Human Nervous System

Chapter Overview

The Human Nervous System is an extensive physiological chapter in Class 12 Biology that elucidates rapid electrical and chemical communication across the body. It begins with the fundamental components of nervous coordination: sensory receptors detecting environmental cues, integrating neural pathways, and effector organs carrying out rapid behavioral or physiological responses. The structural unit-the neuron-is analyzed alongside supporting neuroglial cells (Schwann cells, oligodendrocytes, and astrocytes), contrasting sensory, motor, and interneurons, while detailing the involuntary knee-jerk reflex arc as a model monosynaptic pathway.

The chapter provides an in-depth biophysical account of the nerve impulse, explaining how the resting membrane potential (-70 mV) is sustained through active sodium-potassium ATPase pumps and differential ion permeability. It contrasts this with active membrane potential generation (+50 mV), characterized by voltage-gated sodium influx during depolarization and potassium efflux during repolarization. Synaptic transmission is detailed from calcium influx at the presynaptic knob to neurotransmitter exocytosis across the synaptic cleft, distinguishing excitatory transmitters like acetylcholine from inhibitory transmitters like GABA.

At the system level, the text systematically maps out the Central Nervous System (CNS) and Peripheral Nervous System (PNS). Brain anatomy covers the forebrain (cerebrum, diencephalon, limbic system), midbrain, and hindbrain (cerebellum, pons, medulla oblongata), ventricles, and protective meninges, along with spinal cord reflex tracts. The autonomic nervous system is evaluated by contrasting antagonistic sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) divisions. Finally, the chapter addresses pharmacological drug addiction, tolerance, and withdrawal, as well as neurological disorders and non-invasive diagnostic tools including EEG, CT scans, and MRI.