Neuroscience Lecture Notes: Neurons, Signals, and Brain Structure
Overview
The lecture covers how neurons communicate, the role of myelin, the all-or-none nature of action potentials, synaptic transmission, and the role of neurotransmitters in psychology. It also explains the nervous and endocrine systems, brain imaging technologies, major brain structures (lower and higher), and concepts of plasticity, lateralization, and split-brain studies. Real-world relevance includes mood disorders, treated conditions, and everyday examples like balance, memory, and motivation.
Neurons and Action Potentials
Neuron structure in simple terms: cell body, axon, dendrites (input/output wiring of the neuron).
Myelin sheath: insulation around the axon that protects the signal and speeds transmission; increases signal velocity (saltatory conduction). Without protection, signal integrity can diminish.
Electrical nature of neural input: neurons convey information via electrical impulses (neural input) that behave like waves, with changes in positive and negative charge along the axon.
Threshold and action potential
A neuron fires an action potential only when a stimulus reaches a threshold; below threshold, nothing happens.
Analogy used: a gun trigger must reach a threshold before a bullet fires; pulling the trigger faster does not speed up the bullet—it's an all-or-none event.
Once fired, the action potential propagates along the axon to the terminal end; speed is not increased by a stronger trigger—the amplitude is fixed.
Reaching the terminal end of the axon: the action potential arrives at the synapse, the gap between neurons, initiating chemical signaling.
Signaling speed: although described as a slow process, the transmission is actually very fast, comparable to racing cars in speed.
Stimulus frequency vs. intensity: a stronger stimulus can recruit more neurons to fire or fire more often, but it does not increase the strength or speed of an individual action potential.
Synapses and Neurotransmission
Synapse: the junction (synaptic gap) between sending and receiving neurons where transmission occurs.
Neurotransmitters: chemical messengers released from the presynaptic terminal into the synapse; about ~<50 identified so far, with around ~ ext{≈}100 types suspected overall.
Transmission across the synapse
When the action potential reaches the end of the sending neuron, neurotransmitters are released into the synaptic gap.
Neurotransmitters bind to receptors on the receiving neuron, triggering electrical changes that propagate an impulse if appropriate.
This transmission is electrochemical: mostly electrical in the axon, but chemical at the synapse.
Reuptake/recycling: after neurotransmitters stimulate the receiving neuron, they are taken back up (reuptake) by the sending neuron to be reused.
Major neurotransmitters and their roles
Serotonin: mood regulation, happiness; linked to depression when levels are low; antidepressants can raise serotonin levels.
Noradrenaline (norepinephrine): arousal and the fight/flight response; helps prepare for threats.
Substance P: carries pain signals (ouch).
Endorphins (endogenous morphines): natural painkillers.
Glutamine/Glutamate: involved in motor movement, learning, attention, memory, and emotions (glutamate is the principal excitatory neurotransmitter; transcript uses the term glutamine—note the context).
Dopamine: multiple roles including movement, reward, and mood regulation; too high can relate to hallucinations (psychosis, schizophrenia), too low can relate to Parkinson’s disease (motor impairment).
Other noted roles: norepinephrine in fight/flight; substance P in pain signaling; endorphins in pain relief.
Correlation vs. causation (serotonin example): low serotonin correlates with depression but does not prove causation; antidepressants raise serotonin and can improve mood, yet the exact causal pathways are complex.
Bottom line about neurotransmitters: they have specific psychological effects, but balance is crucial; disruption can lead to mood disorders and mental illness.
The Nervous System: Central and Peripheral
Central Nervous System (CNS): brain and spinal cord.
Peripheral Nervous System (PNS): connects the CNS to the rest of the body; subdivides into:
Somatic nervous system: controls voluntary movements (e.g., reaching for a water bottle).
Autonomic nervous system: regulates involuntary functions (e.g., heart rate, digestion).
Sympathetic division: activated by danger; enhances alertness and prepares for fight/flight (e.g., faster heartbeat, pupil dilation, digestion inhibited).
Parasympathetic division: activated when danger passes; rest-and-digest functions; counterbalances sympathetic activity.
Note: typically one division is active at a time; they are largely mutual opposites.
Neuron types
Sensory neurons: carry information from the environment to the brain.
Motor neurons: carry signals from the brain to muscles and glands to induce movement.
Interneurons: located in the brain and spinal cord; most common type, critical for processing information.
The Endocrine System and Hormones
Hormones vs. neurotransmitters: both chemical messengers, but they differ in speed and pathways.
Neurotransmitters: fast-acting chemical signals used within the nervous system (synaptic transmission).
Hormones: slow-acting chemicals circulated via the bloodstream to target organs.
Pituitary gland as the master gland; hypothalamus as the control center that regulates the pituitary and other endocrine functions.
Hormone example: hunger/appetite involves slow hormonal signaling; stomach signals take time to reach the brain, which can lead to continued eating even after fullness is reached.
Homeostasis: the hypothalamus helps regulate body temperature and fluid intake; normal body temperature around T_{normal} \,=\, 97.6^{\circ}\text{F} with small fluctuations.
The endocrine system as a satellite system under nervous system control; interlinked with hypothalamus and pituitary to regulate other glands.
Brain Imaging and Neuroscience Tools
MRI (magnetic resonance imaging): structural imaging to view brain anatomy.
fMRI (functional MRI): measures blood oxygenation level-dependent (BOLD) signals to infer which brain regions are active during tasks (e.g., visual perception in the occipital lobe).
PET (Positron Emission Tomography): measures metabolic processes (e.g., glucose uptake) to map brain activity, similar functional purposes as fMRI but via different mechanisms.
EEG (electroencephalography): records brain waves; useful in sleep research and other studies of brain activity.
These tools have advanced our understanding of which brain regions are involved in specific functions and how brain activity correlates with behavior and cognition.
The Brain: Lower and Higher Areas
Brain can be conceptually split into lower (older, more primitive) and higher (more sophisticated) structures.
Lower brain structures include:
Brainstem (pons and medulla): essential for basic survival functions (breathing, heart rate).
Reticular Formation (RF): regulates arousal and wakefulness; jet lag and alertness can be influenced by RF activity.
Thalamus: sensory relay station; routes sensory information from the senses (except smell) to the appropriate higher brain areas; also involved in sensory integration.
Cerebellum: coordinates voluntary movements and balance; highly trained in tasks requiring precision.
The limbic system (between lower and higher brain) includes:
Hypothalamus: controls the endocrine system via the pituitary; regulates temperature, hunger, thirst, and basic drives; involved in reward and emotion.
Hippocampus (named for its seahorse shape): processes conscious episodic memories and works with the amygdala to form emotionally charged memories; important for learning and memory.
Amygdala: processes emotions such as fear and aggression; connected to the hypothalamus to modulate emotional and physiological responses.
The hypothalamus and emotional/reward processing: experiments with rats show hypothalamus-driven reward can be highly reinforcing, illustrating the neural basis of motivation and reward.
The Cerebral Cortex: Higher-Order Processing
The cortex is the outer gray matter, highly wrinkled to maximize surface area for ~>2\0{,}000{,}000{,}000 neurons (often written as \approx 2.0\times 10^{10}) and about 3.0\times 10^{11} synapses.
Four lobes of the cortex and their basic functions:
Frontal lobe: high-level functions such as thinking, planning, judgment, speaking, and voluntary motor movements.
Parietal lobe: processing somatosensory information and spatial orientation (sensory cortex adjacent to motor cortex).
Temporal lobe: auditory processing and language-related functions; memory and emotional aspects also involve temporal areas.
Occipital lobe: visual perception and processing; contains the visual cortex.
Sensory and motor cortex layout: primary sensory and primary motor areas lie adjacent, with a body map showing that parts of the body are represented in proportion to the density of neurons dedicated to them (the homunculus). For example, the face and hands have large representations due to high sensory/motor density.
Left vs. right hemisphere asymmetry (lateralization)
Most functions are distributed across both hemispheres, but some functions are lateralized.
Language centers are strongly lateralized to the left hemisphere in most people:
Broca’s area (speech production) and Wernicke’s area (language comprehension) are primarily left-hemisphere regions.
Left-hemisphere dominance for many logical, analytic, and detail-oriented tasks; right-hemisphere dominance for intuition, emotions, and the broader “big picture” view.
Despite lateralization, both hemispheres coordinate via the corpus callosum, a large bundle of axons.
Split-Brain Studies and Lateralization
Corpus callosum connects the two hemispheres; in some epilepsy patients, it was severed to reduce seizures (split-brain patients).
Key findings from split-brain research:
With a disconnect, each hemisphere can process information independently; language centers are usually in the left hemisphere, so language output can be asymmetric.
Visual field experiments show contralateral processing: information from the left visual field goes to the right hemisphere and vice versa.
Left hemisphere tends to verbalize what is seen in the right visual field; the right hemisphere may recognize stimuli but cannot easily name them due to language localization.
When tasks are done with the left hand (controlled by the right hemisphere), participants may draw or respond in ways that reveal right-hemisphere capabilities not accessible to verbal description.
Practical demonstration: patients may name items shown to the right visual field (left hemisphere) but not items shown to the left visual field (right hemisphere); however, the left hand may draw or pick up the correct item when the right hemisphere saw it, illustrating dissociation between language and nonverbal recognition.
Concept of lateralization in daily life: left hemisphere tends toward logical thinking and language; right hemisphere toward emotion and global processing. Both are necessary for full function; neither is superior overall.
Plasticity, Neurogenesis, and Case Examples
Neuroplasticity: the brain’s ability to reorganize and recover functions after damage; younger brains show greater plasticity.
Neurogenesis: growth of new neurons, contributing to recovery by reassigning functions to new brain areas.
Example: a young girl underwent significant brain surgery to remove a large portion of her brain due to life-threatening seizures but recovered well, thanks to neurogenesis and plasticity.
Key Takeaways and Real-World Relevance
Mood and mental health hinge on a balance of neurotransmitters (e.g., serotonin, dopamine, norepinephrine). Alterations can correlate with disorders such as depression or schizophrenia, but causation is complex and not solely determined by one chemical.
The brain functions as an integrated network: electrical signaling within neurons, chemical signaling at synapses, hormonal signaling via the endocrine system, and extensive regional specialization across lobes and systems.
Understanding the difference between fast neurotransmitter signaling and slower hormonal signaling helps explain why some processes (like immediate mood changes) can be rapid whereas others (like hunger signaling) unfold more slowly.
Modern imaging and recording technologies (fMRI, PET, EEG) allow researchers to observe which brain regions are active during specific tasks, supporting theories about brain localization, lateralization, and functional connectivity.
Everyday applications include recognizing how stress responses (fight/flight) affect physiology, the importance of balance for homeostasis, and how brain plasticity underpins rehabilitation after brain injury.
Formulas and Quantitative References (from the lecture)
All-or-none principle of action potentials: an action potential fires fully or not at all; amplitude is constant once threshold is reached.
A simple representation: let $Vm$ be the membrane potential, and $V{th}$ the threshold. Then an action potential occurs if $Vm \ge V{th}$; otherwise, no action potential. Amplitude is approximately constant regardless of stimulus strength beyond threshold.
Neuron and synapse counts referenced:
Neurons in the human brain: \approx 2.0\times 10^{10} (
Synaptic connections: 3.0\times 10^{11})
Neurotransmitter types identified: <50; total suspected types: \approx 100
Brain temperature reference: T_{normal} \approx 97.6^{\circ}\text{F}
Connections to Foundational Principles
Structure-function relationships: anatomy (lobes, brain regions, pathways) underpins cognitive and behavioral functions (language, memory, emotion, motor control).
Systems integration: nervous and endocrine systems work together to regulate behavior, mood, homeostasis, and adaptation to stress.
Neuroplasticity as a cornerstone of learning and recovery: the brain can rewire itself and form new connections after injury or during development.
Ethico-social implications: understanding neurotransmitters and brain mechanisms informs treatment of mood disorders, addiction, and neurological diseases, while highlighting the limits of attributing complex behaviors to single chemicals or regions.
Quick Reference: Key Terms
Action potential, threshold, all-or-none
Synapse, neurotransmitters, reuptake
Serotonin, dopamine, norepinephrine, Substance P, endorphins, glutamate (glutamine in the lecture)
CNS, PNS, somatic, autonomic, sympathetic, parasympathetic
Hypothalamus, pituitary, endocrine system, homeostasis
Thalamus, cerebellum, brainstem (pons, medulla), reticular formation
Limbic system (hippocampus, amygdala)
Cerebral cortex, frontal/parietal/temporal/occipital lobes
Left vs right hemisphere, language centers (Broca’s, Wernicke’s), lateralization
Corpus callosum, split-brain, neuroplasticity, neurogenesis
Imaging: MRI, fMRI, PET, EEG
Jet lag and arousal regulation by reticular formation
If you’d like, I can condense these notes into a shorter study sheet or tailor a version focused on a specific topic (e.g., neurotransmitters only, or brain imaging techniques).