Nerve Cells & Electrochemical Impulses

• Electrochemical impulses in nerve cells facilitate rapid communication.
• Practicing and knowing your learning style are keys to success.

History & Overview

• In the 1900s, Julies Bernstein proposed nerve impulses are electrochemical messages caused by ion movements.
• In 1939, Cole & Curtis discovered squid axons are negatively charged inside relative to the outside.
• When stimulated, the charge becomes positive inside for milliseconds, then returns to negative.

Nerve Impulse

• A nerve impulse is an electrical signal traveling down the axon.
• Dendrites receive messages from other cells, and the cell body is the life-supportenter.
• The axon passes messages away from the cell body to other neurons, muscles, or glands.
• Terminal branches of the axon form junctions with other cells.
• The myelin sheath covers the axon and helps speed up neural impulses.

Cell Membrane

• The cell membrane consists of a lipid bilayer, proteins, carbohydrates chains and phospholipids.
• The outside of the cell is distinct from its inside, which contains cytoplasm.
• Membrane potential is the charge difference between the neuron's outside and inside.
• Resting potential is the membrane potential at rest, averaging -70 mV.
• A membrane is considered polarized when it has a resting potential.

Resting Potential Establishment

• The axon's outside contains lots of Na+ that want to diffuse in.
• The axon's inside contains lots of K+ that want to diffuse out.
• The membrane is more permeable to K+.
• This results in membrane leaks
• More K+ goes out than Na+ coming in, resulting in a more positive charge outside and a more negative charge inside.
• Sodium-potassium pumps maintain the charge difference between the outside and inside.

Sodium Potassium Pumps

• These pumps use active transport to force extra sodium out and potassium in.
• They maintain a higher external voltage and rectify the leaks.
• Three sodium ions move out while two potassium ions move in.
• ATP splitting provides the energy to change the shape of the channel.

Action Potential

• When a nerve impulse arrives, membrane permeability changes.
• Sodium channels open, causing sodium to rush in.
• The inside of the neuron becomes more positively charged.
• The membrane potential becomes +40 mV during an action potential.
• Opening of sodium channels initiates an action potential and this stage is called depolarization.

Repolarization

• Action potential only lasts 0.2 – 2 ms.
• Sodium channels close during repolarization.
• Potassium channels open, causing potassium to rush out.
• The charge inside the axon becomes negative again.

Refractory Period

• During repolarization, it becomes more negative than resting potential, called hyperpolarization (-75mV).
• Eventually, the sodium-potassium pump restores balance and ion position.
• The refractory period, lasting 1 - 10 ms, is the time this takes.
• During the refractory period, another action potential cannot be started.

Transmission of Impulses

• Once threshold stimulus is reached, the response is all-or-none.
• There are no variations in speed or intensity of the action potential.
• One action potential at one axon point triggers an action potential right next to it.
• The action potential only goes one way because the area that it just came from is in its refractory period.
• Ion channels in the axon are voltage-gated.
• Depolarization at one axon segment triggers the ion channel opening in the next segment.
• Consequently, the action potential spreads along the axon as a depolarization wave.

Myelinated Neurons

• Myelinated neurons are commonly found in the PNS, or peripheral nervous system.
• When axons are myelinated, impulses travel by saltatory conduction.
• Action potentials occur only at the nodes of Ranvier in myelin sheaths.
• Myelination results in rapid transmission.

Sensation Intensity

• Stimulus intensity depends on how frequent impulses occur, not voltage.
• The intensity is increased by having more frequent nerve impulses.
• The more intense the stimulus, the greater the impulse frequency.
• Intensity can also be increased with a greater number of neurons are stimulated by more energy.

Nerve cells use electrochemical impulses for rapid communication. Key discoveries in the 1900s revealed the nature of these impulses. A nerve impulse is an electrical signal traveling down the axon, facilitated by the cell membrane's structure.