NERVOUS SYSTEMS - University of Arizona · What do nervous systems do? • detect things in our environment through our senses (INPUT) • ‘tell’ muscles and other tissues what

NERVOUS SYSTEMS

NERVOUS SYSTEMS – Key Concepts

• Cell structure• Neurons: generating and conducting

nerve impulses• Connections between neurons: synapses

and communication• The brain: center of neural integration

What do nervous systems do? • detect things in our environment

through our senses (INPUT)• ‘tell’ muscles and other tissues what to

do (OUTPUT)• integration - nerve cells interacting with

each other

The fundamental unit of all nervous systems is a cell

called a NEURON

input

output

NEURON

the cell body contains the nucleus and most organelles

a neuron has two types of extensions

• dendrite -toward the cell body

• axon - away from cell body

dendrite form varies with function

How neurons work:

• Signals travel down an axon

• Changing the membrane potential: resting action

• Signal is sent to the next neuron - neurotransmitters

Neurons are like rechargeable batteries

Charge/Electrical Potential: outside has more +++ ions,

inside has more ---- ions

How does a cell produce its electrical potential?

by moving ions

Forces important in ion movement across a membrane

1. concentration gradients 2. electrical charges3. ion channels (holes)4. active pumps

1. Concentration differences

molecules tend to move from areas of high concentration to areas of lower concentration

Concentration differences

random distribution of the two types of ions

2. Electrical interactions

• like charges repel

+

+ + _

_

_

•opposites attract

+ _

3. ion channels for Na+ and K+

• there are channels that allow Na+ and K+ to pass through membrane

• the ions move down their concentration gradients

ion-specific channels in the membrane let particular ions pass freely

all types of channels are not represented equally

• K+ ion channels are abundantK+

Some ions and other charged molecules cannot cross

membrane

due to lack of channels or to their large size

4. Na-K - active pump

• the Na-K pump can move these ions across a membrane againsttheir concentration gradient

• this process requires energyenergy

How do these forces work to create resting potential?

1. concentration gradients 2. electrical charges3. ion channels (holes)4. active pumps

Builds up concentration gradients

Na-K pump moves Na+ out and K+ in

inside

outside

Na+

K+

-- charge builds up

big anions cannot pass through membrane

inside

outside

Na+

K+ big anions ---

To balance the charge...

• Cl- ions move out through open ion channels

inside

outside

Na+

K+

Cl-

big anions ---

remember all those open K+ channels?

• K+ leak to outside causing charge imbalance

• Leakage continuesuntil the concentration gradient and electrical forces are balanced

inside

outside

Na+

K+

Cl-

big anions

K+

that is that is how you how you get the get the resting resting potential potential

the voltage can be measured by placing microelectrodes inside and outside the neuron

Review

• What is membrane more permeable to: K+ or Na+ ions?

• What happens if something increases membrane permeability to Na+?

inside

outside

Na+

K+

Cl-

big anions

How is the electrical potential changed - the Action Potential?

Enter

VOLTAGE-GATED CHANNELS

VOLTAGE-GATED CHANNELS

• stay closed unless stimulated

• are opened by specific electrical stimuli

when the Na+ gated channels open...

• Sodium ions rush down their concentration gradient to the inside

inside

outside

Na+

K+

Cl-

big anions

Na+ movement into the neuron causes the action potential

This causes the electrical potential to change, actually

becoming positive.

At the peak - the Na+ gate closes & K+ gate opens.

Na+ gates closed, VG K+ gates open

• even more K+ will moved outside

• starts repolarization

inside

outside

Na+

K+

Cl-

big anions

K+

This is the repolarizing phase.

Propagation of the action potential

from triggering point down the axon

depolarization itself is a voltage change, and it triggers

further depolarization

Propagation of the action potential

Pacific Reef Squid, giant axon

How do we know all this about nerves?

What happens when the action potential reaches the end of the cell?

the synapse

• presynaptic terminal

• synaptic cleft• post-synaptic

membrane

the presynaptic terminal

• synaptic vesicles contain neuro-transmitter

• depolarization triggers release of NT into cleft

Where does the neurotransmitter come from?

From the cell body at the other end of the cell !

• acetylcholine(ACh) is a common neuro-transmitter

• vesicles fuse with membrane, release contents into cleft

• acetycholinediffuses across the narrow cleft

• and binds to acetycholine receptors on the post-synaptic membrane

• the receptor is also an ion channel, specifically a Na+ channel

• the channel is normally closed

• Opens by the binding of acetycholine

• the Na+ ions rush in

• depolarize the post-synaptic membrane

• How is the channel closed?

• An enzyme called acetylcholinesterasebreaks down the neurotransmitter

neurotransmitters can be inhibitory as well as excitatorywhether the post-synaptic nerve fires can depend on the sum of effect

How can cells detect a signal

from the environment?

Channels can be stimulated : pressure,

temperature, chemicals and

light.

Stimulates:action potential

neurotransmitter

NeurotransmittersNeurotransmitters• There have been over 60

neurotransmitters described• Acetylcholine - junction of nerves

and muscles • Serotonin – central nervous system

– pain and mood

Neurotransmitter Mimics

What does a cholinesterase inhibitor do?

• Inhibits breakdown of the ACh in the cleft • What does ACh do?

Recreational drugs: act as or alter the function of neurotransmitters

Problems with neurotransmitter function: neurological diseases and other dysfunctions

The lack of dopamine is involved in Parkinson’s Disease.

The lack of serotonin is involved in clinical depression.

GABA is an inhibitory neurotransmitter

• Lack of GABA causes anxiety• Valium acts to increase release of

GABA at synapses

Neuromodulators: chemical communication between neurons

Endorphins modulate perception of pain.

• Opiates mimic this class of neuromodulators

• One type blocks serotonin uptake; Prozac mimics this action

The Brain - center of neural integration

Brain Lobes

• Frontal• Parietal• Temporal• Occipital

Regions necessary for complete language skills

• Broca’s area - essential for speech• Damage prevents normal speech, but

reading and understanding language OK

BROCABROCA

Regions necessary for complete language skills

• Wernicke’s area• Damage prevents understanding of

written or spoken language• Speech still possible, but doesn’t make

sense

BROCABROCAWERNICKEWERNICKE

Regions necessary for complete language skills

• Auditory, visual, motor areas

MM

AA

VV

PET scans

• positron emission tomography• subject given radioactive oxygen or

glucose• its use will indicate active regions• red= active, blue=inactive

listening to tapes to learn new language: auditory, association

areas

knows language, spelling words:Broca and motor speech areas

hallmark features of humans appeared at different times, not all at once

40,000-nowyes1350Modernsapiens

28k-500kMake fire, complex

tools

Some speech

yes1250-1330Archaic sapiens

.3-1.8 myTools, use fire

Broca +Wernicke

Yes750-1225H. erectus1.5-2.5YesBrocaYes500-800H. habilis

2.5-4.3 myYes~400Australo-pithecus

Yes??Ardipithecus5myNo2-300ccApes

Years agoToolsLanguageBipedalBrain size

(1) What was the most important thing you learned this week?

(2) What was least clear from lecture this week?