Nervous System, Neurons, and Neurotransmitters

Why Your Brain's Communication System Matters for the EPPP

You're sitting at your computer preparing to study, and within milliseconds, your nervous system has already coordinated thousands of signals: visual information about the screen, motor commands to position your hands, emotional responses to the material, and decisions about what to focus on next. Understanding how this message system operates isn't just basic knowledge for the EPPP. It's essential for understanding everything from psychopharmacology to the biological bases of mental disorders.

This material covers roughly 8-10% of the EPPP's content, appearing across multiple domains. More importantly, neurotransmitter function underlies nearly every question about medication effects, substance use disorders, and neurological conditions you'll encounter on the exam.

The Nervous System: Your Body's Information Network

The nervous system divides into two major branches that work together constantly. The central nervous system (CNS) includes your brain and spinal cord, the headquarters where processing and decision-making happen. The peripheral nervous system (PNS) connects everything else to headquarters, carrying messages back and forth between your CNS and the rest of your body.

Spinal cord injury facts for the exam:

• Damage at C1-C5 (neck) = paralysis in all four limbs (quadriplegia)
• Damage at C6-C7 = leg paralysis plus some arm weakness
• Damage at T1 or lower = leg paralysis only (paraplegia)
• Even with paralysis, reflexes often still work

The Peripheral Nervous System's Two Divisions

The PNS splits into two distinct systems with different jobs:

The Somatic Nervous System (SNS) handles voluntary actions you consciously control. {{M}}When you reach for your phone or type a text message{{/M}}, your somatic nervous system is transmitting sensory information (where is the phone?) to your CNS and then carrying motor commands back to your skeletal muscles (move your arm, flex your fingers). This system connects sensory receptors throughout your body to the CNS and connects the CNS to skeletal muscles.

The Autonomic Nervous System (ANS) manages the involuntary functions that keep you alive without conscious effort. Heart rate, digestion, breathing rate, and glandular activity. It communicates between smooth muscles, organs, and the CNS. While these activities are typically automatic, research shows some can be brought under voluntary control through biofeedback training, which has clinical applications for conditions like high blood pressure (hypertension) and anxiety disorders.

Sympathetic vs. Parasympathetic: The Gas and Brake Pedals

The autonomic nervous system contains two subsystems that generally produce opposite effects:

The sympathetic nervous system prepares your body for action (the famous "fight-or-flight" response. {{M}}Picture yourself about to give a major presentation at work{{/M}}. Your pupils dilate (better vision), heart rate increases (more blood flow), respiration quickens (more oxygen), you start sweating (cooling system activated), and digestion slows down (resources redirected to muscles). Sexual arousal also decreases during high sympathetic activation) your body recognizes this isn't the moment for reproduction.

The parasympathetic nervous system handles "rest and digest" activities. {{M}}After that stressful presentation ends{{/M}}, your parasympathetic system brings your body back to baseline: heart rate slows, breathing normalizes, digestion resumes, pupils return to normal size. This is the system active when you're relaxed and recovering.

Here's what trips up many EPPP students: these systems don't simply switch on and off. Both are active to varying degrees most of the time, and they cooperate for certain functions. The classic example is male sexual response. Parasympathetic activation is necessary for erection, while sympathetic activation triggers ejaculation. This cooperative interaction explains why anxiety (high sympathetic activation) can interfere with arousal but also why some antidepressants that affect both systems can cause sexual side effects.

Neurons: The Messengers That Make It All Work

Your nervous system contains two cell types: neurons (which communicate information) and glia (which support neurons by providing structure, insulation, and nutrients). For the EPPP, focus on neurons, particularly their structure and how they communicate.

Neuron Structure: The Basic Blueprint

Every neuron contains three essential components:

• Dendrites: Receive incoming signals from other neurons (the input receivers)
• Soma (cell body): Contains the nucleus and cellular machinery that keeps the neuron alive
• Axon: Transmits signals to other neurons (the output transmitter)

Some axons are wrapped in myelin, a fatty insulation produced by glial cells that speeds up signal transmission a lot. Multiple sclerosis involves myelin destruction, which explains why it causes progressive difficulties with movement, sensation, and coordination. Signals simply can't travel efficiently anymore.

How Information Travels Within a Neuron

Communication inside the nervous system involves two distinct processes. First, conduction within a single neuron (electrochemical). Second, transmission between neurons (chemical). Understanding both is crucial for EPPP questions about neural communication and drug effects.

Conduction within neurons starts when dendrites receive sufficient stimulation. In its resting state, a neuron is polarized. Negatively charged inside relative to outside, {{M}}like a battery waiting to discharge{{/M}}. When stimulation reaches the dendrites, channels in the cell membrane open, allowing positively charged sodium ions to rush inside. This makes the cell less negative (depolarization).

When this depolarization reaches a minimum threshold, complete depolarization occurs, triggering an action potential, an electrical impulse that travels down the axon. After firing, the neuron returns to its resting state and prepares to fire again.

Critical point for the exam: action potentials follow an all-or-none principle. They either occur or they don't, and when they occur, they're always the same intensity. {{M}}It's like a light switch, not a dimmer{{/M}}. There's no "weak" action potential or "strong" action potential. So how does your nervous system encode different stimulus intensities? Through two mechanisms: (1) the frequency of action potentials (rapid firing for intense stimuli), and (2) the number of neurons firing simultaneously.

How Information Travels Between Neurons

Synaptic transmission is how neurons talk to each other, and it's typically chemical (though some synapses are electrical). Here's the sequence EPPP questions often target:

1. An action potential reaches the axon terminal (the end of the axon)
2. This triggers the release of neurotransmitter molecules into the synaptic cleft, the tiny gap between the presynaptic neuron (the sender) and the postsynaptic neuron (the receiver)
3. Neurotransmitter molecules cross this gap and bind to receptors on the postsynaptic neuron
4. This binding has either an excitatory effect (making an action potential more likely) or an inhibitory effect (making an action potential less likely)
5. The neurotransmitter is then cleared from the synaptic cleft through reuptake (absorbed back into the presynaptic neuron) or enzymatic breakdown

This process is where most psychoactive drugs exert their effects. They either enhance or block specific steps in synaptic transmission.

Neuroplasticity: Your Brain's Ability to Adapt

Neuroplasticity refers to the brain's remarkable ability to change its shape and how it works throughout life in response to experience. This concept revolutionized neuroscience and has enormous implications for rehabilitation after brain injury, understanding developmental disorders, and explaining how therapy produces lasting changes.

Four types of neuroplasticity appear in the research literature:

The trade-off in homologous area adaptation matters clinically: when one hemisphere compensates for early damage to the other, it may sacrifice some of its original functions. This explains why early brain injuries sometimes produce unexpected cognitive profiles, the brain adapted successfully but at a cost.

Neurotransmitters: The Chemical Messengers

Neurotransmitters are chemical substances that carry signals between neurons. They're classified as conventional or unconventional based on how they're produced, stored, and released.

Conventional neurotransmitters are stored in small sacs called synaptic vesicles, released when action potentials arrive, and activate postsynaptic receptors. This category includes:

• Small-molecule neurotransmitters: Synthesized right at the axon terminal (dopamine, acetylcholine, glutamate, GABA, norepinephrine, serotonin)
• Neuropeptides: Larger molecules synthesized in the cell body and transported to terminals (endorphins, enkephalins, the brain's natural opioids that reduce pain and create euphoria)

Unconventional neurotransmitters aren't stored in vesicles but are made on demand. They can be released from various locations in the neuron and sometimes travel backward from postsynaptic to presynaptic neurons. The endocannabinoids belong to this category. They bind to the same receptors that respond to THC (the active component in marijuana). Anandamide, nicknamed "the bliss molecule," is a major endocannabinoid involved in pain, emotion, memory, appetite, and sleep. The endocannabinoid system helps keep your body balanced (maintain homeostasis) and plays important roles in immune function and the brain's reward system.

The Major Neurotransmitters You Must Know

Dopamine is both excitatory and inhibitory depending on which receptors it activates. It contributes to movement, personality, mood, sleep, motivation, and reward. For the EPPP, remember these clinical associations:

• Low dopamine in substantia nigra → Parkinson's disease (tremor, rigidity, movement difficulties)
• Low dopamine in prefrontal cortex → ADHD symptoms
• High dopamine in caudate nucleus → Tourette's disorder (tics and involuntary movements)
• Dopamine and addiction: Released in the mesolimbic pathway (reward circuit) during pleasurable activities; alcohol, psychostimulants, and opiates increase dopamine levels, creating their addictive effects

The dopamine hypothesis of schizophrenia has evolved. The original version proposed that schizophrenia results from excessive dopamine or overactive dopamine receptors. The revised version is more nuanced: positive symptoms (hallucinations, delusions) result from too much dopamine activity (hyperactivity) in subcortical regions (especially the striatum), while negative symptoms (flat affect, avolition) result from too little dopamine activity (hypoactivity) in cortical regions (especially the prefrontal cortex). This explains why typical antipsychotics, which block dopamine receptors throughout the brain, effectively reduce positive symptoms but often worsen negative symptoms.

Acetylcholine (ACh) is both excitatory and inhibitory and plays crucial roles in movement, arousal, attention, and memory. Clinical connections include:

• Movement: ACh causes muscle contraction at neuromuscular junctions. Myasthenia gravis destroys ACh receptors, causing progressive muscle weakness
• Memory: Low ACh levels in the entorhinal cortex and hippocampus are linked to early Alzheimer's disease memory loss (this is why cholinesterase inhibitors like donepezil are used to treat Alzheimer's. They prevent ACh breakdown)

ACh has two receptor types that function differently:

Glutamate is the primary excitatory neurotransmitter and contributes to movement, emotions, learning, and memory. Too much glutamate causes "excitotoxicity" (neurons become overexcited and die. This mechanism is implicated in stroke damage, seizures, and neurodegenerative diseases (Huntington's, Parkinson's, Alzheimer's). {{M}}Think of excitotoxicity like overloading an electrical circuit{{/M}}) excessive stimulation damages the system it's supposed to activate.

Norepinephrine is primarily excitatory but inhibits certain receptors. It's essential for the sympathetic fight-or-flight response and involved in attention, arousal, sleep, memory, and mood. The catecholamine hypothesis proposes that some forms of depression result from not enough norepinephrine (deficiency), while mania stems from norepinephrine excess. Many antidepressants (like SNRIs) work partly by increasing norepinephrine availability.

Serotonin (5-hydroxytryptamine or 5-HT) is primarily inhibitory but excites certain receptors. It affects arousal, sleep, sexual activity, mood, appetite, and pain. Low serotonin levels in specific brain regions are linked to:

• Depression and increased suicide risk
• Bulimia nervosa
• Obsessive-compulsive disorder
• Migraine headaches

Interestingly, anorexia nervosa shows a different pattern: higher-than-normal brain serotonin causes anxiety and obsessive thinking, and food restriction lowers serotonin levels, temporarily alleviating these uncomfortable symptoms. This creates a negative reinforcement cycle maintaining the disorder.

Higher-than-normal blood serotonin levels have been found in autism spectrum disorder and in individuals with chronic schizophrenia who also have enlarged ventricles or cerebral atrophy.

Gamma-Aminobutyric Acid (GABA) is the primary inhibitory neurotransmitter, involved in motor control, memory, mood, anxiety, arousal, and sleep. GABA essentially puts the brakes on neural activity:

• High GABA: Memory impairment, daytime drowsiness
• Low GABA: Anxiety, insomnia

Abnormal GABA levels appear in major depression, bipolar disorder, panic disorder, generalized anxiety disorder, PTSD, schizophrenia, and autism spectrum disorder. Benzodiazepines (like Xanax, Valium) work by enhancing GABA activity, which explains their calming, sedating effects.

How Drugs Affect Neurotransmitter Systems

Psychoactive drugs are classified by how they affect neurotransmitters. This classification system appears frequently on EPPP pharmacology questions:

Understanding these categories helps you predict drug effects and side effects. For instance, if you know that dopamine agonists can trigger psychotic symptoms (because they increase dopamine activity, similar to the hyperactivity seen in schizophrenia), you can remember why drugs like L-DOPA (used for Parkinson's) sometimes cause hallucinations as a side effect.

Common Misconceptions That Trip Up EPPP Students

Misconception 1: "The sympathetic and parasympathetic systems are opposites that turn on and off."

Reality: Both systems are active most of the time, working together cooperatively. They balance each other, and both contribute to many functions. The male sexual response example illustrates this perfectly. Both systems are necessary for the complete response.

Misconception 2: "Stronger stimulation creates stronger action potentials."

Reality: Action potentials are all-or-none. Stimulus intensity is encoded by firing frequency and the number of neurons firing, not by action potential intensity. This is fundamental to understanding neural coding.

Misconception 3: "Low serotonin always means depression."

Reality: The relationship between neurotransmitters and disorders is complex. Low serotonin is associated with depression, but anorexia nervosa involves high brain serotonin. No disorder has a single neurotransmitter cause. Multiple systems interact.

Misconception 4: "Neuroplasticity only happens in childhood."

Reality: Neuroplasticity occurs throughout life, though certain types are more pronounced during critical periods. Adult brains continue adapting through learning, practice, and recovery from injury.

Misconception 5: "All neurotransmitters work the same way."

Reality: Conventional and unconventional neurotransmitters have fundamentally different mechanisms. Endocannabinoids, for example, aren't stored in vesicles and can travel backward across synapses. Completely different from how dopamine or serotonin work.

Memory Tips for Exam Success

For the autonomic divisions, remember: Sympathetic = Stress response (fight-or-flight). Parasympathetic = Peaceful, Post-stress recovery.

Here are two mnemonics that make this even stickier:

• SYMPATHetic SYMPATHizes with your Stress by getting your body ready to fight or run away.
• PARAsympathetic helps you PARAchute down from your stress and relax.

Clinical tip: Biofeedback training works by helping people lower their sympathetic arousal.

For action potentials: "All or none, like a phone". It either rings or doesn't; there's no partial ring.

For neurotransmitter associations, create a table you review repeatedly:

For drug classifications: Draw the synapse and label where each drug type acts. Visual memory works better than verbal for many students. Direct drugs bind to receptors; indirect drugs affect availability. Agonists increase activity; antagonists decrease it.

For neuroplasticity types: Create a scenario for each type, {{M}}use mental images of specific patients recovering from specific injuries{{/M}}. The more concrete your mental examples, the better your recall.

Key Takeaways

• The nervous system divides into CNS (brain and spinal cord) and PNS (everything else). The PNS includes somatic (voluntary) and autonomic (involuntary) divisions.

• The sympathetic nervous system activates fight-or-flight responses; the parasympathetic system manages rest-and-digest functions. Both are active simultaneously most of the time.

• Neurons communicate through two processes: electrochemical conduction within neurons (action potentials) and chemical transmission between neurons (synaptic transmission).

• Action potentials follow an all-or-none principle. Stimulus intensity is encoded by firing frequency and number of neurons activated, not action potential strength.

• Neuroplasticity continues throughout life and includes homologous area adaptation, cross-modal reassignment, map expansion, and compensatory masquerade.

• Major neurotransmitters and their clinical associations: Dopamine (movement, reward, Parkinson's, schizophrenia), acetylcholine (memory, movement, Alzheimer's), glutamate (excitatory, excitotoxicity), norepinephrine (arousal, mood), serotonin (mood, appetite, anxiety), and GABA (inhibitory, anxiety, sleep).

• Drug classifications depend on their effects on neurotransmitters: agonists increase effects, antagonists block effects, with direct/indirect variants based on mechanism.

• The relationship between neurotransmitter levels and disorders is complex. Not every disorder involves simple "too much" or "too little" of one neurotransmitter.

Review this material with the understanding that neurotransmitter function underlies questions across multiple EPPP content areas: psychopharmacology, substance use disorders, neurocognitive disorders, and biological bases of behavior. When you encounter a medication question on the exam, immediately ask yourself: "Which neurotransmitter system does this affect, and how?"