The Nervous system
Dr. Paromita Das
217 Biomedical Research Facility
Tallahassee FL
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•The human brain is a network of more than 100
billion individual nerve cells interconnected in systems
that construct our perceptions of the external world
• These nerve cells or neurons are the basic units of the
brain-they are the functional units
• Nerve cells share the same basic architecture and yet
produce complex human behavior
• This is possible due to formation of precise anatomical
circuits
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The nervous system has two classes of cells:
1)Neurons
2)Glia
• Glial cells far outnumber the neurons, they are 10-50
times more in number than neurons
• Glial cells form the supporting network for neurons
providing the brain with structure
Types of Glia:
1)Microglia- phagocytic cells which respond to injury
infections and disease
2) Macroglia- i) oligodendrocytes ii) Schwann cells
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iii) astrocytes
i) Oligodendrocytes : tightly wind around the neuron
in form of a sheath also called the myelin sheath to
form insulation- in the central nervous system
ii) Schwann cells: same function in the peripheral
nervous system
iii) Astrocytes: Star like structure-maintain K+
concentration in extracellular fluid, essential for the
function of neurons
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Functions of Glia
1)Support network for neurons
2)The two types of glia, oligodendrocytes and
Schwann cells produce myelin sheath which insulate
the neurons
3)They are scavengers-remove dead cells and debris
from the environment
4) Release nourishing factors or growth factors
which promote neuronal survival
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Neurons:
A typical neuron has 4 morphologically defined
regions:
1)Cell body (Soma)- contains nucleus and
endoplasmic reticulum
2) Dendrites
3) The axon
4) The presynaptic terminals
The soma or cell body gives rise to dendrites which
are short branching structures and a single axon
which is a long tubular structure
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1)Dendrites – receive electrical signals
2)Axon- is the conducting unit and carries electrical
signals away from the cell body to other neurons
Structure of the neuron:
Know the following terminology and function :
1)Dendrites
2)Axon
3)Synapse- site of contact of neuron with another
neuron or an effector organ
4) Myelin sheath
5) Nodes of Ranvier
6) Axon hillock
7) Presynaptic terminal
8) Postsynaptic terminal
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Neurons are functionally classified into three major
catergories
1)Sensory neurons
2)Motor neurons
3)Interneurons
1) Sensory neurons: carry information from the
periphery to the brain for purpose of perception
2) Motor neurons: carry information/commands from
the brain to muscle or glands (effector cells) to
respond to sensory perception
3) interneurons: are defined as all nerve cells that
are not specifically sensory or motor
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Simplex reflex arc- knee jerk response
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Communication in neurons:
The shape of nerve cell is specialized for reception and
transmission of information
Remember: dendrites-receive electrical signals
axon-propagates information
When a neuron is activated, an electrical impulse is
generated at the axon hillock or the initial segment
The signal is then conducted along the length of the
axon
This electrical signal is called the action potential
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The action potential is propagated to the nerve terminal
also called the presynaptic nerve terminal
This then causes the release of certain chemicals called
Neurotransmitters. The neurotransmitters are released
into the synapse.
The neurotransmitters bind to proteins on
postsynaptic nerve terminals, which further propagate
the electrical signal
At the synapse, neurons communicate with
chemical signals
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In order to understand how action potential is initiated
/generated, we need to understand the concept of
resting membrane potential
Remember: The cell membrane is selectively
permeable to ions
Ions can flow across cell membrane through 3 types
Of ion channels/proteins
1) non-gated- always open at rest
2) Ligand-gated- open in response to binding of
Neurotransmitter
3) Voltage-gated –open in response to changes in
Voltage across the membrane
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At rest (when there is no electrical and chemical
signaling) , non-gated channels are always open
The cell’s exterior has a high concentration of sodium
while the cytosol has a high concentration of potassium
Schematic on white board
So, at rest, the inside of the neuron is more negative
compared to the extracellular environment. The potential
difference that exists due to the uneven distribution of
charge is called the “resting membrane potential”
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At rest, nongated Na+ and
K+ channels
always open
K+
Na+
High Na+
Extracellular
High K+
Cytoplasm
Lot more K+ channels than Na+ channels are open.
Hence more efflux of positive ions take place. Thus net
efflux of positive charge to the outside leads to a more
negative charge in the intracellular side of cell
membrane.
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If this process were to continue for an infinite period of
time, the membrane potential would collapse. To
prevent this, the Na+-K+ ATP pump actively transports 2
K+ into the cell and pumps out 3Na+ out of the cell
K+
Na+
2K+
Na+-K+
ATPase
3Na+
The Na+-K+ ATPase and the non-gated K+ and Na+
channels contribute to the resting membrane potential
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0 mV
1) Depolarization
Voltage-gated Na+
channels open
Membrane potential (Em)
+ 40 mV
2) Repolarization- closing
of Na+ channels and
opening Of voltage-gated
K+ channels
3) After hyperpolarization
-68 mV
Resting membrane potential
Threshold
resting
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1) An action potential is a “all or none event”
2) The action potential is propagated without decrease
in amplitude of response
3) Generally lasts for about 1 millisecond (1/1000th
of a second) after which the membrane comes to
rest
4) A reduction in membrane potential to more positive
is called depolarization. Depolarization enhances
a cell’s ability to generate action potential and hence
is excitatory in nature
5) An increase in membrane potential to more negative
is called hyperpolarization which diminishes the
cell’s ability to generate action potential.
Hyperpolarization is inhibitory.
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--- +++
+++
Local currents
Local signals are propagated to the 1st node of
Ranvier where, if the signal is strong enough, it will
generate an action potential. Local signals can travel
only 1-2 mm distance after which they become weak.
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How does action potential conduct quickly down the
axon?
1)invertebrates- increase in diameter= increase in
conduction
2) mammals- myelin sheath and nodes of Ranvier,
“saltatory conduction” (Latin- Saltus= to jump)
Evolutionary advantage
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Neuron 1
Neuron 2
Voltage-gated calcium channel
synapse
Ca2+
Na+
K+
NT
current
NT
NT
NT
Pre-synaptic nerve terminal
Post-synaptic nerve terminal
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How will the next neuron respond?
• Depends on which neurotransmitter activates which
receptor on the recipient neuron
A) Excitatory neurotransmitter receptors
i) glutamate
ii) aspartate
B) Inhibitory neurotransmitter receptors
i) GABA (valium, alcohol)
ii) glycine
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