DP

Comprehensive Nervous System Study Notes

The Nervous System

  • The nervous system controls everything you do. It consists of three main parts: the brain, the spinal cord, and nerves. It helps you move, think, and feel, and even regulates automatic processes like digestion. It is the body’s command center.

Divisions of the Nervous System

  • Central nervous system (CNS): brain and spinal cord; the CNS is the most complex organ system.

  • Peripheral nervous system (PNS): network of nerves that branches out from the spinal cord; relays information between brain/spinal cord and organs/limbs.

  • The PNS has two main parts:

    • Somatic nervous system: guides voluntary movements.
    • Autonomic nervous system: regulates involuntary activities (without thinking).

  • There are two main groups of nerves:

    • Cranial nerves (12 pairs) originate in the brain and extend through the face, head, and neck; can be sensory, motor, or both.
    • Spinal nerves (31 pairs) branch from the spinal cord; can provide sensory, motor, or both; involved in reflexes.

  • Nerves throughout the body carry electrical signals that enable sensation, movement, digestion, heart rate, and more.

Nerves and Neurons

  • Nerves are made up of:

    • Axons: cord-like fiber bundles inside a nerve.
    • Dendrites: branches that receive electrical impulses.
    • Endoneurium, Perineurium, Epineurium: connective tissue coverings at different levels.

  • In the CNS, oligodendrocytes insulate axons; in the PNS, Schwann cells insulate axons.

  • Neuron types:

    • Sensory neurons: carry signals to the brain.
    • Associative (internuncial) neurons: connect sensory to motor.
    • Motor neurons: carry signals to muscles or glands.

  • Synapse: the space between the dendrites/axon terminals where electrical signals jump via neurotransmitters.

  • Axon terminals release neurotransmitters stored in synaptic vesicles to transmit signals to the next cell.

Neurons and Synapses (Basic Function)

  • The basic functioning unit of the nervous system is the neuron.
  • Neurons communicate via synapses, where neurotransmitters cross the synaptic gap to the next cell.

Neurotransmitters

  • Neurotransmitters are chemical messengers that relay signals from one neuron to the next target cell (neuron, muscle, or gland).

  • They are stored in synaptic vesicles within the axon terminal.

  • Neurotransmitters can have three types of effects:

    • Excitatory: promote firing of the next neuron (e.g., glutamate, epinephrine, norepinephrine).
    • Inhibitory: prevent the next neuron from firing (e.g., GABA, glycine, serotonin).
    • Modulatory: influence effects of other messengers and affect many neurons at once.

  • Major examples and roles:

    • Serotonin: inhibitory; regulates mood, sleep, sexuality, anxiety, appetite, pain; imbalance linked with SAD, anxiety, depression, fibromyalgia; treated with SSRIs and SNRIs.
    • Histamine: regulates wakefulness, feeding, motivation; involved in asthma, bronchospasm, mucosal edema, MS.
    • Dopamine: reward system; affects focus, memory, sleep, mood, motivation; dysfunction linked to Parkinson’s, schizophrenia, bipolar disorder, ADHD; drugs (cocaine, meth, etc.) act on dopamine system.
    • Epinephrine (adrenaline) & Norepinephrine (noradrenaline): fight-or-flight response; increase heart rate, blood pressure, respiration, blood glucose; influence attention and focus; meds may modulate these for ADHD/depression; epinephrine used in anaphylaxis and cardiac arrest.
    • Endorphins: natural pain relief; contribute to “feel good” feelings; low levels may relate to fibromyalgia/headaches.

  • Catecholamines: a group of hormones released by the adrenal glands in response to stress, including epinephrine, norepinephrine, and dopamine; remaining hormones are eliminated via urine.

  • Acetylcholine (ACh): excitatory neurotransmitter in CNS and PNS; regulates heart rate, blood pressure, gut motility; important for muscle contractions, memory, motivation, sexual desire, sleep, and learning; imbalance linked with Alzheimer’s, seizures, and muscle spasms.

Glial Cells

  • Glial cells provide physical and chemical support to neurons and maintain their environment; they support neurons like a secretary/janitorial staff.
  • CNS: 5 types
    • Astrocytes
    • Oligodendrocytes
    • Microglia
    • Ependymal
    • Radial glia
  • PNS: 2 types
    • Schwann cells
    • Satellite cells

Peripheral Nervous System Details

  • PNS includes 12 cranial nerves and 31 spinal nerves.
  • Some cranial nerves carry only sensory fibers (e.g., optic nerve); others are mixed (sensory and motor).
  • Spinal nerves are mixed (sensory and motor) and control reflexes (e.g., pulling hand away from a hot stove).
  • Myelin sheath:
    • Protective outer covering surrounding axons in nerves.
    • CNS: formed by oligodendrocytes.
    • PNS: formed by Schwann cells.

Central Nervous System (CNS) Basics

  • Meninges: protective coverings of brain and spinal cord.
    • Dura mater: outer layer closest to skull.
    • Arachnoid: middle layer with web-like appearance.
    • Subarachnoid space: between arachnoid and pia mater.
    • Pia mater: inner layer closest to brain/spinal cord.
  • Leptomeninges: composed of arachnoid + pia mater.
  • Cerebrospinal fluid (CSF): lubricates, nourishes, and protects CNS; carries waste; produced/stored in ventricles.

Ventricles and CSF

  • Ventricles: four cavities in the brain that produce and store CSF; maintains CNS.
  • CSF production rate: 450\,\text{mL/day} (approx. 15 oz/day).

Parts of the Brain

  • Cerebrum:
    • Largest, most complex brain region; two hemispheres (right/left) with four lobes; responsible for processing sensory information, thoughts, and actions; major part of day-to-day functioning and identity.
  • Thalamus:
    • Located above the brainstem in the middle; body’s information relay station; all senses except smell pass through the thalamus before reaching the cerebral cortex; also involved in sleep, wakefulness, consciousness, learning, and memory.
  • Hypothalamus:
    • Deep in the brain; maintains homeostasis by influencing the autonomic nervous system and managing hormones; the main link between CNS and endocrine system; responds to neural and hormonal signals to maintain internal balance.
  • Cerebellum:
    • Coordinates and regulates a wide range of brain and body functions; contains more than half of the brain’s neurons despite its small size; located at the back of the head; name means “little brain.”
  • Brain Stem:
    • Connects cerebral hemispheres to the spinal cord; three parts:
    • Midbrain: regulates eye movements.
    • Pons: coordinates facial movements and hearing/balance.
    • Medulla Oblongata: regulates breathing, heart rate, blood pressure, and swallowing.
    • Contains the reticular activating system, which controls sleep-wake cycles.

Spinal Cord and Vertebral Column

  • Spinal cord: long, tube-like bundle of tissue; transmits signals between brain and body; damage can affect movement/function.
  • Vertebral column (spine) protects the spinal cord; composed of vertebrae stacked from the pelvis to the skull.
  • Intervertebral disks: cushion discs between vertebrae with tough exterior and gel-like interior; absorb shock.

Autonomic Nervous System (ANS)

  • Autonomic nerve fibers regulate involuntary internal functions (respiration, heart rate, digestion, etc.).
  • Divisions within ANS:
    • Sympathetic nervous system: activates body processes for stress or danger; fight-or-flight response.
    • Parasympathetic nervous system: promotes rest-and-digest processes.
    • Enteric nervous system: manages digestion.

Nerve Fibers and Adrenergic Nerves

  • Sensory nerve fibers carry information from outside to the brain.
  • Somatic nerve fibers carry sensory or motor information (including senses and muscles).
  • Adrenergic fibers are neurons that use adrenaline (epinephrine), noradrenaline (norepinephrine), or dopamine as neurotransmitters.
  • These neurotransmitters are released at the synapse.

Cerebrospinal Fluid (CSF) – Revisited

  • CSF acts as a watery cushion/shock absorber for CNS.
  • It transports nutrients (proteins and carbohydrates) to the brain and spinal cord.
  • Formed continuously within the brain ventricles at about 450\,\text{mL/day}.

Diagnostic Tests (Overview)

  • Arteriography (cerebral angiography): catheter into an artery up to the carotid; dye injected to detect aneurysms, hemorrhage, arteriosclerosis, or stroke signs.
  • Glasgow Coma Scale (GCS): evaluates level of consciousness via eye movements, verbal response, and motor response.
  • CT (CAT) scan: series of X-rays creating a 3D view to identify tumors, bleeding, clots, edema.
  • EEG: measures brain electrical activity; detects epilepsy, tumors, stroke, head injury, infections; can document sleep disorders.
  • EMG: electrical stimulation via needles into muscle to assess peripheral muscle activity; diagnoses diabetic neuropathy, carpal tunnel.
  • Genetic testing/counseling: assesses known genes for neurological disorders; can detect conditions like spina bifida in utero.
  • Lumbar puncture: spinal needle removes CSF to diagnose bleeding/infection and measure CSF pressure.
  • MRI: powerful magnets produce brain/spine images; no radiation; good for multi-angle imaging; contraindicated with ferrous implants or pacemakers.
  • Myelography: spinal needle injects contrast around spinal cord/nerves; real-time X-ray (fluoroscopy) to show compression/irregularities.
  • PET scan: injects radioactive glucose/hormones; images show where material goes, aiding epilepsy or Alzheimer’s diagnosis.
  • Ultrasound/neurosonography: analyzes brain blood flow; helps diagnose stroke, tumors, hydrocephalus, vascular problems; can detect inflammatory processes causing pain.
  • X-rays: used for chest/skull; can reveal fractures or bone structures; assess pressure in the skull.

Conditions of the Head & Meninges (Terminology & Basics)

  • Terms:
    • Cele-: tumor/rupture/herniation
    • Mening-: meninges
    • Hydro-: fluid/water
  • Key structures: skull, dura mater, arachnoid, pia mater, subarachnoid space (contains CSF), spinal meninges, central canal.

Common Neurological Conditions and Diseases

  • Alzheimer’s Disease: neurodegenerative with amyloid plaques; usually begins after age 60; no cure; symptoms include mood changes, disorientation, memory loss, misplacing items, balance problems, personality changes; treated with symptom-relief meds.
  • Amyotrophic Lateral Sclerosis (ALS): also called Lou Gehrig’s disease; neuromuscular disorder causing muscle weakness; progression to difficulty talking, swallowing, moving, and breathing; average life expectancy 2–5 years post-diagnosis.
  • Bell’s Palsy: temporary unilateral facial paralysis due to inflammation/swelling of the seventh cranial nerve; sometimes idiopathic.
  • Cerebral Palsy (CP): developmental brain disorder affecting muscle tone, posture, movement; caused by early brain injury; signs appear in early childhood; forms include:
    • Spastic
    • Dyskinetic
    • Ataxic
    • Mixed
  • Dementia: broad term for decline in mental function; affects thinking, memory, reasoning, personality, mood, behavior; not a single disease; about half of people aged 85+ may have dementia; medications may slow decline.
  • Encephalitis: brain inflammation; caused by viruses, insect-borne infections, or autoimmune reactions; can be life-threatening; treatment often requires hospitalization and intensive care.
    • Types: Viral encephalitis, Arbovirus encephalitis (spread by insects), Bacterial/Fungal encephalitis (less common).
  • Epilepsy: brain disorder with seizures due to abnormal electrical activity; not curable, but manageable with treatment; up to ~70% control seizures with medications.
    • Focused (focal) seizures: may or may not involve loss of consciousness; generalized seizures involve loss of consciousness.
    • Seizure types: Grand Mal (Tonic-Clonic), Petit Mal (absence), Febrile seizures in children.
  • Essential Tremor: most common movement disorder; involuntary shaking that worsens with movement; common after age 40; often distinguished from Parkinson’s.
  • Guillain–Barré Syndrome (GBS): autoimmune; immune system attacks peripheral nerves; causes numbness, tingling, weakness; can progress to paralysis; most people recover with treatment.
  • Headaches: very common; various types:
    • Tension headaches
    • Migraines
    • Cluster headaches
    • Sinus headaches
    • Hormone headaches (estrogen-related during menstrual cycle, pregnancy, menopause)
  • Meningitis: inflammation of meninges; can be bacterial, viral, fungal, parasitic, etc.; bacterial meningitis has high mortality if untreated; viral meningitis usually milder.
  • Multiple Sclerosis (MS): autoimmune disease of CNS; demyelination of nerves; causes muscle weakness, vision changes, numbness, memory issues; no cure; treatment to manage symptoms and slow progression.
  • Neuralgia: nerve pain; shooting, stabbing, burning; can be a symptom of an underlying condition. Trigeminal neuralgia: episodes of severe facial pain.
  • Paralysis: loss of voluntary movement due to disrupted nerve signals; causes include stroke, spinal cord injury, MS; Bell’s palsy is temporary facial paralysis.
    • Paras or partial vs complete paralysis; site of injury defines types: Flaccid (muscles limp) vs Spastic (stiff, with spasms).
  • Forms of Paralysis (site-based):
    • Monoplegia: one limb
    • Hemiplegia: one side of the body
    • Diplegia: same area on both sides (e.g., both arms or both legs)
    • Paraplegia: legs (waist down)
    • Quadriplegia: all four limbs
  • Parkinson’s Disease: age-related neurodegenerative condition targeting basal ganglia; decreased dopamine leads to slowed movements and tremors; treatment aims to manage symptoms; disease involves a shift in brain chemistry and dopamine deficiency.
  • Reye’s Syndrome: rare, causes brain swelling and liver dysfunction after a viral illness (e.g., flu, chickenpox); linked to aspirin use in children; cause unknown.
  • Sciatica: nerve pain from injury/irritation of sciatic nerve; may involve back/buttock pain radiating down leg; sciatic nerve is the longest/biggest nerve; two sciatic nerves (one on each side).
  • Spina Bifida & Myelomeningocele:
    • Spina bifida: neural tube defect; incomplete development of the spine during the first month of pregnancy; often apparent at birth; can occur anywhere along the backbone.
    • Myelomeningocele: type of spina bifida with a fluid-filled sac protruding from the baby’s back containing spinal cord, nerves, and CSF; occurs in early pregnancy.
  • Tourette Syndrome: neurological disorder with involuntary motor and vocal tics; often begins in childhood; may improve with age; genetics likely play a role.
  • Transient Ischemic Attacks (TIA): temporary stroke-like episode with brief loss of blood flow to part of brain; symptoms similar to stroke but resolve within 24 hours (often minutes); medical emergency.
  • Age-Related Body Characteristics:
    • Brain and spinal cord lose nerve cells and weight; slower nerve signaling.
    • Changes to body senses; diminished reflexes and sensation.
    • Waste products accumulate in brain tissue forming plaques/clusters.
    • Memory and thinking slow; higher prevalence of dementia and Alzheimer’s with age.

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