Chapter 12 – Neurology I

Overview of the Nervous System

• Divided anatomically into:
  • Central Nervous System (CNS): brain and spinal cord.
  • Peripheral Nervous System (PNS): nerves and ganglia outside the CNS.
• Basic information flow:
  • Afferent (sensory) pathways → CNS information processing → Efferent (motor) pathways.
• Sensory (afferent) input sources:
  • Somatic receptors – skin, skeletal muscle, joints.
  • Visceral receptors – internal organs.
  • Special receptors – vision, hearing, taste, smell, equilibrium.
• Motor (efferent) output targets (effectors):
  • Skeletal muscle (voluntary control).
  • Smooth muscle and glands (involuntary control).
• Functional divisions of efferent pathways:
  • Somatic Nervous System (SNS) – controls skeletal muscle.
  • Autonomic Nervous System (ANS) – regulates smooth muscle, cardiac muscle and glands.
• Key features of the nervous system:
  • Built primarily from neurons and supporting neuroglia.
  • Communicates via rapid electrochemical impulses (action potentials) and neurotransmitter release.
  • Direct, point-to-point cell communication enables fast, specific responses.

Cellular Components

Neurons

• Structural parts:
  • Cell body (soma) – contains the nucleus and most organelles.
  • Dendrites – receptive extensions that carry graded potentials toward the soma.
  • Axon – single long process that propagates the action potential away from the soma.
  • Synaptic terminals (knobs) – house neurotransmitter-filled vesicles.
• Organization of neuronal cell bodies:
  • CNS clusters = \text{nuclei}.
  • PNS clusters = \text{ganglia}.
• Connectivity:
  • One neuron can form thousands of synapses, producing extensive neural networks.

Neuroglia (Glial Cells)

• Capable of mitosis; outnumber neurons.
• CNS glia and their functions:
  • Oligodendrocytes – myelinate multiple CNS axons.
  • Microglia – phagocytic, migrate to injury/infection via chemotaxis.
  • Astrocytes – regulate extracellular environment; form blood-brain barrier with perivascular feet.
  • Ependymal cells – line ventricles/central canal; help produce & circulate cerebrospinal fluid (CSF).
• PNS glia and their functions:
  • Schwann cells – myelinate single PNS axon segments; vital for axonal regeneration.
  • Satellite cells – surround PNS somata; provide nutrition and structural support.

Neuronal Anatomy & Synaptic Transmission

• Presynaptic neuron → synaptic cleft → postsynaptic cell.
• Synaptic knob contains neurotransmitters packaged in vesicles.
• Action potential arrival triggers exocytosis; neurotransmitters diffuse across the cleft and bind to postsynaptic receptors, converting electrical to chemical signaling.

Structural Classification of Neurons

• Bipolar – one dendrite, one axon; common in special senses (e.g.
  retina, olfactory mucosa).
• Unipolar (pseudo-unipolar) – single process splits into peripheral & central branches; typical of sensory (afferent) neurons.
• Multipolar – one axon, multiple dendrites; most common type; includes all motor neurons and interneurons.

Functional Classification of Neurons

• Sensory (afferent) neurons (usually unipolar):
  • Somatic sensory – input from skin, muscle, joints.
  • Visceral (autonomic) sensory – input from internal organs.
• Interneurons (multipolar):
  • Located entirely within the CNS; integrate and relay information between sensory and motor pathways.
• Motor (efferent) neurons (multipolar):
  • Somatic motor – innervate skeletal muscle.
  • Autonomic motor – innervate smooth muscle, cardiac muscle, glands.

Axonal Injury & Regeneration

• PNS regeneration:
  • Schwann cells create regeneration tubes guiding axon sprouts toward target.
  • Functional recovery possible if alignment and distance are favorable.
• CNS trauma:
  • Injured neurons express apoptotic receptors.
  • Myelin-associated inhibitory proteins prevent axon elongation.
  • Reactive astrocytes form a glial scar, creating physical & chemical barriers to regrowth.
  • Result: limited spontaneous regeneration; focus of current neuro-research.

Vascular & Fluid Interfaces

• Astrocytic perivascular feet envelop capillaries, forming part of the blood-brain barrier that regulates molecular passage into nervous tissue.
• Ependymal cell cilia circulate \text{CSF}, which cushions the CNS and facilitates nutrient/waste exchange.

Practical & Clinical Connections

• Demyelinating disorders:
  • CNS (oligodendrocyte loss) → Multiple Sclerosis.
  • PNS (Schwann cell loss) → Guillain-Barré Syndrome.
• Nerve grafts often exploit Schwann cell-mediated regeneration in the PNS.
• Astrocyte dysfunction is linked to cerebral edema, epilepsy and various neurodegenerative diseases.