NEURON/ NERVE
STRUCTURE &
CLASSIFICATION
Dr. Ayisha Qureshi
A system that controls all of the
activities of the body.
The nervous system is made of:
The brain
The spinal cord
The nerves
The senses
Nervous System
CENTRAL NERVOUS
SYSTEM
Brain
Spinal
Cord
PERIPHERAL
NERVOUS SYSTEM
Sensory
Input
Motor
Output
What makes up the brain, the spinal
cord or your peripheral nerves?
Neurons! Also called Nerves….
INTRODUCTION
•
What is a neuron?
It is the basic structural and functional unit of
the nervous system.
It is a highly differentiated and specialized
excitable tissue.
• The Human NS contains 100 billion neurons.
(Nerve cells and neurons are the same.)
The nerves allow you to react to a
stimulus.
A stimulus is a change in the environment.
Example: A hot stove
Or… tripping over a rock
Functions of the Neurons
• Reception of the stimulus
• Generation of the nerve Impulse
• Transmission of the nerve Impulse
Structure of the A Typical Neuron
A typical neuron thus has the following
parts:
1. Soma or Nerve Cell body
2. Axon with the axon terminals
3. Dendrites
SOMA (Nerve cell body)
• Different shapes
– Fusiform, stellate, oval,
rounded, pyramidal.
• Different sizes
– 5 to 135 micrometers
• Nucleus: typically large
– one nucleolus (usually)
• Perikaryon= cytoplasm
which has:
– Nissl bodies
– Neurofibrils
• All organelles:
– mitochondria, ribosomes,
endoplasmic reticulum, lysosomes &
Golgi apparatus
Nerve Cell Body
• Nissl bodies
– Are rough endoplasmic
reticulum with ribosomes
– Stained with basic dyes
– Composed of RNA &
polysomes.
– Tigroid substance (due to
striped appearance)
– Not present in the axon
– Synthesis of proteins
– Dissolve & disappear if cell
injured (nerve cut, injured,
fatigued, poisoned)
• Neurofibrils
– Formed by clumping of
neurotubules &
neurofilaments
– Delicate threads running
from cytoplasm of the nerve
cell body into the axon and
the dendrite
– Functions:
1. Neuronal microtubules
transport substances from
the cell body to the distal cell
processes.
2. Neurofibrils give support and
shape to the neuron.
Point to remember:
• Nerve cell body is the most vital part-if it is
destroyed the entire neuron dies!
Axon
• Also called axis cylinder or
nerve fiber.
• Longest process
• A single axon arises from a
cone-shaped area of the
neuronal cell body called the
axon hillock
• Axon hillock & first 100 µm
of axon (no myelin sheath) is
called Initial segment.
• Trigger zone: is the name
given to the axon hillock & the
initial segment. It is an area
that shows high excitability
and a nerve impulse is
generated here.
Axon
AXON IS MADE UP OF:
AXON ENDS IN:
• Jelly-like semi fluid substance
called Axoplasm
• Plasma membrane called
Axolemma
• Mitochondria and ER
• No Nissl granules so Does
NOT synthesize proteins.
Terminal Buttons (Synaptic
knob or Bouton Terminaux)
– Axon break up into no. of
terminal branches called
Telodendria or Terminal
filaments
– At their end is a small
swelling called Terminal
knob.
– These knobs contain
granules or vesicles with
neurotransmitter substance
Dendrites
• Short, tree-like, highly
branched tapering
processes of the nerve
cell
• Receive and then carry
impulses to the cell body
• Small knob-like
projections called
dendritic spines
• Have all the components
of the cell body
What are Afferent and Efferent
fibers?
• Afferent
– When nerve fiber
carries impulses from
the periphery towards
the CNS, it is called an
Afferent nerve fiber.
• Efferent
– When the nerve fiber
carries impulses from
the CNS to the
periphery, it is called
an Efferent nerve fiber.
CLASSIFICATION OF
NEURONS:
Classification based on structure
Classification based on Function:
Classification
Physiological/
functional
Motor neurons
Sensory neurons
Interneurons
MYELINATION
What is myelination?
• Myelination is the presence of myelin around the
neuron. Myelin is not part of the structure of the
neuron but consists of a thick layer mostly made
up of lipids, present at regular intervals along the
length of the axon.
• Such fibers are called myelinated fibers.
• The water-soluble ions carrying the current across
the membrane cannot permeate this coat, it act as
an insulator, just like the white coating of the
electric wires and prevents the leakage of ions
from the neuron through its membrane.
How does the process of myelination
occur?
Myelination is carried out by myelin-forming cells
that wrap themselves around the axons in jellyroll fashion. These myelin-forming cells are
Schwann cells in the PNS (peripheral nervous
system) and the Oligodendrocytes in the
CNS (brain & the spinal cord)
Myelination
• Outside CNS
↓
Schwann cells
↓
Neurons CAN
regenerate
↓
Neurons can recover
after injury
• Inside CNS
↓
Oligodendrocytes
↓
Neurons CANNOT
regenerate
↓
Neurons DIE after injury
Outside the CNS: myelinated fibers
• Myelination is not part of the neuron but is done by the schwann
cells.
• As the diagram shows, the nerve cell invaginates the schwann
cell…
• The schwann cell wraps around the axon in concentric spirals.
• Collectively, the various layers form the myelin sheath (a patch of
myelin might be made of upto 300 layers of wrapped lipid bilayers)
Nodes of Ranvier
•
•
•
•
In myelinated nerve fiber, the
myelin sheath is not a
continuous sheath, but is
deficient at regular intervals.
Between the myelinated
regions, at the NODES OF
RANVIER, the axonal
membrane is bare and exposed
to the ECF.
Current can flow across the
membrane only at these bare
spaces to produce action
potentials.
Voltage-gated Na+ channels are
concentrated at these regions.
SALTATORY CONDUCTION
In a myelinated nerve fiber, the nerve impulse
“jumps” from node to node skipping over the
myelinated sections of the axons. This process is
called Saltatory conduction.
Basis: Saltatory conduction propagates nerve
impulse more rapidly because the nerve impulse has
to be generated only at the nodes of ranvier and not
repeatedly. Thus, it is faster.
In unmyelinated fibers, the nerve impulse is like a
grasshopper walking while in a myelinated fiber, the
nerve impulse is like grasshopper jumping.