SCN3230-ABED_1: Module 1 Lesson 7 - 5

Lesson 7 — Synaptic and Neuromuscular Transmission

A Closer Look at Neurotransmitters

Read pages 379 - 380

Neurotransmitters can be either excitatory or inhibitory at the synapse. Sometimes the same neurotransmitter may be both inhibitory and excitatory!

By opening potassium channels or chloride channels in the postsynaptic membrane, inhibitory neurotransmitters make it harder to initiate depolarization in the second neuron because the neuron develops a more negative membrane potential. Just like opening the windows on a winter day and letting the heat out, it is harder to warm the room up.

Excitatory neurotransmitters open up sodium channels and make it easier to start depolarization because the membrane potential becomes less negative. Putting an electric heater into the room makes it easier to warm up the room. Acetylcholine and norepinephrine are excitatory neurotransmitters.

Acetylcholine is a common neurotransmitter in the somatic nervous system, and it is also found between the parasympathetic neurons of the autonomic nervous system. It is also found in some synapses of the brain. Norepinephrine (also called noradrenaline) is an important neurotransmitter in the sympathetic nervous system, and is also found in some synapses in the brain.

Parkinson's Disease

Parkinson's Disease is associated with the neurotransmitter dopamine. Parkinson's Disease occurs when the nerve cells in the part of the brain that controls muscle movement are gradually destroyed and the neurons can no longer produce dopamine to coordinate muscle movements. Consequently, some of the initial symptoms include muscle stiffness, difficulty bending the arms or legs, an unstable, stooped or slumped-over posture, loss of balance, a slow shuffling walk, varying degrees of shaking or muscle tremors, reduced ability to show facial expressions, slow speech, and loss of fine motor skills (such as the ability to write).

The Effect of Drugs on Neurons and Synapses

Read page 383

A drug is a substance that changes the way the body functions. Most drugs, whether they are legal or illegal, affect the neural synapses by either enhancing or decreasing the action of a neurotransmitter. They do this by affecting the vesicles, the receptor proteins, the ion gates in the postsynaptic membrane or the re-absorption of neurotransmitters.

Try This

To investigate in greater detail how drugs affect neurons and synapses, go to the following web address:

http://learn.genetics.utah.edu/content/addiction/
Explore one of the following:

Drugs of Abuse - you will need to use Firefox and enable Flash for this one
Drug Use Changes the Brain Over Time
Drugs Affect Many Brain Pathways
How Drugs Can Kill and How to Stop Them
The Reward Pathway
Meth Mouse
How Pain Works
Opioids and Tolerance

Self-Check

Compare the effects of excitatory and inhibitory neurotransmitters on the postsynaptic membrane.
Explain the relationship between acetylcholine and cholinesterase.

Check your work.

Self-Check Answers

Both excitatory and inhibitory neurotransmitters affect the transmission of the nerve impulse across the synapse. (Similarity) Excitatory neurotransmitters generally make it easier to initiate depolarization in the dendrites of the postsynaptic neuron by opening sodium channels in the postsynaptic membrane and allowing sodium ions to diffuse into the postsynaptic neuron thus initiating the action potential in the second neuron. Inhibitory neurotransmitters make it more difficult to initiate depolarization in the postsynaptic neuron by either opening potassium channels and allowing more potassium ions to leak out of the neuron (hyperpolarizing it), or by opening chloride channels and allowing negative chloride ions to leak into the neuron, therefore making the neuron more negative than its normal resting potential of -60 or -70 mV.
After acetylcholine has opened sodium channels in the postsynaptic membrane and initiated depolarization in the postsynaptic neuron, it is decomposed by the enzyme cholinesterase, and the products are transported into the presynaptic neuron so that they can be re-synthesized into acetylcholine. If cholinesterase is unable to catalyze the decomposition of acetylcholine, the acetylcholine remains in the synaptic cleft and in the receptors and stimulates the postsynaptic neuron continuously, which may cause muscle fatigue.