The formation and the transmission of message in nervous system Farbod Niazi

What is a neuron and an action potential General information What is a neuron and an action potential

What is a neuron The neuron is a nervous system cell that is designed to transmit messages. This cell has three parts: Dendrites Cell body Axon

The path of the message in a neuron Signals are transmitted from dendrites to cell body and from cell body to axon Dendrites cell body axons synaptic terminals BACK

The resting potential When a neuron doesn’t work, it’s at its resting potential. At the resting potential, there is a difference between the electric charge of inside the cell and outside the cell. The cytoplasm of the neuron is slightly more negative than the extracellular fluid(-70 mV) . In fact, the membrane of the cell is polarized because it has a positive pole and a negative pole.

What help the cell to maintain the resting potential There are some channels and pumps that help the neuron to maintain his charge: Chemically-activated channels Voltage-activated channels Passive ion channels Sodium-potassium pumps Back

Passive ion channels Channels with no gates that let specific ions in. At resting potential, the cell is more permeable to potassium ion than to sodium ions, so these channels let more K+ in than Na+. Neuron channels

Sodium-potassium pumps A pump which receives energy to make an exchange between sodium and potassium ions. This pump let two potassium ions in for each three sodium ions that leave the cell. Neuron channels

But how a neuron creates messages Neurons can transmit electrical signals called nerve impulses or action potentials. Every neuron receives signals from its dendrites and transmits it by its axon. This flow continues until it reaches the brain or the spinal cord. The path of a message through a neuron

How the action potential or the nerve impulse is formed When a stimulus is strong enough, it causes a difference of membrane voltage. The cell membrane becomes permeable to sodium ions which are positively charged. This change can fire an action potential and create a nerve impulse.

The creation of a nerve impulse These channels are the main reason of the change in membrane potential

The process of creating a nerve impulse Details The process of creating a nerve impulse

The threshold level Any stimulus that can increase the voltage of cytoplasm to -55 mV or more causes an action potential. When the threshold level is reached at -55 mV, the voltage of the cytoplasm suddenly reaches 0 and overshoots to +35 mV. In fact, the membrane is depolarized.

Voltage-activated channels The change in voltage A stimulus can trigger the voltage-activated channels. Voltage-activated channels are responsible for the sudden change of voltage. There is a specific voltage-activated channel for every ion that pass through the membrane of a neuron. But the potassium and sodium ions voltage-activated channels are essential for the creation of the action potential. Voltage-activated channels

Sodium voltage-activated channels These channels have two gates: Activation gates Inactivation gates When a stimulus is detected, the activation gates open and let some sodium ions in.

Sodium voltage-activated channels When a certain number of ions come into the cell, the threshold level is reached, which means that some other gates start to open.

The opening of sodium voltage-activated channels

Before and after the depolarization

Depolarization is stopped When the membrane voltage reaches 35 mV, the inactivation gates close in response to depolarization and the sodium ions can’t enter the cell anymore. The Na+ can only come in during a brief period when both activation and inactivation gates are open.

The opening and the closing of a voltage-activated channel’s gates

Repolarization What a neuron hates the most is when the membrane potential is positive. That’s exactly the case of a neuron after the depolarization. So, potassium voltage-activated channels open and let K+ ions out

Potassium voltage-activated channels Unlike sodium voltage-activated channels, these channels have only one gate.

Potassium voltage-activated channels In response to depolarization, the only gate of potassium voltage-activated channels opens and potassium ions rush out of the cell and the membrane voltage reaches -70 mV again.

Repolarization Resting potential Threshold level Depolarization

The flow of the nerve impulse When an action potential is initiated, it continues along the cell until the end of the axon where it jumps to another neuron by neurotransmitters.

Resting potential Depolarization Repolarization Resting potential

The Nervous System – Crash Course Biology #26 Sources Author’s name Date of publication Title Publisher or URL Eldra P. Solomon Linda R. Berg Diana W. Martin 2010 Solomon biology Life science Crash course 2012 The Nervous System – Crash Course Biology #26 https://www.youtube.com/watch?v=x4PPZCLnVkA&t=574s&list=PL3EED4C1D684D3ADF&index=26 Eric H. Chudler N.D Neuroscience for kids http://faculty.washington.edu/chudler/neurok.html