Nervous System: Anatomy and Histology

• Nervous tissue consists of nerve cells (neurons) and neuroglia.

Anatomical Divisions of the Nervous System

• The central nervous system (CNS) includes the brain and spinal cord.
• The peripheral nervous system (PNS) includes peripheral nerves and ganglia.
• Peripheral nerves are further divided into cranial and spinal nerves.
• Ganglia are divided into cerebrospinal and autonomic types, with sympathetic and parasympathetic subdivisions of the autonomic ganglia.

Histological Divisions of the Nervous System

• Nerve cells, or neurons, transmit nerve impulses and control body organs.
• Neuroglia occupy the interneuronal space and create a microenvironment suitable for neuronal activity.

Neuron Composition

• Neurons are composed of cell bodies and processes: axons and dendrites.

Cell Body (Perikaryon)

• It contains the nucleus and most cellular organelles.
• Acts as a synthetic and trophic center for the entire neuron.

Cell Body Nucleus

• The nucleus is large and spherical.
• The nucleus is euchromatic with a prominent nucleolus.

Cell Body Cytoplasm

• Cytoplasm contains cytoplasmic organelles, a cytoskeleton, and cytoplasmic inclusions.

Cytoplasmic Contents

• Cytoplasmic organelles include Nissl bodies, the Golgi apparatus, and mitochondria.
• The cytoskeleton includes neurofilaments (intermediate filaments) and microtubules.
• Cytoplasmic inclusions include melanin and lipofuscin pigments.

Nissl Bodies

• They are scattered in the cytoplasm, except in the axon and axon hillock, periphery of the nerve cell, and around the nucleus.
• LM shows basophilic granules.
• EM reveals well-developed rough endoplasmic reticulum (RER) and polyribosomes.
• Function in synthesizing proteins of the nerve cell, including cytoskeletal and transport/secretion proteins.

Golgi Apparatus

• The Golgi apparatus is well developed and located around the nucleus (perinuclear).
• Proteins of the nerve cell are packaged for secretion.

Mitochondria

• Found scattered in the perikaryon, axon, and dendrites.

Neuronal Processes: Axons

• There is usually a single axon per neuron.
• Axons are variable in length, but can be very long or very short.
• Axons have a constant diameter and branch only at their terminal end.
• The E.M. shows mitochondria and neurofibrils but no Nissl granules.
• The function is to conduct nerve impulses away from the cell body.

Neuronal Processes: Dendrites

• There are usually multiple dendrites per neuron.
• Dendrites are usually short.
• Dendrites have an inconstant diameter with extensive branching along their course.
• The E.M. shows mitochondria, neurofibrils, and Nissl granules.
• The function is to conduct nerve impulses to the cell body.

Types of Neurons Based on Number of Processes

• Multipolar neurons
• Bipolar neurons
• Pseudo-unipolar neurons
• Anaxonic neurons

Multipolar Neurons

• Examples include polygonal neurons (sympathetic ganglia), pyramidal neurons (cerebral cortex), and pyriform neurons (cerebellar cortex).

Pseudo-Unipolar Neurons

• One process divides into the axon and dendrite.
• An example is spinal ganglia.

Bipolar Neurons

• Found in the retina and olfactory epithelium.

Anaxonic Neurons

• These neurons have many dendrites but no axon.
• They do not produce action potentials but regulate electrical changes of adjacent neurons.
• An example is interneurons of the CNS and retina.

Neuroglia (Glial Cells)

• There are 10 times more neuroglia than neurons.
• They support neurons and create a microenvironment that is optimal for neuronal activity.
• Types include: Oligodendrocytes, Schwann cells, Astrocytes, and Microglia.

Oligodendrocytes

• They produce myelin in the CNS.
• Cell processes wrap around more than one axon to form myelin.
• They provide electric insulation of neurons in the CNS.

Schwann Cells

• They produce myelin in the PNS.
• Responsible for insulation of nerve impulses.
• Facilitate regeneration of peripheral nerves.

Astrocytes

• They are star-shaped and their processes surround blood vessels.
• Function in nutrition: neurons exchange nutrients and waste products with blood through astrocyte processes.

Microglia

• Located in the white and gray matter of the CNS.
• They are spindle-shaped.
• Function as phagocytic cells.
• Can change into macrophages and phagocytose foreign bodies or bacteria (brain macrophages).

Peripheral Nerves

• Analogous to tracts in the CNS
• Axons are sheathed by Schwann cells, which may or may not form myelin based on axon diameter.
• Nerve fibers can be myelinated or non-myelinated.

Myelinated Nerve Fibers

• The axon has a large diameter.
• The myelin sheath features concentric layers of lipoprotein.
• The sheath is formed by Schwann cells and insulates the axon.
• It provides a suitable environment for action potentials.
• Schwann cells are neurolemmocytes, form a chain of cells that cover the axon only in the PNS, and one cell forms myelin around only a single axon segment.

Unmyelinated Nerve Fibers

• The axon has a small diameter.
• There is no myelin sheath.
• Schwann cells surround portions of many axons.
• Impulse conduction is slower than in myelinated fibers.

Organization of Peripheral Nerves

• The epineurium is a dense irregular fibrous coat that surrounds the nerve from the outside.
• The perineurium surrounds bundles of nerve fibers.
• The endoneurium surrounds individual nerve fibers.

Ganglia Definition

• A collection of nerve cells and nerve fibers covered by a C. T. capsule outside the C. N. S.

Ganglia

• Craniospinal (sensory)
• Autonomic (motor): Sympathetic and Parasympathetic

Spinal Ganglia

• They are located on the dorsal roots of spinal nerves.
• Have a thick C.T. capsule.
• Contain nerve cells that are pseudo-unipolar.
• Nerve cell size is variable, either large or small.
• They contain few nerve cells arranged in groups or rows, numerous satellite cells which form a complete layer, and thick myelinated nerve fibers.

Sympathetic Ganglia

• Located on the sympathetic chain.
• Posses a thin C.T. capsule.
• Nerve cells are multipolar and small.
• There are numerous nerve cells, scattered in their arrangement.
• They contain few satellite cells (incomplete layer), and thin non-myelinated nerve fibres.

Spinal Ganglia and Sympathetic Ganglia: A Comparison

• Spinal ganglia are attached to spinal nerves, while sympathetic ganglia are attached to the sympathetic and parasympathetic chains.
• Spinal ganglia have a thick capsule, while sympathetic ganglia have a thin capsule.
• Spinal ganglia have pseudo-unipolar nerve cells, while sympathetic ganglia have multipolar nerve cells.
• Spinal ganglia have few nerve cells, while sympathetic ganglia have numerous nerve cells.
• Nerve cells are variable in size and arranged in groups in spinal ganglia, and are small and scattered in sympathetic ganglia.
• There are numerous satellite cells in spinal ganglia, while sympathetic ganglia have few satellite cells.
• Spinal ganglia have thick and myelinated nerve fibers, while sympathetic ganglia have thin and non-myelinated nerve fibers.