Neurons are electrically excitable cells that communicate with each other and the body. The human nervous system contains around 100 billion neurons. There are three main types of neurons - sensory neurons relay signals from sense organs to the central nervous system, motor neurons relay signals from the CNS to effector organs, and interneurons connect sensory and motor neurons. Each neuron has a cell body, dendrites that receive signals, and an axon that transmits signals. When a neuron is stimulated, it generates an action potential down its axon via changes in membrane potential. Neurotransmitters are released at synapses to transmit signals between neurons.

1. NEURON
* Electrically excitable cells in the nervous
system that receives information from
other neurons and enables
communication between them and to
different body parts.
*The human nervous system contains
about 100 billion neurons.

2. TYPES OF NEURON
*On the basis of functions
1. Sensory neuron – It relays impulse from sense organs to
the CNS.
2. Motor neuron – It relays impulse from CNS to effector
organs ( muscles ,glands, etc)
3. Interneuron – It is the link between sensory and motor
neurons.
• On the basis of structure
1. Multipolar – It has many dendrites extending from the
cell body and one long axon.
2. Bipolar – It has one dendrite and one axon at opposite
ends of the cell body
3. Unipolar – It has just have only one projection from the
cell body which then further divide into two branches.

3. STRUCTURE OF NEURON
• Each neuron comprise of three main parts –
1. Dendrites – Dendrites are thin widely branched
projections from the cell body or soma.
They are the major input points that receives
information from other neurons.
2. Cyton /soma / cell body-The body of neuron from which
the dendrites and axons project out .
It contains aqueous material called cytoplasm and
suspended structures inside it called cell organelles.
3. Axon- It is a single relatively thicker than dendrites fibre
that relays information to muscle glands etc.

4. • Axon hillock – the axon is joint to the cell body at axon hillock where the nerve impulse
originate.
• Myelin sheath –A protective layer around the axon .
• Those which are covered with myelin sheath are said to be myelinated.
• Those which are not covered with myelin sheath are said to be non- myelinated.
• Functions- It prevents ion leakage and protects the axon.
• Glial cells –Their basic function is to provide nourishment and support to neurons .They are of
2 types
• A) Oligodendrocytes- they produce myelin sheath in CNS.
• B) Schwann cells- they produce myelin sheath in PNS.
• Nodes of Ranvier – It is the gap in the axon, where myelin sheath is absent.
• Function-They enables exchange of Intra cellular and extracellular fluids.
• Telodendrion – the axon splits into many branches at the end of neuron which is called
telodendrion
• Synaptic knob – It is a bulb shaped swelling at the end of telodendrion that contains synaptic
vesicles.
• Synaptic vesicles –These are sac like structure present in synaptic knob that contains
neurotransmitters which are chemical messengers.
• Synapse –It consists of presynaptic membrane + synaptic cleft + post synaptic membrane,
which acts as a point of contact neuron and its target

5. Types of synapse –
There are 4 types of synapse –
1. Axodendritic synapse – Axon of one neuron synapses with the dendrite of
subsequent neuron.
2. Axosoma synapse –Axon of one neuron synapse with the soma of subsequent
neuron.
3. Dendrodendritic – Dendrites of one neuron synapses with the dendrite of
subsequent neuron.
4. Axonaxionic – Axon of one neuron synapses with the axon of subsequent
neuron.

6. EXCHANGE OF INFORMATION
Exchange of information between neurons involves two types of transmission .
1. Axonal transmission –When the information travels along the length of
neuron also called electrical transmission.
2. Synaptic transmission-When the information travels through the gap
between the two neurons ( synaptic cleft ). It is also called chemical
transmission.
NEURAL MEMBRANE
It is the site where most processes involved and functions are triggered.
This lipid bilayer membrane have protein structure inside them which are of 3
types-
A. Receptor protein – It is present to register the presence of neurotransmitter.
B. Channel protein-These are responsible for the passage of ions and proteins.
C. Pump proteins-To pass the ion/ protein from lower to higher concentration.

7. RESTING MEMBRANE POTENTIAL
When a neuron is at rest – that is not receiving any information from other
neurons that is the intracellular fluid is more negatively charged than the
extracellular fluid.This difference in polarity between inside and outside the cell
membrane is called RESTING MEMBRANE POTENTIAL.
Using the intracellular fluid as a reference point the RMP of the neuron is -70
mV that means that inside of the cell is 70 mV more negatively charged than
extracellular fluid.
The outside of the cell contain high concentration of sodium ions( Na+),
chloride ions (Cl-) and calcium ions (Ca+2) where as inside of the cell contains
high concentration of potassium and negatively charged ions.

8. Two passive forces i.e. Diffusion and electrostatic pressure and one active force that is
sodium potassium pump facilitate ion exchange across membrane
1. DIFFUSION –Tendency of molecule to move from higher concentration areas to
low concentration areas till there is an even distribution of molecules.
2. Electrostatic pressure – It refers to the attraction of opposite polarity molecules
and the repulsion of same polarity molecules.
there are 3 cases
a)The concentration of Cl- ions is higher outside the cell so they tend to
diffuse in however the force of diffusion is counteracted by the
electrostatic pressure inside the cell due to high negative charge inside the cell.
Therefore Cl- remains OUTISDE.
b) Similarly k+ ions will tend to diffuse outside the cell but will be counteracted
electrostatic pressure created by the positively charged ions outside the cell.
c) For Na+ ions, Both diffusion and electrostatic pressure would tend to move
Na+ ions inside the cell but it remains outside due to activation of sodium potassium
pump.
3. SODIUM POTASSIUM PUMP- It actively expels 3 Na+ ions outside for 2 k+
Ions inside the cell.

10. ACTION POTENTIAL
The sequential events that occur
when a resting neuron receives
information is collectively referred
to as Action Potential / Spike
potential or firing of neuron.
It occurs in 4 steps-
1. Depolarisation to threshold level
2. Reversal of Membrane polarity
3. Repolarisation to resting
potential.
4. Refractory period.

11. DEPOLARISATION
1. Initially when the neuron is at rest, it exhibits RMP – (-70 mV) that is inside of the cell is more negative then outside.
2. On detecting a stimulus the sodium gates open temporarily as a result of which there is a rapid influx of sodium ions
inside the cell, making it less negative.
3.When it reaches to (-55 mV), the cell membrane undergoes a more radical change in polarisation resulting in more
number of sodium ions pouring inside the cell making It positive (+30 mV)
4. -55mV is the threshold level above which a neuron will pass a message
5. +30 mv is the peak potential.
REVERSAL OF POLARITY
Due to opening of many sodium channels the inside becomes temporarily positive bringing the
membrane potential to +30mV.This is called the reversal of polarity.

12. REPOLARISATION TO RESTING POTENTIAL
1.The K+ channels open and k+ ions leave the cell Because they are repelled by higher levels of Na+
inside the cell.
2. Shortly after the K+ channels open , Na+ channels close and Na+ ions stop entering the cell.
3.The continuous exit of K+ ion outside the cell, causes the inside to become more negative then
outside once again.
4.This phase is called Repolarisation.
REFRACTORY PERIOD
It consist of two phases-
1. Absolute refractory period - The brief period of time during which a neuron is completely resistant
to further stimulation , no matter how intense the stimulus is, calledAbsolute R.P.
2. Relative refractory period –The period during which a neuron can generate an another action
potential only in response to a more intense stimulus then normal.
It corresponds to the part where the membrane potential actually becomes slightly more negative
then normal (-90mV) , a state known as Hyperpolarisation

13. Restoring RMP –
1. Although the distribution of Na+ and K+ ions changes as a result of
depolarisation and action potential the distribution quickly returns to normal
through the sodium potassium pump that acts to maintain the ion
concentration differences of RMP.(-70mV), until changed by a depolarising
stimulus.

14. PROCESS OF NERVE IMPULSE CONDUCTION
1 . SALATORY CONDUCTION –The propagation of an action potential from node to node in
myelinated axons is termed a saltatory conduction.
. AtThe end of each telodendrion, the pre synaptic terminal Calcium channels open and Ca+2 ions
enters inside the cell.
. Calcium entry causes the synaptic vesicles to align themselves along the presynaptic membrane
where they release the neurotransmitter molecules into the synaptic cleft .
.Diffusion carries the neurotransmitter molecules across the synaptic cleft, and they encounter and
attached to specific receptor site on the post synaptic membrane.
.The neurotransmitter have either inhibitory effect or excitatory effect on the post synaptic
membrane.
A) Neurotransmitter that have excitatory influence produces DEPOLARISATION at the post
synaptic membrane by increasing the number of sodium that opens, allowing the more Na+ ions
to enters the cell.This depolarisation is called an Excitatory Post synaptic potential or EPSP.
B) Neurotransmitter that have an inhibitory effect on the post synaptic membrane produces
hyperpolarisation by causing K+ to leave the cell or Cl- to enter.This hyperpolarisation is called an
Inhibitory post synaptic potential or IPSP

15. SUMMATION EFFECT
The net effect of excitation and inhibition determines whether or not the
neuron is activated and will fire.
Unlike action potential, which are all or none events, EPSP and IPSP received by
the neurons are GRADED POTENTIAL with different values at different times.
The various EPSP is received by the post synaptic membrane can be added
together to produce a combine effect of depolarisation, similarly IPSP produces
a combine level of hyperpolarisation .
This process of adding a positive or negative influences on the cell membrane is
called summation.
It is of two types-
1. Spatial summation- Occurs when multiple stimuli exerts their effects at the
same time on different receptor site on the post synaptic membrane.
2. Temporal summation- Occurs when a stimuli (A twice)are close enough in
time that their effects are additive.

16. PRESYNAPTIC EFFECTS
POST SYNAPTIC RECEPTORS
. Pres synaptic facilitation-The enhance release of neurotransmitter from the pre synaptic
membrane caused by the action of another neuron.
. Autoreceptors- A presynaptic receptors whose stimulation decreases the amount of
neurotransmitter release by a pre synaptic neuron.
.Ionotropic receptor – Receptor whose ion channel are rapidly open by the direct action of a
neurotransmitter.
. Metabotropic receptor- A receptor whose ion channel are indirectly open by a second
messenger.
.Second messenger- A chemical that causes changes inside the cell in response to a
neurotransmitter that lead to ion channel opening.
. Pre synaptic inhibition – It is the decrease in neurotransmitter from the pre
synaptic membrane despite the occurrence of an action potential, caused by the
action of another neuron.
. AUTORECEPTORS – Released neurotransmitters acts as an autoreceptors to
decrease the subsequent neurotransmitter release.

17. TERMINATION OF NEUROTRANSMITTER EFFECT
It is done in two ways –
1. Enzymatic degradation- It is the process of breakdown and thus deactivation of
neurotransmitter molecules by an enzyme ( acetylcholinesterase).
2. Neurotransmitter reuptake- It is the process of return of neurotransmitter to the
pre synaptic neuron at the reuptake sites.

18. THANK YOU