Chapter 11: Nervous Tissue

Nervous tissue consists of neurons and glial support cells organized to detect stimuli, process information, and generate coordinated responses throughout the body. The nervous system divides into the central nervous system encompassing the brain and spinal cord, the peripheral nervous system including all external neural tissue, and the enteric nervous system. The peripheral division separates into sensory pathways carrying information toward the central nervous system and motor pathways transmitting commands outward, with the motor system further subdividing into somatic control of skeletal muscles and autonomic regulation of involuntary functions. Individual neurons contain dendrites for receiving signals, a cell body housing the nucleus, and an axon extending to communicate with other cells across synaptic gaps. Glial cells in the central nervous system produce cerebrospinal fluid, maintain protective barriers, remove debris, and generate myelin sheaths that accelerate signal transmission, while peripheral glial cells similarly support axons and facilitate repair. Neuronal signaling depends on electrical gradients established by ion pumps and selective membrane permeability, with the resting potential maintained at approximately negative seventy millivolts. Stimuli open gated ion channels allowing sodium influx causing depolarization or potassium efflux causing hyperpolarization, generating localized graded potentials. When depolarization reaches threshold voltage, voltage-gated channels open in a cascade triggering action potentials that propagate along the axon through continuous conduction in unmyelinated segments or rapid saltatory jumping between nodes in myelinated regions. Synaptic transmission occurs when action potentials trigger calcium influx and neurotransmitter release into the synaptic cleft, where chemical messengers bind postsynaptic receptors generating excitatory potentials that depolarize or inhibitory potentials that hyperpolarize the receiving cell. Individual synaptic events integrate through temporal summation from repeated stimulation or spatial summation from multiple simultaneous inputs, enabling neurons to process complex information and determine whether threshold excitation occurs at the axon hillock.