How the Human Brain Works: Anatomy & Neuron Function

Dopamine and serotonin are two of the most clinically significant neurotransmitters. Dopamine functions can be remembered using DOPAMINE: Drive (motivation/reward), Psychosis (antipsychotics block dopamine), Parkinsonism (dopamine deficiency causes Parkinson's), Attention (stimulants improve focus), Motor (motor function control), Inhibition of prolactin, Narcotics (addiction), Extrapyramidal side effects. Dopamine is involved in the reward pathway from the ventral tegmental area to the nucleus accumbens. Serotonin functions follow HEAD, RED, FED: Head (depression, anxiety, migraines), Red (platelet function affecting bleeding), Fed (gut motility and nausea control, with 90% of body serotonin in GI tract). Serotonin syndrome (HEAD, RED, DEAD) has significant mortality and contributed to 80-hour work week regulations for interns.

Synaptic vesicles are membrane-bound structures in axon terminals that store neurotransmitters. When an action potential arrives, calcium influx causes synaptic vesicles to fuse with the presynaptic membrane and release their contents into the synaptic cleft. Common neurotransmitters include acetylcholine, dopamine, serotonin, and norepinephrine. Each neurotransmitter has specific effects on the postsynaptic cell. Synaptic transmission is terminated by: 1) Enzymatic degradation - enzymes break down neurotransmitters in the synaptic cleft; 2) Reuptake - presynaptic neurons reabsorb neurotransmitters; 3) Diffusion - neurotransmitters diffuse away from the synaptic cleft.

Synaptic transmission involves: (1) Action potential arrival at presynaptic terminal, (2) Calcium influx triggering vesicle fusion, (3) Neurotransmitter release into synaptic gap, (4) Binding to postsynaptic receptors, (5) Signal transmission. Major neurotransmitters include: serotonin (mood, sleep, appetite), glutamate (memory, learning), GABA (inhibition), dopamine (reward, movement), acetylcholine (muscle contraction, learning), and norepinephrine (arousal, attention).

When energy begins to flow in neurons, neurotransmitters are released to transmit signals between neurons. Neurotransmitters are chemical substances that allow energy to cross the gap (synapse) between neurons. Examples include adrenaline (associated with excitement and stress), acetylcholine, dopamine (associated with pleasure and learning), and serotonin. The specific neurotransmitter released determines the nature of the thought produced.

Synaptic transmission is the process by which neurons communicate with each other or with effector cells like muscles and glands through chemical messengers called neurotransmitters. The process involves an action potential reaching the axon terminal, triggering voltage-gated calcium channels to open and causing calcium influx, which then causes neurotransmitter-containing vesicles to fuse with the membrane and release neurotransmitters into the synaptic cleft. These neurotransmitters diffuse across the synaptic cleft and bind to specific receptors on the post-synaptic cell, opening chemically-gated ion channels that can either depolarize (excitatory) or hyperpolarize (inhibitory) the post-synaptic membrane. The effect of neurotransmitters depends on the type of receptor present on the post-synaptic cell, not the neurotransmitter itself. In the peripheral nervous system, acetylcholine serves as the primary neurotransmitter for somatic motor neurons and parasympathetic post-ganglionic neurons, while norepinephrine serves as the primary neurotransmitter for sympathetic post-ganglionic neurons. In the central nervous system, glutamate is the main excitatory neurotransmitter and GABA and glycine are the main inhibitory neurotransmitters. Fast synaptic transmission (direct receptor activation) is essential for rapid responses like sensory-motor coordination and communication, while slow synaptic transmission (indirect receptor activation through second messengers) is important for learning and memory formation.

Neuroplasticity refers to the brain's ability to modify neural connections. Long-term potentiation (LTP) strengthens synapses between frequently activated neurons, making future activation easier—this underlies memory formation. Long-term depression (LTD) weakens rarely used synapses, facilitating forgetting. These processes occur at the cellular level and represent the biological basis for learning and memory. The strength of a synapse correlates with how often it has been used, creating a molecular mechanism for experience-dependent brain changes.

Donald Hebb proposed in 1949 the principle that states: 'Neurons that fire together, wire together.' This means the connection between two neurons strengthens when both become active simultaneously. According to this rule, a synapse between two neurons fortifies when presynaptic and postsynaptic activity coincides—when the postsynaptic neuron is stimulated at the same time it emits action potentials. In 1973, Bliss and Lømo discovered that hippocampal neuron responses would increase after applying brief high-frequency electrical pulses. This lasting enhancement in synaptic strength became known as long-term potentiation (LTP). LTP exhibits specificity, associativity, and operativity, making it an ideal candidate for explaining the physiological changes underlying learning and memory.

Synaptic plasticity - the ability of synapses to strengthen or weaken with activity - is fundamental to learning and memory. Long-term potentiation (LTP) and long-term depression (LTD) are key cellular mechanisms. Hebbian plasticity states that synapses strengthen when presynaptic and postsynaptic neurons fire together.

Synaptic plasticity—the ability of synapse strength to change based on neural activity history—is the foundation of learning and memory. Short-term changes include facilitation, augmentation, post-tetanic potentiation (increased strength), and depression/habituation (decreased strength). Long-term changes include long-term potentiation (LTP) and long-term depression (LTD). LTP involves high-frequency stimulation causing glutamate release, NMDA receptor activation, calcium influx, calmodulin kinase activation, protein phosphorylation, and AMPA receptor upregulation. LTD involves climbing fiber input modifying mossy fiber input to Purkinje cells, smoothing movement coordination through reduced synaptic responses.

Long-Term Potentiation (LTP) is the strengthening of synapses through coactivation, representing a key mechanism of neural plasticity. LTP explains how the brain adapts to damage, recovers functions, and encodes learning and memory. Learning is defined as behavioral change from experience, while memory is the consolidation of that learning. Hebb's Law states that neurons that fire together wire together, providing the fundamental principle underlying synaptic strengthening.

Neurodegenerative diseases like Alzheimer's and Parkinson's involve progressive neuron loss. Alzheimer's features accumulation of amyloid and tau proteins causing neuron death and brain atrophy, visible as enlarged sulci and ventricles. Parkinson's affects dopaminergic neurons in the substantia nigra, reducing dopamine in the striatum. Both diseases show long asymptomatic periods before symptoms appear—Alzheimer's takes about 10 years, Parkinson's requires 60% neuron loss. Brain imaging (MRI, PET) reveals structural and functional changes underlying these conditions.

This section provides a detailed explanation of Parkinson's and Alzheimer's diseases. Parkinson's disease results from degeneration of dopamine-producing neurons in the substantia nigra and is the second most common neurodegenerative disorder after Alzheimer's. The pathophysiology involves accumulation of defective alpha-synuclein proteins inside neurons. The disease progresses slowly over 10-20 years, initially affecting motor function with tremors and rigidity, eventually leading to complete disability. Treatment with L-DOPA helps for the first 10-15 years but eventually requires higher doses with more side effects. Unlike Alzheimer's, Parkinson's primarily affects motor functions rather than higher cognitive functions. Risk factors include genetic predisposition, poor lifestyle factors, and repeated head trauma. The speaker notes that while we cannot yet cure these diseases, understanding risk factors can help in prevention and early intervention.

Neurodegenerative diseases like Alzheimer's and Parkinson's involve progressive neuron loss and brain atrophy, with Alzheimer's affecting 50 million people worldwide (2/3 of dementia cases) and Parkinson's over 10 million; while no cure exists, modifiable risk factors include hypertension, diabetes, obesity, and poor diet, while protective factors include physical exercise, Mediterranean diet, cognitive training, and social engagement, with current treatments being symptomatic rather than disease-modifying.

Neurodegenerative diseases like Parkinson's and Alzheimer's are increasingly affecting younger populations, not just the elderly. A documented case involved a 12-year-old girl diagnosed with Parkinson's. A definitive Parkinson's diagnosis is typically only established when approximately 80% of neurons have died. Early warning signs include loss of charm, restless leg syndrome, tremors, shuffling gait (indicating 60% neuronal damage), slowed speech, and difficulty swallowing. The gut microbiome plays a crucial role, with approximately 50% of dopamine produced in the gut and transported to the brain. Impaired detoxification leads to free radical accumulation, and heavy metals like mercury from vaccines can accumulate in the cerebral cortex.

Alzheimer's disease accounts for two-thirds of dementia cases, causing progressive memory loss starting from the hippocampus and spreading to the cortex. Parkinson's disease affects up to 1% of people over 60, involving dopamine neuron death in the substantia nigra. Both diseases share common pathological features of protein accumulation (amyloid-beta and tau in Alzheimer's, alpha-synuclein in Parkinson's) leading to neuronal death and brain atrophy. Alzheimer's presents with memory loss, cognitive decline, and personality changes, while Parkinson's manifests with tremor, rigidity, bradykinesia, and non-motor symptoms like loss of smell appearing years earlier.