Neural Zones

How Neurons connect

The Synapse

• A functional connection between surfaces• Signal transmission zone• Synapse – synaptic cleft, presynaptic cell, and

postsynaptic cell• Synaptic cleft – space in between the presynaptic and

postsynaptic cell• Postsynaptic cell – neurons, muscles, and endocrine

glands• Neuromuscular junction – synapse between a motor

neuron and a muscle

The Synapse• Axon terminal: found in motor neurons• Axon varicosities: ie swellings. Arranged like beads on a string and

contain neurotransmitter containing vesicles• En passant synapse: CNS. Consists of a swelling along the axon• Spine synapse: presynaptic cell connects with a dendritic spine on

the dendrite of the postsynaptic cell

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The Synapse• Axodentritic: between axon terminal of one neuron and the dendrite

of another• Axosomatic: between the axon terminal of one neuron and the cell

body of another• Dendrodendritic: between dendrites of neurons (often are electricla

synapses)• Axoaxonic: between an axon terminal of a presynatpic neuron and

the axon of a postsynaptic neuron.

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Diversity of Signal Conduction

So far:• Electrotonic• Action potentials• Saltatory conduction• Chemical and electrical synapses

Diversity of Synaptic Transmission

Electrical and Chemical Synapses

Electrical synapse Chemical synapse

Rare in complex animals Common in complex animals

Common in simple animals

Rare in simple animals

Fast Sloooooow

Bi-directional ↔ Unidirectional →

Postsynaptic signal is similar to presynaptic

Postsynaptic signal can be different

Excitatory Excitatory or inhibitory

Electrical synapsescells connect via gap junctions- membranes are separated by 2 nm- gap junctions link the cytosol of two cells - provide a passageway for movement of very

small molecules and ions between the cells- gap junction channels have a large conductance- NO synaptic delay (current spread from cell to cell is instantaneous)

- important in some reflexes - chemical synapses do have a significant delay ie slow- commonly found in other cell types as well i.e. glia- can be modulated by intracellular Ca2+ , pH, membrane voltage,

calmodulin- clusters of proteins that span the gap such that ions and small

molecules can pass directly from one cell to another 

More about electrical synapsescells connect via gap junctions

- made up of 6 protein subunits arranged around a central pore, made up of the connexin protein

- the two sides come together to make a complete unit of 12 proteins around the central pore

Chemical Synapse Diversity

Vary in structure and location

Chemical Synapse• most common type of synapse• electrical signal in the presynaptic cell is communicated to the

postsynaptic cell by a chemical (the neurotransmitter)• separation between presynaptic and postsynaptic membranes is

about 20 to 30 nm• a chemical transmitter is released and diffuses to bind to receptors

on postsynaptic side• bind leads (directly or indirectly) to changes in the postsynaptic

membrane potential (usually by opening or closing transmitter sensitive ion channels)

• the response of the neurotransmitter receptor can depolarizes (excitatory postsynaptic potential; epsp) or hyperpolarizes (inhibitory postsynaptic potential; ipsp) the post-synaptic cell and changes its activity

• significant delay in signal (1 msec) but far more flexible than electrical synapse  

More about chemical Synapses• Some types of chemical synapse include• Excitatory - excite (depolarize the postsynaptic cell• Inhibitory - inhibit (hyperpolarize the postsynaptic cell)• Modulatory - modulates the postsynaptic cells response to other

synapses

General sequence of events

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1. Nerve impulse arrives at presynaptic terminal

2. Depolarization causes voltage-gated Ca 2+ channels to open- increases Ca 2+ influx, get a transient elevation of internal Ca 2+ ~100 mM

3. Vesicle exocytosis- increase in Ca 2+ induces fusion of synaptic vesicles to membrane- vesicles contain neurotransmitters

4. Vesicle fusion to membrane releases stored neurotransmitter

5. Transmitter diffuses across cleft to postsynaptic side

6. Neurotransmitters bind to receptor either:i) ligand-gated ion channel or ii) receptors linked to 2nd messenger systems

7. Binding results in a conductance change - channels open or close or - binding results in modulation of postsynaptic side

Cont…….

General sequence of events

8. Postsynaptic response - change in membrane potential (e.g. muscle contraction in the case of a motorneuron at a neuromuscular junction)

9. Neurotransmitter is removed from the cleft by two mechanismsi) transmitter is destroyed by an enzyme such as acetylcholine esteraseii) transmitter is taken back up into the presynaptic cell and recyclede.g. - acetylcholine esterase, breaks down acetylcholine in cleft, choline is recycled back into the presynaptic terminal

General sequence of events

Neurotransmitters

Characteristics• Synthesized in neurons• Released at the presynaptic cell following

depolarization• Bind to a postsynaptic receptor and causes

an effect

Neurotransmitters, Cont.

More than 50 known substances

Categories• Amino acids• Neuropeptides• Biogenic amines• Acetylcholine• Miscellaneous …..

Neurons can synthesize many kinds of neurotransmitters

Neurotransmitters

Neurotransmitters cont.

Neurotransmitter Action

Inhibitory neurotransmitters• Cause hyperpolarization• Make postsynaptic cell less likely to generate

an AP

Excitatory neurotransmitters• Cause depolarization• Make postsynaptic cell more likely to generate

an AP

Amount of Neurotransmitter

Influenced by AP frequency which influences Ca2+ concentration

Control of [Ca2+] • Open voltage-gated Ca2+ channels [Ca2+] • Binding with intracellular buffers [Ca2+]• Ca2+ ATPases [Ca2+]

High AP frequency influx is greater than removal high [Ca2+] many synaptic vesicles release their contents high [neurotransmitter]

Signal Strength

Influenced by neurotransmitter amount and receptor activity

Neurotransmitter amount: Rate of release vs. rate of removal• Release: due to frequency of APs• Removal

• Passive diffusion out of synapse• Degradation by synaptic enzymes• Uptake by surrounding cells

• Receptor activity: density of receptors on postsynaptic cell

Ca2+ Regulates Neurotransmitter Release

Graded Potentials via Neurotransmitters• Vary in magnitude depending on the

strength of the stimulus

• e.g., more neurotransmitter more ion channels will open

• Can depolarize (Na+ and Ca2+ channels) or hyperpolarize (K+ and Cl- channels) the cell

Graded Potentials

Graded Potentials Travel Short Distances

Neurotransmitter Receptor FunctionIonotropic • Ligand-gated ion channels• Fast• e.g., nicotinic ACh

Metabotropic• Channel changes shape• Signal transmitted via

secondary messenger• Ultimately sends signal to

an ion channel• Slow• Long-term changes

Second Messenger again

• When activated by a ligand the catalytic domain starts a phosphorylation cascade

• Named based on the reaction catalyzed

Second Messengers to know

Neurotransmitter receptorsDifferent types of neurotransmitter receptors

Functional Type Ligand Ion Channel

Excitatory Receptors Acetylcholine Na+/K+ 

Glutamate Na+/K+; Ca2+ 

Glutamate Na+/K+ 

Serotonin Na+/K+

Inhibitory Receptors Aminobutyric acid, GABA Cl- 

Glycine Cl-

Removal of Neurotransmittera) broken down by enzyme

- acetylcholine esterase breaks down acetylcholine in the synaptic cleft

- many nerve gases and insecticides work by blocking acetylcholine esterase – Yikes!

- prolongs synaptic communication

b) recycled by uptake

- most neurotransmitters are removed by Na+/neurotransmitter symporters

- due to a specific neurotransmitter transporter

- recycled by uptake into presynaptic terminal or other cells  (glial cells will take up neurotransmitters)

c) diffusion: simple diffusion away from site  

Neurotransmitters - stages1. Synthesis

- all small chemical neurotransmitters are made in the nerve terminal

- responsible for fast synaptic signalling

- synthetic enzymes + precursors transported into nerve terminal

- subject to feedback inhibition (from recycled neurotransmitters

- can be stimulated to increase activity (via Ca2+ stimulated phosphorylation)

2. Packaging into vesicles

- neurotransmitters packaged into vesicles

- packaged in small "classical" vesicles

- involves a pump powered by a pH gradient between outside and inside of vesicle

- pump blocked by drugs and these block neurotransmitter release

Presynaptic vesiclesTwo groups  i) low molecular weight, non-peptide  e.g. acetylcholine, glycine, glutamate

 ii) neuropeptide (over 40 identified so far and counting…..) 

Presynaptic vesicles• There are 2 types of secretory vesicles

• We will only talk about small chemical synaptic vesicles

• Neuropeptides are made and packaged in the cell body and transported to synapse)

• Small chemical neurotransmitter vesicles• responsible for fast synaptic signaling• store non-peptide neurotransmitters,

  e.g. acetylcholine, glycine, glutamate• enough vesicles in the typical nerve terminal to transmit a few

thousand impulses• exocytosis only occurs after an increase of internal Ca 2+ (due to

depolarization) and at active zones (regions in the presynaptic membrane adjacent to the cleft) 

Presynaptic vesicles

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Vesicle Exocytosis• A group of 6 to 7 proteins work together to respond to Ca 2+ influx

and regulate vesicle fusion• after exocytosis the synaptic vesicle membranes are

reinternalized by endocytosis and reused (reloaded with neurotransmitter by a transmitter transporter system)

• vesicles are also transported from the cell body to the nerve terminal- transmitter is synthesized in the terminal and loaded into the vesicles- enzymes and substrates necessary are present in the terminal- i.e. acetylcholine, acetyl-CoA + choline used by choline acetyltransferase 

Vesicle Exocytosisnon-peptide transmitters • exocytosis only occurs after an increase of internal Ca 2+ (due

to depolarization)• at active zones (regions in the presynaptic membrane

adjacent to the synaptic cleft)

peptide-transmitters (same as for non-peptide transmitters except:)• exocytosis is NOT restricted to active zones• exocytosis is triggered by trains of action potentials

 

SNARE hypothesis

Two families:i) V-SNARE - vesicle membrane proteinsii) T-SNARE - target membrane proteins

SNARE is an acronym for SNAP receptor (SNAP stands for soluble N-ethylmaleimide-sensitive factor attachment proteins).

SNARES are involved in the mediation of protein transport between various plant organelles by small membrane vesicles.

The SNARE Hypothesis for Transport Vesicle Targeting and FusionThe SNARE Hypothesis for Transport Vesicle Targeting and Fusion

SNARE hypothesis

1. Vesicle docking occurs between the V-SNARE and T-SNARE proteins

2. The combined proteins act as a receptor for an ATPase that utilizes ATP to generate the "docked" form

3. One of the proteins is a Ca2+ sensor such that when Ca2+ enters the synapse the vesicle fuses with the plasma membrane and releases its contents

4. The membrane and proteins are then recycled through endocytosis (clatharin coat and dynamin etc.) and reused.

Synaptic Plasticity

• Change in synaptic function in response to patterns of use

• Synaptic facilitation – APs neurotransmitter release

• Synaptic depression – APs neurotransmitter release

• Post-tetanic potentiation (PTP) – after a train of high frequency APs neurotransmitter release

Long-term potentiation

Diversity of Signal Conduction

So far:• Electrotonic• Action potentials• Saltatory conduction• Chemical and electrical synapses

Also:• Shape and speed of action potential• Due to diversity of Na+ and K+ channels

Ion Channel Isoforms

• Multiple isoforms• Encoded by many genes• Variants of the same protein• Voltage-gated K+ channels are highly diverse (18 genes

encode for 50 isoforms in mammals)• Na+ channels are less diverse (11 isoforms in mammals)

Channel Density

Higher density of voltage-gated Na+ channels Lower threshold Shorter relative refractory period

Voltage-Gated Ca2+ Channels

• Open at the same time or instead of voltage-gated Na+ channels

• Ca2+ enters the cell causing a depolarization

• Ca2+ influx is slower and more sustained

• Slower rate of APs due to a longer refractory period

• Critical to the functioning of cardiac muscle

Chemical synapses: post-synaptic mechanisms

Postsynaptic Membranes and ion channels

Ligand gated ion channels – a review

a. Resting K+ channels: responsible for generating the resting potential across the membrane

b. Voltage- gated channels: responsible for propagating action potentials along the axonal membrane

Two types of ion channels in dendrites and cell bodies are responsible for generating electric signals in postsynaptic cells.

c. Has a site for binding a specific extracellular neurotransmitter d. Coupled to a neurotransmitter receptor via a G protein.

More things to know about Ion channelsAll the ion channels in question have a common feature

• A pore that allows the ion(s) in question to flow across the lipid bilayerThe pore is specific to a certain ion or ions• Leak K+ channel only allows K+ ions to flow across the membrane

• Examples: Acetylcholine (ACH) receptor allows Na+ to flow and the glycine receptor allows Cl- to flow through the channel

• Ligand gated ion channels are different than voltage-gated ion channels in that they are chemically gated ie via neurotransmittersBinding a small chemical triggers the opening of the ion channel

• i) Na+ channels - excitatory (generates an excitatory postsynaptic potential)ii) Cl- channels - inhibitory (generates an inhibitory postsynaptic potential) 

• Important: The specificity of a transmitter response is a function of the receptor type NOT the transmitter itself. (i.e. Ach can be excitatory when binding to one type of AchR (NMJ)) and inhibitory when binding to another type of receptor

Acetylcholine

Primary neurotransmitter at the vertebrate neuromuscular junction

Acetylcholine – general info• Motor neuron transmitter at the

neuromusccular junction (NMJ) in vertebrates

• Present in brain (10% of synapses)• Packaged in high numbers in vesicles

1,000 to 10,000 molecules per vesicle at the NMJ

• Like all small chemical transmitters Ach is synthesized and packaged into vesicles in the synapse

• The NMJ pre-synaptic side is packed full of vesicles in the axon terminal

• Many vesicles are released per action potential to ensure a large safety margin so that the muscle fiber (i.e. the postsynaptic cell) will depolarize to beyond threshold.

Acetylcholine – receptor• Officially called the nicotinic ACH receptor

(nAChR) because nicotine binds to this receptor and activates it

• Ligand gated ion channel• has a depolarizing effect because Na+ is

the dominant ion through these channels

Acetylcholine – receptor• generates an excitatory postsynaptic potential which at the NMJ

(motor end plate) is often called an "end plate potential“

EPP - end plate potentialAka Excitatory Junctional Potential (EJP)

End plate potentials (EPPs) evoked by stimulation of a motor neuron are normally above threshold and therefore produce an action potential in the postsynaptic muscle cell.

nACHR – a closer look• Most of the mass of the protein protrudes from the outer (synaptic)

surface of the plasma membrane• The M2 alpha helix (red) in each subunit is part of the lining of the ion

channel• Aspartate and glutamate side chains at both ends of each M2 helix

form two rings of negative charges that help exclude anions from and attract cations to the channel. The gate, which is opened by binding of acetylcholine, lies within the pore.

Aspartate

The Neuromuscular junction

The Neuromuscular junction

• Arrival of an action potential at the terminus of a presynaptic motor neuron induces opening of voltage-gated Ca2+ channels

• subsequent release of acetylcholine, which triggers opening of the ligand-gated nicotinic receptors in the muscle plasma membrane

• The resulting influx of Na+ produces a localized depolarization of the membrane

• leading to opening of voltage-gated Na+ channels and generation of an action potential

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The Neuromuscular junction

Synapses in the brain or central nervous system (CNS)

• A single synapse on a target is seldom found in brain• Large neurons in the brain typically receive many inputs (1000 to

80,000 per cell)• The inputs are integrated in the receiving neuron such that a "decision"

is made to pass on the information onto other cells - this "decision" is often whether or not to generate an action potential

• each synaptic input usually only gives only a small depolarization, so many inputs must cooperate (summate) to reach threshold to fire an action potential

EPSP An excitatory impulse, an excitatory post-synaptic potential raises the membrane potential above rest

1.An excitatory impulse at a synapse on the soma causes a depolarization of the whole soma including the beginning of the axon. The beginning of the axon is also known as the spike initialization zone or axon hillock and is packed with Na+ channels, an epsp of +15 to +20 mV triggers an action potential in the zone2. Due to the absence of voltage-gated Na+ channels in the soma and dendrite of most neurons it is very unlikely that an action potential will be generated in these regions 

IPSP •An inhibitory impulse is called an ipsp (inhibitory post-synaptic potential) and lowers the membrane potential below rest (hyperpolarizes)•Synaptic transmission triggers the opening ligand gated Cl- channels or indirectly through other mechanisms the opening of K+ channels•Cl- flows into the cell•K+ flows out of the cell •Both increase the negative charge within the cell, hyperpolarizes the soma•Brings membrane potential further away from threshold and so it is harder to trigger an action potential therefore inhibitory•An ipsp on the dendrite will have less effect due to current loss than an ipsp in the soma 

CNS Major ligand gated ion channels and their neurotransmitters

Glutamate - amino acid•Most common excitatory neurotransmitters in central nervous system•Neurotransmitter of NMJ in invertebrates (locust, giant axon of squid)•Glutamate receptor - at least 3 different ligand gated ion channel receptors for glutamate - all generate epsps as Na+ is the dominant ion that flows after the channel is open

 GABA - aminobutyric acid•Major inhibitory neurotransmitter in the brain•In some areas of cortex 1 in 5 neurons are GABAergic•GABA receptors- again many different types of receptors - the more common GABA receptors are Cl - channels- usually inhibitory causes an inhibitory postsynaptic potential (IPSP)- reversal potential is the same as ECl - usually around - 70 mV

• Note: reversal potential is synonymous with equilibrium potential  •cont…..

CNS

Glycine - simplest amino acid•Major inhibitory neurotransmitter in the brainstem and spinal cord•Glycine Receptor- major receptor is a Cl - channel- inhibitory- like GABA receptor in that usually causes IPSPs- blocked by strychnine (rat poison) which literally causes convulsions and death as now the motor neurons are not inhibited and the muscles contract without control. Yikes! 

Cable properties again

• Dendrites extend 0.5 to 1 mm in all directions from soma and receive signals from a large area

• 80-90% of all presynaptic terminals terminate on dendrites• Most can't produce action potentials (too few or no Na+ channels) • Transmit current by passive spread down dendrites to the soma• Therefore the membrane potential decreases as move along dendrite

due to current loss thanks to our friends ri, rm and cm• Dendrites have no voltage gated Na+ channels and cell bodies ie soma

have little or no voltage-gated Na+ channels current flow is solely dependent on the Cable Properties of the dendrites and soma

Things to remember

• 1) loss of current across membrane (leaky membranes)• dependent on the internal resistance (ri) and the membrane resistance

(rm)• the length or space constant describes this property

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• 2) loss of current (charge) due to capacitance properties of the membrane

• cell membrane acts as a capacitor• it takes time and current (charge) to charge the membrane capacitor• the time constant describes this effect

  τ = Rm x Cm  

• details are in the lecture on cable properties

im rr /

Summation - CNS •The postsynaptic effects of most synapses in the brain are not as large as those at the neuromuscular junction•In the CNS the postsynaptic potentials are usually far below the threshold for generating postsynaptic action potentials•Neurons in the central nervous system are typically innervated by thousands of synapses, and the postsynaptic potentials produced by each active synapse can summate together (in space and in time) to bring the membrane to threshold for firing an action potential

Motor neurons and summation

• Each motor neuron synapses with multiple muscle fibers • The motor neuron and the fibers it contacts defines the motor unit

Summation Summation of multiple epsps to bring the membrane potential to threshold for an action potential.

Summation A microelectrode records the postsynaptic potentials produced by the activity of two excitatory synapses (E1 and E2) and an inhibitory synapse (I)

Electrical responses to synaptic activation1. Stimulating either excitatory synapse (E1 or E2) produces a subthreshold

EPSP, whereas stimulating both synapses at the same time (E1 + E2) produces a suprathreshold EPSP that evokes a postsynaptic action potential

2. Activation of the inhibitory synapse alone (I) results in a hyperpolarizing IPSP

3. Summing this IPSP with the EPSP produced by one excitatory synapse (E1 + I) reduces the amplitude of the EPSP, while summing it with the suprathreshold EPSP produced by activating synapses E1 and E2 keeps the postsynaptic neuron below threshold, so that no action potential is evoked.

 

Summation