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.