11 - Nervous Tissue

TARGET DECK: MED::I::Morphology and Development::Histology::11 - Nervous Tissue

Overview

Key Facts

One of the four main tissues of the body

Ectodermal origin (neural crest cells)

Main function: communication

Composition

Component	Role	Size Range
Neurons	Reception & transmission of impulses	$5 μ m$ – $150 μ m$
Neuroglia	Physical & metabolic support	—

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Nervous tissue has {1:ectodermal (neural crest cell)} origin and its main function is {1:communication}.

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Neurons range in size from {1:5 µm} (smallest) to {1:150 µm} (largest).

Anatomical Organization

Division	Structures
CNS	Brain + spinal cord
PNS	Cranial & spinal nerves + ganglia

PNS has afferent (sensory) and efferent (motor) components

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The CNS consists of the {1:brain and spinal cord}; the PNS consists of {1:cranial and spinal nerves and ganglia}.

Neurons

Definition

Neurons are the cells responsible for the reception and transmission of nerve impulses to and from the CNS.

Parts of a Neuron

Most neurons are composed of three distinct parts:

Cell body (soma / perikaryon)
Multiple dendrites
A single axon

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The three main parts of a neuron are: {1:cell body (soma)}, {1:dendrites}, and {1:a single axon}.

Cell Body (Soma / Perikaryon)

Perikaryon = Greek peri (around) + karyon (nucleus)
Contains the nucleus and surrounding cytoplasm
Specialized for reception and integration of information
The cell body is the most conspicuous region, but the largest volume of cytoplasm is actually in the processes

Nucleus

Large, usually spherical, centrally located
Finely dispersed chromatin → rich synthetic activity
Smaller neurons may have condensed, inactive heterochromatin
Well-defined nucleolus commonly present

Cytoplasm (Ultrastructure)

Abundant rough endoplasmic reticulum (RER) with parallel cisternae arrays — especially prominent in large motor neurons
Polyribosomes scattered throughout cytoplasm
Stacked RER cisternae + polyribosomes stain with basic dyes → basophilic clumps = Nissl bodies (visible under LM)
RER present in dendritic regions (scattered short/branching cisternae)
RER absent at the axon hillock — only SER is present in the axon

Nissl Bodies

Nissl bodies are clumps of RER + polyribosomes in the soma and dendrites. They are absent at the axon hillock — this is a key LM landmark.

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Nissl bodies are formed by stacked {1:rough endoplasmic reticulum cisternae} and {1:polyribosomes} stained with basic dyes.

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RER (Nissl substance) is absent at the {1:axon hillock}; only {1:smooth ER} is present in the axon.

Dendrites

Projections from the cell body specialized for receiving stimuli from sensory cells, axons, and other neurons
Often multibranched (arborized) → receive multiple stimuli simultaneously
Impulses received by dendrites are transmitted toward the soma
Form a dendritic tree collectively
Covered by small protrusions: dendritic spines → establish axonal synaptic connections

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Dendritic spines establish {1:axonal synaptic connections}.

Axon

Each neuron has a single axon
Varying diameter; up to 100 cm in length
Conducts impulses away from the soma to other neurons, muscles, or glands
May also receive stimuli from other neurons, modifying its behavior
Originates from the soma at the axon hillock
Ends in a terminal arborization = telodendron
Each terminal branch of the telodendron has an enlarged ending: synaptic terminal or synaptic bouton
Axon terminals (end bulbs) approach other cells to form synapses

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The axon originates from the soma at the {1:axon hillock} and terminates in the {1:telodendron}, whose enlarged endings are called {1:synaptic boutons}.

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The axon conducts impulses {1:away from} the soma; dendrites conduct impulses {1:toward} the soma.

Neuron Classification

Morphological Classification

Type	Processes	Location / Example
Multipolar	1 axon + ≥2 dendrites	Motor neurons, pyramidal cells (cerebral cortex), Purkinje cells (cerebellum) — most common
Bipolar	1 axon + 1 dendrite	Olfactory epithelium, retina, inner ear
Pseudounipolar	1 axon splitting into 2 branches	Sensory ganglia of PNS (dorsal root ganglion)

Pseudounipolar Detail

The single axon splits into:

One central process (→ CNS)

One peripheral process (from sensory endings)
Impulses generated in sensory endings travel through both branches to the CNS.

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The most common type of neuron is the {1:multipolar} neuron, which has one axon and {1:two or more} dendrites.

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Bipolar neurons are found in specialized tissues such as the {1:olfactory epithelium}, {1:retina}, and {1:inner ear}.

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Pseudounipolar neurons are found in the {1:sensory ganglia of the PNS (dorsal root ganglia)}.

Functional Classification

Type	Direction	Examples
Motor neurons (efferent)	CNS → effectors	Somatic efferent, visceral efferent
Sensory neurons (afferent)	Receptors → CNS	Somatic afferent, visceral afferent
Interneurons (intercalated)	Between sensory & motor	Communicating/integrating network

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Interneurons form a {1:communicating and integrating} network between {1:sensory and motor} neurons.

Neuroglia (“Nerve Glue”)

Info

Neuroglia are 10× more common than neurons. Main function: support.

Summary Table

Cell	Location	Function
Satellite cells	PNS (ganglia)	Surround neuron cell bodies; maintain controlled microenvironment; regulate $O_{2}$ , $C O_{2}$ , nutrients, neurotransmitter levels
Schwann cells	PNS (nerve fibers)	Myelinate PNS axons (1 Schwann cell : 1 axon); guide axon regrowth; form neurolemma
Oligodendrocytes	CNS	Myelinate CNS axons (1 oligodendrocyte : multiple axons); structural support
Astrocytes	CNS	Support/brace neurons; anchor to blood supply; guide migration; potassium spatial buffering; regulate ion/nutrient/gas concentrations; absorb & recycle neurotransmitters; form scar tissue
Microglia	CNS	Phagocytosis of debris, pathogens, dead cells; maintained by self-renewal
Ependymal cells	CNS (ventricles & central canal)	Line ventricles (brain) and central canal (spinal cord); assist CSF production & circulation; have motile cilia

Key Comparison: Myelination

Schwann cells (PNS): 1 cell myelinates 1 internode of 1 axon

Oligodendrocytes (CNS): 1 cell myelinates internodes of multiple axons

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In the PNS, myelination is performed by {1:Schwann cells}; in the CNS, by {1:oligodendrocytes}.

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A single oligodendrocyte can myelinate {1:multiple axons}, whereas a single Schwann cell myelinates only {1:one axon}.

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Astrocytes regulate extracellular potassium concentration through a process called {1:potassium spatial buffering}.

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Microglia maintain their population by {1:self-renewal} and are functionally/developmentally {1:unrelated to monocytes}.

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Ependymal cells line the {1:ventricles of the brain} and the {1:central canal of the spinal cord} and bear {1:motile cilia}.

Astrocyte Subtypes

Subtype	Location
Fibrous astrocytes	White matter
Protoplasmic astrocytes	Gray matter

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Fibrous astrocytes are found in {1:white matter}; protoplasmic astrocytes are found in {1:gray matter}.

Shared Functions (CNS ↔ PNS Analogues)

Function	PNS Cell	CNS Cell
Myelination	Schwann cells	Oligodendrocytes
Ion/nutrient/gas regulation	Satellite cells	Astrocytes

Myelin Sheath

Structure

Myelin = plasmalemma of the Schwann cell (or oligodendrocyte) wrapped multiple times around the axon
Prevents leakage of the action potential
EM: alternating lines at $12 nm$ intervals:
- Major dense line — fused cytoplasmic surfaces of the Schwann cell membrane
- Intraperiod line — apposing outer leaflets of the Schwann cell membrane

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The major dense line of myelin represents the fused {1:cytoplasmic surfaces} of the Schwann cell plasma membrane.

Myelin Formation

Oligodendrocyte (or Schwann cell) concentrically wraps its membrane around the axon
Wrapping may continue for >50 turns
During wrapping, cytoplasm is squeezed back → cytoplasmic surfaces contact each other → major dense line

Developmental Note

Motor nerves: nearly completely myelinated at birth

Sensory roots: myelinated several months after birth

Some CNS tracts: not fully myelinated until several years after birth

Nerves are not myelinated simultaneously during development

Unmyelinated Nerve Fibers

Predominate in gray matter; axons are thin
In PNS: a single Schwann cell houses several unmyelinated axons in individual cytoplasmic invaginations — no myelin produced
Entire axolemma is freely exposed to interstitial tissue; partially protected by a basal lamina surrounding the Schwann cell

Conduction Speed Comparison

Type	Mechanism	Max Speed
Myelinated	Saltatory conduction (node to node)	$120 m / s$
Unmyelinated	Continuous conduction	$15 m / s$

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Myelinated nerves conduct impulses via {1:saltatory conduction} at up to {1:120 m/s}; unmyelinated nerves conduct continuously at up to {1:15 m/s}.

Peripheral Nerves

Connective Tissue Investments

Peripheral nerves are bundles of nerve fibers (axons) surrounded by three connective tissue sheaths:

Layer	Coverage	Composition
Epineurium	Entire nerve	Dense irregular collagenous CT; type I collagen + fibroblasts; contains arteries, veins, lymphatics
Perineurium	Each fascicle	Dense CT (thinner than epineurium); several concentric layers of neuroepithelial perineurial cells joined by tight junctions → forms blood-nerve barrier; basal lamina of type IV collagen + laminin
Endoneurium	Individual axons	Type III collagen fibrils; few fibroblasts, macrophages, mast cells, endoneurial capillaries

Blood-Nerve Barrier

The perineurium is responsible for maintaining the homeostatic microenvironment of the endoneurium via tight junctions between perineurial cells.

Mnemonic — Nerve Layers (outside → in)

“Every Penguin Enjoys” = Epineurium → Perineurium → Endoneurium

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From outermost to innermost, the connective tissue layers of a peripheral nerve are: {1:epineurium}, {1:perineurium}, and {1:endoneurium}.

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The blood-nerve barrier is formed by {1:tight junctions} between {1:perineurial cells} of the perineurium.

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The endoneurium contains {1:type III collagen} and surrounds {1:individual axons and their Schwann cells}.

Virtual microscopy links (UniBo):

Synapses

Definition

Synapses are sites where nerve impulses are transmitted from a presynaptic cell to a postsynaptic cell. Transmission can be electrical or chemical; chemical synapses are the most common.

Types of Synaptic Contacts

Type	Connection
Axodendritic	Axon → dendrite
Axosomatic	Axon → soma
Axoaxonic	Axon → axon
Dendrodendritic	Dendrite → dendrite

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The four main types of synaptic contacts are: {1:axodendritic}, {1:axosomatic}, {1:axoaxonic}, and {1:dendrodendritic}.

Structure of the Synapse

Presynaptic terminal (bouton terminal): bulbous expansion at the axon end
Presynaptic cytoplasm contains:
- Mitochondria
- Elements of smooth ER
- Abundance of synaptic vesicles ( $40$ – $60 nm$ diameter) filled with neurotransmitter
Peptide neurotransmitters are manufactured in the cell body and transported to the terminal via anterograde transport
Non-peptide neurotransmitters are manufactured and packaged near the axon terminal
Enzymes in axoplasm protect neurotransmitters from degradation
Cone-shaped presynaptic densities project from the membrane into the cytoplasm → form the active site
A reserve pool of synaptic vesicles adheres to actin microfilaments
Cell adhesion molecules (CAMs) act as signaling molecules at both pre- and postsynaptic membranes

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Synaptic vesicles are {1:40–60 nm} in diameter and are filled with {1:neurotransmitter}.

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Peptide neurotransmitters are synthesized in the {1:cell body} and transported to the axon terminal by {1:anterograde transport}.

Mechanism of Neurotransmitter Release

Action potential reaches the presynaptic membrane
Voltage-gated $C a^{2 +}$ channels open → $C a^{2 +}$ enters
$C a^{2 +}$ influx causes synaptic vesicles (under SNARE protein influence) to fuse with the presynaptic membrane
Neurotransmitter released into the synaptic cleft via exocytosis
Excess membrane recaptured via clathrin-mediated endocytosis
Endocytic vesicle fuses with smooth ER → membrane recycled

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Synaptic vesicle fusion with the presynaptic membrane is triggered by {1:Ca²⁺ influx} and mediated by {1:SNARE proteins}.

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After exocytosis, excess presynaptic membrane is recaptured by {1:clathrin-mediated endocytosis}.

Postsynaptic Events

Postsynaptic membrane contains neurotransmitter receptors (ligand-gated ion channels)
Neurotransmitter binding → ion channel opening → altered membrane permeability → reversal of membrane potential
Depolarization = excitatory response
Hyperpolarization = inhibitory response
Glial cells increase synaptogenesis, synaptic efficacy, and action-potential firing

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Binding of a neurotransmitter to postsynaptic receptors causes either {1:depolarization} (excitatory) or {1:hyperpolarization} (inhibitory) of the postsynaptic membrane.

Autonomic Nervous System (ANS)

Overview

The ANS is a motor system controlling viscera by innervating smooth muscle, cardiac muscle, and glands.
Unlike the somatic system (1 neuron: CNS → effector), the ANS has 2 neurons between CNS and effector organ.

Divisions of the ANS

Feature	Sympathetic	Parasympathetic
Physiological role	”Fight or flight”	Homeostasis
Heart rate	↑	↓
Blood pressure	↑	↓
Respiration	↑	↓
Blood flow to skeletal muscle	↑	↓
Pupils	Dilated	Constricted
Visceral function	↓	↑
Postganglionic neurotransmitter	Noradrenaline (adrenergic)	Acetylcholine (cholinergic)

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The sympathetic nervous system prepares the body for {1:“fight or flight”}; the parasympathetic nervous system promotes {1:homeostasis}.

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In contrast to the somatic motor system (1 neuron CNS → effector), the ANS uses {1:two neurons} between the CNS and the effector organ.

CNS Organization

Gray Matter vs. White Matter

	Gray Matter	White Matter
Contents	Neuronal cell bodies	Myelinated axon fibers
Location in cerebrum	Cerebral cortex (outermost layer)	Inside
Location in spinal cord	Inside (H-shape)	Outside

Brain Regions

Cerebral cortex: contains several layers of pyramidal neurons
Cerebellar cortex: contains Purkinje cells and granule cells

Spinal Cord

Gray matter: H-shaped, inside
White matter: outside
Central canal: lined by ependymal cells
Dorsal: sensory components
Ventral: motor components

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In the spinal cord, gray matter is located {1:inside (H-shaped)} and white matter is located {1:outside}; this is the {1:reverse} of the arrangement in the cerebrum.

PNS Organization Summary

Ganglia

Type	Neurons	Glial Cells
Autonomic ganglia	Multipolar neurons	Satellite cells
Sensory ganglia (e.g., DRG)	Pseudounipolar neurons	Satellite cells

Nerves

Bundles of nerve fibers surrounded by endoneurium, perineurium, and epineurium (see Peripheral Nerves section above).

📋 TLDR — Nervous Tissue Summary

Nervous tissue is of ectodermal (neural crest) origin; composed of neurons ( $5$ – $150 μ m$ ) and neuroglia (10× more numerous).
The CNS = brain + spinal cord; PNS = cranial/spinal nerves + ganglia (afferent/efferent).
Neurons have: soma (nucleus, Nissl bodies/RER, perinuclear cytoplasm), dendrites (receive impulses, toward soma), single axon (transmit impulses, away from soma, up to 100 cm).
Nissl bodies (RER + polyribosomes) are absent at the axon hillock.
Morphological types: multipolar (most common), bipolar (retina/olfactory/ear), pseudounipolar (sensory ganglia).
Functional types: motor (efferent), sensory (afferent), interneurons (integrating).
Neuroglia: satellite cells & Schwann cells (PNS); astrocytes, oligodendrocytes, microglia, ependymal cells (CNS).
Myelin: formed by Schwann cells (PNS, 1:1) and oligodendrocytes (CNS, 1:many); prevents AP leakage; saltatory conduction up to $120 m / s$ vs. $15 m / s$ unmyelinated.
Peripheral nerve layers (out → in): epineurium → perineurium (blood-nerve barrier, tight junctions) → endoneurium (surrounds individual axons).
Synapses: presynaptic terminal has vesicles ( $40$ – $60 nm$ ) + SNARE proteins; $C a^{2 +}$ influx triggers exocytosis; postsynaptic response = depolarization (excitatory) or hyperpolarization (inhibitory).
ANS: 2-neuron chain; sympathetic = fight-or-flight (adrenergic); parasympathetic = homeostasis (cholinergic).
CNS: gray matter (cell bodies) = cortex (brain) / inside (spinal cord); white matter (myelinated axons) = inside (brain) / outside (spinal cord).

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11 - Nervous Tissue

Overview

Composition

Anatomical Organization

Neurons

Parts of a Neuron

Cell Body (Soma / Perikaryon)

Nucleus

Cytoplasm (Ultrastructure)

Dendrites

Axon

Neuron Classification

Morphological Classification

Functional Classification

Neuroglia (“Nerve Glue”)

Summary Table

Astrocyte Subtypes

Shared Functions (CNS ↔ PNS Analogues)

Myelin Sheath

Structure

Myelin Formation

Unmyelinated Nerve Fibers

Conduction Speed Comparison

Peripheral Nerves

Connective Tissue Investments

Synapses

Types of Synaptic Contacts

Structure of the Synapse

Mechanism of Neurotransmitter Release

Postsynaptic Events

Autonomic Nervous System (ANS)

Divisions of the ANS

CNS Organization

Gray Matter vs. White Matter

Brain Regions

Spinal Cord

PNS Organization Summary

Ganglia

Nerves

📋 TLDR — Nervous Tissue Summary

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