The nervous system is divided into central (brain and spinal cord) and peripheral (sensory and motor nerves) components. Sensory (afferent) neurons detect stimuli and send information to the CNS. Motor (efferent) neurons carry commands from the CNS to muscles. The spinal cord can execute rapid local reflexes without brain involvement. The brain provides centralized integration and allows flexible behavioral control.
Trace a simple reflex arc showing sensory neuron, synapse in spinal cord, and motor neuron. Study dermatome and myotome maps showing sensory and motor organization. Compare reflex latency with voluntary movement latency. Examine spinal cord lesion effects.
Peripheral nerves are just passive wires / all information must reach the brain / spinal cord reflexes are less important than brain control / sensory and motor organization are completely separate.
The nervous system is the body's information infrastructure, and understanding its organization starts with a single architectural distinction: the central nervous system (CNS) — the brain and spinal cord — is the processing center, while the peripheral nervous system (PNS) — the network of nerves branching out through the body — is the input/output system that connects the CNS to everything else. Everything you sense, every movement you make, and every internal regulation your body performs runs through this two-part system.
Think of it like a company. The brain is headquarters: it integrates vast amounts of information, plans, decides, and issues complex commands. The spinal cord is the regional office: it handles local, time-sensitive operations without needing to route every decision through headquarters. The peripheral nerves are the field agents — sensors and actuators in every tissue and organ, constantly sending reports inward and receiving instructions outward. Afferent (sensory) neurons carry signals from the periphery toward the CNS — from skin, muscles, joints, and organs, reporting what is happening in the environment and inside the body. Efferent (motor) neurons carry signals from the CNS outward to muscles and glands, producing movement and regulation.
The spinal cord's ability to act locally is most clearly demonstrated in the reflex arc. When you touch something hot and pull your hand away before you consciously register pain, that withdrawal happens at the spinal level: sensory neurons in your hand synapse directly onto interneurons in the spinal cord, which activate motor neurons to the arm muscles — all without waiting for a signal to travel up to the brain and back down. The brain learns about the event a fraction of a second later, after the protective action has already occurred. This architecture makes sense for survival: fast, stereotyped responses to immediate threats do not need centralized deliberation. The brain's role is to provide flexible, context-sensitive behavior that reflexes cannot — planning, learning, inhibiting reflexes when appropriate, and coordinating complex sequences of movement.
This organizational framework is the foundation for everything else in biological psychology. The distinction between CNS and PNS predicts what happens after injury: damage to peripheral nerves can often regenerate (slowly), while damage to the CNS — brain or spinal cord — is typically permanent because mature central neurons do not regenerate in the same way. The division into sensory and motor pathways predicts the pattern of deficits after specific injuries: damage to the dorsal (back) portion of the spinal cord disrupts sensation while sparing movement; damage to the ventral (front) portion disrupts movement while sparing sensation. Understanding this architecture turns isolated anatomical facts into a coherent, predictive map of how the nervous system works and how it fails.
This is a foundational topic with no prerequisites.
No prerequisites — this is a starting point.