Synaptic Functions and Neurotransmission

Yazeed Alwelaie MBBS

November 14, 2012

Outlines

• Basic Functions of the Synapses

• Types of Synapses

• The Physiologic Anatomy of the Synapses

• Mechanism of Neurotransmission

• Synaptic Transmitters

Introduction

200-400 billion

stars!

Courtesy of rikenresearch.jp

100 billion

neurons1014 Synapses!

How is Information Transmitted in the Nervous System?

• Nerve action potentials, simply called “nerveimpulses”

• Each impulse may be blocked, changed or integrated

• Collectively, these functions can be classified assynaptic functions of neurons

Voltage-gated

Sodium and

Potassium Channels

National Institutes of Health (US); Biological Sciences Curriculum Study.Bethesda (MD): National Institutes of Health (US); 2007-

Types of Synapses

Overview

• Almost all the synapses used for signaltransmission are chemical synapses

• Neurotransmitter acts on the membrane of thenext neuron to excite the neuron, inhibit it, ormodify its sensitivity

Neuroscience. 2nd edition.Purves D, Augustine GJ, Fitzpatrick D, et al., editors.Sunderland (MA): Sinauer Associates; 2001.

• Electrical synapses, in contrast, are characterized bydirect open fluid channels that conduct electricity fromone cell to the next

From: Electrical SynapsesCopyright © 2001, Sinauer Associates, Inc.

Electrical Synapse

“One-way conduction”

• Chemical synapses always transmit signals in onedirection (exceedingly important characteristic)

Physiologic Anatomy of the Synapses

Presynaptic Terminals

• Sometimes called terminal knobs, boutons, end-feet, or synapticknobs

Function of Receptor Proteins on Postsynaptic Neuron

1. Binding component: bindsthe transmitter coming fromthe presynaptic terminal

2. Ionophore component: canbe either an ion channel or a“second messenger”activator

Neuroscience. 2nd edition.Purves D, Augustine GJ, Fitzpatrick D, et al., editors.Sunderland(MA): Sinauer Associates; 2001

Excitatory or Inhibitory Receptors in the Postsynaptic Membrane

Excitation

• Opening of sodium (Na+) channels > flow of positivecharges interiorly > raised membrane potential tothreshold for excitation

• Decreased conduction through chloride (Cl-) channels >decreased diffusion of negative charges to the inside >more positive membrane potential

Inhibition

• Opening of chloride (Cl-) channels > influx of negativecharges to the inside > more negative membranepotential, which is inhibitory

• Increased conductance of potassium (K+) out of theneuron > less positive charges inside > increasednegativity

Role of Calcium ions

Three Stages of a

Neuron

Neurotransmitters

Overview

• More than 50 chemical substance have been proved

• Two groups: small-molecule (rapidly acting) andneuropeptides (slowly acting)

Small-Molecule Transmitters

• Synthesized in the cytosol of the presynapticterminal

• Absorbed by means of active transport intotransmitter vesicles

• Continuous recycling of vesicles

1. Acetylcholine

• Typical small-molecule transmitter

Acetyle CoA Choline

Acetylcholine + CoA

Synthes

is

Acetylcholine Degradati

on

Acetate Choline

• Acetylcholine is secreted by neurons in many areas ofthe nervous system:

1. Large pyramidal cells from the motor cortex

2. Basal Ganglia

3. Somatic neurons

4. All Preganglionic autonomic neurons

5. Parasympathetic postganglionic neurons

• Excitatory centrally, mixed action peripherallydepending on type of receptor

2. The Amines

Inhibitory

Excitatory

Excitatory

Inhibitory

3. Amino Acids

• Glycine: Secreted mainly at synapses in the spinalcord. It is always inhibitory

• GABA: secreted in the spinal cord, cerebellum, basalganglia and cortex. It is always inhibitory

• Glutamate: secreted by sensory pathways enteringthe CNS, as well as cerebral cortex. It is alwaysexcitatory

4. Nitric Oxide (NO)

• Secreted in areas responsible for long-term behaviorand for memory

• NO has different chemical properties. It is a diffusiblegas

• It regulates cerebral and peripheral blood flow

Neuropeptides

• Slow acting > much more prolonged actions

• They are synthesized as integral parts of large-protein molecules by ribosomes in the cell body

Small Molecule Neurotransmitter SubstancesAcetylcholine (ACh) Dopamine (DA) Norepinephrine (NE)

Serotonin (5-HT) Histamine Epinephrine

Amino AcidsGamma-aminobutyric

acid (GABA)Glycine Glutamate

Aspartate

Neuroactive Peptides - partial list

bradykinin beta-endorphin bombesin calcitonin

cholecystokinin enkephalin dynorphin insulin

gastrin substance P neurotensin glucagon

secretin somatostatin motilin vasopressin

oxytocin prolactin thyrotropin angiotensin II

sleep peptides galanin neuropeptide Ythyrotropin-

releasing hormone

gonadotropnin-

releasing hormone

growth hormone-

releasing hormone

luteinizing

hormone

vasoactive

intestinal peptide

Soluble GasesNitric Oxide (NO) Carbon Monoxide

Summary of Neurotransmitters