Wednesday, March 19, 2025

THE NERVOUS SYSTEM

THE NERVOUS SYSTEM

1. ANATOMY OF THE NERVOUS SYSTEM

1.1 Basic Terminology and Definitions

  • Nervous System → The body's primary control and communication network, transmitting electrical and chemical signals for sensory, motor, and integrative functions.
  • Neuron (Nerve Cell) → Fundamental unit responsible for impulse conduction.
    • Structural Components:
      • Dendrites → Receive input.
      • Cell Body (Soma) → Contains the nucleus, synthesizes proteins.
      • Axon → Transmits impulses away from the cell body.
      • Axon Terminals → Release neurotransmitters for synaptic transmission.
  • Types of Neurons:
    • Sensory (Afferent) Neurons → Carry impulses from receptors to CNS.
    • Motor (Efferent) Neurons → Carry impulses from CNS to muscles/glands.
    • Interneurons → Connect neurons within the CNS for processing.
  • Glial Cells (Neuroglia) → Non-neuronal supportive cells.
    • CNS Glial Cells:
      • Astrocytes → Maintain blood-brain barrier, regulate extracellular environment.
      • Oligodendrocytes → Myelinate CNS neurons.
      • Microglia → Act as macrophages for CNS immune defense.
      • Ependymal Cells → Line ventricles, produce cerebrospinal fluid (CSF).
    • PNS Glial Cells:
      • Schwann Cells → Myelinate PNS neurons.
      • Satellite Cells → Support neuronal cell bodies in ganglia.
  • White Matter → Myelinated axons; responsible for rapid signal conduction.
  • Gray Matter → Unmyelinated neuron cell bodies, dendrites; responsible for integration and processing.
  • Nucleus (CNS) → Cluster of neuronal cell bodies.
  • Ganglion (PNS) → Collection of neuron cell bodies outside CNS.
  • Tracts (CNS) → Bundles of axons transmitting signals within CNS.
  • Nerves (PNS) → Bundles of axons transmitting signals in PNS.
  • Synapse → Junction where neurotransmission occurs between neurons.

1.2 Divisions of the Nervous System

1.2.1 Central Nervous System (CNS)

  • Brain → Main control center of the body.
  • Spinal Cord → Conduit for nerve signals, controls reflexes.

1.2.2 Peripheral Nervous System (PNS)

  • Cranial Nerves (12 pairs) → Control head, neck, and thoracic/abdominal organs.
  • Spinal Nerves (31 pairs) → Conduct sensory and motor information between CNS and body.
  • Autonomic Nervous System (ANS) → Involuntary control of visceral functions.
    • Sympathetic Nervous System → "Fight or Flight" response, increases HR, BP.
    • Parasympathetic Nervous System → "Rest and Digest," conserves energy, lowers HR.
  • Enteric Nervous System (ENS) → Independent control of gut functions.

2. ANATOMY OF THE BRAIN

2.1 Cerebrum (Forebrain) - Higher Function Center

  • Hemispheres: Right & Left connected by the Corpus Callosum.
  • Lobes and Their Functions:
    • Frontal Lobe → Motor control, decision-making, speech (Broca’s Area).
    • Parietal Lobe → Sensory perception, spatial awareness.
    • Temporal Lobe → Hearing, memory, language comprehension (Wernicke’s Area).
    • Occipital Lobe → Vision processing.

2.2 Diencephalon - Relay & Control Center

  • Thalamus → Sensory relay to cortex.
  • Hypothalamus → Controls hormones, hunger, thirst, body temperature.
  • Epithalamus (Pineal Gland) → Melatonin secretion, circadian rhythms.
  • Subthalamus → Motor regulation, part of the basal ganglia.

2.3 Brainstem - Vital Function Center

  • Midbrain → Contains Substantia Nigra (Dopamine production, Parkinson’s disease association).
  • Pons → Relays motor commands, regulates breathing.
  • Medulla Oblongata → Controls heart rate, BP, respiration.

2.4 Cerebellum - Coordination & Balance

  • Fine motor control, equilibrium, coordination.

3. SPINAL CORD ANATOMY & TRACTS

3.1 Organization

  • Extends from foramen magnum to L1-L2 vertebral level.
  • 31 Pairs of Spinal Nerves:
    • Cervical (C1-C8)
    • Thoracic (T1-T12)
    • Lumbar (L1-L5)
    • Sacral (S1-S5)
    • Coccygeal (Co1)

3.2 Gray Matter Organization

  • Dorsal Horn → Sensory processing.
  • Ventral Horn → Motor control.
  • Lateral Horn (T1-L2) → Autonomic function.

3.3 White Matter Tracts

  • Ascending (Sensory) Pathways:
    • Dorsal Column (Gracile & Cuneate) → Fine touch, proprioception.
    • Spinothalamic Tract → Pain, temperature.
  • Descending (Motor) Pathways:
    • Corticospinal (Pyramidal) Tract → Voluntary movement.
    • Extrapyramidal Tracts → Involuntary motor control.


4. PHYSIOLOGY OF THE NERVOUS SYSTEM (CONTINUED)

4.1 Neuronal Physiology

  • Resting Membrane Potential (RMP) (~ -70 mV)

    • Maintained by Na⁺/K⁺ ATPase pump (3 Na⁺ out, 2 K⁺ in).
    • K⁺ leak channels play a major role in RMP.

  • Action Potential (AP) Mechanism

    • Depolarization → Na⁺ influx via voltage-gated channels (~ +30 mV).
    • Repolarization → K⁺ efflux restores negative potential.
    • Hyperpolarization → K⁺ overshoot before returning to RMP.
    • Refractory Periods
      • Absolute Refractory Period → No AP possible.
      • Relative Refractory Period → Stronger stimulus needed.

  • Synaptic Transmission

    • Electrical Synapse → Direct ion flow via gap junctions (e.g., cardiac, smooth muscle).
    • Chemical Synapse → Neurotransmitters released into synaptic cleft.
    • Excitatory Neurotransmitters → Glutamate, Acetylcholine.
    • Inhibitory Neurotransmitters → GABA, Glycine.

  • Saltatory Conduction

    • Occurs in myelinated axons.
    • Nodes of Ranvier allow APs to "jump," increasing speed.

4.2 Sensory System (Ascending Pathways)

  • Dorsal Column-Medial Lemniscus Pathway (DCML) → Fine touch, vibration, proprioception.

  • Spinothalamic Tract

    • Lateral → Pain & temperature.
    • Anterior → Crude touch & pressure.

  • Receptors:

    • Meissner’s Corpuscles → Fine touch.
    • Merkel’s Discs → Pressure.
    • Pacinian Corpuscles → Vibration.
    • Ruffini Endings → Stretch.

4.3 Motor System (Descending Pathways)

  • Pyramidal Tracts → Voluntary movement.

    • Corticospinal Tract → Limb & trunk movement.
    • Corticobulbar Tract → Head & neck movement.

  • Extrapyramidal Tracts → Involuntary movement & posture.

    • Reticulospinal Tract → Reflexes, muscle tone.
    • Vestibulospinal Tract → Balance.
    • Tectospinal Tract → Head & eye movement.

  • Upper Motor Neuron (UMN) vs Lower Motor Neuron (LMN) Lesions

    • UMN Lesion → Spastic paralysis, hyperreflexia, Babinski sign (+).
    • LMN Lesion → Flaccid paralysis, hyporeflexia, fasciculations.

5. PATHOLOGY OF THE NERVOUS SYSTEM

5.1 Neurodegenerative Disorders

  • Alzheimer’s Disease
    • Accumulation of beta-amyloid plaques, tau tangles.
    • Memory loss, cognitive decline.
  • Parkinson’s Disease
    • Loss of dopaminergic neurons in substantia nigra.
    • Bradykinesia, resting tremor, rigidity.
  • Huntington’s Disease
    • CAG trinucleotide repeat expansion in HTT gene.
    • Chorea, dementia, psychiatric symptoms.

5.2 Cerebrovascular Disorders

  • Stroke (CVA)
    • Ischemic Stroke → Due to thrombus/embolism.
    • Hemorrhagic Stroke → Due to vessel rupture (e.g., aneurysm).
  • Transient Ischemic Attack (TIA)
    • Temporary ischemia without infarction.

5.3 Infectious Diseases

  • Meningitis → Infection of meninges (bacterial, viral, fungal).
  • Encephalitis → Infection of brain parenchyma (HSV-1 common).

5.4 Demyelinating Diseases

  • Multiple Sclerosis (MS)
    • Autoimmune demyelination in CNS.
    • Optic neuritis, weakness, bladder dysfunction.
  • Guillain-Barré Syndrome (GBS)
    • Autoimmune demyelination in PNS.
    • Ascending paralysis, areflexia.

5.5 Peripheral Nerve Disorders

  • Diabetic Neuropathy → Chronic hyperglycemia damages peripheral nerves.
  • Carpal Tunnel Syndrome → Median nerve compression at the wrist.

6. CLINICAL EXAMINATION OF THE NERVOUS SYSTEM

6.1 General Inspection

  • Level of Consciousness (Glasgow Coma Scale).
  • Speech & Language (Aphasia, dysarthria).
  • Gait Analysis (Ataxic, spastic, hemiplegic gait).

6.2 Cranial Nerve Examination

  • CN I (Olfactory) → Smell test.
  • CN II (Optic) → Visual acuity, fields, funduscopy.
  • CN III, IV, VI (Oculomotor, Trochlear, Abducens) → Eye movements, pupil reflexes.
  • CN V (Trigeminal) → Facial sensation, jaw movement.
  • CN VII (Facial) → Facial symmetry, taste (anterior 2/3 tongue).
  • CN VIII (Vestibulocochlear) → Hearing, balance.
  • CN IX, X (Glossopharyngeal, Vagus) → Gag reflex, swallowing.
  • CN XI (Accessory) → Shoulder shrug, head rotation.
  • CN XII (Hypoglossal) → Tongue movements.

6.3 Motor System Examination

  • Muscle Bulk & Tone (UMN vs LMN lesions).
  • Strength Testing (0-5 MRC scale).
  • Reflexes
    • Hyperreflexia (UMN Lesion) → Stroke, MS.
    • Hyporeflexia (LMN Lesion) → GBS, peripheral neuropathy.
  • Babinski Sign → (+) in UMN lesions.

6.4 Sensory System Examination

  • Fine Touch, Pain, Temperature → DCML vs Spinothalamic tract testing.
  • Proprioception & Vibration → Posterior column disorders (e.g., B12 deficiency).

6.5 Coordination & Cerebellar Examination

  • Finger-Nose Test, Heel-Shin Test → Dysmetria in cerebellar lesions.
  • Rapid Alternating Movements → Dysdiadochokinesia in cerebellar disorders.
  • Romberg’s Test → (+) in sensory ataxia (DCML lesion).

6.6 Autonomic Function Testing

  • Orthostatic BP Changes → Autonomic failure in Parkinson’s.
  • Sweat, Pupillary Reflexes → Autonomic neuropathy (Diabetes, GBS).

7. SUMMARY & HIGH-YIELD POINTS

✔ Anatomy: Brain, spinal cord, PNS, pathways, lobes, and nuclei.
✔ Physiology: Neuronal function, sensory & motor pathways.
✔ Pathology: Stroke, neurodegeneration, infections, demyelination.
✔ Clinical Examination: Cranial nerves, reflexes, coordination, gait.


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