• Review:• What decision is made by

a neuron?• At what location the

decision is made?• How the decision is

made?• Simple decision rule:

if Em > threshold (-40mV) fire an action potential

• After action potential is initiated, it propagates to the axon terminal in all-or-none manner

-

-

Inject current

Measure Em

Threshold = -40mV

Em

Peak = +30mV

Resting Em= -80mV

Inject current Let us simulate excitatory input by injecting

positive ions (electrical current) into neuron

What happens at the contact between two neurons?

1. Synapse (Greek “syn”= together + Greek “hapto”= to clasp)

2. Presynaptic neuron

3. Postsynaptic neuron

Let’s start with something simple: The only monosynaptic reflex in our body

• Stretch reflex or Patellar reflex or knee-jerk reflex

• Why doctors check for this reflex?

• Decreased knee-jerk reflex is a clinical sign of demyelination of peripheral nerves as occurs in untreated syphilis infection

• In patients with cerebellar disease, the leg can oscillate up to 8 times

• Dorsal root• From Latin

Dorsalis = Back

• Ventral from Latin ventralis= belly, abdomen

• SAME DAVESensory – DorsalAfferent – AfferentMotor – VentralEfferent - Efferent

• In both spine and the brain: the back (posterior) is always sensory

• the front (ventral) is always motor

Ventral

Dorsal

• Sensor: Muscle spindle is a stretch receptor = the length sensor

ΔL

Em

Freq

uenc

y (r

ate)

of

actio

n po

tenti

als

Stimulus = ΔL

Fmax

• Conclusion: Information is coded in frequency of action potentials (firing frequency): the higher the frequency the stronger quadriceps muscle would contract

• We talk of a reflex when consciousness is not involved.• A patient could be unconscious, but the reflex will work• Other examples of a reflex?• Crossed extensor reflex (a withdrawal reflex) after stepping on a nail• Note that the firing of the motor neuron can be inhibited by the brain

Reflexes

Types of synapses• The problem: we need transmit signal

from one cell to another• In the stretch reflex the signal needs

to be transmitted the stretch receptor axon terminal to a motor neuron

• From the motor neuron axon terminal to the muscle

• The first type of connection that evolution tried was an electrical contact: gap junction

• Ions and small molecules (up to 2nm) can diffuse without leaking to extracellular space

• A kind of local plumbing system

• Most cell in the body are connected; except red blood cells, neurons and skeletal muscles.

• Gap junctions couple neighboring cells in the heart, smooth muscles, in glands, liver, epithelia; at some stages of early development they couple all cells in an embryo.

Electrical synapses also sometimes connect neurons

• Example: crayfish giant motor synapse, that mediates emergency tail flip – the escape reaction of crayfish.

• 4 giant axons of the ventral nerve cord make contact with axons of giant motor neurons in each segment all abdominal flexor muscles become activated simultaneously and without delay

• (b) Photomicrograph of cross section of ventral nerve cord in abdomen of crayfish made near the level of the third root (arrow in a); the largest axons near the top are giant fibers used in escape responses

• (c) Relationship between lateral giant and motor giant axons as they make electrical synaptic contact near the third root of an abdominal ganglion in the crayfish

• A gap junction can be regulated by:• 1. A number of small molecules• 2. By insertion or removing gap junctions from the membrane

Electrical synapse Chemical synapse

• Until 1920 scientists did not know how neurons communicated.• Scientists formed two camps: they called themselves soups and

sparks. Soups were in favor of chemical transmission and sparks were in favor of electrical communication. They were arguing for 50 years and before 1920 sparks seemed to be winning.

• The solution came to Otto Loewi in a dream in 1920…

Loewi’s Experiment Demonstrating Chemical Transmission

• Stimulated vagus nerve slowed heart A

• Injected solution from heart A into heart B Heart B rate slowed

• Stimulated sympathetic nerve sped up heart A.

• Injected solution from heart A into heart B Heart B rate sped up

• Loewi’s conclusion– Heart uses chemical

messengers, not electrical signal to control heart rate.

– Loewi got a Nobel prize in 193617

A B

Which synapse is electrical? chemical?

• Electron microscopy of synapses

Electrical synapse Chemical synapse

• Why wasn’t evolution satisfied with electrical synapses?– Gap junctions synaptic transmission is fast– They can be regulated

• PROBLEM: Gap junctions are straight forward transducers of electricity:– Pre depolarized Post is depolarized; – Post depolarized Pre depolarized

• It is impossible to build a highly compartmentalized

system with just electrical synapses• Evolution favored more complex systems. It is easier to

build a complex system with chemical synapses: – cells are electrically isolated; – chemicals (neurotransmitters) are used to transmit signal

between cells.– Some neurotransmitters depolarize the postsynaptic cell;

some hyperpolarize– Some neurotransmitters have only transient effect (1ms);

others long-lasting effect (100ms)

Electrical synapse Chemical synapse

Electrical synapse

Chemical synapse

A

B

Action potential in cell A

A

B

Em cell A

Em cell B

Em cell A

Em cell B

No delay

delay

Differences between chemical

and electrical synapse

Electrical synapse

Chemical synapse

A

B

Action potential in cell A

A

B

Em cell A

Em cell B

Action potential in cell B

Em cell A

Em cell B

Em cell A

Em cell B

Em cell A

Em cell B

Differences between chemical

and electrical synapse

From now on we’ll be talking about chemical synapse only. Rules of the game at a chemical synapse

1. Excitatory synapse always depolarization

2. Inhibitory synapse always hyperpolarization

Em

Em

Excitatory synapse

Action potentials in the presynaptic cell 1 are 100ms apart no temporal summation

Action potentials in the presynaptic cell 1 are 5ms apart temporal summation of EPSP results in action potential in the postsynaptic cell

Simultaneous action potentials in presynaptic cell 1 and 2 spatial summation of EPSP results in action potential in the postsynaptic cell

Inhibitory postsynaptic potential (IPSP) reduces the probability of action potential in the postsynaptic cell

Summary

1. Postsynaptic potentials last longer than action potentials

2. Postsynaptic potentials are local potentials – they are not usually amplified

3. Postsynaptic potentials are graded potentials: the more APs arrived the greater EPSP (as opposed to all-or-none APs)

4. Since not amplified postsynaptic potentials as decreasing in amplitude as one measures change in Em away from a synapse

axondendrite

synapse

Em

AP in presynaptic cell

10mV 5mV 3mV 1mV

• Usually one finds a number of inhibitory synapses right at the axon hillock

• So that even if there a lot of excitatory inputs, inhibitor can decide whether to fire AP or not

The mechanism of neurotransmitter release

• Let’s look at neuromuscular junction• The goal of the neuron: to depolarize muscle so

that muscle fires an action potential• Transmission is chemical neurotransmitter has to

be delivered to the surface of the muscle

• Neurotransmitter is stored inside synaptic vesicles • What triggers the release of vesicles?

• Increase of Ca++ concentration from 0.0001mM to 1mM in close vicinity to vesicles triggers exocytosis

• Neurotransmitter packed inside vesicles at high concentration is spilled on the postsynaptic cell

• There are usually huge number of postsynaptic receptors

• These receptors can be coupled to ion channels or to other proteins that transmit the signal to the postsynaptic cell

• POSTSYNAPTIC RECEPTOR: In the case of a neuromuscular junction (nmj), ACh receptor itself is a channel

• Following binding of two ACh molecules, the channel opens• Sodium enters the muscle and ….what happens?• Positive sodium depolarizes the muscle, making it more likely to

fire an action potential.

• Positive sodium depolarizes the muscle • membrane potential more positive• more sodium channels open • Even more sodium ions enter the cells • membrane potential even more positive on so on ==• Positive feedback loop == explosion == gun powder

–

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+ +

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[K+]=150mM[Na+]=15mM

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+30mV

• The release of one vesicle usually is not enough to trigger AP in a muscle.

• Many vesicles and many release zones have to exocytose to result in muscle AP and a corresponding twitch.

• Each neuronal AP releases MANY vesicles, • Each neuronal AP normally results in a muscle AP

• If Ach is allowed to hang around ACh receptor for too long new signal will not get through.

• Therefore ACh has to be quickly removed from synaptic cleft

• Name several possible mechanisms?

• ACh is broken down by ACh esterase (AChE)• AChE is located in postjunctional folds (AChR are on the surface)• ACh is broken into choline and acetate• Choline is transported back into the neuron by choline transporter

AChEAChEAChEAChEAChE

Cholinetransporter

Inside the axon terminal:choline + AcetylCoA ACh + CoA

ACh is then packaged into vesicles

AChR AChR AChR AChR

• Electron microscope autoradiograph of the vertebrate nmj, showing localization of ACh receptors (black developed grains) at the top one-third of the postsynaptic junctional folds.

• AChE is at the bottom of postsynaptic junctional folds

Find two errors

Find two errors

Disease: Myasthenia gravis

• G. myo=muscle + asthenia=weakness

• Severe dropping of eyelids

• Patient cannot move his eyes to look to either side

• Severity varies within a single day, day to day…

Disease: Myasthenia gravis

• Autoimmune disease in which antibodies are produced against the nicotinic AChR

• Viral or bacterial antigens may share epitopes with the AChR• This reduces the number of receptors, increases receptor

turnover and degradation

• Normally an AP in a motor axon releases enough vesicles to induce muscle AP.

• In fact the number of vesicles can be reduced by 25% before it fails to initiate AP

• In MG the number of AChR goes down and infoldings are reduced more ACh is broken down by AChE

Disease: Myasthenia gravis

Disease: Myasthenia gravis

• The weakness is reversed by drugs that inhibit AChE• E.g. neostigmine block the breakdown of ACh by AChE and

temporarily increases the levels of ACh at the neuromuscular junction

Garrett: Brain & Behavior 4e

Neurotransmitter Function

Biogenic amines (contain NH2 amino group)

Acetylcholine (Ach) Neurotransmitter at the neuromuscular junction; in brain, involved in learning, etc. Dopamine (DA) Involved in reward circuits in the PFC. Abnormalities in DA system are implicated in

schizophrenia and Parkinson’s disease. Cocaine blocks DA reuptake. Attention deficit disorder drug Ritalin also blocks DA reuptake. Amphetamine (Adderall) modulates DA as well as 5-HT and NE).

Serotonin (5-HT) Involved in regulation mood, sleep and arousal, sexuality, aggression, body temperature. Abnormalities in 5-HT system are implicated in depression, obsessive-compulsive disorder. Psychedelic drugs like LSD activate 5-HT receptors as well as some DA receptors.

Norepinephrine (NE) Released during stress. Neurotransmitter in the brain to increase arousal and attentiveness to events in the environment. ADD drug Strattera affects only NE.

Epinephrine (Epi) A stress hormone related to norepinephrine. Also called Adrenalin.Histamine (His) Modulates sleep: antihistamines induce sleepiness (Dramamine)

Amino Acids

Glutamate (Glu) The major excitatory neurotransmitter in the brain and spinal cord.

Gamma-aminobutyric acid (GABA)

The major inhibitory neurotransmitter. Its receptors respond to alcohol and the class of tranquilizers called benzodiazepines. Deficiency in GABA or receptors is one cause of epilepsy.

Glycine (Gly) Inhibitory neurotransmitter in the spinal cord and lower brain. Works by opening Chloride channels that hyperpolarizes the cell. The poison strychnine causes convulsions and death by blocking Gly binding site on the Chloride channels.

Types of neurotransmitters (You are not responsible for learning different types of neurotransmitters, their function, and location)

Neurotransmitter Function

Neuropeptides

Endorphins (over 85 different neuropeptides)

Endogenous opioids (5 amino acid polypeptide) reduce pain and enhance reinforcement. Exogenous opioids: morphine, heroine.

Substance P Neurotransmitter in neurons sensitive to pain.

Neuropeptide Y Initiates eating and produces metabolic shifts.

Gases

Nitric Oxide Viagra enhances male erections by increasing nitric oxide’s ability to relax blood vessels and produce penile engorgement.

CO carbon monoxide

• Every molecule of neurotransmitter has a number of different target receptors, increasing the system complexity

• Presynaptically• Transmitter release facilitation• Depression• Calcium channel regulation• Autoreceptors provide feedback

mechanism• AP duration (AP longer more Ca++ in

more release• Regulate the number of active zones

per synapse• Regulate the number of synapses

between the cells• Modify recycling of neurotransmitter

Regulation of the synaptic strength• Postsynaptically• Modification of

receptors via ligands• Modify receptor

subunit composition• Change number of

receptors

Regulate the number of synapses between the cells: grow dendritic spines

Sleep and memory

• Sleep is good for memory consolidation (conversion of memory into permanent memory).

• Memory improvement is most dramatic for procedural memory (riding a bike, skating, playing the piano).

• Even a short nap can really improve memory.

Poisons

• Botulinum toxin (from Latin “sausage”) fusion protein are cut vesicles are not released

• Most potent toxin on the planet. Medial lethal dose 1ng/1kg (compare to strychnine 1,000,000ng/kg)

• You only need 300gram to kill every person on earth

Botulinum toxin cuts Syntaxin, SNAP-25, and Synaptobrevin

Clostridium botulinum

• Canned food creates ideal anaerobic environment for the bacterium Clostridium botulinum allows the spores to germinate, after which the bacteria can multiply and produce toxin

• Do not eat food from cans that are swollen!

• The spores survive boiling at 100°C• To kill spores canning companies

use high pressure to raise temperature to 121°C for 3 min

• Bottox

Poisons

• Botulinum toxin (from Latin “sausage”) fusion protein are cut vesicles are not released

• Black widow spider toxin vesicles are released without Ca++• Cobratoxin binds tightly to nAChR, preventing activation of nAChR• Organophosphates (biological weapons): irreversible inhibitors of AChE ACh

is not broken down desensitization of AChR

AChEAChEAChEAChEAChE

Cholinetransporter

AChR AChR AChR AChR

Botulinum toxin

Black widow spider toxin

Cobratoxin

Organophosphates

Why all victims die?

Conclusions

• Most interesting staff – short-term

memory– long-term

memory– learning

happens at the synapses.

Example: growth of neuronal connections

Within the first day after plating one of the short processes of a hippocampal neuron becomes defined as the single axon of the cell that grows out very rapidly. Both rate and direction of outgrowth are influenced by the nature of the substrate the axon encounters. This movie follows the elongation of the axon on a uniform substrate over a period of 16 hours. Note how the axon elongates in spurts.

Time lapse: 16.5 hours (one loop of movie)