Nervous Tissue: Structure, Function, and Support in Anatomy & Physiology

Lesson 12: Nervous Tissue

12.1 Overview of the Nervous System

The nervous system is a complex network responsible for coordinating body activities and responding to internal and external stimuli. It is divided into major anatomical and functional subdivisions.

• Central Nervous System (CNS): Consists of the brain and spinal cord. It is the main control center for processing information.

• Peripheral Nervous System (PNS): Composed of nerves and ganglia outside the CNS.

  • Nerve: A bundle of nerve fibers (axons) wrapped in fibrous connective tissue.

  • Ganglion: A knot-like swelling in a nerve where neuron cell bodies of the PNS are concentrated.

Functional Divisions of the Peripheral Nervous System

• Sensory (afferent) division: Carries signals from receptors to the CNS.

  • Somatic sensory division: Signals from skin, muscles, bones, and joints.

  • Visceral sensory division: Signals from internal organs (viscera) such as heart, lungs, stomach, and urinary bladder.

• Motor (efferent) division: Carries signals from the CNS to effectors (glands and muscles).

  • Somatic motor division: Signals to skeletal muscles; controls voluntary muscle contraction and reflexes.

  • Visceral motor division (Autonomic Nervous System, ANS): Signals to glands, cardiac, and smooth muscle; controls involuntary responses called visceral reflexes.

    • Sympathetic division: Prepares the body for action ("fight or flight").

    • Parasympathetic division: Calms the body and conserves energy.

    • Enteric plexus: Coordinates and communicates within the digestive tract.

12.2 Properties of Neurons

Neurons are specialized cells that transmit electrical and chemical signals throughout the body. They possess unique properties and can be classified by function and structure.

Universal Properties of Neurons

• Excitability: Ability to respond to stimuli.

• Conductivity: Ability to produce and transmit electrical signals.

• Secretion: Release of neurotransmitters to communicate with other cells.

Functional Classes of Neurons

• Interneurons: Process information within the CNS; connect motor and sensory pathways. Most abundant (about 90% of all neurons).

• Sensory (afferent) neurons: Detect stimuli and transmit information toward the CNS.

• Motor (efferent) neurons: Send signals from the CNS to muscles and glands (effectors).

Structure of a Neuron

• Cell body (neurosoma, soma, perikaryon): Contains the nucleus and organelles (mitochondria, lysosomes, Golgi complex, rough ER).

• Cytoskeleton: Contains microtubules and neurofibrils (actin bundles) that compartmentalize rough ER into chromatophilic substance.

• Inclusions: May include glycogen granules, lipid droplets, melanin, or lipofuscin.

• Neurites: Extensions from the cell body.

  • Dendrites: Numerous, tree-like branches; primary sites for receiving signals.

  • Axon (nerve fiber): Long, cylindrical extension; specialized for rapid conduction.

    • Originates at the axon hillock.

    • Contains axoplasm (cytoplasm) and axolemma (membrane).

    • Ends in terminal arborization and axon terminals (terminal boutons) forming synapses.

Structural Classes of Neurons

• Multipolar neuron: One axon, multiple dendrites; most common in CNS.

• Bipolar neuron: One axon, one dendrite; found in olfactory cells, retina, and ear.

• Unipolar neuron: Single process splits into peripheral and central processes; sensory neurons.

• Anaxonic neuron: Many dendrites, no axon; found in brain, retina, adrenal gland.

Axonal Transport

• Axonal transport: Two-way passage of materials along an axon.

  • Anterograde transport: Movement away from cell body, down the axon; driven by kinesin.

  • Retrograde transport: Movement toward the cell body; driven by dynein.

  • Example: Rabies virus uses retrograde transport to infect the CNS.

12.3 Supportive Cells (Neuroglia)

Neuroglia, or glial cells, are non-neuronal cells that support, protect, and assist neurons. They are essential for the proper functioning of the nervous system.

• Bind neurons together and form supportive tissue framework.

• Guide migrating neurons during development.

• Cover mature neurons (except at synapses) to prevent unwanted contact and ensure precise conduction pathways.

Types of Glia in the CNS

• Oligodendrocytes: Form myelin sheaths in the CNS.

• Ependymal cells: Line internal cavities of the brain; secrete and circulate cerebrospinal fluid (CSF).

• Microglia: Macrophages that engulf debris and defend against pathogens.

• Astrocytes: Most abundant; provide structural support, regulate blood flow, supply energy, secrete growth factors, influence synaptic signaling, regulate tissue fluid composition, and form scar tissue (astrocytosis/sclerosis).

Types of Glia in the PNS

• Schwann cells (neurolemmocytes): Envelop axons, form myelin sheath, assist in regeneration of damaged fibers.

• Satellite cells: Surround neuron cell bodies in ganglia; provide insulation and regulate chemical environment.

Glial Cells and Brain Tumors

• Tumors: Masses of rapidly dividing cells.

• Mature neurons rarely form tumors due to lack of mitosis.

• Brain tumors arise from:

  • Meninges (protective membranes of CNS)

  • Metastasis from nonneuronal tumors

  • Mitotically active glial cells

• Gliomas: Tumors of glial cells; grow rapidly, highly malignant, and difficult to treat due to blood-brain barrier.

• Treatment: Radiation or surgery.

12.3b Myelin

The myelin sheath is a spiral layer of insulation around axons, crucial for rapid signal conduction.

• Formed by Schwann cells in the PNS and oligodendrocytes in the CNS.

• Composed of plasma membrane (20% protein, 80% lipid).

• Production is called myelination; begins in fetal development, rapid in infancy, complete by late adolescence.

Myelination in the PNS

• Schwann cell spirals around a section of a single axon, laying up to 100 layers of membrane.

• Neurilemma: Thick, outermost coil containing Schwann cell nucleus and cytoplasm.

Myelination in the CNS

• Oligodendrocyte wraps processes around portions of multiple axons.

• Myelination spirals inward toward the axon.

Segmentation of Myelin Sheath

• Multiple Schwann cells or oligodendrocytes are needed to myelinate one axon.

• Node of Ranvier: Gap between myelinated segments.

• Internodal segments: Myelin-covered segments.

• Initial segment: Bare section between axon hillock and first glial cell.

• Trigger zone: Axon hillock and initial segment; initiates action potential.

Diseases of the Myelin Sheath

• Multiple Sclerosis (MS):

  • Oligodendrocytes and myelin sheaths in CNS deteriorate, possibly due to autoimmune response.

  • Myelin replaced by scar tissue, disrupting nerve conduction (symptoms: double vision, tremors, numbness, speech defects).

  • Onset: 20-40 years; fatal 25-30 years after diagnosis.

• Tay-Sachs Disease:

  • Hereditary disorder, mainly in infants of Eastern European Jewish ancestry; usually fatal before age 4.

  • Abnormal accumulation of glycolipid GM2 in myelin sheath due to missing lysosomal enzyme.

  • Disrupts nerve conduction, causing blindness, loss of coordination, dementia.

Regeneration of Damaged Axons

• Damaged axons in the PNS can regenerate with the help of Schwann cells.

• Process involves retraction of growth processes, regrowth of muscle fibers, and restoration of function.

Summary Table: Types of Glial Cells

Type	Location	Main Function
Oligodendrocytes	CNS	Form myelin sheaths
Ependymal cells	CNS	Line cavities, secrete CSF
Microglia	CNS	Phagocytosis, defense
Astrocytes	CNS	Support, regulate environment, form scar tissue
Schwann cells	PNS	Form myelin, aid regeneration
Satellite cells	PNS	Insulate cell bodies, regulate environment

Key Equations

• Action Potential Initiation: $V_{m} = V_{rest} + ext{depolarization}$ Where $V_{m}$ is membrane potential, $V_{rest}$ is resting potential.

• Rate of Axonal Transport: $ext{Rate} = rac{ ext{Distance}}{ ext{Time}}$

Example Application

• Clinical: Understanding myelin diseases such as MS and Tay-Sachs is crucial for diagnosis and treatment in neurology.

• Research: Neuroglia are targets for therapies in brain tumors and neurodegenerative diseases.

Additional info: The notes above expand on the original slides with definitions, examples, and academic context for clarity and completeness.