Natural Science

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Natural Science

• 04 Nerve Impulses

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Some animations may have been used from
neurons.med.utoronto.ca/index.swf under the
conditions stated below
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• Nerve Conduction
• Nerve conduction is when nerve impulses are made
  and move along our neurons.
• None of our thoughts, or memories would be
  possible without nerve conduction.

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• Nerve conduction is an electrochemical process.
• It uses electricity made with chemical molecules.
  
• The electricity in the brain is not produced by
  electrons flowing like they do through an
  electrical wire.
• Instead, the brains electricity is caused by the
  movements of electrically charged molecules
  through the neurons membranes.

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• The membrane of a neuron, like that of any other
  cell, contains tiny holes known as channels. It
  is through these channels that charged molecules
  pass through the neural membrane.
• But the channels in neurons are so specialized
  that they can coordinate the movements of these
  charges across the membrane so as to conduct
  nerve impulses.

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• Here is a simplified sequence of events by which
  a nerve impulse is conducted.
• (see figure one at this site)

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• In the resting state the channels in the neural
  membrane create an unequal distribution of
  charges.

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• The nerve impulses open and close ion channels.
  Ions move across the membrane and change the
  electrical potential across the membrane. For a
  short time the inside becomes more positive than
  the outside.

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• Other channels quickly re-establish the resting
  potential, but next to this area the potential is
  changed. http//thebrain.mcgill.ca/flash/d/d_01/d
  _01_m/d_01_m_fon/d_01_m_fon.html

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• Ion Channels and Nerve Impulses

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• Ion transport proteins have a special role in the
  nervous systems because voltage-gated ion
  channels and ion pumps are needed to form a nerve
  impulse.

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• Ion channels use energy to build and maintain a
  concentration gradient of ions between the
  extracellular fluid and the cells cytosol.

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• Channel proteins in the plasma membrane. Membrane
  channel proteins (or channel proteins), allow the
  movement of specific ions across the cell
  membrane.

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• The concentration gradient results in electrical
  potential energy building up across the membrane,
  the basis for the conduction of a nerve impulse.

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• This concentration gradient results in a net
  negative charge on the inside of the membrane and
  a positive charge on the outside.

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• Ion channels and ion pumps are very specific
  they allow only certain ions through the cell
  membrane.

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• For example, potassium channels will allow only
  potassium ions through, and the sodium-potassium
  pump acts only on sodium and potassium ions.

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• All cells have an electrical charge which is due
  to the concentration gradient of ions that exists
  across the membrane.

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• All cells have an electrical charge which is due
  to the concentration gradient of ions that exists
  across the membrane.

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• The number of positively charged ions outside the
  cell membrane is greater than the number of
  positively charged ions in the cytosol. This
  difference causes a voltage difference across the
  membrane.

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• The number of positively charged ions outside the
  cell membrane is greater than the number of
  positively charged ions in the cytosol. This
  difference causes a voltage difference across the
  membrane.

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• The number of ions outside is greater than the
  number of ions in the cytosol. This causes a
  voltage difference across the membrane.

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• Voltage is electrical potential energy that is
  caused by a separation of opposite charges, in
  this case across the membrane.
• The voltage across a membrane is called membrane
  potential.

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• Voltage is electrical potential energy that is
  caused by a separation of opposite charges, in
  this case across the membrane.
• The voltage across a membrane is called membrane
  potential.

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• Membrane potential is the basis for the
  conduction of nerve impulses along the cell
  membrane of neurons. Ions that are important in
  the formation of a nerve impulse include sodium
  (Na) and potassium (K).

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• Membrane potential is the basis for the
  conduction of nerve impulses along the cell
  membrane of neurons. Ions that are important in
  the formation of a nerve impulse include sodium
  (Na) and potassium (K).

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• How Diffusion Works
• and quiz

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• How Facilitated Diffusion Works
• And quiz

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• How the Sodium Potassium Pump Works
• And quiz

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• Receptors Linked to a Channel Protein
• And quiz

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• For review look at 3. Ions and Ion Transport

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• Resting Potential

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• When a neuron is not conducting a nerve impulse,
  it is said to be at rest. The resting potential
  is the resting state of the neuron, during which
  the neuron has an overall negative charge.

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• In neurons the resting potential is approximately
  -70 milliVolts (mV). The negative sign indicates
  the negative charge inside the cell relative to
  the outside.

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• The reasons for the overall negative charge of
  the cell include

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• The sodium-potassium pump removes Na ions from
  the cell by active transport. A net negative
  charge inside the cell is due to the higher
  concentration of Na ions outside the cell than
  inside the cell.

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• The sodium-potasium pump is also called the
  Na/K-ATPase (fully sodium-potassium adenosine
  triphosphatase, also known as the Na/K pump, or
  sodium pump, for short).
• It is an enzyme (EC 3.6.3.9) located in the
  plasma membrane in all animals.
• As its name suggests, it uses ATP in its action.

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• Animation of the sodium-potassium pump is at
• http//highered.mcgraw-hill.com/sites/0072495855/s
  tudent_view0/chapter3/animation__sodium-potassium_
  exchange_pump__quiz_1_.html

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• Na, K-ATPase and other ion pumps must work all
  the time in our body. If they were to stop, our
  cells would swell up, and might even burst, and
  we would rapidly lose consciousness. A great deal
  of energy is needed to drive ion pumps - in
  humans, about 1/3 of the ATP that the body
  produces.

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• Since neurons purposely allow Na to flood into
  the cell and K to go out, we can assume that
  neurons use much more energy for pumping

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• Ion pumps are affected by chemical substances.
• Digitalis plants contain a substance that
  inhibits Na, K-ATPase which results in an
  accumulation of sodium ions in cells.
• Used as a pharmaceutical, it causes reinforced
  heart muscle activity.

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• Most cells have potassium-selective ion channel
  proteins that remain open all the time. The K
  ions move down the concentration gradient
  (passively) through these potassium channels and
  out of the cell, which results in a build-up of
  excess positive charge outside of the cell.

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• There are a number of large, negatively charged
  molecules, such as proteins, inside the cell.

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• For review click on 4. Resting Membrane Potential

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• Potassium (K), which is positively charged,
  passes most easily through a neural membrane in
  its resting state.
• Sodium (Na) and chloride (Cl-), which has a
  negative charge, have more difficulty passing
  through the membrane.
• Large, negatively charged molecules inside the
  neuron cannot get out but also influence the
  membranes electrical potential.
• The calcium ion (Ca) also plays an important
  role, but in the process of synaptic
  transmission.

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• Membrane ion-channel and ion-pumping proteins.

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• Action Potential

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• An action potential is an electrical charge that
  travels along the membrane of a neuron. It is
  made when a neurons membrane potential is
  changed by chemical signals from a nearby cell.

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• In an action potential, the cell membrane
  potential changes quickly from negative to
  positive as sodium ions flow into and potassium
  ions flow out of the cell through ion channels.

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• The movement of an action potential down an axon.

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• The movement of an action potential down an axon.

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• A chemical message from another nerve causes the
  sodium ion channels at one point in the axon to
  open. Sodium ions rush across the membrane and
  cause the interior of the axon to become
  positively charged (depolarized) because the cell
  now contains more positive charges. Potassium ion
  channels then open and potassium ions flow out of
  the cell, which end the action potential. The
  action potential then moves down the axon
  membrane toward the synapse.

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• The cell becomes depolarized. An action potential
  works on an all-or-nothing basis. This means, the
  membrane potential has to reach a certain level
  of depolarization, called the threshold,
  otherwise an action potential will not start.

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• This threshold potential varies, but is generally
  about 15 millivolts (mV) more positive than the
  cell's resting membrane potential. If a membrane
  depolarization does not reach the threshold
  level, an action potential will not happen.

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• The first channels to open are the sodium ion
  channels, which allow sodium ions to enter the
  cell. The resulting increase in positive charge
  inside the cell (up to about 40 mV) starts the
  action potential.

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• The first channels to open are the sodium ion
  channels, which allow sodium ions to enter the
  cell. The resulting increase in positive charge
  inside the cell (up to about 40 mV) starts the
  action potential.

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• Potassium ion-channels then open up, allowing
  potassium ions out of the cell, which ends the
  action potential.

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• Potassium ion-channels then open up, allowing
  potassium ions out of the cell, which ends the
  action potential.

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• Both of the ion channels then close, and the
  sodium- potassium pump restores the resting
  potential of -70 mV.

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• Both of the ion channels then close, and the
  sodium- potassium pump restores the resting
  potential of -70 mV.

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• The action potential will move down the axon
  toward the synapse like a wave.

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• The action potential will move down the axon
  toward the synapse like a wave.

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• Video
• http//highered.mcgraw-hill.com/sites/0072495855/s
  tudent_view0/chapter14/animation__the_nerve_impuls
  e.html

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• Quiz 1
• Quiz 4

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• In myelinated neurons, ion flows occur only at
  the nodes of Ranvier. As a result, the action
  potential signal is quickly pushed along the axon
  membrane, from node to node, rather than
  spreading smoothly along the membrane, as they do
  in axons that do not have a myelin sheath.

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• The quick pushing of the action potential signal
  along the axon membrane, from node to node, is
  called saltatory conduction.

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• Comparison
• http//faculty.stcc.edu/AandP/AP/AP1pages/nervssys
  /unit11/saltator.htm

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• Saltatory conduction
• Saltatory conduction is faster than smooth
  conduction. Some typical action potential
  velocities

Fiber Diameter AP Velocity
Unmyelinated 0.2-1.0 micron 0.2-2 m/sec
Myelinated 2-20 microns 12-120 m/sec
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• The action potential is increased at the Nodes of
  Ranvier. This is due to clustering of Na and K
  ion channels at the Nodes of Ranvier.

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• Unmyelinated axons do not have Nodes of Ranvier
  and ion channels in these axons are spread over
  the entire membrane surface.

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• Video-
• The Schwann Cell and Action Potential
• http//www.knowmia.com/watch/lesson/433

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• For review go to this page and click on 5. Action
  Potential

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• Communication Between Neurons

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• Neurons communicate with each other at
  specialized junctions called synapses. Synapses
  are also found at junctions between neurons and
  other cells, such as muscle cells.

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• Neuromuscular junction
• Axon
• Synaptical junction
• Muscle fiber
• Myofibrils

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• There are two types of synapses
• - chemical synapses use chemical signaling
  molecules as messengers
• - electrical synapses use ions as messengers
• We will look at chemical synapses.

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• The axon terminal of one neuron usually does not
  touch the other cell at a chemical synapse.
  Between the axon terminal and the receiving cell
  is a gap called a synaptic cleft.

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• The transmitting cell is called the presynaptic
  neuron, and the receiving cell is called the
  postsynaptic cell or if it is another neuron, a
  postsynaptic neuron.

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• The brain has a huge number of synapses. Each of
  the 1012 (one trillion) neurons, including glial
  cells, has on average 7,000 synaptic connections
  to other neurons.

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• It has been estimated that the brain of a three
  year-old child has about 1016 synapses (10
  quadrillion). This number declines with age, and
  levels off by adulthood.

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• An adult has between 1015 and 5 x 1015 synapses
  (1 to 5 quadrillion).

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• Neurotransmitter Release

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• A neurotransmitter is a chemical message that is
  used to relay electrical signals between a neuron
  and another cell.

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• Neurotransmitter molecules are made inside the
  presynaptic neuron and stored in vesicles at the
  axon terminal.

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• Some neurons make only one type of
  neurotransmitter, but most neurons make two or
  more types of neurotransmitters.

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• When an action potential reaches the axon
  terminal, it causes the neurotransmitter vesicles
  to fuse with the terminal membrane, and the
  neurotransmitter is released into the synaptic
  cleft.

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• The neurotransmitters then diffuse across the
  synaptic cleft and bind to receptor proteins on
  the membrane of the postsynaptic cell.

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• The synaptic cleft. Neurotransmitter that is
  released into the synaptic cleft diffuses across
  the synaptic membrane and binds to its receptor
  protein on the post synaptic cell.

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• Neuromuscular junction
• presynaptic terminal
• sarcolemma
• synaptic vesicles
• Acetylcholine receptors
• mitchondrion

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• At a synapse, neurotransmitters are released by
  the axon terminal. They bind with receptors on
  the other cell.

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• The location of synapses. Synapses are found at
  the end of the axons (called axon terminals) and
  help connect a single neuron to thousands of
  other neurons. Chemical messages called
  neurotransmitters are released at the synapse and
  pass the message onto the next neuron or other
  type of cell.

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• Neurotransmitter Action

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• Many types of neurotransmitters exist.
• Neurotransmitters can have an excitatory or
  inhibitory effect on the postsynaptic cell.

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• Common Neurotransmitters and Their Receptors

Name Receptor Name and Type Ions Involved
Glutamate (glutamic acid) Glutamate receptors (ligand-gated ion channels and G protein-coupled receptors) Ca2, K, Na
Acetylcholine Acetylcholine receptors (ligand-gated ion channel) Na
Norepinephrine (noradrenaline) Adrenoceptors (G protein-coupled receptors) Ca2
Epinephrine (adrenaline) Adrenoceptors (G protein-coupled receptors) Ca2
Serotonin (5-hydroxytryptamine) 5-HT receptors 5-HT3 is a ligand-gated ion channel 5-HT1, 5-HT2, 5-HT4, 5-HT5A, 5-HT7 are G protein-coupled receptors K, Na
Gamma-aminobutyric acid (GABA) GABAA and GABAC (ligand-gated ion channels) GABAB (G protein-coupled receptors) Cl- K
Histamine Histamine receptors (H1, H2, H3, H4) (G protein-coupled receptors)
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• An excitatory neurotransmitter initiates an
  action potential and an inhibitory
  neurotransmitter prevents one from starting.

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• Glutamate is the most common excitatory
  transmitter in the body while GABA and glycine
  are inhibitory neurotransmitters.

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• Acetycholine

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• The release of acetylcholine, an excitatory
  neurotransmitter causes an inflow of positively
  charged sodium ions (Na) into the postsynaptic
  neuron.

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• This inflow of positive charge causes a
  depolarization of the membrane at that point. The
  depolarization then spreads to the rest of the
  postsynaptic neuron.

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• The effect of a neurotransmitter also can depend
  on the receptor it binds to. So, a single
  neurotransmitter may be excitatory to the
  receiving neuron, or it may inhibit such an
  impulse by causing a change in the membrane
  potential of the cell.

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• Synapses too can be excitatory or inhibitory and
  will either increase or decrease activity in the
  target neuron, based on the opening or closing of
  ion channels.

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• Neurotransmitter receptors can be gated ion
  channels that open or close through
  neurotransmitter binding or they can be
  protein-linked receptors.

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• Protein-linked receptors are not ion channels
  instead they cause a signal transduction that
  involves enzymes and other molecules (called
  second messengers) in the postsynaptic cell.

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• Video-
• http//highered.mcgraw-hill.com/sites/0072495855/s
  tudent_view0/chapter14/animation__transmission_acr
  oss_a_synapse.html

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• Quiz 3

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• Neurotransmitter Reuptake

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• Many neurotransmitters are removed from the
  synaptic cleft by neurotransmitter transporters
  in a process called reuptake. Reuptake is the
  removal of a neurotransmitter from the synapse by
  the pre-synaptic neuron.

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• Reuptake happens after the neurotransmitter has
  transmitted a nerve impulse.
• Without reuptake, the neurotransmitter molecules
  might continue to stimulate or inhibit an action
  potential in the post-synaptic neuron.

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• A synapse before and during reuptake.

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• Re-uptake is carried out by transporter proteins
  which bind to the released transmitter and
  actively transport it across the plasma membrane
  into the pre-synaptic neuron.

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• Video-
• Synaptic Transmission
• http//www.youtube.com/watch?v9vZI_XKca88feature
  related

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• Reuptake of neurotransmitter as a medical target.

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• The reuptake of neurotransmitter is the target of
  some types of medicine.
• For example, serotonin is a neurotransmitter that
  is produced by neurons in the brain.

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• Serotonin is believed to play an important role
  in the regulation of mood, emotions, and
  appetite.

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• After release into the synaptic cleft, serotonin
  molecules either attach to the serotonin
  receptors (5-HT receptors) of the post-synaptic
  neuron, or they attach to receptors on the
  surface of the presynaptic neuron that produced
  the serotonin molecules, for reuptake.

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• Reuptake is a form of recycling because the
  neuron takes back the released neurotransmitter
  for later use.

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• Medicines called selective serotonin reuptake
  inhibitors (SSRIs) block the reuptake of the
  neurotransmitter serotonin.
• This blocking action increases the amount of
  serotonin in the synaptic cleft, which prolongs
  the effect of the serotonin on the postsynaptic
  neuron.

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• Some scientists hypothesize that decreased levels
  of serotonin in the brain are linked to clinical
  depression and other mental illnesses.
• So SSRI medications such as sertraline and
  fluoxetine are often prescribed for depression
  and anxiety disorders.

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• Another way that a neurotransmitter is removed
  from a synapse is digestion by an enzyme.

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• L-Monoamine oxidases (MAO) (EC 1.4.3.4) are a
  family of enzymes that catalyze the oxidation of
  monoamines
• They are found bound to the outer membrane of
  mitochondria in most cell types in the body.
• They belong to the protein family of
  flavin-containing amine oxidoreductases.

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• In humans there are two types of MAO MAO-A and
  MAO-B.
• Both are found in neurons and astroglia.
• Outside the central nervous system
• MAO-A is also found in the liver,
  gastrointestinal tract, and placenta.
• MAO-B is mostly found in blood platelets.

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• They are well known enzymes in pharmacology,
  since they are the substrate for the action of a
  number of monoamine oxidase inhibitor drugs.
• Both MAOs are inactivate monoaminergic
  neurotransmitters, for which they display
  different specificities.

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• Serotonin, melatonin, norepinephrine, and
  epinephrine are mainly broken down by MAO-A.
• Phenethylamine and benzylamine are mainly broken
  down by MAO-B.
• Both forms break down dopamine, tyramine, and
  tryptamine equally

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• MAO dysfunction (too much or too little MAO
  activity) is thought to be responsible for a
  number of psychiatric and neurological disorders.

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• For example, unusually high or low levels of MAOs
  in the body have been associated with depression,
  schizophrenia, substance abuse, attention deficit
  disorder, migraines, and irregular sexual
  maturation.

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• Monoamine oxidase inhibitors are one of the major
  classes of drug prescribed for the treatment of
  depression.
• MAO-A inhibitors act as antidepressant and
  antianxiety agents, whereas MAO-B inhibitors are
  used alone or in combination to treat Alzheimers
  and Parkinsons diseases.

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• MAO is also heavily depleted by use of tobacco
  cigarettes.

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• Neurotransmitters and Disease

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• Before we begin
• We often think of disease (or the cause of
  disease) as a bad functioning of the cells or
  systems.
• But the proper functioning may be the underlying
  cause of disease.

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• Before we begin
• We often think of disease (or the cause of
  disease) as a bad functioning of the cells or
  systems.
• But the proper functioning may be the underlying
  cause of disease.
• When we look at diseases which are caused by
  compulsive or addiction related behaviors,
  remember that they may be based on the proper
  functioning of neurotransmitters or their
  receptors.

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• Diseases that affect nerve communication can have
  serious consequences.

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• A person with Parkinson's disease has a
  deficiency of the neurotransmitter dopamine.
• Progressive death of brain cells that produce
  dopamine increases this deficit, which causes
  tremors, and a stiff, unstable posture.

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• L-dopa is a chemical related to dopamine that is
  given as a medicine to ease some of the symptoms
  of Parkinsons disease.
• The L-dopa acts as a substitute neurotransmitter,
  but it cannot reverse the disease.

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• The soil bacterium Clostridium tetani produces a
  neurotoxin that causes the disease tetanus.
• The bacteria usually get into the body through an
  injury caused by an object that is contaminated
  with C. tetani spores.

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• The C. tetani neurotoxin blocks the release of
  the neurotransmitter GABA, which causes skeletal
  muscles to relax after contraction.
• When the release of GABA is blocked, the muscle
  tissue does not relax and remains contracted.

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• Tetanus can be fatal when it affects the muscles
  used in breathing.
• Tetanus is treatable and can be prevented by
  vaccination.

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• Electrical Synapses

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• An electrical synapse is a link between two
  neighboring neurons that is formed at a narrow
  gap between the pre- and postsynaptic cells
  called a gap junction.

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• At gap junctions, cells are about 3.5 nm from
  each other, a much shorter distance than the 20
  to 40 nm distance that separates cells at
  chemical synapses.

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• Electrical synapses. The image at the bottom
  right shows the location of gap junctions between
  cells.

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• Each gap junction has many channels which cross
  the plasma membranes of both cells. Gap junction
  channels are wide enough to allow ions and even
  medium sized molecules like signaling molecules
  to flow from one cell to the next.

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• For example, when positive ions move through the
  channel into the next cell, the extra positive
  charges depolarize the postsynaptic cell.

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• Signaling at electrical synapses is faster than
  the chemical signaling that occurs across
  chemical synapses.

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• Ions directly depolarize the cell without the
  need for receptors to recognize chemical
  messengers, which occurs at chemical synapses.

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• Such fast communication between neurons may
  indicate that in some parts of the brain large
  groups of neurons can work as a single unit to
  process information.

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• Electrical synapses are numerous in the retina
  and cerebral cortex.

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