Nerves

By the end of this class you should understand:

• The divisions of the nervous system and how they interconnect

• The general role of glial cells and the specific function of selected glial cells

• The structure and function of a neuron• The mechanism of an action potential and

neuron signaling• The function of myelin in the nervous system

Nervous System

• The nervous system is one of the two control systems of the body– The other being the

endocrine system– These two systems have a

lot of interaction!• Made of neurons and

neuroglia– Nervous tissue!

Nervous System Organization

Neuroglia

• Also known as glial cells• Serve many functions

related to protecting the neurons– Physical protection– Myelination (more on that

later)– Chemical protection– Antimicrobial protection

Neurons

• There are three classifications of neurons– Sensory Neurons– Interneurons– Motor Neurons

• Interneurons form the central nervous system (CNS)

• Sensory and motor neurons form the peripheral nervous system (PNS)

Neuron Structure

• Neurons have three general structures:– Soma (cell body)– Axon (signal

transmission)– Dendrite (signal

reception)• Some neurons look

rather different but all have these three parts in some way

Electrical Activity

• Recall that muscle and nervous tissue are electrically active

• They maintain a potential across their cell membrane (outside has different electrical charge than inside)– This is also called a

voltage

Membrane Potential

• There are many negatively charged particles both inside and outside the cell– Outside: chloride, inside:

proteins• Whichever side of the

membrane has more positively charged particles will be positive– The other side will be

negative even though it clearly has many positively charged particles as well

Positive Ions• Sodium and potassium are

two positively charged particles found throughout the body

• Neurons have sodium-potassium pumps that pump three sodiums out of the cell for every two potassiums they pump into the cell– Creates a net positive charge

outside the cell

Sodium/Potassium Balance• Our diet must include both

sodium and potassium on a regular basis– This is why sodium tastes good– The kidneys can help adjust this

balance• Cramping and nervous problems

can result from chronic or acute imbalances of ions– Sweating profusely can lose ions,

which is why it is recommended to rehydrate with salt pills or gatorade

Resting Membrane Potential• When a neuron is at rest, it is

constantly pumping sodium out and potassium in to create a net positive charge on the outside of the cell– Typically the inside is -70mV

compared to the outside• This requires a lot of daily

ATP– Neurons can only use glucose,

so blood sugar must be maintained at a constant level

Graded potentials• Neuron potentials can

be altered by many different structures– Sensory neurons can

have their potentials altered by the stimulus they are supposed to detect (pressure, heat, stretch, light, etc)

• These stimuli may cause the membrane potential to be reduced to -55 mV

Threshold Voltage

• -55mV is usually the threshold voltage, at which point the behavior of certain membrane proteins changes

• These membrane channels are voltage-gated which means they open or close depending on the cell voltage

• When the voltage hits -55mV, the voltage-gated sodium channels open

THINK FAST!

• When the voltage-gated sodium channels open, what happens to the:– Sodium?– Potassium?– Membrane Voltage?

• Bonus thought: what would happen if these voltage-gated sodium channels were blocked?

Voltage-Gated Ion Channels• The sodium will

rush into the cell– Potassium ions

cannot fit through the sodium channel and so remain concentrated inside the cell

• Net effect: the voltage is now positive inside the cell!

Voltage-gated Potassium Channels

• The voltage change also opens voltage-gated potassium channels– These open more slowly

than the sodium channels• When they open,

potassium rushes out of the cell– Sodium channels inactivate

quickly, whereas potassium channels inactivate slowly

Voltage Changes

• The potassium rushing out of the cell counterbalances the sodium rushing in– Cell voltage drops back to -

70mV or more and the voltage-gated channels close

• This brief flicker back and forth of voltage is called an action potential– All-or-none due to the

voltage-gated ion channels

Action Potential• The action potential can

happen again only once the sodium and potassium have been replaced back to the outside and inside of the cell– Sodium-potassium pump

required for this– The delay before firing

again is called the refractory period

Action Potential• So what is the purpose of

the action potential??– Must send a signal

somehow!• Each voltage flip (action

potential) on the membrane triggers the neighboring membrane proteins to flip their voltage– Sends a rapid wave down

the cell membrane

Speed of Conduction

• Action Potentials may be sped up if there is myelin on the axon– In peripheral nervous system,

Schwann cells provide the myelin and can also regrow the axon if it is damaged

• Myelin conducts the voltage from one ion-channel node to the next much more quickly than if they were continuous

Neurotransmitter

• Recall from muscles that a neuron will release a neurotransmitter called acetylcholine onto a muscle

• All neurons release a neurotransmitter at the end of the axon!– Acetylcholine is most common

and usually stimulating– Dopamine and serotonin are

commonly used in the brain and may be stimulating or inhibiting

– There are many others!

That’s our show!

• See you Wednesday for more of the nervous system including the brain!