Sponges get by without nerves; almost nothing else does
A nervous system coordinates action and sensation by shuttling electrochemical signals through the body, usually in tandem with hormones. Sponges, placozoans, and mesozoans are the rare multicellular animals that lack one entirely—everyone more complex wires itself somehow.
Vertebrates split their wiring into two territories. The brain and spinal cord form the central nervous system, while the peripheral system is mostly nerves: sheathed bundles of axons running out to every corner of the body. Motor (efferent) nerves carry commands away from the brain, sensory (afferent) nerves carry reports inward, and the peripheral side divides again into somatic and autonomic branches. The autonomic branch has three parts: sympathetic for emergencies, parasympathetic for rest, and enteric for running the gut.
The working unit is the neuron, which fires electrochemical impulses along its axon. At an electrical synapse the pulse passes straight to the next cell; at a chemical synapse it triggers the release of neurotransmitters that can excite, inhibit or subtly tune the receiver. Chains of neurons build circuits and larger networks that produce perception and steer behaviour. Glial cells supply structure and metabolic support, and a neurovascular unit adjusts blood flow in the brain so busy neurons get the energy they demand.
Nervous tissue first appeared in wormlike creatures roughly 550 to 600 million years ago. Living species now span a huge range, from worms with only a few hundred nerve cells up to roughly 300 billion in an African elephant. Comb jellies and cnidarians such as corals, hydras and jellyfish get by with a diffuse nerve net, while nearly every other animal has a brain, a central cord or a pair of cords, and radiating nerves. Sponges, placozoans and mesozoans have no nervous system at all, though sponges and even slime molds use cell-to-cell signals that foreshadow neural ones.
Nerves are thick enough that ancient Egyptians, Greeks and Romans noticed them, but their inner makeup stayed hidden until microscopes showed axons wrapped in membranes and grouped into fascicles. When the system fails, the cause may be a genetic defect, trauma, toxins, infection or plain ageing. In the periphery the commonest fault is broken nerve conduction, seen in diabetic neuropathy and in demyelinating conditions such as multiple sclerosis.
Source: Nervous system
