Neural Conduction and Synaptic Transmission (i.e., Electricity and Chemistry)

Neurons Figure 2.5 A typical neuron and synapse Klein/Thorne: Biological Psychology © 2007 by Worth Publishers Figure 2.6 The four major types of synapses Klein/Thorne: Biological Psychology © 2007 by Worth Publishers

Neural Conduction An Electrical Process

Resting Membrane Potential

Figure 4.2 Recording the resting membrane potential of a neuron Klein/Thorne: Biological Psychology © 2007 by Worth Publishers

Ions and the resting membrane potential K+ Potassium ions, positive charge

Ions and the resting membrane potential Na+ Sodium ions, positive charge

Ions and the resting membrane potential Cl- Chloride ions, negative charge

Ions and the resting membrane potential Inside the neuron K+ Protein- Outside the neuron Na+ Cl-

What the ions naturally want to do Force of diffusion It’s getting crowded in here Electrostatic pressure Opposites attract Similarities repel Figure 4.4 The influence of diffusion and electrostatic pressure on the movement of ions into and out of the neuron Klein/Thorne: Biological Psychology © 2007 by Worth Publishers

What the neural membrane is making the ions do Differential permeability Playing favorites K+ and Cl- pass through easily Na+ -- not so easy to pass through the membrane Proteins: not a chance! Ion channels: like doors

What the neural membrane is making the ions do Sodium-potassium pump Three Na+ out for every two K+ cells in Figure 4.5 The sodium-potassium pump Klein/Thorne: Biological Psychology © 2007 by Worth Publishers

Putting it all together… Na+ ions Want to go inside neuron because There are fewer of them inside (force of diffusion) There is a negative charge inside (opposite to their positive charge) But Neuron’s membrane not very permeable to Na+ ions Sodium-potassium pump keeps kicking them out Therefore, most Na+ ions stay outside neuron

Putting it all together… K+ ions Want to go outside neuron because There are fewer of them outside (force of diffusion) Neuron’s membrane very permeable to K+ ions But There is a positive charge outside (similar to their positive charge), so they are repelled by the outside Sodium-potassium pump keeps kicking them back into neuron Therefore, most K+ ions stay inside neuron

Putting it all together… Cl- ions: can’t make up their minds Want to go inside neuron because There are fewer of them inside Neuron’s membrane very permeable to Cl- ions Also want to stay outside of neuron because The charge outside is positive (and their own charge is negative Therefore, Cl- ions keep going back and forth, distribution of Cl- ions is held at equilibrium. _____________________________________________

Postsynaptic Potentials Getting the membrane potential to change from -70 mv

Figure 2.6 The four major types of synapses Klein/Thorne: Biological Psychology © 2007 by Worth Publishers Figure 4.11 Overview of synaptic transmission Klein/Thorne: Biological Psychology © 2007 by Worth Publishers

Like relay team passing a baton Causes something called “postsynaptic potentials” to happen

Postsynaptic potentials can do one of 2 things… Depolarize neuron Hyperpolarize neuron

Postsynaptic potentials Depolarize Decrease resting potential Become less negative E.g., from -70 mV to – 67 mv De/hyper polarize receptive membrane, why called “post” synaptic

Postsynaptic potentials Increase likelihood that neuron will fire Excitatory postsynaptic potentials: EPSPs De/hyper polarize receptive membrane, why called “post” synaptic

Postsynaptic potentials Hyperpolarize Increase the resting potential Become more negative E.g., from -70 mV to -72 mV De/hyper polarize receptive membrane

Postsynaptic potentials Decrease likelihood that neuron will fire Inhibitory postsynaptic potentials: IPSPs De/hyper polarize receptive membrane

Characteristics of EPSPs and IPSPs Notes

There are a bunch of EPSPs and IPSPs happening in the same neuron at once

How EPSPs or IPSPs add up Spatial summation A bunch of EPSPs/IPSPs combine together Figure 4.14 Spatial summation and temporal summation Klein/Thorne: Biological Psychology © 2007 by Worth Publishers

How EPSPs or IPSPs add up Temporal summation When EPSPs/IPSPs are coming in real fast, the next one happens before the previous one fades away They add together over time Figure 4.14 Spatial summation and temporal summation Klein/Thorne: Biological Psychology © 2007 by Worth Publishers

Getting a neuron to fire EPSPs and IPSPs travel until they reach near the axon hillock

Getting a neuron to fire Remember… EPSP make membrane’s resting potential less negative (e.g., from -70 mv to -68 mv) IPSPs make membrane’s resting potential more negative (e.g., from -70 mv to -75 mv) When combined they cancel each other out, and whichever is stronger wins

Getting a neuron to fire: Example 1 EPSPs add up to change resting potential from -70 mv to -60 mv (change of +10 mv) IPSPs add up to change resting potential from -70 mv to -75 mv (change of -5 mv)

Getting a neuron to fire: Example 1 Net difference of +5 mv, from -70 mv to -65 mv The resting membrane potential to become less negative The end result is the membrane is depolarized

Figure 4.7 Changes in the membrane potential during the action (spike) potential Klein/Thorne: Biological Psychology © 2007 by Worth Publishers

Getting a neuron to fire: Example 2 EPSPs add up to change resting potential from -70 mv to -68 mv (change of +2 mv) IPSPs add up to change resting potential from -70 mv to -75 mv (change of -5 mv)

Getting a neuron to fire: Example 2 Net difference of -3 mv, from -70 mv to -73 mv The resting membrane potential to become more negative The end result is the membrane is hyperpolarized

Figure 4.7 Changes in the membrane potential during the action (spike) potential Klein/Thorne: Biological Psychology © 2007 by Worth Publishers

Getting a neuron to fire The end result that matters is how the EPSPs and IPSPs cancel each other out near the axon hillock

Getting a neuron to fire If it so happens that, near the axon hillock The net combination of EPSPs/IPSPs Depolarizes the membrane (makes it less negative) To a point called threshold potential

Figure 4.7 Changes in the membrane potential during the action (spike) potential Klein/Thorne: Biological Psychology © 2007 by Worth Publishers

Action potential Membrane becomes depolarized to about 40 mV All-or-nothing

Voltage-activated ion channels When a neuron’s membrane reaches the threshold of excitation, ion channels open Na+ ions (previously could not permeate the membrane) can now rush into the neuron As a result, membrane potential goes to about 40mv

Voltage-activated ion channels K+ ions (they start out being inside the neuron) now rush out of the neuron Force of diffusion When membrane potential is now positive, also driven out by electrostatic pressure

Refractory period Absolute refractory period Lasts 1 to 2 milliseconds Impossible for another action potential to happen Figure 4.7 Changes in the membrane potential during the action (spike) potential Klein/Thorne: Biological Psychology © 2007 by Worth Publishers

Refractory period Relative refractory period Possible for another action potential to happen But need extra-strength stimulation Figure 4.7 Changes in the membrane potential during the action (spike) potential Klein/Thorne: Biological Psychology © 2007 by Worth Publishers

Action potential travels down axon Figure 4.9 Propagation of the action potential along an unmyelinated axon Klein/Thorne: Biological Psychology © 2007 by Worth Publishers

Action potential travels down axon Action potentials are nondecremental – do not become weaker as they travel Travel very slowly

Action potential only causes those ion channels in one small spot of the membrane to open To travel down the axon, needs to nudge the adjacent ion channels

Conduction of Action Potential in Myelinated Axons Figure 4.10 Propagation of the action potential along an myelinated axon Klein/Thorne: Biological Psychology © 2007 by Worth Publishers

Conduction of Action Potential in Myelinated Axons This time, Action potential travels rapidly Action potential simply hops from one node of Ranvier to another Saltatory conduction (saltare = dance) Action potential grows weaker as it travels But, still strong enough to initiate another action potential at the next node of Ranvier _________________________________________

Synaptic Transmission of Signals A Chemical Process

Chemical signals In your nervous system there are chemicals called “neurotransmitters.” Neurons produce neurotransmitters.

Neurotransmitters Neurotranmsitters are packed into synaptic vesicles. Synaptic vesicles are found at the terminal buttons.

Release of neurotransmitters Action potential travels down axon and reaches synapse This causes Ca2+ (calcium) ion channels to open Figure 4.11 Overview of synaptic transmission Klein/Thorne: Biological Psychology © 2007 by Worth Publishers

Release of neurotransmitters Ca2+ ions cause synaptic vesicles to join to presynaptic membrane Vesicles release neurotransmitters into synaptic cleft Neurotransmitters get passed on to the next neuron Figure 4.11 Overview of synaptic transmission Klein/Thorne: Biological Psychology © 2007 by Worth Publishers

It’s like playing pinball What happens next? It’s like playing pinball

Back to where we started… Neurotransmitter binds with receptor, causes ion channels to open If Na+ channels open, then Na+ ions enter neuron, depolarizes membrane  EPSP

Back to where we started… If chloride channels open, the Cl- ions enter neuron, hyperpolarizes membrane  IPSP If potassium channels open, the K+ ions leave the neuron, hyperpolarizes membrane  IPSP

What happens to the leftover neurotransmitters? Reuptake Neurotransmitters return to presynaptic buttons Figure 4.18 Termination of neural transmission Klein/Thorne: Biological Psychology © 2007 by Worth Publishers

What happens to the leftover neurotransmitters? Degradation Neurotransmitters broken apart in the synapse by enzymes Figure 4.18 Termination of neural transmission Klein/Thorne: Biological Psychology © 2007 by Worth Publishers

Chemicals in the Nervous System Neurotransmitters Chemicals in the Nervous System

Neurotransmitters Acetylcholine (Ach) Gamma-aminobutyric acid (GABA) Muscles Memory: Alzheimer’s Gamma-aminobutyric acid (GABA) Seizures Huntington’s disease

Neurotransmitters Epinephrine (aka adrenaline) Norepinephrine Activation of cardiovascular system

Neurotransmitters Dopamine Serotonin Schizophrenia Parkinson’s Depression Aggression _____________________________________

Agonists and Antagonists

Agonists Synthesis of neurotransmitter Helps with release Obstructs autoreceptor Pretends to be neurotransmitter Prevents reuptake Figure 4.16 Autoreceptors Klein/Thorne: Biological Psychology © 2007 by Worth Publishers

Antagonists Obstacle to synthesis Obstacle to neurotransmitter release Fools autoreceptor Blocks receptor

How some drugs work Cocaine Agonist of norepinephrine and dopamine Prevents reuptake of leftover norepinephrine and dopamine Therefore, effects of these neurotransmitters are increased

How some drugs work Botulinium toxin Antagonist of acetylcholine Prevents acetylcholine from being released Therefore, effects of these neurotransmitters are decreased Small amounts used to paralyze certain muscles