1.2

Functional Divisions and Primary Components of the Nervous System

• Definition and Primary Communication Mechanism

  • The body's nervous system is an electrochemical communication network that takes in information from the external world and internal body tissues, processes this information to make decisions, and sends back commands and instructions to bodily tissues.

  • Communication within this network occurs via specialized nerve cells known as neurons, which transmit chemical messengers called neurotransmitters.

• Main Functional Divisions

  • Central Nervous System (CNS):

  • Composed exclusively of the brain and the spinal cord.

  • Functions as the primary decision maker of the human body.

  • Peripheral Nervous System (PNS):

  • Composed of all nerve structures outside the CNS.

  • Responsible for gathering sensory information from the environment and body tissues, as well as transmitting executive decisions from the CNS outward to other body parts.

• Structure and Role of Nerves

  • Nerves are cable-like electrical bundles composed of numerous aggregated axons (the elongated neuron extensions that pass messages to other neurons, muscles, or glands).

  • Nerves serve as the physical linkages connecting the CNS with sensory receptors, muscles, and internal glands.

  • Example — Optic Nerve: Bundles approximately $1 \times 10^6$ ($1\text{ million}$) individual axons into a single continuous cable to convey visual information directly from the retina of the eye to the brain (Mason & Kandel, $1991$).

• Illustrative Real-World Scenario — Dual Emotional and Physiological Arousal

  • An individual entering a magnetic resonance imaging (MRI) scanner for a shoulder scan experienced extreme claustrophobia upon lying on their back within the confined scanner space.

  • Sympathetic Surge: Sudden onset of panic caused a pounding heart rate and an intense urge to escape.

  • Parasympathetic Reversal: Shortly thereafter, an internal calming influence counteracted the alarm, slowing the heart rate and relaxing the body, though physiological arousal fluctuated prior to completing the full $20\text{ minutes}$ confinement.

Functional Classes and Characteristics of Neurons

Information travels through the nervous system via three distinct operational classes of neurons:

• Sensory Neurons (Afferent Neurons)

  • Direction of Travel: Carry incoming sensory messages inward from bodily tissues and sensory receptors to the brain and spinal cord for processing.

  • Population Scale: The human nervous system contains a few million sensory neurons.

• Motor Neurons (Efferent Neurons)

  • Direction of Travel: Carry outgoing instructional messages and motor commands outward from the CNS to the body's muscles and glands.

  • Population Scale: The human nervous system contains a few million motor neurons.

• Interneurons

  • Location and Function: Situated within the CNS (brain and spinal cord), interneurons process information internally between sensory inputs and motor outputs.

  • Role in Human Complexity: Human cognitive, emotional, and behavioral complexity resides predominantly within these internal processing cells.

  • Population Scale: Interneurons vastly outnumber sensory and motor neurons, totaling billions and billions of cells.

Subdivisions and Mechanisms of the Peripheral Nervous System

The Peripheral Nervous System is organized into two primary functional components: the somatic nervous system and the autonomic nervous system.

• Somatic Nervous System

  • Primary Function: Enables voluntary control over skeletal muscles.

  • Mechanism: Constantly reports the real-time operational state of skeletal muscles to the brain and delivers motor commands back to initiate movement.

  • Example: When a friend taps an individual on the shoulder, the somatic nervous system conveys the sensory input to the brain and carries back instructions that trigger the neck muscles to turn the head toward the stimulus.

• Autonomic Nervous System (ANS)

  • Primary Function: Controls internal organ muscles and involuntary glandular activity (e.g., heartbeat, glandular secretions, and digestion).

  • Etymology: The term "autonomic" signifies "self-regulating." While consciously overriding the ANS is possible, it generally operates autonomously (analogous to a self-driving vehicle).

  • Sympathetic Nervous System (Arousal Division):

  • Primary Function: Expends energy and arouses the body to prepare for action ("fight or flight").

  • Specific Physiological Effects: Accelerates heartbeat, elevates blood pressure, inhibits/slows digestion, raises blood glucose/sugar levels, and stimulates perspiration to cool the body.

  • Triggers: Threatening, challenging, or alarming scenarios, such as taking an AP® Psychology exam, being confined inside an MRI machine, or facing an impending hazard.

  • Parasympathetic Nervous System (Calming Division):

  • Primary Function: Conserves energy and calms the body to facilitate internal maintenance ("rest and digest").

  • Specific Physiological Effects: Produces exact opposite physiological changes to the sympathetic division (e.g., decelerates heartbeat, restores active digestion, reduces blood pressure).

  • Homeostasis:

  • The sympathetic and parasympathetic divisions operate as a dynamic dual system to preserve a stable, balanced internal physiological state known as homeostasis.

  • Case Example — $2018$ Hawaii Missile Alarm Response:

  • In $2018$, residents in Hawaii received an urgent emergency alert regarding an incoming North Korean nuclear ballistic missile launch (Nagourney et al., $2018$).

  • The threat triggered intense sympathetic ANS arousal, leading individuals to believe they were about to die.

  • Upon notification $38\text{ minutes}$ later that the alert was a false alarm, parasympathetic activation initiated bodily recovery and calming.

Architecture and Dynamics of the Central Nervous System

• Neural Networks in the Brain

  • Cognitive Function: The brain serves as the ultimate center for human thinking, feeling, and voluntary action.

  • Neuron Count: The brain contains an estimated $128\text{ billion}$ neurons (Barrett, $2020$), each forming synaptic connections with thousands of other neurons to construct an ever-changing wiring web.

  • Networking Principles: Neurons aggregate into specialized functional work groups called neural networks (similar to human population clusters in cities, as noted by Stephen Kosslyn and Olivier Koenig in $1992$, p. $12$).

  • Structural Architecture: Cells build short, rapid connections with adjacent neurons; layers of cells inside a network establish connections with specific cells in subsequent layers.

  • Synaptic Plasticity and Learning: Learning complex skills (such as playing a violin, speaking a second language, or solving mathematical equations) occurs as repeated experience reinforces specific neural pathways. This principle is summarized by neuropsychologist Donald Hebb ($1949$): "neurons that fire together, wire together."

• The Spinal Cord and Reflex Architecture

  • Function: Serves as a two-way information superhighway physically bridging the PNS and the brain.

  • Fiber Tracts: Ascending neural fibers convey incoming sensory information upward to the brain; descending neural fibers convey motor-control instructions back down to bodily structures.

  • Reflexes: Reflexes are automatic, involuntary responses to sensory stimuli governed by localized spinal pathways.

  • Reflex Arc Structure: A simple spinal reflex arc consists of a single sensory neuron, a single motor neuron, and typically a single spinal interneuron.

  • Knee-Jerk Reflex: A classic simple reflex arc traveling from sensory receptors in the knee to the spinal cord (CNS) and back out through motor neurons to the leg muscles. This circuit is entirely spinal and can function in a headless warm body.

  • Pain Reflex Mechanism:

  1. Thermal energy from a candle flame excites heat-sensitive skin receptors.

  2. Neural impulses travel along sensory neurons to interneurons located within the spinal cord.

  3. Spinal interneurons immediately trigger motor neurons connected to the arm muscles.

  4. The arm contracts and jerks the hand away from the heat source before the sensory impulse reaches the brain or initiates the conscious perception of pain.

  • Consequences of Spinal Cord Severance:

  • If the top section of the spinal cord is completely severed, the brain is disconnected from sensory and motor pathways below the level of injury.

  • The individual loses all voluntary motor movement and sensory awareness (including sensation of pain or pleasure) in body regions linked to the cord below the lesion.

  • Spinal reflexes (such as the knee-jerk reflex) remain operational, but the tap is executed without conscious perception because sensory signals cannot reach the brain.