How nerve cells “talk” to each other Neurotransmission How nerve cells “talk” to each other

Dendrites Stimulus Axon hillock Nucleus Cell body Presynaptic cell Figure 48.4 Dendrites Stimulus Axon hillock Nucleus Cell body Presynaptic cell Axon Signal direction Synapse Synaptic terminals Figure 48.4 Neuron structure and organization. Synaptic terminals Postsynaptic cell Neurotransmitter 2

Transmission of a signal Think dominoes! start the signal knock down line of dominoes by tipping 1st one  trigger the signal propagate the signal do dominoes move down the line?  no, just a wave through them! re-set the system before you can do it again, have to set up dominoes again  reset the axon

Transmission of a nerve signal Neuron has similar system protein channels are set up once first one is opened, the rest open in succession all or nothing response a “wave” action travels along neuron have to re-set channels so neuron can react again

Cells: surrounded by charged ions Cells live in a sea of charged ions anions (negative) more concentrated within the cell Cl-, charged amino acids (aa-) cations (positive) more concentrated in the extracellular fluid Na+ channel leaks K+ K+ Na+ K+ Cl- aa- + – K+

Cells have voltage! Opposite charges on opposite sides of cell membrane membrane is polarized negative inside; positive outside charge gradient stored energy (like a battery) + This is an imbalanced condition. The positively + charged ions repel each other as do the negatively - charged ions. They “want” to flow down their electrical gradient and mix together evenly. This means that there is energy stored here, like a dammed up river. Voltage is a measurement of stored electrical energy. Like “Danger High Voltage” = lots of energy (lethal). – – +

Measuring cell voltage Voltage = measures the difference in concentration of charges. The positives are the “hole” you leave behind when you move an electron. Original experiments on giant squid neurons! unstimulated neuron = resting potential of -70mV

How does a nerve impulse travel? Stimulus: nerve is stimulated reaches threshold potential open Na+ channels in cell membrane Na+ ions diffuse into cell charges reverse at that point on neuron positive inside; negative outside cell becomes depolarized The 1st domino goes down! – + Na+

How does a nerve impulse travel? Wave: nerve impulse travels down neuron change in charge opens next Na+ gates down the line “voltage-gated” channels Na+ ions continue to diffuse into cell “wave” moves down neuron = action potential Gate + – channel closed channel open The rest of the dominoes fall! – + Na+ wave 

How does a nerve impulse travel? Re-set: 2nd wave travels down neuron K+ channels open K+ channels open up more slowly than Na+ channels K+ ions diffuse out of cell charges reverse back at that point negative inside; positive outside Set dominoes back up quickly! + – Na+ K+ wave  Opening gates in succession = - same strength - same speed - same duration

How does a nerve impulse travel? Combined waves travel down neuron wave of opening ion channels moves down neuron signal moves in one direction      flow of K+ out of cell stops activation of Na+ channels in wrong direction Ready for next time! + – Na+ wave  K+

How does a nerve impulse travel? Action potential propagates wave = nerve impulse, or action potential brain  finger tips in milliseconds! In the blink of an eye! + – Na+ K+ wave  K+ gates open more slowly than Na+ gates

Voltage-gated channels Ion channels open & close in response to changes in charge across membrane Na+ channels open quickly in response to depolarization & close slowly K+ channels open slowly in response to depolarization & close slowly Structure & function! + – Na+ K+ wave  Na+ channel closed when nerve isn’t doing anything.

How does the nerve re-set itself? After firing a neuron has to re-set itself Na+ needs to move back out K+ needs to move back in both are moving against concentration gradients need a pump!! + – Na+ K+ wave  A lot of work to do here! Na+ K+

How does the nerve re-set itself? Sodium-Potassium pump active transport protein in membrane requires ATP 3 Na+ pumped out 2 K+ pumped in re-sets charge across membrane ATP Dominoes set back up again. Na/K pumps are one of the main drains on ATP production in your body. Your brain is a very expensive organ to run! That’s a lot of ATP ! Feed me some sugar quick!

Neuron is ready to fire again Na+ K+ aa- resting potential + –

Falling phase of the action potential 3 Figure 48.11-5 Key Na K 4 Falling phase of the action potential 3 Rising phase of the action potential 50 Action potential 3 Membrane potential (mV) Threshold 4 2 50 1 1 5 2 Depolarization Resting potential 100 Figure 48.11 The role of voltage-gated ion channels in the generation of an action potential. Time OUTSIDE OF CELL Sodium channel Potassium channel INSIDE OF CELL Inactivation loop 1 Resting state 5 Undershoot 17

Falling phase of the action potential 3 Figure 48.11-4 Key Na K 4 Falling phase of the action potential 3 Rising phase of the action potential 50 Action potential 3 Membrane potential (mV) Threshold 4 2 50 1 2 Depolarization Resting potential 100 Figure 48.11 The role of voltage-gated ion channels in the generation of an action potential. Time OUTSIDE OF CELL Sodium channel Potassium channel INSIDE OF CELL Inactivation loop 1 Resting state 18

Axon Plasma membrane Action potential 1 Cytosol Action potential 2 Figure 48.12-3 Axon Plasma membrane Action potential 1 Cytosol Na Action potential K 2 Na Figure 48.12 Conduction of an action potential. K Action potential K 3 Na K 19

Action potential graph Resting potential Stimulus reaches threshold potential Depolarization Na+ channels open; K+ channels closed Na+ channels close; K+ channels open Repolarization reset charge gradient Undershoot K+ channels close slowly 40 mV 4 30 mV 20 mV Depolarization Na+ flows in Repolarization K+ flows out 10 mV 0 mV –10 mV 3 5 Membrane potential –20 mV –30 mV –40 mV Hyperpolarization (undershoot) –50 mV Threshold –60 mV 2 –70 mV 1 Resting potential 6 Resting –80 mV

What happens at the end of the axon? Impulse has to jump the synapse! junction between neurons has to jump quickly from one cell to next How does the wave jump the gap? Synapse

Concept 48.4: Neurons communicate with other cells at synapses At electrical synapses, the electrical current flows from one neuron to another At chemical synapses, a chemical neurotransmitter carries information across the gap junction Most synapses are chemical synapses © 2011 Pearson Education, Inc.

Synaptic terminals of pre- synaptic neurons Figure 48.16 Postsynaptic neuron Synaptic terminals of pre- synaptic neurons 5 m Figure 48.16 Synaptic terminals on the cell body of a postsynaptic neuron (colorized SEM). 23

Presynaptic cell Synaptic cleft Figure 48.15 Presynaptic cell Postsynaptic cell Axon Synaptic vesicle containing neurotransmitter 1 Postsynaptic membrane Synaptic cleft Presynaptic membrane 3 Figure 48.15 A chemical synapse. K Ca2 2 Voltage-gated Ca2 channel Ligand-gated ion channels 4 Na 24

Chemical synapse Events at synapse ion-gated channels open action potential depolarizes membrane opens Ca++ channels neurotransmitter vesicles fuse with membrane release neurotransmitter to synapse  diffusion neurotransmitter binds with protein receptor ion-gated channels open neurotransmitter degraded or reabsorbed axon terminal action potential synaptic vesicles synapse Ca++ Calcium is a very important ion throughout your body. It will come up again and again involved in many processes. neurotransmitter acetylcholine (ACh) receptor protein muscle cell (fiber) We switched… from an electrical signal to a chemical signal

Nerve impulse in next neuron K+ Post-synaptic neuron triggers nerve impulse in next nerve cell chemical signal opens ion-gated channels Na+ diffuses into cell K+ diffuses out of cell switch back to voltage-gated channel K+ Na+ ion channel binding site ACh Here we go again! – + Na+

Neurotransmitters There are more than 100 neurotransmitters, belonging to five groups: acetylcholine, biogenic amines, amino acids, neuropeptides, and gases A single neurotransmitter may have more than a dozen different receptors © 2011 Pearson Education, Inc.

Neurotransmitters Acetylcholine transmit signal to skeletal muscle Epinephrine (adrenaline) & norepinephrine fight-or-flight response Dopamine widespread in brain affects sleep, mood, attention & learning lack of dopamine in brain associated with Parkinson’s disease excessive dopamine linked to schizophrenia Serotonin Nerves communicate with one another and with muscle cells by using neurotransmitters. These are small molecules that are released from the nerve cell and rapidly diffuse to neighboring cells, stimulating a response once they arrive. Many different neurotransmitters are used for different jobs: glutamate excites nerves into action; GABA inhibits the passing of information; dopamine and serotonin are involved in the subtle messages of thought and cognition. The main job of the neurotransmitter acetylcholine is to carry the signal from nerve cells to muscle cells. When a motor nerve cell gets the proper signal from the nervous system, it releases acetylcholine into its synapses with muscle cells. There, acetylcholine opens receptors on the muscle cells, triggering the process of contraction. Of course, once the message is passed, the neurotransmitter must be destroyed, otherwise later signals would get mixed up in a jumble of obsolete neurotransmitter molecules. The cleanup of old acetylcholine is the job of the enzyme acetylcholinesterase.

Neurotransmitters Weak point of nervous system any substance that affects neurotransmitters or mimics them affects nerve function gases: nitrous oxide, carbon monoxide mood altering drugs: stimulants amphetamines, caffeine, nicotine depressants quaaludes, barbiturates hallucinogenic drugs: LSD, peyote SSRIs: Prozac, Zoloft, Paxil poisons Selective serotonin reuptake inhibitor

NEUROTRANSMITTERS

Table 48.2 Major Neurotransmitters 31