Exercise 15 – Histology of Nervous Tissue

Lab Materials & Preparatory Resources

• Needed for Exercise 15:

  • Neuron model (if available)

  • Compound light microscope with oil-immersion feature

  • 100\times oil-immersion lens and oil

  • Prepared slides:

    • Ox spinal-cord smear (for large motor neurons)

    • Teased myelinated peripheral nerve fibers (to see Schwann cells)

    • Purkinje cells (from cerebellum)

    • Pyramidal cells (from cerebral cortex)

    • Dorsal-root ganglion (for sensory pseudounipolar neurons)

    • Peripheral nerve, cross-section (to study layers)

• Digital learning tools (Mastering A&P → Study Area → Lab Tools):

  • Practice Anatomy Lab, Building Vocabulary, Pre-Lab Quiz, Art-Labeling, PAL practical questions

Learning Outcomes & Competency Checklist

• Understand the different roles of neurons (nerve cells) and neuroglia (supporting cells).

• List the 6 types of neuroglial cells and know if they are in the CNS (Central Nervous System) or PNS (Peripheral Nervous System).

• Identify and label all visible parts of a neuron, both large and microscopic.

• Describe what dendrites, axons, and axon terminals do.

• Track how a nerve signal crosses a synapse (connection point).

• Explain why myelin sheaths are important and how Schwann cells create myelin in the PNS.

• Classify neurons by their structure (multipolar, bipolar, unipolar) and their function (sensory, motor, interneuron).

• Tell the difference between:

  • A Nerve vs a Tract

  • A Ganglion vs a CNS nucleus

• Identify endoneurium, perineurium, and epineurium on a microscope image and explain their roles.

Key Terminology & Abbreviations

• CNS = Central Nervous System (brain & spinal cord)

• PNS = Peripheral Nervous System (all nerves and structures outside CNS)

• Neuroglia = “nerve glue”; supporting cells that do not carry electrical signals

• Myelin sheath gaps = Nodes of Ranvier (gaps in myelin)

• Chromatophilic substance = Nissl bodies (clusters of rough ER in neurons)

• Axon hillock = Cone-shaped area where the axon originates from the cell body

Overview of the Nervous System

• Main control system of the body; it takes in external and internal signals, stores information, and produces responses (movement, hormones).

• Two main parts:

  • CNS: Brain and spinal cord; handles information processing.

  • PNS: Includes sensory receptors, nerves (axon bundles), and ganglia (neuron cell body clusters).

Neuroglia (Glial Cells)

• General characteristics:

  • More numerous than neurons; they protect, support, and provide nutrients.

  • They DO NOT generate or send electrical signals.

• CNS neuroglia (Figure 15.1 a–d):

  • Astrocytes ▸ Most common type; manage ion and neurotransmitter levels outside neurons, help nutrients pass between capillaries and neurons; star-shaped cells with “end-feet” that wrap around capillaries and neurons.

  • Oligodendrocytes ▸ Form myelin around CNS axons; one cell can myelinate several axon segments.

  • Microglial cells ▸ Immune cells of the CNS; act as scavengers, removing waste and germs.

  • Ependymal cells ▸ Line brain ventricles and the central canal of the spinal cord; they produce and circulate cerebrospinal fluid (CSF).

• PNS neuroglia (Figure 15.1 e):

  • Schwann cells (neurolemmocytes) ▸ Form myelin around PNS axons; crucial for nerve repair.

  • Satellite cells ▸ Surround neuron cell bodies in PNS ganglia; help control the environment (nutrients, ions) around these bodies.

Neurons: Anatomy & Histology

• Basic working unit of the nervous system; highly excitable, they transmit nerve impulses.

• Common microscopic parts (Figure 15.2):

  • Cell body (soma): The main part, responsible for making proteins and receiving signals.

  • Prominent nucleus with nucleolus (often called an “owl’s-eye” due to its appearance).

  • Neurofibrils ▸ Provide structural support and help transport materials inside the cell.

  • Chromatophilic substance ▸ Rough ER clusters where proteins are made.

  • Processes (extensions):

    • Dendrites ▸ Many short, branched extensions; they are the main receiving areas for signals (contain receptors); they carry small electrical changes (graded potentials) toward the cell body.

    • Axon (nerve fiber) ▸ A single, often long extension; it generates and conducts nerve impulses.

      • Axon hillock ▸ The starting point of the axon from the cell body, where the electrical signal (action potential) begins.

      • Collaterals ▸ Occasional branches coming off the main axon.

      • Axon terminals ▸ The end parts of the axon that release neurotransmitters (chemical messengers).

      • Synapse ▸ The junction where a neuron connects with another neuron or an effector (muscle/gland); there’s a small gap (\approx 20\,\text{nm}) called the synaptic cleft; signals travel in one direction here using chemicals.

Myelination

• Myelin ▸ A fatty, protein-rich covering that electrically insulates axons, which greatly increases the speed of nerve impulse conduction (called saltatory conduction).

• PNS myelination process (Figure 15.3):

  1. A Schwann cell wraps around an axon.

  2. The Schwann cell keeps rotating, wrapping its plasma membrane repeatedly.

  3. The cell's cytoplasm is squeezed out, forming a compact myelin sheath; the remaining cytoplasm forms the outer layer (neurolemma).

• Myelin sheaths are not continuous; they have Nodes of Ranvier (gaps) at regular spaces.

• CNS myelination is done by oligodendrocytes; one glial cell can myelinate >60 axon segments.

• Why gaps are important:

  • Voltage-gated \text{Na}^+ channels are concentrated at these gaps, allowing the impulse to jump from node to node (saltatory conduction), which saves energy and speeds up signal transmission.

Neuron Classification (Structural)

• Based on the number of extensions from the cell body (Figure 15.5):

  • Multipolar ▸ Have many dendrites and one axon; most common type in the CNS (e.g., pyramidal cells, Purkinje cells); includes most motor neurons.

  • Bipolar ▸ Have one dendrite and one axon; rare, found in specialized sensory areas (like retina, inner ear).

  • Unipolar (pseudounipolar) ▸ Have a single process that branches into two (one going to the PNS, one to the CNS); cell bodies are in dorsal-root ganglia; they act as sensory neurons; the peripheral branch acts like a dendrite.

Functional Classification of Neurons

• Sensory (afferent)

  • Carry impulses from receptors to the CNS.

  • Mostly unipolar; their cell bodies are in PNS ganglia.

• Motor (efferent)

  • Carry impulses from the CNS to effectors (muscles, glands, organs).

  • Multipolar; their cell bodies are in the CNS (gray matter).

• Interneurons (association neurons)

  • Found only within the CNS; they connect sensory and motor pathways.

  • Make up over 99\% of all neurons; multipolar and highly branched.

Structure of Peripheral Nerve & Connective-Tissue Wrappings

• In the PNS, axon bundles are called nerves; in the CNS, they are called tracts.

• Three layers of connective tissue surrounding nerves (Figure 15.8):

  • Endoneurium ▸ A delicate connective tissue layer around each individual axon (myelinated or unmyelinated) and its myelin sheath; provides independent electrical insulation; contains capillaries.

  • Perineurium ▸ A thicker connective tissue layer that wraps around a bundle of axons (called a fascicle); it helps maintain the blood–nerve barrier.

  • Epineurium ▸ A tough, irregular connective tissue layer that encloses multiple fascicles to form the entire nerve; it connects to the dura mater closer to the CNS.

• Nerves are classified by signal direction:

  • Sensory nerves (carry only afferent signals) – rare (some cranial nerves).

  • Motor nerves (carry only efferent signals) – e.g., ventral roots of the spinal cord.

  • Mixed nerves – most peripheral nerves, including all 31 pairs of spinal nerves, carry both sensory and motor signals.

Laboratory Activities & Microscopy Tips

• Activity 1: Identify neuron parts on a model and ox spinal-cord smear

  • Look for the prominent nucleolus, chromatophilic substance, and tell the difference between a dendrite and an axon.

• Teased peripheral nerve (longitudinal view)

  • Identify Schwann cell nuclei (long and thin), the myelin sheath, and Nodes of Ranvier; note that the nodes are fairly regularly spaced for saltatory conduction.

• Activity 2: Examine specialized neurons

  • Pyramidal cells (cerebral cortex): Have a triangular cell body and a long dendrite pointing upwards; classified as multipolar.

  • Purkinje cells (cerebellum): Have a large flask-shaped cell body with many branching dendrites; multipolar.

  • Dorsal-root ganglion cells: Have a round cell body surrounded by satellite cells; unipolar (pseudounipolar).

• Activity 3: Peripheral nerve cross-section

  • Tell apart the myelinated axon (dark ring), endoneurium (thin light ring), perineurium (thicker boundary around axon bundles), and epineurium (outermost layer).

Pre-Lab Quiz: Question Breakdown & Correct Answers

1. Neuroglia of PNS include … c. satellite cells and Schwann cells

2. Branching receptive neuron processes … c. dendrites

3. Axons running through CNS form tracts of white matter

4. Neuron with many processes … b. Multipolar

5. Within a nerve, each axon covered by … a. endoneurium

Concept Connections & Practical / Clinical Notes

• Schwann-cell myelination failure (e.g., Guillain-Barré syndrome) → leads to blocked signals → muscle weakness.

• Multiple sclerosis attacks CNS myelin (oligodendrocytes), affecting white-matter tracts; symptoms depend on which tracts are damaged (vision, movement, sensation).

• Regeneration: PNS axons can regrow through a