Researchers at Texas Children’s Hospital have found that the middle part of the brain guides the hippocampal region — the brain’s front part that is crucial for memory and navigation. The study, published in Nature, explains the underlying steps of a new neural pathway that governs how signalling occurs across the brain’s regions. This study will help shed light on how a person learns new things or behaves in new environments.
It is known that the brain’s neurons can reorganise their structure and connections in response to learning or adapting to new input from the external environment. This self-organising activity of the brain, or neuroplasticity, happens due to synaptic plasticity, or repeated communication between neurons at non-contact junctions called synapses. The repeated communication exchange between the neurons forms a neural network in response to an external signal.
“It [the result of the study] completely changes our view of how learning-dependent changes in the brain occur and reveals new realms of possibilities that will transform and guide how we approach neurological and neurodegenerative disorders in the future,” said Dr Jeffrey Magee, professor at Baylor College of Medicine, in a statement.
The brain adaptability – or how it rewires in response to changes in the external environment, information, and experiences – has baffled scientists for long. However, various synaptic plasticity theories, like Hebbian’s theory that says “neurons that fire together, wire together”, were proposed to explain the same.
In 2021, Dr Jaffery Magee’s research team from the Baylor College of Medicine found evidence of change in the neurons’ activity of the hippocampus in response to adapting to a new activity or environment and termed it behavioural timescale synaptic plasticity (BTSP). The result also showed that an internal signal triggered the communication process between neurons in response to the external factor.
In the follow-up research in 2022, the research team conducted experiments on mice to identify the internal brain region that triggers the activity of neurons in the hippocampus. Brain neuron activity while the rodents ran on a treadmill in two different environments, were recorded.
First, sugar water was kept as a reward on the treadmill. After the mice ran each lap the position of the reward was changed. Researchers observed that the mice ran at the same speed to get at the reward. When the sugar water bottle was fixed on the treadmill, however, the mice slowed down when approaching it.
A powerful microscope recorded the mice’s brain activity in the experiment. The researchers observed an increased density and activity of specific neurons (place cells) in the front part of the brain, which is responsible for the changes in the behaviour of mice.
“The behaviour change was accompanied by increased density and activity of place cells around the hippocampal region. This indicated that changes in place cells can lead to adaptive reorganisation of mice’s brains,” Dr Christine Grienberger, a post-doctoral fellow in Dr Jaffery Magee’s lab at Texas Children’s Hospital, said in the statement.
However, the signal for the neuron activity was not observed in the hippocampus. Therefore, the researchers hypothesized that it could be coming from the brain’s middle – the entorhinal cortex – which acts as a gateway for the entry and exit of information to the hippocampus region.
To test the hypothesis, they switched off the function of a specific group of nerve fibres of the brain’s middle region and noticed that the activity and density of the place cells in the hippocampus also diminished immediately. Hence, the researchers concluded that the middle part of the brain controls the front part of the brain during neural activity.