recap

44.3 recap

Chemical synapses involve the release of neurotransmitter molecules stored in vesicles in the presynaptic terminal. APs reaching that terminal cause the fusion of vesicles with the presynaptic membrane, releasing neurotransmitter that can then bind to receptors on the postsynaptic membrane and influence its membrane potential. At excitatory synapses, the neurotransmitter causes depolarization of the postsynaptic membrane, and at inhibitory synapses the neurotransmitter hyperpolarizes or stabilizes the postsynaptic membrane potential. There is a great diversity of neurotransmitters and their receptors. At electrical synapses the AP moves directly between pre- and postsynaptic cells.

learning outcomes

You should be able to:

  • Describe the events at a neuromuscular synapse, beginning with the action potential in the presynaptic neuron and ending with the response of the postsynaptic cell.

  • Compare the characteristics of electrical versus chemical synapses.

  • Discuss how the properties of transmission differ for electrical and chemical synapses, and why chemical synapses present a greater variety of postsynaptic responses than do electrical synapses.

  • Discuss how the properties of chemical synapses present a variety of postsynaptic responses.

  • Describe the reasons why synaptic transmission is typically transitory.

Question 1

What are the two types of cation channels in the motor end plate, and what are their functions?

The motor end plate contains chemically gated Na+ channels and voltage-gated Na+ channels. The chemically gated channels respond to the neurotransmitter acetylcholine to depolarize the motor end plate. The voltage-gated channels respond to that local depolarization by generating action potentials that spread to the adjacent muscle cell membrane.

Question 2

How can the same neurotransmitter be excitatory to some postsynaptic neurons and inhibitory to others?

The action of a neurotransmitter depends on the receptor types in the postsynaptic membrane, and they can initiate excitatory or inhibitory responses in the postsynaptic neuron.

Question 3

Sarin is a colorless, odorless liquid that is highly lethal. It irreversibly inhibits the enzyme acetylcholinesterase. Explain why victims of sarin poisoning generally die of asphyxiation.

Asphyxiation is the inability to breathe. Breathing requires regular activity in the motor neurons controlling the respiratory muscles. The inhibition of acetylcholinesterase by sarin results in the accumulation of acetylcholine in the synapses between the respiratory motor neurons and the respiratory muscles, reducing the ability of those muscles to relax between breaths.

Question 4

Explain why parts of nervous systems that integrate complex information use chemical rather than electrical synapses.

Electrical synapses are fast but do not integrate information well. Electrical synaptic input does not allow temporal summation of inputs, as electrical synapses require a large area of contact between pre- and postsynaptic cells for effective transmission thus limiting the numbers of synapses that can be formed between two neurons. Finally, electrical synapses cannot provide inhibitory input.

Now that you understand how neurons communicate with each other, we will explore how they are organized into circuits and systems that process sensory information and generate responses to that information.