Cells of the Nervous System - Comprehensive Notes

Cells of the Nervous System

Overview

  • The nervous system consists of two main types of cells:
    • Neurons: Structural and functional units that conduct nerve impulses.
    • Neuroglia (or Glial cells): Non-conductive cells that support neurons.

Neurons

Structure
  • Billions of nerve cells (neurons) in the body.
    • Approximately 100 billion in the brain.
    • Approximately 13.5 million in the spinal cord.
  • Neurons vary in size, shape, and structure, depending on their role and location.
  • Three basic parts:
    • Cell body (Soma)
    • Dendrites
    • Axon
Function
  • Carry out the functions of the nervous system by conducting nerve impulses.
  • Transmit electric and chemical signals (electrochemical energy) to other neurons.
Cell Body (Soma)
  • Structure:
    • Core of the neuron containing a nucleus and specialized cytoplasmic organelles.
    • Enclosed by a membrane for protection and interaction with the surroundings.
  • Function:
    • Carries genetic information.
    • Maintains the neuron's structure.
    • Provides energy.
Dendrites
  • Structure:
    • Fibrous roots branching from the cell body which increases surface area to receive signals from other neurons.
    • Number of dendrites varies.
  • Function:
    • Receive and process signals from the axons of other neurons.
Axon
  • Structure:
    • Long, tail-like structure joining the cell body at the axon hillock.
    • Axon hillock: control area initiating neuron electrical impulses.
    • Many axons are surrounded by myelin sheath (segmented, white, fatty substance).
    • Myelin facilitates quick and effective electrical signal conduction.
    • Unmyelinated regions between myelin segments are called nodes of Ranvier.
    • Action potentials jump from one node to the next.
    • In the peripheral nervous system, Schwann cells produce myelin.
    • End of the axon contains axon terminals with synaptic knobs.
    • Synaptic vesicles store neurotransmitter molecules.
  • Function:
    • Carries electrical impulses from the cell body to axon terminals, which then pass the impulse to another neuron.

Types of Neurons

Function
  • Sensory (or Afferent) Neurons: Carry action potentials towards the CNS.
  • Interneurons (or association or relay neurons): Carry action potentials from one neuron to another.
  • Motor (or Efferent) Neurons: Carry action potentials away from the CNS.
Structure
  • Pseudounipolar Neuron:
    • Single structure extends from the soma and branches into two distinct structures.
    • Most sensory neurons are pseudounipolar, with one extension connected to dendrites (receives sensory information) and another transmitting information to the spinal cord.
  • Bipolar Neuron:
    • One axon and one dendrite extending from the cell body.
  • Multipolar Neuron:
    • Most common type.
    • One axon and multiple dendrites that branch directly from the cell body.

Glial Cells

Function
  • Do not conduct nerve impulses.
  • Far more numerous than neurons and capable of mitosis.
  • Support, nourish, and protect neurons.
  • Surround neurons, hold them in place, and provide structural support.
  • Supply nutrients and oxygen to neurons.
  • Aid in waste removal.
  • Destroy pathogens and assist cell death.
  • Insulate neuronal axons.
  • Carry out signal conduction and modification.
Types of Glial Cells
  • Satellite cells: Regulate nutrient and neurotransmitter levels around neurons in ganglia.
  • Schwann cells: Myelinate axons in the peripheral nervous system.
  • Oligodendrocytes: Myelinate axons in the central nervous system and provide structural framework.
  • Astrocytes: Maintain the blood-brain barrier and preserve the chemical environment by recycling ions and neurotransmitters.
  • Microglia: Remove cell debris, wastes, and pathogens.
  • Ependymal cells: Line ventricles (brain) and central canal (spine) and are involved in cerebrospinal fluid production.