Heartbeats Can Modulate Changes in Neuronal Activity
Post by Laura Maile
The takeaway
The brain monitors not only changes in the external environment but also the internal state of the body. Neuronal activity synchronizes with pressure pulsations in the surrounding vasculature, giving neurons the capacity to directly detect heartbeats.
What's the science?
Interoception is the ability for the brain to sense the internal state of the body. There are several examples of neurons being directly or indirectly activated by interoceptive feedback - neurons in the olfactory bulb that are mechanically sensitive to airflow during respiration and cardiac baroreceptor cells that detect blood pressure and heartbeat, relaying that information to other neurons. Scientists have previously identified mechanosensitive ion channels in neurons within different regions of the brain that allow cells to perceive mechanical force. It is not yet clear whether cells in the brain can directly detect heartbeat pressure pulsations through mechanosensation, rather than indirectly receiving feedback from the heart itself. This week in Science, Jammal Salameh and colleagues examined neurons of the rat olfactory bulb to determine whether the neurons themselves could detect pressure pulsations mimicking the heartbeat via mechanically sensitive ion channels.
How did they do it?
The authors perfused the vascular system of rats through the aorta with oxygenated artificial cerebrospinal fluid, allowing them to mimic the pressure pulsations caused by the heartbeat. They then inserted a recording electrode into the olfactory bulb tissue and measured local field potentials (LFPs), signals representing the activity of a group of neurons adjacent to the probe, and observed the relationship between neural activity and pump-induced perfusion pressure. They determined whether the neural oscillations were localized to a specific area by recording from nine locations on the surface of the olfactory bulb, and then from multiple layers. The authors next tested whether mechanosensitive ion channels like Piezo2 produced neural activity by injecting a compound that blocks the activity of various known mechanosensitive ion channels. They analyzed the spontaneous activity of olfactory bulb mitral cells, and simultaneously measured LFP oscillations. Finally, they examined whether blood pressure pulsations activate olfactory bulb neurons in mice by simultaneously measuring heart electrocardiogram, respiration via nasal airflow, and olfactory bulb neuron activity.
What did they find?
The authors found neural oscillations for which activity was temporally correlated with the pressure pulsations induced by the heart-mimicking pump. They localized the LFPs to a specific layer of the olfactory bulb called the mitral cell layer, composed of mitral cells that represent a major output channel of the olfactory bulb. Injecting a blocker of mechanically sensitive ion channels abolished slow LFP oscillations, but did not affect overall neural activity. This indicates that synaptic transmission is not involved and that the modulation of activity by pressure pulsations is below the threshold of neuronal firing. They detected synchronization between spontaneous olfactory bulb mitral cell activity and the perfusion pressure pulsations. They also found a direct correlation between LFP oscillations and mitral cell excitatory currents. Together, this suggests that rhythmic pressure pulsations stimulate mitral cell activity. Finally, in awake animals, they detected neuronal firing that was entrained to the heartbeat in a subset of neurons. This heartbeat entrainment was also observed in the hippocampus and prefrontal cortex.
What's the impact?
This study found that a subset of olfactory bulb neurons are directly modulated by heartbeat-induced pressure pulsations of the brain vasculature. This is the first study to show increases in the activity of neurons in response to vascular pulsations, demonstrating how the brain can detect and respond to cardiac activity. We still don’t fully understand the link between the mind and body, but this study adds to our understanding of the brain’s capacity to directly monitor the activity of the heart.
