Neurotransmission

Properties of Electricity

• Electrical currents flow consists of charged particles

• Like charges repel, opposites attract

• Currents only flow through materials that conduct electricity

• Voltage measures potential for charge to move.

Ohm's Law

Current = Potential/Resistance

How does Nerve Conduction differ to Wire Conduction?

Speed of electrical signals in nerves is 1 million times slower than in wires. (Von Helmholtz 1849)

How can cells signal electrically?

• Movement of Ions

• Electrically charged particles

Cell Membrane

Electrically charged lipid membrane encasing cells. Plays a role in allowing movement of ions to cause electrical impulses.

Equilibrium Potential

Membrane potential where there is no net flow of an ion. It is dictated by concentration difference and ion charge.

Membrane Potential

Set by electrochemical gradient and permeability of membrane to different ions. At equilibrium it is ~ 70 mV.

What maintains ion gradients?

Sodium Potassium pumps

Features of Electrical Signals

• Can be measured using a voltmeter

• Fast

• Negative resting membrane potential

Action Potential

Self-regenerating electrical wave in neurons. They are “All or Nothing”

Process of Action Potentials

1. Threshold Potential reached

2. Depolarisation due to sodium channels opening

3. Repolarisation due to inactivation of sodium channels and opening of voltage-gated potassium channels

4. Hyperpolarisation as voltage-gated potassium channels are still open

5. Sodium Channels reactivated allowing an Action Potential to fire again

Ion Channels

Membrane holes allowing selective ion passage based on gradients. (Na+ Outside, K+ Inside)

Absolute Refractory Period

When sodium channels are inactivated and no more action potentials can be fired.

Relative Refractory Period

Where some sodium channels are inactivated and can only be reopened by a strong stimuli.

What does action potential speed depend on?

• How fast membrane potential changes due to:

  • Membrane Resistance (to leakage)

  • Capacitance of cell

• How far depolarisations can spread along the axon due to:

  • Membrane Resistance

  • Diameter (larger = faster)

Myelination

Membrane Insulation enhancing speed and efficiency of action potential propagation as less charge is lost.

Saltatory Conduction

Where action potentials efficiently travel from one Node of Ranvier to another

Process of Synaptic Transmission

Presynaptic Cell

1. Action Potential arrives

2. Membrane depolarisation opens voltage-gated calcium channels

3. Calcium inside cells cause vesicles of neurotransmitter to fuse with membrane

4. Neurotransmitter diffuses through the synaptic cleft

   
   

   Synaptic Cell

5. Neurotransmitter binds to ligand gated ion channels

6. Ions flow through channel either depolarising or hyperpolarising the post-synaptic membrane

Glutamate Receptors

Receptors generating excitatory postsynaptic potentials in response to ion flow via depolarisation of dendrites

NMDA Receptor

Ion channels that bind glutamate and open when depolarised. They let in calcium which causes changes to synapses.

Temporal Summation

Where many stimuli overtime can reach a threshold

Spatial Summation

Where stimuli from different synapses combine to generate an action potential.

GABA

Main inhibitory neurotransmitter in the brain. It opens chloride channels allowing negative charge to generate inhibitory post-synaptic potential, making it harder for action potentials to form.

Soma

Part of an axon that integrates signals from the dendrites to decide whether to fire an action potential.

Synaptic Integration

Summation of excitatory and inhibitory inputs to determine firing of action potential.

Neuronal Networks

Interconnected neurons influencing computations and information representation.

Lateral Inhibition

Where an excited neuron can reduce the activity of neighbouring neurons. It can prevent action potentials from forming.