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

Dr...
Basic structure of Neuron
Dendrites

END BUTTEN

soma

Node of
ranvier

AXON

Axon hillock

Dr Alamze MBBS, M.Phil, MPH.
soma
• The soma, is the
central part of the
neuron.
• It contains the
nucleus of the cell,
and therefore is
where most pro...
The Dendrites
•

•

•

The dendrites of a
neuron are cellular
extensions with many
branches,
This is where the
majority of...
axon
• The axon is a finer, cablelike projection which can
extend tens, hundreds, or
even tens of thousands of
times the d...
Axon Hillock.
• The part of the axon where it
emerges from the soma is called the
'axon hillock'.
• Besides being an anato...
Classification
• Structural classification
• Classification on basis of function;
Classification by action on other neuron...
Structural classification
• Unipolar or
Pseudounipolar :
dendrite and axon
emerging from same
process.

Dr Alamze MBBS, M....
Bipolar:
• single axon
and single
dendrite on
opposite ends
of the soma.

Dr Alamze MBBS, M.Phil, MPH.
Structural classification
• Multipolar: more than
two dendrites

Dr Alamze MBBS, M.Phil, MPH.
• Axons and dendrites in the
central nervous system are
typically only about a micrometer
thick,
• while some in the perip...
Classification on function;
• receptor or sensory neurons,
• motor neurons
• and interneurons.

Dr Alamze MBBS, M.Phil, MP...
Sensory or afferent NEURONS
• (carrying toward the
brain)
• These neurons are
specialized & sensitive to
a particular phys...
Motor or efferent neurons
• Motor or efferent
(carrying away from the
brain) neurons receive
impulses from other
neurons a...
Alpha Motor Neurons.
•

•

•

The alpha motor neurons give
rise to large type A alpha (Aα)
motor nerve fibers, averaging
1...
Gamma Motor Neurons.
• gamma motor neurons
are located in the spinal
cord anterior horns.
These gamma motor
neurons transm...
Interneurons or intrinsic neurons
• Interneurons or
intrinsic neurons form
the largest group in
the nervous system.
• They...
Classification by action on other neurons
• Excitatory neurons excite their target
neurons. Excitatory neurons in the brai...
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 neurostriat...
Phasic or bursting
• Neurons that fire in bursts are called
phasic.
• Fast spiking. Some neurons are notable
for their fas...
Thin-spike.
• Action potentials of some neurons are
more narrow compared to the others. For
example, interneurons in prefr...
Classification by
neurotransmitter released
•
•
•
•

Some examples are cholinergic,
GABAergic,
Glutamatergic
and dopaminer...
Glial cells

• In addition to neurons, the nervous system is
populated with another category of cells, glial
cells.
• Glia...
Myelin sheaths
• Oligodendrocytes and Schwann cells form
the myelin sheaths that insulate axons in
the central and periphe...
synapses

• The human brain has a huge number of
synapses.
• Each of the 1012 neurons (1,000 billion, i.e.
1 trillion) has...
Dr Alamze MBBS, M.Phil, MPH.
synaptic transmission
•
•
•

Neurons communicate via chemical and electrical synapses, in a process
known as synaptic tran...
Dr Alamze MBBS, M.Phil, MPH.
Dr Alamze MBBS, M.Phil, MPH.
stained pyramidal neurons in cerebral cortex.

Dr Alamze MBBS, M.Phil, MPH.
Neuron
Neuron
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  1. 1. Neurons: The Basic Functional Unit Dr Alamze MBBS, M.Phil, MPH.
  2. 2. 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 Dr Alamze MBBS, M.Phil, MPH. characteristics
  3. 3. • The flow of information moves in the following direction: • dendrite to soma to axon to terminal buttons to synapse. Dr Alamze MBBS, M.Phil, MPH.
  4. 4. Basic structure of Neuron Dendrites END BUTTEN soma Node of ranvier AXON Axon hillock Dr Alamze MBBS, M.Phil, MPH.
  5. 5. 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 Dr Alamze MBBS, M.Phil, MPH.
  6. 6. 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.
  7. 7. 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. Dr Alamze MBBS, M.Phil, MPH.
  8. 8. 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 Dr Alamze MBBS, M.Phil, MPH. •'axon hillok'.
  9. 9. Classification • Structural classification • Classification on basis of function; Classification by action on other neurons Classification by neurotransmitter released Dr Alamze MBBS, M.Phil, MPH.
  10. 10. Structural classification • Unipolar or Pseudounipolar : dendrite and axon emerging from same process. Dr Alamze MBBS, M.Phil, MPH.
  11. 11. Bipolar: • single axon and single dendrite on opposite ends of the soma. Dr Alamze MBBS, M.Phil, MPH.
  12. 12. Structural classification • Multipolar: more than two dendrites Dr Alamze MBBS, M.Phil, MPH.
  13. 13. • 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. • Dr Alamze MBBS, M.Phil, MPH.
  14. 14. Classification on function; • receptor or sensory neurons, • motor neurons • and interneurons. Dr Alamze MBBS, M.Phil, MPH.
  15. 15. 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). Dr Alamze MBBS, M.Phil, MPH.
  16. 16. 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. Dr Alamze MBBS, M.Phil, MPH.
  17. 17. 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. Dr Alamze MBBS, M.Phil, MPH.
  18. 18. 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, Dr Alamze MBBS, M.Phil, MPH.
  19. 19. 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 Dr Alamze MBBS, M.Phil, MPH.
  20. 20. 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. Dr Alamze MBBS, M.Phil, MPH.
  21. 21. Classification by discharge patterns • Neurons can be classified according to their electrophysiological characteristics Dr Alamze MBBS, M.Phil, MPH.
  22. 22. Tonic or regular spiking Some neurons are typically constantly (or tonically) active. Example: interneurons in neurostriatum. Dr Alamze MBBS, M.Phil, MPH.
  23. 23. 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. Dr Alamze MBBS, M.Phil, MPH.
  24. 24. Thin-spike. • Action potentials of some neurons are more narrow compared to the others. For example, interneurons in prefrontal cortex are thin-spike neurons. Dr Alamze MBBS, M.Phil, MPH.
  25. 25. Classification by neurotransmitter released • • • • Some examples are cholinergic, GABAergic, Glutamatergic and dopaminergic neurons. Dr Alamze MBBS, M.Phil, MPH.
  26. 26. 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 Dr Alamze MBBS, M.Phil, MPH.
  27. 27. Myelin sheaths • Oligodendrocytes and Schwann cells form the myelin sheaths that insulate axons in the central and peripheral nervous systems, respectively. Dr Alamze MBBS, M.Phil, MPH.
  28. 28. 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 Dr Alamze MBBS, M.Phil, MPH. trillion
  29. 29. Dr Alamze MBBS, M.Phil, MPH.
  30. 30. 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. Dr Alamze MBBS, M.Phil, MPH.
  31. 31. Dr Alamze MBBS, M.Phil, MPH.
  32. 32. Dr Alamze MBBS, M.Phil, MPH.
  33. 33. stained pyramidal neurons in cerebral cortex. Dr Alamze MBBS, M.Phil, MPH.
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