Name: Chanaka Lakshan

Student ID: BS/15/10/15

Facilitator: Dr.Ariff

Subject: Human Physiology

Stream: Biomedical Science

Neuron Structure & Nerve Impulse Propagation

ContentNeuron StructureIon ChannelsResting Membrane potentialElectrochemical force for Na & KPositive feedback loop & interruptionAction Potential Graph & ExplanationAbsolute Refractory PeriodRelative Refractory PeriodConduction velocitySummary

Neuron StructureNeuron is a Specialized type of cell found in the body. A typical neuron is divided into ; 1.Cell body, 2.Dendrites, 3.Axon.

Cell BodyIt Is main nutritional and metabolic region of neuron.It receives signals from other cells and sends them towards the axon.

DendritesBranched dendrites receive signals coming in from other cells and send them towards the axon.

AxonIt generates an action potential, an outgoing signal also called a nerve impulse, and conducts it to the next cell. Axon is transmitting or conductive region of the neuron.

Ion ChannelsCell membrane is a lipid bilayer which large integral proteins embedded. Some of these proteins have watery pores called Ion Channels through which ion can pass. Ion Channels control the movement of ions through the neuronal cell membrane. They are;

1.Selective

2.Passive or Active

3.Regionally Located

4.Functionally Unique

Selective ion channels select ions for passage based on charge, size ,attraction & holding of water by the ion.

Active Channels have Gates that can Open & Close It. Passive Channels are always open. Voltage gated Channels have Gates that are Controlled

by Voltage. Neuronal cell membrane have more +ve ions outside &

more –ve ions inside. This difference makes membrane potential across cell membrane.

Chemically gated channels have neurotransmitters that bind to it.

Chemically gated channels controlled by acetylcholine & GABA bind to them causing it to open.

When neuron is at rest, Voltage gated channels are closed.

In Action potential, it open & Close. Ions move through Open Channels.

Ion ChannelsLocation Function

•Cell membrane on Dendrites.

•Cell body & Axon.

Passive Channels

•Dendrites.

•Cell body.

Chemically gated

Channels

•Axon Hillock, all along unmyelinated axon.

•Nodes of Ranvier in myelinated axons.

Voltage gated

Channels

•Makes Resting Membrane Potential.

•.

Passive Channels

•Makes synaptic Potentials.

•

Chemically gated

Channels

•Generation & Propagation of Action Potentials.

•.

Voltage gated

Channels

Resting Membrane Potential (Resting voltage)1.It is the relatively static  Membrane Potential of quiescent cells.

2.It is opposed to the specific dynamic electrochemical phenomena called action potential and graded membrane potential.

3.It is a relatively stable, ground value of transmembrane voltage in animal and plant cells.

4.It is results from the membrane of both Na+ & K+ ions. Activation gates closed & inactivation gates open in voltage gated Na+ channels, and voltage gated k+ channels closed.

5.During resting potential, the cell’s cytoplasm is negatively charged relative to the outside of the cell. It depends on distribution of both Na+ & K+ across cell membrane.

6.For many neurons, it is close to -70mV.

7.Na+ & K+ Pump Essential to Maintain It. ( When 3 Na+ go Out, 2 K+ comes In. )

Electrochemical Force for Na & K1.Chemical Force & Electric Force are either known as Electrochemical gradient.

2.When they are equal, the equilibrium potential occurs. At that potential, no net movement of K+ is occurred.

3.Forces from Concentration gradient & electrical potential combine to produce a large Electrochemical gradient drives Na+ into cell.

4.Positively charged Na+ ions that have entered the neuron make the membrane potential more positive than -90mV; which is the equilibrium potential for K+.

Positive Feedback Loop & Interruption*If the stimulus to the axon hillock is great enough, neuron depolarizes by about 15mV and reaches a point called Threshold.

*At that point, action potential is created.

*When & Only When a neuron reaches Threshold, a Positive Feedback Loop established.

*At Threshold, process of depolarization becomes initiate & positive feedback loop established and generate an action potential.

*Action potential always have the same amplitude & same duration & It is a all-or-none event. *Then, more Na flow into the cell, therefore cell depolarize further and opens still more voltage gated Na Channels.

*Therefore, At threshold, depolarization opens more voltage gated Na+ Channels.

*This positive feedback loop produces the rising phase of action potential. This ends when positive feedback loop is interrupted. This breakdown is done by 2 processes;

1.Inactivation of voltage gated Na+ channels .

2.Opening of voltage gated k+ channels.

Depolarization

Open Voltage gated Na+ Channels

Inward Flow Of Na+

Action Potential Graph & ExplanationRestvoltage gated Na+ & k+ channels closed.

Depolarization

voltage gated Na+ channels open rapidly, resulting In movement of Na+ into the cell.

Peak

Voltage gated Na+ channels begin to inactivate & Voltage gated K+ channels begin to open. This initiates repolarization.

Repolarization

With less Na+ moving to cell and more K+ out to cell, membrane potential becomes more Negative, moving towards resting value.

Hyperpolarization

Some voltage gated K+ channels remain open, resulting in movement of K+ out of the cell.

Absolute Refractory Period1.The absolute refractory period is the interval from the beginning of the action potential until the fiber is able to conduct another action potential.

2.After the neuron has generated an action potential, it can not generate another one. Many Na+ channels are inactive & will not open, no matter what voltage is applied to the membrane. Most K+ channels are open.

3.The neuron cannot generate an action potential because Na+ cannot move in through inactive channels & because K+ continues to move out through open voltage gated channels.

4.A neuron cannot generate an action potential during the absolute refractory period.

Relative Refractory Period 1.Immediately after the absolute refractory period, the cell can generate an action potential, but only if it is depolarized to a value more positive than normal threshold.

2.This is true because some Na+ channels are still inactive & some K+ channels are still open.

Conduction Velocity

*It is the speed with which an action potential is propagated.

It depends on;

1.The diameter of the axon.

2.How well the axon is insulated with myelin.

Diameter-As it increases, internal resistance to flow of charge decreases & Action Potential travel faster.

Myelination-In it charge flows only at nodes ,so Action Potential generated only at nodes.

More speed is gained by Myelination than increasing

diameter.

Summary; A neuron is an electrically excitable  specialized type of cell that processes and transmits information

through electrical and chemical signals  with the presence of synapses, which are complex membrane junctions that transmit signals to other cells with the aid of Cell body, Dendrites & Axon.

Ion Channels control the movement of ions through the neuronal cell membrane. Resting Membrane Potential is the relatively static  Membrane Potential of quiescent cells. It is opposed to the specific dynamic electrochemical phenomena called action potential and graded

membrane potential. Chemical Force & Electric Force are either known as Electrochemical gradient. When they are equal, the equilibrium potential occurs. At that potential, no net movement of K+ is

occurred. If the stimulus to the axon hillock is great enough, neuron depolarizes by about 15mV and reaches a

point called Threshold. At Threshold, process of depolarization becomes initiate & positive feedback loop established and

generate an action potential. This positive feedback loop produces the rising phase of action potential. This ends when positive

feedback loop is interrupted. An action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly

rises and falls, following a consistent trajectory. The absolute refractory period is the interval from the beginning of the action potential until the fiber

is able to conduct another action potential. Relative Refractory Period immediately after the absolute refractory period, the cell can generate an

action potential, but only if it is depolarized to a value more positive than normal threshold. Conduction Velocity depends on diameter and myelination of the action. It is the speed with which an action potential is propagated.

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