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.