ELECTROCHEMICAL IMPULSE

1
ELECTROCHEMICAL IMPULSE

• Luigi Galvani, 18th century muscle of dead frog
  would twitch if electricity passed through it
• These experiments lead to lots of research in the
  field of electrical conductivity of muscle tissue
  and the body
• 1840 Emil Dubois-Reymond, German physiologist,
  made instruments that could measure current in
  nerves and muscles.
• 1906, Willem Einthoven, Dutch physiologist, made
  first electrocardiogram (ECG) that measured
  electrical impulses in the heart
• 1929 Hans Berger, German physiologist, measured
  electrical changes associated with brain
  activity, the electroencephalaograph (EEG) was
  born.
• Julius Bernstein suggested nerve impulses were an
  electrochemical message created by the movement
  of ions through the nerve cell membrane.
• 1939 Cole and Curtis, evidence to back up
  Bernstein's theory. Found rapid change in the
  potential (voltage) across a squid neuron when it
  was excited.

2
RESTING POTENTIAL

• Found that the resting potential of the nerve was
  -70 mV.
• More negative charges on the inside of the nerve
  cell than outside.
• When the nerve became excited, the potential went
  up to 40 mV and this was termed the action
  potential.
• The action potential did not last long and the
  nerve cell went back to its resting potential.
• It has been found that it is the movement of
  positive ions that causes the potential to change
  in a nerve cell, not the negative ions.
• The highly concentrated potassium ions want to
  diffuse out of the nerve cell, while the highly
  concentrated sodium ions want to diffuse in...why
  does the potential change if they both have the
  same charge?
• The resting membrane is more permeable to
  potassium diffusion than sodium diffusion.
• This means more potassium is moving out than
  sodium moving in and consequently the outside of
  the nerve cell is more positive than the inside.

3
NERVE IMPULSE

• This leads to why the resting potential is -70
  mV. There are fewer positive ions inside the
  nerve cell than outside.
• The resting membrane is said to be charged or
  polarized.
• When the nerve cell becomes excited, it becomes
  more permeable to sodium than potassium.
  Scientists believe that sodium and potassium
  gates open and close opposite of one another. As
  one type of gate opens, the other closes.
• Sodium rushes into the cell which causes a
  reversal of charge called a depolarization.
• Once the voltage becomes positive, the sodium
  gates close. That is why the max action
  potential under normal situations is only 40 mV.
• Sodium-potassium pumps actively restore the
  original resting potential by moving sodium out
  and potassium back in. This is called
  repolarization.

4
NERVE IMPULSE

• Nerve cells cannot transport a second message
  until the resting potential is reset. This is
  called the refractory period, the time it takes
  the nerve cell to be repolarized.
• Depolarization moves along the axon of the nerve
  cell in a wave.
• The critical amount of electricity that is
  required from a nerve cell to fire is known as
  the threshold level. Stimuli below this level do
  not initiate a response.
• Any amount of stimulus above the threshold level
  gets the same response from the nerve cell.
• Nerve firing is an all-or-none response. It
  fires maximally or not at all.
• Homework Handout Questions 1-15

5
SYNAPTIC TRANSMISSION

• The spaces between neurons and adjacent neurons
  or effectors are known as synapses.
• Synapses usually involve many neurons.
• The nerve impulse moves along the presynaptic
  neuron and causes chemicals called
  neurotransmitters to be released into the
  synapse. They diffuse across the synaptic cleft
  and attach to membrane receptors on the
  postsynaptic neuron. This causes the
  depolarization to continue on.
• The diffusion of neurotransmitters is a slow
  process, so a neural response that involves many
  synapses takes a relatively longer time than a
  simple reflex arc.
• Acetylcholine is an example of a
  neurotransmitter.
• It is an excitory neurotransmitter as it causes
  depolarization to continue in the postsynaptic
  neuron by opening sodium gates.

6
SYNAPTIC TRANSMISSION

• In order to return the postsynaptic neuron to
  resting potential, the sodium gates must be
  closed. This is indirectly done by
  cholinesterase, an enzyme that breaks down
  acetylcholine and thus shuts the sodium gates.
• Many neurotransmitters can have an inhibitory
  action on a neuron by making postsynaptic neurons
  more permeable to potassium. This causes even
  more potassium to leave the cell and thus causes
  even more potassium to leave the cell and thus
  causes the potential to be even more negative or
  hyperpolarized.
• Summation is when two or more neurons are needed
  to create an action potential in a further
  neuron. The sum of their firing causes an action
  potential in the postsynaptic neuron.
• Homework Questions 16 - 24