NERVE CELLS AND NERVE IMPULSES STRUCTURE

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NERVE CELLS AND NERVE IMPULSESSTRUCTURE
FUNCTIONS OF THE CELLS OF THE NERVOUS SYSTEM
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• The most striking differences between humans and
  other animals are in the size and complexity of
  our brain
• The human brain possesses billions of neurons,
  each of which may communicate with thousands of
  other neurons in information-processing networks
  that make even the most elaborate computer look
  primitive

• Photograph of Einsteins Brain

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Color scanning electron micrograph of brain
cells. Neurons large cells with long, thin
branches Glial others (Photo Researchers, Inc.)

• We will try and understand how the brain
  accomplishes all of these tasks but understanding
  the physical structures that make up the system-
  the Cells
• There are two basic types of cells in the brain,
  Glial cells and Neurons

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Glial cells

• These make up 90 of the cells in the brain
• Support, nurture and protect neurons
• There are several types

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Glial Cells Astrocyte

• Astrocytes
• Clean up waste in the brain-clean up dead neurons
  after TBI, stroke, etc-Once the dead tissue is
  removed, a framework of astrocytes fills in the
  area- scar tissue

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Glial CellsRadial Glia

• Radial glia
• type of astrocyte which guides the migration of
  neurons and growth of their axons and dendrites
  during embryonic development

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Glial CellsOligodendrocyte

• Oligodendrocytes
• principle function is to provide support to axons
  and produce the myelin sheath surrounds and
  insulates certain neurons
• Schwann Cells produce myelin

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Glial Cells

• Traditionally, glial cells were not believed to
  play a role in function. However, newer research
  has suggested that they may play a critical role
  in memory formation!!!

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The Neuron

• Neurons will vary in shape depending upon the
  shape of their cell body and the dendritic
  branching that occurs- over 1000 basic shapes of
  neurons glial cells have been identified in
  the brain
• Neurons vary greatly in size, shape and function.
  More recent studies have shown that their shape
  changes throughout life as a function of learning
  and experience- plasticity

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Th
Neurons primarily use glucose, a simple sugar,
for their nutrition (fuel) A typical neuron has
1,000-10,000 synapses, thus, it is connected to
this many other neurons This means there are
over 1000 trillion connections in the brain- more
than there are stars in the universe!
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• In many ways neurons are the same as typical
  cells of the body. We will be focusing on the way
  that they are different 
• 1. They process information. This information
  processing is accomplished electrochemically.
• Electrical charge Action Potential (5 of AA
  battery)
• Chemical Neurotransmitter
• 2. They do not regenerate to the extent that
  other cells do
• The issue of Stem Cells

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Types of Neurons

• Neurons can be classified based on their function
  as Motor, Sensory, or Interneurons
• Sensory specialized at one end to be highly
  sensitive to a particular type of stimulation
  (i.e. visual, auditory, touch)- different kinds
  of sensory neurons have different
  structures(input)

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• Motor receives excitation from other neurons
  and conducts impulses from its soma in the
  spinal cord to the muscles and glands (output)
• Interneuron connects sensory and motor neurons

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• Neurons have all the parts of a typical cell
  (Nucleus, Cell Membrane, Ribosomes, etc)- We will
  focus on the parts of the neuron that are
  different than the traditional cell

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• Soma (Cell Body) contains the nucleus and other
  structures vital for the life processes of the
  cell
• Dendrites receives the message for the
  postsynaptic neuron from the presynaptic neuron
  neurons can have any number of these (Antenna)
• Axon conducts the action potential (Wire)
• Neurons generally have no more than one axon,
  which may have branches- some can be amazingly
  long (i.e. from your spinal cord to your feet)
  or short
• Myelin sheath insulating material that covers
  some neurons, increasing their speed of
  conductivity-has periodic breaks called the Nodes
  of Ranvier

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Neurons

• Presynaptic terminal (Terminal buttons) hold
  and release the neurotransmitter into the synapse
• Diffusion movement of ions from an area of
  higher co
• Synapse the junction where the neurons meet and
  into which neurotransmitters are released

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The Nerve ImpulseThe action potential
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• The neuron works like an electric battery and
  thus, works by changes in its voltaage
• A neuron fires, when, due to stimulation from
  another neuron (presynaptic neuron), a
  postsynaptic neurons membrane potential passes
  the threshold of excitation, causing another
  action potential to form
• The membrane of the cell is specialized to
  control the exchange of molecules between the
  inside and outside of the cell-this exchange is
  what is responsible for the formation of the
  action-potential
• The best way to understand how this happens is to
  start with understanding the anatomy and
  physiology of the neuron at rest, when it is
  not active

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Ions (Na, K, Cl-) are what are responsible for
the initiation and transmission of an action
potential The Resting Potential (-70 mv) is the
result of the Concentration Gradient of the ions
that are inside and outside the cell membrane
Compared with its surroundings, the inside of
a resting neuron has a lower concentration of
Na neurons and a higher concentration of K
neurons The outside has a higher concentration
of Na neurons and a total number of positively
charged ions
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• Thus, the -70mv represents a measurement of the
  inside charge relative to the outside
• Sodium-Potassium pump helps maintain this ionic
  imbalance by actively transporting three sodium
  ions out for every two potassium ions it pumps in
• (Cl-) chloride can pass through open leak
  channels at rest, thus entering the cell
• Ca2- acts as a powerful intracellular signaling
  molecule once it enters the cells through its
  ion channels

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Neuron at Resting Potential

• Ion channels, specialized proteins embedded in
  the membrane which control the rate of passage
  for certain ions
• These channels are often gated, opened or
  closed
• When open, the ions can enter and pass through
  their channels by diffusion

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The Action Potential

• The resting potential remains stable until the
  neuron is stimulated, which takes place at the
  synapses by Neurotransmitters
• When the neurotransmitter binds with the
  receptor of the post-synaptic dendrite, it
  results in the opening of the Na channel,
  starting a cascade of events which lead to the
  action potential being propagated down the axon

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Action Potential (Cont.)

• The sodium ion channel opens and Sodium ions
  rush in resulting in an active change in the
  charge, becoming more positive (Step 1)
  Depolarization
• Then, the K channels begin to open K begins
  to flow out (Step 2)
• When the action potential reaches its peak
  (1msec) 40mV, the sodium channels close. This
  stops the flow of positive ionsfrom outside to
  inside of the cell. However, the K channels are
  still open, thus, the K ions are still flowing
  out of the cell, leading to a plunge of the
  membrane potential (Step 3) Hyperpolarization
• When the membrane potential reaches its resting
  state, the K channels close (Step 4)
• Now the Sodium-potassium pump transports Na out
  of the cell and K into the cell, so that it is
  ready for the next action potential (Step 5)

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• In many cases, the membrane potential becomes
  even more negative than the resting potential for
  a brief period, this state is called when the
  cell is hyperpolarized
• Refractory period after opening, the sodium
  channels become inactivated as the potential
  moves positive. They cannot open again until they
  are reset by the hyperpolarization period at
  the end of the action potential.
• Prevents back propagation of the action
  potential and thus, that it moves in one direction

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• In order for the neuron to function properly,
  these steps must occur exactly!!! Any
  disruption of the process, disrupts the
  functioning of the neuron, and thus, experience
  (i.e. anesthetics)
• Local Anesthetics (i.e. Novocain)- attach to the
  Na channels of the membrane, preventing Na ions
  from entering- block action potentials in the
  affected areas!
• General anesthetics decrease brain activity by
  opening certain K channels more widely than
  normal- K exits as fast as Na enters,
  preventing most action potentials

• The figure shows a biological membrane at its
  melting point. The green molecules are liquid,
  and the red are solid. Molecules of anesthetics
  reduce the number of red areas so that the sound
  pulse can no longer transport its signal. The
  nerve is anesthetised. (Credit Illustration by
  Heiko Seeger, PhD.)

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• Propagation of the action potential the
  transmission of it down the axon- travels at
  speeds up to 220 mph!
• Unmyelinated axons The action potential than
  travels down the length of the axon in a
  wave-like fashion As a section of the axon
  undergoes the above process, it increases the
  membrane potential of the neighboring section and
  causes it to spike. The action potential gives
  birth to a new action potential at each point
  along the axon. This is like a mini-chain
  reaction which continues down the length of the
  axon until it reaches the synapse. Dominoes
• Myelinated axons myelin is a fatty outer layer
  which insulates and protects the axon- made up of
  oligodendrocytes and schwann cells- A process
  called Saltatory Conduction results in faster
  transmission

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• Saltatory conduction the action potential is
  regenerated only at the nodes- the jumping of
  action potentials from node to node is
  considerably faster than the regeneration of an
  action potential at each point along the axon,
  hence neurons without myelin
• M.S. destroys the myelin sheath, slowing down
  action potential or stopping them completely
• A myelinated axon sodium channels almost
  exclusively at its nodes, thus, it lacks these
  in the areas in between the nodes. When the
  myelin is destroyed, the action potential dies
  between the nodes

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All or None Principle

• All or none principle if a neuron fires, then
  the action potential is the same regardless of
  the amount of excitation received from the inputs
• Thus, neurons do not code information by the
  strength of the signal, but by the rate of firing
  of the neuron (e.g. muscular contraction higher
  rate means stronger response)- thus, a stronger
  stimulus means a higher rate of responding, not a
  stronger action potential
• Changes in the rate of firing are often studied
  (i.e.LTP)