Neurons and Neuroglia: Structure, Function, and Clinical Relevance

Cell Types in Nervous Tissue

Neurons and Neuroglia

The nervous system is composed of two primary cell types: neurons and neuroglia (also called glial cells). Neurons are responsible for transmitting electrical signals, while neuroglia provide support, protection, and nutrition for neurons.

• Neurons: Specialized for conduction of action potentials; typically not replaced once lost.

• Neuroglia (e.g., astrocytes): Supportive cells that maintain the environment around neurons, produce myelin, and can divide (source of gliomas).

Structure of a Neuron

Basic Anatomy

Neurons have a unique structure that enables them to transmit nerve impulses efficiently.

• Dendrites: Receive incoming signals; short, highly branched, and non-myelinated.

• Cell Body (Soma): Contains the nucleus, mitochondria, Golgi apparatus, lysosomes, Nissl bodies (rough ER), neurofibrils, and lipofuscin.

• Axon: Single, long process that conducts impulses away from the cell body; contains axon hillock (trigger zone), axoplasm, axolemma, axon collaterals, axon terminals, synaptic knobs, and varicosities.

Example: Pyramidal cells (cerebral cortex) and Purkinje cells (cerebellum) are specialized neuron types with distinct dendritic trees.

Functional and Structural Classification of Neurons

Structural Classification

• Anaxonic: No obvious axon; found in the brain.

• Pseudounipolar: Single process splits into two branches; sensory neurons.

• Bipolar: One axon and one dendrite; found in the retina.

• Multipolar: One axon, multiple dendrites; most common type.

Functional Classification

• Sensory (Afferent): Transmit impulses from receptors to the CNS.

• Motor (Efferent): Transmit impulses from CNS to effectors (muscles/glands).

• Association (Interneurons): Connect neurons within the CNS; most abundant (20 billion).

Organization of the Nervous System

CNS vs. PNS

• CNS (Central Nervous System): Brain and spinal cord; contains tracts (bundles of axons).

• PNS (Peripheral Nervous System): Nerves and ganglia; nerves are bundles of axons, ganglia are clusters of cell bodies.

Synapses and Varicosities

Synaptic Structure and Function

• Synaptic Knob: Axon terminal containing synaptic vesicles with neurotransmitters.

• Synaptic Cleft: Gap between presynaptic and postsynaptic membranes.

• Varicosities: Swellings along autonomic nerve fibers that release neurotransmitters to smooth muscle.

Axonal Transport

Mechanisms and Clinical Relevance

Axonal transport moves materials between the neuron cell body and axon terminals.

• Slow Transport: 1–3 mm/day; moves enzymes and cytoskeletal components.

• Fast Transport: 20–400 mm/day; moves organelles (e.g., mitochondria).

• Anterograde (Kinesin): From cell body to axon terminal.

• Retrograde (Dynein): From axon terminal to cell body; can transport viruses (e.g., herpes, rabies) and toxins (e.g., tetanus).

Example: Rabies virus travels retrogradely from a bite site to the CNS, then anterogradely to salivary glands.

Neuroglia: Types and Functions

Central Nervous System (CNS)

• Astrocytes: Form scar tissue, regulate the blood-brain barrier, and maintain extracellular environment.

• Oligodendrocytes: Produce myelin sheaths in the CNS.

• Microglia: Phagocytic cells that remove debris and pathogens.

• Ependymal Cells: Line ventricles and produce cerebrospinal fluid (CSF).

Peripheral Nervous System (PNS)

• Satellite Cells: Support neuronal cell bodies in ganglia.

• Schwann Cells: Produce myelin sheaths around peripheral axons; assist in axon repair.

Myelination

Myelin Sheaths and Nodes of Ranvier

• Myelin: 80% lipid (e.g., sphingolipids), 20% protein; increases speed of nerve impulse conduction.

• Nodes of Ranvier: Gaps in myelin sheath where action potentials are regenerated.

• Schwann Cells (PNS): Wrap around axons, forming a neurilemma (outermost layer containing nucleus and cytoplasm).

• Oligodendrocytes (CNS): Extend processes to multiple axons; no neurilemma, no regeneration after injury.

Example: Myelination begins at 14 weeks gestation and continues until about age 20, explaining why babies cannot walk at birth.

Diseases Associated with Myelin Sheath Destruction

Multiple Sclerosis (CNS)

• Autoimmune demyelinating disease; myelin is destroyed and replaced by scar tissue (sclerosis).

• Symptoms depend on lesion location: vision loss, impaired coordination, movement, and bladder/bowel control.

• Diagnosis: MRI shows lesions; treatment may include immunomodulatory drugs (e.g., betaseron).

Guillain-Barré Syndrome (PNS)

• Progressive but reversible demyelination; often follows viral infection.

• Symptoms: Loss of sensation, muscle weakness, respiratory difficulty.

Heavy Metal Poisoning

• Lead and mercury accumulate in the CNS, causing glial cell damage and demyelination.

Diphtheria

• Caused by Corynebacterium diphtheriae; toxin destroys Schwann cells, leading to paralysis.

• Prevention: Vaccine available.

Repair of Peripheral Neurons

Regeneration in the PNS vs. CNS

• Neurons do not undergo mitosis after 6 months of age.

• PNS: Repair possible if cell body and Schwann cells remain intact; Schwann cells form a regeneration tube.

• CNS: No regeneration due to absence of neurilemma and inhibitory signals from glia; astrocytes form scar tissue.

Steps in Peripheral Nerve Repair

1. Wallerian degeneration: Distal axon and myelin degenerate.

2. Macrophages remove debris; Schwann cells proliferate.

3. Axon sprouts grow through regeneration tube formed by Schwann cells.

4. Axon reestablishes connection with target cell if successful.

Summary Table: Types of Neuroglia

Location	Type	Main Function
CNS	Astrocytes	Support, blood-brain barrier, scar formation
CNS	Oligodendrocytes	Myelinate CNS axons
CNS	Microglia	Phagocytosis
CNS	Ependymal cells	Produce cerebrospinal fluid
PNS	Satellite cells	Support neuron cell bodies in ganglia
PNS	Schwann cells	Myelinate PNS axons, assist in repair

Key Equations and Concepts

• Speed of Nerve Impulse: Up to 280 miles/h (approx. 120 m/s in myelinated axons).

• Saltatory Conduction: Action potentials jump from node to node in myelinated axons, increasing speed.

• Axonal Transport Rates:

  • Slow: $1-3\ \text{mm/day}$

  • Fast: $20-400\ \text{mm/day}$

Additional info: This guide integrates and expands upon the provided notes and images, offering a comprehensive overview of neuron and neuroglia structure, function, and clinical relevance for Anatomy & Physiology students.