Nervous Tissue: Structure, Function, and Types

Posted on Aug 6, 2024 in Biology

Nervous Tissue: Structure, Function, and Types

Nervous tissue is a specialized tissue responsible for communication within the body. It originates from the ectoderm and consists of two main cell types: neurons and glial cells.

Functions of Nervous Tissue

  • Detection and Transmission of Nerve Impulses: Neurons receive stimuli through receptors and transmit these impulses to the central nervous system (CNS). They also carry responses back to effector organs.
  • Support and Protection: Glial cells provide structural support, nutrition, and protection for neurons.

Neurons

Neurons are the functional units of the nervous system. They consist of a cell body (soma), an axon, and dendrites.

  • Cell Body (Soma): Contains the nucleus and other organelles, including Nissl substance, Golgi apparatus, and lysosomes.
  • Axon: A long process that transmits nerve impulses away from the cell body.
  • Dendrites: Short, branched processes that receive nerve impulses from other neurons.

Characteristics of Neurons

  • Large Nucleus: Neurons have a prominent nucleus.
  • Neurofibrils: Fine filaments that provide structural support.
  • Synthesize and Release Neurotransmitters: Neurons communicate with each other through chemical messengers called neurotransmitters.
  • Limited Regeneration: Neurons have a limited ability to regenerate.

Glial Cells

Glial cells, also known as neuroglia, are supportive cells that play a crucial role in the function of the nervous system.

  • Ependymal Cells: Line the ventricles of the brain and central canal of the spinal cord, producing cerebrospinal fluid.
  • Oligodendrocytes: Produce myelin sheaths around axons in the CNS, enhancing nerve impulse conduction.
  • Astrocytes: Star-shaped cells that form the blood-brain barrier, providing a selective barrier between the blood and the brain. They also provide structural support and nutrition to neurons.
  • Microglia: Macrophages of the nervous system, responsible for removing debris and pathogens.

Types of Synapses

Synapses are junctions between neurons where nerve impulses are transmitted. There are three main types of synapses:

  • Axo-somatic: Axon to cell body
  • Axo-dendritic: Axon to dendrite
  • Axo-axonic: Axon to axon

Classification of Neurons

Neurons can be classified based on various criteria:

Myelination

  • Myelinated: Axons covered with a myelin sheath, which increases the speed of nerve impulse conduction.
  • Non-myelinated: Axons without a myelin sheath.

Shape of Cell Bodies

  • Stellate: Star-shaped, commonly found in the spinal cord.
  • Purkinje: Pear-shaped, found in the cerebellum.
  • Pyramidal: Pyramid-shaped, found in the cerebral cortex.
  • Spherical: Round-shaped.

Number of Processes

  • Unipolar/Pseudounipolar: Found in spinal ganglia, with one axon and one dendrite.
  • Bipolar: Found in the retina of the eye, with one axon and one dendrite.
  • Multipolar: Found in the spinal cord, with one axon and multiple dendrites.

Axon Length

  • Golgi Type I: Long axon.
  • Golgi Type II: Short axon.

Function

  • Sensory: Transmit nerve impulses from receptors to the CNS.
  • Motor: Transmit nerve impulses from the CNS to effector organs (muscles or glands).
  • Associative: Connect different parts of the brain.
  • Neuroendocrine: Secrete hormones, such as those produced by the pituitary gland.

Nervous Fibers

Nervous fibers are composed of axons and their associated glial cells.

Function of Nervous Fibers

  • Dendrites: Short processes that transmit nerve impulses from the periphery to the CNS.
  • Axons: Long processes that transmit nerve impulses from the CNS to the periphery.

Types of Nervous Fibers

Myelinated Nerve Fibers

Myelinated nerve fibers are found in both the CNS and peripheral nervous system (PNS). They consist of an axon, myelin sheath, Schwann sheath, and endoneurium.

  • Myelin Sheath: A fatty, insulating layer that increases the speed of nerve impulse conduction.
  • Schwann Cells: Produce myelin in the PNS.
  • Oligodendrocytes: Produce myelin in the CNS.
  • Nodes of Ranvier: Gaps in the myelin sheath that allow for saltatory conduction, a faster mode of nerve impulse transmission.

Non-myelinated Nerve Fibers

Non-myelinated nerve fibers are also found in both the CNS and PNS. They consist of an axon, Schwann sheath, and endoneurium.

  • Olfactory Nerves: Responsible for the sense of smell.
  • Smaller Axons of the CNS: Some axons in the CNS are not myelinated.
  • Peripheral Postganglionic Autonomic Fibers: Part of the autonomic nervous system, which controls involuntary functions.
  • Fine Sensory Fibers: Found in the skin, muscles, and viscera.

Further Classification of Nervous Fibers

Myelinated and non-myelinated axons can be further classified based on:

Number of Processes

  • Unipolar/Pseudounipolar: Found in spinal ganglia, with one axon and one dendrite.
  • Bipolar: Found in the retina of the eye, with one axon and one dendrite.
  • Multipolar: Found in the spinal cord, with one axon and multiple dendrites.

Axon Length

  • Golgi Type I: Long axon, such as the giant pyramidal cells of Betz in the motor cortex.
  • Golgi Type II: Short axon, such as interneurons.

Function

  • Motor Neurons: Transmit nerve impulses to muscles, glands, and blood vessels.
  • Sensory Neurons: Receive nerve impulses from the periphery (internal and external environment of the body).
  • Interneurons: Connect motor neurons and sensory neurons.

Chemical Composition of Myelin

Myelin is composed of various chemical components, including:

  • Cholesterol: An essential component of myelin.
  • Water: Myelin is about 40% water.
  • Proteins: Myelin contains various proteins, including myelin basic protein, myelin oligodendrocyte glycoprotein, and proteolipid protein.
  • Lipids: Myelin is rich in lipids, including galactocerebroside, sphingomyelin, and phospholipids.

Neuroglia

Neuroglia, also known as glial cells, are non-neuronal cells that support and protect neurons.

Functions of Neuroglia

  • Support: Astrocytes provide structural support to neurons.
  • Nutrition: Ependymal cells produce cerebrospinal fluid, which provides nutrients to neurons.
  • Protection: Microglia act as macrophages, removing debris and pathogens.
  • Myelin Production: Oligodendrocytes produce myelin in the CNS, enhancing nerve impulse conduction.
  • Homeostasis: Neuroglia play a role in maintaining the homeostasis of the nervous system.

Types of Neuroglia

  • Astrocytes: Star-shaped cells that form the blood-brain barrier, provide structural support, and regulate the chemical environment around neurons.
  • Oligodendrocytes: Small, circular cells that produce myelin in the CNS.
  • Microglia: Elongated cells that act as macrophages in the CNS.
  • Ependymal Cells: Line the ventricles of the brain and central canal of the spinal cord, producing cerebrospinal fluid.

Staining of Neuroglia

  • Protoplasmic Astrocytes: Stain with silver nitrate (AgNO3).
  • Fibrous Astrocytes: Stain with gold chloride (AuCl3).

Origin of Neuroglia

Neuroglia cells originate from the ectodermal layer, except for microglia, which originate from hematopoietic stem cells.

The ratio of glial cells to neurons is approximately 10:1, highlighting the importance of these supportive cells in the function of the nervous system.