Cells & Tissues: Nervous Tissue

Module 1: Cells & Tissues

Introduction to Nervous Tissue

• Nervous tissue is recognized as the last of the four primary tissue types.
• It exists as two principal parts:
  • Central Nervous System (CNS): comprises the brain and spinal cord.
  • Peripheral Nervous System (PNS): involves nerves, sensory receptors, and ganglia.
• Cells, known as neurons, are characterized by:
  • Being widely scattered with limited contact mainly at their extremities.
  • The ability to transmit signals (action potentials) by reversing the polarity of the cell membrane, a process known as depolarization.
• Specialized contact points called synapses facilitate signal transmission from cell to cell.
• Nervous tissue cells feature various cellular processes (cytoplasmic extensions).
• Characteristics of nervous tissue can differ depending on whether it is part of the CNS or the PNS.

Types of Cells in Nervous Tissue

• The key cell types present in nervous tissue include:
  1. Neurons (N)
  2. Glial Cells (Supporting Cells)
  • Oligodendrocytes (O)
  • Astrocytes (A)
  • Microglia (M)
  • Ependymal Cells (E)
  • Schwann Cells
  • Satellite Cells

Summary of Part One

• The nervous system divides into the CNS and PNS, characterized by highly cellular structures consisting of:
  • Neurons
  • Glial cells with extended cytoplasmic processes
• CNS glial cells: oligodendrocytes, astrocytes, microglia, and ependymal cells.
• PNS glial cells: Schwann cells and satellite cells.
• In subsequent parts, the structure of neurons will be examined.

Neurons

Typical Neuron Structure

• Neuronal cell bodies (soma) exist only within:
  • The CNS (brain, spinal cord)
  • Ganglia, which are accumulations of neuronal cell bodies in the PNS.
• Notably, nerve cell bodies are absent in peripheral nerves.

Types of Neurons

• Neurons are classified based on structure and function into:
  1. Multipolar Neuron: Possesses multiple cell processes stemming from the soma; primarily involved in motor functions.
  2. Unipolar Neuron: Contains a single cell process from the soma; primarily senses signals and relates to sensory neurons.
  3. Bipolar Neuron: Has two processes off the soma and is connected to sensory functions in special senses like vision, taste, smell, and hearing.

Synapses and Neuron Communication

• Neurons communicate with other cells through synapses, which are specialized contact points between:
  • The axon terminal of the pre-synaptic neuron and a post-synaptic cell (such as another neuron's dendrite or a skeletal muscle cell).

Summary of Part Two

• Key points learned include:
  • Neurons comprising cell bodies (soma) which house the majority of the neuron's cytoplasm.
  • Dendrites, defined as cytoplasmic extensions that receive information.
  • The axon serves as a long, thin cytoplasmic process transmitting the action potential to its terminals, which may be myelinated or unmyelinated.
  • Axon terminals are contact points forming synapses for communication.
  • Types of neurons: multipolar, unipolar, and bipolar, each with distinct applications in the nervous system.
  • Next part focuses on supporting cells of nervous tissue, specifically glial cells.

Supporting Cells of Nervous Tissue

Glial Cells Overview

• Following types of glial cells in nervous tissue:
  1. Oligodendrocytes: Produce myelin in CNS.
  2. Microglia: Small support cells that act as resident macrophages, fulfilling an innate immunological function.
  3. Ependymal Cells: Line the brain’s ventricles and the spinal cord's central canal; contribute to producing and monitoring cerebrospinal fluid (CSF).
  4. Astrocytes: Star-shaped cells with extensive processes that support neurons and blood vessels in the CNS; play a crucial role in forming scar tissue following damage.
  5. Schwann Cells: Also referred to as neurolemmocytes, are found in the PNS and can myelinate parts of axons, with each axon potentially requiring numerous Schwann cells.
  6. Satellite Cells: Support neurons within ganglia in the PNS and surround neuronal cell bodies akin to satellites orbiting a planet.

Myelination of Axons

• Myelin is produced by:
  • Oligodendrocytes in the CNS
  • Schwann Cells in the PNS
• Myelin wraps around axons, creating a phospholipid-rich sheath, essential for electrical insulation.
• Node of Ranvier: Areas between myelin sheaths where voltage-gated ion channels congregate, allowing for rapid signal transmission.
• Myelinated axons undergo saltatory conduction, where action potentials leap from one node to another, increasing propagation speeds, whereas unmyelinated axons typically demonstrate continuous conduction, which is slower and traverses the entire axon length.

Summary of Part Three

• Two pivotal cells create myelin sheaths for axons:
  • Oligodendrocytes for CNS
  • Schwann/Neurolemmocytes for PNS.
• Concentration of ion channels occurs between myelin sheaths at nodes, enhancing conduction speed due to action potentials “jumping” between nodes.
• Astrocytes serve as primary supportive cells in the CNS; microglia function as resident macrophages, while ependymal cells line ventricles and central canals.
• Satellite cells are supportive in ganglia. The subsequent part contrasts CNS and PNS structures.

CNS vs PNS Tissue

Structure of CNS

• Composed of two types of matter:
  • White Matter: Primarily contains tracts of myelinated axons, responsible for transmitting information.
  • Grey Matter: Composed of neuronal cell bodies and neuropil, which involve axons, dendrites, glial cells, and cellular processes, and is primarily where information is received.

Grey Matter Composition

• Contains:
  • Neurons
  • Neuropil, being everything interspersed between the neurons, including axons, dendrites, glial cells, and their processes.

White Matter Composition

• Comprises:
  • Predominantly myelinated axon tracts
  • Glial cells including oligodendrocytes, astrocytes, and microglial cells.

Nervous Tissue in PNS

• Defined by the structure of nerves, which entails:
  • Axons: Include both myelinated and unmyelinated fibers.
  • Schwann Cells: As a glial component of the PNS.
  • Connective Tissue:
  • Endoneurium: Loose connective tissue (LCT) surrounding individual Schwann cells and axons.
  • Perineurium: Dense irregular connective tissue (DICT) surrounding a fascicle of axons.
  • Epineurium: DICT encapsulating multiple fascicles of axons.

Summary of Part Four

• CNS consists of both brain and spinal cord with:
  • White matter containing tracts of myelinated axons.
  • Grey matter encompassing neuronal cell bodies and supporting glial structures.
• In PNS, nerves are comprised of axons, Schwann cells, and connective tissue layers (endoneurium, perineurium, and epineurium).

Conclusion of Module 1: Cells & Tissues

• This concludes Module 1 with the understanding of:
  • The structures and functions of both CNS and PNS cells.
  • The roles of various neural and supportive cells in nervous tissue.
  • Critical differences between the types of matter in the CNS and structural organization of nerves in the PNS.
• Congratulations on completing Module 1!