THE SECOND BRAIN

 

The human gut, or, more properly, the entire Alimentary Canal (AC), from the esophagus to the anus, is innervated by two separate nets of neurons (afferent, efferent, and interneuron types). This neural system is known as the Enteric Nervous System (ENS), and, remarkably, although it is linked to the Central Nervous System (CNS; brain/spinal cord), it is able to operate entirely on its own, if these CNS links are severed.

The ENS controls the peristaltic movement of the esophagus, stomach, and intestine, influences AC sphincter opening and constricting, and has influence over absorption of nutrients, and, in the colon, fluids. The ENS also affects the amount of hydrochloric acid that goblet cells generate and dump into the stomach, assists in localizing AC infections and guiding immune responders to the sites, and shuts down much of the blood flow to the gut and nearly stops digestion during times of great physical exertion or stress, in order to provide more oxygen-rich blood to the brain and muscles, and more glucose to the brain.

The sensory neurons of the ENS help mediate hunger by providing feedback to the CNS (especially the Thalamus) regarding stomach fullness and gut activity. Sensory neurons also warn the brain about potential lesions, blockages, constrictions, or constipation, via pain signals. When the stomach or small intestine contains some toxic, infectious, or otherwise objectionable material, sensory neurons of the ENS signal the Vagus Nerve to initiate reverse peristalsis, resulting in vomiting. These are just a few of the many functions of the ENS neurons. The ENS connects to the CNS in two ways; to the Parasympathetic Nervous System via the Vagus Nerve and to the Sympathetic Nervous System via the Prevertebral Ganglion. Again, however, if these connections are ablated somehow, the ENS functions very well on its own.

To finally answer your question, the ENS has an amazing 500,000,000 or so individual neurons composing its networks, which has led to it being misleadingly referred to as the “second brain”. While this is a whole bunch of neurons, the human brain contains around 200 times the number of neurons present in the ENS, and, just as important, the neural connections, pathways, specialized structures, and neurotransmitter variations in the brain are far more complex than in the ENS. Interestingly, the spinal cord “only” contains around 100 million neurons, or 1/5 the number of neurons in the ENS.