PSYC101: The Brain and Behavior - Dr. Taylor Bosch (Fall 2025)

The Nervous System: Characteristics

  • The nervous system is defined as the body's electrochemical communication circuitry.

  • It possesses several key characteristics:

    • Complexity: Involves intricate interconnections and processes.

    • Integration: Various parts work together seamlessly.

    • Adaptability, or Plasticity: The brain has a special capacity for change, allowing it to adapt to new experiences or injury.

    • Electrochemical Transmission: Information is transmitted through both electrical impulses and chemical signals.

  • Neuroscience is the scientific field dedicated to studying the nervous system.

Divisions of the Nervous System

  • The nervous system is broadly divided into two major parts:

    • Central Nervous System (CNS): Comprised of the brain and the spinal cord.

    • Peripheral Nervous System (PNS): A vast network of nerves that connects the brain and spinal cord to all other parts of the body, including organs, muscles, and glands.

Divisions of the Peripheral Nervous System

  • The Peripheral Nervous System (PNS) is further subdivided into two main divisions:

    • Somatic Nervous System:

      • Consists of nerves that transmit information from the skin and muscles to the Central Nervous System (CNS).

      • Also conveys information from the CNS back to the muscles, allowing for voluntary movement.

      • Afferent nerves (or Sensory nerves): These nerves carry information from sensory receptors (e.g., touch, pain, temperature) to the brain and spinal cord.

      • Efferent nerves (or Motor nerves): These nerves carry information out of the brain and spinal cord to other areas of the body, primarily muscles, to initiate action.

    • Autonomic Nervous System:

      • Responsible for taking messages to and from the body's internal organs.

      • Monitors and regulates various involuntary internal body processes, such as heartbeat, digestion, and respiration.

Divisions of the Autonomic Nervous System

  • The Autonomic Nervous System is also divided into two complementary parts that typically work in opposition to maintain homeostasis:

    • Sympathetic Nervous System:

      • Arouses the body to mobilize it for action, often referred to as the "fight or flight" response.

      • Involved in the experience of stress, preparing the body for perceived threats.

    • Parasympathetic Nervous System:

      • Calms the body down after a sympathetic response.

      • Conserves the body's energy, promoting rest and digestion.

  • Stress: Defined as the body's physiological and psychological response to stressors—circumstances and events perceived as threatening.

Functional Divisions of the Human Nervous System (Hierarchical Overview)

  • Nervous System

    • Central (Brain and Spinal Cord)

    • Peripheral

      • Autonomic (Controls self-regulated action of internal organs and glands)

        • Sympathetic (Arousing)

        • Parasympathetic (Calming)

      • Somatic (Controls skeletal muscles)

        • Sensory Input

        • Motor Output

Neurons and Glial Cells

  • Within each division of the nervous system, information is transmitted through the combined action of nerve cells, chemicals, and electrical impulses.

  • Neurons:

    • These are the specialized nerve cells that handle the information-processing function.

    • Considered the basic building block of the nervous system.

    • The human brain contains an astonishing number of neurons, approximately 86 billion.

  • Glial cells:

    • These cells provide essential support and nutritional benefits to neurons.

    • They perform various other crucial functions, such as myelination and waste removal.

Specialized Neuron Structure

  • Every neuron typically consists of three main parts:

    • Cell body (Soma):

      • The central part of the neuron that contains the nucleus.

      • The nucleus directs the manufacture of necessary substances for the neuron's function and maintenance.

    • Dendrites:

      • Treelike or branchlike fibers that project from the cell body.

      • Their primary role is to receive information from other neurons and orient these signals toward the cell body.

    • Axon:

      • A typically long, slender projection that carries information away from the cell body toward other cells.

    • Myelin sheath:

      • A layer of fat cells that encases and insulates most axons.

      • This insulation significantly increases the speed at which electrical impulses travel down the axon.

The Neural Impulse: Resting Potential

  • To transmit information, a neuron generates and sends brief electrical impulses through its axon to communicate with subsequent neurons.

  • The neuron creates these electrical signals by precisely moving positively and negatively charged ions back and forth across its outer membrane.

  • Resting potential: This refers to the stable, slightly negative electrical charge maintained by an inactive neuron, ready to fire but not currently transmitting an impulse.

The Neural Impulse: Action Potential

  • A neuron becomes activated when an incoming impulse from other neurons raises its internal voltage sufficiently.

    • During activation, positively charged sodium ions ( ext{Na}^+) rapidly flow into the neuron.

    • Following this influx, positively charged potassium ions ( ext{K}^+) move out, which helps return the neuron's charge to a negative state, repolarizing it.

  • Action potential: This is the brief wave of positive electrical charge that sweeps down the axon, representing the neuron's electrical signal.

  • All-or-nothing principle: Once the electrical impulse reaches a specific threshold level of intensity, it fires completely and moves down the axon without losing any of its intensity. It either fires at full strength or not at all; there are no partial action potentials.

Understanding the Neural Impulse with a Metaphor

  • Imagine the neuron's membrane has ion channels, like doors.

  • Outside Cell: High concentration of positive sodium ions ( ext{Na}^+).

  • Inside Cell: Relatively negative charge, with positive potassium ions ( ext{K}^+) and other negatively charged molecules.

  • When activated, specific ion channels open, allowing ext{Na}^+ to rush into the cell, making the inside temporarily positive.

  • Then, other channels open, letting ext{K}^+ flow out of the cell, restoring the negative charge inside.

  • This rapid change and flow of ions constitutes the electrical signal, or action potential, moving down the neuron.

Synapses and Neurotransmitters

  • Neurons communicate with each other not through direct contact, but through chemical signals carried across a tiny space between them.

  • Synapses: This is the specialized junction or gap where communication occurs. It is the point of contact between the axon terminal of the sending neuron and the dendrite or cell body of the receiving neuron.

  • Neurotransmitters: These are chemical messengers. They are stored in tiny sacs called synaptic vesicles within the neuron's terminal buttons. Their role is crucial in transmitting information across the synaptic gap from one neuron to another.

Neurochemical Messengers (Key Neurotransmitters)

  • Acetylcholine ( ext{ACh})

    • Function: Enables muscle action, plays a vital role in learning, and memory.

    • Examples of Malfunctions: With Alzheimer's disease, $S ext{ACh}$-producing neurons deteriorate significantly. Neurotoxins like Latrotoxin can result in spasticity, while Clostridium botulinum (Botox) can lead to flaccid paralysis by affecting $S ext{ACh}$ signaling.

  • Dopamine

    • Function: Influences movement, learning, attention, and emotion.

    • Examples of Malfunctions: An oversupply is linked to schizophrenia. An undersupply is associated with tremors and the loss of motor control seen in Parkinson's disease.

  • Serotonin

    • Function: Affects mood, hunger, sleep, and arousal.

    • Examples of Malfunctions: An undersupply is strongly linked to depression. Many antidepressant drugs work by raising serotonin levels in the brain.

  • Norepinephrine

    • Function: Helps control alertness and arousal.

    • Examples of Malfunctions: An undersupply can depress mood.

  • GABA (gamma-aminobutyric acid)

    • Function: A major inhibitory neurotransmitter, meaning it calms neural activity.

    • Examples of Malfunctions: An undersupply is linked to seizures, tremors, and insomnia.

  • Glutamate

    • Function: A major excitatory neurotransmitter, critically involved in memory formation and learning.

    • Examples of Malfunctions: An oversupply can overstimulate the brain, potentially producing migraines or seizures (which is why some people choose to avoid MSG, monosodium glutamate, in food).

Drugs and Neurotransmitters

  • Drugs can significantly interfere with the natural functions of neurotransmitters in the brain.

    • Agonist: A type of drug that mimics or increases a neurotransmitter's effects. It might bind to receptor sites and activate them, or prevent the reuptake of neurotransmitters, making more available in the synapse.

    • Antagonist: A type of drug that blocks a neurotransmitter's effects. It might occupy the receptor sites but not activate them, thereby preventing the natural neurotransmitter from binding and exerting its influence.

How Neurons Communicate (Summary of Synaptic Transmission)

  1. Action Potential Arrival: Electrical impulses (action potentials) travel down a neuron's axon until they reach a tiny junction known as a synapse.

  2. Neurotransmitter Release and Binding: When an action potential arrives at the axon terminal, it stimulates the release of neurotransmitter molecules into the synaptic gap. These molecules then cross the gap and bind to specific receptor sites on the receiving neuron. This binding allows electrically charged atoms to enter the receiving neuron, either exciting it to fire a new action potential or inhibiting it from doing so.

  3. Neurotransmitter Deactivation: Excess neurotransmitters remaining in the synaptic gap are either reabsorbed by the sending neuron (a process called reuptake), drift away from the synapse, or are broken down by enzymes present in the gap.

Brain Damage, Plasticity, and Repair

  • Plasticity: The brain possesses a remarkable special capacity for change, adapting its structure and function in response to experience, development, or injury.

  • Recovery from brain damage can depend on several crucial factors:

    • The age of the individual (younger brains often exhibit greater plasticity).

    • The extent or severity of the damage.

    • The presence and type of intervention or rehabilitation efforts.

  • Three primary ways that repair can take place in the brain include:

    • Collateral sprouting: Healthy axons sprout new branches to compensate for damaged ones.

    • Substitution of function: Another part of the brain takes over the functions of the damaged area.

    • Neurogenesis: The process by which new neurons are generated, primarily in specific brain regions.

Neural Networks

  • The brain operates through complex neural networks, where the activity of one neuron is intricately linked with that of many others.

  • These networks allow for the processing of inputs and generation of outputs through highly interconnected pathways.

How Researchers Study the Brain and Nervous System

  • Advancements in technology have provided numerous brain imaging techniques and methods to study neural activity:

    • X-Ray: Basic imaging that can show structural problems like fractures.

    • Computer Axial Tomography (CAT scan or CT scan): Uses X-rays from different angles to create cross-sectional images, detailed for bone and soft tissue structures.

    • Positron-Emission Tomography (PET scan): Measures metabolic activity by detecting radioactive tracers, showing which brain areas are active during tasks.

    • Magnetic Resonance Imaging (MRI): Uses magnetic fields and radio waves to generate detailed images of brain structures.

    • Functional Magnetic Resonance Imaging (fMRI): Detects changes in blood flow to particular brain areas, allowing researchers to observe brain activity in real-time during cognitive tasks.

    • Electroencephalograph (EEG): Records the brain's electrical activity through electrodes placed on the scalp, useful for studying sleep, seizures, and cognitive states.

    • Transcranial Magnetic Stimulation (TMS) and virtual lesions: A non-invasive technique that uses magnetic fields to stimulate or temporarily inhibit specific brain regions, creating temporary