Neurons are the basic structural and functional units of the nervous system. They have three main parts - a cell body containing the nucleus, dendrites which receive signals, and an axon which transmits signals. Neurons communicate via electrical and chemical signals across synapses. There are different types of neurons classified by structure and function, including sensory neurons, motor neurons, and interneurons. Neurons are formed through neurogenesis, migrate to their destinations, differentiate, and form neural networks. Diseases can cause neuron death. Key neurotransmitters mediate signaling between neurons.

1. Neurons – Structure , Classification and
Functions
Pavithra Narasimhan

3. Structure of Neuron
The central nervous system ( includes the
brain and spinal cord) is made up of two
basic types of cells:
 neurons Pic (1) and glia (4) & (6).
Glia outnumber neurons in some parts of
the brain, but neurons are the key players in
the brain.
Neurons are information messengers.
They use electrical impulses and chemical
signals to transmit information between
different areas of the brain, and between the
brain and the rest of the nervous system.
Everything we think and feel and do would
be impossible without the work of neurons
and their support cells, the glial cells called
astrocytes (4) and oligodendrocytes (6).
Neurons have three basic parts: a cell
body and two extensions called an axon
(5) and a dendrite (3).
Within the cell body is a nucleus (2),
which controls the cell’s activities and
contains the cell’s genetic material.
 The axon looks like a long tail and
transmits messages from the cell.
 Dendrites look like the branches of a
tree and receive messages for the cell.
Neurons communicate with each other
by sending chemicals, called
neurotransmitters, across a tiny space,
called a synapse, between the axons and
dendrites of adjacent neurons

4. .

6. Neurogenesis
• Neurons are born in areas of the brain that are rich in concentrations of neural
precursor cells (also called neural stem cells). These cells have the potential to
generate most, if not all, of the different types of neurons and glia found in the brain.
• Neural stem cells increase by dividing in two and producing either two new stem
cells, or two early progenitor cells, or one of each.
• When a stem cell divides to produce another stem cell, it is said to self-renew. This
new cell has the potential to make more stem cells.
• When a stem cell divides to produce an early progenitor cell, it is said to
differentiate. Differentiation means that the new cell is more specialized in form and
function. An early progenitor cell does not have the potential of a stem cell to make
many different types of cells. It can only make cells in its particular lineage.
• Early progenitor cells can self-renew or go in either of two ways. One type will give
rise to astrocytes. The other type will ultimately produce neurons or
oligodendrocytes.

8. Neural Migration
Once a neuron is born it has to travel to the place in the brain
where it will do its work.
How does a neuron know where to go? What helps it get there?
neurons use at least two different methods to travel:
Some neurons migrate by following the long fibers of cells called
radial glia. These fibers extend from the inner layers to the outer
layers of the brain. Neurons glide along the fibers until they
reach their destination.
Neurons also travel by using chemical signals. There are special
molecules on the surface of neurons -- adhesion molecules -- that
bind with similar molecules on nearby glial cells or nerve axons.
These chemical signals guide the neuron to its final location.

9. Neurons migrate by riding along extensions (radial glia) until they reach their final
destinations.

10. Differentiation
 Once a neuron reaches its destination, this is the final step of differentiation is the part of
neurogenesis.
 Neurons are responsible for the transport and uptake of neurotransmitters - chemicals that relay
information between brain cells.
 Depending on its location, a neuron can perform the job of a sensory neuron, a motor neuron, or an
interneuron, sending and receiving specific neurotransmitters.
 In the developing brain, a neuron depends on molecular signals from other cells, such as astrocytes, to
determine its shape and location, the kind of transmitter it produces, and to which other neurons it will
connect. These freshly born cells establish neural circuits - or information pathways connecting neuron
to neuron - that will be in place throughout adulthood.
 But in the adult brain, neural circuits are already developed and neurons must find a way to fit in.
 As a new neuron settles in, it starts to look like surrounding cells. It develops an axon and dendrites
and begins to communicate with its neighbors.

11. Stem cells differentiate to produce different
types of nerve cells.

12. Death of a Neuron
Although neurons are the longest living cells in the body, large numbers of them die during migration and differentiation.
The lives of some neurons can take abnormal turns. Some diseases of the brain are the result of the unnatural deaths of
neurons.
 In Parkinson’s disease, neurons that produce the neurotransmitter dopamine die off in the basal ganglia, an area of the
brain that controls body movements. This causes difficulty initiating movement.
 In Huntington’s disease, a genetic mutation causes over-production of a neurotransmitter called glutamate, which kills
neurons in the basal ganglia. As a result, people twist and writhe uncontrollably.
 In Alzheimer’s disease, unusual proteins build up in and around neurons in the neocortex and hippocampus, parts of
the brain that control memory. When these neurons die, people lose their capacity to remember and their ability to do
everyday tasks. Physical damage to the brain and other parts of the central nervous system can also kill or disable
neurons.
 - Blows to the brain, or the damage caused by a stroke, can kill neurons outright or slowly starve them of the oxygen
and nutrients they need to survive.
 - Spinal cord injury can disrupt communication between the brain and muscles when neurons lose their connection to
axons located below the site of injury. These neurons may still live, but they lose their ability to communicate.

13. Diseased vs Dying Neuron

14. Classes of Neuron
Structurally
Unipolar Neurons
Bipolar Neurons
Multipolar Neurons
Functionally
Sensory Neurons
Motor Neurons
Interneurons
(Association Neurons)

15.  Sensory neurons are neurons responsible for converting external stimuli from the
environment into corresponding internal stimuli.
 Motor neurons are neurons located in the central nervous system, and project their axons
outside of the CNS to directly or indirectly control muscles.
 Interneurons act as the "middle men" between sensory and motor neurons, which convert
external stimuli to internal stimuli and control muscle movement, respectively.

16. The connections between neurons form a highly complex network through which
signals or impulses are communicated across the body
The basic kinds of connections between neurons are chemical synapses and
electrical gap junctions, through which either chemical or electrical impulses are
communicated between neurons.
Neural networks are primarily made up of axons, which in some cases deliver
information as far as two meters.
Neural Networks

17.  Networks formed by interconnected groups of neurons are capable of a wide variety of
functions; in fact the range of capabilities possible for even small groups of neurons are
beyond our current understanding.
 Modern science views the function of the nervous system both in terms of stimulus-response
chains, and in terms of intrinsically generated activity patterns within neurons.
 Cell assembly, or Hebbian theory, asserts that "cells that fire together wire together,"
meaning neural networks can be created through associative experience and learning.

19. Neurotransmitters are often referred to as the body’s chemical messengers. They are the molecules used
by the nervous system to transmit messages between neurons, or from neurons to muscles.
Communication between two neurons happens in the synaptic cleft (the small gap between
the synapses of neurons). Here, electrical signals that have travelled along the axon are briefly converted
into chemical ones through the release of neurotransmitters, causing a specific response in the receiving
neuron.
A neurotransmitter influences a neuron in one of three ways: excitatory, inhibitory or modulatory.
An excitatory transmitter promotes the generation of an electrical signal called an action potential in the
receiving neuron, while an inhibitory transmitter prevents it. Whether a neurotransmitter is excitatory or
inhibitory depends on the receptor it binds to.
Neurotransmitters

20.  A neurotransmitter influences a neuron in one of three ways: excitatory, inhibitory or modulatory.
 An excitatory transmitter promotes the generation of an electrical signal called an action potential in the
receiving neuron, while an inhibitory transmitter prevents it.
 Whether a neurotransmitter is excitatory or inhibitory depends on the receptor it binds to.
 Neuromodulators are a bit different, as they are not restricted to the synaptic cleft between two neurons, and so
can affect large numbers of neurons at once.
 Neuromodulators therefore regulate populations of neurons, while also operating over a slower time course
than excitatory and inhibitory transmitters.
 Most neurotransmitters are either small amine molecules, amino acids, or neuropeptides. There are about a
dozen known small-molecule neurotransmitters and more than 100 different neuropeptides, and
neuroscientists are still discovering more about these chemical messengers. These chemicals and their
interactions are involved in countless functions of the nervous system as well as controlling bodily functions.

21. Key Neurotransmitters
 Acetylcholine - It plays a major role in the peripheral nervous system, where it is released by motor neurons and
neurons of the autonomic nervous system. It also plays an important role in the central nervous system in maintaining
cognitive function. Damage to the cholinergic neurons of the CNS is associated with Alzheimer disease.
 Glutamate is the primary excitatory transmitter in the central nervous system. Conversely, a major inhibitory
transmitter is its derivative γ-aminobutyric acid (GABA), while another inhibitory neurotransmitter is the amino acid
called glycine, which is mainly found in the spinal cord.
 Many neuromodulators, such as dopamine, are monoamines. There are several dopamine pathways in the brain, and
this neurotransmitter is involved in many functions, including motor control, reward and reinforcement, and
motivation.
 Noradrenaline (or norepinephrine) is another monoamine, and is the primary neurotransmitter in the sympathetic
nervous system where it works on the activity of various organs in the body to control blood pressure, heart rate, liver
function and many other functions.
 Neurons that use serotonin (another monoamine) project to various parts of the nervous system. As a result, serotonin
is involved in functions such as sleep, memory, appetite, mood and others. It is also produced in the gastrointestinal
tract in response to food.
 Histamine, the last of the major monoamines, plays a role in metabolism, temperature control, regulating various
hormones, and controlling the sleep-wake cycle, amongst other functions

22. References
• https://www.ninds.nih.gov
• https://qbi.uq.edu.au
• https://www.medicalnewstoday.com/articles
• https://www.sciencedirect.com/