Nervous Tissue Histology

Unit 6: Nervous Tissue & Nervous System Histology

This unit provides an overview of nervous system histology, focusing on the cellular components andtheir organization.

6.1 Introduction to the Nervous System

• Santiago Ramón y Cajal:Recognized as the Father of modern neuroscience.

• Key contributions: Discovered the axonal growth cone and provided evidence for the neuron theory.

• Nobel laureate in Medicine (1906) along side Camillo Golgi.

Functions of the Nervous System

• Sensory Function: Carried out by sensory or afferent neurons, receiving stimuli from internal and external environments.

• Integrative Function: Performed by interneurons, processing and interpreting sensory information.

• Motor Function: Executed by motor or efferent neurons, sending commands to effectors.

• Effectors: Muscles and glands that respond to motor commands.

6.2 Histology of the Nervous Tissue

6.2.1 Types of Cells

The nervous system is composed of two main types of cells: Neurons and Neuroglia.

Neurons: The Structural and Functional Units

• Highly specialized and excitable cells.

• Electrical excitability: Respond to stimuli by generating action potentials.

• Mature neurons generally do not undergo mitosis (cell division).

• New neurons can arise from neural stem cells inspecific brain regions.

• Energy source: Primarily glucose.

• Regeneration: Limited; the part of an axon connected to the soma can regenerate over short distances if cut.

Structure & Function of Neurons

1. Cell Body (Soma/Perikaryon)

• Functions: Nutrition and metabolism, reception of signals.

• Structure:

  • Single nucleus with a prominent nucleolus.

  • Cytoplasm contains Nissl bodies (RER), lysosomes, mitochondria, Golgi apparatus, and synaptic vesicles.

  • Contains a cytoskeleton (microtubules, neurofibrils).

  • Siteof high protein synthesis.

2. Dendrites

• Functions: Reception of signals via membrane receptors.

• Structure:

  • Highly branched processes, often covered by dendritic spines.

  • "Dendron" means "tree".

  • Cytoplasm contains Nissl bodies and mitochondria.

3. Axon

• Functions: Propagation of signals (nerve impulses).

• Structure:

  • Long, thin & cylindrical projection.

  • Axoplasm (cytoplasm): Contains mitochondria, microtubules, neurofibrils; no protein synthesis.

  • Axolemma: Cell membrane of the axon.

  • Axon Hillock (AH): Cone-shaped region connecting axon to soma.

  • Initial Segment (IS): Closest to AH.

  • Trigger Zone: Junction between AH and IS, where action potentials typically originate.

  • Axon Collaterals: Side branches.

  • Axon Terminals (Telodendria): End branches containing synaptic vesicles filled withneurotransmitters.

  • Synaptic End Bulbs and Varicosities: Swellings at the axon terminals that store neurotransmitters.

Synapse

• A neuronal junction; the site of communication between aneuron and its effector.

• Categorized by connection: Axo-somatic, Axo-dendritic, Axo-axonic.

• Neurotransmitter is the chemical released atthe synapse.

Axonal Transport

• Purpose: Transport of neurotransmitters and repair proteins along the axon.

• Slow Axonal Flow:

  • Anterograde: Away from cell body.

  • Movement: 0.2-4 mm/day (cytosolic proteins).

• Fast Axonal Flow:

  • Movement: 200-400 mm/day(cargoes vesicles), uses ATP and microtubules.

  • Anterograde: Soma → axon terminal.

  • Retrograde: Axon terminal → soma.

Types of Neurons

1. Structural Classification (by number of processes)

• Multipolar: Several dendrites, 1 axon (most common type).

• Bipolar: 1 main dendrite, 1 axon (e.g., retina, inner ear, olfactoryarea).

• Unipolar (Pseudounipolar): 1 process that branches into dendritic and axonal parts (e.g., sensory receptors, soma in ganglia).

2. Functional Classification

• Sensory (Afferent) Neurons:

  • Receive signals from environment, skin, muscles, joints, organs.

  • Transmit signals towards the CNS.

  • Primary receptor function.

• Interneurons (AssociationNeurons):

  • Connect sensory to motor neurons.

  • Located in the CNS.

  • Constitute 90% of all neurons.

  • Perform integrative or associative functions.

• Motor (Efferent) Neurons:

  • Send signals from CNS to muscles and glands (effectors).

  • Conducting function.

Neuroglia (Glial Cells)

• Smaller than neurons and 25 times more numerous.

• Do not propagate action potentials.

• Possess the ability to divide.

Neuroglia of the Central Nervous System (CNS)

1. Astrocytes

• Star-shaped cells. Two types: fibrous (white matter) and protoplasmic (gray matter).

• Functions:

  • Form part of the blood-brain barrier (BBB) by covering blood capillaries.

  • Provide structural support.

  • Protect neurons from toxic substances.

  • Metabolize neurotransmitters.

  • Regulate chemical environment (Ca²⁺, K⁺ balance).

  • Involved in memory and learning (synapses modulation).

  • Secrete neurotrophic factors for growth and cellular migration.

  • Crucial for brain vascularization.

2. Oligodendrocytes

• Most common glial cell type in theCNS.

• Function: Form and maintain myelin sheath around axons in the CNS.

• Analogous to Schwann cells in the PNS.

3. Microglia

• Derived from cells of the immune system (mesoderm).

• Small cells with few projections, found near blood vessels.

• Functions:

  • Phagocytic role: Clear away dead cells, bacteria, and biological debris.

  • Cytotoxic: Release substances (ROS) that eliminate bacteria/viruses.

4. Ependymal Cells

• Epithelial cells (simple cuboidal to columnar) with microvilli and cilia, forming the EPENDYMA.

• Functions:

  • Produce, monitor, and circulate cerebrospinal fluid (CSF).

  • Form the epithelial lining of brain ventricles and the central canal of the spinal cord.

  • Forma leaky barrier between CSF and interstitial fluid.

  • Form the choroid plexus (selectively permeable barrier with blood vessels).

  • Can acquire neural stem cell properties.

Neuroglia of the Peripheral Nervous System (PNS)

1. Satellite Cells

• Flat cells surrounding neuronal cell bodies in peripheral ganglia.

• Functions:

  • Support neurons in PNSganglia.

  • Regulate exchange of substances between neuronal soma and interstitial fluid.

  • Participate in signaling processing and transmission in sensory ganglia.

2. Schwann Cells

• Non-specific shaped cells.

• Functions:

  • Myelination of axons in the PNS. Each Schwann cell surrounds only one axon in the PNS.

  • Crucial for regeneration of PNSneurons.

  • Non-myelinating Schwann cells can enclose many unmyelinated axons.

6.2.2 Myelination

• Myelin Sheath: A multi-layered lipidand protein covering that electrically insulates axons.

• Purpose: Increases the speed of nerve impulse propagation.

• Formed by Oligodendrocytes in the CNS and Schwann Cells in thePNS.

Schwann Cells vs. Oligodendrocytes Summary

6.2.3 Components of Nervous Tissue Grouped Together

Grey and White Matter

• White Matter: Composed primarily of myelinated processes, appearing white due to myelin.

• Gray Matter: Contains neuron cell bodies, dendrites, axon terminals, unmyelinated axons, and neuroglia; appears darker.

• Spinal Cord: Gray matter forms an H-shaped inner core surrounded by white matter.

• Brain:A thin outer shell of gray matter covers the surface (cortex); also found in clusters called nuclei inside the CNS.

Neuronal Cell Bodies form Clusters

• Within the PNS: Ganglion (ganglia).

• Within the CNS: Nucleus (nuclei).

Axons form Bundles

• Within the PNS: Nerves (e.g., cranial nerves, spinal nerves).

• Within the CNS: Tracts.

Main Structures of the PNS

• Nerves: Bundles of thousands of axons outside the CNS.

  • 12 pairs ofcranial nerves emerge from the brain.

  • 31 pairs of spinal nerves emerge from the spinal cord.

• Ganglia: Small masses of nervous tissue outside the brain and spinal cord, primarily containing neuronal cell bodies.

• Enteric Plexuses: Networks of nerves in the gastrointestinal tract, often called the "brain of the gut".

• Sensory Receptors: Specialized cells or parts of neurons that monitor internal/external environment changes.