Pinel Chapter 4 Lecture 4a

THE GENERATION OF ACTION POTENTIALS

Lecture 4a Outline
1.      Measuring the Membrane Potential
2.      Squid Giant Motor Neurons
3.      Resting Membrane Potential
4.      Four Factors That Underlie the Resting Potential
     a.      Random Motion
     b.      Electrostatic Pressure
     c.      Differential Permeability of the Membrane
     d.      SodiumPotassium Pumps
5.      Postsynaptic Potentials
6.      Generation of Action Potentials
7.      Ionic Events Underlying Action Potentials
Relevant Video Segments
Segment 25.             Nervous System as a Communication Network.
Segment 12.             Myelination.

Lecture 4a

1.      Measuring the Membrane Potential     CH04F02.BMP  Display Meaasure of  Membrane Potential

Understanding how neurons work comes from study of membrane potential
  (i.e. the difference in electrical charge between inside and outside of  the neuron);
To record a membrane potential two electrodes are needed:   intracellular and extracellular
     Intracellular electrodes must be microelectrodes usually made from salinefilled micropipettes by heating and pulling apart a fine glass tube with an electrode puller

2.     Squid Giant Motor Neurons

What we know about the Resting Potential with respect to neural function was revealed by          studying the squid giant motor neuron;

   a. its axon is 0.5 mm diameter, compared to 0.015 mm in mammalian motor neurons;
   b. motor neurons are multipolar neurons that terminate on a muscle
    (remember these are squid "giant motor neurons", not "giant  squid" motor neurons;
    c.  fortunately, most of what has been discovered about squid giant motor neurons holds for other multipolar neurons






















Resting Membrane Potential  CH04F03.BMP  Ion Concetration in RP


     When both electrodes are outside a neuron, the difference between the  electrical potentials at their tips is zero;  CH04F04.BMP  Ionic Distribution across Membrane

  as the intracellular electrode penetrates the neuron, the potential jumps to about 70 millivolts
       (the inside is 70 millivolts less than the outside)
               this is the resting potential of the neuron
     RP results from the fact that positively and negatively charged ions become distributed unequally on the two sides of the neural membrane:
    (1) the concentration of Na+ is higher outside,
    (2) the concentration of Cl is higher outside,
    (3) the concentration of K+ is higher inside, and
    (4) various negatively charged protein ions are trapped inside








4.      Four Factors interact to produce the Resting Potential:     CH04F04.BMP



Two passive (nonenergyconsuming) factors act to distribute ions equally across the      membrane (homogenizing factors):

Random Motion (passive) ions in solution are normally in random motion thus,
any time there is an accumulation of a particular class of ions in one area, the probability is increased that random motion will drive concentrated ions out of this area
    and probability is decreased that random motion will drive more ions into the area
     2. Electrostatic Pressure (passive)like charges repel and opposite charges attract
     therefore electrostatic pressure disperses any accumulation of  positive or negative                 charges in an area

  b.  Additionally, One passive and one active factor act to distribute ions unequally across the membrane
     1.Differential Permeability of the Membrane (passive)ions pass through the cell membrane at special pores called ion channels
When neurons are at rest, the membrane is:
      a) totally resistant to the passage of protein ions,
      b) extremely resistant to the passage of Na+ ions,
      c) moderately resistant to the passage of K+ ions,
      d) and only slightly resistant to the passage of Clions

       2.  SodiumPotassium Pumps (active) active (energyconsuming) mechanisms in the neural membrane continuously transfer Na+ ions out of the neuron and K+ ions in

The four factors above combine to generate a Resting Potential of 70 mV.

5.      Postsynaptic Potentials - How signals are created in neurons:
     a.  First,  postsynaptic potentials (PSPs) are produced by the action of  neurotransmitters released by presynaptic neurons;
      input across some synapses is excitatory,
      across others input is inhibitory
OVERHEAD T20; postsynaptic potentials

Excitatory postsynaptic potentials (EPSPs) are depolarizations;
  i.e. they increase the likelihood that a neuron will fire

Inhibitory postsynaptic potentials (IPSPs) are hyperpolarizations;
  they decrease the likelihood that a neuron will fire
PSPs have three important properties:

  (1) they are graded - i.e. their amplitude proportional to intensity of input
    i.e. stronger stimuli produce bigger EPSPs and IPSPs);
  (2) they are transmitted decrementally (as they passively spread from their site of generation (a synapse), they get weaker as they go, like sound through air);

  (3) they are transmitted rapidly (like electricity through a cable, so rapidly that transmission is usually regarded as being instantaneous)





6.      Generation of Action Potentials

     A neuron's action potentials (APs; firing; spikes) are triggered at the axon hillock when neuron is depolarized to the point that the membrane potential at the hillock reaches about -65 mV;
     this is the threshold of excitation for many neurons (umbral)

 Unlike EPSPs and IPSPs, APs are not graded;

     they are allornone (they occur full blown or not at all)

Most neurons receive hundreds of synaptic contacts;

  what happens at any one synapse has very little effect on the firing of the neuron.     

Whether or not a neuron fires is determined by the adding together (integration) of what goes on at many presynaptic neuron synapses

  there are two kinds of neural integration: CH04F06.BMP
  (1) spatial summation (EPSPs + EPSPs; IPSPs + IPSPs; EPSPs + IPSPs) and




















 (2) temporal summation (EPSPs + EPSPs; IPSPs + IPSPs)  CH04F07.BMP



 In a functioning neuron, spatial and temporal summation go on continuously;

Synapses closer to the axon hillock have a larger effect on firing due to the decremental transmission of postsynaptic potentials

7.  Ionic Events Underlying Action Potentials  CH04F08.BMP

     When threshold of excitation is reached, voltagegated Na+ channels open momentarily, and Na+ ions rush into the neuron under tremendous pressure
due both to concentration gradient and the electrostatic gradient;
     this drives the membrane potential to about +50 millivolts at which point K+ channels open momentarily and K+ ions are driven out of the neuron by the +50 millivolt charge and by their high internal concentration     

     this repolarizes the neuron and leaves it slightly hyperpolarized for a few milliseconds;
The +50 millivolt charge also draws some Clions into the neuron because only a few ions adjacent to the membrane are involved in the generation of an action potential, the resting potential is readily reestablished by the random motion of ions

Conclusion

 Today we have seen how action potentials are generated;
 Next lecture we will see how action potentials travel along an axon and trigger the release of neurotransmitter molecules from its buttons;
 We will also see how neurotransmitter molecules travel across the synapse and elicit postsynaptic potentials;
 thus returning us to the starting point of today's lecture and completing the cycle of neural conduction and synaptic transmission.


Pinel Lecture 4b  -- XONAL CONDUCTION AND SYNAPTIC TRANSMISSION

Outline:
1.     Review
2.     Conduction of Action Potentials
3.     Refractory Periods
4.     Synaptic Contacts and Transmission
a.     Diversity of Synaptic Contacts
b.     Synaptic Transmission
5.     Neurotransmitters and Receptors
a.     Amino Acid Neurotransmitters
b.     Monoamine Neurotransmitters
c.     Acetylcholine
d.     Soluble Gas Neurotransmitters
e.     Neuropeptide Neurotransmitters

Lecture Notes

1.     Review

     You have already learned that neurotransmitters induce EPSPs and IPSPs on the postsynaptic membranes of the dendrites and cell body;
     that these graded potentials are transmitted instantly and decrementally to the axon hillock;
     that these EPSPs and IPSPs are integrated (summated); and that if the sum of the EPSPs and IPSPs at the hillock is a depolarization great enough to bring the membrane potential to its threshold of excitation, an all-or-none action potential is generated.

     Today, you will learn how action potentials travel along the axon and how signals produced by them are transmitted across synapses.

2  Conduction of Action Potentials

once an AP is generated at the axon hillock, it is transmitted along the axon;

the purpose of axons is to transmit APs from the soma to the terminal buttons of the neuron

transmission of all-or-none APs along an axon is not like the transmission of graded postsynaptic potentials;

transmission of EPSPs and IPSPs is passive (like electricity through a cable), thus it is instantaneous and decremental;
whereas transmission of an AP along an axon is active, thus it is slower and nondecremental

to understand axonal transmission, think of the axon as a row of voltage-gated sodium channels

     when voltage-gated sodium channels on the hillock membrane open, Na+ ions rush in and a full blown AP is generated;

     the electrical disturbance thus created is transmitted passively to the next sodium channels along the axon, and like trap doors they open and another full-blown potential is generated there--and so on.

     in reality, the sodium channels are so tightly packed that it is best to think of APs as waves of depolarization spreading down an axon

-     because the conduction of APs is an active process, there are two key differences between the conduction of APs and the conduction of PSPs:
(1) AP conduction is slower; and
(2) AP conduction is nondecremental; that is,
APs arriving at the end of the axon are just as large as those generated at the hillock

-     APs also spread from the hillock back through the cell body and dendrites, but because the ion channels in the cell body and dendrites are chemical-gated rather than voltage-gated, transmission of action potentials through cell bodies and dendrites is passive

     many of the neurons in the CNS have no axons; they are interneurons; they have no action potentials; they are small, difficult to study, and not well understood; for example, in the human visual system, incoming signals are transmitted through four layers of neurons before reaching one with an axon

     Transmission in the normal direction (orthodromic stimulation), from the hillock to the buttons, is called anterograde transmission;
however, if the buttons are electrically stimulated, APs can be generated, and these are actively transmitted back to the hillock; this is called retrograde transmission (antidromic stimulation)

-     I mentioned previously in the course that many axons are myelinated by oligodendroglia in the CNS and by Schwann cells in the PNS;

     myelination insulates the semipermeable axon membrane blocking the flow of ions through the axon at all but the nodes of Ranvier; paradoxically this actually improves transmission

-     in myelinated axons, APs travel passively (decrementally and rapidly) between the nodes of Ranvier but at each node there is a "pause" while a full-blown AP is generated

-     this is called saltatory conduction ("saltatory" means to skip or jump);

     because much of the transmission of APs in myelinated axons is passive (from node to node), transmission in myelinated axons is faster and it requires less energy

     Larger axons conduct faster than smaller; myelinated axons conduct even faster:

(1) large myelinated mammalian axon (e.g., axons of sensory and motor neurons; diameter  0.015 mm) transmit at about 100 meters per second (about 224 miles per hour),

(2) small unmyelinated mammalian axons conduct at about 1 meter per second (2.24 mph); and

(3) squid giant motor axons (diameter = 0.5 mm), which are unmyelinated, conduct at 25 meters per second (56 mph)


3.     Refractory Periods

     For a brief period of time (about 1 millisecond) after the onset of an action potential, another action potential cannot be elicited at the same neuron, no matter how intense the stimulation; this period is called the absolute refractory period

     -a wave of "absolute refractoriness" spreads down the axon behind the action potential; a part of the membrane that has just participated in the transmission of an action potential cannot fire again until it has been repolarized;

     this keeps the spread of the action potential down the axon from reversing because the absolute refractory period is about 1 millisecond, neurons cannot normally fire more frequently than 1,000 times per second

     after the absolute refractory period, there is a period of time during which the neuron can fire again, but it takes a greater than normal level of stimulation to do it;
this is called the relative refractory period the relative refractory period is the reason why more intense stimulation produces more rapid firing


4.     Synaptic Contacts and Transmission

     a.     The Diversity of Synaptic Contacts - don't think for a minute that all synapses are the same.

-     in addition to axosomatic and axodendritic synapses there are: (1) axoaxonic synapses, (2) dendrodendritic synapses, (3) dendroaxonic synapses, (4) synapses between the main shafts of axons, (5) nondirected synapses

     Some dendrodendritic synapses are reciprocal (they can transmit in either direction)

     Many neurons have auto receptors in their presynaptic membranes; these are stimulated by the neuron's own neurotransmitter and are thought to mediate negative feedback


-     some synapses occur on little buds on dendrites; these buds are called dendritic spines; other dendritic synapses occur right on the dendrite shaft

axoaxonic synapses mediate presynaptic inhibition;

          postsynaptic inhibition is mediated by axodendritic and axosomatic synapses

-     most synapses that are discussed in textbooks are directed synapses (synapses where the site of release and the target site are in close apposition)

-     there are also nondirected synapses; for example, some presynaptic axons have a string-of-beads appearance and the neurotransmitter is widely dispersed from each bead to many targets in the general area; this arrangement is common for monoamines

-     in a sense, the neuroendocrine system has the most nondirected synapses; neurotransmitters (i.e., neurohomones) are released into the circulatory system by neurons and dispersed throughout the body










Synaptic Transmission (use Digital Image Archive, Figure CHO4FO9.BMP)

-     much of what we know about synaptic transmission comes from the study of neuromuscular junctions;

     muscle cells are conveniently large and, unlike neurons, most muscle cells receive only one synapse;

     acetylcholine is the neurotransmitter at neuromuscular junctions;

     the arrival of an AP at a terminal button opens voltage gated calcium channels in the button membrane, and Ca+ ions enter the button

     the entry of the Ca+ ions causes the synaptic vesicles to fuse with the presynaptic membrane and empty their contents into the synaptic cleft--a process called exocytosis

     Within the synapse the released neurotransmitter substance moves to the post-synaptic membrane; there it binds to receptors for it in the postsynaptic membrane;


     there are specific receptors for each neurotransmitter;
     as the synaptic vesicles combine with the presynaptic membrane during exocytosis, the button gets larger and larger, but bits of presynaptic membrane break off back into cytoplasm;

     in some cases the membrane bits are turned into vesicles and filled with neurotransmitter right in the button by a cisterna;

     in other cases, the bits are transported back to the cell body for recycling by the Golgi apparatus (use Digital Image Archive, Figure CH03F06.BMP)

     this is called reuptake and is fundamentally important in neural function (e.g. SSRI)

-     the binding of the neurotransmitter to its receptors can influence the postsynaptic neuron in one of two fundamentally different ways: (use Digital Image Archive, Figure CHO4F13.BMP)


     (1) it can directly influence chemical-gated channels in the postsynaptic membrane and induce brief EPSPs or IPSPs; or
     (2) it can trigger chemical reactions in the cytoplasm of the postsynaptic neuron that lead to the production of chemicals, called secondary messengers (e.g., cyclic AMP), which can have more enduring and far-reaching effects on the sensitivity of the neuron














-     neurotransmitters are deactivated in the synapse by one of two mechanisms: (use Digital Image Archive, Figure CHO4F14.BMP)


     (1) first, it was found that acetylcholine is broken down  in the synapse by the enzyme acetylcholinesterase, and it was assumed that all neurotransmitters were deactivated by enzymes;

     (2) it now appears that all other neurotransmitters are deactivated by reuptake into the presynaptic neuron, where they are recycled

5.     Neurotranamitters and Receptors
     (use Digital Image Archive, Figure CHO4F17.BMP)


-     neurotransmitters are of two types,
     large molecule neurotransmitters and small-molecule neurotransmitters

-     most small-molecule neurotransmitters have punctate, point-to-point effects; they are released in a pulse into synaptic clefts each time an action potential reaches a button

      they are synthesized in the cytoplasm of terminal buttons

-     large-molecule neurotransmitters are released gradually in response to general increases in neuron firing; their effects are usually widespread because they are often released into extracellular fluid, the ventricles, or the bloodstream; they are thought to function as neuromodulators

-     it was initially assumed that there is only one kind of receptor for each neurotransmitter;
     it is now clear that each neurotransmitter binds to more than one class of receptor;

     for example, muscarinic (found in internal organs) and nicotinic (found at neuromuscular junctions) bind to acetylcholine receptors, with each subtype binding, then producing fundamentally different responses

receptor subtypes are located in different brain areas;

this allows the same neurotransmitter to signal differently at various locations;
postsynaptic neurons are differentially influenced based on the receptor subtype

-     ion-channel linked receptors chemically open or close an ion channel inducing an immediate postsynaptic potential, these receptors are not prevalent but are fast acting

-     G-protein linked receptors consist of a protein chain that winds in and out of the cell membrane seven times and each is located next to a guanine sensitive protein

-     the binding breaks away the G-protein and leads to one of two actions;
the subunit may move inside the surface of the membrane, binding to an ion channel to induce an EPSP or IPSP; or,
the binding may trigger the synthesis of a second messenger that diffuses through the cytoplasm to bind to ion channels (for an EPSP or IPSP), and influence the metabolic activities of the cell, or bind to DNA in the nucleus to influence gene expression

-     G-protein linked receptors are more prevalent, are slower acting, longer lasting, and more diffuse than ion-channel linked receptors

a.     Amino Acid Neurotransmitters

-     amino acids are the individual building blocks of proteins; they also serve as the transmitters at fast-acting, point-to-point synapses

     there is conclusive evidence that glutamate, aspartate, glycine. and gamma-aminobutyric acid (GABA) are neurotransmitters

we get glutamate, aspartate, and glycine from the proteins that we eat;
GABA is synthesized from glutamate, thus diet is important to "healthy" neurotransmission

b.     Monoamine Neurotransmitters

-     monoamine neurotransmitters are formed by slight modification to amino acid molecules; thus the name "monoamine" (one amine)
     they are often released from string-of-beads axons, and they have slow, lingering, diffuse effects; neurons that release monoamines typically have their cell bodies in the brain stem (e.g., the nigrostriatal dopamine pathway)

     There are four monoamine neurotransmitters and they belong to one of two subclasses:
     catecholamines or indolamine

1) there are three catecholamine neurotransmitters: dopamine, norepinephrine, and epinephrine;

     all three are synthesized from the amino acid tyrosine;
     tyrosine is converted to L-DOPA, to dopamine, to norepinephrine, to epinephrine

2)     there is one indolamine neurotransmitter: serotonin; it is synthesized from the amino acid tryptophan

c..     Acetylcholine

-     acetylcholine (ACH) is the small molecule transmitter at neuromuscular junctions, at many synapses in the ANS, and at some CNS synapses;
 it is created by addition of an acetyl group to a choline molecule; thus the name;
     acetylcholine is the only neurotransmitter known to be deactivated in the synapse by enzymatic degradation rather than by reuptake; it is deactivated by (acetylcholinesterase) (ACHE)

d.     Soluble Gas Neurotransmitters

-     this class of recently identified neurotransmitters includes nitric oxide and carbon monoxide

-     the gasses are produced in the neural cytoplasm, diffuse immediately through the cell membrane into the extracellular fluid and into nearby cells to stimulate production of second messengers
-     they are difficult to study as they act rapidly and are immediately broken down, existing for only a few seconds


e.     Neuropeptide Transmitters

-     peptides are short chains of 10 or fewer amino acids; about 40 or 50 peptides are putative neurotransmitters;
     they are the largest neurotransmitters
-     many peptides are released by endocrine glands into the bloodstream as well as by neurons; thus they have far-reaching diffuse effects

-     peptides are synthesized by being cleaved from polypeptide chains containing between 10 and 100 amino acids; [proteins are chains of over 100 amino acids]
-     neuropeptides are thought to function as neuromodulators; they are thought to adjust a neuron sensitivity to fast-acting point-to-point neurotransmitters

-     coexistence (neuropeptides are released from neurons that also release small-molecule neurotransmitters); it had been previously assumed that each neuron releases only one neurotransmitter


Summary (use Digital Image Archive, Figure CHO4F18BMP),

-     review the seven steps of synaptic transmission illustrated in this figure.




Suggested Websites for Lecture 4b:

Synaptic Transmission: hup://www.csuchico. edu/psy/BioPsych/neurotransmission.hlml
From the Department of Psychology at Cal State, Chico, a nice page on synaptic transmission; includes some interesting neurotrivia and pointing quizes in several places.

Neurotransmitter Systems:
http://www.uams.edu/department_of_psychiatIy/syllabus/NEUROTRA/Trans95.htm
From Dr. Jeremy Clothier at the University of Arkansas, good page on neurotransmitter systems. A bias toward the biogenic amines, perhaps.. .but some informative text and good figures of synthetic pathways and terminal distribution for each system under study. See also:

http://weber.u. washington. edu/~chudler/chnt1.html
More from the Dr. Eric Chudler at the University of Washington, a good overview of synaptic transmission including synthesis, release, and degradation of neurotransmitters and neuropeptides