9.2 Electrochemical Impulses

• Nerves impulses are similar to electrical impulses but are slightly slower.

• They stay the same strength throughout the entire length of the neuron however.

• Nerve impulses are electrochemical messages made by the movement of ions through the nerve cell membrane.

Comparison of Nervous and Endocrine System

• communicates with electrical impulses and neurotransmitters

• reacts quickly to stimuli, usually within 1 to 10 msec

• stops quickly when stimulus stops

• adapts relatively quickly to continual stimulation

• has relatively local, specific effects on target organs

• communicates with hormones carried in blood

• reacts more slowly to stimuli, often taking seconds to days

• may continue responding long after stimulus stops

• adapts relatively slowly; may continue responding for days or weeks

• sometimes has very general, widespread effects on many organs in the body

Nervous system Endocrine system

Synaptic Transmission

• Electrical Synapse – direct connection between the cytoplasm of the two

adjacent cells through gap junctions– Relatively quick– Common in invertebrates

• Chemical Synapse – far more prevalent in invertebrates and vertebrates – action potential will initiate a sequence of events in

the plasma membrane of the synaptic knob

• Early experiments used tiny electrodes inside neurons of giant squids. Results showed:– A fast change in electrical potential

difference across the membrane every time the nerve was excited.

– Resting membrane potential was about -70mV, but when the nerve was exited, it changed to +40mV.

– This change in potential is called the action potential. It did not last more than a few ms though before it returned to its resting potential (non-stimulated state).

How is an impulse is transmitted?• The nerve cell membrane is permeable to ions

which move across the membrane and set up an electrical chemical potential.

• When a nerve becomes excited, a rapid change in the potential difference is detected.

• This potential difference is called the action potential (+40 mV) and it travels along the neuron from dendrite to axon.

• After the nerve impulse travels down the axon the neuron returns to its original potential difference called the resting potential (-70 mV).

How do

The Details: Resting Potential

• A neuron at rest is polarized (more positive outside the membrane than inside).

• The difference in charge between the outside and the inside of the membrane in most neurons is –70 mV. This is caused by an unequal concentration of positive ions across the membrane of the neuron.

• The highly concentrated K+ ions inside the neurons have a tendency to diffuse (leak) outside of the nerve cells.

• Similarly the highly concentrated Na+ ions outside the neurons have a tendency to diffuse into the nerve cells.

• The diffusion of Na+ and K+ is unequal however.• The resting membrane is 50 X more permeable to K+

than it is to Na+, so more K+ diffuses out than Na+ in. The outside is therefore relatively more positive and is said to be polarized.

http:

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Resting Potential

Action Potential– When the nerve cell is excited, the membrane

becomes more permeable to Na+ than K+ .– It is believed that Na+ gates in the membrane

are opened and the K+ gates close.– There is a rapid flow of Na+ into the cell,

creating a depolarization.– The polarity of the membrane is reversed.– Once the voltage inside the cell becomes

positive, the Na+ gates slam shut and the inflow of Na+ is stopped.

• Na+ channels open and Na+ ions flood into the neuron.

• K+ channels close at the same time and K+ ions can no longer leak out.

• The interior of the neuron in that area becomes positive relative to the outside • Depolarization causes the electrical potential to change from –70 mV to + 40 mV

The sodium-potassium pump that is in the membrane restores the resting membrane by transporting Na+ out of the neuron while moving K+ into the neuron in a ratio of 3 Na+ to 2 K+ ions.

• This pump requires energy from ATP. The process is called repolarization.

Animation: The Nerve Impulse

Repolarization•The spike in voltage causes the K+ pumps to open and K+ ions rush out

•The inside becomes negative again.

Refractory period

• Once the action potential has peaked, a refractory period occurs while the neuron returns to its resting potential (polarized conditions).

• The membrane becomes impermeable to Na+ ions and the Na+/K+ pump will pump the Na+ ions back out of the neuron.

• Nerves conducting an impulse cannot be activated until the condition of the resting membrane is restored.

• The period of depolarization must be completed and the nerve must repolarize before the next action potential is conducted.

• The time it takes for the cell to repolarize (refractory period) is about 1 to 10 ms.

• .

Refractory Period

Local Po

• So many K+ ions get out that the charge goes below the resting potential.

The Refractory period

Section of Graph

Activity Description

a original resting potential

b gradual depolarizationSODIUM CHANNELS OPEN

c rapid depolarization (becomes positive in or negative on the outside)MORE SODIUM CHANNELS OPEN

d excessive charge inside cell is rapidly lostSODIUM CHANNELS CLOSE & POTASSIUM CHANNELS OPEN

e temporarily drops below original resting potential before restorationPOTASSIUM CHANNELS CLOSE

MOVEMENT OF THE ACTION POTENTIAL

• In order for the impulse to move along the axon, the impulse must move from one zone of depolarization to adjacent regions.

• The positively charged ions that rush into the nerve cell during depolarization, are then attracted to the adjacent negative ions, which are aligned along the inside of the nerve membrane.

• A similar attraction occurs along the outside of the nerve membrane.

Why does the current move in one direction?

• The electric current passes outward over the membrane in all directions BUT the area to one side is still in the refractory period and is not sensitive to the current.

• Therefore, the impulse moves from the dendrites toward the axon.

What causes the inside of a neuron to become negatively charged?

Positively charged ions are lost from inside of the resting membrane faster than they are added.

•http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter14/animation__the_nerve_impulse.html

THRESHOLD LEVELS AND THE ALL-OR-NONE RESPONSE

• The threshold level is the minimum level of a stimulus required to produce a response.

• The all-or-none response refers to the fact that a nerve responds completely or not at all to a stimulus.

• If there is not enough potential to reach the threshold level, no response is noted.

• A potential stimulus must be above a critical value (threshold level) to produce a response.

• Increasing the intensity of the stimuli above threshold will not produce an increased response.

• Intensity of impulse & speed of transmission remain the same.

Neurons, Synapses, Action Potentials, and Neurotransmission - The Mind Project

Propagation of APin unmyelinated fibre

• action potential will begin in one spot • membrane at that location will undergo a

depolarization • The depolarizing membrane will cause adjacent

channels in the membrane to open causing the depolarization of that membrane

• membrane that originally underwent depolarization will be repolarizing

• and then enter a period of refraction

Propagation of AP in myelinated fibre

• Saltatory conduction due to myelin sheath

• Charge jumps from one node to the next

• http://www.blackwellpublishing.com/matthews/actionp.html

Differentiating Between Warm & Hot• The more intense the stimulus, the greater

the frequency of impulses.• Intense stimuli excite more neurons.

– Different neurons will have different threshold levels.

– This affects the number of impulses reaching the brain.

Threshold Levels & the All-or-None Response

SYNAPTIC TRANSMISSION

• There are small spaces between neurons and between a neuron and an effector. These spaces are called synapses.

• Small vesicles that contain chemicals called neurotransmitters are found in the end plates of nerves.

• Impulses move along the axon and release neurotransmitters from the end plate.

Neurotransmitters– Chemicals that are

produced within a neuron, are released by a stimulated neuron, and cause an effect on adjoining neurons.

– There are two types of neurotransmitters:

1. Small molecule neurotransmitters:

2. Neuropeptides

• These neurotransmitters are released from the presynaptic neuron and diffuse across the synaptic cleft, creating a depolarization of the dendrites of the postsynaptic neuron.

• This does slow down nerve transmission, so the greater number of synapses, the slower the speed of transmission.

Synaptic Transmission

Neuronal communication

• Acetylcholine is an example of an excitatory neurotransmitter found in the end plates of many nerve cells. It can act as an excitatory neurotransmitter on many postsynaptic neurons by opening the sodium channels.

• Once acetylcholine has done its job, and the action potential has moved to the postsynaptic neuron, we need cholinesterase to break down the acetylcholine so that the sodium channels can close.

Close to Home Animation: Cocaine

Close to Home Animation: Alcohol

• Not all neurotransmitters are excitatory. Some are inhibitory neurotransmitters.

• Inhibitory neurotransmitters work by opening the potassium channels, letting out even more potassium from the postsynaptic cell. The neuron is said to be hyperpolarized because it becomes more negative in the cell.

• These inhibitory neurotransmitters help prevent postsynaptic neurons from being active.

• The interaction of excitatory and inhibitory neurotransmitters is what allows you to throw a ball. As the triceps receive excitatory impulses and contracts, the biceps receive inhibitory and relaxes.

Summation

• Summation is the effect produced by the accumulation of neurotransmission from two or more neurons.

• http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter45/animations.html#

Animation: Chemical Synapse (Quiz 1)

ANIMATIONS Neurons and Synapse Action

• McGraw-Hill

http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter45/animations.html#

• Channel gating during an action potential

http://www.blackwellpublishing.com/matthews/channel.html

Propagation of the action potential.

http://www.blackwellpublishing.com/matthews/actionp.html

Synaptic vesicle fusion and neurotransmitter release.

http://www.blackwellpublishing.com/matthews/nmj.html