1 Fundamentals of the Nervous System and Nervous Tissue: Part B11 Fundamentals of the Nervous System and Nervous Tissue: Part B
2 ELECTRICITY! What’s the difference in terms?Action potential Voltage Current Charge
3 NERVOUS SYSTEM PART 2 “Electricity” terms’ Membrane potentialThreshold potential Action potential Voltage-gated channels Chemical gated channels Local potential Unmyelinated axons Myelinated axons
4 Imagine a simplistic batteryImagine a simplistic battery. The positive and negative charges are separated by a barrier or resistance. ++++++
5 VOLTAGE is the difference in charges between the 2 chambers++++++ ++++++ One volt battery The greater the difference in charges The greater the VOLTAGE or POTENTIAL Twelve volt battery!!!!!
6 We intuitively know that there is POTENTIAL danger in a high VOLTAGE…but not unless we touch it a certain way….. ++++++ Twelve volt battery!!!!!
7 Because if we connect the sides, as with a wire (or two fingers…), the currents now have a way to reach each other. The charges move. Moving charges are called a current. ++++++
8 As the current flows, the bulb lights upAs the current flows, the bulb lights up. Also, charges start to even out on each side of the battery. ++++++
9 As the charges become more similar on each side of the battery, there is less difference between the sides, and so less voltage. ++++++
10 When the charges are the same on each side of the battery, the current cannot flow. The battery is dead.
11 +++++++++++++++++++++++++Think of an nerve cell like a battery. In a living cell at REST, intracellular fluid is more negative than the extracellular fluid Living cell
12 OUTSIDE. INSIDE Hi Na+. Low Na+ Hi Cl-. Low Cl- Hi Ca++OUTSIDE INSIDE Hi Na+ Low Na+ Hi Cl- Low Cl- Hi Ca++ Low Ca++ Low K+ Hi K+
13 +++++++++++++++++++++++++When the voltage on the inside and outside of the cell are different, opening CHANNELS in the cell, will cause charges to flow (from hi to low) in a current and change the membrane voltage. Changing the membrane voltage (potential) can act as a signal. A POSTIVE CHANGE excites the neuron. Outside: Hi Na+ Inside: Low Na+
14 +++++++++++++-++++++++++If the nerve cell gets POSITIVE inside, even briefly, this is like a SIGNAL to the cell to do something When did we see this happen in a muscle cell? Living cell
15 +++++++++++++-++++++++++The muscle cell got positive when the nerve cell sent ACh to bind to the muscle cell, and Na+ entered the cell. This was the start of the signal for contraction to begin. Living cell
16 +++++++++++++++++++++++++If it ever evens out permanently between inside and outside, the cell is dead. Living cell Dead cell
17 We know a cell is negative inside…but HOW? There are 3 main reasons.Voltmeter Plasma membrane Ground electrode outside cell Microelectrode inside cell Axon Neuron Figure 11.7
18 Why is the resting membrane potential negative inside the cellWhy is the resting membrane potential negative inside the cell? Reason 1. There are many more BIG anions (negatively charged proteins) inside the cell than outside. - Extracellular fluid Intracellular fluid has many anions - - - - -
19 Why is the resting membrane potential negative inside? Reason 2.There are many leaky K+ channels in neurons
20 OUT Na+ , Ca++, Cl-, K+ Ion concentrations IN Na+, Ca++ Cl-, K+Which way would K+ leak through leaky channels? OUT Na+ , Ca++, Cl-, K+ IN Na+, Ca++ Cl-, K+
21 MANY Positive K+ ions flow OUT leaky channels So, the inside of the cell becomes more NEGATIVE
22 Why K+ does not keep leaving forever!MANY Positive K+ ions flow OUT leaky channels So, the inside of the cell becomes more NEGATIVE compared to the outside of the cell Opposites attract! As the cell becomes more negative inside, some K+ are drawn back in by the negative charge! The balance point of K+ flowing out and in leaves the cell at about -70mV as the RESTING MEMBRANE POTENTIAL.
23 There are leaky Na+ channels, but we are going to ignore them.MANY Positive K+ ions flow OUT leaky channels FEW positive Na+ ions flow in leaky channels There are 75x more K+ leak channels than Na+ leak channels. So mostly K+ leaks out.
24 Leakage channels occur all over the neuron, including the dendrites, soma, and axon
25 Why is the resting membrane potential negative? Reason 3.The sodium potassium pump! It pumps OUT more + ions than it pumps in (click here). 3 Na+ out 2 K +in
26 Review What are the three reasons that the resting membrane potential is -70mV? 1. 2. 3.
27 Your patient’s IV Putting the incorrect fluid in your patients IV can change the concentrations of ions inside and outside the cell. This causes change in voltage inside the cell, preventing or causing signals from occurring. This can cause severe complications in your patient.
28 What excites a neuron? Excitation has to do with chemical-gated channels. These channels occur mostly on dendrites.
29 Opening chemical gated channels in the Depolarizing stimulus Inside positive Inside negative Depolarization Resting potential Time (ms) Opening chemical gated channels in the dendrites makes a small change to the voltage. An upward change is a DEPOLIZATION, exciting the cell. Figure 11.9a
30 Opening chemical gated channels in the Hyperpolarizing stimulus Resting potential Hyper- polarization Opening chemical gated channels in the dendrites makes a small change to the voltage. A downward change is a HYPERPOLARIZATION. This change does not excite the cell. Time (ms) Figure 11.9b
31 Summary of channel locationsLeakage channels occur all over the neuron, including the dendrites, soma, and axon Chemically gated channels occur mostly on dendrites and call body Voltage gated channels occur mainly on the axon
32 How do cells use changing voltages to communicate?To communicate with the next cell, a neuron must send an AP down the axon, which releases NT. -70 mV
33 A nearby cell releases NT, which opens some chemical gate Na+ channels… Na+ will flow IN. Why? Na+ will depolarize the cell. Why? neurotransmitte -70mV
34 Imagine that very vesicle of this NT depolarizes (excites) the dendrite by 3mV. These small changes are called LOCAL potentials. neurotransmitter -67mV
35 The area of depolarization spreads through the dendrites…but not very far… And gets smaller (decays) the further it goes…. -67 mV
36 -------------- Starting to fade….-69 mV The change of voltage is local and stays in dendrites. It does not travel along the axon to the next cell.
37 However if NT is dropped repeatedly, or by more than one neuron the effect increases.-70 mV What will the new voltage be here?
38 If enough excitatory NT is applied, the cell becomes very depolarized-55 mV
39 If enough excitatory NT is applied, the cell becomes very depolarized-55 mV
40 To send the Depolarization down the axon, we need another type of channel to open.Voltage-gated channels occur mainly on the axon They are opened and closed by a particular voltage
41 . There is a VERY high concentration of voltage-gated channelson the axon hillock.
42 -------------- At -55Mv, or THRESHOLDmany Voltage-Gated Na+ channels open on the axon hillock, in a place called the trigger zone! -55 mV
43 As Na+ rushes IN, this makes a big + charge inside the cell This depolarization is the first part of the AP -55 mV
44 Each depolarized area opens the next set of voltage gated Na+ channels.-70 mV
45 Each depolarized area opens the next set of voltage gated Na+ channelsEach depolarized area opens the next set of voltage gated Na+ channels In this way, the POSTIVE WAVE of the action potential is carried to the end of the axon, and NT is released. . -70 mV NT released
46 Behind the wave, the slower opening K+ channels open and allow K+ to leave, making the cell more negative again, and it goes back to rest. -70 mV NT released
47 -70 mV NT released
48 The cell returns to rest at -70mV No more + charge reaches the terminal. No further NT is released.
49 --------------------------------------------------To start local potentials, add 5 drops of ACh so cell depolarizes in small steps called LOCAL POTENTIALS. +30 mV -55 -70 Add ACh Time in milliseconds (mS)
50 --------------------------------------------------Eventually the cell reaches threshold (-55mV) Action potential: At -55, Na+ VG channels open Na+ enters cell At +30 Na+ VG channels close Na+ stops entering cell At +30 K+ VG channels open K+ leaves cell At -70 K+ VG channels close K+ stops leaving cell +30 mV -55 -70 Add ACh Time in milliseconds (mS)
51 At -55, Na+ VG channels open. Na+ enters cell Action potential: At -55, Na+ VG channels open. Na+ enters cell At +30 Na+ VG channels close Na+ stops entering cell At +30 K+ VG channels open K+ leaves cell At -70 K+ VG channels close K+ stops leaving cell Depolarization (Na+ enters) Repolarization (K+ leaves) After-hyperpolarization Stimulus Time (ms) Figure 11.14
52 Action potential analogiesAn action potential occurs at every point along the axon where Na+ an K+ leave the cell If you were standing inside the axon, you would see the approaching depolarization like a wave, coming down the shore… Some people think of an Action Potential carrying the + charge to the axon tip like a row of falling dominost would wash over you and keep going all the way to the axon tip.
53 The AP cannot travel backwardsBecause the channels that just opened behind the traveling wave cannot reopen again right away. (refractory period).
54 Graded potentials Action potentialsOccur only in dendrites and soma Occur only in axon Travel a short distance to axon hillock Travel a long distance to axon tip Channels opened by chemical, light, sound, touch,taste Channels opened by voltage Can add on each other or decay Always the same size, all or none Up to 15 mV in size (until threshold) 100mV in size Can be excitatory (EPSP) Can be inhibitory (IPSP) Are only excitatory
55 How does the brain think complicated thoughts?Just like you, neurons are always making decisions based on incoming information… Some negative some positive Some thoughts make you act, others stop you from acting.
56 Which side wins?!
57 The brain contains excitatory and inhibitory neurotransmittersThe brain contains excitatory and inhibitory neurotransmitters. Excitatory NTs tend to turn ON the next neuron in line. Inhibitory NTs tend to turn OFF the next neuron in line.
58 Inhibitory NTs open K+ channels or Cl- channelsInhibitory NTs open K+ channels or Cl- channels. What would an inhibitory NT do to the RMP? -70 mV
59 Inhibitory NT push the RMP further from threshold, making it less likely an AP will form.-85 mV
60 Add up all the local potentials (3mV each) Would the cell fire an action potential?
61 Summary: how do YOU make decisions?The brain uses Excitatory NT to turn on some cells, and Inhibitory NT to turn off others. It is the SUM of positive and negative inputs that decides the output. IT’S LIKE A VOTE!!! Analogy: some friends want you to go to the movies, and others do not. If each friend is one vote, how do you determine if you go or not?
62 We can plug a lamp in using a pretty long cord, and the light will come on.battery Bright light 50 yards? BUT if we add longer and longer cords it will eventually fail. The current will NOT make it all the way to the end. battery Dimmer light 500 yards? battery NO light 5000 yards? THE CURRENT RUNS DOWN AS IT TRAVELS
63 If the current runs down as it travels, thenhow does current get from the Hoover Dam to your house? P ow e r D I s t a n c e
64 The current must be boosted along the way with booster stationsThe current must be boosted along the way with booster stations! The current is jacked up at intervals, and so gets to your home power D i s t a n c e
65 Axons must also carry the current VERY long distances!But if an axon was just a wire, the BIG POSTIVE Current would run out before it reached the end of theaxon, and no NT could be released! P ow e r D I s t a n c e
66 So, there are booster stations in an axon, too!power D i s t a n c e
67 The Voltage gated Na+ channels act as booster stations, making new ACTION POTENTIALS all the way down the axon.
68 Just as the current is about to run out, it is boosted up at the next voltage gated Na+ channel !
69 The voltage gated Na+ channels OPEN, allowing a burst of POSITIVE Na+ ions into the cell. Depolarizing the cell. NT Ca++
70 Many UNMYELINATED axons transmit current this wayMany UNMYELINATED axons transmit current this way. But since each boost takes TIME, it is precious time is lost in transmitting information. NT Ca++
71 What if there were FEWER booster stationsWhat if there were FEWER booster stations? Would it take longer or shorter time to send the signal? NT Ca++
72 Myelin is made of a glial cell membrane that covers some of the voltage gated channels. Fewer voltage gated Na+ channels are exposed. NT Ca++
73 The charge runs QUICKLY under the myelin. NT Ca++
74 The voltage DOES drops as it runs under the myelin, but it IS boosted at the next set of channels back to +30 mV. Quick Quick NT Slow boost Slow boost
75 The myelinated axons needs only 4 APsIn summary, a myelin covered axon transmits the action potential 30x quicker than a naked axon! The bare axon needs 6 APs slow slow slow slow slow slow quick slow quick slow quick slow quick slow quick The myelinated axons needs only 4 APs
76 Saltatory conduction down an axon the jerky motion is like “hopping” alongSlow fast slow fast slow fast slow Node myelin node myelin node myelin node At the node: VG channels have to open to allow in sodium (positive charge). A slow but POWERFUL BOOST. Under myelin: there are no channels. The positive charge zooms to the next node inside the axon. FAST but the current runs down.
77 Conduction Velocity Larger diameter axons transmit the AP faster. SlowEven faster
78 Conduction Velocity But add myelin, and it is faster stillnode node node node The myelinated axon is faster, even tho it is thinner. This saves space in the brain!
79 Conduction velocity Fast axons (larger diameter and myelinated)serve pathways where speed is essential such as skeletal reflexes Slower (smaller diameter and unmyelinated) serve internal organs (viscera, glands, blood vessels)
80 Coding for Stimulus IntensityALL action potentials are 30mV high!! So, how does the brain tell the difference between a weak stimulus and a strong one? A dim light or a bright one? A quiet sound or a loud sound? Weak stimuli send many APs to brain ___I___I___I___I Strong stimuli send many APs to brain __I_I_I_I_I_I_I_I_I Strong stimuli increase the number of AP going to the brain.
81 If AP travels very fast under the myelin, why isn’t the entire axon covered in one unbroken sheath of myelin? Take a minute to think about the answer to this question on your own. Now turn to your neighbor (neighbors) and discuss your answers. Do we have a group that would like to volunteer to give their answer?
82 Multiple Sclerosis (MS)An autoimmune disease that mainly affects young adults Symptoms: visual disturbances, weakness, loss of muscular control, speech disturbances, and urinary incontinence Myelin sheaths in the CNS become nonfunctional scleroses Shunting and short-circuiting of nerve impulses occurs Impulse conduction slows and eventually ceases