7.2 Nervous Systems and Neuron Structure

Animal nervous systems vary from diffuse nerve nets in cnidarians to centralized brains and cords. Their shared logic is input, integration and output. Receptors transduce environmental or internal stimuli into electrical changes. Sensory neurons carry information toward an integrating region. Relay neurons connect circuits within that region. Motor neurons carry commands to effectors such as muscles or glands.

In humans, the central nervous system consists of brain and spinal cord, while peripheral nerves connect it to receptors and effectors. The cerebral hemispheres contribute to language, memory, reasoning and voluntary action; the cerebellum coordinates movement and balance; the medulla contributes to automatic functions such as ventilation and heart-rate control; and the hypothalamus links neural information to endocrine control through the pituitary.

The spinal cord can integrate a reflex without waiting for conscious analysis. In a withdrawal reflex, sensory input activates relay and motor pathways that protect the body rapidly, while other signals ascend to the brain and contribute to conscious perception. Calling a reflex 'unconscious' describes how it is initiated, not an absence of information reaching the brain.

A typical motor neuron has many dendrites and a cell body receiving input, plus one long axon carrying action potentials toward terminals. The cell body contains the nucleus and most protein-synthesis machinery. Axon terminals form synapses with effectors or other neurons. A sensory neuron's cell body commonly lies to one side of a long process, positioning it between receptor endings and the CNS rather than at one terminal.

Axons may be wrapped by myelin-forming glial cells. In peripheral nerves, Schwann-cell membrane repeatedly wraps an axon, creating a lipid-rich insulating sheath separated by nodes of Ranvier. Myelin reduces current loss and changes where action potentials are regenerated. Glia are therefore active contributors to neural performance, not merely packing around the 'real' signalling cells.