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Nervous System

The Neuron

By

Abhishek Jaguessar

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The Neuron

The basic functionalunit of the nervoussystem.

Function: Sendimpulses to and fromthe CNS and PNS andthe effectors(muscles/glands)

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Neuron Structure

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Dendrite Fine hair-like extensions on the end of aneuron.

Function: receive incoming stimuli.

Cell Body or Soma The control center of the neuron.

Function: Directs impulses from the dendrites to theaxon.

Nucleus Control center of the Soma.

Function: Tells the soma what to do.

Axon Pathway for the nerve impulse (electrical message)from the soma to the opposite end of the neuron.

Myelin Sheath An insulating layer around an axon. Madeup of Schwann cells.

Nodes of Ranvier Gaps between schwann cells.

Function: Saltatory Conduction (Situation where speedof an impulse is greatly increased by the messagejumping the gaps in an axon).

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Types of Neurons

There are 3 types of neurons.1. Sensory Neurons Neurons located near receptor

organs (skin, eyes, ears).

Function: receive incoming stimuli from the

environment.2. Motor Neurons Neurons located near effectors

(muscles and glands)

Function: Carry impules to effectors to initiate aresponse.

3. Interneurons Neurons that relay messages betweenother neurons such as sensory and motor neurons. (foundmost often in Brain and Spinal chord).

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Types of Neurons

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Nerves

Nerves Collections of neurons that arejoined together by connective tissue.

Responsible for transferring impulses fromreceptors to CNS and back to effectors.

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How Do Neurons Operate?

Neuron at Rest Resting Potential

Occurs when the neuron is at rest.

A condition where the outside of the membrane ispositively(+)charged compared to the inside which is negatively(-)charged.

Neuron is said to be polarized.

Neuron has a voltage difference of -70 mV

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How is resting potential maintained?

Ion Distribution

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How is resting potential maintained?

At rest, the sodium gates are closed.

Membrane is 50 times more permeable to K+

ions causing them to leak out.

This causes outside of membrane to have anabundance of + charges compared to inside. Theinside of the membrane is negative compared tothe outside. This is helped by the (-) proteinsetc.

The sodium-potassium pump pulls 2 K+ ions infor 3 Na+ ions sent out. This further creates acharge difference!!

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Action Potential

The mechanism by which neurons sendimpulses. They are comprised ofelectrical signals generated at the soma

and moving along the axon toward theend opposite the soma (motor neurons)

Action potentials occur in two stages:

Depolarization

Repolarization

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Depolarization in an action potential

When the neuron is excited past its Threshold thefollowing events occur:

Sodium ions (Na+) rush into the axon.

This neutralizes the negative ions inside.

The inside of the axon becomes temporarily (+) whilethe outside becomes temporarily (-). The reversal ofcharge is known as depolarization

Nearby Sodium (Na+) channels open to continue thedepolarization.

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Lets Have a look.

http://www.mcgrawhill.ca/school/booksites/biology/student+resources/toc/unit+4+homeostasis+maintaining+dynamic+equilibrium/chapter+12+the+nervous+system/cool+stuff+to+see+and+do

/movie+channel+behaviour.php

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Repolarization

This is the restoring of the (+) charge on the outsideof the axon and (-) on the inside.

Potassium gates open and potassium floods out.

This generates positive charge on the outside of membrane.

Sodium Channels Close (no + charges can get inside)

The Sodium/Potassium pump rapidly moves Sodium out ofthe cell.

Further creates the (+) charge outside with a (-) charge inside.

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Refractory Period

Brief period of time between the triggeringof an impulse and when it is available foranother.

NO NEW action potentials can be createdduring this time.

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Saltatory Conduction The jumping of an impulse between the Nodes of

Ranvier thus dramatically increasing its speed.

Only occurs in axons having Myelin.

2m/s 120 m/s

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All or None Response

If an axon is stimulated above itsthreshold it will trigger an impulse downits length.

The strength of the response is notdependent upon the stimulus.

An axon cannot send a mild or strongresponse. It either responds or does not!!!