Synaptic Transmission Quiz

Synaptic Transmission

• Synapse is a functional connection between a neuron (presynaptic) and another cell (postsynaptic)
• Electrical and chemical synapses exist
• Electrical synapses are rare in the nervous system
• Synaptic transmission at chemical synapses occurs via neurotransmitters (NTs)

Learning Objectives

• Compare convergence and divergence in neural arrangement and understand their significance
• Describe the transmission of impulses at the synapse
• Distinguish excitatory (EPSP) and inhibitory (IPSP) postsynaptic potentials

Synapse = Cell-to-Cell Conduction

• A functional connection between a neuron (presynaptic) and another cell (postsynaptic)
• Electrical and chemical synapses exist
• Electrical synapses are rare in the nervous system
• Synaptic transmission at chemical synapses is via neurotransmitters

Electrical Synapse

• Depolarization flows from presynaptic to postsynaptic cell through channels called gap junctions
• Formed by connexin proteins
• Very fast conduction
• Found in smooth and cardiac muscles, brain, and glial cells

Chemical Synapse

• Synaptic cleft separates terminal bouton of presynaptic from postsynaptic cell
• Pre-synaptic terminal releases neurotransmitters
• Synthesis of neurotransmitters
• Ca²⁺ influx triggers release of neurotransmitters
• NTs are in synaptic vesicles
• Vesicles fuse with bouton membrane, releasing NT by exocytosis
• Neurotransmitter diffuses across synaptic cleft
• Amount of NT released depends upon frequency of AP
• Postsynaptic cell has specific receptors for the neurotransmitter

Synaptic Transmission 1

• Presynaptic neuron brings impulse to the synapse to stimulate or inhibit a postsynaptic neuron
• This is synaptic transmission
• Action potentials travel down the axon to depolarize the bouton
• Voltage-gated Ca²⁺ channels open; Ca²⁺ diffuses inward
• Triggers exocytosis of vesicles, releasing NTs
• First activating calmodulin, then protein kinase
• Phosphorylating regulatory proteins promotes vesicle fusion and exocytosis
• NT release is rapid because vesicles are docked at release sites before APs arrive
• Neurotransmitters diffuse across the synaptic cleft, binding to specific receptors

Synaptic Transmission 2

• NT (ligand) diffuses across cleft
• Binds to receptor proteins on postsynaptic membrane
• Opening chemically-regulated ion channels
• Depolarizing membrane causes EPSPs (excitatory postsynaptic potentials)
• Hyperpolarizing membrane causes IPSPs (inhibitory postsynaptic potentials)
• These are graded potentials

Synaptic Transmission 3

• EPSPs and IPSPs summate
• If membrane potential (MP) in postsynaptic cell reaches threshold at axon hillock, a new action potential (AP) is generated
• Axon hillock has many voltage-gated channels and is where APs are initiated

Synaptic Transmission 4

• Action potentials are conducted by axons
• Opens voltage-gated Ca²⁺ channels
• Release of excitatory neurotransmitters
• Opens chemically gated channels
• Inward Na⁺ diffusion causes depolarization (EPSP)
• Localized, decremental conduction of EPSP
• Opens voltage-gated Na⁺ and then K⁺ channels
• Conduction of action potentials

Synaptic Potentials

• Synaptic potentials are graded and can depolarize or hyperpolarize the receiving cell membrane
• EPSP (excitatory postsynaptic potential) opens Na⁺ channels, depolarizing the membrane
• IPSP (inhibitory postsynaptic potential) opens K⁺ channels, hyperpolarizing the membrane

Synaptic Inhibition

• Presynaptic inhibition: one neuron synapses onto axon or bouton, inhibiting release of another neuron's neurotransmitter
• Postsynaptic inhibition: GABA and glycine produce IPSPs, dampening EPSPs, making it harder to reach threshold
• K⁺ efflux or Cl⁻ influx causes hyperpolarization

Neurotransmitters

• Acetylcholine (ACh): autonomic ganglion, stimulates muscle contractions
• Monoamines: norepinephrine, epinephrine, dopamine, serotonin
• Amino acids: glycine, glutamic acid, GABA
• Peptides: enkephalins, substance P

Neuropeptides

• Neurons in the CNS synthesize neuropeptides
• Act as neurotransmitters or neuromodulators
• Enkephalins bind to opiate receptors, relieving pain
• Beta-endorphin binds to opiate receptors, lasting longer
• Substance P helps perceive pain

Major Neurotransmitters

• Acetylcholine is a major neurotransmitter in the peripheral nervous system

Acetylcholine (ACh)

• Most widely used NT
• Used in brain and ANS, used at all neuromuscular junctions
• Has nicotinic and muscarinic receptor subtypes
• These can be excitatory or inhibitory

Ligand-Gated Channels

• Contain both a NT receptor site and an ion channel
• Open when ligand (NT) binds to its receptors

Nicotinic ACh Channel

• Formed by 5 polypeptide subunits
• 2 subunits contain ACh binding sites
• Opens permitting diffusion of Na⁺ and K⁺
• Inward flow of Na⁺ dominates, producing EPSPs

G Protein-Coupled Channels

• NT receptor is not part of the ion channel
• Is a 1 subunit membrane polypeptide
• Activates ion channel indirectly through G-proteins

Muscarinic ACh Channel

• Binding of 1 ACh activates G-protein cascade, affecting gated K⁺ channels
• Opens some, causing hyperpolarization
• Closes others, causing depolarization

Acetylcholinesterase (AChE)

• Inactivates ACh, terminating its action; located in the cleft

Integration of Signals

• Interneurons are organized into neuronal pools (groups of neurons that work together)
• Convergence: a single neuron receives impulses from two or more fibers; allows collection and processing of information
• Divergence: an impulse from one neuron stimulates two or more fibers, amplifying response

Convergent Pathway

• Many presynaptic neurons converge to influence a smaller number of postsynaptic neurons

Divergent Pathway

• One presynaptic neuron branches to affect a larger number of postsynaptic neurons

Summation: Convergent Integration

• Graded potentials are below threshold
• Graded potentials arrive at the trigger zone, summating to create an AP
• Spatial summation
• Temporal summation
• Summation can be EPSP or IPSP

EPSP

• Graded in magnitude
• Have no threshold
• Na⁺ or Ca²⁺ influx causes depolarization
• Summate
• Have no refractory period

Temporal Summation

• EPSPs fade quickly but closely-timed EPSPs sum before they fade

Spatial Summation

• EPSPs fade quickly over time and distance
• EPSPs from different synapses occur in postsynaptic cell at the same time

Postsynaptic Inhibition

• GABA and glycine produce IPSPs
• IPSPs dampen EPSPs, making it harder to reach threshold
• Presynaptic inhibition: one neuron synapses onto an axon or bouton, inhibiting the release of a neuron's neurotransmitter
• K⁺ efflux or Cl⁻ influx causes hyperpolarization