THE HUMAN NERVOUS SYSTEM — A COMPLETE, DETAILED LESSON The human nervous system is the body’s electrochemical communication network, responsible for sensing the environment, processing information, generating behavior, and maintaining internal stability. It is composed of neurons and glial cells, organized into the central nervous system (CNS) and peripheral nervous system (PNS). 1. Major Divisions of the Nervous System A. Central Nervous System (CNS) Includes the brain and spinal cord, housed within the skull and vertebral column. Brain Weighs ~3 pounds, ~60% fat, and contains ~100 billion neurons. Divided into cerebrum, cerebellum, and brainstem. Spinal Cord A long, thin bundle of nervous tissue that relays signals between brain and body. Integrates reflexes without requiring conscious brain input. B. Peripheral Nervous System (PNS) All neural tissue outside the CNS. Includes nerves and ganglia. Subdivisions: Somatic Nervous System — voluntary control of skeletal muscles. Autonomic Nervous System — involuntary control of organs, glands, and smooth muscle. Sympathetic (“fight or flight”) Parasympathetic (“rest and digest”) 2. Neurons — The Functional Units Neurons are specialized cells that transmit information using electrical impulses and chemical signals. Structure Dendrites — receive incoming signals. Cell body (soma) — integrates signals. Axon — long projection that sends signals; can be up to 1 meter long. Myelin sheath — increases conduction speed. Axon terminals — release neurotransmitters. Signal Transmission Electrical: action potentials travel along axons. Chemical: neurotransmitters cross synapses to activate the next cell. 3. Glial Cells — The Support System Glial cells provide structural support, insulation, immune defense, and metabolic assistance. Types include: Astrocytes — regulate environment, form blood–brain barrier. Oligodendrocytes — myelinate CNS axons. Schwann cells — myelinate PNS axons. Microglia — immune cells of the CNS. 4. Gray Matter vs. White Matter Gray matter: neuron cell bodies; responsible for processing and interpreting information. White matter: myelinated axons; responsible for transmitting signals. In the brain: gray outside, white inside. In the spinal cord: white outside, gray inside. 5. Major Brain Regions and Their Functions A. Cerebrum Largest part of the brain; responsible for higher functions like memory, reasoning, emotions, and voluntary movement. Cerebral Cortex Outer gray matter layer with folds (gyri) and grooves (sulci). Divided into frontal, parietal, temporal, and occipital lobes. Deep Structures Basal nuclei — planning and coordinating movement. Limbic system — emotion, memory, behavior. B. Diencephalon Includes: Thalamus — sensory relay station. Hypothalamus — homeostasis, hormones, hunger, temperature. C. Brainstem Controls vital functions: Breathing Heart rate Blood pressure Sleep cycles D. Cerebellum Coordinates: Balance Posture Fine motor control 6. Spinal Cord Organization Ascending tracts: carry sensory information to the brain. Descending tracts: carry motor commands from the brain. Reflex arcs: rapid, automatic responses processed in the spinal cord. 7. How the Nervous System Works (Functionally) The nervous system performs three major functions: 1. Sensory Input Detects internal and external stimuli through receptors. 2. Integration Processes information in the CNS. 3. Motor Output Activates muscles or glands to respond. 8. Voluntary vs. Involuntary Control Somatic Nervous System (Voluntary) Controls skeletal muscle movement. Autonomic Nervous System (Involuntary) Regulates: Heartbeat Breathing Digestion Metabolism 9. Development of the Nervous System The nervous system begins forming around 18 days after conception as the neural plate. Key developmental facts: Most neurons are generated prenatally and are not replaced later. ~2.5 million neurons are produced per minute during development. Early reflexes appear by the second prenatal month. 10. How the CNS and PNS Work Together The CNS processes information; the PNS delivers it. Examples: Touching a hot surface → PNS sends signal → CNS interprets → PNS triggers withdrawal reflex.