Nervous Tissue Part I

Nervous Tissue Part I: Fundaments of the Nervous System

I. Organization of the Nervous System

The nervous system is broadly divided into two main parts:

A. Central Nervous System (CNS)
  • Components: Consists of the Brain and Spinal Cord.
  • Function: Acts as the main processing center for the entire nervous system.
B. Peripheral Nervous System (PNS)
  • Components: Comprises all nervous tissue outside the CNS.
  • Divisions: The PNS has two distinct functional parts:
    • 1. Somatic Nervous System (SNS)
      • Sensory Information: Conveys sensory information from the skin, muscles, and joints to the CNS.
      • Motor Information: Conveys motor information from the CNS to skeletal muscles, enabling voluntary movement.
    • 2. Autonomic Nervous System (ANS)
      • Function: Involuntarily conveys information to smooth muscle, cardiac muscle, and glands.
      • Subdivisions (from diagram):
        • Sympathetic Nervous System
        • Parasympathetic Nervous System
      • Nerve types (from diagram):
        • Afferent nerves
        • Efferent nerves
C. Major Divisions Hierarchy (As per diagram)

The nervous system branches into:

  1. Central Nervous System (CNS)
    • Brain
    • Spinal cord
  2. Peripheral Nervous System (PNS)
    • Afferent nerves (carrying sensory information towards the CNS)
    • Efferent nerves (carrying motor commands away from the CNS)
    • Somatic Nervous System
      • Afferent nerves
      • Efferent nerves
    • Autonomic Nervous System
      • Afferent nerves
      • Efferent nerves
      • Parasympathetic nervous system
      • Sympathetic nervous system

II. Nervous Tissue

The nervous system is comprised of specialized nervous tissue, which includes two major cell types:

A. Neuron
  • Size & Function: Generally larger cells responsible for propagating an action potential (nerve impulse).
  • Excitability: Neurons respond to stimuli (e.g., environmental changes) by rapidly altering their cell membrane ionic gradient, making them 'excitable' cells.
  • Action Potential: The spreading (propagating) of this change along the cell membrane is called a nerve impulse or action potential.
  • Division: The vast majority of neurons do not divide after early development.
  • Components:
    • Cell body (soma or perikaryon)
    • Processes (neurites), including dendrites and axons
B. Glial Cells (Neuroglia)
  • Prevalence: Constitute the majority of cells in the nervous system.
  • Action Potential: Unlike neurons, glial cells do not propagate an action potential.
  • Examples seen in diagrams: Astrocyte, Microglia, Schwann Cell (Neurolemmocyte).

III. The Neuron: Detailed Anatomy

A. Cell Body (Soma or Perikaryon)
  • Nucleus: Features a large, euchromatic nucleus with a prominent nucleolus, indicating high metabolic activity.
  • Nissl Bodies: Characterized by the presence of prominent Nissl bodies, which are aggregates of rough endoplasmic reticulum (rER) and free ribosomes. These are essential for protein synthesis.
  • Other Organelles: Contains other typical cellular organelles necessary for cell function.
B. Neuropil
  • Definition: A fibrous intercellular network that surrounds the cells of the CNS.
  • Composition: Consists of processes (dendrites and axons) of neurons and glial cells.
C. Processes (Neurites)

Neurites extend from the cell body and are categorized into dendrites and axons.

1. Dendrites
  • Function: Main sites for signal reception and processing.
  • Structure: Possess numerous dendritic spines, which significantly increase the receptor surface area. They are typically short and highly branched.
  • Contents: Their cytoplasmic contents are similar to those of the cell body (e.g., Nissl bodies).
2. Axon
  • Function: Primarily responsible for conducting the action potential away from the cell body.
  • Components:
    • Axolemma: The plasma membrane of the axon.
    • Axoplasma: The cytoplasm of the axon.
    • Axon Hillock: The specialized region of the cell body where the axon originates. It lacks Nissl bodies.
    • Initial Segment: Located just past the axon hillock, this is the region where excitatory and inhibitory stimuli are summed to determine if an action potential will be generated.
    • Terminal Arborization & Boutons: The axon typically branches extensively at its end (terminal arborization), with each branch terminating in a synaptic bouton (axon terminal), which forms synapses with other neurons or effector cells.
  • Axonal Transport: Essential for moving cytoplasmic components (substances) within the axon:
    • Anterograde transport: Movement away from the soma (cell body) towards the axon terminal.
    • Retrograde transport: Movement towards the soma from the axon terminal.

IV. Neuron Classification

Neurons can be classified in two primary ways:

A. By Function
  • Sensory (Afferent) Neurons: Transmit sensory information from the periphery to the CNS.
  • Motor (Efferent) Neurons: Transmit motor commands from the CNS to effector organs (muscles, glands).
  • Interneurons: Act as intermediaries, connecting neurons within the CNS. They are the most numerous type of neuron.
B. By Structure
  • Multipolar Neurons: Characterized by one axon and multiple dendrites. This is the most common type (e.g., motor neurons).
  • Bipolar Neurons: Possess one axon and one dendrite, typically extending from opposite poles of the cell body (e.g., in the retina and olfactory epithelium).
  • Pseudounipolar (or Unipolar) Neurons: Have a single process that emerges from the cell body and then divides into two branches:
    • Peripheral process: Functions like a dendrite, receiving sensory input from the periphery.
    • Central process: Functions like an axon, carrying the action potential towards the CNS. These are typically sensory neurons.

V. Synapse

A synapse is a specialized junction between two neurons, or between a neuron and an effector cell (e.g., muscle or gland).

A. Types of Synapses

Synapses are classified based on the parts of the neuron involved:

  • Axodendritic: Axon terminal of one neuron forms a synapse with a dendrite (often a dendritic spine) of another neuron.
  • Axosomatic: Axon terminal synapses with the cell body (soma) of another neuron.
  • Axoaxonic: Axon terminal synapses with the axon of another neuron, often modulating the activity of the postsynaptic axon terminal.
B. Components of a Chemical Synapse

Syntheses are typically formed by three main components:

  • Presynaptic Terminal: The end of the axon of the neuron transmitting the signal. It contains synaptic vesicles filled with neurotransmitters.
  • Synaptic Cleft: A narrow gap between the presynaptic and postsynaptic membranes.
  • Postsynaptic Terminal: The membrane of the neuron receiving the signal, containing receptors for neurotransmitters.
  • Chemical Messengers: Neurotransmitters are the chemical messengers released into the synaptic cleft.
C. Synaptic Transmission Steps

Synaptic transmission is the process by which a neuron communicates with another neuron or an effector cell, primarily through chemical signals (neurotransmitters). It involves the following sequence:

  1. Action Potential Arrival: An action potential (electrical/nervous impulse) arrives at the axon terminal of the presynaptic neuron.
  2. Ca2+Ca^{2+} Entry: The depolarization caused by the action potential opens voltage-gated Ca2+Ca^{2+} channels in the presynaptic membrane. This leads to an influx of Ca2+Ca^{2+} ions into the presynaptic terminal.
  3. Neurotransmitter Release: The increase in intracellular Ca2+Ca^{2+} concentration triggers the fusion of neurotransmitter-containing synaptic vesicles with the presynaptic membrane, releasing their contents (neurotransmitters) into the synaptic cleft via exocytosis.
  4. Diffusion Across Synaptic Cleft: The released neurotransmitters rapidly diffuse across the synaptic cleft.
  5. Binding to Receptors: Neurotransmitters bind to specific ligand-gated ion channels (receptors) located on the postsynaptic membrane.
  6. Permeability Alteration & Graded Potentials: The binding of neurotransmitters opens these ligand-gated ion channels, altering the cell membrane permeability of the postsynaptic neuron. This ion movement results in localized changes in membrane potential, known as graded potentials.
    • Excitation/Depolarization: If the ion movement makes the postsynaptic membrane more positive (e.g., Na+Na^{+} influx), it leads to excitation or depolarization, increasing the likelihood of an action potential.
    • Inhibition/Hyperpolarization: If the ion movement makes the postsynaptic membrane more negative (e.g., Cl−Cl^{-} influx or K+K^{+} efflux), it leads to inhibition or hyperpolarization, decreasing the likelihood of an action potential.
  7. Signal Termination: Neurotransmitter levels in the synaptic cleft are rapidly reduced to terminate the signal. This occurs through three main mechanisms:
    • Reuptake: Neurotransmitters are reabsorbed by the presynaptic neuron.
    • Enzymatic Degradation: Specific enzymes in the synaptic cleft break down the neurotransmitter.
    • Diffusion: Neurotransmitters diffuse away from the synapse.

These processes ensure precise control over neural communication.