NM Unit 11

Take Home Messages in Unit 11

1) The brain and spinal cord are plastic or adaptable 2) The brain and spinal cord constantly change and adapt based on experience 3) Neural plasticity is ongoing. Nervous tissue is plastic before and after injury and the mechanisms for learning and re-learning (after injury) are similar 4) The experiences resulting in neural plasticity can be both positive and negative; the nervous system can learn to walk or sit in a wheelchair depending on its experiences 5) Damaged neurons can either repair themselves (via a direct mechanism) in order to recover function, or, both nearby (direct) and distant neurons (indirect mechanism) can reorganize and take over the function of the damaged neurons to restore function

Cortical Remapping-Peripheral Injury

After an amputation of the arm (i.e. a peripheral injury), the face area of the somatosensory cortex enlarges and shifts into the area represented by the arm (recall that the face area is adjacent to the arm area on the sensory homunculus). In other words, because the arm area of the brain is no longer needed to control the arm (the persons arm was amputated), the face area of the brain decides to put the arm area to good use and takes over the region. Interestingly, researchers have demonstrated that touching the face of an individual who has had their arm amputated will evoke a phantom limb sensation! Thus the individual feels their arm, even though it no longer exists, when you touch their face (this is due to the fact that the face region of the sensory cortex took up residency in the arm area of the cortex).

patient movement compensations

Compensations are a natural response to the environment. For instance, if a patient has difficulty moving in bed, they will naturally reach for the bedrails or use the electric buttons to adjust their position. In addition, compensations may be necessary in many instances, especially when recovery is prolonged and the patient may wish to return home. The nervous system, however, does not differentiate which experiences to respond to. Neural plasticity constantly occurs in response to all experiences. Therefore, PT interventions may help the patient re-learn to walk, but sitting in the wheel chair for many hours is essentially teaching the patient's nervous system how to 'sit in a wheelchair'. Once again, I am oversimplifying; however both the musculoskeletal and neuromuscular systems will adapt to experience. The experience may be practicing stepping and climbing stairs (positive neuroplasticity)...or it may be sitting in a slumped posture in a wheelchair (negative neuroplasticity).

Indirect Mechanisms (restorative) for neurologic recovery after brain injury

Completely different neural circuits reorganize and take on the functions of the damaged circuits. For example, if the shoulder representation on the motor cortex is undamaged it can remap and reorganize to also represent the wrist if that area of the brain was permanently damaged. In turn, this reorganization would restore motor function to the wrist (see Cortical Reorganization).

Collateral Sprouting of undamaged neurons (AKA: Reactive Synaptogenesis)

Healthy neurons that live near the damaged neurons sprout new connections to neurons that were previously innervated by the damaged neurons

Cortical Remapping-Brain Injury

In individuals with brain injury due to stroke, neuroimaging techniques (fMRI, PET, transcranial magnetic stimulation) have shown that the facial cortex is activated when they move their formally paretic hand. Thus the hand representation on the cortex shifted over to the face region which was undamaged. Said differently, the undamaged face area of the cortex was able to reorganize and begin to control the arm for recovery of arm function. Secondary sensory and motor areas may take over function for damaged primary motor areas. Although when this occurs the recovered motor function is not as coordinated as before the injury. Thus secondary areas of the brain are not very effective at restoring normal movement after brain injury. In individuals with stroke, uncrossed motor pathways from the undamaged hemisphere may contribute to motor recovery on the affected side (recall that 10% of the corticospinal pathway remains uncrossed).

FACTORS THAT INFLUENCE NEURAL PLASTICITY

Neural plasticity may be influenced by a number of factors Age: Persons of all ages have the capacity for neural plastic change; however, this potential may be greater in young persons. Characteristics of the neurologic lesion (size, growth or rate of change): The chance of recovery from a small lesion may be greater, assuming the functional area has not been entirely removed. Slowly developing lesions (like a brain tumor) appear to cause less functional loss than lesions that occur rapidly (like a stroke or a blow to the head secondary to a MVA). Basically, if the lesion occurs slowly, the nervous system has time to adapt. Consider how someone may have a brain tumor for a period of time, but not know it because it is slow growing. In contrast, a stroke is a very immediate and catastrophic event. There is no time for the nervous system to adapt to the event. Effect of Experience: Enriched subjects, or those with experience in enriched environments, may have developed neural circuitry that is more varied than that of 'restricted' subjects. This could provide them with a greater ability to re-organize the nervous system after a lesion, or to use alternate pathways to perform a skill. An enriched environment is one that exposes an individual or animal to a variety of experiences. For example a child that is exposed to different types of environments and interactive toys, games, objects of varying color, texture, difficulty level, etc. will build more neural connections in their brain than a child who remains confined to a single room with one or two simple toys. These additional connections and pathways in the "enriched" child's brain will serve a protective function as an adult in the event of a brain injury. In other words, the enriched child will have more neural pathways on reserve to restore motor function than in the child raised in a "boring, non-varying" environment. Type of Training: Training must be of skilled activities with intense practice. The key word in this later statement is skilled ! An individual who routinely go's to the gym to do biceps curls will increase the strength of the biceps but this elbow flexion-extension activity will NOT drive neuroplastic changes in the brain. The task of flexing and extending the elbow is not a skilled activity, as it doesn't require much "brain power", so to speak, to carry out and thus the brain does not change in response to these unskilled basic motor tasks. Conversely, a task such as doing a back walk over on a balance beam, or playing a musical instrument, involves a lot of skill and "brain power". With intense practice of these skilled activities neural plastic changes will occur in the brain. This is an important point to consider when designing exercise programs for your patient. Always put functional activities like the ones pictured below (which require a lot of skill and practice to master when you have brain damage) at the forefront!!

Denervation Supersensitivity

Postsynaptic neuron forms more receptors in response to less presynaptic input ( REFER to your Motor Control text on page 93 for explanation of denervation supersensitivity in Parkinson's disease)

Direct Mechanisms (restorative) for neurologic recovery after brain injury

Recovery of the injured neuron itself (see Neural Regeneration for details and images) Intact neurons that are nearby the damaged neurons take on the identical functions of the damaged neurons (see collateral sprouting and unmasking for details and images).

Unmasking of silent or unused neural pathways

Redundant pathways exist and some remain silent, after injury these silent pathways are unmasked to restore neurological function Occurs immediately and quickly Consider the following analogy. You are driving south along the interstate and reach a road block due to a car accident (i.e. the neuron is damaged). You immediately get off the interstate and jump on a parallel back road and continue going south towards your destination. Both the interstate and the back road run north-south and are very close to each other. They serve the same function; allow people to travel north south between cities. Until the accident on the interstate, the back road was rarely used (i.e. a silent neural pathway), however, it is immediately available when the interstate is blocked off.

Neural Regeneration (AKA: Regenerative synaptogenesis)

The Injured axon itself begins sprouting or regenerating Axons have difficulty regrowing over long distances and when they do they do not show normal function Regeneration occurs very slowly and there is limited evidence that it exists in the CNS (it occurs commonly in the PNS). However, recently a man named Terry Wallis began to speak after 19 years of relative silence in a minimally conscious state (MCS; recall the definition for the MCS from your TBI units). It is hypothesized, based on his brain scans and theories of neural plasticity, that his axons were busy "regrowing" over this prolonged period of recovery. I would encourage you to Google his name if you are intrigued by this unusual and rare recovery.

Neural plasticity

is a term that describes the adaptive capabilities of the central nervous system. The central nervous system (CNS) can adapt and change through reorganization of its structure and function. Neural reorganization occurs ALL the time. The nervous system adapts as you learn new skills and knowledge. Consider that you are training for a new sport activity. The efficiency and function of the neural connections changes and improves as your newly acquired skills do Neural changes that occur may be both positive and/ or negative. Consider the positive effects of therapy and training. Rehabilitation and practice of skilled activities promotes positive neural changes that facilitate recovery of function. Conversely, when an individual experiences a prolonged decrease in activity levels, which may occur following neurologic injury, the brain will also respond to this disuse and negative neural changes will occur. Thus, neural plasticity is the nervous system's ability to adapt, but it can change in either positive or negative ways based on the input (or lack of input) it receives.