Hi, I am Shikha Laloraya, Professor of Biochemistry, at IISc.
Welcome to this lecture on nerve cells that are specialized cells of the nervous system
and that generate and conduct electrical impulses in the body.
Nerve cells or neurons are cells of the nervous system and they are specialized for communication.
The nervous system includes the central nervous system and the peripheral nervous system.
The central nervous system as you know consists of the brain and the spinal cord and this receives,
processes, and stores information. The peripheral nervous system consists of sensory,
that is carries information to the brain and the spinal cord, and motor divisions. Motor
means they carry information from the central nervous system to other parts of the body.
So, nerve cells or neurons are highly specialized cells that transmit signals throughout the body.
They are non-dividing cells that permanently stay in G0 phase throughout adult life;
these cells are specialized for communication. They generate and conduct electrical impulses
termed action potentials. These cells are very long and highly polarized.
This illustration here shows an artist's impression of nerve cells or neurons.
So, there are actually three categories of neurons in the nervous system,
one is a sensory neurons, these neurons respond to stimulus and transmit this information
to the central nervous system in the form of electrical impulses.
Motor neurons transmit impulses from the central nervous system to other tissues
and organs of the body. And interneurons are connecting neurons they transmit information
between parts of the central nervous system. For example, they receive input from sensory neurons,
integrate this information and then they transmit it to other neurons. Nerve cells are
highly differentiated and they have processes such as axons, this long process over here
is referred to as an axon, and dendrites that connect with other cells are for communication.
The dendrites are relatively shorter projections, whereas the axons can be very long.
Shown here is the structure of a neuron. Neurons consist of a cell body,
an axon and one or more dendrites.
The dendrites, they receive information from receptors or impulses from other neurons. Motor
neurons and interneurons have got many dendrites projecting from the cell body. The dendrites of
the sensory neurons interestingly connect directly to an axon. An axon is a long tube
originating from the cell body, as you can see, at a projection referred to as the axon hillock.
At the other end the axon branches into extensions termed axon terminals
and each of the axon terminal ends in a small, rounded axon bulb.
The cytoskeleton is particularly important in neurons for shaping it and it has got
a specialized organization. So, I already mentioned neurons are polarized cells and
they have these two types of processes extending from the cell body, the dendrites, which are the
short processes that receive signals from other cells and the axons, a single long process that
carries impulses to other cells such as other neurons or effector cells such as muscle cells.
So, these axons and dendrites are supported by microtubules and neurofilaments. They have a
plasma membrane and a small amount of cytoplasm as well and a lot of transport also goes on
inside these processes. Microtubules in nerve cells are different in that they are not anchored
to the centrosome. Stable microtubules in both the axons as well as the dendrites
terminate in the cytoplasm and their ends are stabilized by capping proteins.
In the axon the microtubules are oriented with their plus ends pointing towards the
tip of the axon, that is away from the cell body, which is a normal orientation
for a microtubule. In dendrites though microtubules are oriented in both directions
with their plus and minus ends pointing both towards and away from the cell body.
Microtubule associated proteins or MAPs, which are important normally for the stability
and properties of microtubules, they cap the plus and minus ends. And they also stabilize the
microtubules by binding along their length for example the tau protein in axons and
the MAP2 in dendrites. So, axons and dendrites have distinct distributions
of MAPs; axons have tau proteins but no MAP2, whereas dendrites have MAP2 but no tau proteins.
In addition, neurofilaments are intermediate filaments,
which are 10 to 12 nanometers in diameter intermediate between microtubules and actin.
In neurons, intermediate filaments are formed of these neurofilament proteins
NF-L, NF-M and NF-H, which stands for light medium and heavy forms, and alpha-internexin.
Shown here is a structure of a myelinated motor neuron of the peripheral nervous system.
Now in the nervous system only 20% of the cells are neurons, the remaining cells are
supporting cells that provide protection and support to the function of the neurons,
and these are termed neuroglial cells and these cells do not generate or transmit impulses.
So, in the peripheral nervous system, the axons of many of the neurons are enclosed and protected
by specialized neuroglial cells termed as Schwann cells, which is shown over here.
The Schwann cells, they produce a fatty insulating material which is termed myelin.
So, this myelin forms a protective layer around the axon and this layer is referred to
as a myelin sheet, and this wraps different segments of the axon, you can see here.
And between the adjacent Schwann cells
are uninsulated sections term nodes, this section or region is referred to as nodes of Ranvier.
So, myelination increases the speed and efficiency of propagation of action potential in nerve cells
and it also protects them. In the central nervous system, the sheath is
also there but it is produced by cells termed oligodendrocytes, instead of Schwann cells.
Neurons initiate action potentials. An action potential
is a sudden temporary reversal of voltage difference across the cell membrane.
The action potential, which is an electrical impulse, sweeps down the axon until it reaches
the axon terminals. The details of this process are more appropriate for
a more specialized discussion, but when the action potential reaches the axon terminals,
the information must be converted to another form, for transmission to the appropriate
target cell, which could be a muscle cell or another neuron or glands in the body.
So, this elicits the release of chemical signals termed neurotransmitters at the
junction between the two cells, which is referred to as a synapse. This process of
transmission of information from a neuron to its target is termed synaptic transmission.
For example, nerves can activate muscle cells by synaptic transmission across a
nerve and a muscle cell and also they can communicate with other nerve cells as well.
Seen here is a synapse between two neurons. At the synapse, the neuron,
which is sending the information in the blue here is referred to as a presynaptic neuron
and the cell which is receiving the information is the postsynaptic neuron.
And in addition, this fluid-filled gap between the pre and the post-synaptic
membranes of the synapse is referred to as the synaptic cleft.
Now the axon bulb of the terminus of the presynaptic neuron has got neurotransmitters,
which are stored in vesicles. Arrival of the action potential at the axon bulb
causes calcium channels to open up in the membrane and calcium enters into the axon bulb.
The entry of calcium causes these vesicles to fuse with the presynaptic membrane and therefore the
neurotransmitters are released into the synaptic cleft.
The neurotransmitters bind to receptors on the surface of the postsynaptic membrane
and this results in ion channels for example for sodium ions, to open up and
sodium ions can enter and they produce a graded depolarization of the postsynaptic membrane.
The neurotransmitters can have excitatory or inhibitory effects on the postsynaptic cell.
Some common neurotransmitters are listed here;
one of them is acetylcholine. Acetylcholine is released at neuromuscular junctions
in the autonomic nervous system and in the brain and it has excitatory action on muscles.
And it is excitatory or inhibitory elsewhere depending on the receptors that are present.
Acetylcholine travels from the neuromuscular junction and it binds to acetylcholine receptors,
which are activated and they can generate the muscle contraction.
Interestingly in an autoimmune disease Myasthenia gravis, autoantibodies
block, alter or destroy the receptors for acetylcholine at the neuromuscular junction,
which prevents the muscle contraction from occurring. These antibodies in this disease
are produced by the body's own immune system as it is an autoimmune disease.
Serotonin is another neurotransmitter which is released in areas of the brain and spinal cord,
and it has usually inhibitory effects involved in mood, sleep cycle and appetite. Dopamine is
released in areas of the brain and parts of the peripheral nervous system, and it could also be
either excitatory or inhibitory depending on the receptors present and it has a role in emotions.
Glutamate is an important neurotransmitter which is released in the areas of the brain and spinal
cord, and it is usually excitatory and it is the main neurotransmitter present in the brain.
And also, there are several other neurotransmitters not listed here.
An interesting property of the neural processes of neurons is self-avoidance. What this means is
that the dendrites and axons of the same neuron avoid making contact or connections with each
other. Axons and dendrites from the same neuron will not make the connections with each other
but prefer to make the connections with axons and dendrites from other neighbouring neurons. So,
this helps introducing the formation of purposeless synapses with itself and
also it helps in these processes to spread out more widely and innervate a larger territory.
So, how can these processes distinguish between self and non-self? This is an
interesting question. For this, there is a class of proteins term DSCAM
in Drosophila and protocadherins in vertebrates, which are very important. In Drosophila there
are nearly 30000 variants of DSCAM proteins that can be produced by alternative splicing
and when two cell surfaces express the identical isoform this results in repulsion.
So, neighbouring processes from different neurons are unlikely to
express the same DSCAM variant. So, only the processes of the same cell
repel each other whereas the process of other cells can establish connections with each other.
The protocadherin locus in vertebrates codes for 58 cadherin-like transmembrane
proteins which are expressed in different combinations in individual neurons. The
neighbouring dendrites of different neurons, they express different protocadherins
and therefore they can avoid repulsion and they can establish connections with each other.
I hope with this lecture you have learned something
about the structure, the complexity and the function of the nerve cells or neurons. Thank you.