Section 2: Signal Transmission Between the Neurons

Neurotransmission 1.Chemical synapse (Classical Synapse) Predominates in the vertebrate nervous system 2.Non-synaptic chemical transmission 3.Electrical synapse Via specialized gap junctions Does occur, but rare in vertebrate NS Astrocytes can communicate via gap junctions

Chemical Synapse Terminal bouton is separated from postsynaptic cell by synaptic cleft. Vesicles fuse with axon membrane and NT released by exocytosis. Amount of NTs released depends upon frequency of AP.

Non-synaptic chemical transmission The postganglionic neurons innervate the smooth muscles.  No recognizable endplates or other postsynaptic specializations;  The multiple branches are beaded with enlargements (varicosities) that are not covered by Schwann cells and contain synaptic vesicles; Fig. : Ending of postganglionic autonomic neurons on smooth muscle

Non-synaptic chemical transmission continued  In noradrenergic neurons, the varicosities are about 5m, with up to 20,000 varicosities per neuron;  Transmitter is apparently released at each varicosity, at many locations along each axon;  One neuron innervate many effector cells. Fig. : Ending of postganglionic autonomic neurons on smooth muscle

Electrical Synapse Impulses can be regenerated without interruption in adjacent cells. Gap junctions: Adjacent cells electrically coupled through a channel. Each gap junction is composed of 12 connexin proteins. Examples: Smooth and cardiac muscles, brain, and glial cells.

Electrical Synapses Electric current flow- communication takes place by flow of electric current directly from one neuron to the other No synaptic cleft or vesicles cell membranes in direct contact Communication not polarized- electric current can flow between cells in either direction

Electrical Synapse Chemical Synapse Purves, 2001

I The Chemical Synapse and Signal Transmission

The chemical synapse is a specialized junction that transfers nerve impulse information from a pre synaptic membrane to a postsynaptic membrane using neurotransmitters and enzymes

Synaptic connections ~100,000,000,000 neurons in human brain Each neuron contacts ~1000 cells Forms ~10,000 connections/cell How many synapses?

Neurotransmitter- communication via a chemical intermediary called a neurotransmitter, released from one neuron and influences another Synaptic cleft- a small gap between the sending (presynaptic) and the receiving (postsynaptic) site Chemical Synapses

Synaptic vesicles- small spherical or oval organelles contain chemical transmitter used in transmission Polarization- communication occurs in only one direction, from sending presynaptic site, to receiving postsynaptic site Chemical Synapses

1. Synaptic Transmission Model Precursor transport NT synthesis Storage Release Activation Termination ~diffusion, degradation, uptake, autoreceptors

Postsynaptic Membrane Presynaptic Axon Terminal Terminal Button Dendritic Spine

(1) Precursor Transport

(2) Synthesis enzymes/cofactors _ NT

(3) Storage in vesicles

NT Terminal Button Dendritic Spine Vesicles Synapse

(4) Release Terminal Button Dendritic Spine Synapse Receptors

Terminal Button Dendritic Spine AP Synapse

Exocytosis Ca2+

Each vesicle contains one quanta of neurotransmitter (approximately 5000 molecules) – quanta release

(5) Activation

(6) Termination

(6.1) Termination by... Diffusion

(6.2) Termination by... Enzymatic degradation

(6.3) Termination by... Reuptake

(6.4) Termination by... Autoreceptors A

Autoreceptors On presynaptic terminal Binds NT same as postsynaptic receptors different receptor subtype Decreases NT release & synthesis Metabotropic receptors

Synaptic Transmission AP travels down axon to bouton. VG Ca2+ channels open. Ca2+ enters bouton down concentration gradient. Inward diffusion triggers rapid fusion of synaptic vesicles and release of NTs. Ca2+ activates calmodulin, which activates protein kinase. Protein kinase phosphorylates synapsins. Synapsins aid in the fusion of synaptic vesicles.

Synaptic Transmission (continued) NTs are released and diffuse across synaptic cleft. NT (ligand) binds to specific receptor proteins in postsynaptic cell membrane. Chemically-regulated gated ion channels open. EPSP: depolarization. IPSP: hyperpolarization. Neurotransmitter inactivated to end transmission.

2 EPSP and IPSP

(1)Excitatory postsynaptic potential (EPSP) An AP arriving in the presynaptic terminal cause the release of neurotransmitter; The molecules bind and active receptor on the postsynaptic membrane;

(1)Excitatory postsynaptic potential (EPSP) Opening transmitter-gated ions channels ( Na+) in postsynaptic- membrane; Both an electrical and a concentration gradient driving Na+ into the cell; The postsynaptic membrane will become depolarized(EPSP).

EPSP No threshold. Decreases resting membrane potential. Closer to threshold. Graded in magnitude. Have no refractory period. Can summate.

(2) Inhibitory postsynaptic potential (IPSP) • A impulse arriving in the presynaptic terminal causes the release of neurotransmitter; •The molecular bind and active receptors on the postsynaptic membrane open CI- or, sometimes K+ channels; • More CI- enters, K+ outer the cell, producing a hyperpolarization in the postsynaptic membrane.

(IPSPs): No threshold. Hyperpolarize postsynaptic membrane. Increase membrane potential. Can summate. No refractory period.

3 Synaptic Inhibition Presynaptic inhibition: Amount of excitatory NT released is decreased by effects of second neuron, whose axon makes synapses with first neuron’s axon. Postsynaptic inhibition

(1) Postsynaptic inhibition Concept: effect of inhibitory synapses on the postsynaptic membrane. Mechanism: IPSP, inhibitory interneuron Types: Afferent collateral inhibition( reciprocal inhibition) Recurrent inhibition.

1) Reciprocal inhibition Activity in the afferent fibers from the muscle spindles (stretch receptors) excites (EPSPs) directly the motor neurons supplying the muscle from which the impulses come. Postsynaptic inhibition At the same time, inhibits (ISPSs) those motor neurons supplying its antagonistic muscles.

1) Reciprocal inhibition The latter response is mediated by branches of the afferent fibers that end on the interneurons. Postsynaptic inhibition The interneurons, in turn, secrete the inhibitory transmitter (IPSP) at synapses on the proximal dendrites or cell bodies of the motor neurons that supply the antagonist.

Postsynaptic inhibition Neurons may also inhibit themselves in a negative feedback fashion. Each spinal motor neuron regularly gives off a recurrent collateral that synapses with an inhibitory interneuron which terminates on the cell body of the spinal neuron and other spinal motor neurons. The inhibitory interneuron to secrete inhibitory mediator, slows and stops the discharge of the motor neuron. Postsynaptic inhibition 2) Recurrent inhibition

(2) Presynaptic inhibition Concept: the inhibition occurs at the presynaptic terminals before the signal ever reaches the synapse. The basic structure: an axon-axon synapse (presynaptic synapse), A and B. Neuron A has no direct effect on neuron C, but it exert a Presynaptic effect on ability of B to Influence C. The presynatic effect May decrease the amount of neuro- transmitter released from B (Presynaptic inhibition) or increase it (presynaptic facilitation). (2) Presynaptic inhibition B A A A B C C

Presynaptic inhibition The mechanisms: • Activation of the presynaptic receptors increases CI- conductance,  to decrease the size of the AP reaching the excitatory ending,  reduces Ca2+ entry and consequently the amount of excitatory transmitter decreased. • Voltage-gated K+ channels are also opened, and the resulting K+ efflux also decreases the Ca2+ influx.

Presynaptic Inhibition Excitatory Synapse + A B A active B more likely to fire Add a 3d neuron ~

Presynaptic Inhibition Excitatory Synapse + A B C - Axon-axon synapse C is inhibitory ~

Presynaptic Inhibition Excitatory Synapse + A B C - C active less NT from A when active B less likely to fire ~

4 Synaptic Facilitation: Presynaptic and Postsynaptic

(1) Presynaptic Facilitation Excitatory Synapse + A B A active B more likely to fire ~

Presynaptic Facilitation Excitatory Synapse + A B C + C active (excitatory) more NT from A when active (Mechanism:AP of A is prolonged and Ca 2+ channels are open for a longer period.) B more likely to fire ~

(2) Postsynaptic facilitation: neuron that has been partially depolarized is more likely to undergo AP.

Vm EPSP + + - Record here Depolarization more likely to fire ~ Time -65mv - 70mv AT REST - Time

5 Synaptic Integration EPSPs can summate, producing AP. Spatial summation: Numerous PSP converge on a single postsynaptic neuron (distance). Temporal summation: Successive waves of neurotransmitter release (time).

(1) Spatial Summation The accumulation of neurotransmitter in the synapse due the combined activity of several presynaptic neurons entering the Area (Space) of a Convergent Synapse. A space (spatial) dependent process.

vm Spatial + Summation + + - Multiple synapses Time -65mv - 70mv AT REST - Time

(2) Temporal Summation The accumulation of neurotransmitters in a synapse due to the rapid activity of a presynaptic neuron over a given Time period. Occurs in a Divergent Synapse. (explain later) Is a Time (Temporal) dependent process.

Vm Temporal + Summation + - Repeated stimulation same synapse ~ Time -65mv - 70mv AT REST - Time

(3) EPSPs & IPSPs summate CANCEL EACH OTHER Net stimulation EPSPs + IPSPs = net effects ~

+ EPSP IPSP - + - 70mv -

6. Divergent and Convergent Synapse

Divergent Synapse A junction that occurs between a presynaptic neuron and two or more postsynaptic neurons (ratio of pre to post is less than one). The stimulation of the postsynaptic neurons depends on temporal summation).

Convergent Synapse A junction between two or more presynaptic neurons with a postsynaptic neuron (the ratio of pre to post is greater than one). The stimulation of the postsynaptic neuron depends on the Spatial Summation. Presynaptic neurons Postsynaptic neuron

II Neurotransmitters and receptors

1. Basic Concepts of NT and receptor Neurotransmitter: Endogenous signaling molecules that alter the behaviour of neurons or effector cells. Neuroreceptor: Proteins on the cell membrane or in the cytoplasm that could bind with specific neurotransmitters and alter the behavior of neurons of effector cells

Vast array of molecules serve as neurotransmitters The properties of the transmitter do not determine its effects on the postsynaptic cells The properties of the receptor determine whether a transmitter is excitatory or inhibitory

A neurotransmitter must (classical definition) Be synthesized and released from neurons Be found at the presynaptic terminal Have same effect on target cell when applied externally Be blocked by same drugs that block synaptic transmission Be removed in a specific way Purves, 2001

Classical Transmitters (small-molecule transmitters) Non-classical Transmitters Biogenic Amines Acetylcholine Catecholamines Dopamine Norepinerphrine Epinephrine Serotonin Amino Acids Glutamate GABA (-amino butyric acid) Glycine Neuropeptides Neurotrophins Gaseous messengers Nitric oxide Carbon Monoxide D-serine

Agonist A substance that mimics a specific neurotransmitter, is able to attach to that neurotransmitter's receptor and thereby produces the same action that the neurotransmitter usually produces. Drugs are often designed as receptor agonists to treat a variety of diseases and disorders when the original chemical substance is missing or depleted.

Antagonist Drugs that bind to but do not activate neuroreceptors, thereby blocking the actions of neurotransmitters or the neuroreceptor agonists.

Same NT can bind to different -R different part of NT ~ Receptor A Receptor B

Specificity of drugs Drug B Drug A NT Receptor A Receptor B

Five key steps in neurotransmission Synthesis Storage Release Receptor Binding Inactivation Purves, 2001

Synaptic vesicles Concentrate and protect transmitter Can be docked at active zone Differ for classical transmitters (small, clear-core) vs. neuropeptides (large, dense-core)

Neurotransmitter Co-existence (Dale principle) Some neurons in both the PNS and CNS produce both a classical neurotransmitter (ACh or a catecholamine) and a polypeptide neurotransmitter. They are contained in different synaptic vesicles that can be distinguished using the electron microscope. The neuron can thus release either the classical neurotransmitter or the polypeptide neurotransmitter under different conditions.

Purves, 2001

Receptors determine whether: Synapse is excitatory or inhibitory NE is excitatory at some synapses, inhibitory at others Transmitter binding activates ion channel directly or indirectly. Directly ionotropic receptors fast Indirectly metabotropic receptors G-protein coupled slow

2. Receptor Activation Ionotropic channel Metabotropic channel directly controls channel fast Metabotropic channel second messenger systems receptor indirectly controls channel ~

(1) Ionotropic Channels NT neurotransmitter

Ionotropic Channels NT Pore

Ionotropic Channels NT

Ionotropic Channels NT

(2) Metabotropic Channels Receptor separate from channel G proteins 2d messenger system cAMP other types Effects Control channel Alter properties of receptors regulation of gene expression ~

(2.1) G protein: direct control NT is 1st messenger G protein binds to channel opens or closes relatively fast ~

G protein: direct control GDP

G protein: direct control GTP Pore

(2.2) G protein: Protein Phosphorylation external signal: nt external signal: NT norepinephrine b adrenergic -R Receptor trans- ducer primary effector Receptor trans- ducer primary effector GS adenylyl cyclase 2d messenger 2d messenger cAMP secondary effector secondary effector protein kinase

G protein: Protein Phosphorylation C G GDP PK

G protein: Protein Phosphorylation C G GTP ATP cAMP PK

G protein: Protein Phosphorylation C G GTP Pore ATP P cAMP PK

(3) Transmitter Inactivation Reuptake by presynaptic terminal Uptake by glial cells Enzymatic degradation Presynaptic receptor Diffusion Combination of above

Summary of Synaptic Transmission Purves,2001

Basic Neurochemistry

3. Some Important Transmitters

(1) Acetylcholine (ACh) as NT

Acetylcholine Synthesis acetyltransferase choline + acetyl CoA ACh + CoA

Acetylcholinesterase (AChE) Enzyme that inactivates ACh. Present on postsynaptic membrane or immediately outside the membrane. Prevents continued stimulation.

The Life Cycle of Ach

Ach - Distribution Peripheral N.S. Central N.S. - widespread Excites somatic skeletal muscle (neuro-muscular junction) Autonomic NS Ganglia Parasympathetic NS--- Neuroeffector junction Few sympathetic NS – Neuroeffector junction Central N.S. - widespread Hippocampus Hypothalamus ~

Ach Receptors ACh is both an excitatory and inhibitory NT, depending on organ involved. Causes the opening of chemical gated ion channels. Nicotinic ACh receptors: Found in autonomic ganglia (N1) and skeletal muscle fibers (N2). Muscarinic ACh receptors: Found in the plasma membrane of smooth and cardiac muscle cells, and in cells of particular glands .

Acetylcholine Neurotransmission “Nicotinic” subtype Receptor: Membrane Channel for Na+ and K+ Opens on ligand binding Depolarization of target (neuron, muscle) Stimulated by Nicotine, etc. Blocked by Curare, etc. Motor endplate (somatic) (N2), all autonomic ganglia, hormone producing cells of adrenal medulla (N1)

Acetylcholine Neurotransmission “Muscarinic” subtype Receptor: M1 Use of signal transduction system Phospholipase C, IP3, DAG, cytosolic Ca++ Effect on target: cell specific (heart , smooth muscle intestine ) Blocked by Atropine, etc. All parasympathetic target organs Some sympathetic targets (endocrine sweat glands, skeletal muscle blood vessels - dilation)

Acetylcholine Neurotransmission “Muscarinic” subtype: M2 Use of signal transduction system via G-proteins, opens K+ channels, decrease in cAMP levels Effect on target: cell specific CNS Stimulated by ? Blocked by Atropine, etc.

Cholinergic Agonists Direct Indirect Muscarine Nicotine AChE Inhibitors ~

Cholinergic Antagonists Direct Nicotinic - Curare Muscarinic - Atropine

Ligand-Operated ACh Channels N Receptor

G Protein-Operated ACh Channel M receptor G Protein-Operated ACh Channel

(2) Monoamines as NT

Monoamines Catecholamines – Indolamines - Dopamine - DA Norepinephrine - NE Epinephrine - E Indolamines - Serotonin - 5-HT

Mechanism of Action ( receptor)

Epi a1 G protein PLC IP3 Ca+2

Norepinephrine (NE) as NT NT in both PNS and CNS. PNS: Smooth muscles, cardiac muscle and glands. Increase in blood pressure, constriction of arteries. CNS: General behavior.

Adrenergic Neurotransmission 1 Receptor Stimulated by NE, E, blood vessels of skin, mucosa, abdominal viscera, kidneys, salivary glands vasoconstriction, sphincter constriction, pupil dilation

Adrenergic Neurotransmission 2 Receptor stimulated by, NE, E, ….. Membrane of adrenergic axon terminals (pre-synaptic receptors), platelets inhibition of NE release (autoreceptor), promotes blood clotting, pancreas decreased insulin secretion

Adrenergic Neurotransmission 1 receptor stimulated by E, …. Mainly heart muscle cells, increased heart rate and strength

Adrenergic Neurotransmission  2 receptor stimulated by E .. Lungs, most other sympathetic organs, blood vessels serving the heart (coronary vessels), dilation of bronchioles & blood vessels (coronary vessels), relaxation of smooth muscle in GI tract and pregnant uterus

Adrenergic Neurotransmission  3 receptor stimulated by E, …. Adipose tissue, stimulation of lipolysis

(3) Amino Acids as NT Glutamate acid and aspartate acid: Excitatory Amino Acid (EAA) gamma-amino-butyric acid (GABA) and glycine: Inhibitory AA

(4) Polypeptides as NT CCK: Substance P: Promote satiety following meals. Substance P: Major NT in sensations of pain.

(5) Monoxide Gas: NO and CO Nitric Oxide (NO) Exerts its effects by stimulation of cGMP. Involved in memory and learning. Smooth muscle relaxation. Carbon monoxide (CO): Stimulate production of cGMP within neurons. Promotes odor adaptation in olfactory neurons. May be involved in neuroendocrine regulation in hypothalamus.