Histology of Nervous Tissues

Histology of Nervous Tissues

• Nervous tissue receives stimuli from internal and external environments, processing and responding through various effector organs.
• Neurons are the functional units of the nervous system.
• Neuroglial cells support the nervous system.

Learning Outcomes

• Classify nerve tissue.
• Describe the general structure and different types of neurons.
• Detail the structure and types of supporting cells (glia).
• Detail the classification of receptors.

Introduction: Tissues of the Body

• Basic tissues types are shown in the image.

Neurons and Neuroglial Cells

• Neurons receive stimuli, analyzed, integrated, and coordinated responses.
• Neurons are the basic functional unit of the nervous system.
• Neurons are responsible for sending electrical impulses.
• Neuroglial cells support the nervous system.

Types of Neuroglia

• Central Nervous System (CNS): Ependymal cells, Oligodendrocytes, Astrocytes, Microglia
• Peripheral Nervous System (PNS): Satellite cells, Schwann cells

Neuron Characteristics

• Neurons possess many processes.
• They are excitable and conductive.
• They receive stimuli and conduct impulses.
• The electrical nerve impulse spreads through neurons.
• Neurons cannot divide (lack mitosis) after injury.
  • Axons (when severed) can regenerate.

Neuron Structure

• Cell body/perikaryon (soma): Contains the nucleus, nucleolus, and organelles.
• Neurites/Processes:
  • Dendrites: highly branched extensions that receive signals
  • Axons: long extensions that transmit signals away from the cell body
• Terminal button: Small swellings at the end of the axon.

Neuron ~ Characteristics (continued)

• Dendrites are short extensions of the cell body, highly branched and tapering.
  • Some dendrites end in special sensory receptors or attach to other cells.
• They get thinner further away from the cell body.
• They contain Nissl substances (RER).
• Axons are long, cone-shaped extensions arising from the axon hillock.
  • No Nissl granules.
  • Tubular and a uniform diameter.
  • Conduction of impulses increases when the diameter is higher.
  • Impulses travel away from the cell body.

Classifications of Neurons

• A: Configuration of neurites (number of processes)
  • Bipolar
  • Pseudounipolar
  • Multipolar
• B: Functional role
• C: Cell size
• D: Neurotransmitter released

Neuron Types

• Bipolar Neurons: One axon, one dendrite, sensory function (smell, sight, balance).
• Pseudounipolar Neurons: Single process branching into two, sensory function (dorsal root ganglia).
• Multipolar Neurons: More than two processes, motor function (spinal cord, cerebral cortex, cerebellar cortex).
  • Pyramidal cells, Purkinje cells

Neurotransmitter Released

• Types and description: Cholinergic, Adrenergic, GABAergic, Dopaminergic, Serotonergic, Glycinergic.
  • Examples Most somatic motor neurons, most postganglionic sympathetic motor neurones, some neurons in the cerebellum, cerebral cortex etc.

Neuroglia / Glia Cells

• Glial cells support neurons. They are non-excitable. They are smaller than neurons, but more abundant by a factor of 10.
• Glial cells provide the microenvironment necessary for neuronal activity.
• Functions: structural and nutritional support; electrical insulation; enhancement of impulse conduction velocity.
• Varieties: Astrocytes, Oligodendrocytes, Microglia cells, Ependymal cells, Satellite cells, and Schwann cells.

Nerve Fibers

• Two types: Myelinated and Unmyelinated
• Myelinated fibers: large diameter, wrapped by Schwann cells for insulation.
• Unmyelinated fibers: small diameter, axon enveloped in Schwann cell cytoplasm.
• Oligodendrocytes myelinate axons in the CNS; Schwann cells in the PNS.

Nerve Bundles

• Epineurium surrounds the entire nerve.
• Perineurium surrounds nerve fiber fascicles.
• Endoneurium surrounds individual nerve fibers.

Synapse

• Forms from the interaction of the axon terminal and cell wall of the target cell.
• Neurone – neurone and neurone – effector cells (muscle and gland) contact site.
• Neurotransmitter release is at the axon terminal.

Sensory Receptor Endings

• Specialized nerve endings (or separate cells) for signal transduction.
  • Transduce stimuli (internal or external) into nerve impulses.
• Stimuli are then passed to the CNS for sensory and motor responses.
  • Classification of types of sensory endings: exteroceptors, interoceptors, and proprioceptors.
• Types: Free nerve endings, Pacinian corpuscles, Meissner corpuscles, Ruffini end-organs and Golgi tendon organs, Krause corpuscles, Muscle Spindles

Clinical Application

• Dead neurons are removed by macrophages.
• Hyperplasia and hypertrophy of astrocytes can occur, leading to gliosis.
• Axon regeneration may occur if the damage is far from the cell body.
• Gliomas are invasive and have incomplete removal, recurrence, and interference of function.

References

• A list of references are provided.