Neural Tissue: Structure, Function, and Organization of the Nervous System

Neural Tissue

Introduction to Neural Tissue

Neural tissue is a specialized tissue found in the nervous system, responsible for transmitting electrical and chemical signals throughout the body. It is essential for sensation, movement, cognition, and homeostasis.

• Functions: Sensory input, integration of information, motor output, and regulation of bodily functions.

• Main Divisions: Central Nervous System (CNS) and Peripheral Nervous System (PNS).

Organization of the Nervous System

CNS and PNS Structure and Function

• Central Nervous System (CNS): Composed of the brain and spinal cord; integrates and processes information.

• Peripheral Nervous System (PNS): Consists of cranial and spinal nerves; transmits signals between the CNS and the rest of the body.

• Functional Divisions:

  • Somatic Nervous System: Controls voluntary movements via skeletal muscles.

  • Autonomic Nervous System: Regulates involuntary functions (e.g., heart rate, digestion); subdivided into sympathetic and parasympathetic divisions.

Cell Types in Neural Tissue

Neurons

Neurons are the primary signaling cells of the nervous system, specialized for the conduction of electrical impulses.

• Structure:

  • Cell Body (Soma): Contains the nucleus and Nissl bodies (rough endoplasmic reticulum).

  • Dendrites: Receive incoming signals.

  • Axon: Conducts impulses away from the cell body; may be myelinated or unmyelinated.

  • Axon Hillock: Site of action potential initiation.

• Classification:

  • Multipolar Neurons: Many dendrites, one axon (most common in CNS).

  • Bipolar Neurons: One dendrite, one axon (e.g., retina, olfactory epithelium).

  • Unipolar (Pseudounipolar) Neurons: Single process splits into two branches (sensory neurons in PNS).

Glial Cells

Glial cells support, protect, and nourish neurons. They are more numerous than neurons in the CNS.

• Astrocytes: Maintain blood-brain barrier, regulate ion balance, provide metabolic support.

• Oligodendrocytes: Form myelin sheaths in the CNS.

• Microglia: Immune cells of the CNS; phagocytose debris and pathogens.

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

• Schwann Cells: Form myelin sheaths in the PNS.

• Satellite Cells: Support neurons in PNS ganglia.

Neuron Structure and Microanatomy

Key Features of Neurons

• Nucleus: Large, euchromatic with prominent nucleolus (indicates high metabolic activity).

• Nissl Bodies: Aggregates of rough ER; site of protein synthesis.

• Dendritic Spines: Small protrusions on dendrites; sites of synaptic input.

Motor Neurons

• Function: Transmit impulses from CNS to muscles or glands.

• Myelin Sheath: Produced by oligodendrocytes (CNS) or Schwann cells (PNS); increases conduction velocity.

Synapses and Neurotransmission

Synapse Structure and Function

• Synapse: Specialized junction where a neuron communicates with another cell.

• Types:

  • Axodendritic: Axon to dendrite.

  • Axosomatic: Axon to cell body.

  • Axoaxonic: Axon to axon.

• Mechanism:

  • Action potential arrives at presynaptic terminal.

  • Calcium influx triggers neurotransmitter release.

  • Neurotransmitter binds to receptors on postsynaptic membrane, altering membrane potential.

Excitatory vs. Inhibitory Synapses

• Excitatory Synapses:

  • Neurotransmitter (e.g., glutamate) opens cation channels.

  • Depolarization leads to action potential generation.

• Inhibitory Synapses:

  • Neurotransmitter (e.g., GABA) opens anion channels.

  • Hyperpolarization makes action potential generation less likely.

Neurotransmitters

Major Neurotransmitters and Their Functions

• Acetylcholine (ACh): Used by cholinergic neurons; acts on muscarinic and nicotinic receptors.

• Catecholamines:

  • Norepinephrine (NE), Epinephrine (EPI), Dopamine (DA): Synthesized from tyrosine; used by adrenergic neurons.

• Serotonin (5-HT): Synthesized from tryptophan; involved in mood, sleep, and cognition.

• Amino Acid Neurotransmitters:

  • GABA, Glycine (GLY): Inhibitory in CNS.

  • Glutamate (GLU), Aspartate (ASP): Excitatory in CNS.

White Matter and Gray Matter

Distinction and Significance

• White Matter: Composed mainly of myelinated axons; responsible for signal transmission.

• Gray Matter: Contains neuronal cell bodies, dendrites, and unmyelinated axons; site of synaptic integration.

• Location: In the spinal cord, gray matter is central (butterfly-shaped), white matter is peripheral; in the brain, gray matter forms the cortex and nuclei, white matter is internal.

Myelin Sheath and Nodes of Ranvier

Formation and Function

• Myelin Sheath: Multilayered lipid-rich covering that insulates axons, increasing conduction velocity.

• Formation:

  • CNS: Oligodendrocytes myelinate multiple axons.

  • PNS: Schwann cells myelinate a single axon segment.

• Nodes of Ranvier: Gaps in the myelin sheath where action potentials are regenerated (saltatory conduction).

Saltatory Conduction Equation:

Conduction velocity increases with myelination and axon diameter:

$v \propto \sqrt{d}$

where $v$ is conduction velocity and $d$ is axon diameter.

• Unmyelinated Fibers: Slower conduction; Schwann cells envelop but do not myelinate multiple axons.

Glial Cells of the CNS and PNS

Types and Functions

• Astrocytes: Support neurons, regulate extracellular environment, form blood-brain barrier.

• Oligodendrocytes: Myelinate CNS axons.

• Microglia: Immune defense in CNS.

• Ependymal Cells: Line ventricles, produce CSF.

• Schwann Cells: Myelinate PNS axons.

• Satellite Cells: Support neurons in PNS ganglia.

Connective Tissue Coverings of Nerves

Organization in the PNS

• Endoneurium: Surrounds individual axons.

• Perineurium: Surrounds bundles (fascicles) of axons.

• Epineurium: Outermost layer, surrounds entire nerve.

Layer	Location	Function
Endoneurium	Around individual axons	Provides support and insulation
Perineurium	Around fascicles	Maintains blood-nerve barrier
Epineurium	Around entire nerve	Protects nerve from external damage

Specialized Neurons and Pathology

Pyramidal and Purkinje Cells

• Pyramidal Neurons: Found in cerebral cortex; involved in motor control and cognition.

• Purkinje Cells: Located in cerebellum; regulate motor coordination.

Clinical Example: Cerebellar Agenesis

• Definition: Absence of the cerebellum from birth.

• Imaging: MRI scans reveal missing cerebellar tissue.

• Symptoms: Motor deficits, balance issues, but variable cognitive effects.

Summary Table: Neuron and Glial Cell Types

Cell Type	Location	Function
Neuron	CNS & PNS	Signal transmission
Astrocyte	CNS	Support, blood-brain barrier
Oligodendrocyte	CNS	Myelination
Microglia	CNS	Immune defense
Ependymal cell	CNS	CSF production
Schwann cell	PNS	Myelination
Satellite cell	PNS	Support in ganglia

Key Terms and Definitions

• Action Potential: Rapid change in membrane potential that propagates along the axon.

• Synapse: Junction between two neurons or a neuron and another cell.

• Myelin: Lipid-rich sheath that insulates axons.

• Node of Ranvier: Gap in myelin sheath where action potentials are regenerated.

• Neurotransmitter: Chemical messenger released at synapses.

Additional info:

• Some details, such as the conduction velocity equation and the summary tables, were inferred and expanded for academic completeness.

• Images referenced in the slides were described and their academic context provided.