Module 1 The Nervous System

The Effect of Drugs on Neurons and Synapses

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.

 

Read

To understand how drugs interrupt communication, read page 383 of your text.

 

Try This

Explore one of the following:

• Drugs of Abuse
• Physiology of a High
• Hardwiring an Addict
• Changes Last Long after Use
• How PET Scans can Measure Brain Activity
• Death by Overdose

Self-Check

Synaptic and neuromuscular transmitters are important and often challenging concepts to master. To ensure your understanding, you should do the following self check.

Use the following information to answer the next two questions.

 

Opiate Drugs

The human body naturally produces its own opiate-like substances and uses them as neurotransmitters. These substances include endorphins, enkephalins, and dynorphin, often collectively known as endogenous opioids. Endogenous opioids modulate our reactions to painful stimuli. They also regulate vital functions such as hunger and thirst and are involved in mood control, immune response and other processes.

 

The reason that opiates such as heroin and morphine affect animals so powerfully is that these exogenous substances bind to the same receptors as our endogenous opioids. There are three kinds of receptors widely distributed throughout the brain: mu, delta, and kappa receptors.

 

These receptors, through secondary messengers, influence the likelihood that ion channels will open, which in certain cases reduces the excitability of neurons.  This reduced excitability is likely the source of the euphoric effect of opiates, and appears to be mediated by the mu and delta receptors.

 

This euphoric effect also appears to involve another mechanism in which the GABA (gamma aminobutyric acid) inhibitory interneurons of the ventral tegmental area come into play. By attaching to their mu receptors, exogenous opioids reduce the amount of GABA released. Normally, GABA reduces the amount of dopamine released in the nucleus accumbens in the brain. By inhibiting the effects of this inhibitor, the opiates ultimately increase the amount of dopamine produced and the amount of pleasure experienced.

 

Source:  The Brain from Top to Bottom: http://www.thebrain.mcgill.ca/flash/i/i_03/i_03_m/i_03_m_par/i_03_m_par_heroine.html#drogues

 

1.  The two receptors thought to be involved in the euphoric effects of opiate drugs are

1. mu and delta
2. exogenous and endogenous
3. dopamine and GABA
4. endorphins and enkephalins

2.   Which of the following naturally occurring substances regulate hunger and thirst?

1. mu, delta, and kappa receptors
2. exogenous opioids
3. endogenous opioids
4. dopamine

Use the following information to answer the next two questions.

 

Avoiding a Collision

You are driving down a highway at night when a deer jumps in front of your vehicle. You slam on the brakes and avoid a collision.

3. Which row below best describes the initial reaction of your autonomic nervous system to this situation?
   
   

   Row	System Involved	Neurotransmitter	Response
   a.	Sympathetic Nervous System	Norepinephrine	Heart rate increases; pupils of eyes dilate
   b.	Parasympathetic Nervous System	Acetylcholine	Heart rate increases; pupils of eyes dilate
   c.	Sympathetic Nervous System	Acetylcholine	Heart rate increases; pupils of eyes constrict
   d.	Parasympathetic Nervous System	Norepinephrine	Heart rate decreases; pupils of eyes constrict

   
   
   
4. Which row below best describes the reaction of your autonomic nervous system several minutes after the incident?
   
   

   Row	System Involved	Neurotransmitter	Response
   a.	Sympathetic Nervous System	Norepinephrine	Heart rate increases; pupils of eyes dilate
   b.	Parasympathetic Nervous System	Norepinephrine	Heart rate increases; pupils of eyes dilate
   c.	Sympathetic Nervous System	Acetylcholine	Heart rate decreases; pupils of eyes constrict
   d.	Parasympathetic Nervous System	Acetylcholine	Heart rate decreases; pupils of eyes constrict

 

Use the following information to answer the next question.

5. Destruction of the synaptic vesicles of Neuron 1 will
   
   
   1. block the nerve impulse at W
   2. cause X to be constantly stimulated
   3. prevent depolarizations from occurring at Y
   4. result in the action of cholinesterase in Neuron 2

Use the following information to answer the next question.

6. If the structures labeled Q were absent, what effect on neural transmission would be expected?
   
   
   1. The axon would not release acetylcholine.
   2. The axon would not become depolarized.
   3. The speed of transmission would be reduced.
   4. No action potential would arrive to release neurotransmitter.

Use the following information to answer the next question.

 

The disease myasthenia gravis causes a person to experience muscular weakness because of the failure of neuromuscular junctions to transmit signals from nerve fibres to muscle fibres. The weakness is due to a reduced sensitivity to acetylcholine, which is necessary to stimulate the muscle fibre. People suffering from this disease are often treated with neostigmine, an anticholinesterase drug, which can result in some normal muscular activity within minutes.—Guyton and Hall, 1996

7. Neostigmine is effective in treating this disease because it
   
   
   1. binds with cholinesterase to form acetylcholine
   2. binds with cholinesterase to increase acetylcholine production
   3. reduces the amount of active cholinesterase, thereby increasing the amount of acetylcholine available to stimulate muscle contraction
   4. increases the amount of active cholinesterase, thereby increasing the amount of acetylcholine available to stimulate muscle contraction

Use the following information to answer the next question.

 

Observations About a Synapse and Synaptic Transmission

1. Only axon terminals release neurotransmitters.
2. A neurotransmitter diffuses from an axon terminal across the synapse to the dendrites or cell body.
3. Many transmissions across a synapse in a short time may cause fatigue of synaptic transmission.
4. Electron micrographs of a synapse show that there is no direct connection between the axon terminal of a presynaptic neuron and the dendrites or cell body of a postsynaptic neuron.

8. The assumption that axon terminals contain a limited amount of neurotransmitter could account for observation
   
   
   1. 1
   2. 2
   3. 3
   4. 4

Use the following information to answer the next question.

 

Alternative medicine, such as aromatherapy, is becoming increasingly popular in western society.  Aromatherapy uses natural oils and plant extracts. The scents of the oils and extracts are inhaled or the fragrant oils are massaged into the skin. Proponents of aromatherapy hypothesize that odours affect the brain and its release of neurochemicals. These neurochemicals may then relieve pain.

 

Hypothesized Steps in Aromatherapy Action

1. Olfactory neurons depolarize.
2. Olfactory receptors are stimulated.
3. Neurochemicals affect pain interpretation.
4. Neurochemicals are released from axon terminals.

Numerical Response

9. If it is assumed that the hypothesis is correct, the order in which the steps above would occur to result in pain relief in a person having just inhaled the scent from an aromatherapy oil or extract is ______, ______, ______, and ______.

10. While working in a lab, you accidentally mixed some unknown chemicals together and a vapour was produced which diffused throughout the lab.  Soon the lab animals in the room collapsed, unable to move. You made a quick examination of the animals and found their musculature to be very loose and relaxed. You concluded that the vapour became internalized in the animals by way of the lungs to the blood, which then distributed the vapour throughout the animals’ bodies. The symptoms shown by the lab animals may lead you to conclude that the vapour could have
    
    
    1. stimulated the action of the enzyme cholinesterase
    2. inhibited the production of acetylcholine by the axon endings of neurons
    3. inhibited the formation of synaptic transmission chemical produced by the dendrites
    4. stimulated the axon endings to secrete large amounts of acetylcholine