Brain and Behaviour (3): 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: Fluid collected after vagus nerve stimulation of one frog heart could slow heart rate when applied to a different frog heart, proving chemical transmission of nerve impulses.

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 receptors on the postsynaptic neuron →

6. Ion channels in the postsynaptic neuron open →

   1. Positive Ions flow in: Excitatory effects OR

   2. Negative Ions flow in: inhibitory effects

7. Remaining neurotransmitters are removed through reuptake or deactivating enzymes

GABA

Main inhibitory neurotransmitter in the brain, which reduces neuronal excitability. It is synthesised from Glutamine.

Glutamate

Main excitatory neurotransmitter in the brain. It binds to NMDA & AMPA receptors, allowing synaptic transmission. It is synthesised from Glutamine.

Acetylcholine

Neurotransmitter that plays a role in the nervous system. It binds to nicotinic & muscarinic receptors which influence neuronal excitability.

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 metabotropic protein receptors located on the presynaptic terminal that regulate neurotransmitter release via a negative feedback mechanism.