Nervous Tissue

Human Anatomy: Chapter 14 - Nervous Tissue

Introduction

• Authors: Michael P. McKinley, Valerie Dean O'Loughlin, Elizabeth E. Pennefather-O'Brien, Ronald T. Harris

• Dr. R. Knudsen presents information on Nervous Tissue as published by McGraw Hill Education, Fourth Edition.

Outline

• Organization of the Nervous System

• Cytology of Nervous Tissue

• Myelination of Axons

• Axon Regeneration

• Nerves

• Synapses

• Neural Integration and Neuronal Pools

• Development of the Nervous System

Organization of the Nervous System

• The nervous system is categorized into two main types:

  • Structural organization

  • Functional organization

• It operates as a cohesive unit despite these categories.

Structural Organization

• Central Nervous System (CNS):

  • Comprises the brain and spinal cord.

• Peripheral Nervous System (PNS):

  • Comprises cranial nerves, spinal nerves, and ganglia.

Illustrative Diagram

• Figure 14.1 demonstrates the relationship between the CNS and PNS including their respective components.

Functional Organization of the Nervous System

• The CNS and PNS accomplish three core functions:

  1. Collecting Information:

  • Receptors detect changes in internal and external environments and relay this to the CNS.

  1. Processing and Evaluating Information:

  • The CNS determines responses based on received information.

  1. Responding to Information:

  • The CNS initiates nerve impulses to effectors to respond to environmental changes.

Functional Divisions

• Sensory Nervous System (Afferent):

  • Transmits information from receptors to the CNS.

• Motor Nervous System (Efferent):

  • Transmits information from the CNS to effectors.

Functional Organization Diagram

• Figure 14.2 depicts the organization, outlining how sensory information is received and motor responses are generated.

Sensory Nervous System

• Also referred to as the afferent system, it:

  • Receives sensory information from receptors in the PNS and sends it to the CNS.

  • Subdivisions include:

  • Somatic Sensory: Involves general senses (touch, pain, pressure, vibration, proprioception).

  • Visceral Sensory: Involves impulses from the visceral organs.

Motor Nervous System

• Known as the efferent system, it:

  • Sends impulses from the CNS to muscles and glands.

  • Subdivisions include:

  • Somatic Motor: Involves voluntary control of skeletal muscles.

  • Autonomic Motor: Involves involuntary control of cardiac muscle, smooth muscle, and glands.

Neurons

• Fundamental cells of the nervous system, responsible for transmitting nerve impulses.

• Characteristics:

  • High metabolic rate.

  • Extreme longevity (ability to live for many years).

  • Nonmitotic (do not undergo mitosis).

• Structure:

  • Cell Body: Contains nucleus and organelles.

  • Dendrites: Branching structures that receive nerve impulses.

  • Axon: Long fiber that transmits impulses away from the cell body.

Detailed Neuron Anatomy

• A typical neuron comprises:

  • Dendrites: Short branched processes that collect signals.

  • Axon Hillock: The region where the axon connects to the cell body.

  • Axon Collaterals: Side branches of the axon.

  • Telodendria: Terminal branches of the axon.

  • Synaptic Knobs: Bulbous ends of telodendria involved in neurotransmitter release.

Cytology of Nervous Tissue

• The nervous system consists of two types of cells:

  • Neurons (nerve cells): Excitable cells that initiate, transmit, and receive nerve impulses.

  • Glial Cells (neuroglia): Non-excitable cells that protect and support neurons, capable of mitosis.

Neuron Structure Details

• Cell Body Contents:

  • Nucleus

  • Nucleolus

  • Mitochondria

  • Rough ER (Nissl Bodies) for protein synthesis.

• Dendrites function as inputs, receiving messages.

• Axons act as outputs, sending signals to other cells. Some neurons may be anaxonic, lacking an axon.

Neuron Classification - Structure

• Unipolar Neurons: One short process branches like a T.

• Bipolar Neurons: Two processes (one axon and one dendrite).

• Multipolar Neurons: Many dendrites and one axon; the most common type.

Functional Classification of Neurons

• Sensory (Afferent): Carries impulses from sensory receptors to the CNS.

• Motor (Efferent): Carries impulses from the CNS to muscles or glands.

• Interneurons: Facilitate communication between sensory and motor neurons.

Glial Cells Overview

• Also known as neuroglia.

• Found in both CNS and PNS; smaller and more numerous than neurons.

• Function: Support, nourish, and protect neurons, involved in healing and defense.

• Types in CNS:

  1. Astrocytes: Maintain the blood-brain barrier, provide structural support.

  2. Ependymal Cells: Line brain cavities, assist in CSF production.

  3. Microglial Cells: Phagocytes that clean up debris.

  4. Oligodendrocytes: Myelinate CNS axons.

Glial Cells Functions in CNS

• Astrocytes: Regulate neuronal environment, form scar tissue, support metabolic functions.

• Ependymal Cells: Produce and circulate CSF, line brain ventricles.

• Microglial Cells: Act as immune defense agents in the CNS, involved in cleanup processes.

• Oligodendrocytes: Myelinate multiple CNS axons, enhancing signal transduction speed.

Glial Cells in the PNS

• Types:

  1. Satellite Cells: Surround neuron cell bodies in ganglia, regulating neuronal microenvironment.

  2. Neurolemmocytes (Schwann Cells): Myelinate PNS axons, similar to oligodendrocytes but for peripheral nerves.

Myelination of Axons

• It facilitates faster nerve impulse conduction via saltatory conduction.

• Forms the myelin sheath around axons, produced by:

  • Neurolemmocytes in the PNS

  • Oligodendrocytes in the CNS

Axon Regeneration

• Axons can regenerate if the cell body remains intact and the neurilemma is present.

• Regeneration outcomes depend on damage extent and site.

• Neurolemmocytes are crucial for the regeneration process.

Wallerian Degeneration Process

1. Trauma severs the axon.

2. The proximal portion swells and seals off.

3. The distal portion disintegrates but neurilemma survives.

4. Neurilemma forms a regeneration tube.

5. Axon regenerates with remyelination.

Protective Coverings of Nerves

• Nerves are structured as cable-like bundles of parallel axons, surrounded by three connective tissue layers:

  1. Endoneurium: Surrounds each axon.

  2. Perineurium: Encases each fascicle.

  3. Epineurium: Covers the entire nerve.

Nerve Structure Visual

• Figures illustrate the structural organization of the nerve including components like endoneurium, perineurium, and epineurium.

Synapses

• These are crucial junctions connecting one axon to another neuron, muscle cell, or gland.

• A typical synapse includes:

  • Presynaptic Neuron: Neuron sending the signal.

  • Postsynaptic Neuron: Neuron receiving the signal.

  • Synaptic Cleft: The space between the two cells.

Types of Synapses

• Electrical Synapses: Direct connections allowing for fast transmission, often found in smooth muscle.

• Chemical Synapses: Involving neurotransmitters for signal transmission, e.g. Acetylcholine (ACh) at neuromuscular junctions.

Neural Integration and Neuronal Pools

• Neurons form patterns called neuronal pools, which are classified based on their functional organization:

  • Converging Circuit: Multiple inputs leading to a single output.

  • Diverging Circuit: Single input spreading to multiple outputs.

  • Reverberating Circuit: Output re-stimulating an earlier stage in the pathway.

  • Parallel-After-Discharge Circuit: Input diverging into multiple parallel pathways that converge on one output.

Development of the Nervous System

• The development begins during the third week gestation and originates from the ectoderm:

  • The thickened ectoderm forms the neural plate, which evolves into the neural tube and related structures.

• Neural crest cells migrate and differentiate to form various components of the nervous system.