Neuron

Neurons: The Basic Functional Unit

Dr Alamze MBBS, M.Phil, MPH.

Neuron

• A neuron is the functioning unit of the
nervous system;
• are electrically excitable cells in the nervous
system that function to process and transmit
information.
• In vertebrate animals, neurons are the core
components of the brain, spinal cord and
peripheral nerves.
• Although there are many types of neurons
among the estimated ten billion in the human
brain, each typically shares the same
characteristics

• The flow of information moves in the following
direction:
• dendrite to soma to axon to terminal buttons to
synapse.


Basic structure of Neuron
Dendrites

END BUTTEN

soma

Node of
ranvier

AXON

Axon hillock


soma
• The soma, is the
central part of the
neuron.
• It contains the
nucleus of the cell,
and therefore is
where most protein
synthesis occurs.
• The nucleus ranges
from 3 to 18
micrometers in
diameter

The Dendrites
•

•

•

The dendrites of a
neuron are cellular
extensions with many
branches,
This is where the
majority of input to the
neuron occurs.
However, information
outflow (i.e. from
dendrites to other
neurons) can also
occur. Dr Alamze MBBS, M.Phil, MPH.

axon
• The axon is a finer, cablelike projection which can
extend tens, hundreds, or
even tens of thousands of
times the diameter of the
soma in length.
• The axon carries nerve
signals away from the
soma (and also carry
some types of information
back to it).
• Many neurons have only
one axon, but this axon
may - and usually will undergo extensive
branching, enabling
communication with
many target cells.

Axon Hillock.
• The part of the axon where it
emerges from the soma is called the
'axon hillock'.
• Besides being an anatomical
structure, the axon hillock is also the
part of the neuron that has the
greatest density of voltagedependent sodium channels.
• This makes it the most easily-excited
part of the neuron and the spike
initiation zone for the axon
• in neurological terms it has the
greatest hyperpolarized action

•'axon hillok'.

Classification
• Structural classification
• Classification on basis of function;
Classification by action on other neurons
Classification by neurotransmitter released


Structural classification
• Unipolar or
Pseudounipolar :
dendrite and axon
emerging from same
process.


Bipolar:
• single axon
and single
dendrite on
opposite ends
of the soma.


Structural classification
• Multipolar: more than
two dendrites


• Axons and dendrites in the
central nervous system are
typically only about a micrometer
thick,
• while some in the peripheral
nervous system are much thicker.
• The soma is usually about 10–25
micrometers in diameter and
often is not much larger than the
cell nucleus it contains.
• The longest axon of a human
motoneuron can be over a meter
long, reaching from the base of
the spine to the toes.
• Sensory neurons have axons that
run from the toes to the dorsal
columns, over 1.5 meters in
adults.
•

Classification on function;
• receptor or sensory neurons,
• motor neurons
• and interneurons.


Sensory or afferent NEURONS
• (carrying toward the
brain)
• These neurons are
specialized & sensitive to
a particular physical
stimulation
• such as light (vision),
• sound (audition),
• chemical (olfaction),
• or pressure (touch).

Motor or efferent neurons
• Motor or efferent
(carrying away from the
brain) neurons receive
impulses from other
neurons and transmit this
information to muscles or
glands.
• These neurons are of two
types, alpha motor
neurons and gamma
motor neurons.

Alpha Motor Neurons.
•

•

•

The alpha motor neurons give
rise to large type A alpha (Aα)
motor nerve fibers, averaging
14 micrometers in diameter;
these fibers branch many
times after they enter the
muscle and innervate the large
skeletal muscle fibers.
Stimulation of a single alpha
nerve fiber excites from three
to several hundred skeletal
muscle fibers, which are
collectively called the motor
unit.


Gamma Motor Neurons.
• gamma motor neurons
are located in the spinal
cord anterior horns.
These gamma motor
neurons transmit
impulses through much
smaller type A gamma
(Aγ) motor nerve fibers,
averaging 5 micrometers
in diameter, which go to
small, special skeletal
muscle fibers called
intrafusal fibers,

Interneurons or intrinsic neurons
• Interneurons or
intrinsic neurons form
the largest group in
the nervous system.
• They form
connections between
themselves and
sensory neurons
before transmission of
control to motor
neurons

Classification by action on other neurons
• Excitatory neurons excite their target
neurons. Excitatory neurons in the brain are
often glutamatergic.
• Inhibitory neurons inhibit their target neurons.
Inhibitory neurons are often interneurons.
These neurons use such neurotransmitters as
dopamine, acetylcholine, serotonin and others.


Classification by discharge
patterns
•
Neurons can be classified according to
their electrophysiological characteristics


Tonic or regular spiking
Some neurons are typically constantly (or
tonically) active.
Example: interneurons in neurostriatum.


Phasic or bursting
• Neurons that fire in bursts are called
phasic.
• Fast spiking. Some neurons are notable
for their fast firing rates, for example some
types of cortical inhibitory interneurons,
cells in globus pallidus.


Thin-spike.
• Action potentials of some neurons are
more narrow compared to the others. For
example, interneurons in prefrontal cortex
are thin-spike neurons.


Classification by
neurotransmitter released
•
•
•
•

Some examples are cholinergic,
GABAergic,
Glutamatergic
and dopaminergic neurons.


Glial cells

• In addition to neurons, the nervous system is
populated with another category of cells, glial
cells.
• Glial cells are approximately 10 times more
plentiful than neurons.
• But since they are approximately one-tenth the
size, they take up equal space.
• Glia is a Greek term meaning glue.
• glial cells served as the putty that held the
neurons together

Myelin sheaths
• Oligodendrocytes and Schwann cells form
the myelin sheaths that insulate axons in
the central and peripheral nervous
systems, respectively.


synapses

• The human brain has a huge number of
synapses.
• Each of the 1012 neurons (1,000 billion, i.e.
1 trillion) has on average 7,000 synaptic
connections to other neurons.
• It has been estimated that the brain of a
three-year-old child has about 10 16
synapses (10,000 trillion).
• This number declines with age, stabilizing
by adulthood.
• Estimates vary for an adult, ranging from
1015 to 5 x 1015 synapses (1,000 to 5,000
trillion

synaptic transmission
•
•
•

Neurons communicate via chemical and electrical synapses, in a process
known as synaptic transmission.
The fundamental process that triggers synaptic
transmission is the
action potential,
a propagating electrical signal that is generated by exploiting the electrically
excitable membrane of the neuron.


stained pyramidal neurons in cerebral cortex.


Neuron

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