Introduction to Nervous Tissue

Introduction to Nervous Tissue

• The nervous system is a complex network of neurons and supporting cells, coordinating body activities by transmitting signals between different body parts.
• Main functions include sensory input (collecting information from sensory receptors), integration (processing and interpreting sensory data), and motor output (activating effector organs to produce a response).

Divisions of the Nervous System

• Central Nervous System (CNS): brain and spinal cord, responsible for integration of sensory information and coordinating responses, involving higher functions like thought and emotion.
• Peripheral Nervous System (PNS): all nerves outside the CNS, including sensory and motor neurons.
  • Somatic Nervous System: controls voluntary movements of skeletal muscles.
  • Autonomic Nervous System: regulates involuntary functions like heart rate and digestion, further divided into sympathetic and parasympathetic nervous systems.

Nervous System (Detailed Diagram)

• CNS (Central Nervous System): responsible for sensory activities, memory, and emotions.
  • Brain
  • Spinal Cord
• PNS (Peripheral Nervous System): connects the CNS with the rest of the body.
  • Autonomic Nervous System (ANS): involuntary movements.
  • Somatic Nervous System (SNS): voluntary movements.
  • Parasympathetic Division: rest and relax actions
  • Sympathetic Division: flight/fight response
  • Enteric Division: digestive system

Neuron Structure

• Neuron: basic unit of the nervous system, crucial for signal transmission.
  • Cell Body (Soma): contains the nucleus, cytoplasm, and essential organelles.
  • Dendrites: short, branching extensions that receive incoming signals from other neurons.
  • Axon: a long, slender projection that carries impulses.

Axonal Structure and Myelination

• Axon Characteristics: varies in length and diameter, influencing signal speed; surrounded by the axolemma (axonal membrane) for signal propagation.
• Myelin Sheath: composed of glial cells (oligodendrocytes in CNS, Schwann cells in PNS), providing insulation and increasing conduction velocity of action potentials.

Specialized Features and Synaptic Communication

• Nodes of Ranvier: gaps in the myelin sheath that facilitate rapid signal conduction (saltatory conduction).
• Synaptic Terminals: end points of axons where neurotransmitters are released, essential for communication with target cells (neurons, muscles, glands).
• Organelles in Neurons: include mitochondria for energy, rough endoplasmic reticulum (Nissl bodies) for neurotransmitter production.

Structure of Myelinated vs. Unmyelinated Nerve Fibers

• Myelinated Nerve Fibers: composed of multiple layers of lipid-rich membrane from glial cells (insulates the axon, allowing for faster signal transmission). Includes nodes of Ranvier with concentrated ion channels.
• Unmyelinated Nerve Fibers: lack a myelin sheath; slower conduction velocity due to continuous conduction along the entire length of the axon.

Axonal Transport

• Definition: The process by which materials are moved along the axon, essential for neuron function and health.
• Importance: Supports the transport of organelles, proteins, and neurotransmitters between the cell body and synaptic terminals.
  • Anterograde Transport: movement of materials from the cell body to the axon terminal.
  • Retrograde Transport: movement of materials from the axon terminal back to the cell body.
• Mechanisms: utilise motor proteins (kinesin for anterograde, dynein for retrograde) along microtubules to transport materials.

Neuroglia

• Definition: non-neuronal cells in the nervous system that support, protect, and maintain neurons.
  • Central Nervous System (CNS) Glia: Includes astrocytes, oligodendrocytes, microglia, ependymal cells.
  • Peripheral Nervous System (PNS) Glia: Includes Schwann cells and satellite cells.

Central Nervous System (CNS) Glia

• Astrocytes: star-shaped cells with numerous processes, providing structural support, regulating blood flow, maintaining the blood-brain barrier, and modulating neurotransmitter levels.
• Oligodendrocytes: fewer processes compared to astrocytes; myelinate multiple axons, forming and maintaining the myelin sheath, facilitating faster conduction.
• Microglia: small, with branching processes; immune cells of the CNS, acting as macrophages, removing debris and dead neurons; involved in immune response and inflammation.

Peripheral Nervous System (PNS) Glia

• Schwann Cells: wrap around single axons (or support unmyelinated fibers); form the myelin sheath, aiding in rapid signal transmission and nerve regeneration.
• Satellite Cells: flattened cells surrounding neuronal cell bodies in ganglia; provide structural support and regulate the microenvironment around neurons, contributing to homeostasis.

Neurotrophins

• Definition: family of proteins essential for the growth, survival, and differentiation of neurons, playing a critical role in the development and maintenance of the nervous system.

List of Neurotrophins and Their Functions

• Nerve Growth Factor (NGF): Promotes survival and growth of sympathetic and sensory neurons.
• Brain-Derived Neurotrophic Factor (BDNF): Supports neuron survival and encourages synapse formation, vital for learning and mood regulation.
• Neurotrophin-3 (NT-3): Essential for the survival and differentiation of various neuronal populations.
• Neurotrophin-4 (NT-4): Promotes neuron survival and enhances synaptic plasticity, similar to BDNF..
• Ciliary Neurotrophic Factor (CNTF): Supports motor and sensory neuron survival and provides neuroprotection.
• Glial Cell Line-Derived Neurotrophic Factor (GDNF): Promotes the survival of dopaminergic neurons; maintains peripheral neuron health.