Over the course of four weeks, Amie Hesbach and Gillian Tabor took us on a journey of understanding neuroplasticity and neurodynamics, and the ways in which they can be applied to the equine patient – every equine patient.
Any injury in equines or canines, whether orthopaedic or neurological in nature, will result in changes in the nervous system, both peripherally at the site of the injury and within the spinal cord and brain. There are a variety of ways in which we can address these changes to the nervous system as vet rehab therapists and physiotherapists to ensure full rehabilitation.
Below, I share a few of the most prominent points discussed during the course of our webinar series on Equine Neurodynamics.
Injury and the Nervous System
During the first webinar of our series, Amie Hesbach introduces neuroplasticity and its relevance in patients that have sustained an injury or experience pain. As Vetrehabbers, we are intimately familiar with pain pathways and the neurological component of the pain experience, but there is a larger puzzle piece that we may be missing in our patients.
Immediately following an injury, we know that there will be an influx of neurochemicals in the nervous system, which will lead to changes in the nervous system. Structural changes will also occur on a neural level, including as a result of oedema and inflammation.
In the event of an injury to a joint, the surrounding muscles will experience a reflexive inhibition to protect the joint from further damage, which leads to muscle weakness and atrophy over time if left untreated. This is known as Arthrogenic Muscle Inhibition.
In the weeks following an injury, we can expect the injured area to become atrophied and degenerate, while nerves may become demyelinated and apoptosis may occur. Whether an injury is neurological or orthopaedic in origin, the impact on the nervous system will be profound and ultimately detrimental if not corrected.
Arthrogenic Muscle Inhibition
AMI is a neurophysical phenomenon in which healthy muscle tissue becomes reflexively inhibited following an injury to the joint. AMI can occur as a result of injury in any joint, and can significantly impede the recovery of muscle function and ultimately, rehabilitation.
In patients experiencing AMI following joint injury, surgery or pathology, we will see:
- muscle weakness;
- reduced muscle activation or failure to activate muscle; and
- muscle atrophy
Muscle function is dependent on the availability of motor neurons in the muscle and on the ability to voluntarily recruit them. Patients with joint injury usually present with fewer motor neurons available for recruitment, or a lower motor neuron pool excitability, as well as a reduced ability to voluntarily recruit motor neurons, or a central activation failure. This inhibition is reflexive and involuntary and is controlled by the presynaptic mechanisms within the spinal cord, or a lowered spinal reflexive excitability during acute stages of healing as a result of tissue damage, joint laxity, joint effusion, pain or inflammation within a joint.
While the motor neuron pool excitability will return to normal over time, central activation does not return to normal without intervention and remains lowered or inhibited, increasing the risk of reinjury in the patient.
The Cortical Map
The cortical map is a representation of the body on the surface of the brain, or the cortex, from both a sensory and a motor perspective. Certain areas of the body take up more space on the cortex, with a higher volume of sensors or sensitisation, the most prominent of which would be the face, mouth, tongue and hands in people. Most animals have a high cortical volume for the face, whiskers, muzzle, mouth, ears and tongue, and will have representation across the rest of their bodies according to their specific function and use.
The cortical map is not ‘fixed’ and can change throughout the course of an animal’s life, depending on the amount of sensory input a certain area receives, external stimulation, and motor output and learning of a specific area.
When we incorporate motor learning into a patient’s life, we can influence the cortical map. Some of the ways in which we can do this are through the use of repetition, experience and motivation.
Following an injury, the cortical map will be re-organised. This re-organisation will be determined by the site and size of the injury, as well as the function of the neurological tracks travelling to the brain. Changes in the cortical map, and our interventions on it, will impact a horse’s movement, recovery and function following rehabilitation.
During the initial recovery phase, as well as during the later remodelling phases of an injury, there will be increased potential for neuroplasticity in the cortex and ultimately for functional recovery.
Cortical silencing occurs in the absence of sensory feedback, motor activation or autonomic activity. Over time, the space in the cortex that used to be devoted to a specific area will be taken over by other areas of the body, leaving areas of the body underrepresented or completely absent from cortical representation.
Neuroplasticity and opportunities for recovery
The nervous system can recover in three ways:
- Neurogenesis or regeneration
The regenerative process is slow and unpredictable, but it is possible for new neurons to form throughout adulthood. The possibility of neurogenesis is reliant on the cellular environment. From both a medical and nutritional perspective, we increase the potential for regeneration within the cellular environment. Inadequate or incomplete regeneration will result in impaired ability, mobility or permanent deficits. From a Vetrehabber’s perspective, we can utilise high-intensity exercise together with environmental stimulation to increase BDNF expression and potentially stimulate neurogenesis indirectly.
Neuroplasticity is the dynamic ability of the nervous system, brain and neurons to modify or change their activity in response to intrinsic or extrinsic stimuli by developing, optimising and re-organising their structure, functions or connections and network. We can use mental gymnastics during recovery from injury or development of the horse to cause a change in the nervous system.
Compensation involves the substitution of an original behavioural strategy to complete a task. In both the nervous and muscular systems there are many options available for compensation that can be taught or utilised by the patient. Teaching compensatory strategies will only be applicable in certain scenarios, and during the rehabilitation process, we must ask ourselves how much compensation we should be allowed.
Retrain the Brain
We can incorporate specific strategies to target learning and change at the level of the brain, instead of only at the level of the musculoskeletal system.
Through environmental stimulation, motivation and repetition, we can start to make inroads in retraining the brain. By adding novel stimuli and touch therapies, we can further influence the cortical map. Through stimulation of certain muscle groups, we can reconnect and strengthen neurological pathways.
Until we connect the brain and the body, we will continue to be frustrated in achieving successful rehabilitation outcomes in certain cases.
We need to adjust the way we look at and approach our patients. It is essential that we realise that in the event of an injury or long-term compensation and pain, there will be changes that have occurred at the level of the brain. Without retraining the brain, we will continue to fall short of full recovery or rehabilitation in our patients.