User:SarahReed54/Cortical Remapping

Cortical remapping, also referred to as cortical reorganization, is what happens when a cortical map becomes affected by a stimulus, changes and creates a 'new' cortical map.

Definition
Cortical remapping in the somatosensory system happens when there has been an increase in sensory input and there has been a decrease in sensory input due to deafferentation or amputation. Every part of the body is connected to a corresponding area in the brain which creates a cortical map. When something happens to disrupt the cortical maps such as an amputation or a change in neuronal characteristics, the map is no longer relevant. The part of the brain that is in charge of the amputated limb or neuronal change will be dominated by adjacent cortical regions that are still receiving input, thus creating a remapped area.

History
Dr. Wilder Penfield, a neurosurgeon, was one of the first to map the cortical maps of the human brain. When performing brain surgeries on conscious patients, Penfield would touch either a patient’s sensory or motor brain map, located on the cerebral cortex, with an electric probe, to determine if a patient could notice either a specific sensation or movement in a particular area on their body. Penfield also discovered that the sensory or motor maps were topographical; areas of the body adjacent to one another would likely be adjacent on the cortical maps.

Due to Penfield’s work, the scientific community concluded that the brain must be fixed and unchangeable because a specific area of the brain corresponds to a particular point on the body. However, this conclusion was challenged by Michael Merzenich, who many call “the world’s leading research in brain plasticity.” In 1968, Merzenich and two neurosurgeons, Dr. Ron Paul and Herbert Goodman, conducted an experiment to determine effects on the brain after a large bundle of peripheral nerves in adolescent monkeys’ hands were cut and began to regenerate again. They knew that the peripheral nervous system could regenerate itself and sometimes during that process the neurons would ‘rewire’ themselves by accident. These ‘wires’ would accidently connect to a different axon, stimulating the wrong nerve. This results in a “false localization” sensation, where if the patient is touched on a specific area of the body, that touch is actually felt on a different part of the body than expected. To better understand this phenomnen in the brain they used microelectrodes to micromap the monkey’s cortical map of its hand. The peripheral nerves were cut and sewn close together to observe evidence of axon ‘wires’ crossing during regeneration. After seven months, the cortical map of the monkeys' hands were remapped and instead of finding hand maps that were a jumble of ‘wires’, due to the expected ‘wire’ crossing, the maps appeared to be essentially normal. The new maps were arranged as if the intended ‘wire’ crossing never even had a chance of actually crossing. They concluded if a cortical map was able to ‘normalize’ itself when stimulated with an irregular input that the adult brain must be plastic.

This experiment helped inspire questioning of the scienctific “truth” that the adult brain is fixed and cannot continue to change outside of the critical period, especially by Merzenich. Later in his career, Merzenich conducted an experiment that highlighted the existence of cortical remapping and neuroplasticty. Merzenich and fellow neuroscientist, Jon Kaas, conducted an experiment where they cut the median nerve of a monkey’s hand, which delivers sensation to the middle of the hand, to see what the median nerve map would look like when all input was cut off after a period of two months. When the hand was remapped, it was found that when the middle of the hand was touched no activity occurred at the median nerve location. But when the sides of the monkey’s hand were touched, activity was found in the median nerve location on the map. This meant that the cortical remapping had occurred at the median nerve; the nerves that correlated to the outsides of the monkey’s hand had remapped themselves to take over the ‘cortical real estate’ that was now available due to the median nerve being disconnected.

Mechanism
Remapping can occur in the sensory or motor system. The mechanism for each system may be quite different.

Sensory System
Sensory system remapping can potentially self-organize due to the spatiotemporal structure of input. This means that the location and timing of the input is critical for remapping in the sensory system. A study by Gregg Recanzone demonstrates this by seeing if a monkey could distinguish between a stimulus of high and low frequency vibrations. Both low and high frequency vibrations were delivered to the tip of a finger of the monkey at a fixed location. Over time, the monkey got better and better at identifying the differences in vibration frequency. When the finger was mapped, they found that map of the finger had been degraded. Because the stimuli was done at a fixed location, everything was excited and therefore selected. This created a cortical map that was crude with no refinement. The experiment was then conducted again, except the location of the high and low vibrations were varied at different parts of the finger tip. As before, the monkey improved overtime. When the monkey's finger was remapped it was found that the crude map from before had been replaced with an elegant map of the finger-tip showing all the different places stimulation had occurred on different locations of the fingertip. This study showed that over time a map that could be created from a localized stimulus and then that same map could be altered by location variable stimulus.

Motor System
Motor system remapping, as compared to sensory system remapping, receives more limited feedback that can be difficult to interpret. When looking at motor system maps, you find that the last pathway for movement to occur in the motor cortex does not actually activate the muscles directly, but causes decreased motor neuron activity. This means, there is a possibility that remapping in the motor cortex can come from changes in the brainstem and spinal cord, locations that are difficult to experiment on, due to challenging access.

A study done by Anke Karl helps demonstrate why the motor system may be dependent on the sensory system in regards to cortical remapping. The study found a strong connection between motor and somatosensory cortical remapping after amputation and phantom limb pain. The study made an assumption that somatosensory cortex reorganization can affect plasticity in the motor system, because stimulation of the somatosensory cortex prompts long term potentiation in the motor cortex. The study concluded that reorganization of the motor cortex may only be subsidiary to cortical changes in the somatosensory cortex. This helps support why the feedback received by the motor system is limited and difficult to determine for cortical remapping.

Application
Cortical remapping helps individuals regain function from injury.

Phantom Limbs
Phantom limbs are sensations felt by amputees that make it feel like their amputate extremity is still there. Sometimes amputees can experience pain from their phantom limbs; this is called phantom limb pain (PLP).  Recently, a study by Tamar R. Makin suggests that instead of PLP being caused by maladaptive plasticity, it may actually be pain induced. The maladaptive plasticity hypothesis suggests that once afferent input is lost from an amputation, cortical areas bordering the same amputation area will begin to invade and take over the area, affecting the primary sensorimotor cortex, seeming to cause PLP. Makin now argues that chronic PLP may actually be ‘triggered’ by “bottom-up nociceptive inputs or top-down inputs from pain-related brain areas” and that the cortical maps of the amputation remain in-tact while the “inter-regional connectivity” is distorted.

Stroke
The mechanisms involved in stroke recovery mirror those related to brain plasticity. Tim H. Murphy describes it as, “Stroke recovery mechanisms are based on structural and functional changes in brain circuits that have a close functional relationship to those circuits affect by stroke." Neuroplasticity after a stroke is enabled by new structural and functional circuits that are formed through cortical remapping.

A stroke occurs when there is not enough blood flow to the brain, causing debilitating neurological damage. The tissue that surrounds the infarct (stroke damaged area) has reduced blood flow and is called the penumbra. Though the dendrites in the penumbra have been damaged due to the stroke they can recover during the restoration of blood flow (reperfusion) if done with hours to a few days of the stroke due to time sensitivity. Due to reperfusion in the peri-infarct cortex (found next to the infarct), the neurons can help with active structural and functional remodelling after stroke.

Cortical remapping is activity-dependent and competitive. The recovering peri-infarct regions that have bad circuits are competing with healthy tissue for cortical map space. An in vivo study by Murphy, was done using mice to help identify the sequence and kinetics of the peri-infarct cortical remapping after stroke. The study showed that eight weeks after a stroke had occurred in the forelimb sensory cortex of a mouse, the 'surviving' portion was able to promptly relay enhanced sensory signals to the motor cortex, which resulted in the remapping of sensory function. The mouse that experienced a stroke had remapped responses that lasted longer and spread farther from the motor cortex than those of the control. This means that recovery of the sensorimotor functions after stroke and cortex remodeling suggests changes in the temporal and spatial spread of sensory information.

A model for stroke recovery suggest by Murphy, involves beginning with homeostatic mechanisms (neurons receive proper amount of synaptic input) at the start of stroke recovery. This will restart activity in stroke-affected areas through structural and functional circuit changes. Activity-dependent synaptic plasticity can then strengthen and refine circuits when some of the sensory and motor circuitry is spared. Regions of the brain with partial function can have their circuits recover over a few days to weeks through remapping. Cortical remapping after a stroke is comparable to initial brain development. For example, remapping that occurs in motor recovery after a stroke is similar to an infant learning skilled movement patterns. Though this is very important information on developing recovery plans for stroke patients, it is important to keep in mind the circuitry of a stroke patient is quite different from that of a developing brain and could be less receptive.