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Neuroanatomy and Neurophysiology Review

Types of Nervous Cells

  • Two main types of cells:
    • Neurons: Primary cells for information processing and communication.
    • Structure:
      • Variable size and structure.
      • Primary fuel: Glucose (insulin is not required for uptake in the CNS).
      • Components:
      • Cell body (soma): Located mainly in CNS; densely packed cell bodies in the CNS are called nuclei; in PNS, they are called ganglia.
      • Dendrites: Carry nerve impulses toward the cell body (afferent). Dendritic zone receives stimuli that facilitate conduction.
      • Axons: Carry impulses away from the cell body (efferent). The initial segment is where action potentials (AP) begin; typically, there is one axon per neuron.
      • Myelin: Insulated segments made by Schwann cells.
      • Endoneurium: Layer of connective tissue around each axon.
      • Neurolemma: Membrane between the myelin sheath and endoneurium.
      • Nodes of Ranvier: Interruptions in the myelin sheath where ion flow happens, facilitating saltatory conduction.
      • Saltatory conduction: Faster ion flow between myelin segments.
    • Functional Classification:
      • Sensory neurons: Transmit impulses from sensory receptors to CNS (afferent).
      • Interneurons: Transmit impulses from neuron to neuron.
      • Motor neurons: Transmit impulses from CNS to effector organs (efferent).
      • Neuromuscular junctions: Specialized synapses in skeletal muscle.
    • Neuroglial cells: Supporting cells in CNS.
    • Astrocytes: Fill spaces between neurons and around blood vessels.
    • Oligodendroglia (oligodendrocytes): Produce myelin in CNS.
    • Ependymal cells: Line cavities filled with cerebrospinal fluid (CSF) in the CNS.
    • Microglia: Clean up debris via phagocytosis.
    • Schwann cells: Myelinate axons in the PNS.

Overview of Nervous System

  • CNS: Composed of the brain and spinal cord.
  • PNS: Includes cranial and spinal nerves.
    • Afferent pathways: Carry impulses toward the CNS.
    • Efferent pathways: Carry impulses away from the CNS.
    • Somatic nervous system: Regulates voluntary motor control of skeletal muscles.
    • Autonomic nervous system: Regulates internal environments involuntarily.
    • Components:
      • Preganglionic: Myelinated fibers.
      • Postganglionic: Unmyelinated fibers.
    • Divisions:
      • Sympathetic: Prepares the body for stressful situations (fight or flight).
      • Parasympathetic: Promotes relaxation and energy conservation (rest and digest).

Sympathetic Nervous System Functions

  • Innervation: Cell bodies in thoracolumbar region.
  • Nerve fibers: Short; synapse near spinal cord.
  • Responses:
    • Dilation of pupils, increased heart rate, and blood pressure.
    • Blood flow diverted from gut to muscles.
    • Pathological Significance:
    • Lesion effects include increased blood flow leading to red, warm skin, lack of sweating, and pupil constriction on the lesion's side.
    • Horner's Syndrome: Caused by sympathetic trunk injury.

Parasympathetic Nervous System Functions

  • Innervation: Cell bodies in craniosacral region, while fibers are longer and synapse near end organs.
  • Functions: Restores energy by promoting digestion and relaxation.
  • Neurotransmitters: Include acetylcholine, norepinephrine, and others affecting neuronal excitability.

Nerve Injury and Regeneration

  • Wallerian degeneration: Occurs when an axon is severed.
    • Injury effects: Depends on injury type, location, and inflammatory response.
    • Regeneration potential: Better near cell body; crushing injuries typically have better outcomes than cuts.
  • Changes in axons:
    • Distal (below cut): degeneration and myelin sheath reforms.
    • Proximal (above cut): Increased metabolic activity and new terminal sprouts form 7-14 days post-injury.

Synapse Basics

  • Nerve Impulse: Generated by neurons through membrane potential changes.
  • Resting Membrane Potential (RMP): Neurons at rest have permeability favoring potassium influx and sodium efflux.
  • Synapses: Region between neurons for impulse transmission.
    • Presynaptic neurons: Carry impulses toward the synapse.
    • Postsynaptic neurons: Carry impulses away.
  • Action Potential: Triggered by depolarization of the neuron, leading to sodium influx and potassium efflux.

Reticular Formation and its Functions

  • Reticular formation: Network regulating vital reflexes (e.g., cardiovascular and respiratory functions); contributes to wakefulness via the reticular activating system.

Forebrain Anatomy and Functions

  • Cerebrum: Largest brain part, with gray (cell bodies) and white matter (myelinated fibers).
  • Four lobes:
    • Frontal: Important for goal-oriented behavior and voluntary motor control.
    • Parietal: Processes somatic sensory data.
    • Occipital: Houses visual processing centers.
    • Temporal: Involved in auditory processing and memory.

Brainstem Functions

  • Includes: Midbrain, pons, and medulla oblongata.
  • Functions:
    • Midbrain: Relay for sensory/motor tracts and auditory/visual reflexes.
    • Pons: Controls respiration and contains cranial nerves.
    • Medulla: Regulates reflexes like HR and respiration.

Spinal Cord Anatomy

  • Spinal Cord: Communicates between the brain and body, conducting reflexes.
  • Regions: Cervical, thoracic, lumbar, sacral, coccygeal.
  • Gray Matter:
    • Dorsal horn: Sensory neurons.
    • Lateral horn: ANS cell bodies.
    • Ventral horn: Efferent pathways.
  • White Matter: Contains spinal tracts for sensory and motor impulses.

Cranial Nerves Overview

  • I: Olfactory (S) - Smell
  • II: Optic (S) - Vision
  • III: Oculomotor (M) - Eye movement
  • IV: Trochlear (M) - Eye muscle control
  • V: Trigeminal (M, S) - Face sensation, mastication
  • VI: Abducens (M) - Eye abduction
  • VII: Facial (M, S) - Facial expression, taste
  • VIII: Vestibulocochlear (S) - Hearing
  • IX: Glossopharyngeal (M, S) - Pharynx and salivary gland function
  • X: Vagus (M, S) - Internal organ function
  • XI: Spinal Accessory (M) - Neck muscle control
  • XII: Hypoglossal (M) - Tongue movement

Meninges Structure & Function

  • Meninges: Protective layers surrounding the brain/spinal cord (Dura mater, Arachnoid, Pia mater).
  • Spaces: Subdural, subarachnoid (contains CSF), and epidural.

Blood Supply to the Brain

  • Receives 800-1000 mL/min; regulated primarily by CO2 levels.
  • Vascular Anatomy: Internal carotid and vertebral arteries contribute to a circle (circle of Willis).
    • Arteries:
    • ACA: Supplies frontal lobe.
    • MCA: Supplies temporal and parietal lobes.
    • PCA: Supplies occipital lobe.

Blood-Brain Barrier

  • Function: Selectively restricts harmful substances from entering brain and CSF, maintains biochemical stability.
  • Clinical Implications: Certain drugs can or cannot cross this barrier due to permeability changes during conditions like hypertension and high-dose anesthetics.