Nervous System Overview and Cellular Components

Overview of the Nervous System

  • Introduction to the Central Nervous System (CNS) and Peripheral Nervous System (PNS).
  • Overview of neuron and glial cell classifications, structures, appearances, locations, and functions in relation to neuropil.
  • Detailed descriptions of six types of glial cells.
  • Components reviewed: neuronal cytoskeleton, Nissl substance, organelles, and lipofuscin granules.
  • Discussion of myelin, synapses, and neuromuscular junctions in the context of neuromuscular physiology.
  • Comparative histology between grey and white matter with respect to gross appearances and locations in cerebrum, cerebellum, and spinal cord.
  • Identification and description of connective tissue coverings of peripheral nerves.
  • Introduction to terminologies related to neurodegeneration of peripheral nerves.

Learning Objectives

By the end of this session, you will be able to:

  1. List the parts of a neuron and their functions and recognize these parts on light and/or electron micrographs.
  2. Classify neurons based on shape and function.
  3. Describe anterograde and retrograde axonal transport.
  4. List the names, locations, and functions of six types of neuroglial cells.
  5. Compare the creation and structure of myelin sheaths in the PNS versus CNS.
  6. Distinguish between white matter and grey matter and their components.
  7. Draw the structure of the peripheral nerve, explain the function of each part, and recognize these on micrographs.
  8. Describe the process of nerve regeneration after injury.
  9. Discuss clinical correlations, including gliosis, multiple sclerosis, viral transport, brain tumors, and rabies inclusion bodies.
  10. Recognize all labeled structures on histologic sections.

General Overview of the Nervous System

  • Two major parts of the nervous system:
    • Central Nervous System (CNS): Comprising the brain and spinal cord.
    • Peripheral Nervous System (PNS): Encompassing:
    • Somatic nervous system: spinal nerves & cranial nerves
    • Autonomic nervous system: sympathetic nerves, parasympathetic nerves
    • Enteric nervous system: in the gut.

Central Nervous System (CNS)

Principal Structures:

  1. Cerebrum (cortex)
    • Largest part of the brain associated with higher brain functions (thought and action), comprising 85% of the brain's weight and 77% of its volume.
  2. Cerebellum (little cerebrum)
    • Accounts for approximately 10% of brain volume, containing over 50% of total neurons in the brain; controls balance and posture.
  3. Brainstem
    • Contains nuclear centers for many vital life-supporting functions.
  4. Spinal cord.

Peripheral Nervous System (PNS)

Functional Components:

  1. Sensory Component
    • Transmits electrical impulses (signals) to the CNS.
  2. Motor Component
    • Transmits impulses from the CNS to body structures; further divided into:
    • Somatic-motor fibers: innervate voluntary skeletal muscle.
    • Autonomic-motor fibers: innervate involuntary muscles, cardiac muscle, and some glands, subdivided into sympathetic and parasympathetic.

Neurons and Glial Cells

Two Cell Types in Both CNS & PNS:

  1. Neurons
    • Greek 'neûron' meaning sinew or nerve; regarded as the structural and functional unit of the nervous system.
    • Specialized to receive, integrate, and send impulses; can communicate with other neurons and excite muscle tissues.
  2. Glial Cells
    • Greek 'glia' meaning glue; six types identified (4 in CNS and 2 in PNS).
    • Short processes with diverse functions including nutrition, support, and protection of neurons.
    • More abundant than neurons by 5-10 times in the brain.

Neuropil

  • A fibrous intercellular network surrounding CNS cells, resembling collagen under light microscopy.
  • Composed of cellular processes from neurons and glial cells.
  • No fibroblasts or collagen present in the CNS.

Neuron Structure

Components of a Neuron:

  1. Cell body (soma or perikaryon):
    • Contains the nucleus, rough and smooth endoplasmic reticulum, etc.
  2. Dendrites:
    • Responsible for receiving information.
  3. Axon:
    • Single process responsible for transmitting information to other neurons or muscle.
Method of Classification by Function:
  • Sensory Neurons
  • Motor Neurons
  • Interneurons
Types of Neurons by Shape:
  1. Multipolar Neurons:
    • One axon and two or more dendrites; most neurons in the CNS and all motor neurons.
  2. Bipolar Neurons:
    • One dendrite and one axon; common in sensory neurons of retina, olfactory mucosa, inner ear.
  3. Unipolar (pseudo-unipolar) Neurons:
    • One process bifurcated near the cell body; found in dorsal root ganglia (DRG), most cranial ganglia.

Neuronal Structures and Functions

Neuronal Cell Body:

  • Contains a large nucleus with pale euchromatin, prominent nucleolus.
  • Rich in rough endoplasmic reticulum forming Nissl bodies, basophilic.

Nissl Bodies (Nissl Substance):

  • Composed of rough endoplasmic reticulum and ribosomes, indicative of neuronal activity.

Lipofuscin Granules:

  • Yellow-brown cytoplasmic inclusions from undigested material accumulations; associated with aging.

Neuronal Nuclei:

  • Spherical, large with pale euchromatin indicating high synthetic activity.

Dendrites:

  • Extensive branching process, receiving signals from other neurons via synapses.
  • Form dendritic spines, increasing surface area for synaptic interaction.
  • Dendritic number and structure can be altered by factors such as nutrition and genetic conditions (trisomy).

Axons:

  • Longer than dendrites, consistent diameter, arising from axon hillock; initial nerve impulse originates here.

Action Potential

  • Capable of travelling long distances, transmitting signals to other neurons, muscles, glands.
  • Myelin: Insulating lipoprotein sheath enhancing impulse transmission via saltatory conduction across Nodes of Ranvier, which allow ion flow.
  • Created by oligodendrocytes (CNS) and Schwann cells (PNS).

The Synapse

  • Structure allowing neuron communication via electrical or chemical signals.
  • Types:
    • Chemical Synapse: Conversion of electrical activity into neurotransmitter release.
    • Electrical Synapse: Connected by gap junctions enabling direct electric communication.

Neuromuscular Junction

  • Connection between motor neurons and skeletal muscle fibers, crucial for voluntary movement.
  • Inhibition process by Botox involves blocking synaptic vesicles at this junction.

Cytoskeleton in Neurons

Actin:

  • Essential for neuronal growth, guidance, branching, morphogenesis, and synapse stability.

Intermediate Filaments:

  • Present in neuron cell body and processes; neurofilaments are cell-specific types.

Microtubules:

  • Serve as tracks for axonal transport, critical due to the long distances materials must travel from cell body to axon terminals.

Retrograde Axonal Transport:

  • Assists in neurotrophic signaling and injury response, transporting vital nutrients back to the neuron.

Glial Cells - Overview

  • Supporting cells of the nervous system responsible for neuronal integrity and function. Types include:
    • Oligodendrocytes: Form myelin sheaths in CNS.
    • Astrocytes: Most abundant; maintain homeostasis and structural support.
    • Microglia: Resident immune cells of CNS; protect against pathogens.
    • Ependymal Cells: Line brain ventricles; assist in CSF production.

CNS Glial Cells

Oligodendrocytes:

  • Compact, rounded with short processes; insulate CNS axons and allow for rapid action potential propagation.
  • Abundant smooth endoplasmic reticulum for lipid synthesis.

Astrocytes:

  • Star-shaped, numerous cytoplasmic processes; involved in forming blood-brain barrier, regulating ion concentrations, aiding in neuronal growth and repair.
  • They create a regulatory network for synaptic activity and neurovascular coupling.

Microglial Cells:

  • Macrophages in the CNS, recruited from bone marrow; vital in monitoring and responding to CNS health.

Ependymal Cells:

  • Epithelial-like cells lining the brain's ventricles; assist in CSF propulsion and maintenance.
  • CSF: critical for nourishment and protection of the CNS.

Cerebrospinal Fluid (CSF)

  • Produced primarily in the choroid plexus; circulates and provides protective cushioning.
  • Continuous production and reabsorption are vital for CNS function.

Clinical Considerations

Gliosis:

  • Reactive proliferation of glial cells in response to CNS damage, often impeding neuronal regeneration.
  • Prominent in disorders like multiple sclerosis and following strokes.

Brain Tumors:

  • Incidence: 10-17 per 100,000 for cranial, 1-2 per 100,000 for spinal.
  • Types include gliomas: astrocytomas, oligodendrogliomas, ependymomas.

CNS Gray and White Matter

  • Gray matter contains neuronal cell bodies; white matter rich in myelinated axons, attributed to myelin's whitish hue.
  • Gray matter has dark staining due to Nissl bodies; white matter appears lighter.

Peripheral Nervous System (PNS)

Structure and Function:

  • PNS serves as an interface between the CNS and the rest of the body, comprising ganglia and nerves. Ganglia relay sensory information; nerves transmit signals.

Peripheral Ganglia:

  • Collections of neuronal cell bodies outside the CNS; includes autonomic and sensory ganglia.

Satellite Cells:

  • Glial cells in PNS, providing support and nutrients, similar in function to astrocytes in CNS.

Schwann Cells:

  • Myelinate axons in the PNS; critical for regeneration in response to nerve injury. Encapsulate axons and form myelin sheaths.
Myelination and Nodes of Ranvier:
  • Schwann cells each myelinate segments of axons (internodes). Entail regeneration capabilities unique to PNS compared to CNS.

Nerve Bundle Structure:

  1. Endoneurium: Thin layer around individual fibers.
  2. Perineurium: Surrounds groups of fibers (fascicles).
  3. Epineurium: Outer layer encompassing entire nerve; composed of dense fibrous connective tissue.

Neural Regeneration

After PNS Injury:

  • Neuronal processes can grow, creating new synapses and restoring function. Address the proximal and distal changes, including Wallerian degeneration.
  • Growth via Schwann cells facilitating the reestablishment of neural pathways, growth rate of 3-4 mm/day.
  • Factors affecting regeneration: scar tissue, mismatched sensory and motor nerve fibers.

CNS vs. PNS Regeneration:

  • PNS: High regeneration capacity due to supportive glial environment.
  • CNS: Limited by oligodendrocyte efficiency, blood-brain barrier restricting macrophage access, and inhibitory signals from myelin debris.