Neurotransmission

Structure of a Neuron

• Dendrites: Recipients of information from other neurons

• Soma (Cell Body): Contains machinery that controls processing in the cell and integrates information

• Axon: Carries information (action potentials) from the soma → Axon terminals. They branch to connect multiple neurones.

• Axon Terminals: Communication point with other neurons found at the end of axons (where synapses are found)

Neuronal Membrane

• 5nm thick lipid bilayer that separates the extracellular and intracellular environment.

• It acts as the boundary of soma, dendrites and axons and their terminals.

• Contains protein structures that detect substances outside the cell and allows access of certain substances into the cell

Types of Synapse

1. Electric Synapse:

   • Rare in adult mammals (e.g. in retina)

   • Small junctions between neurons (3nm) which is spanned by proteins that communicate between neurons through ion flow

2. Chemical Synapse

   • Common in adult mammals

   • Junction between neurons (20-50nm)

   • Chemicals are released from the presynaptic neuron → postsynaptic neuron

Evidence for Chemical Transmission

Loewi: Application of fluid following vagus nerve stimulation → slowed heart rate.

Neurotransmitters

Chemicals that transmit information from the presynaptic → postsynaptic neuron

4 Properties of Neurotransmitters

1. Pharmacology – What binds and how drugs interact (agonists/antagonists)

2. Kinetics – Rate of binding and channel gating

3. Selectivity – Which ions are fluxed

4. Conductance – Rate of ion movement

Steps of Chemical Transmission

1. Depolarisation - Impulse (Action Potential) travels down axon →

2. Voltage-gated Ca²⁺ channels open and calcium ions enter the neuron

3. Vesicles move to the presynaptic membrane

4. Exocytosis - the vesicle fuses with the membrane and the neurotransmitter is released into the synaptic cleft (diffusion)

5. Neurotransmitter binds to the postsynaptic neuron

6. Ion channels in the postsynaptic neuron open → excitatory/inhibitory effects

7. Remaining neurotransmitters are removed through reuptake or deactivating enzymes

Differences between Ionotropic & Metabotropic receptors

Ionotropic (e.g. GABA):

• Fast

• Direct ion channel opening

• Immediate effect

Metabotropic (Dopamine/Serotonin):

• Slow

• Indirect ion channel opening via G-protein activation, activating molecules → reactions → signal amplification

• Amplified, lasting effect

Autoreceptors

(Typically) G-coupled protein receptors located on the presynaptic terminal that respond to neurotransmitters in the synaptic cleft and regulate internal processes controlling synthesis and release of neurotransmitter via a negative feedback mechanism.