Neurons – Electrochemical Communication

Electrochemical Communications communication between neurons Neurotransmitters movie

Preview of today’s lecture

Structure of a synapse

presynaptic membrane

postsynaptic membrane

synaptic cleft

Structure of a synapse

synaptic vesicles are produced by Golgi apparatus in the soma

OR

by recycled matter in the cisternae of the terminal button: Pinocytosis

Neurons – from electrical to chemical

• axon terminal contains synaptic vesicles

• vesicles release neurotransmitters across the synaptic cleft• the released neurotransmitter leads to post-synaptic potentials (hyperpolarization or depolarization) that alter the firing rate of the receiving neuron (decrease or increase)

Structure of the synapse

• as viewed under an electron microscope

Structure of the synapse

• synaptic vesicles fusing with the presynaptic membrane

Structure of the synapse

• synaptic vesicles fusing with the presynaptic membrane

Neurochemicals

• Neurotransmitters - chemical substance released from the end of a neuron during the propagation of a nerve impulse; it relays information from one neuron to another.

• Neuromodulators – secreted in larger amounts and diffuse further (composed of peptides)

• Hormones – produced in endocrine glands – released into extracellular fluid to be taken up by specific target cells

Binding

• only specific neurotransmitters will bind with specific receptor sites – like a key in a lock

• chemical that attaches to a binding site is a ligand

• neurotransmitters are naturally produced ligands

• neurotoxins are also ligands and various drugs have their effect in the same manner – artificially produced ligands (e.g., LSD)

Neurons – from electrical to chemical

Only specific neurotransmitters will bind with the post-synaptic membrane.

Binding sitesAxodendritic – synapse on the dendrite of the neuronAxosomatic – synapse on the somaAxoaxonic – synapse on the axon

Axodendritic Axosomatic Axoaxonic

Receptors

• neurotransmitter specific postsynaptic receptors

• open to allow ions to flow into the postsynaptic neuron

• two main types• ionotropic• metabotropic

Ionotropic receptors

• receptor site has its own ion channel

• contain sodium channels

• fast acting and short lasting

Metabotropic receptors

• indirect method• located nearby G-

proteins• G-proteins in turn

activate an ion channel

• slower to begin and longer lasting

Metabotropic receptors

• G-proteins can also activate second messengers – enzymes that in turn activate an ion channel

Excitatory or inhibitory post-synaptic potentials.

• the change in the electric charge can be more positive than the resting potential (excitatory) or more negative than the resting potential (inhibitory)

Inhibitory

• once neurotransmitters are bound to the post synaptic membrane the electrical charge is now altered in the receiving neuron

Excitatory or inhibitory post-synaptic potentials.

Post-synaptic potentials

• determined by the ion channel opened by the neurotransmitter and not the transmitter itself

• graded – the potential dissipates with distance traveled

• smaller in magnitude than action potentials

• action potentials are always excitatory – post-synaptic potentials can be either excitatory or inhibitory

Post-synaptic potentials

• excitatory PSP – typically related to sodium ion channels (rush of Na+ into the cell makes it more positively charged)

• inhibitory PSP typically related to potassium ion channels (extra K+ maintained inside cell by sodium-potassium pump leaks out making the cell more negatively charged)

• action of Cl– channels depends on the state of the receiving neuron – if depolarised Cl– will bring the cell back to a resting state

Terminating the PSP

• reuptake – rapid removal of neurotransmitter from the synaptic cleft

• SSRIs (selective seratonin reuptake inhibitors – e.g, Prozac) prolong the PSP by inhibiting reuptake

Summation of post-synaptic potentials.• whether the PSP leads to the excitation or inhibition of the

neuron depends on the combined effects of many PSPs

Neural integrationSpatial integration: equal excitatory and inhibitory input will cause no change

Neural integrationTemporal integration: ripples can combine to make bigger ripples

Autoreceptors

• autoreceptors respond to neurotransmitters they produce

• regulate synthesis and release of other transmitters

• metabotropic

• usually inhibitory – may control amount of neurotransmitter released

Other types of synapses

• axoaxonic – modulate the neurotransmitters in the presynaptic neuron

• gap junctions – electrical synapses – the synaptic cleft is much smaller – ions pass directly from one neuron to another

Why do you need to know all this?

• different disease processes involve different aspects of the basic electrochemical transmission of neural information

• Parkinson’s Disease – dopamine deficiency

• Multiple Sclerosis – affects the myelin sheath of white matter

• Epilepsy – abnormal electrical stimulation

• Alzheimer’s Disease – neurofibrillary tangles may affect the transport of neurotransmitters

Review Questions

1 ) NeuromodulatorsA) are rarely of a peptide form.B) are secreted from a neuron and only effect an adjacent neuron.C) are inevitably inhibitory.D) are secreted from neurons, but dispersed widely in the brain.E) are typically secreted in very small amounts compared to neurotransmitters.

2 ) Most ________ are secreted into the extracellular fluid from endocrine glands or tissues.A) neurotransmittersB) neuropeptidesC) modulatorsD) hormonesE) Pheromones

3 ) Large synaptic vesicles are produced in theA) soma.B) dendrites.C) terminal buttons.D) dendritic spines.E) neuroglia.

4 ) Which of the following is true of neurotransmitter function?A) Neurotransmitters diffuse widely in the brain to exert changes in metabolism.B) Neurotransmitters directly alter ion channels using a second-messenger chemical.C) Neurotransmitters are released into the synapse from the cistaerna.D) Neurotransmitters open ion channels in the postsynaptic membrane.E) Neurotransmitters alter ion channel activity for minutes.

5 ) Which of the following will produce an EPSP?A) opening a sodium channelB) closing a sodium channelC) opening a potassium channelD) opening a manganese channelE) closing a calcium channel

Nice review animation

Review Questions

For Next Time

Start reading Chapter 3

Structure of the Nervous System

Movie Behaving Brain