Structure and function of neurons

Structure and function of cells of nervous system

Brain - It is the organ that moves the muscles. It is very flexible in that it makes us behave in different ways according to the environment , let us learn from our experiences and all these abilities are possible by billions of cells found in nervous system called neurons.

Types of neurons:

Sensory neurons - A neuron that detect changes in the external or internal environment , gathers these information and sends them to the CNS

Motor neurons - located within the central nervous system and control  contraction of muscles and secretion of the glands , control movements of  body

Interneuron - located entirely within the CNS . This is of two types:

1.Local interneurons - Form circuits with nearby neurons and analyse small pieces of information

2.Relay interneurons - Connect circuits of local interneurons from one region of the brain to another

Cells of nervous system:

Most important cells of nervous system:

1.Neurons and their supporting cells

2.Blood - brain barrier

Neurons:

Neurons or nerve cells are the information processing and information transmitting element of the nervous system.

Basic structure of a typical neuron consists of:

A. Cell body or Soma - the cell body containing the nucleus

B. Dendrites - tree like structure attached to the Soma receive information from the terminal button of the neurones

C .Axons - long tube covered in balance sheet transfer the information from cell body to the terminal button. The message that it carries is called action potential, it starts at the end of the cell body and travel towards the terminal button.

It is always of same size and duration when it reaches where axon branches ,it splits but does not reduce its size .

Axoplasm - The jelly like substance in the axon

Types of neurons according to the way in which their axon and dendrites leave the soma :

1. Multipolar neuron - Most common types in the CNS ,In this the soma give rise to one axon and many dendrites attached to the Soma

2. Bipolar neuron - A neuron with one axon and one dendrites attached to the soma , these are usually sensory neurones

3. Unipolar neuron -  A neuron with one stalk attached , which divides into two branches with one branch receiving sensory information and other sending information to the CNS .

Dendrites of unipolar neurons mostly detect the touch temperature changes

D. Terminal buttons -  It is present at the end of the branch of an axon , forms synapses with another neuron and sends information to that neuron , it also secretes a chemical called neurotransmitters ,when an action potential travelling down the axon reaches there .

Any individual neuron receive information from the terminal button of axons of other neuron and the terminal button of its axon forms synapse with the other neuron.

Internal structure of neuron:

A. Membrane - Boundary of the cell made of double layer of lipid substance.
Embedded in the membrane are  variety of protein molecules with special functions:

 Some of these protein detects substances outside the cell , some pass information and some control access to the interior of the cell, some acts as transporters carrying certain molecules in and out of the cell ,these are important in transmitting the information.

Importance of proteins in cell function

1.    It provides shape - by cytoskeleton formed by microtubules and protein fibres linked to each other forming a cohesive mass and gives the cell its shape

2.    Serves as enzyme - it causes particular molecules to join together or split apart

     3. Help transporting substance within the cell

 

The system that transport items rapidly and effectively through       the axoplasm called is called axoplasmic transport

The transport is done by long protein strands called microtubules which is bundle of 13 filaments arrange around a hollow Core

It is of two types:

Anterograde axoplasmic transport which is movement from the cell body to the terminal buttons , this is very fast

Retrograde axoplasmic transport carries substance from the terminal button back to the soma , this is half as fast as the anterograde transport .

Energy for both the transport is provided by mitochondria through ATP

B. Cytoplasm - Jelly like substance containing specialised structure
Such as:
Mitochondria -It breaks down the nutrients , provide energy to the cell . It produces chemical adenosine tri-phosphate ( ATP ) which is used as energy source in the cell

C. Nucleus - Structure in centre of the cell containing the chromosome. chromosome is a strand of DNA carries genetic information , portion of chromosome called genes is the functional unit of chromosome , it directs synthesis of proteins

                 Supporting cells of nervous system

Neurones contain only half the volume of the CNS . Rest consists of supporting cells. They provide nutrients ,support and protection to the neurons

Glia - also called glial cells , surrounds the neurons ,holds them together , control supply of nutrients , insulate neurons from another so that information don't get mixed , destroy and removes carcasses of dead neurones

Three Types of glial cells

1. Astrocytes provide physical support , control the chemical composition of the fluid surrounding the neurons and provide nourishment ,serves as a matrix that holds neurons together in place

When neurons  die , astrocytes clean up the debris. When  astrocytes contact a piece of debris from a dead neuron ,they push themselves against it, engulfing and digesting it ,this process is called phagocytosis

2. Oligodendrocytes in CNS provide support to axon and produce myelin sheath , myelin is 80% lipid and 20% protein. It forms a tube surrounding the axon , these tubes consists of series of segments ,with small portion of uncoated axon between the segments.

 These bare portions of axon are called nodes of Ranvier

These serves to facilitate rapid conduction of Action potential ,as these bare portion enable impulses to jump from one node to another called saltatory conduction.

3. Microglia - Smallest glial cells, serves as one of the representation of the immune system in the brain, protect brain from microorganisms.

4. Schwann cells (In pns) - Support axon and produce myelin in PNS.

Blood brain barrier

Discovered by Paul Ehrlich. It is a semi permeable barrier between the blood and the brain .Produced by the cells in the walls of the brain capillaries.

The presence of blood brain barrier makes it easier to regulate the composition of fluid that enters brain. Blood - brain barrier is not uniform throughout the CNS, it is relatively permeable in area called area postrema , region of the medulla where the blood - brain barrier is weak ,poisons can be detected there and can initiate vomiting

Communication within the neurons


Measuring electrical potential of Axons:

A Micro electrode is used to measure the activity of an individual neuron .

inside of Axon is negatively charged as compared to the outside .Difference is -70 MV.  Thus, the inside of the membrane of an Axon is -70 MV called the membrane potential. It is the difference in electrical potential inside and outside the cell (Potential is stored up energy)

The message that it conducts down the axon consists of brief change in the membrane potential

To study these changes and potential we can use an Oscilloscope, it measure voltage and also produce a record of these voltage graphically.

Resting potential-It is the membrane potential when the membrane is at rest, approximately -70 mv.

If the membrane potential is altered, the membrane potential suddenly reverses itself so that the inside becomes more positive
and the outside becomes more negative.  The membrane potential then quickly returns to normal but it overshoot as it passes is resting potential (-70mv) and becomes hyperpolarized that is become more negative for a short time this whole process takes about 2 milliseconds

This very Rapid reversal of membrane potential is called action potential

The voltage level the triggers an action potential is called threshold of excitation



Reason for membrane potential:

1. Force of diffusion - The process whereby molecules distribute themselves evenly throughout the medium by moving from regions of high concentration to regions of low concentration

2.Force of electrostatic pressure- When some substance called electrolytes dissolved in water it splits into parts called ions .Ions are of 2 types : Cations - Positively charged and Anions - Negatively charged.

The force exerted by attraction or repulsion between the ions is called electrostatic pressure .

The force moves ions from higher concentration to lower concentration region.

3. Ions in extra and intracellular fluid

Intracellular fluid - Fluid within the cell

Extracellular fluid -Fluid outside the cell

Four important ions of these ions are:

A. Organic ions (A-)
B. Chloride ions(Cl-)
C. Sodium ions(Na+)
D. Potassium ions(K+)

Organic ions (A-)  ,negatively charged are found only in the intracellular fluid

Potassium ions ( K+) are found predominantly in the intracellular fluid

Sodium (Na+) and chloride (Cl-) ions are also present in the intracellular fluid but lesser in number however these are present in predominantly or in larger number outside or extracellular fluid

Reasons for such distributions of ions inside and outside of the cells:

A. Organic ions- is present only in intracellular fluid because the membrane is not permeable or impermeable to it and it contributes to membrane potential

B. Potassium ions - it is present predominantly in intracellular fluid that is inside the Axon , so the force of diffusion tries to push it outside of the cell but outside of the cell is more positively charged than the inside so the electrostatic pressure tends to keep it inside.

As K+ is positively charged and outside the cell is positively charged and as we know similar forces repel each other so potassium ions remains inside the cell.

C. Chloride - Present in greater concentration outside the cell ,so, the force of diffusion push it inside the cell but because inside is more negatively charged and chloride is also negatively charged and similar charges repels each other so, they remain where they are.  The two opposite force of diffusion and force of electrostatic pressure tends to balance each other and so it remains where it is.


D. Sodium ions - Present in higher concentration outside the cell and is pushed inside the cell by force of diffusion.

Since, sodium is positively charged and inside of the cell is negatively charged so, electrostatic force does not prevent it from entering the cell because the negative charge inside attract the positively charged sodium Ion .

But in spite of the fact that two forces tends to push sodium inside the cell it is present in higher concentration outside of the cell .

This is because of the presence of Sodium Potassium pump

Sodium Potassium pump continuously pushes sodium out of the cell.

The pump consists of large number of Sodium Potassium Transporter proteins embedded in the membrane which works by the energy produced by ATP molecules. The pump exchanges Sodium for Potassium, pushing three sodium ions outside of the cell for every  two potassium inside the cell.


                   The Action Potential

Both electrostatic and diffusion forces push sodium into the cell but because the membrane is not so permeable to Sodium and also the Sodium - Potassium pump keeps its outside , So, the intracellular level of sodium is low.

However, during the action potential the membrane becomes more permeable to sodium , forces of diffusion and electrostatic process would cause sodium to rush inside the cell . The sudden influx of positively charged sodium ions would change the membrane potential and cause a action potential to occur.


Reason for this increase in permeability of sodium ions:

This is due to a type of protein molecules which provide ion channels that allows ions to enter or leave the cells . The more number of Ion channels are open more is the permeability.

Process of action potential

(1) When the threshold of excitation is reached the opening of sodium ion channels increases the permeability of sodium and it rushes inside the cell

The sodium ions change the membrane potential from -70 mv to approximately +40 mv, decreasing the membrane potential or depolarization .

Because these channels are opened by changes in the membrane potential it is also called voltage dependent Ion channels

(2) The potassium channel in the membrane are less sensitive and require higher depolarization to open so they open later than the sodium channels

(3) When action potential reaches its peak sodium channels are blocked and cannot open until the membrane once again goes back to its resting potential so at this point sodium cannot enter the cell.

(4) Now the potassium (K+) ion channel opens and because of depolarization the inside is more positively charged so potassium ion is rushed outside of the cell by diffusion in electrostatic force.

(5) While returning to its normal value the membrane overshoots is resting value because the potassium ions moves outside the cell this is called hyperpolarization and then gradually returns to normal and extra potassium ions soon diffuses away and membrane becomes at resting potential.

Conduction of action potential:

Basic characteristics of action potential :

1.     Follows the all or none law all or none law states that an action potential will either occur or does not occur ,no in between

2.     Once fire it always goes down the axon to its end

3.      Action potential always remains the same size without increasing or decreasing in size.

4.     When an action potential reaches a point where the axon branches it splits but does not decreases in size .

5.     Also follows the rate law , high rate of firing (firing refers to production of action potential) causes of strong effect ,low firing rate cause low effect.

6.     Saltatory conduction- In this the action potential jumps from one nodes of ranvier to another.

Benefits of saltatory conduction-it consumes less energy and it conducts message at a very high speed.

Communication between neurons:

Neurons communicate primarily through synaptic transmission which is the transmission of message from one neuron to another through a synapse

Synapse- the space\gap formed between the dendrites of a neuron and terminal button of another neuron

Neurotransmitters -information is transmitted by neurotransmitter released by the terminal buttons

Neurotransmitter produced postsynaptic potential that increases or decreases the rate of firing

postsynaptic potential is of two types :

depolarization and hyperpolarization

Postsynaptic potential - changes in the membrane of the postsynaptic neuron produced by the release of neurotransmitter from the presynaptic membrane at the synapse .

Neurotransmitters are natural ligands and attach to a particular binding site at postsynaptic membrane

Presynaptic membrane - is the membrane of the terminal button and releases the neurotransmitter

Postsynaptic membrane -is a member of the opposite neuron (dendrite) , opposite to the presynaotic membrane  which receives the neurotransmitter

Synaptic cleft- the space between presynaptic membrane and postsynaptic membrane

Synaptic vesicles - bead like structures found in terminal button producing and containing neurotransmitters

Dendritic spine -structures present on the dendrites which forms a synapse with the terminal button

             Structure of synapse

synapse can be formed at three places:

On dendrites called axodendritic

on Soma call axosomatic

on axon called axoaxonic

The presynaptic membrane located on terminal button and the postsynaptic membrane located on opposite neuron receives the neurotransmitters. The two membrane faces each other across the synaptic cleft

Release of Neurotransmitters:

Synaptic vesicles fuses with membrane ,then break open and spill their contents into the synaptic cleft.

Activation of neurons

Neurotransmitter producer depolarization or hyperpolarization in presynaptic membrane by:Diffusing across the synaptic cleft ,and attaching to the binding site in the postsynaptic membrane.

Binding site is present in the receptor molecules, in the postsynaptic membrane called postsynaptic receptor

During the binding process the postsynaptic receptor opens neurotransmitter dependent Ion channels mainly sodium or potassium channels.

With the opening of channel specific ions rushes into the cell.

In short, neurotransmitter causes the iron channel to open, causes certain ions to enter the cell and changing the membrane potential.

Neurotransmitters main job is to open the Ion channels.

 It does this by 2 method:

1.Direct method – In this ,The neurotransmitter molecules attaches to the ion channels which causes it to open this is also called Ionotropic receptor

2.Indirect method - Some receptors contain metabotropic receptors , these don't open the Ion channels by themselves but start a change of events which causes it to open.

Metabotropic receptors are located near another protein call G- protein.

When neurotransmitter attached to metabotropic receptors it activates G-protein, G-protein inturn activates an enzyme ,that enzyme stimulates production of chemical called second messenger .

second messenger because a neurotransmitter in the first messengers.

Second messenger then causes the Ion channels to open .

Cyclic AMP was the first second messenger discovered

Postsynaptic potential


It is of two types:

 1.Depolarization (excitatory)

 2.Hyperpolarization (inhibitory)

Whether the postsynaptic membrane is excitatory and inhibitory is determined by the type of Ion channels they open in the postsynaptic membrane.

Three major types of Ion channels in the postsynaptic membrane are :

1.Sodium,

2.Potassium and

3.Chloride

Sodium- Opening of sodium channel causes excitatory post synaptic potentian .When sodium channels are open sodium comes in the cell and causes depolarization which is an excitatory postsynaptic potential.

Potassium channel causes inhibitory postsynaptic potential(IPSP).  when potassium ion channels are open potassium ion moves out of the cell causing hyperpolarization which is an inhibitory postsynaptic potential

Chloride the opening of chloride channel causes the chloride to enter the cell . The inflow of chloride brings the membrane back to its resting potential of -70 MV . It neutralizes the post synaptic potential.

Termination of postsynaptic potential

It is terminated by two method :

1.Re-uptake - Rapid removal of neurotransmitter from the synaptic cleft by re-entering  in the terminal button.

2.Enzymatic deactivation- The destruction of the neurotransmitter by an enzyme after its release


Effect of postsynaptic potential

1.Excitatory post synaptic potential (EPSP)- causes a neuron to fire

2.Inhibitory post synaptic potential (IPSC)- causes a neuron to stop firing

The rate of firing of a neuron depends upon the relative activity of excitatory and inhibitory postsynaptic potential.

The interaction effect of excitatory and inhibitory postsynaptic potential on a particular neurone is called neural integration

Therefore if the activity of excitatory postsynaptic potential goes up rates of firing also goes up .If activity of inhibitory synapse goes up rate of firing goes down.

Autoreceptors - receptors on the neurons that response to the neurotransmitters that they themselves produced is called autoreceptors .

The autoreceptors don't control Ion channels, they only control internal process such a synthesis and release of neurotransmitter. These are metabotropic receptors.


Axoaxonic synapse

Axodendritic and axosomatic synapses causes  IPSP and EPSP.

However ,the type of synapse, the axoaxonic synapse alters the amount of neurotransmitter released by the terminal button .

They causes pre synaptic modulation, i.e, modulates the amount of neurotransmitter released.

If it increases the neurotransmitter release it causes presynaptic facilitation.

If it decreases the neurotransmitter released it is called presynaptic inhibition

Neuromodulators - Beside releasing neurotransmitters the terminal button of neuron also release neuromodulators. It travels faster and disperses more widely than a neurotransmitter secreted in large amount.