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Ganglia: These are small clusters of neuron cell bodies located outside the central nervous system (CNS) and serve as relay points or processing stations in the peripheral nervous system (PNS).
- Function as local opportunities for processing neural signals prior to relaying them to the CNS.

Brain Layers:
- Cortex: The gray matter surface of the brain where much of the processing occurs.
- Core Structure: The interior of the brain primarily consists of white matter.
- White Matter: Comprised mainly of myelinated axons connecting different brain regions.

Neurons: The fundamental units of the nervous system that transmit signals through electrical impulses.
- The human body contains the longest neuron extending from the spinal cord to the toes, indicating the vast length of neuronal pathways.

Nervous System: Described as the master controlling communication system of the body; responsible for rapid, precise, and short-lived signals compared to the slower and longer-lasting signals of the endocrine system.
- Endocrine System: Utilizes hormones released into the bloodstream affecting various cells, unlike the direct communication of the nervous system.

Neural Impulses: Generated through the movement of ions, primarily sodium and potassium, across neuron membranes.
- Neurotransmitter Release: In a synapse, neurotransmitters released from the presynaptic neuron bind to receptors on the postsynaptic neuron, initiating an ion flow that can lead to an action potential.

Parts of a Neuron:
- Cell Body (Soma): Contains the nucleus and organelles.
- Dendrites: Tree-like structures that receive signals from other neurons.
- Axon: A long projection that transmits impulses away from the cell body to other neurons or muscles.
- Axon Terminals: Ends of axons where neurotransmitter release occurs.

Neurons are typically classified into three major types:
- Multipolar Neurons: Most common type, with multiple dendrites and one axon; primarily found in the CNS.
- Unipolar Neurons: Have a single process that splits into two branches (sensory neurons); often found in the PNS ganglia.
- Bipolar Neurons: Characteristically have two distinct processes; found in sensory organs (retina, olfactory areas).

Central Nervous System (CNS): Comprises the brain and spinal cord, acting as the processing center for sensory inputs and determining motor outputs.
- Gray Matter: Contains neuronal cell bodies and is situated typically in the interior of structures such as the spinal cord.
- White Matter: Made up of myelinated axons, functioning like highways for signal transmission.

Peripheral Nervous System (PNS): Contains all the neural elements outside the CNS, including sensory (afferent) and motor (efferent) neurons.
- Subdivisions:
- Sensory Division: Carries sensory information from sensory receptors to the CNS.
  - Somatic Sensory: Relays information from the skin, muscles, and joints.
  - Visceral Sensory: Informs the CNS about conditions within the internal organs.
- Motor Division: Transmits signals from the CNS to effectors (muscles or glands).
  - Somatic Motor: Controls voluntary movements; innervates skeletal muscles.
  - Autonomic Motor (Involuntary): Controls involuntary functions of internal organs.
  - Sympathetic Branch: Activates fight or flight responses.
  - Parasympathetic Branch: Promotes rest and digest functions.

Reflex Actions: Quick responses that occur via the spinal cord without direct involvement of the brain, demonstrating a direct sensory to motor pathway.
- Withdrawal Reflex Example: Touching a hot object stimulates sensory neurons that send signals to the spinal cord, immediately activating motor neurons to withdraw the hand.

Types of Glial Cells:
- Oligodendrocytes: Myelinate axons in the CNS; can wrap multiple axons.
- Astrocytes: Supportive cells that maintain the blood-brain barrier and assist in recycling neurotransmitters.
- Microglia: Act as immune cells in the CNS, scavenging for pathogens and debris.

Meninges: Protective layers surrounding the CNS; inflammation of these layers indicates meningitis.

The nervous system functions by transmitting electrical signals through neurons, classified into sensory, motor, and interneurons, with the CNS integrating inputs and outputs while the PNS facilitates communication to and from the body.
A clear understanding of the structure and function of neurons, glial cells, and the division of the nervous system is essential for grasping the fundamental mechanisms of communication within the body.