AP Bio- Communication 4: Neurons

Neurons

Make Sure You Can

Big Questions

How is a neuron's structure related to its function?

Why are neurons an “animals only” phenomenon?

Compare the regulatory structures and functions of the nervous and endocrine systems

Diagram the processes by which nervous signals are transmitted by and between neurons.

Label all parts of a neuron

Explain the causes and effects of major disruptions to neuron function

The need for fast,

long-distance messages

Any Questions?

Yes!

• Why are axons so long?
• Why have synapses at all?
• How do “mind altering drugs” work? caffeine, alcohol, nicotine, marijuana…
• Do plants have a nervous system?
• Do they need one?

Nucleus!

Action Potential

Characteristics of a nervous system

• fast
• accurate
• reset quickly

A nerve signal

• Electrical (ion exchange)
• Binary (all or nothing)

Dendrites

Think dominoes!

1. start the signal-

knock down line of dominoes by tipping 1st one

2. propagate the signal-

do dominoes move down the line? no, just a wave through them!

3. re-set the system-

before you can do it again, have to set up dominoes again

Action potential generation:

wave = nerve impulse

brain to fingertips in milliseconds!

Incoming signals enter here.

Each dendrite is connected to another neuron at a synapse

The Neuron has a similar system

protein channels are set up

1. once the first one is opened, the rest open in succession

2. a “wave” action travels along neuron

3. have to re-set channels so neuron can react again

Opposite charges on opposite sides of cell membrane

membrane is polarized ("membrane potential")

• negative inside; positive outside
• charge gradient
• stored energy (like a battery)

Cells live in a sea of charged ions

more concentrated within the cell

• anions (negative)
• K+
• charged amino acids (aa-)

more concentrated in the extracellular fluid

• cations (positive)
• Na+
• Cl-

An "All or None" Signal

How membrane potential is measured

Lots of pump proteins and ion channels

Neural membrane polarity is constantly fluctuating around the resting potential.

For an action potential to be generated, the membrane must be depolarized to a "threshold potential."

Neurons Do These Things!

Maintain concentrations of ions

Reset concentrations following nerve signal

Different kinds of "gates" control the openings to the channels:

• mechanical gates: Opened by force
• voltage gates: Opened by voltage
• ligand gates: Opened by ligand binding

Axon Hillock

Structure fits function

• many entry points for signal
• one path out
• transmits signal

Cell Body

The start of the axon.

sensory neuron Interneuron Motor Neuron

Where an Action Potential begins (if it's going to).

Action Potential Generation

Contains all organelles for the neuron. Signals move from Dendrites to cell body to axon to nerve terminals

Axon

1. Neuron is at rest:

All gated channels are closed

2. Neuron Depolarizes:

Na+ gated channels begin to open.

3. Rising Phase of AP:

Na+ gated channels all open.

4. Falling Phase of AP:

Na+ gates shut. K+ gates open

5. Undershoot:

K+ gates begin to close. Na/K

pump restores resting potential

Signal moves from black to red

Myelin Sheath

Fun facts about neurons

Node of Ranvier

"Na+ in, K+ out"

Layers of Myelin insulate neurons.

Speeds Transmission of AP's

Glial cells help neurons

Summary

The Action Potential moves through the Nodes of Ranvier by saltatory conduction

• Most specialized cell in animals
• Longest cell
• blue whale neuron: 10-30 meters
• giraffe neuron: 5 meters
• human neuron: 1-2 meters

Multiple Sclerosis:

Results from De-mylenation of motor neurons

Saltatory Conduction

Action potential travel by jumping ("saltations") between Schwann cells.

1. Resting potential

2. Stimulus reaches threshold potential

3. Rising (Depolarization):

Na+ channels open; K+ channels closed

4. Falling:

Na+ channels close;

K+ channels open

5. Undershoot:

K+ channels close slowly, reset charge gradient

Neurons stained with "Brainbow" flourescence

Action Potential Propagation

Axon coated with Schwann cells insulates axon, speeds signal

Signal hops from node to node

Saltatory conduction:

Myelinated vs. Unmyelinated

150 m/sec vs. 5 m/sec (330 mph vs. 11 mph)

1. Wave: nerve impulse travels down neuron

• Change in charge opens next Na+ gates down the line
• “voltage-gated” channels
• Na+ ions continue to diffuse into cell
• “wave” moves down neuron = action potential

Schwann Cell

So Cool!

Nerve Terminals

2. After firing a neuron has to re-set itself

• Na+ needs to move back out
• K+ needs to move back in
• both are moving against concentration gradients
• need a pump!! We have one!

Impulse has to jump the synapse!

junction between neurons

has to move quickly from one cell to next

3. Re-set: 2nd wave travels down neuron

• K+ channels open more slowly than Na+ channels
• K+ ions diffuse out of cell
• charges reverse back at that point
• negative inside; positive outside

Combined waves travel down neuron.

A wave of opening ion channels moves down neuron.

The signal moves in one direction.

The flow of K+ out of cell stops activation of Na+ channels in wrong direction.

Major Neurotransmitters

Acetylcholine

transmit signals to skeletal muscle

Epinephrine (adrenaline) & norepinephrine stimulate release

fight-or-flight response

Dopamine

widespread in brain

affects sleep, mood, attention & learning

lack of dopamine in brain associated with Parkinson’s disease

excessive dopamine linked to schizophrenia

Serotonin

widespread in brain

affects sleep, mood, attention & learning

Events at synapse

• action potential depolarizes membrane
• opens Ca++ channels
• neurotransmitter vesicles fuse with membrane
• release neurotransmitter to synapse (diffusion)
• neurotransmitter binds with protein receptor
• ion-gated channels open
• neurotransmitter degraded or reabsorbed

Modes of action of some of the major venoms

Weak point of nervous system

any substance that affects neurotransmitters or mimics them affects nerve function

gases: nitrous oxide, carbon monoxide

mood altering drugs:

stimulants

amphetamines, caffeine, nicotine

depressants

quaaludes, barbiturates

hallucinogenic drugs: LSD, peyote

SSRIs: Prozac, Zoloft, Paxil

poisons

Cone Snail Venom Blocks Na+ channels on post synaptic neurons

Snake venom (green) blocking the active site of acetylcholinesterase

Incoming Impulses Come in 2 Flavors Excitatory or Inhibitory

Post-synaptic neuron

triggers nerve impulse in next nerve cell

chemical signal opens ion-gated channels

Na+ diffuses into cell

K+ diffuses out of cell

switch back to voltage-gated channel

Complexity Emerges!