Neurophysiology PT 2

Transmission Across the Synapse step 1-2

Transmission Across the Synapse step 1-2

*Action potential reaches the axon terminal

*Voltage gated Ca2+ channels open and Ca2+ floods into the terminal

Transmission Across the Synapse step 3-4

*Synaptic vesicles fuse with the plasma membrane and release neurotransmitters into the synaptic cleft

*Neurotransmitters diffuse across the synaptic cleft and bind to receptors on ligand gated channels initiating a postsynaptic potential

Neurotransmitter effects on Postsynaptic Potentials

(Binding of…)

*Binding of neurotransmitters cause a graded potential (localized change in the membrane)

*Depending on how the neurotransmitter affects the membrane potential determines if it will excite or inhibit the postsynaptic neuron

Excitatory postsynaptic potentials

(Postsynaptic Potentials)

(What becomes less negative?)

• binding of neurotransmitter opens Na+ channels and causes depolarization

  • Membrane potential becomes less negative and closer to reaching threshold potential therefore closer to firing an action potential

Inhibitory Postsynaptic Potentials

(Postsynaptic Potentials)

• binding of neurotransmitters cause hyperpolarization of the membrane therefore moving away from threshold and reducing the ability to initiate an action potential

  • Causes K+ or Cl- channels to open

  • K+ rushes out or Cl- rushes in, both causing the inside to become more negative

Summation

*A single EPSP(Excitatory) cannot induce an action potential but they can be summed

*The axon hillock adds up all the potentials

Types of Summation

Temporal summation

Spatial summation

Temporal summation

a presynaptic neuron increases the frequency of impulses and more neurotransmitters are released in quick succession

Spatial summation

• postsynaptic neuron is stimulated by multiple presynaptic neurons at the same time

  • IPSPs and EPSPs can also be summed and cancel each other out

Modulator Neurons

• The effectiveness of the presynaptic input can be affected by another neuron.

• Allows a specific presynaptic neuron to be inhibited or enhanced without affecting the input from other neurons or effecting all targets

• Allows for tuning

Presynaptic inhibition

the amount of neurotransmitter released from neuron “A” is decreased

Presynaptic facilitation

the amount of neurotransmitter released from neuron “A” is enhanced

Neurotransmitter receptors mediate changes in membrane potential according to

(Effects of Neurotransmitters)

*The amount of neurotransmitter released

*The amount of time the neurotransmitter is bound to receptors

*Neurotransmitters will affect the membrane potential as long as they are bound so they must be deactivated

Three ways neurotransmitters are inactivated:

(Deactivation of Neurotransmitters)

*Through reuptake by presynaptic axon terminals or astrocytes

* enzymes

*They diffuse away from synapse

Termination of Neurotransmitter Effects

Acetylcholine

Norepinephrine

dopamine

serotonin

Acetylcholine

• Degraded by the enzyme acetylecholinesterase found in the synaptic cleft

• Ach=Acetate + Choline

• Choline is actively transported back into the presynaptic terminal and recycled

  • Choline + acetyl CoA=Ach

Norepinephrine, dopamine, serotonin

• Catechol-O-methyltransferase (COMT) deactivates these neurotransmitters

• Taken back up by presynaptic terminal

  • Repackaged or broken down by monoamine oxidase (MAO)

What do MAO inhibitors do?

• Inhibit the breakdown of NE and E to keep them in the synaptic cleft longer

• Used as antidepressants

• Now there are neurotransmitter specific MAOI’s with fewer side effects.

Classification of Neurotransmitters by Chemical Structure

• Acetylcholine (ACh)

• Biogenic amines – catecholamines, serotonin

• Amino acids – glutamate, glycine, GABA

• Peptides – endorphins, substance P

• Messengers: ATP and dissolved gases NO

Classification by Function

Excitatory neurotransmitters

Inhibitory neurotransmitters

Some neurotransmitters have both excitatory and inhibitory effects

How are having both excitatory and inhibitory effects determined by?

• Determined by the receptor type of the postsynaptic neuron

• Example: acetylcholine

  • Excitatory at neuromuscular junctions with skeletal muscle (nicotinic receptor)

  • Inhibitory in cardiac muscle (muscarinic receptor)

Neurotransmitter Receptor Mechanisms

Direct

Indirect

Direct

• neurotransmitters that open ion channels

  • Promote rapid responses “fast synapses”

  • Examples: ACh and amino acids

Indirect

• neurotransmitters that act through second messengers

  • Promote long-lasting effects, “slow synapses”

  • Examples: biogenic amines, peptides, and dissolved gases

Types of Circuits in Neuronal Pools

Divergent

Convergent

Reverberating

Parallel after-discharge

Divergent

• one incoming fiber stimulates multiple fibers, often amplifying circuits

  • Ex: balance and posture

Convergent

• opposite of divergent circuits, resulting in either strong stimulation or inhibition

  • Ex: process sensory information

Reverberating

• chain of neurons containing collateral synapses with previous neurons in the chain

  • Ex: short term memory (repetition)

Parallel after-discharge

• incoming neurons stimulate several neurons in parallel arrays

  • Plexus, in case a neuron gets damaged