The Nervous System

The nervous system detects, integrates, and responds to information. It is the organ system that produces sensation, coordinates movement, generates thought, maintains homeostatic regulation, and — centrally for this module — produces the experience of pain. Everything in somatics, pain neuroscience, and pharmacodynamics depends on understanding how the nervous system is organized and how it processes information.

Divisions

The nervous system has two anatomical divisions:

Central nervous system (CNS)

The brain and spinal cord — the processing centers where sensory information is integrated, decisions are made, and motor commands are generated.

The brain is organized into regions with distinct but interconnected functions:

• Cerebral cortex — the outer layer of the brain; the seat of conscious experience, voluntary movement, language, and reasoning. The somatosensory cortex (behind the central sulcus) processes touch, pressure, temperature, and pain location/intensity. The motor cortex (in front of the central sulcus) generates voluntary movement commands. The prefrontal cortex (front of the brain) handles planning, decision-making, and contextual evaluation — including the cognitive dimension of pain assessment.
• Limbic system — a group of structures involved in emotion, memory, and motivation. The amygdala processes threat detection and fear. The hippocampus encodes and retrieves memories. The anterior cingulate cortex processes the affective (unpleasantness) dimension of pain. These structures explain why pain is not just a sensation but an emotional experience — and why emotional states modulate pain intensity.
• Thalamus — the central relay station; nearly all sensory information passes through the thalamus on its way to the cortex.
• Hypothalamus — the master regulator of homeostasis; controls body temperature, hunger, thirst, circadian rhythm, and the hormonal stress response (hypothalamic-pituitary-adrenal axis).
• Brainstem — connects the brain to the spinal cord; contains centers for consciousness (reticular activating system), breathing (respiratory centers), cardiovascular regulation, and — critically — descending pain modulation (periaqueductal gray, rostral ventromedial medulla). The brainstem is where descending signals that amplify or suppress pain transmission originate.
• Cerebellum — coordinates movement, balance, and motor learning. Involved in the smooth, coordinated execution of movement that somatic practices work to refine.

The spinal cord runs from the brainstem through the vertebral column. It contains:

• Ascending pathways — carry sensory information (including nociceptive signals) from the body to the brain
• Descending pathways — carry motor commands from the brain to the muscles, and modulatory signals from the brainstem to the dorsal horn
• The dorsal horn — the posterior region of the spinal cord’s gray matter; the first major processing center for sensory input, including nociception. The dorsal horn is not a passive relay — it is where gate-control mechanisms, descending modulation, and central sensitization all operate. Understanding the dorsal horn is essential for understanding pain.
• The ventral horn — the anterior region; contains motor neurons whose axons exit the spinal cord and travel to skeletal muscles

Peripheral nervous system (PNS)

Everything outside the brain and spinal cord — the nerves that connect the CNS to the rest of the body.

Somatic nervous system — the nerves controlling voluntary movement and conscious sensation:

• Sensory (afferent) nerves carry information from sensory receptors (in skin, muscles, joints, viscera) to the spinal cord and brain. Different fiber types carry different information at different speeds: A-beta fibers (touch/pressure, fast), A-delta fibers (sharp pain, fast), C fibers (dull pain, temperature, slow).
• Motor (efferent) nerves carry commands from the spinal cord to skeletal muscles.

Autonomic nervous system — the nerves controlling involuntary functions: heart rate, blood pressure, digestion, respiration, pupil dilation, glandular secretion. Two divisions:

• Sympathetic — prepares for action; increases heart rate, dilates bronchi, releases glucose, diverts blood to muscles, dilates pupils. Activated by threat, exercise, or stress.
• Parasympathetic — promotes maintenance and recovery; decreases heart rate, constricts bronchi, promotes digestion, constricts pupils. Dominated by the vagus nerve (cranial nerve X), which is central to polyvagal theory and to the autonomic regulation that Somatic Experiencing works with.

The autonomic nervous system is not under voluntary control — but it can be influenced by practices that alter autonomic input. Deep, slow breathing activates the parasympathetic system through vagal stimulation. Somatic awareness training improves interoceptive sensitivity to autonomic states. These are not metaphors — they are interventions on the autonomic nervous system through its known input pathways.

Signal flow

The nervous system operates through a consistent flow:

1. Sensory receptors detect a stimulus (pressure, temperature, chemical, stretch)
2. Sensory neurons carry the signal as action potentials along afferent nerves to the spinal cord
3. The spinal cord processes and modulates the signal (dorsal horn gating, interneuron activity)
4. Ascending pathways carry the processed signal to the brain
5. The brain integrates the signal with context, memory, emotion, and expectation — and produces a conscious experience (sensation, pain) and/or generates a motor response
6. Descending pathways carry motor commands and modulatory signals back to the spinal cord
7. Motor neurons carry commands to muscles, producing movement

This flow is not a one-way conveyor belt. At every stage, the signal is modulated — amplified, suppressed, or transformed. The brain does not passively receive what the periphery sends. It actively shapes what reaches consciousness and what does not. This is why pain is a product of the nervous system, not a message from the tissues.