Electrochemical Impulse SBI 4U Electrochemical Impulse

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 Potential Action 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 Steps Nerve excited Na+ gates open allowing more sodium ions inside cell, while K+ gates close Rapid inflow of + ions causes charge reversal (a.k.a. Depolarization) Inside of cell now positive so Na+ gates close 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

Higher threshold of B, so two neurons are excited 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