Quick ReviewQuick Review

What’s another name for neurons?Can you name the parts of a neuron?

Review QuestionsHow do nerve cells communicate with each other?Nerve cells are also called ____________How many types of neurons are there? Which nervous system are they related to?What is a resting potential?

Taking a closer look at Taking a closer look at the the AXONAXON

Myelin SheathMyelin SheathA protective layer that wraps around the axonAllows impulse to travel faster Made by Schwann cells“Schwann cells myelinate parts of an axon”Gaps are called Nodes of Ranvier

nerve impulses -- unmyelinated axons 1 m/s

- myelinated axons 100 m/s.

Myelin SheathMyelin Sheath

in PNSin PNSGives fibers their white glistening appearance Myelin serves as an excellent insulatorRegenerate nerves

Damaged axon may use myelin sheath as a passageway to regenerate new fiber

in CNSin CNSNerve regeneration does not occur

Myelinated Axon – in CNSMyelinated Axon – in CNS2 types of nervous tissue (Made up of many neurons):

Gray matter White matter

-gray because it contains cell bodies + short-Not myelinated

-white because it contains myelinated axons -Carries messages from one part of the CNS to another-Lies deep within the gray matter (in the brain)

The Nerve ImpulseConveys messages in our bodyUnit: millivolts (mV)

Measures the electrical potential difference (polarity) between 2 points

Saltatory Impulse TransmissionSaltatory Impulse Transmission

Flow is due to ion movements at the nodesIONS: Electrically charged chemicalsNa + (outside axon)K +Cl –Therefore, the voltage (mV) is created by the polarity (difference of charge) of the inside and outside of an axon

(Inside axon)electrical potential

difference

Saltatory Impulse TransmissionSaltatory Impulse Transmission

Neurons have a semi-permeable membrane for ions to pass throughAxomembrane: membrane of the axon- contain Na/K pumps and gatesAxoplasm: cytoplasm of the axon

(fluid inside the axon)

Saltatory Impulse TransmissionSaltatory Impulse Transmission“jumping” transmission“jumping” transmission

Saltatory Impulse TransmissionSaltatory Impulse Transmission

When neurons (axon) are at rest – not sending a nerve impulse it is called the

A neuron in the resting state is polarized

Resting Potential is always negative on the inside of the axon (-65 mV)

RESTING POTENTIAL

The Resting PotentialThe Resting Potential

At rest, axomembrane is NOT permeable to these ions because the gates are closed.Due to natural unequal distribution of ions a slight negative charge results

[Na+] > [K+]outside

The Resting PotentialThe Resting Potential

Na/K pumps (proteins) are found on the axomembrane

pumps Na+ out and K+ into the axonKeeps the unequal distribution of the ions

Membrane is permeableto K+ therefore, it leaksout (outside is alwaysmore positive thaninside)

The Action PotentialThe Action Potential

Only takes 2 millisecondsRapid change in polarity (electrical difference)Occurs when there is a intense stimulus that reach the threshold value

ALL-OR-NONE (yes-impulse or none at all)

The Action PotentialThe Action PotentialWhat happens next:1. DEPOLARIZATION

Na+ gates (in the axomembrane) becomes permeable to Na+- Na+ moves down its concentration gradient into the axon- Membrane potential -65mV +40mV

2. REPOLARIZATIONK+ gates open

- K+ moves down its concentration gradient out of the axon - Membrane potential reverts back to +40mV -65mV- Since there is more negative ions inside compared to positive ions

Refractory PeriodRefractory Period

Prevents the action potential from moving backwardsDuring this period, Na+ gates can’t be opened

The Na+/K+ pump is busy restoring the resting potential by pumping Na+ ions out and K+ ions in through the axomembrane.

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