PSY 100 General Psychology
Lecture Guide 2: Basic Neuroscience
Michael L. Thomas, PhD
1 Introduction and Learning Objectives
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2 The Neuron
A neuron is one of the body’s many types of cells (e.g., skin cells, blood cells, muscle cells). This particular cell type specializes in communication. Neurons transmit sensory information from the sensory organs to the brain, carry motor commands from the brain to the body’s muscles and glands, and—most relevant to this course—communicate with each other, forming the basis of thought.
Neurons are composed of structures including the cell body, dendrites, an axon, and terminal buttons.
The cell body (soma) is the metabolic center of the neuron (i.e., it produces energy). It is essential for life support and plays a role in transmitting information. Cell body structures include the following:
- Nucleus – contains the cell’s genetic material
- Endoplasmic Reticulum – synthesizes proteins
- Golgi Apparatus – packages proteins
- Mitochondria – site of respiration and energy production
- Neurofilaments – maintain structure and shape
- Microtubules – transport material within the cell
Dendrites are short, bushy branches that extend from the cell body. Their main function is to receive information from other neurons.
An axon passes information along the neuron from the cell body to the terminal buttons. Many axons are covered by a sheath of myelin, but the myelin does not uniformly enclose the axon. At regular intervals, unmyelinated sections called Nodes of Ranvier are present. This alternation between myelinated and unmyelinated segments is crucial for increasing conduction velocity.
🧠 Clinical insight: In Multiple Sclerosis (MS), the immune system mistakenly attacks the myelin sheath—the fatty insulation around axons—disrupting the smooth flow of electrical signals. Damage to myelin results in slowed or blocked signals, leading to symptoms such as muscle weakness, vision problems, and difficulty with coordination and balance.
Terminal buttons are where neurotransmitters are released to communicate with other neurons. Neurons are not physically connected to each other but communicate across synapses. Synapses are thin (20–40 nanometer [nm]) fluid-filled gaps surrounding neurons. Neurotransmitters are chemical messengers that enable communication between neurons by transmitting signals across synapses. The neurotransmitter molecules diffuse across the synapse and bind to receptor sites on the dendrites of the post-synaptic neuron.
There are about 86 billion neurons in the human brain. For comparison, the number of stars in the Milky Way is somewhere between 200 and 400 billion. How many synapses are needed to connect the brain’s 86 billion neurons? Estimates suggest that the adult human brain contains as many as 100 trillion synapses, enabling the vast network of connections that underlie thought, behavior, and perception.
🧠 Quick Note About Terminology: It’s easy to confuse the terms neuron and nerve. Remember this: a neuron is a single nerve cell, while a nerve is a bundle of axons. More broadly, axons in the peripheral nervous system (PNS) are generally called nerves, axons in the central nervous system (CNS) are generally called tracts, cell bodies in the PNS are generally called ganglia, and cell bodies in the CNS are generally called nuclei. We’ll define the concepts of PNS and CNS more clearly later in the lecture.
3 Glial Cells
Other types of cells in the nervous system include glial cells. Glial cells, also glia or neuroglia, are non-neuronal cells in the nervous system. There is approximately a 1:1 relationship of glia to neurons (higher ratios of 10-50:1 are an oft-cited myth). The term glia, from the Greek word meaning glue, reflects the nineteenth-century presumption that these cells held the nervous system together in some way. While glia do broadly support nervous tissue, they play more important roles in neuronal signaling, myelination, and clearing debris. There are four types of glia in the central nervous system.
- Microglia
- Small immune-like cells that act as scavengers.
- Remove debris and respond to injury or disease.
- Activated microglia are involved in neuroinflammation and can be measured using neuroimaging.
- Astrocytes
- Star-shaped cells that maintain the chemical environment for neuronal signaling.
- Regulate neurotransmitter levels, ion balance (e.g., potassium), and provide metabolic support.
- Oligodendrocytes
- Wrap axons in the CNS with myelin, a fatty sheath that speeds neural transmission.
- Each oligodendrocyte can myelinate multiple axons.
- Ependymal Cells
- Line the spinal cord and brain ventricles (the ependyma).
- Help produce and circulate cerebrospinal fluid (CSF).
Glial cells in the peripheral nervous system include satellite cells which are thought to play a similar role to astrocytes, and schwann cells which play a similar role to oligodendrocytes.
4 Neural Communication
Neurons communicate with one another using action potentials—brief electrical impulses that travel along the neuron’s axon. These impulses are generated by a series of chemical events, much like how a battery operates.
A neuron at rest maintains a resting potential of approximately −70 millivolts (mV). When it receives enough excitatory input from other neurons, and the internal voltage reaches a threshold of around −50 mV, an action potential is triggered. This is an all-or-none response, meaning it either occurs fully or not at all—unlike excitatory postsynaptic potentials (EPSPs) or inhibitory postsynaptic potentials (IPSPs), which are graded.
These electrical changes are made possible by ion channels embedded in the neuron’s semi-permeable membrane. These channels allow electrically charged particles (ions) to flow in and out, creating the voltage changes necessary for the action potential. Some receptors also act via signal proteins, initiating changes within the neuron’s internal chemistry.
Once the action potential reaches the axon terminal, the signal must jump the synaptic cleft. This is achieved through neurotransmitters stored in synaptic vesicles. When the action potential arrives at the terminal, these vesicles fuse with the presynaptic membrane and release their neurotransmitter contents into the cleft.
The neurotransmitters then diffuse across the symapes and bind in a lock-and-key fashion to specific receptors on the postsynaptic membrane of the next neuron. Depending on the type of neurotransmitter and receptor, this interaction generates either an EPSP (which increases the likelihood of firing) or an IPSP (which decreases it).
💊 Perspective: Since neurotransmitters are the chemical basis of communication in the brain, understanding and manipulating them has become a cornerstone of neurology and psychiatry—medical fields devoted to the treatment of neurological and mental disorders, respectively. While it might feel unsettling to think that our thoughts, emotions, and perceptions are fundamentally rooted in chemical and electrical activity, this view provides powerful insights into mental health, behavior, and brain function. From antidepressants to anesthesia, much of medicine is built on modulating neurotransmission.
🌐 Watch this! Follow this link to a Khan Academy video that provides a descirption of the neuron action potential.
5 Neurotransmitters
Neurotransmitters are chemical messengers that neurons use to communicate. More than 50 endogenous substances may function as neurotransmitters in the nervous system.
One important distinction is between excitatory and inhibitory signaling. Excitatory neurotransmitters generally increase the likelihood that a neuron will fire, whereas inhibitory neurotransmitters generally decrease it. A useful analogy is that excitation acts somewhat like a gas pedal, while inhibition acts like a brake.
Importantly, neurotransmitters do not usually have a single psychological function. Their effects depend on the receptors they bind to, where in the brain they are released, and which neural circuits are involved.
Glutamate: The primary excitatory neurotransmitter in the brain. It is especially important for learning, memory, and synaptic plasticity. Drugs such as PCP and ketamine alter glutamate signaling.
GABA (Gamma-aminobutyric acid): The primary inhibitory neurotransmitter in the brain. GABA helps regulate neuronal activity and prevents excessive excitation. Its effects are enhanced by drugs such as benzodiazepines (e.g., Xanax) and alcohol.
Serotonin: Involved in mood, sleep, appetite, pain, and many other functions. Although serotonin is often associated with depression, the idea that depression is simply caused by “low serotonin” is an oversimplification.
Dopamine: Involved in motivation, reinforcement learning, attention, movement, and reward-related behavior. Changes in dopamine systems are associated with conditions such as schizophrenia, Parkinson’s disease, and addiction.
Common misconception: Dopamine is sometimes called the brain’s “pleasure chemical,” but it may be more accurate to think of it as helping the brain learn what is worth pursuing and motivating behavior toward important outcomes.
Norepinephrine: Important for attention, alertness, arousal, and responses to stress. Norepinephrine systems are also targeted by some medications used to treat ADHD and depression.
Endorphins: Naturally occurring opioid-like chemicals that reduce pain and can contribute to feelings of well-being. Drugs such as morphine and fentanyl activate the same general receptor system.
Acetylcholine (ACh): Involved in muscle movement, attention, learning, and memory. The degeneration of acetylcholine-producing neurons is one feature of Alzheimer’s disease.
🧪 How drugs change neurotransmission: Psychoactive drugs can influence neurotransmitters in several ways. They may mimic a neurotransmitter, block its receptor, increase its release, prevent its reuptake, or prevent its breakdown. Cocaine, for example, blocks the reuptake of dopamine, norepinephrine, and serotonin.
🤔 Think about it: If you had to design the human brain from scratch, but had only a handful of neurotransmitters to work with, how would you design the system so that humans could still experience and produce such a wide variety of mental and behavioral processes? Hint: Think about how computers can generate extremely complex outputs from a small set of basic signals, such as binary code (
0and1). The brain is not literally binary, but it also creates complexity through patterns and combinations of relatively simple signals. Also, there are many more neurotransmitters and neuromodulators than the ones listed here. However, this relatively small group accounts for many of the major signaling systems commonly discussed in psychology and neuroscience.
6 Divisions of the Nervous System
The nervous system is generally divided into two major components:
Central Nervous System (CNS) – “Mission Control” 🎧. The CNS is composed of the brain and spinal cord. It is the core decision-making system of the body.
Peripheral Nervous System (PNS) – “Space Shuttle” 🚀. The PNS links the CNS to the body’s tissues and sensory organs. This includes the cranial nerves.
The PNS itself is broken down into two subsystems.
The somatic nervous system carries messages between the CNS and the sensory receptors, muscles, and surface of the body. Sensory (afferent) nerves transmit information from sensory organs to the CNS (e.g., temperature, pain, touch), while motor (efferent) nerves carry commands from the CNS to muscles, enabling both voluntary and reflexive movements.
The autonomic nervous system regulates internal organs and glands. It enables the CNS to monitor and control functions such as heart rate, respiration, digestion, and other involuntary processes.
The autonomic system is further divided into:
- Sympathetic nervous system – the “fight or flight” system. It:
- Accelerates heartbeat
- Slows digestion
- Raises blood sugar
- Dilates arteries
- Increases perspiration
These changes prepare the body to respond to threats or stress.
- Slows digestion
- Parasympathetic nervous system – the “rest and digest” system. It:
- Slows heartbeat
- Stimulates digestion
- Conserves energy
- Supports normal bodily maintenance
- Stimulates digestion
📈 Fun fact: Lie detector machines (polygraphs) measure physiological responses associated with activation of the sympathetic nervous system. They don’t detect lying directly, but instead record bodily reactions—such as increased heart rate or sweating—that may occur when a person feels discomfort or stress while being deceptive.
🌐 Explore this! Follow this link to a innerbody website that provides a 2D and 3D model of the nervous system. Spend some time clicking on the structures listed under the “Anatomy Explorer”. Read through the site’s descriptions to learn more about the brain and nervous system structures discussed in this lecture guide—some of which will be explored further in the next one.
7 Brain Imaging and Neural Recording Techniques
A number of different techniques are used to measure and quantify characteristics of the CNS. These include:
Electroencephalography (EEG): Measures electrical activity of the brain via electrodes placed on the scalp; useful for tracking rapid changes in brain activity over time. Also used to measure event-related potentials (ERPs) which are commonly studied in cognitive neuroscience.
Magnetoencephalography (MEG): Records magnetic fields produced by neuronal activity; provides high temporal resolution and better spatial localization than EEG.
Positron Emission Tomography (PET): Uses radioactive tracers to visualize metabolic processes in the brain, such as glucose or neurotransmitter activity.
Magnetic Resonance Imaging (MRI): Provides detailed structural images of the brain using magnetic fields; does not measure brain activity.
Functional Magnetic Resonance Imaging (fMRI): Measures changes in blood oxygenation (BOLD signal) to infer brain activity; offers high spatial resolution but relatively low temporal resolution.
Functional Near-Infrared Spectroscopy (fNIRS): Uses near-infrared light to detect blood oxygenation changes; a portable, noninvasive way to measure brain activity, especially in surface cortical areas. Offers high temporal relation but poorer spatial resolution relative to fMRI.
EEG and fMRI are two of the most commonly used tools in the fields of Cognitive Neuroscience and Neuropsychology. EEG is particularly effective at measuring changes in brain waves—that is, fluctuations in electrical activity generated by neurons—associated with different cognitive states. In contrast, fMRI is especially useful for identifying where in the brain activity increases or decreases, by tracking changes in blood flow that reflect underlying neural activity.
The video below shows brain activity recorded from a single individual undergoing fMRI while performing a cognitive task. In the video, you may be able to see how changes in brain activity result in very subtle shifts in blood oxygenation, as indicated by the color coding of brain voxels (the small 3D units used to measure activity) over time.
🎥 Watch this!
8 Neuromodulation and Brain Stimulation Techniques
Electroconvulsive Therapy (ECT): A psychiatric treatment in which controlled electric currents are passed through the brain to induce brief seizures; primarily used for severe depression.
Transcranial Magnetic Stimulation (TMS): Noninvasive technique that uses magnetic fields to stimulate neurons in targeted brain regions; used in research and clinical treatment of depression. Unlike ECT, TMS is not designed to cause seizures and typically produces only momentary or minor effects on brain function.
Transcranial Direct Current Stimulation (tDCS): Applies a low electrical current to the scalp to modulate cortical excitability; used experimentally to enhance cognitive function or treat psychiatric symptoms. tDCS also produces only momentary or minor effects on brain function.
🌐 Watch this! OPTIONAL Follow this link to a TED Talk by Raaj Chatterjee, who discusses how TMS is used to treat some mental health disorders.
9 Neuropsychological Testing and Case Studies
For centuries, physicians and researchers have been defining the functions of specific brain regions through careful observation and testing of individuals with naturally occurring brain injuries or disorders. This is sometimes referred to as the clinical-pathological approach.
One of the most famous case studies in psychology was conducted by Brenda Milner, a neuropsychologist who used careful cognitive assessments to investigate the profound memory impairments observed in a brain surgery patient nicknamed H.M.. H.M. underwent surgery to treat his intractable epilepsy, which was becoming increasingly debilitating. Unfortunately, in an effort to remove the epileptic focus (i.e., the source of the seizures), the surgeons removed too much brain tissue—particularly in the medial temporal lobe and hippocampus—leaving H.M. with a profound case of anterograde amnesia, or the inability to form new memories. From the day of his surgery to the end of his life, H.M. was unable to remember meeting new people, experiencing new events, or learning new skills. We’ll discuss more about this case later in the course when we cover memory.
Neuropsychological testing is both a research strategy and a clinical tool. Neuropsychological testing refers to administering a structured set of tasks designed to measure a person’s cognitive, motor, sensory, and emotional functioning. These tests are typically used to evaluate how different areas of the brain are functioning and can help identify patterns of strengths and weaknesses associated with brain injury, neurological illness, or developmental disorders.
10 Summary
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