AXONAL CONDUCTION TO CARRY MESSAGES TO OTHER CELLS Axons can range in length from a fraction of an inch to several feet, but they all have the same function: TO CARRY MESSAGES TO OTHER CELLS How does the neuron send its signal down the axon? NEURAL IMPULSES Every time an impulse travels down the axon, the axon has “fired” or “responded.” This impulse is called the ACTION POTENTIAL. It is either “All or None.”

Axon chemistry Ions = some of the molecules in and around the axon carry an electrical charge, either positive or negative. The fluid inside the axon contains protein molecules (anions) with a negative charge (P-). These particular protein molecules are found only in the intracellular fluid. Other electrically charged ions are present in the fluid both inside and outside the axon.

Cell Membrane in resting state Ion concentrations Cell Membrane in resting state K+ Na+ Cl- A- Outside of Cell Inside of Cell Key words: ion concentrations; cell membrane; intracellular fluid; extracellular fluid; Na+; Cl-; K+ Slide ten represents a schematic of the typical concentrations of the intracellular and extracellular fluids. There are large concentrations of sodium and chloride ions concentrations of on the outside of the cell (relative to inside the cell). There are large concentrations of potassium ions and protein molecules on the insde of the cell (relative to concentrations on the outside of the cell).

The Axon Membrane To understand how impulses travel, you need to know that the axon’s membrane is selectively permeable. That means that some molecules can pass easily through the membrane, while others can only pass through special channels controlled by gates.

The Cell Membrane is Semi-Permeable Cell Membrane at rest Na+ Cl- K+ A- Outside of Cell Inside of Cell Potassium (K+) can pass through to equalize its concentration Sodium and Chlorine cannot pass through Result - inside is negative relative to outside - 70 mv Key words: Cell membrane; semi-permeable; K+; Na+; Cl- The cell membrane is semi-permeable. That is, when the neuron is at rest, the cell membrane allows some ions (K+) to pass freely through the cell membrane, whereas other ions (such as Na+ and Cl-) cannot. Hit enter once and K+ ions will slowly pass through the cell membrane. After K+ animation is finished, hit enter again and animation showing that Na+ and l- ions cannot pass through the membrane will occur.

Axon chemistry, continued THE RESTING POTENTIAL When the axon is in its resting state, there are more negative particles inside the axon than outside the axon, largely because of the presence of protein anions inside but not outside the axon. This produces an electrical charge of -70 mV (the resting potential) across the membrane. The interior of the axon is electrically negative compared to the exterior.

Resting Potential At rest the inside of the cell is at -70 millivolts With inputs to dendrites inside becomes more positive if resting potential rises above threshold an action potential starts to travel from cell body down the axon Figure shows resting axon being approached by an AP

Axon chemistry, continued Remember there are several types of positively charged and negatively charged molecules on both sides of the axon membrane. Our focus will be on the sodium (Na+) and potassium (K+) ions, because their movement through the membrane is the central issue.

Although sodium (Na+) ions are present on both sides of the membrane, they are more concentrated in the extracellular fluid. Potassium ions (K+) are more concentrated in the intracellular fluid.

Axon polarization Remember that the overall balance between the positive and negative charges is such that the axon is electrically negative with respect to the outside. We say that the axon is polarized in its resting state (the resting potential) at -70 mv.

Membrane channels Scattered throughout the axon membrane are sodium channels (yellow) and potassium channels (purple). Ions diffuse through these channels. Sodium/potassium pumps (yellow/purple) capture ions and actively push them through the membrane.

SODIUM GATES The positively charged sodium ions are repelled by the other positive charges outside the axon, and attracted to the negative charges inside, but are kept out by the sodium gates in the cell membrane.

Depolarization When the neuron fires, the part of the axon nearest the cell body opens its sodium gates, allowing the sodium ions to rush in. The influx of these positively charged ions depolarizes that part of the axon, meaning it makes that part of the axon electrically positive relative to the extracellular fluid. This in turn causes the next section of the axon to open its gates and become depolarized, and the impulse (the action potential) moves down the axon like a wave.

The Action Potential Graded potentials are generated at the dendrites and are conducted along the membrane to the axon hillock If the summated activity at the axon hillock raises the membrane potential past threshold, an action potential (AP) will occur During the AP, NA+ ions flow into the cell raising the membrane potential to +40 mV, producing the spike The restoration of the membrane potential to -70 mV is produced by an opening of channels to K+ The AP is conducted along the axon toward the terminals © 2004 John Wiley & Sons, Inc.

Depolarization AP opens cell membrane to allow sodium (Na+) in Graphic from Hockenbury slides AP opens cell membrane to allow sodium (Na+) in Inside of cell rapidly becomes more positive than outside This depolarization travels down the axon as leading edge of the AP

REPOLARIZATION As the impulse travels to the next section of the axon, the sodium gates in the first section of the axon close up again. Other gates open to allow some of the potassium ions, repelled by the sudden positive shift, to flow out of the axon, thereby repolarizing the membrane.

Repolarization follows depolarization Graphic from Hockenbury slides After depolarization potassium (K+) moves out restoring the inside to a negative voltage This is called repolarization The rapid depolarization and repolarization produce a pattern called a spike discharge

Hyperpolarization Repolarization leads to a voltage below the resting potential, called hyperpolarization Now neuron cannot produce a new action potential This is the refractory period

RETURN TO RESTING POTENTIAL Sodium/potassium pumps push most of the sodium ions out of the axon and transport most of the potassium ions back inside the axon. This restores the electrochemical situation that existed before the action potential reached this portion of the axon.

Sending the messages This process of temporary depolarization followed by repolarization continues on down the axon. The action potential travels at speeds ranging from 1 to 200 mph, depending on the type of neuron. More intense stimulus = more frequent firing

THE SYNAPSE When one neuron communicates with another neuron, they do not touch. There is a small gap between the neurons. This gap is the synaptic gap or synaptic cleft. The actual synapse consists of three elements: the presynaptic membrane which is formed by the terminal button of an axon the synaptic cleft or gap. the postsynaptic membrane which is composed of a segment of dendrite or cell body This is where synaptic transmission takes place.

How does the signal jump across the synaptic gap? Sending Neuron Synapse Axon Terminal End of the axon = axon terminal Tip of axon terminals = terminal buttons/synaptic knobs Terminal buttons contain tiny sacs = synaptic vesicles Inside vesicles = neurotransmitters

NEUROTRANSMITTERS When the neural impulse reaches the axon terminal, the vesicles open up and release their neurotransmitter molecules into the synaptic cleft. Stimulating the receptors Each neurotransmitter is like a piece of a jigsaw puzzle (or key for a lock). Each receptor (binding site) will accept only a certain type of neurotransmitter.

EXCITATORY & INHIBITORY SYNAPSES If the message sent to the next neuron has an excitatory post-synaptic potential/EPSP), it is MORE likely that an action potential will fire. If the message sent to the next neuron has an inhibitory post-synaptic potential/IPSP) it is LESS likely that an action potential will fire. SO, the rate of firing of the target neuron is influenced by the ratio of excitatory and inhibitory impulses that it is receiving.

Table 2.1 Myers: Psychology, Ninth Edition Copyright © 2010 by Worth Publishers

Inactivation of Neurotransmitters The action of neurotransmitters can be stopped by four different mechanisms: 1. Diffusion: the neurotransmitter drifts away, out of the synaptic cleft where it can no longer act on a receptor. 2. Enzymatic degradation (deactivation): a specific enzyme changes the structure of the neurotransmitter so it is not recognized by the receptor. For example, acetylcholinesterase is the enzyme that breaks acetylcholine into choline and acetate. Enzymatic degradation    

Inactivation of Neurotransmitters, continued 3. Glial cells: astrocytes remove neurotransmitters from the synaptic cleft 4. Reuptake: the whole neurotransmitter molecule is taken back into the axon terminal that released it. This is a common way the action of norepinephrine, dopamine and serotonin is stopped...these neurotransmitters are removed from the synaptic cleft so they cannot bind to receptors

Some drugs are shaped like neurotransmitters Antagonists : Inhibit; fit the receptor but not well enough to stimulate receptor; block the NT action e.g. beta blockers Agonists : Excite; fit receptor well and act like the NT e.g. nicotine, morphine

EXCITATORY & INHIBITORY SYNAPSES When NT binds to receptor, ions enter. Positive ions (NA+ ) depolarize the neuron, giving it an excitatory post-synaptic potential/EPSP); makes it MORE likely that an action potential will fire. Negative ions (CL-) hyperpolarize the neuron, giving it an inhibitory post-synaptic potential/IPSP); makes it LESS likely that an action potential will fire. SO, the rate of firing of the target neuron is influenced by the ratio of excitatory and inhibitory impulses that it is receiving.