Electrochemical Impulse

SBI 4U

Some Interesting Facts about the Neuron

Longevity – can live and function for a lifetime

Do not divide – fetal neurons lose their ability to undergo mitosis; neural stem cells are an exception

High metabolic rate – require abundant oxygen and glucose

How do nerve cells pass along a message? Nerve impulses remain as strong at

the end of a nerve as they were at the beginning

Use cellular energy to generate current (which is the message)

Deals with a change in electrical potential energy across a membrane

Action vs Resting PotentialAction potential voltage

difference across a membrane when a nerve is excited

Resting potential voltage difference during resting stage

How do nerve cell membranes become charged? Molecular Level of the Nerve Cell

Neurons have a large amount of both + and – ions both inside and outside cell (unlike most cells)

Negative ions don’t contribute much charge (too big)

Key Focus:▪ Electrochemical message is caused by an

unequal concentration of + ions across the nerve cell membranes

How do nerve cell membranes become charged? Note

Sodium likes to diffuse into cell Postassium likes to diffuse out of cell▪ Both diffuse at the same time▪ However, diffusion is unequal; cell more permeable to

potassium, so more potassium diffuse out Since more K+ out, exterior of membrane more

positive than interior Conversely the excess negative ions

accumulate along the inside of the membrane▪ Creates a polarized membrane

How a charge is generated Ion gates allow ions into and out of

cell Steps1. Nerve excited2. Na+ gates open allowing more sodium ions

inside cell, while K+ gates close3. Rapid inflow of + ions causes charge reversal

(a.k.a. Depolarization)4. Inside of cell now positive so Na+ gates close5. Sodium-potassium pump restores resting

membrane by transporting 3 Na+ out for every 2 K+ in (a.k.a. Repolarization)

Process of Depolarization and Repolarization

Note how the action

potential is

moving away

from the site of origin

Refractory Period

Before a nerve can produce another action potential, it must repolarize

This recovery time is called the refractory period

Often lasts from 1 to 10 ms

Movement of Action Potential Once a nerve is stimulated, the

message needs to be carried along the length of the axon

Therefore, depolarization has to move from the zone it initiated in to adjacent regions

How does this happen?

Movement of Action Potential Once an action depolarization has

occurred, there are now more positive ions on the inside of the membrane

These positive ions are attracted to the negative ions in the adjoining regions that have not been stimulated

As these positive ions move toward the negative ions (the resting membrane), the nerve impulse is carried with them

This resting membrane then undergoes depolarization

Movement of Action Potential

Movement of Action Potential Once depolarization happens, it stimulates

the sodium channels to open which results in the movement in the action potential

This wave of depolarization therefore moves along the length of the nerve membrane

Depolarization of the membrane causes the sodium channels to close, the potassium ions to reopen

Note that every wave of depolarization is followed by a repolarization and a refractory period

Movement of Action Potential

Nodes of Ranvier How does one action potential move down the axon? Nodes of Ranvier are located between myelinated sections of

the axon Figure 8.15, p.358 Nodes contain many Na+ channels Nodes are the specific site of triggering an action potential Remember: sodium ions ENTER via the channels This DEPOLARIZES the membrane THRESHOLD occurs Prior membrane can not be stimulated. Why not? Next membrane in front can be. Once Threshold occurs,

action potential is triggered. Signal continues down axon. This process can also be referred to as : Saltatory

Conductions No jumping in non-myelinated axons. So which is faster?

Movement of an Action Potential, cont’d

Threshold Levels

Threshold Level A minimum level of stimulus is required

to produce a response Varies depending on the neuron

All-or-None Response Once you reach the threshold

level, adding more stimulus will not elicit a greater response

A nerve muscle fibre responds completely or not at all to a stimulus Note: the intensity of the response

will change depeding on:▪ If the brain recognizes a change in the

frequency of responses ▪ if neurons that have a higher

threshold are excited

Higher threshold of B, so two neurons are

excited

Synaptic Transmission

Messages need to be transmitted between neurons

Occurs via synapses Small spaces between neurons, or between

neurons and effectors (e.g. Muscles) As impulse moves along axon it reaches

the endplate and releaes vesicles that contain neurotransmitters (via exocytosis)

Neutrotransmitters are released from the presynaptic neuron, travel across the synaptic cleft, and create a depolarization of the post synaptic neuron

Synaptic Transmission

Example of Neurotransmitter Acetylcholine

Acts as excitatory neurotransmitter on many post synaptic neurons

Opens sodium channels, helping with depolarization

Potential problem keeps sodium channels open, so keeps cell in constant state of depolarization; not able to respond to next impulse since no refractory period▪ Solution: membrane enzyme cholinesterase destroys

acetylcholine

Real world application: insecticides blocks cholinesterase so insect heart remains contracted