Nervous System Cells Notes

Introduction to the Nervous System

  • Functions alongside the endocrine system for communication.
  • Composed of:
    • Brain
    • Spinal cord
    • Nerves

Organization of the Nervous System

According to Relative Position

  • Central Nervous System (CNS)
    • Structural and functional core of the entire nervous system.
    • Consists of brain and spinal cord.
    • Integrates sensory information, evaluates it, and initiates an outgoing response, e.g., dropping a hot item.
  • Peripheral Nervous System (PNS)
    • Nerves located in outer regions of the nervous system.
    • Cranial Nerves: Originate from the brain.
    • Spinal Nerves: Originate from the spinal cord.
    • Function to detect changes in environments, evaluate information, and initiate responses.

According to Pathways

  • Afferent Division: Incoming sensory pathways (toward CNS).
  • Efferent Division: Outgoing motor pathways (away from CNS).

According to Effectors Regulated

  • Somatic Nervous System (SNS) (voluntary):
    • Carries information to skeletal muscles.
    • Somatic sensory division provides feedback information to CNS.
  • Autonomic Nervous System (ANS) (involuntary):
    • Carries information to smooth muscle, cardiac muscle, and glands.
    • Sympathetic Division: Prepares body for immediate threats (fight-or-flight).
    • Parasympathetic Division: Coordinates normal resting activities (rest and repair).

Cells of the Nervous System

Glial Cells (Neuroglia)

  • Function: Surround, support, and insulate neurons.
  • Major types:
    • Astrocytes: Largest and most numerous, connect neurons to blood supply.
    • Microglia: Small, engulf and destroy debris and pathogens.
    • Ependymal Cells: Line fluid-filled cavities; some produce and circulate fluid.
    • Oligodendrocytes: Protect nerve fibers and produce myelin sheath in CNS.
    • Schwann Cells: Produce myelin sheath in PNS; gaps are nodes of Ranvier.

Neurons

  • Excitable cells initiating and conducting impulses; major components:
    • **Cell Body (Perikaryon)
    • Dendrites: Receive impulses and carry signals toward the cell body.
    • Axon: Sends impulses away from the cell body, can be myelinated.
    • Telodendria: Distal tips that terminate in synaptic knobs.
  • Classification of Neurons:
    • Structural:
    • Multipolar: One axon and several dendrites.
    • Bipolar: One axon and one dendrite (least common).
    • Unipolar: One process splitting into two almost immediately.
    • Functional:
    • Afferent (Sensory) Neurons: Conduct impulses to CNS.
    • Interneurons: Located in CNS; process information.
    • Efferent (Motor) Neurons: Conduct impulses away from CNS to effectors.

Reflex Arc

  • Definition: Conduction route including receptors, CNS, and effectors.
  • Types:
    • Three-neuron arc: Includes afferent, interneurons, and efferent neurons.
    • Two-neuron arc: Simplest form (afferent and efferent).
  • Synapses: Where nerve signals transmit; includes electrical and chemical types (more common).

Nerves and Tracts

Nerves

  • Bundles of peripheral nerve fibers held by connective tissue:
    • Epineurium: Surrounds entire nerve.
    • Perineurium: Surrounds bundles of fibers (fascicles).
    • Endoneurium: Surrounds individual nerve fibers.

Tracts

  • Bundles of nerve fibers in CNS; called tracts rather than nerves.

Gray and White Matter

  • White Matter: Myelinated nerves (PNS) and tracts (CNS).
  • Gray Matter: Contains cell bodies and unmyelinated fibers.

Repair of Nerve Fibers

  • Mature neurons cannot divide, leading to potential permanent damage.
  • Limited repair possible if cell body and neurilemma remain intact.

Action Potential

  • Membrane potential in neurons conducting impulses, also known as nerve impulse.

Neurotransmitters

  • Chemicals released during nerve impulse facilitating neuron communication.
  • Classification:
    • By function: Excitatory (promoting action) and inhibitory (regulating action).

Types of Neurotransmitters

  • Small-Molecule Neurotransmitters:
    • Acetylcholine: Unique structure, plays both excitatory and inhibitory roles.
    • Amines: Affect learning, emotions, and motor control.
    • Amino Acids: Common neurotransmitters in CNS.
    • Nitric Oxide (NO): Derived from amino acids.
  • Large-Molecule Neurotransmitters (Neuropeptides):
    • Composed of 2 or more amino acids, can act as modulators affecting other neurotransmitters.

Development of Nervous System Cells

  • Develops from the ectoderm, most rapid during gestation and the first two years of life.