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There are two kinds of cells in the nervous system: glial cells and neurons. Glial cells, which make up the support structure of the nervous system, perform four functions:
- Provide structural support to the neurons
- Insulate neurons
- Nourish neurons
- Remove waste products
The other cells, neurons, act as the communicators of the nervous system. Neurons receive information, integrate it, and pass it along. They communicate with one another, with cells in the sensory organs, and with muscles and glands.
Each neuron has the same structure:
Each neuron has a soma, or cell body, which is the central area of the neuron. It contains the nucleus and other structures common to all cells in the body, such as mitochondria.
The highly branched fibers that reach out from the neuron are called dendritic trees. Each branch is called a dendrite. Dendrites receive information from other neurons or from sense organs.
The single long fiber that extends from the neuron is called an axon. Axons send information to other neurons, to muscle cells, or to gland cells. What we call nerves are bundles of axons coming from many neurons.
Some of these axons have a coating called the myelin sheath. Glial cells produce myelin, which is a fatty substance that protects the nerves. When an axon has a myelin sheath, nerve impulses travel faster down the axon. Nerve transmission can be impaired when myelin sheaths disintegrate.
At the end of each axon lie bumps called terminal buttons. Terminal buttons release neurotransmitters, which are chemicals that can cross over to neighboring neurons and activate them. The junction between an axon of one neuron and the cell body or dendrite of a neighboring neuron is called a synapse.
The Reflex Arc
In the spinal cord, the reflex arc is a neural pathway that enables an automatic, rapid response to stimuli by coordinating neurons within the central and peripheral nervous systems. This pathway involves three types of neurons: sensory neurons, which detect a stimulus and send signals to the spinal cord; interneurons, which process the information within the spinal cord; and motor neurons, which transmit the response signal from the spinal cord to the muscles, resulting in a swift, involuntary action such as pulling one’s hand away from a hot surface. The reflex arc exemplifies how these neurons work together seamlessly to protect the body from harm without the delay of routing signals through the brain.
Communication Between Neurons
Neurons communicate by sending electrical and chemical signals to each other or other cells, a process referred to as neural transmission. This process enables functions like movement, sensation, and cognition. For communication to occur effectively, neurons follow specific principles, such as the all-or-none law, and experience various phases, such as depolarization, the refractory period, and reuptake of neurotransmitters. When neural transmission is disrupted, neurological disorders can arise.
The All-or-None Law
Neural impulses conform to the all-or-none law, which means that a neuron either fires and generates an action potential, or it doesn’t. Neural impulses are always the same strength—weak stimuli don’t produce weak impulses. If stimulation reaches a certain threshold, or minimum level, the neuron fires and sends an impulse. If stimulation doesn’t reach that threshold, the neuron simply doesn’t fire. Stronger stimuli do not send stronger impulses, but they do send impulses at a faster rate.
The Resting Potential
Nerves are specially built to transmit electrochemical signals. Fluids exist both inside and outside neurons. These fluids contain positively and negatively charged atoms and molecules called ions. Positively charged sodium and potassium ions and negatively charged chloride ions constantly cross into and out of neurons, across cell membranes. An inactive neuron is in the resting state. In the resting state, the inside of a neuron has a slightly higher concentration of negatively charged ions than the outside does. This situation creates a slight negative charge inside the neuron, which acts as a store of potential energy called the resting potential. The resting potential of a neuron is about -70 millivolts.
The Action Potential
When something stimulates a neuron, sodium ion channels in the cell membrane open up. This allows positively charged sodium ions to flow into the neuron, changing the internal charge of the cell and making the inside more positive compared to the outside. This process is known as depolarization. For a limited time after neuron stimulation, there are more positively charged ions inside than in the resting state. Once the charge difference reaches a critical level, it creates an action potential, an electrical impulse that travels along a neuron’s axon and allows for the transmission of signals between neurons or from neurons to muscles and glands. After this, the sodium channels close, and the neuron begins to rest. During the period when the channels remain closed, the neuron can’t send impulses.
Refractory Period and Resting Potential
After an action potential occurs, the neuron enters a brief refractory period (which lasts about 1-2 milliseconds). The refractory period has two phases. During the absolute refractory period, which occurs immediately after an action potential, the neuron cannot fire another action potential, no matter how strong the stimulus. This is because the sodium channels are inactive. Following the absolute refractory period comes a relative refractory period, during which the neuron can fire another action potential, but only if the stimulus is stronger than usual. This phase occurs as the neuron is still returning to its resting potential. The neuron works to restore its resting potential by pumping positively charged ions out of the cell, reestablishing the slightly-negative charge inside the neuron. Once the resting potential is restored, the neuron is ready to fire again when a new stimulus reaches the required threshold.
Reuptake and Synaptic Transmission
When the action potential reaches the end of the neuron, the signal is transmitted across a small gap between two cells called the synaptic cleft. The signal-sending cell is called the presynaptic neuron, and the signal-receiving cell is called the postsynaptic neuron.
Neurotransmitters are the chemicals that allow neurons to communicate with each other. These chemicals are kept in synaptic vesicles, which are small sacs inside the terminal buttons. When an action potential reaches the terminal buttons, which are at the ends of axons, neurotransmitter-filled synaptic vesicles fuse with the presynaptic cell membrane. As a result, neurotransmitter molecules pour into the synaptic cleft. When they reach the postsynaptic cell, neurotransmitter molecules attach to matching receptor sites. Neurotransmitters work in much the same way as keys. They attach only to specific receptors, just as certain keys fit only certain locks.
When a neurotransmitter molecule links up with a receptor molecule, there’s a voltage change, called a postsynaptic potential (PSP), at the receptor site. Receptor sites on the postsynaptic cell can be excitatory or inhibitory:
Excitatory: The binding of a neurotransmitter to an excitatory receptor site results in a positive change in voltage, called an excitatory postsynaptic potential or excitatory PSP. This increases the chances that an action potential will be generated in the postsynaptic cell.
Inhibitory: Conversely, the binding of a neurotransmitter to an inhibitory receptor site results in an inhibitory PSP, or a negative change in voltage. In this case, it’s less likely that an action potential will be generated in the postsynaptic cell.
Unlike an action potential, a PSP doesn’t conform to the all-or-none law. At any one time, a single neuron can receive a huge number of excitatory PSPs and inhibitory PSPs because its dendrites are influenced by axons from many other neurons. Whether or not an action potential is generated in the neuron depends on the balance of excitation and inhibition. If, on balance, the voltage changes enough to reach the threshold level, the neuron will fire.
Neurotransmitter effects at a synapse do not last long. Neurotransmitter molecules soon detach from receptors and are usually returned to the presynaptic cell for reuse in a process called reuptake.
Disorders Related to Neural Transmission
Disruption of neural transmission can lead to neurological disorders. In multiple sclerosis (MS), for example, the protective myelin sheath surrounding neurons's axons becomes damaged, slowing or blocking the transmission of neural impulses. This causes symptoms such as muscle weakness, coordination problems, and vision loss. Another disorder, myasthenia gravis, is an autoimmune condition that disrupts the communication between nerves and muscles. Antibodies attack receptors at the neuromuscular junction, preventing motor neurons from triggering muscle contractions, which leads to muscle weakness and fatigue.
Neurotransmitters
So far, researchers have discovered more than 60 different neurotransmitters, and new ones are still being identified. The nervous system communicates accurately because there are so many neurotransmitters and because neurotransmitters work only at matching receptor sites. Different neurotransmitters do different things.
|
Neurotransmitter |
Major Functions |
Excess is associated with: |
Deficiency is associated with: |
|
Acetylcholine (the first neurotransmitter discovered) |
muscle movement, attention, arousal, memory, emotion |
|
Alzheimer’s disease |
|
Dopamine |
voluntary movement, learning, memory, emotion |
Schizophrenia, poor impulse control |
Parkinsonism, lack of motivation |
|
Serotonin |
sleep, wakefulness, appetite, mood, aggression, impulsivity, sensory perception, temperature regulation, pain suppression |
|
Depression, anxiety, sleep problems, digestive problems |
|
Endorphins |
pain relief, pleasure |
|
Fibromyalgia, self-harm, sleep problems |
|
Norepinephrine |
learning, memory, dreaming, awakening, emotion, stress-related increase in heart rate, stress-related slowing of digestive processes |
|
Depression, ADHD, memory problems, sleeping problems |
|
Gamma-aminobutyric acid (GABA) |
main inhibitory neurotransmitter in the brain |
Anxiety |
Schizophrenia, Autism Spectrum Disorder, Depression, Epilepsy |
|
Glutamate |
main excitatory neurotransmitter in the brain and most abundant neurotransmitter in the brain, plays key roles in cognitive functions like thinking, learning, and memory |
Multiple sclerosis, Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, Fibromyalgia |
Learning and memory issues, low concentration, mental exhaustion, low energy |
|
Substance p |
Involved in pain signaling |
Inflammatory bowel disease, major depression, fibromyalgia |
|
The Endocrine System
In addition to neurotransmitters, hormones also play a significant role in regulating behavior and mental processes, though they operate through the endocrine system rather than the nervous system. The endocrine system is a network of glands that produce and release hormones into the bloodstream, regulating various bodily functions such as growth, metabolism, and mood. Hormones are chemical messengers produced by various glands, tissues, and organs in the body, which travel through the bloodstream to regulate physiological processes, behavior, and emotions. Hormones act more slowly than neurotransmitters, but their effects tend to be longer-lasting. The following describes various hormones produced by the endocrine system:
Adrenaline is produced by the adrenal glands. It prepares the body for the “flight-or-fight” response by increasing heart rate, blood flow, and energy availability during stressful or emergency situations.
Leptin is produced by adipose tissue (fat cells). It regulates hunger, energy balance, and body weight by signaling the brain to reduce appetite and increase energy expenditure when fat stores are sufficient.
Ghrelin is produced primarily by the stomach. It stimulates appetite by signaling the brain to increase hunger, particularly before meals.
Melatonin is produced by the pineal gland in the brain. It helps regulate sleep-wake cycles by signaling the body when it is time to sleep, particularly in response to darkness.
Oxytocin is produced by the hypothalamus and released by the posterior pituitary gland. It plays a key role in promoting social bonding, trust, and emotional connections and in facilitating childbirth and lactation.
Psychoactive Drugs and Neurotransmitter Function
Psychoactive drugs, as opposed to medicinal drugs, have psychological effects, meaning that they change sensory experience, perception, mood, thinking, and behavior. Psychoactive drugs are sometimes called recreational drugs, though some have legitimate medical uses. These drugs interact with the brain’s neural transmission by modifying how neurotransmitters are released, received, or recycled. Different types of psychoactive drugs can either enhance or inhibit neural communication, impacting processes like mood regulation, attention, and motor control. Depending on their effects on neurotransmitters, psychoactive drugs can function as agonists, antagonists, or reuptake inhibitors.
Agonists are substances (psychoactive drugs or chemicals) that enhance neural firing by mimicking or increasing the activity of neurotransmitters. These substances bind to receptor sites on neurons, imitating the action of natural neurotransmitters and stimulating the same effects. For example, opioids like morphine act as agonists by binding to endorphin receptors, increasing feelings of pleasure, and reducing pain. Agonists effectively amplify the regular activity of neurotransmitters, leading to heightened neural communication and intensified physiological or psychological responses.
Nicotine is an acetylcholine agonist, which means that it mimics acetylcholine closely enough to compete for acetylcholine receptors. When both nicotine and acetylcholine attach to a receptor site, the nerve fibers become highly stimulated, producing a feeling of alertness and elation.
Antagonists are chemicals that block the action of a particular neurotransmitter. They bind to receptors but can’t produce postsynaptic potentials. Because they occupy the receptor site, they prevent neurotransmitters from acting. An example of this is naloxone, which acts as an opioid antagonist by blocking the receptors that opioids would normally bind to, reversing the effects of an opioid overdose. Antagonists reduce or halt neural activity, which can lead to a decrease in functions such as pain perception, alertness, or muscle movement, depending on the system they affect. In some cases, antagonists are used to treat conditions like schizophrenia or anxiety by blocking excessive neurotransmitter activity. However, they can also cause side effects related to the suppression of normal neural functions.
Some psychoactive drugs function as reuptake inhibitors, meaning they block the reabsorption of neurotransmitters back into the presynaptic neuron after they have been released into the synapse. Normally, after neurotransmitters have transmitted their signal to the postsynaptic neuron, they are reabsorbed by the presynaptic neuron for reuse or breakdown. Reuptake inhibitors disrupt this process, allowing neurotransmitters to remain active in the synaptic gap for a longer time, which enhances their effects on the postsynaptic neuron. This mechanism is commonly utilized by selective serotonin reuptake inhibitors (SSRIs), which are used to treat depression and anxiety by increasing serotonin levels in the brain and improving mood and emotional stability. Similarly, norepinephrine-dopamine reuptake inhibitors (NDRIs) affect dopamine and norepinephrine levels, boosting energy and concentration. By blocking reuptake, these drugs intensify and prolong the action of neurotransmitters, but they can also lead to side effects such as agitation, insomnia, or digestive disturbances.
Physiological and Psychological Effects of Psychoactive Drugs
Psychoactive drugs can have significant psychological and physiological effects on the body and brain. Depending on the type of drug, they can either stimulate or depress neural activity, alter perceptions, or relieve pain. These effects vary based on the type of drug and its interaction with neurotransmitters.
Stimulants, such as caffeine and cocaine, typically increase neural activity by enhancing the release of excitatory neurotransmitters like dopamine and norepinephrine. This results in heightened alertness, increased energy, and improved focus. However, overuse of stimulants can lead to anxiety, irritability, and dependence.
Depressants, such as alcohol, reduce neural activity by enhancing the action of inhibitory neurotransmitters like GABA. This results in relaxation, reduced anxiety, and sedation. While depressants can help calm the nervous system, overconsumption can impair cognitive and motor functions, leading to dangerous consequences such as alcohol poisoning or addiction.
Hallucinogens alter the brain’s neurotransmitter systems, particularly those involving serotonin and glutamate, to distort perception, mood, and thought processes. These drugs affect sensory pathways, causing users to experience hallucinations—vivid images, sounds, or sensations that aren’t real. Hallucinogens, such as LSD, psilocybin (found in certain mushrooms), and marijuana, primarily interact with serotonin receptors, especially in regions of the brain that regulate mood, perception, and cognition. By overstimulating these receptors, hallucinogens can cause altered states of consciousness, changes in the perception of time and space, and heightened emotional responses. In some cases, they may also affect glutamate, which plays a role in learning and memory. Although hallucinogens do not typically lead to addiction, they can have unpredictable psychological effects, including anxiety, paranoia, or flashbacks, even after the drug has left the system.
Opioids, such as heroin and morphine, act as powerful pain relievers by mimicking the body’s natural endorphins and binding them to opioid receptors in the brain. These drugs are highly effective at reducing pain but carry a high risk of addiction and dependence. Opioid addiction can lead to severe physical and psychological withdrawal symptoms, including intense cravings, nausea, and anxiety.
Tolerance and Addiction
Prolonged use of psychoactive drugs can lead to tolerance, a lowered responsiveness to the drug’s effects, requiring higher doses to achieve the same results. This can result in addiction, a condition in which individuals become physically and/or psychologically dependent on the drug. Addiction often leads to withdrawal symptoms—ranging from headaches and irritability to severe physical distress or even death—if the drug is no longer consumed, making it difficult to quit without assistance.
