Nervous System Overview

Body Cavity and Nervous System Overview

  • Body Cavity: Includes the cranial cavity (housing the brain) and vertebral canal (housing the spinal cord).
    • Central Nervous System (CNS): Comprising the brain and spinal cord.
    • Derived from embryonic dorsal hollow nerve cord.

Peripheral Nervous System (PNS)

  • Definition: Comprises all nerves originating from the brain and spinal cord.
    • Cranial Nerves: 12 pairs arising from the brain.
    • Spinal Nerves: Emerge from the spinal cord.
  • Clusters of neurons outside the CNS are known as ganglia.

Structure and Function of the Nervous System

  • Functional Division: Consists of sensory input, data integration, and motor output.
    • Sensory Neurons: Monitor internal (interoceptors) and external (exteroceptors) environmental conditions, sending information to the CNS.
    • Data Integration: CNS processes sensory input and decides on responses.
    • Motor Output: Involves somatic (voluntary) and autonomic (involuntary) responses.

Afferent and Efferent Divisions

  • Afferent Division: Sensory neurons that send impulses towards the CNS.
  • Efferent Division: Motor neurons that send impulses away from the CNS.
    • Somatic Sensory Neurons: Touch, temperature, pressure, etc., from the skin.
    • Visceral Sensory Neurons: Monitor changes within organ systems, including digestion, olfaction, and gustation.

Autonomic Nervous System (ANS)

  • Divisions: Sympathetic (fight or flight) and Parasympathetic (rest and digest).
    • Sympathetic Division: Increases heart rate, redistributes blood flow towards muscles.
    • Parasympathetic Division: Conserves energy, promotes digestion.

Types of Neurons

  • Neurons (excitable cells transmitting impulses):

    • Unique characteristics: high metabolic rate, long lifespan (non-dividing post-fetal), and high energy needs.
    • Dendrites: Receptive regions for signals from other neurons.
    • Cell Body: Contains the nucleus and organelles needed for neuron function.
    • Axon Hillock: Trigger zone for action potentials.
    • Axon: Long projection leading to terminal arborizations and synaptic boutons (release neurotransmitters).

  • Neurotransmitters: Chemicals like acetylcholine secreted at synapses to relay neural signals.

  • Action Potentials: All-or-nothing signals generated when neuronal threshold is reached.

Neuronal Structure Types

  • Multipolar Neurons: Most common, with multiple dendrites and one axon (e.g., motor neurons).
  • Bipolar Neurons: Rare, with one dendrite and one axon (e.g., in special sensory organs).
  • Unipolar Neurons: Sensory neurons with one process that divides into peripheral and central branches.

Neuroglial Cells (Glial Cells)

  • Outnumber neurons and support neuronal function.
  • Include:
    • Astrocytes: Provide structural support, regulate neurotransmitter levels, and form the blood-brain barrier.
    • Microglial Cells: Act as phagocytes, removing debris and pathogens.
    • Ependymal Cells: Line brain ventricles and produce cerebrospinal fluid (CSF).
    • Oligodendrocytes: Form myelin sheaths in the CNS.
    • Schwann Cells (PNS): Form myelin sheaths in the PNS and surround nonmyelinated axons.
  • Satellite Cells: Surround neuron cell bodies in the PNS, regulating their environment.

Myelination

  • Myelin Sheath: Insulating layer that facilitates rapid signal transmission in neurons.
    • Saltatory Conduction: Action potentials jump between myelin sheath gaps (nodes of Ranvier) for faster transmission.
    • Nonmyelinated axons conduct impulses more slowly due to continuous conduction.

Nerves and Neural Integration

  • A nerve is a bundle of axons in the peripheral nervous system, organized into fascicles.
  • Connective tissues include:
    • Endoneurium: Surrounds individual axons.
    • Perineurium: Encloses fascicles of axons.
    • Epineurium: Surrounds the entire nerve.

Reflex Arcs

  • Monosynaptic Reflexes: Include a direct synapse between sensory and motor neurons (e.g., patellar reflex).
  • Polysynaptic Reflexes: Involve one or more interneurons (e.g., withdrawal reflex).
    • The structure of the reflex arc includes receptors, sensory neurons, integration centers (CNS), motor neurons, and effectors.

Types of Neural Circuits

  • Converging Circuits: Multiple inputs converge on a single output.
  • Diverging Circuits: One input diverges to multiple outputs.
  • Reverberating Circuits: Provide rhythmic outputs (e.g., breathing).
  • Parallel After-Discharge Circuits: Can integrate information simultaneously, useful in complex reasoning.

Summary

  • The nervous system is structured in a way that determines its functionality, with a focus on how information is processed and responded to through different neuronal types and networks.