The nervous system consists of the brain, spinal cord, and the nerves outside of the brain and spinal cord. Traditionally, the human nervous system is divided into two major components:
- The central nervous system: This consists of the brain and spinal cord.
The peripheral nervous system: This is the complete collection of nerves outside of the brain and spinal cord. The main function of this division of the nervous system is to connect the central nervous system to the rest of the body. The peripheral nervous system is typically divided up into two subdivisions:
- The somato-sensory nervous system: This division of the peripheral nervous system is generally considered to be under the voluntary control of the person and includes nerves that are attached to skeletal muscles.
- The autonomic nervous system: This is the part of the peripheral nervous system that is not generally considered to be under voluntary control. This system is further divided into two subdivisions: the sympathetic nervous system, which typically stimulates or speeds up functions in the body, and the parasympathetic nervous system, which typically slows down various functions.
The nerves that are part of the central nervous system are generally referred to as neurons, whereas outside the central nervous system, they are generally referred to simply as nerves. This system is a complex collection of nerves and other specialized cells that transmit signals throughout the body. One can think of it as the body’s system of electrical wiring, as neurons typically function through electrical and chemical activities.
Control of the nervous system is done mainly through the brain, and the spinal cord is used to transmit messages from the brain to different parts of the body and to receive information from different parts of the body. The nervous system has three general functions:
- Receive input. Collect input from the environment or the body (sensory information).
- Make sense of input. Interpret the sensory input (perception).
- Act. Respond in an appropriate manner to the sensory inputs.
Responses to sensory input can consist of numerous actions depending on the specific division of the nervous system that is being utilized. Abuse of numerous drugs can have devastating effects on the nervous system, particularly the central nervous system. Because the brain is the master control unit for the entire sensory system and the rest of the body, the vast amount of research that is performed on the effects of drugs and alcohol on the nervous system is targeted at the effects of these substances on the brain.
NeuroplasticityNeuroplasticity is simply a term that describes the remarkable ability of the brain to alter its structure and functioning in response to the experiences one has. Research has identified the plasticity of the brain, and there have been decades of research investigating how the brain changes in response to the experiences of the person. This feature of the brain has been a significant part of how researchers understand the functions of the brain and the development of human behavior.
Many of the alterations that occur in the brains of individuals who abuse drugs and alcohol are alterations that represent the plasticity of the brain. Other changes may be due to the toxic effects of the drugs themselves.
The notion that specific changes occur in everyone or that there is a “one-size-fits-all” rule, such that individuals who use any specific drug will develop specific issues is not well substantiated by the research. Other influences besides the use of drugs will come into play in everyone, and there will be significant variation in how each individual responds to the use of drugs or alcohol. These influences include genetics, past experience, the type of drug being used, the amount and frequency of use, and the effects of other experiences.Most any drug can alter the brain in some manner. Certain drugs have chemical properties and mechanisms of action that make them more likely to produce significant alterations in the brain and to produce significant neurological issues for the person over time. While the list of these drugs is quite extensive, the major drugs that can seriously affect brain functioning include:
- Stimulants: central nervous system stimulants, such as cocaine and methamphetamine
- Depressants: central nervous system depressant drugs, particularly opiate drugs, benzodiazepines, and alcohol
- Inhalants: substances like aerosol sprays, paint thinner, and paint
- Hallucinogens: drugs that primarily produce hallucinations
- Nicotine: tobacco products can significantly alter brain functioning.
- Cannabis: many variations of cannabis products
Other drugs that do not neatly fit into the above categories can also significantly alter the brain, such as ecstasy (MDMA). Very often, so called designer drugs (drugs that are manufactured in private laboratories) contain substances that can be potentially toxic to the brain. These substances are not manufactured under strict guidelines or supervision by a governing body that is interested in the safety of the individuals using the drug.
The Effects of Long-Term Substance Abuse on the Central Nervous System
Entire volumes of books have been devoted to the specific effects of different drugs on the brain. There are several general methods in which chronic abuse of most drugs affects the neurons in the brain. These include:
Alterations in the “reward pathways”: Although the term reward pathway is not fully accurate, it continues to be used to describe a specific neural pathway that is involved in the experience of reinforcement and the repetition of certain types of behaviors that generate positive results. The alterations that occur in the reward pathways of the brain are most often used as evidence that addiction is a brain disease, although this notion is contested by some researchers. When a person performs any type of reinforcing or pleasurable act, including using drugs of abuse, this particular area of the brain is activated.
Areas of the brain that are involved in reinforcement include the ventral tegmental area, the nucleus accumbens, and the prefrontal cortex. These structures are rich with neuronal receptors for the neurotransmitter dopamine. When a pleasurable or reinforcing behavior is performed, the neurotransmitter dopamine released in this area is believed to be responsible for the experience of pleasure or reinforcement. This pleasurable experience leads to a higher probability that the behavioral be repeated, strengthening the brain pathways that are stimulated by this particular experience.
Continued use of drugs and alcohol leads to structural changes in this area of the brain that make it more likely that the person will attempt to repeat this behavior (using the drug of choice). The structural changes that occur can be very resistant to change, although long-term abstinence can weaken these connections somewhat. Nonetheless, the very act of using drugs, feeling the euphoria associated with using drugs, and then continuing to use them to repeat that feeling result in changes in this area of the brain that many researchers believe drives addictive behaviors.
In addition, many researchers claim that because the structural alterations that occur in this area of the brain are long lasting, it makes addictive behaviors very hard to change.
Compensatory changes in brain pathways: When a neurotransmitter like dopamine is consistently released in significant amounts within the brain, the system makes alterations to account for these increased amounts. The opponent process theory applied to drug abuse indicates that when one set of neurotransmitters (e.g. dopamine) is continually released in large amounts, the system will automatically make adjustments to achieve a state of balance. Levels of neurotransmitters that have the opposite effect of dopamine will be increased.
Over time, structural changes can lead to significant problems with the person experiencing pleasurable feelings from activities that normally gave them pleasure, such as engaging in social actions with others or hobbies they used to like. Instead, they will seek out their drug of choice in order to experience pleasure. When the amount of the drug in the person’s system decreases and the levels of dopamine decline, the other neurotransmitters that have been increasing their efficiency will exert their effects, and individuals will experience negative feelings (believed to be the physical/neurological basis of the withdrawal syndrome).
Increased chance of brain disease or brain damage: The term encephalopathy is used to describe brain damage or disease of the brain that leads to changes in a person’s cognition, emotional state, or mental faculties. Encephalopathy can result from the direct effects of drugs on the brain; for example, methamphetamine can produce significant brain damage when used chronically. Encephalopathy can also occur due to damage to other organs in the body, such as the liver, heart, and kidneys, and the ramifications of other organ damage and its effect on the brain.
Typically, individuals who are diagnosed with any form of encephalopathy have some outward behavioral manifestations, such as significant problems with memory, significant emotional issues (e.g., depression), or some other cognitive or emotional issue. There are over 100 different types of encephalopathy, and many of these can occur as a result of chronic drug abuse. Some may be reversible, and others may be permanent.
The well-known condition Wernicke-Korsakoff encephalopathy includes mental confusion, dense amnesia, problems walking, and nystagmus. It is caused by a lack of vitamin B1. This form of encephalopathy is known to occur in chronic alcoholics who neglect their diet. If the symptoms of the disorder are recognized and treatment can be initiated early enough (vitamin supplementation), the symptoms may resolve. If not, they may be permanent.
Drugs like alcohol also can damage the cerebellum, a structure at the posterior portion of the brain that is heavily involved in movement and aspects of thinking. Individuals with chronic alcohol use disorders may demonstrate problems with movement as a result of cerebellar damage.
- Issues with the vascular system of the brain: Chronic abuse of many drugs can result in the development of cardiovascular issues. This can also extend to the vascular system of the brain. The vascular system is responsible for delivering blood containing oxygen and nutrients to the different structures in the brain. The use of numerous types of drugs can lead to blockages, hemorrhages, and hardening of the arteries that can result in a potential stroke or damage to certain areas of the brain due to decreased oxygen delivery or lack of oxygen altogether.
- Changes in the structure of neurons: Chronic use of drugs and alcohol can lead to changes in the structure of the nerves in the brain, the neurons, or other supporting structures such as glial cells. This change in the structure of the neurons can lead to difficulties with neurons communicating with one another, and it can result in numerous functional issues, including problems with emotional functioning (the development of depression, psychosis, etc.), impulse control, attention, and memory. It can be noted that chronic abuse of drugs and alcohol affects numerous neurons in the brain and does not affect single neurons or specific groups of neurons.
- Increased risk for infections, other diseases, or head trauma: It is well-known that the chronic use of drugs or alcohol leads to an increased risk to develop various infectious diseases or other diseases including cancer. Substance abusers are also at a higher risk to experience traumatic brain injuries, which can affect the structure and functioning of the brain permanently.
Although most drugs of abuse do affect the functioning of the availability of dopamine to some extent, different classes of drugs will affect different neurotransmitters, which will lead to structural changes in the pathways of the brain that primarily use those neurotransmitters (in addition to the reward pathway). For example, chronic use of some drugs may be more likely to produce problems with attention and concentration that can interfere with the memory process, whereas other drugs may not affect the attention and concentration of the person significantly, but may affect the memory process of encoding and retrieving material.
The ability of the brain to repair itself is limited, particularly in adults. Individuals who have long-term issues with substance abuse may experience some resolution to cognitive, emotional, or other problems that are believed to be due to brain damage, but some individuals may continue to experience issues with different areas of functioning. In most cases, the best way to address the neurological effects associated with a substance use disorder is to remain abstinent from drugs and alcohol, stay in recovery, and remain in recovery for years.