💪Physiology of Motivated Behaviors Unit 2 – Neural Communication in the Brain

Key Concepts and Terminology

• Action potential: electrical signal that propagates along the axon of a neuron, allowing for communication between neurons
• Neurotransmitters: chemical messengers released by neurons that bind to receptors on target cells, influencing their activity (dopamine, serotonin, GABA)
• Synapse: junction between two neurons where neurotransmitters are released and received, facilitating communication
• Neuroplasticity: brain's ability to reorganize and form new neural connections in response to experiences, learning, and environmental stimuli
• Motivated behaviors: goal-directed actions driven by internal states, such as hunger, thirst, and sexual desire (feeding, mating, drug-seeking)
• Reward system: network of brain regions involved in processing rewarding stimuli and reinforcing behaviors (nucleus accumbens, ventral tegmental area)
• Homeostasis: maintenance of stable internal conditions in the body, regulated by various physiological processes and neural circuits (body temperature, fluid balance)
• Neuromodulators: substances that modify the activity of neurons and neural circuits, influencing behavior and cognitive processes (hormones, neuropeptides)

Neuron Structure and Function

• Soma (cell body): contains the nucleus and organelles necessary for protein synthesis and cellular metabolism
• Dendrites: branched extensions that receive signals from other neurons, allowing for integration of multiple inputs
• Axon: long, thin projection that carries electrical signals away from the soma to the axon terminals
  • Axon hillock: region at the base of the axon where action potentials are generated
  • Myelin sheath: insulating layer around the axon that increases the speed of signal transmission
• Axon terminals: specialized structures at the end of the axon that release neurotransmitters into the synapse
• Neurons can be classified based on their structure (unipolar, bipolar, multipolar) and function (sensory, motor, interneurons)
• Glia: non-neuronal cells that support and protect neurons, regulate synaptic transmission, and maintain homeostasis (astrocytes, oligodendrocytes, microglia)
• Neurons maintain a resting membrane potential of around -70 mV, which is essential for their excitability and signal transmission

Synaptic Transmission

• Electrical synapses: direct connections between neurons that allow for rapid and bidirectional signal transmission, commonly found in the brain and retina
• Chemical synapses: more common type of synapse where neurotransmitters are released from the presynaptic neuron and bind to receptors on the postsynaptic cell
• Synaptic vesicles: membrane-bound organelles that store and release neurotransmitters at the presynaptic terminal
• Synaptic cleft: narrow space between the pre- and postsynaptic neurons where neurotransmitters diffuse
• Neurotransmitter release is triggered by the arrival of an action potential at the axon terminal, causing an influx of calcium ions
• Neurotransmitters can have excitatory (increasing the likelihood of an action potential) or inhibitory (decreasing the likelihood) effects on the postsynaptic neuron
• Synaptic plasticity: changes in the strength of synaptic connections that underlie learning and memory (long-term potentiation, long-term depression)
• Neurotransmitter reuptake and degradation mechanisms ensure the termination of synaptic transmission and prevent excessive signaling

Neurotransmitters and Receptors

• Glutamate: primary excitatory neurotransmitter in the brain, involved in learning, memory, and synaptic plasticity
  • NMDA and AMPA receptors: ionotropic glutamate receptors that mediate fast excitatory transmission and synaptic plasticity
• GABA (gamma-aminobutyric acid): main inhibitory neurotransmitter in the brain, regulating neuronal excitability and synchronization
• Dopamine: neuromodulator involved in reward processing, motivation, and motor control, released by neurons in the midbrain (substantia nigra, ventral tegmental area)
• Serotonin: neuromodulator that regulates mood, sleep, appetite, and cognitive functions, released by neurons in the raphe nuclei
• Acetylcholine: neurotransmitter involved in attention, learning, and memory, as well as motor control in the peripheral nervous system
• Norepinephrine: neuromodulator released by neurons in the locus coeruleus, involved in arousal, attention, and stress response
• Neuropeptides: short chains of amino acids that act as neuromodulators, influencing various behaviors and physiological processes (oxytocin, vasopressin, endorphins)
• Receptors can be classified as ionotropic (ligand-gated ion channels) or metabotropic (G protein-coupled receptors), depending on their mechanism of action

Neural Circuits and Networks

• Neural circuits: interconnected groups of neurons that process specific types of information and generate specific outputs (sensory, motor, cognitive)
• Feedforward circuits: unidirectional flow of information from input to output neurons, allowing for rapid processing and response generation
• Feedback circuits: recurrent connections that allow the output of a circuit to influence its own input, enabling self-regulation and modulation
• Parallel processing: simultaneous processing of different aspects of information by separate neural circuits, allowing for efficient and complex computations
• Convergence: integration of multiple inputs onto a single neuron, allowing for the detection of complex patterns and features
• Divergence: distribution of a single neuron's output to multiple target neurons, enabling the broadcast of information to different brain regions
• Oscillations: rhythmic patterns of neural activity that facilitate communication and synchronization between brain regions (theta, alpha, beta, gamma)
• Neural networks can exhibit emergent properties, such as pattern recognition, decision-making, and adaptive behavior, that arise from the collective activity of many interconnected neurons

Brain Regions Involved in Motivated Behaviors

• Hypothalamus: central regulator of homeostatic processes, including hunger, thirst, sleep, and reproductive behaviors
  • Arcuate nucleus: contains neurons that sense metabolic signals and regulate feeding behavior (NPY, POMC neurons)
  • Suprachiasmatic nucleus: master circadian pacemaker that entrains physiological and behavioral rhythms to the light-dark cycle
• Nucleus accumbens: key component of the reward system, integrating motivational and emotional information to guide goal-directed behaviors
• Ventral tegmental area (VTA): midbrain region that contains dopaminergic neurons projecting to the nucleus accumbens and prefrontal cortex, involved in reward processing and motivation
• Amygdala: limbic structure that processes emotional information and mediates fear and anxiety responses, as well as reward-related behaviors
• Prefrontal cortex: higher-order brain region involved in decision-making, planning, and impulse control, regulating the expression of motivated behaviors
• Hippocampus: crucial for the formation and retrieval of declarative memories, which can influence the learning and expression of motivated behaviors
• Insula: cortical region that integrates interoceptive signals with emotional and cognitive information, contributing to the subjective experience of craving and desire
• Basal ganglia: group of subcortical nuclei involved in motor control, habit formation, and action selection, modulating the execution of motivated behaviors

Neuroplasticity and Learning

• Structural plasticity: changes in the physical structure of neurons and synapses, such as the growth of new dendritic spines or the formation of new synaptic connections
• Functional plasticity: changes in the strength and efficacy of synaptic transmission, mediated by mechanisms such as long-term potentiation (LTP) and long-term depression (LTD)
• Hebbian plasticity: "neurons that fire together, wire together" - simultaneous activation of pre- and postsynaptic neurons leads to the strengthening of their synaptic connection
• Spike-timing-dependent plasticity (STDP): form of Hebbian plasticity where the precise timing of pre- and postsynaptic activity determines the direction and magnitude of synaptic modification
• Neuromodulators, such as dopamine and norepinephrine, can modulate the induction and expression of synaptic plasticity, facilitating learning and memory formation
• Experience-dependent plasticity: changes in neural circuits and synaptic connections that occur in response to sensory, motor, and cognitive experiences (environmental enrichment, skill learning)
• Critical periods: developmental windows of heightened plasticity during which specific experiences can have profound and lasting effects on brain structure and function
• Adult neurogenesis: formation of new neurons in specific brain regions throughout life, such as the hippocampus and olfactory bulb, contributing to learning and adaptability

Research Methods and Techniques

• Electrophysiology: recording of electrical activity from individual neurons (patch-clamp) or populations of neurons (extracellular recording, EEG, MEG)
  • Single-unit recording: measuring the activity of individual neurons using microelectrodes, allowing for the study of neural coding and information processing
  • Local field potentials (LFPs): recording of the summed electrical activity of nearby neurons, reflecting the collective activity of neural populations
• Optogenetics: use of light-sensitive proteins (opsins) to control the activity of specific neuronal populations with high temporal and spatial precision
• Chemogenetics: use of engineered receptors (DREADDs) that can be activated by specific ligands, allowing for the selective modulation of neuronal activity
• Functional neuroimaging: non-invasive techniques that measure brain activity and connectivity, such as functional magnetic resonance imaging (fMRI) and positron emission tomography (PET)
• Tract tracing: use of anatomical tracers to map the connections between different brain regions, revealing the structure of neural circuits
• Behavioral assays: tests designed to measure specific aspects of motivated behaviors, such as food intake, drug self-administration, or social interaction
• Genetic manipulations: use of transgenic animals, viral vectors, or genome editing tools (CRISPR-Cas9) to study the role of specific genes and proteins in neural function and behavior
• Computational modeling: development of mathematical and computational models to simulate and predict the behavior of neurons, circuits, and networks, providing insights into the mechanisms underlying motivated behaviors