
There are no intermediate signals.
When a neuron fires, it generates a signal called the ‘action potential’.
If the sum of the resultant incoming signals, is not strong enough,
the neuron will not fire an action potential.
If the sum of the resultant incoming signals, is strong enough,
the neuron will fire, an outgoing action potential.
In taking this decision, by the summation of the incoming signals,
the neuron is acting like a complex bio transistor.
This micro intelligence is built into the neuron.
The neuron does not make, physical contact with the next neuron.
There is a small gap between the end of the axon, of one neuron,
and the dendrite of the next neuron.
This gap is called as the synaptic cleft.
To bridge this gap, the action potential, which is a electrical signal,
is translated into a chemical message.
This chemical messenger is called as a neurotransmitter.
The neurotransmitter diffuses across the synaptic cleft.
It reaches the membrane of the dendrite of the next neuron.
The signal is now reconverted, as a membrane potential, which is processed,
by the receiving neuron.
The receiving neuron, sums up all the signals, and if threshold is reached,
generates a new action potential.
This action potential is then carried by the second neuron, to other neurons.
This is the basic process by which neurons communicate with each other.
The communication is an electro chemical process.
The brain communicates signals to the muscles in our body.
This is called as the motor system.
The motor neuron communicates the signal to the target muscle.
This signal carried to the muscles, from the brain, is also an action potential.
The motor neuron, innervates the muscles.
This is the way that the brain controls the muscles in the body.
The brain receives sensory signals from the sensory organs.
The eyes, the ears, the nose, tongue, and the skin,
convert sensory information, into electrical signals, or action potentials.
This is communicated to the brain.
Any type of sensory information received by the brain is an action potential.
In the brain neurons are communicating with one another.
There are about hundred billion neurons, in the brain.
There are about hundred trillion connections, between the neurons.
There is a complex flow of signals, between the hundred millions neurons.
All these signals, are action potentials.
There is essentially one type of electrical signal signalling,
that the entire nervous system uses, regardless of function.
This is the action potential.
The action potential is an electrical signal.
It can be represented as a wave.
We can plot time in the X axis,
and voltage in the Y axis.
The starting point is the resting membrane potential.
This is minus 70 millivolts.
This is the starting point of the wave.
This wave starts to rise to the value, of plus 10 millivolts.
Then it starts to drop.
It drops till it reaches a voltage of minus 80 millivolts.
Then it rises to minus 70 millivolts.
This is the resting membrane potential.
One cycle of an action potential wave, starts from the resting membrane potential,
and comes back to the resting membrane potential.
This wave propagates, along the nerve fibre, or axon.
This wave is the basic signalling unit, of the action potential.
The neuron uses voltage gated ion channels,
to generate the action potential wave.
A voltage gated ion channel, is a protein, that is embedded,
in the membrane of the axon.
Proteins perform amazing variety, of functions, in living systems.
When a certain voltage is reached, these protein channels open,
to allow a certain ion to pass through.
When another voltage is reached, these protein channels close.
This acts like a gate.
The gates open, at a certain voltage, and close at a certain voltage.
These protein channels, are called as voltage gated ion channels.
There is a higher concentration, of sodium ions, in the outer membrane of the neuron.
When a voltage of minus 55 millivolts is reached, these channels open.
When the gate is opened, sodium ions tend to rush in.
Sodium ions are positively charged.
The influx of these positively charged ions,
causes the voltage to be less negative.
This is called as depolarisation.
The threshold voltage of a neuron is minus 55 millivolts.
When this voltage is reached,
it causes the voltage gated sodium channels to open.
This initiates the action potential.
This is called as the threshold voltage.
If the net incoming signals result, in a voltage of minus 60 millivolts,
the sodium gated voltage channels will not open.
No action potential will be initiated.
This is how the neuron, achieves a “all or none” response.
As more positively charged sodium ions come in,
the voltage becomes more and more positive.
From minus 70 millivolts, the voltage rises to about plus 10 millivolts.
Around this voltage, the voltage gated sodium channels, start to close.
No more sodium ions can now flow in.
This is the peak of the wave.
There is a higher concentration, of potassium ions inside the membrane.
When the voltage reaches, plus 10 millivolts,
the voltage gated potassium channels open.
This causes potassium ions, to rush out of the neuron.
Potassium ions are positively charged, when they exit the cell,
the membrane potential becomes more and more negative.
The voltage starts to drop from plus 10 millivolts.
As more potassium ions come in,
the voltage continuous to drop, till it reaches about minus 80 millivolts.
At this voltage, the voltage gated potassium channels start to close.
No more potassium ions, are allowed to flow out.
This corresponds to the bottom, of the action potential wave.
Minus 80 millivolts, is less than the threshold voltage of minus 70 millivolts.
At this stage both the sodium,
and the potassium voltage gated ion channels, are closed.
No ions can move through the voltage gated sodium and potassium channels.
The sodium potassium pump is still active.
It pumps out three sodium ions, and pumps in two potassium ions.
This results in a small increase in the membrane voltage.
The sodium and potassium leak channels, or diffusion channels are open.
These mechanisms restore the voltage, from minus 80 millivolts.
To the resting membrane potential of minus 70 millivolts.
This completes a full cycle, of the action potential wave.
This wave propagates through the axon.
In un-myelinated axon, the voltage gated sodium and potassium channels,
are located close together.
The first set of sodium and potassium channels generates,
the first action potential wave.
The charge diffuses for a short distance along the axon,
which triggers the next set of sodium channels to open.
This is followed by the opening of the potassium channel,
and closing of the sodium channel.
This creates the second wave.
By alternately opening and closing, voltage gated ion channels,
the neuron is able to propagate the signal throughout the length of the axon.
The same basic mechanism is used, throughout the nervous system,
to communicate the action potential wave.
This is the fundamental concept of signal transmission, in a nerve fibre or axon.
Many nerve fibres, have a myelin sheath covering it.
The myelin sheath has many functions.
One of them is to provide insulation, for the nerve fibre.
Un-myelinated axons do not have a myelin sheath.
These axons have voltage gated, sodium and potassium channels,
situated close to each other.
By sequential opening, and closing of the voltage gated channels,
the action potential is able to propagate along the axon.
Opening and closing of voltage gated channels, take a little time.
They are relatively slower, compared to the speed of ion conduction.
The speed of propagation of action potential, in un-myelinated axons,
is relatively slower.
This form of communication is used for short distance,
inter-neuron communication.
Myelinated axons have a coating of a myelin sheath, on the nerve fibre.
The myelin sheath, acts like a insulation for the nerve fibre.
It prevents ion movement across the membrane of the axon.
The myelin sheath is not continuous.
It has some gaps in it.
These gaps are called, as nodes of Ranvier.
The nodes of Ranvier has many voltage gated sodium and potassium channels.
The propagation of action potential, proceeds in a slightly different manner,
in myelinated axons.
The action potential is generated at the axon hill.
The potential travels in the first myelinated segment, by diffusion.
That is the charged ions move along the axon, due to the potential difference.
This type of conduction, is very fast.
As it moves along the myelinated segment, the signal gets weaker.
Before it gets too weak, it reaches the next node of Ranvier.
Here it meets a dense set of voltage gated sodium and potassium channels.
This triggers a new action potential.
The full cycle and strength of the action potential is regenerated.
The new action potential, travels along with the myelinated axon,
till it reaches the next node of Ranvier.
This way the action potential moves from one node to another.
This type of conduction is very fast.
It is suitable for long distance communication for neurons with long axons.
This is specially true for many nerve fibres, in the peripheral nerve system.
For example, there is a nerve fibre which extends all the way,
from the bottom of the spine, to the tip of the toe.
To be of practical use, signals need to travel very fast, over this distance.
This is achieved by myelinated axons.
This type of communication, is known as saltatory conduction.
Saltatory conduction is widely used in the peripheral nervous system.
We had discussed that there is only one type of action potential in the nervous system.
Once an action potential is generated in a the axon hill of a neuron,
it is communicated with the same strength, to the end of the axon terminal.
There are no weak or strong signals.
Even along a myelinated axon the signal, is regenerated in full strength,
in the nodes of Ranvier.
This way there is effectively no loss, in the strength of the signal.
It is a marvellous design of nature where the signal is always a unique signal.
Regardless of the location or the function, the entire nervous system,
uses a singularly unique signal, called the action potential.
There are about hundred billion neurons in our nervous system.
To work as a whole effective system, they need to communicate with each other.
Most of the neurons, do not make physical contact,
with the neuron that they are communicating with.
There is a small gap between the end of the axon terminal, and the dendrite,
of the receiving neuron.
This gap is called as the synaptic cleft.
The communication across the synaptic cleft is by a chemical messenger,.
This chemical messenger is a biochemical molecule,
It is called as a neurotransmitter.
Communication across the neurons is electro chemical.
In the neuron the signal is electrical.
In the synapse the signal is chemical.
When an action potential reaches a synapse,
it causes a neurotransmitter, to be released.
This neurotransmitter is released into the synaptic cleft.
It diffuses across the synaptic cleft, and reaches the membrane of the dendrite.
Here the chemical message is reconverted into a electrical signal.
The neuron can receive signals from many other neurons.
The receiving neuron sums up these signals,
if the net signal, reaches threshold, an action potential is generated.
This is the manner in which neurons communicate with each other.
The neuron communicating the message, is called the pre-synaptic neuron.
The membrane at the synapse junction, is called the pre-synaptic membrane.
When an electrical signal, reaches the end of an axon terminal,
it reaches the synapse.
The synapse has containers called vesicles, which stores a neurotransmitter.
The neurotransmitter is a biochemical, which is used for transmitting signals.
In the resting state, the neurotransmitter, is stored inside the vesicles.
There are many types of neurotransmitters.
For the purpose of our discussion, we will take acetylcholine, as an example.
Acetylcholine is widely used, in the peripheral nervous system.
Acetylcholine is chemically produced in the synapse, from precursor molecules.
Voltage gated calcium channels, are located in the synapse.
When a graded potential reaches the synapse, the calcium gated ion channels open.
This causes calcium ions to flow in.
The calcium initiates a biochemical process,
which leads the vesicles, containing the neurotransmitter acetylcholine,
to bind and fuse, with the presynaptic membrane.
The vesicle then opens, and releases acetylcholine, into the synaptic cleft.
The synaptic cleft is a very thin opening.
It is typically 10 to 20 nanometers thick.
The neurotransmitter acetylcholine, diffuses across this synaptic cleft.
The neuron receiving the message, is called the post synaptic neuron.
The membrane on the post synaptic neuron, is called as the post synaptic membrane.
The post synaptic membrane, has receptors for neurotransmitters.
There are specific receptors for specific neurotransmitters.
In our example, these receptors are acetylcholine receptors.
These receptors are ligand gated ion channels.
The ligand is the chemical messenger or neurotransmitter.
These receptors are proteins, embedded in the post synaptic membrane.
When these channels bind, with a specific neurotransmitter,
the ion channels open.
This causes specific ions to flow in.
In our example, acetylcholine binds with a acetylcholine receptor.
This leads to the ion channel opening.
Positively charged sodium ions flow in, through the open ion channel.
This generates a graded potential, in the post synaptic membrane.
The chemical signal, is converted to an electrical signal.
The signal started as an electrical signal, in the presynaptic neuron.
It is converted into a chemical signal, by releasing a neurotransmitter.
The post synaptic neuron reconverts the chemical signal,
to an electrical signal.
This example illustrates the basic mechanism, of neuron to neuron communication.
A variety of neurotransmitters, are used by the human nervous system.
Acetylcholine is a common neurotransmitter used in the peripheral nervous system.
Some other examples of neurotransmitters are:
Catecholamines like:
Dopamine.
Epinephrine.
Norepinephrine.
Serotonin.
Histamine.
Amino acids like:
Glutamate.
Aspartate.
Glycine.
GABA.
Purines like:
ATP.
ADP.
Adenosine.
Neurotransmitters and receptors act like a lock and key mechanism.
Typically each neurotransmitter, will have a corresponding receptor.
Receptors are proteins, that are embedded, in the post synaptic membrane.
The neurotransmitter released, from the pre synaptic membrane,
binds to a specific receptor, in the post synaptic membrane.
The combination of a neurotransmitter, and a receptor,
will have a unique, well defined function.
This function will be to let specific ions move in or out of the membrane.
Some examples of the ions, that can move in or out,
are sodium ions, potassium ions, chloride ions, etc.
The movement of ions, across the membrane,
will cause a potential difference, in the post synaptic neuron.
This is the signal, that is communicated to receiving neuron.
The post synaptic membrane, will have multiple receptors,
embedded in the membrane.
When there are more receptors, for a particular neurotransmitter,
the receiving membrane becomes more sensitive to the neurotransmitter.
When there are less receptors, for a particular neurotransmitter,
the receiving membrane becomes less sensitive, to the neurotransmitter.
The amount of neurotransmitters released, and the number of receptors,
for that neurotransmitter, alters the behaviour of the synapse.
This altered behaviour is responsible for memory and learning.
A neuron can receive signals from hundreds and thousands of other neurons.
The receiving membrane , can have a variety of receptors.
These receptors can be for a variety of neurotransmitters.
This can cause different types of signals,
called excitatory post synaptic potential,
or inhibitory post synaptic potential,
to be received, by the post synaptic neuron.
The neuron can receive more than one type of signal.
We will discuss the type of signals and their processing.
When a certain neurotransmitter, binds to a specific receptor,
which causes the voltage potential to increase,
it is called as a excitatory post synaptic potential.
It is abbreviated as EPSP.
This can be due to a net positive ion inflow.
It can also be due to a net negative ion outflow.
An EPSP causes the potential to increase,
from a resting potential of minus 70 millivolts,
towards the threshold potential of minus 55 millivolts.
For example, acetylcholine, when it binds with its corresponding receptor,
causes an influx of positively charged sodium ions.
This creates an increase in the potential, and generates an EPSP.
When a certain neurotransmitter, binds to a specific receptor,
which causes the voltage potential to decrease,
it is called as a inhibitory post synaptic potential.
It is abbreviated as IPSP.
This can be due to a net negative ion inflow.
An IPSP causes the potential to decrease,
towards the resting membrane potential of minus 70 millivolts.
For example, a neurotransmitter can bind to a receptor,
and cause negatively charged, chloride ions to flow in.
We note, that an IPSP, has the opposite effect, of a EPSP.
Interestingly, the brain uses a combination of EPSP, and IPSP,
to control target organs.
By using this combination a much finer control, is achieved.
Also by using this combination, the control becomes very smooth.
It causes the excitatory signal, to rise gradually and smoothly.
It causes the inhibitory signal, to decrease gradually and smoothly.
To take an example, we are able to lift a cup, gently without jerking,
by the intricate combination of command signals sent out to the hand muscles.
The autonomous nervous system, controls many internal organs,
of the body, like heart, lungs, liver, etc.
The autonomous nervous system, has a sympathetic,
and a parasympathetic division.
They control the organ by using a combination of opposing,
excitatory and inhibitory signals.
These divisions use, EPSP’s and IPSP’s for this purpose.
The action potential is the basic unit of a signal.
The action potential, is an all or none, signal.
There are no strong action potential or weak action potential.
It is like a binary 0 or 1.
There is another elegant mechanism, to send strong signals.
Strong signals are a series of high frequency action potentials.
For example, if we touch something warm,
it will generate a few, infrequent action potentials.
This is interpreted by the brain, as a weak signal.
If we touch something hot,
it will generate a large number, of frequent action potentials.
This is interpreted, by the brain, as a strong signal.
The same concept is used by the brain, to send and receive,
strong and weak signals.
When these signals reach the synapse, they release neurotransmitters.
Strong signals will release, more and sustained quantity of neurotransmitters.
Weak signals will release, less neurotransmitters.
This is the mechanism, to convey signal strength, across the synapse.
Signals are received, in the post synaptic membrane.
When a neurotransmitter binds, to a receptor,
it generates a graded potential.
The neurotransmitter, that binds to the receptor,
opens the channel, allowing ions to flow.
If the neurotransmitter stays bound to the receptor,
the channel will continue to remain open.
This does not happen.
The neurotransmitter leaves the receptor.
There are various mechanisms, which causes the neurotransmitter,
to leave the receptor.
In some cases, it might just diffuse away.
In some cases, an enzyme will chemically break down the receptor.
When the neurotransmitter leaves the receptor,
the ligand gated ion channel closes.
When the channel closes, the ions flow stops.
The neurotransmitter, not only opens the gate, but also shuts it.
This causes a finite graded potential to be generated.
When one molecule of neurotransmitter gets released,
and the ion channel closes,
another molecule of the neurotransmitter, can bind to the receptor.
This causes another graded potential, to be generated.
This process can keep repeating.
These repeated graded potentials are summed up by the receiving neuron.
This is called temporal summation.
In the presynaptic neuron, a strong action potential,
will have a high frequency signal.
This causes more neurotransmitter, to be released,
at the presynaptic membrane.
When more neurotransmitters reach the post synaptic membrane,
they cause more graded potentials, to be generated.
These potentials under go temporal summation.
The strength of the signal, gets conveyed across the synapse,
in this manner.
The post synaptic neuron can receive signals from many neurons.
This can happen in the post synaptic membranes of the receiving neuron.
The dendrites in the neuron, effectively increase the surface area of the cell body,
or the soma of the neuron.
This enables hundreds and thousands, of other neurons,
to connect to the receiving neuron.
Signals are received in many locations.
All these signals are summed up by the receiving neuron.
This is called spatial summation.
Neurons play a role in communicating with other neurons.
A neuron also has another important function.
The neuron is the receiving centre for hundreds and thousands of signals.
This is specially true for neurons in the brain, where complex processing takes place.
The received signals,
can be an EPSP or IPSP.
It can be a strong signal or a weak signal, and many levels in-between.
All these signals converge, in the soma.
The receiving neuron sums up, all these signals.
If the net result reaches a threshold, of minus 55 millivolts,
it will generate an action potential.
If the net signal is sub threshold, no signal will be generated.
The output of a neuron, is a binary zero or one.
During the process of summation,
the neuron is actually performing, a complex biochemical computation.
In a sense, the neuron can be thought of as a computing unit.
The brain has about 100 billion neurons.
These neurons are involved in parallel processing of signals.
In this sense the brain acts like a complex parallel processing computer.
However, we must bear in mind, that the brain has a unique design,
which cannot be compared with man made computers.
The brain is the most complex, and sophisticated living organ.