Summary: A new study reveals how neurons in the GI tract coordinate activity to propel waste through the digestive system.
Millions of neurons in the gastrointestinal tract coordinate their activity to generate the muscle contractions that propel waste through the last leg of the digestive system, according to a study of isolated mouse colons published in Journal of Neuroscience. The newly identified neuronal firing pattern may represent an early feature preserved through the evolution of nervous systems.
The enteric nervous system (ENS) is known as the “second brain” or the brain in the gut because it can operate independently of the brain and spinal cord, the central nervous system (CNS). It has also been called the “first brain” based on evidence suggesting that the ENS evolved before the CNS.
Despite the known role of the ENS in generating motor activity in the colon, observing ENS neurons in action has been a challenge.
Nick Spencer and colleagues combined a new neuronal imaging technique with electrophysiology records of smooth muscle to reveal a pattern of activity that involves many different types of neurons firing simultaneously in repetitive bursts to activate the muscle cells at the same rate. They demonstrate how this rhythmic activity generates so-called colonic migrating motor complexes to transport fecal pellets through the mouse colon.
These findings identify a previously unknown pattern of neuronal activity in the peripheral nervous system.
Funding: The study was funded by National Health and Medical Research Council of Australia.
Source: David Barnstone – SfN
Publisher: Organized by NeuroscienceNews.com.
Image Source: NeuroscienceNews.com image is credited to Spencer et al., JNeurosci (2018).
Original Research: Abstract for “Identification of a rhythmic firing pattern in the enteric nervous system that generates rhythmic electrical activity in smooth muscle” by Nick J Spencer, Timothy J Hibberd, Lee Travis, Lukasz Wiklendt, Marcello Costa, Hongzhen Hu, Simon J Brookes, David A Wattchow, Phil G Dinning, Damien J Keating and Julian Sorensen in Journal of Neuroscience. Published May 28 2018
Identification of a rhythmic firing pattern in the enteric nervous system that generates rhythmic electrical activity in smooth muscle
The enteric nervous system (ENS) contains millions of neurons essential for organization of motor behaviour of the intestine. It is well established the large intestine requires ENS activity to drive propulsive motor behaviours. However, the firing pattern of the ENS underlying propagating neurogenic contractions of the large intestine remains unknown. To identify this, we used high resolution neuronal imaging with electrophysiology from neighbouring smooth muscle. Myoelectric activity underlying propagating neurogenic contractions along murine large intestine (referred to as colonic migrating motor complexes, CMMCs) consisted of prolonged bursts of rhythmic depolarizations at a frequency of ∼2 Hz. Temporal coordination of this activity in the smooth muscle over large spatial fields (∼7mm, longitudinally) was dependent on the ENS. During quiescent periods between neurogenic contractions, recordings from large populations of enteric neurons, in mice of either sex, revealed ongoing activity. The onset of neurogenic contractions was characterized by the emergence of temporally synchronized activity across large populations of excitatory and inhibitory neurons. This neuronal firing pattern was rhythmic and temporally synchronized across large numbers of ganglia at ∼2 Hz. ENS activation preceded smooth muscle depolarization, indicating rhythmic depolarizations in smooth muscle were controlled by firing of enteric neurons. The cyclical emergence of temporally coordinated firing of large populations of enteric neurons represents a unique neural motor pattern outside the central nervous system. This is the first direct observation of rhythmic firing in the ENS underlying rhythmic electrical depolarizations in smooth muscle. The pattern of neuronal activity we identified underlies the generation of CMMCs.
How the enteric nervous system (ENS) generates neurogenic contractions of smooth muscle in the gastrointestinal (GI) tract has been a long-standing mystery in vertebrates. It is well known that myogenic pacemaker cells exist in the GI-tract (called Interstitial cells of Cajal, ICC) that generate rhythmic myogenic contractions. However, the mechanisms underlying the generation of rhythmic neurogenic contractions of smooth muscle in the GI-tract remains unknown. We developed a high resolution neuronal imaging method with electrophysiology to address this issue. This technique revealed a novel pattern of rhythmic coordinated neuronal firing in the ENS that has never been identified. Rhythmic neuronal firing in the ENS was found to generate rhythmic neurogenic depolarizations in smooth muscle that underlie contraction of the GI-tract.