Ch_15 (1)

Chapter 15: Structure and Function of the Neurologic System

Overview of the Nervous System

• Central Nervous System (CNS): Consists of the brain and spinal cord.

• Peripheral Nervous System (PNS): Comprises nerves outside the CNS.

  • Afferent Nerves: Carry information towards the CNS.

  • Efferent Nerves: Carry information away from the CNS.

• Systems:

  • Somatic: Involves somatosensory and motor systems.

  • Autonomic: Consists of sympathetic and parasympathetic divisions.

  • Special Senses: Include sight, balance, hearing, smell, taste.

Cells of the Nervous System - Neurons

• Neurons:

  • Primary communicating cells in the nervous system.

  • Terminally differentiated (cannot divide).

  • Polarized (have axons vs. dendrites).

  • Numerous types exist, essential for signal transmission.

Neuron Classification

• Pseudounipolar neurons: Have a single process that splits into two branches (dendrites and axon).

• Bipolar neurons: Have two processes, one dendrite and one axon.

• Multipolar neurons: Have multiple dendrites and one axon, most common type.

Cells of the Nervous System - Glia

• Glial Cells: 5 to 10 times more numerous than neurons.

  • CNS Glia:

    • Astrocytes: Support and nourishment for neurons.

    • Oligodendrocytes: Myelinate CNS axons.

    • Microglia: Immune defense of the CNS.

    • Ependymal Cells: Line the ventricles of the brain.

  • PNS Glia:

    • Schwann Cells: Myelinate PNS axons.

    • Satellite Cells: Support ganglia in the PNS.

Cells of the Nervous System - Nerve Injury and Regeneration

• Injury and Recovery Process:

  • Wallerian Degeneration: Occurs distal to injury, leading to axon degradation.

  • Proliferation of Schwann Cells: Creates a conduit for regrowth.

  • Regeneration of Axons: Possible only in PNS if the lesion is small (approximately 1 mm/day growth).

Nerve Impulse Generation

• Resting Membrane Potential: Changes when dendrites and cell body receive stimuli.

• Action Potential:

  • Occurs once the threshold is reached, leading to the release of neurotransmitters.

  • All-or-none principle: action potentials are uniform in strength.

  • Neurotransmitter release depends on postsynaptic receptor type.

Nerve Impulse - Summation

• Temporal Summation: Occurs when multiple signals arrive at a single postsynaptic neuron over time.

• Spatial Summation: Occurs when multiple signals arrive simultaneously from different presynaptic neurons.

Nerve Impulse - Synapses

• Synaptic Transmission: Involves the release of neurotransmitters across the synaptic cleft, which bind to postsynaptic receptors.

• Excitatory and Inhibitory Effects:

  • Depolarization (EPSP): Increases likelihood of action potential.

  • Hyperpolarization (IPSP): Decreases likelihood of action potential.

Central Nervous System - Brain

• Major Divisions:

  • Forebrain: Comprises Telencephalon (cerebral hemispheres) and Diencephalon (thalamus).

  • Midbrain: Relays motor and sensory signals.

  • Hindbrain: Contains Metencephalon (cerebellum and pons) and Myelencephalon (medulla).

• Cerebral Structures:

  • Lobes: Frontal, parietal, occipital, and temporal lobes with specific functions for sensory processing, motor control, and higher cognition.

  • Frontal Lobe: Involved in reasoning, motor functions, and speech production (Broca's area).

Autonomic Nervous System

• Regulates involuntary body functions.

  • Parasympathetic Division: Responsible for rest-and-digest activities.

  • Sympathetic Division: Responsible for fight-or-flight responses.

• Neurotransmitters:

  • Acetylcholine in parasympathetic fibers.

  • Norepinephrine for sympathetic responses.

Central Nervous System - Blood Supply

• Blood Supply: Includes anterior and posterior cerebral arteries, and various branches to supply blood to the brain and spinal cord.

• Tight Junctions: Protect the brain through the blood-brain barrier.

Tests of Nervous System Function

• Imaging Techniques:

  • X-rays, MRIs, CT scans, and PET scans to visualize structures and functions.

• Electrophysiological Tests: EEG, MEG, and evoked potentials to assess electrical activity.