Power Point Lecture Slides prepared by Janice Meeking

ELECTRICITY! What’s the difference in terms? • Action potential • Voltage • Current •

NERVOUS SYSTEM PART 2 • “Electricity” terms’ • Membrane potential • Threshold potential • Action potential • Voltage-gated channels • Chemical gated channels • Local potential • Unmyelinated axons • Myelinated axons Copyright © 2010 Pearson Education, Inc.

++++++ Copyright © 2010 Pearson Education, Inc. --------------------- Imagine a simplistic battery. The positive

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 Copyright © 2010 Pearson Education, Inc. ++++++ --------------------- Twelve volt battery!!!!!

We intuitively know that there is POTENTIAL danger in a high VOLTAGE…but not unless we touch it a certain way…. . ++++++ --------------------- Twelve volt battery!!!!! Copyright © 2010 Pearson Education, Inc.

+-+-+-+ ++++++ Copyright © 2010 Pearson Education, Inc. +-+----------------- Because if we connect the sides, as with a wire (or two fingers…), the currents now have a way to reach other. The charges move. Moving charges are called a current.

As the current flows, the bulb lights up. Also, charges start to even out

+-+-+-++-+-+-+ ++++++ Copyright © 2010 Pearson Education, Inc. +-+-+-++-+-+-+ ------ As the charges become more similar on each side of the battery, there is less difference between the sides, and so less voltage.

When the charges are the same on each side of the battery, the current

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 Copyright © 2010 Pearson Education, Inc. -------------------

OUTSIDE Hi Na+ Hi Cl. Hi Ca++ Low K+ Copyright © 2010 Pearson Education,

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+ Copyright © 2010 Pearson Education, Inc.

If the nerve cell gets POSITIVE inside, even briefly, this is like a SIGNAL to the cell to do something. +++++++-+++++ Living cell --------+---------- When did we see this happen in a muscle cell? Copyright © 2010 Pearson Education, Inc.

The muscle cell got positive when the nerve cell sent ACh to bind to the muscle cell, and +++++++-+++++ Living cell --------+---------- Na+ entered the cell. This was the start of the signal for contraction to begin. Copyright © 2010 Pearson Education, Inc.

If it ever evens out permanently between inside and outside, the cell is dead.

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 Copyright © 2010 Pearson Education, Inc. Figure 11. 7

Why 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 - Copyright © 2010 Pearson Education, Inc. - -

Why is the resting membrane potential negative inside? Reason 2. There are many leaky

Ion concentrations Which way would K+ leak through leaky channels? OUT Na+ , Ca++,

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 -70 m. V as the RESTING MEMBRANE POTENTIAL. Copyright © 2010 Pearson Education, Inc.

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 75 x more K+ leak channels than Na+ leak channels. So mostly K+ leaks out. Copyright © 2010 Pearson Education, Inc.

Leakage channels occur all over the neuron, including the dendrites, soma, and axon Copyright

Why is the resting membrane potential negative? Reason 3. The sodium potassium pump! It

Review What are three reasons that the resting membrane potential is -70 m. V?

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. Copyright © 2010 Pearson Education, Inc.

What excites a neuron? Excitation has to do with chemical-gated channels. These channels occur

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. Copyright © 2010 Pearson Education, Inc. Figure 11. 9 a

Hyperpolarizing stimulus Resting potential Hyperpolarization Opening chemical gated channels in the Time (ms) dendrites makes a small change to the voltage. A downward change is a HYPERPOLARIZATION. This change does not excite the cell. Copyright © 2010 Pearson Education, Inc. Figure 11. 9 b

Summary of channel locations Leakage 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 Copyright © 2010 Pearson Education, Inc.

How do cells use changing voltages to communicate? To communicate with the next cell,

A nearby cell releases NT, which opens some chemical gate Na+ channels…. . -70

• neurotransmitter -67 m. V ------- Imagine that very vesicle of this NT depolarizes (excites) the dendrite by 3 m. V. These small changes are called LOCAL potentials. Copyright © 2010 Pearson Education, Inc.

The area of depolarization spreads through the dendrites…but not very far…. . -67 m.

Starting to fade…. -69 m. V ------- The change of voltage is local and

However if NT is dropped repeatedly, or by more than one neuron the effect

If enough excitatory NT is applied, the cell becomes very depolarized -55 m. V

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 Copyright © 2010 Pearson Education, Inc.

. There is a VERY high concentration of voltage-gated channels on the axon hillock.

At -55 Mv, or THRESHOLD ------- -55 m. V many Voltage-Gated Na+ channels open

As Na+ rushes IN, this makes a big + charge inside the cell -55

Each depolarized area opens the next set of voltage gated Na+ channels. • -70 m. V • ++++ NT released In this way, the POSTIVE WAVE of the action potential is carried to the end of the axon, and NT is released. Copyright © 2010 Pearson Education, Inc.

Behind the wave, the slower opening K+ channels open • -70 m. V -----+++++ NT released and allow K+ to leave, making the cell more negative again, and it goes back to rest. Copyright © 2010 Pearson Education, Inc.

-70 m. V Copyright © 2010 Pearson Education, Inc. ------+ NT released

The cell returns to rest at -70 m. V -70 m. V ------------- No

To start local potentials, add 5 drops of ACh so cell depolarizes in small steps called LOCAL POTENTIALS. +30 m. V -55 ------------------------- -70 Add ACh Copyright © 2010 Pearson Education, Inc. Time in milliseconds (m. S)

Eventually the cell reaches threshold (-55 m. V) 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 m. V -55 ------------------------- -70 Add ACh Copyright © 2010 Pearson Education, Inc. Time in milliseconds (m. S)

Depolarization (Na+ enters) 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 Repolarization (K+ leaves) After-hyperpolarization Stimulus Time (ms) Copyright © 2010 Pearson Education, Inc. Figure 11. 14

Action potential analogies • An 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. Copyright © 2010 Pearson Education, Inc.

The AP cannot travel backwards • Because the channels that just opened behind the

Graded potentials Action potentials Occur 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 m. V in size (until threshold) 100 m. V in size Can be excitatory (EPSP) Can be inhibitory (IPSP) Are only excitatory Copyright © 2010 Pearson Education, Inc.

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. Copyright © 2010 Pearson Education, Inc.

• Which side wins? ! -------- Copyright © 2010 Pearson Education, Inc. ++++++

The 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. Copyright © 2010 Pearson Education, Inc.

Inhibitory NTs open K+ channels or Cl- channels. What would an inhibitory NT do

Inhibitory NT push the RMP further from threshold, making it less likely an AP

Add up all the local potentials (3 m. V each) Would the cell fire

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? Copyright © 2010 Pearson Education, Inc.

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. Dimmer light battery 500 yards? battery 5000 yards? NO light THE CURRENT RUNS DOWN AS IT TRAVELS Copyright © 2010 Pearson Education, Inc.

If the current runs down as it travels, then how does current get from

The current must be boosted along the way with booster stations! The current is

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 Copyright © 2010 Pearson Education, Inc. s t a n c e

So, there are booster stations in an axon, too! p o w e r

The Voltage gated Na+ channels act as booster stations, making new ACTION POTENTIALS all

Just as the current is about to run out, it is boosted up at

The voltage gated Na+ channels OPEN, allowing a burst of POSITIVE Na+ ions into

Many UNMYELINATED axons transmit current this way. But since each boost takes TIME, it

What if there were FEWER booster stations? Would it take longer or shorter time

Myelin is made of a glial cell membrane that covers some of the voltage

The charge runs QUICKLY under the myelin. + + ++++++++ + + Ca++ Copyright

The voltage DOES drops as it runs under the myelin, but it IS boosted at the next set of channels back to +30 m. V. + + Quick + + Slow boost Copyright © 2010 Pearson Education, Inc. Quick + + Slow boost NT

In summary, a myelin covered axon transmits the action potential 30 x quicker than a naked axon! The bare axon needs 6 APs slow slow quick slow quick The myelinated axons needs only 4 APs Copyright © 2010 Pearson Education, Inc.

Saltatory conduction down an axon the jerky motion is like “hopping” along Slow fast Node myelin slow node fast myelin slow 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. Copyright © 2010 Pearson Education, Inc.

Conduction Velocity • Larger diameter axons transmit the AP faster. Slow Faster Even faster

Conduction Velocity • But add myelin, and it is faster still node The myelinated

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) Copyright © 2010 Pearson Education, Inc.

Coding for Stimulus Intensity • ALL action potentials are 30 m. V 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 • Strong stimuli send many APs to brain __I_I_I_I_I • Strong stimuli increase the number of AP going to the brain. Copyright © 2010 Pearson Education, Inc.

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? Copyright © 2010 Pearson Education, Inc.

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 Copyright © 2010 Pearson Education, Inc.