Neurons are the basic structural and functional units of the nervous system. They transmit electrical and chemical signals and have three main parts - the cell body, dendrites, and axon. The cell body contains the nucleus. Dendrites receive signals and the long axon conducts signals away from the cell body. Neurons communicate with each other via synapses, where neurotransmitters are released by the presynaptic neuron and bind to receptors on the postsynaptic cell. This allows signals to be transmitted electrically along neurons and chemically between neurons.

2. introduction
• A neuron ( also known as a neurone or nerve cell) is
an electrically excitable cell that processes and
transmits information by electrical and
chemical signallnig.
• Neurons are the core components of the nervous
system, which includes the brain, spinal cord, and
peripheral ganglia.
• Human brain comprises tens of billions of
neurons, each linked to thousands of other neurons
via the chemical channels called synapse.

3. structure
• There are many, many different types of neurons but
almost all have certain structural and functional
characteristics in common.
• A neuron consists of three main parts the cell body or
perikaryon or soma, dendrites and axons.
• The cell body is the central region which is the most
important part of the neuron containing the nucleus of
the cell.
• The soma is, the site of major metabolic activity in
the neuron.

4. • The size of neuronal somas range widely from 0.005
mm to 0.1 mm in mammals.
• Collections of cell bodies (somas) give the greyish
appearance to the gray matter of the brain.

5. • The protoplasm of cell body contains peculiar angular
granules, which stain deeply with basic dyes, such as
methylene blue; these are known as Nissl’s granules.
• These granules disappear (chromatolysis) during fatigue or
after prolonged stimulation of the nerve fibers connected
with the cells. They are supposed to represent a store of
nervous energy, and in various mental diseases are
deficient or absent.
• Thought to be involved in the synthesis
of neurotransmitters such as acetylcholine.

6. • Sections of motor
neuron spinal cord
showing Nissl bodies
on nissl staining.

7. • Dendrites are extensions that carry impulses toward the cell body
and are referred to as being afferent fibers.
• They effectively increase the surface area of a neuron to increase
its ability to communicate with other neurons.

8. • An axon is one of two types of protoplasmic protrusions that
extrude from the cell body of a neuron .
• Unlike dendrites axons are long, slender projection of a nerve
cell, or neuron, that conducts electrical impulses away from the
neuron's cell body or soma.
• Axons are distinguished from dendrites by several
features, including shape, length , and function.
• The point where the axon arises from a cell body is termed as
axon hillock.
• Axoplasm is the cytoplasm within the axon of a neuron.

9. • The axolemma is the cell membrane surrounding an axon. It is
responsible for maintaining the membrane potential of the
neuron, and it contains ion channels through which ions can flow.
• In vertebrates, the axons of many neurons are sheathed
in myelin, which is formed by either of two types of glial
cells: Schwann cells ensheathing peripheral neurons and
oligodendrocytes insulating those of the central nervous system
• The myelin sheath functions to:
– Protects the axon and electrically isolates it
– Increases the rate of Action Potential transmission (saltation)
• Along myelinated nerve fibers, gaps in the sheath known
as nodes of Ranvier occur at evenly-spaced intervals.

10. • Terminally the Axon branch sparsely, forming collaterals. Each
collateral may split into telodendria which end in a synaptic
knob, which contains synaptic vesicles – membranous bags of
NTs.
• Axons make contact with other cells via the synaptic knob—
usually on dendrites of other neurons but sometimes muscle or
gland cells—at junctions called synapses.
• The region between the two connecting neurons is known as the
synaptic gap or snaptic cleft or neural junction.

13. Classification of neurons
STRUCTURAL CLASSIFICATION
 BASED ON POLARITY
 Unipolar : type of neuron in which only one protoplasmic
process (neurite) extends from the cell body.
– Found mostly in inverterbrate
– In humans mostly found in dorsal root ganglia
 Pseudounipolar : contains an axon that has split into two
branches; one branch runs to the periphery and the other to the
spinal cord.
 Bipolar: An axon and a single dendrite on opposite ends of the
soma
– are specialized sensory neurons for the transmission of
special senses,hence abundant in sensory pathways for
smell, sight, taste, hearing and vestibular functions

14.  Multipolar: An Axon along with more than two dendrites
– Multipolar neurons constitute the majority of neurons in
the brain
– Subdivided in to golgi I and golgi II types
– Includes motor neurons and interneurons.

15. FUNCTIONAL CLASSIFICATION
BASED ON CONDUCTION DIRECTION
 Afferent neurons –
– Also called sensory neurons.
– Convey information from tissues and organs into the
central nervous system
 Efferent neurons –
– Also called as motor neurons.
– Carry nerve impulses away from the central nervous
system to effectors such as muscles or glands.
– According to their targets, motor neurons are classified
into three broad categories: Somatic motor neurons,
Special visceral motor neurons ,General visceral motor
neurons.
–Somatic motor neurons are further divided in to α motor
neuron (innervating extrafusal muscle fibre) and γ motor
neuron (innervating intrafusal muscle fibre)

16.  Interneuron-
– also called as relay neuron or local circuit neuron.
– connects afferent neurons and efferent neurons in neural
pathways.

17.  BASED ON NEUROTRANSMITTER PRODUCTION
 Cholinergic neurons —secreting acetylcholine
 GABAergic neurons — secreting gamma aminobutyric acid.
 Glutamatergic neuron — secreting glutamate
 Dopaminergic neurons — secreting dopamine . Loss of dopamine
neurons in the substantia nigra has been linked to Parkinson's
disease
 Serotonergic neurons — secreting serotonin. A lack of serotonin
at postsynaptic neurons has been linked to depression.
 BASED ON UNIQUE SHAPE AND FUNCTION
 Betz cells – large motor neurons located within the fifth layer of
the grey matter in the primary motor cortex, M1.
 Purkinje cells - some of the largest neurons in the
human brain, found within the Purkinje layer in the cerebellum.

18.  Renshaw cells - neurons with both ends linked to alpha motor
neurons. Target of the toxin of Clostridium tetani
 Pyramidal neurons (pyramidal cells) - type of neurons with
triangular soma found in areas of the brain including cerebral
cortex, the hippocampus, and in the amygdala.
 Basket cells - inhibitory GABAergic interneurons found in
several brain regions: the molecular layer of
the cerebellum, the hippocampus, and the cortex.

19. • Neurons collectively form a nerve.
• a nerve is usually made up from a variety of fascicles .
• each fascicle is encased by perineurium. Inside the fascicle are a
group of axons bathed in endoneurial fluid.
• Between the fascicles is a fatty material called
the interfascicular epineurium. The nerve is then wrapped in the
main epineurium

20. Neuronal communication
 Neurons are the information/signal relay system of our nervous
system
 Once stimulated neurons need to conduct information in two
ways:
1. From one end of a neuron to the other end.
2. Across the minute space separating one neuron from another
neuron/muscle end plate (synaptic cleft).
 The 1st is accomplished electrically via Action Potential
generation.
 The 2nd is accomplished chemically via neurotransmitters

21. Electrical conduction
RESTING MEMBRANE POTENTIAL
• The relatively static membrane potential of quiescent cells is
called the resting membrane potential.
• Resting membrane potential of nerve cell = -70 mV

22. • Resting membrane potential is maintained by the ionic
distribution across the neuron cell membrane
• Ions involved mainly are the potassium and sodium ion.
• concentration gradients of Na+ & K+
– Na+ 10x greater outside
– K+ 30x greater inside

23. • At rest more K+ move out than Na+ move in.
• K+ ions diffuse out leave behind excess negative charge inside.
• Sodium-potassium pump
– Na+ out - K+ in (more Na+ out than K+ in)
– contributes to loss of (+).

24. THE ACTION POTENTIAL :
• The action potential is generated by ion flux through voltage
gated channels.

26. GENERATION OF ACTION POTENTIAL AND IT’S
CONDUCTION IN NEURON:
• A excitatory stumulus generates at a dendrite of neuron
• This transmitter acts on the membrane excitatory receptor to
increase the membrane’s permeability to Na+.
• Influx of Na+ inside the cell, causing resting membrane potential
to move toward positive side.
• This positive increase in voltage above the normal resting
neuronal potential—that is, to a less negative value—is called the
graded potential or excitatory postsynaptic potential (or EPSP).
• Dendrites and somata typically lack voltage-gated
channels, which are found in abundance on the axon hillock and
axolemma.So Action potential is not generated here.

27. • The positive charge carried by the Na+ spreads as a wave of
depolarization through the cytoplasm in the form EPSP.
• If the initial amplitude of the EPSP is sufficient, it will spread all
the way to the axon hillock where Voltage-gated channels resides
in high number.
• At the axon hillock If the arriving potential change is
suprathreshold, an Action potential will be initiated and it will
travel down the axon to the synaptic knob where it will cause NT
exocytosis.
• If the potential change is subthreshold, then no AP will ensue
and nothing will happen.
• The EPSP can undergo spatial summation or temporal
summataion so as to reach suprathreshold level and excite an
action potential.

28. • Temporal summation :The same presynaptic neuron stimulates the
postsynaptic neuron multiple times in a brief period. The
depolarization resulting from the combination of all the EPSPs may be
able to cause an AP.
• Spatial summation : Multiple neurons all stimulate a postsynaptic
neuron resulting in a combination of EPSPs which may yield an AP

30. • If an AP gets generated at the axon hillock, it will travel all the
way down to the synaptic knob.
• The manner in which it travels depends on whether the neuron is
myelinated or unmyelinated.
• Unmyelinated neurons undergo the continuous conduction of an
AP whereas myelinated neurons undergo saltatory conduction of
an AP.
• In continous conduction, the wave of de- and repolarization
simply travels from one patch of membrane to the next adjacent
patch.

31. • Action potential propagation in unmyelinated axon

32. • Saltatory conduction (from the Latin saltare, to hop or leap) is
the propagation of action potentials along myelinated
axons from one node of Ranvier to the next node.
• It ncreasing the conduction velocity of action potentials without
needing to increase the diameter of an axon.
• MS  destruction of mylin sheath by own immune system
(progressive loss of signal conduction, muscle control & brain
function)

34. Chemical conduction
• One neuron will transmit information to another neuron or to a
muscle or gland cell by releasing chemicals called
neurotransmitters.
• The site of this chemical interplay is known as the synapse.
• Three Types of Synapses occur between neurons
– Axodendritic Synapse
• Axon to dendrite
– Axosomatic Synapse
• Axon to cell body
– Axoaxonic Synapse
• Axon to terminal endings

35. • An AP reaches the axon
terminal of the presynaptic
cell and causes V-gated
Ca2+ channels to open.
• Ca2+ rushes in, binds to
regulatory proteins &
initiates NT exocytosis.
• NTs diffuse across the
synaptic cleft and then bind
to receptors on the
postsynaptic membrane and
initiate some sort of
response on the
postsynaptic cell.

36. • Different neurons can contain different NTs.
• Different postsynaptic cells may contain different receptors.
– Thus, the effects of an NT can vary.
• Some NTs cause cation channels to open, which results in a
graded depolarization.
• Some NTs cause anion channels to open, which results in a
graded hyperpolarization.
• A graded depolarization will bring the neuronal VM closer to
threshold. Thus, it’s often referred to as an excitatory
postsynaptic potential or EPSP
• Graded hyperpolarizations bring the neuronal VM farther
away from threshold and thus are referred to as inhibitory
postsynaptic potentials or IPSPs

37. • EPSP and IPSP