Impulse Conductionin Neurons

Learning Targets• By the end of this lesson, you should

be able to:• Explain what polarized, depolarized, and

repolarized mean with respect to neurons.• Explain the events of impulse conduction.• Explain the events that occur at a synapse.• Describe some effects that drugs have on

synaptic transmission.

Cell Membrane Potential In a resting neuron,

a greater number of positive ions are located

outside the cell, causing the membrane to be electrically charged (polarized).

Polarization is maintained by the cell’s ability to control whether or not some ions are allowed to cross the cell’s membrane through protein channels.

-70 mV resting membrane potential

axon

Events of Impulse Conduction• (1) If a stimulus causes enough of a

change in the membrane potential (-55 mV), an action potential is achieved.

• -55mV is the threshold voltage

Events of Impulse Conduction

• (2) A stimulus opens Na+ channels in the cell membrane allowing Na+ to flow into the cell. This causes the resting potential to rise (depolarization).

Events of Impulse Conduction3. The action potential triggers the opening

of Na+ channels in adjacent areas of the axon, causing the nerve impulse to continue down the axon in a domino-like effect.

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Events of Impulse Conduction

4. K+ channels open, allowing K+ to flow out of the cell. The potential drops, returning to its starting point (repolarization).

5. A single depolarization-repolarization cycle takes approximately .001 seconds.

The All-or-None Response

• The action potential acts according to an all-or-none principle: It has to reach a threshold voltage (-55 mV) before it will fire, and then it fires completely.

• If threshold potential is not reached, the neuron will NOT fire (failed initiation)

Unmyelinated neurons conduct impulses over the entire membrane surface.

Myelinated neurons conduct impulses only at the nodes of Ranvier, allowing the impulse to move many times faster as the impulse jumps from one node to another (up to 150 m/s).

Synaptic Transmission

1. When an impulse reaches the end of an axon, calcium ions diffuse into the axon terminal, causing synaptic vesicles to dock and fuse with the neuron’s membrane.

2. Neurotransmitters are released into the synaptic cleft.

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Synaptic Transmission3. Neurotransmitters attach to

receptors on the postsynaptic membrane, causing ion channels within that membrane to open.

4. Ions flow into the postsynaptic cell, eliciting a response.

5. Neurotransmitters in the synaptic cleft are either broken down by enzymes or reabsorbed by the presynaptic neuron (reuptake).

Click image for animation

Learning Targets• By the end of this lesson, you

should be able to:• Explain what polarized,

depolarized, and repolarized mean with respect to neurons.

• Explain the events of impulse conduction.

• Explain the events that occur at a synapse.

• Describe some effects that drugs have on synaptic transmission.

Summary• (1) The neuron maintains its

resting potential (-70 mV)• (2) A threshold stimulus is

received. (-55 mV)• (3) Sodium ion channels

open.• (4) Sodium ions diffuse

inward, depolarizing the membrane.

• (5) Potassium ion channels open/ Sodium ion channels close.

• (6) Potassium ions diffuse outward, repolarizing the membrane.

Summary• (7) The resulting action

potential causes a local bioelectric current that stimulates adjacent portions of the membrane.

• (8) An action potential reaches the axon terminal, causing calcium ion channels to open.

• (9) Neurotransmitters are released into the synaptic cleft.

Summary• (10) Neurotransmitters

attach to receptors on the postsynaptic membrane, causing ion channels to open.

• (11) Ions flow into the postsynaptic cell, eliciting a response.

• (12) Enzymes break down neurotransmitters still in synaptic cleft or neurotransmitters are taken back up by the presynaptic membrane (reuptake).

Some common neurotransmitters…• Acetylcholine

– Ex. Skeletal muscle movement• Norepinephrine

– Ex. Alertness, fight or flight• Dopamine

– Ex. Pleasure, reward– Too much= Schizophrenic

symptoms– Too little= Parkinson’s symptoms

• Serotonin– Feelings of well-being &

happiness, obsessions & compulsions

• Endorphins– Involved in inhibition of pain– May be responsible for “runner’s

high” Click image

• Here are summaries of the effect of select street drugs on the brain. Some of the introductory information is derived from About.com. Select authoritative references for information about effects of drugs on the brain include:

• Heroin• Heroin is a highly addictive opiate (like morphine). Brain cells can become

dependent (highly addictive) on this drug to the extent that users need it in order to function in their daily routine. While heroin use starts out with a rush of pleasure, it leaves the use in a fog for many hours afterwards. Users soon find that their sole purpose in life is to have more of the drug that their body has become dependant on.

• Marijuana• The parts of the brain that control emotions, memory, and judgment are affected

by marijuana.  Smoking it can not only weaken short-term memory, but can block information from making it into long term memory. It has also been shown to weaken problem solving ability.

• Alcohol• Alcohol is no safer than drugs. Alcohol impairs judgment and leads to memory

lapses. It can lead to blackouts. It distorts vision, shortens coordination, and in addition to the brain can damage every other organ in the body.

• Cocaine• Cocaine, both in powder form and as crack, is an extremely addictive stimulant. An

addict usually loses interest in many areas of life, including school, sports, family, and friends. Use of cocaine can lead to feelings of paranoia and anxiety. Although often used to enhance sex drive, physical effect of cocaine on the receptors in the brain reduce the ability to feel pleasure (which in turn causes the dependency on the drug).

• Inhalants• Inhalants, such as glue, gasoline, hair spray, and paint thinner, are sniffed. The

effect on the brain is almost immediate. And while some vapors leave the body quickly, others will remain for a long time. The fatty tissues protecting the nerve cells in the brain are destroyed by inhalant vapors. This slows down or even stops neural transmissions. Effects of inhalants include diminished ability to learn, remember, and solve problems. 

• Ecstasy• Extended use of this amphetamine causes difficulty differentiating reality and

fantasy, and causes problems concentrating. Studies have found that ecstasy destroys certain cells in the brain.  While the cells may re-connect after discontinued use of the drug, they don't re-connect normally. Like most drugs, this one impairs memory and can cause paranoia, anxiety, and confusion.

What about Drugs?• A drug may…• (1) mimic the

neurotransmitter• (2) block the section of the

neurotransmitter– Anesthetics block the opening

of ion channels*Anesthetics interrupt impulses from passing

thru the affected region and reaching the brain, preventing sensations of touch & pain*

• (3) block the re-uptake of the neurotransmitter– Cocaine blocks dopamine re-

uptake– Antidepressants block

norepinephrine &/or serotonin re-uptake

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