1 Introduction and Learning Objectives

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Learning Objectives.

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.



Labeled diagram of a neuron showing its major parts, including dendrites, cell body, axon, myelin sheath, action potential direction, and terminal buttons. Illustrates how neurons transmit signals to each other.
Figure 1. Diagram of a neuron, including dendrites, axon, myelin sheath, and terminal buttons. This image shows how electrical signals travel down the axon and how neurons connect via synapses. Source: OpenStax



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:

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.



Illustration of a synapse showing neurotransmitter release. Vesicles release neurotransmitters into the synaptic cleft where they bind to receptors on the postsynaptic neuron.
Figure 2. Visualization of a synapse. Neurotransmitters (yellow) are released from vesicles in the presynaptic neuron and cross the synaptic cleft to bind to receptors (blue) on the postsynaptic neuron. Source: Wikimedia Commons.



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.



Image of the Milky Way galaxy.
Figure 3. The Milky Way. Source: Wikimedia Commons.



🧠 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.

  1. 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.
  2. Astrocytes
    • Star-shaped cells that maintain the chemical environment for neuronal signaling.
    • Regulate neurotransmitter levels, ion balance (e.g., potassium), and provide metabolic support.
  3. Oligodendrocytes
    • Wrap axons in the CNS with myelin, a fatty sheath that speeds neural transmission.
    • Each oligodendrocyte can myelinate multiple axons.
  4. Ependymal Cells
    • Line the spinal cord and brain ventricles (the ependyma).
    • Help produce and circulate cerebrospinal fluid (CSF).


Diagram comparing different subtypes of glial cells.
Figure 4. Glial cell subtypes and function. Source: Lago-Baldaia I, Fernandes VM and Ackerman SD (2020) More Than Mortar: Glia as Architects of Nervous System Development and Disease. Front. Cell Dev. Biol. 8:611269. doi: 10.3389/fcell.2020.611269.



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.



Graph showing the phases of the action potential, including resting potential, depolarization, peak, repolarization, and hyperpolarization. Membrane potential is plotted over time.
Figure 5. Graph of an action potential. The curve represents the membrane potential over time, including depolarization (rising phase), repolarization (falling phase), and a brief period of hyperpolarization before returning to the resting state. Source: Wikimedia Commons.



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 facilitate communication between neurons. Over 50 endogenous substances in the brain may function as neurotransmitters. Some of the more important include the following:

Glutamate: The most abundant excitatory neurotransmitter; crucial for learning and memory. Disrupted by drugs like PCP and ketamine.

GABA (Gamma-aminobutyric acid): The main inhibitory neurotransmitter; plays a key role in reducing neuronal excitability. Enhanced by benzodiazepines (e.g., Xanax) and alcohol.

Serotonin: Regulates mood, sleep, and appetite. Low levels are linked to depression.

Dopamine: Involved in reward, motivation, and motor control. Implicated in schizophrenia and Parkinson’s disease.

Norepinephrine: Important for attention, arousal, and mood. Linked to ADHD.

🧪 Note: Many drugs of abuse (e.g., cocaine) boost dopamine, serotonin, and norepinephrine by blocking their reuptake.

Endorphins: Natural painkillers that bind to opioid receptors. Inhibit pain transmission and promote euphoria. Mimicked by drugs like morphine and fentanyl.

Acetylcholine (ACh): Involved in movement, attention, and memory. Decreased levels are seen in Alzheimer’s disease.

6 Divisions of the Nervous System



Detailed diagram of the human nervous system showing both the central nervous system (brain and spinal cord) and the peripheral nervous system (cranial nerves, spinal nerves, and their branches).
Figure 5. The nervous system is divided into two major parts: (a) the Central Nervous System and (b) the Peripheral Nervous System. Source: OpenStax.



The nervous system is generally divided into two major components:

  1. 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.

  2. 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.

  1. 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.

  2. 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.



Diagram comparing parasympathetic and sympathetic nervous system effects on different body functions, including pupil size, salivation, heart rate, digestion, bronchi, and bladder function.
Figure 3. Diagram of the autonomic nervous system. The parasympathetic nervous system supports ‘rest and digest’ functions, while the sympathetic nervous system prepares the body for ‘fight or flight’ responses. Source: OpenStax..



The autonomic system is further divided into:

  1. Sympathetic nervous system – the “fight or flight” system. It:
  1. Parasympathetic nervous system – the “rest and digest” system. It:

📈 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.

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fMRI Time Series Data.

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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Lecture summary.