Nervous System Disorders

Core Concepts

Neurodevelopmental conditions

Autism spectrum disorder (ASD) appears early in life, affecting social communication and behavior, often with restricted or repetitive interests. "Spectrum" is the key idea: traits and support needs differ enormously from person to person, and it likely reflects many genetic and environmental influences rather than a single cause. Attention-deficit/hyperactivity disorder (ADHD) involves persistent inattention, hyperactivity, and impulsivity that interfere with daily functioning — a difference in executive function networks. Neither is caused by "bad parenting" or vaccines — science repeatedly contradicts that claim.

Degenerative disorders: neurons die, function fades

Alzheimer's disease, the most common cause of dementia, progressively erodes memory and cognition. Its two hallmark findings are amyloid plaques (protein clumps between neurons) and neurofibrillary tangles (twisted tau protein inside neurons). These are descriptive findings, not settled causes, but they capture the theme: proteins misfold, neurons die, starting in the hippocampus and spreading.

Parkinson's disease is a movement disorder: tremor at rest, rigidity, slow movement (bradykinesia), and postural instability. Its best-known mechanism is the progressive loss of dopamine-producing neurons in the substantia nigra; dopamine normally helps the basal ganglia smooth movement, so its loss produces the symptoms. "Lose a neurotransmitter, lose a function."

Multiple sclerosis (MS) is an autoimmune disease attacking myelin in the CNS. Because myelin speeds conduction, demyelination slows or blocks signaling, and symptoms appear wherever lesions occur — blurred vision, numbness, weakness, or coordination problems. MS often follows a relapsing-remitting course as remyelination partially restores conduction.

Amyotrophic lateral sclerosis (ALS), or Lou Gehrig's disease, progressively destroys motor neurons in the cortex and cord. Muscles weaken and waste while sensation stays intact — the person can feel everything while losing the ability to move.

Stroke: the brain's plumbing fails

A stroke kills brain tissue through interrupted blood supply. Ischemic stroke (more common) is a blockage — clot or embolism — cutting off oxygen and glucose; hemorrhagic stroke is bleeding from a ruptured vessel, damaging tissue by oxygen loss and pressure. Because the brain stores almost no energy, neurons begin dying within minutes. Symptoms depend on the territory: motor tracts cross in the medulla, so a left-hemisphere stroke typically produces right-side weakness, and language-area damage produces speech problems. A transient ischemic attack (TIA) is a brief, reversible episode — a warning sign.

Seizures and epilepsy

A seizure is a burst of excessive, synchronized neuronal firing. Seizures have many causes (fever in young children, electrolyte disturbances, toxins, epilepsy). Epilepsy is recurrent, unprovoked seizures; not all seizures convulse — some are brief lapses of awareness. The biology connects to signaling: anything shifting the excitation–inhibition balance toward excitation (too much glutamate, too little GABA) lowers the threshold for runaway firing.

Psychiatric conditions as brain disorders

Schizophrenia involves psychosis: hallucinations, delusions, and disorganized thinking. The dopamine hypothesis — excess dopamine signaling in certain pathways drives psychotic symptoms — is a simplified model that has guided research for decades but is not the whole story. Depression involves persistent low mood and loss of interest; the monoamine hypothesis proposes that reduced serotonin, norepinephrine, and dopamine signaling contributes, which is why many treatments raise monoamine levels. Both hypotheses are actively debated simplifications — beware single-chemical explanations.

Infectious and traumatic disorders

Meningitis is inflammation of the meninges, usually from infection; bacterial meningitis is typically more severe than viral and can progress rapidly, with fever, headache, and stiff neck as classic signs. Spinal cord injury damages ascending and descending tracts, and the level predicts the outcome: cervical damage can affect both arms and legs (tetraplegia); lower damage affects the legs (paraplegia). Traumatic brain injury (TBI), including concussion, disrupts brain function through mechanical forces, from brief confusion to lasting impairment.

How It Works / Step-by-Step Process

Why demyelination slows a message (MS walkthrough):

1. A motor cortex neuron sends a command down a myelinated axon toward a muscle.
2. Normally the action potential jumps between nodes of Ranvier — saltatory conduction.
3. In MS, immune cells attack myelin segments, exposing the axon and disrupting the jumps.
4. The signal must creep through damaged stretches as slow continuous conduction — or stop entirely.
5. The muscle receives a weak, delayed, or absent command — weakness or numbness in whatever body part that pathway serves.
6. If remyelination partially occurs during recovery, conduction improves — why MS symptoms come and go.

Common Confusions