The Nervous system

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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
• Neurons
• 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
• Microglia- phagocytic cells which respond to
  injury
• infections and disease
• 2) Macroglia- i) oligodendrocytes ii) Schwann
  cells
• iii) astrocytes

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• 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
• Support network for neurons
• 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
• 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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• Dendrites receive electrical signals
• 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
• Dendrites
• Axon
• 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
• Sensory neurons
• Motor neurons
• 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
• non-gated- always open at rest
• 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 cells 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 differenc
e that exists due to the uneven distribution
of charge is called the resting membrane
potential
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At rest, non-gated Na and K channels always open
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
The Na-K ATPase and the non-gated K and
Na channels contribute to the resting membrane
potential
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40 mV
2) Repolarization- closing of Na channels and
opening Of voltage-gated K channels
1) Depolarization Voltage-gated Na channels open
Membrane potential (Em)
0 mV
3) After hyperpolarization
-68 mV
Threshold
resting
Resting membrane potential
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• An action potential is a all or none event
• 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 cells 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 cells 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?
• 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 2
synapse
Pre-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