Section 43.3 (Exam 4) Communication Flashcards
Neurons Communicate with Other Cells
Neurons communicate with other neurons or target cells at synapses. Name the two types of synapses.
Electrical synapse
Chemical synapse
Describe an electrical synapse.
Electrical synapse: action potential spreads directly to the postsynaptic cell via current flow through gap junctions, causing depolarization, which may trigger action potentials in subsequent neurons
Describe a chemical synapse.
Chemical synapse: neurotransmitters released by the presynaptic cell bind to ligand-gated ion channels on the postsynaptic membrane causing depolarization (via Na⁺ or Ca²⁺ influx) or hyperpolarization (via K⁺ efflux or Cl⁻ influx), leading to excitation or inhibition of APs, respectively
They are the most common type of synapse in vertebrates
What are neuromuscular junctions, and which neurotransmitter do they use?
chemical synapses between motor neurons and skeletal muscle cells
the neurotransmitter they use is acetylcholine (ACh)
How does acetylcholine (ACh) function in neuromuscular junctions?
ACh diffuses across the synaptic cleft to the motor end plate on the muscle cell
What does an AP at an axon terminal of a neuron of a neuromuscular junction cause?
Slight depolarization causes voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal
Ca2+ influx triggers the fusion of vesicles containing ACh with the presynaptic membrane, releasing the ACh into the synaptic cleft via exocytosis
What is the synaptic cleft?
The space between the pre-synaptic and post-synaptic cell
What ACh do upon entering the synaptic cleft?
ACh diffuses across the synaptic cleft and binds to receptors (AChRs) on the motor end plate
What are AChRs?
AChRs (acetylcholine receptors) are chemically/ligand gated channels that allow Na+ and K+ to flow through (they are not ion-specific)
An increase in Na+ depolarizes the membrane
If AChRs are non-selective channels, how come K+ doesn’t flow out of the cell and negate the influx of Na+ ions?
Both the concentration gradient and the electrical gradient drive Na+ into the cell
The concentration gradient favors K+ leaving the cell, but since the resting potential is negative, the electrical gradient pulls K+ back into the cell
These opposing forces mean K+ does not leave the cell as readily as Na+ enters
What does the amount of depolarization at a neuromuscular junction depend on?
number of AChRs
How is an action potential at a chemical synapse terminated?
Neurotransmitters must be cleared from the synaptic cleft after release in order to stop their action
Breakdown occurs via enzymes: ACh is destroyed by acetylcholinesterase (AChE)
What would happen if AChE (acetylcholinesterase) were inhibited?
ACh stays in the synaptic cleft and causes spastic muscle paralysis and death
Some nerve gases and insecticides inhibit AChE
A neuron has many synapses and may receive different chemical messages. Are synapses at neuromuscular junctions excitatory or inhibitory? Why?
Synapses between motor neurons and muscle cells are excitatory
ACh always causes depolarization
What would be necessary for a synapse to be inhibitory?
the postsynaptic response must be hyperpolarization
