One of the most important and interesting, but also very challenging, parts of the job of a Physical Therapist is the treatment of people with spinal cord injuries. A deep understanding of the nervous system, its structure and its function, is a prerequisite before he/she starts applying any kind of physiotherapeutic intervention. In this article and the presentation that goes with it, I briefly present the structure of the nervous system, its major tracts/pathways, as well as a very nice and simple mind map on how to assess and what symptoms to expect after a complete or incomplete spinal cord injury. My notes include my own work and are a based on the lectures I attended and the study notes I received during in the Anatomy and Neurology courses in the European School of Physiotherapy in the Hogeschool van Amsterdam by Dr. Jaap Bakker (MD) and Bert Loozen (PT, MSc). In order to follow and understand this article, you should have also this powerpoint presentation I have created.
A. The Neuron
Having in mind the functional unit of the nervous system, the neuron (slide 2, 3), we can create two conceptual models of a neuron based on the structure and function they have in the human body (slide 4). The first model is a neuron with a long axon and short dendrites. I will use this model for the neurons that transfer action potentials from the Central Nervous System (CNS) to the peripheral organs. The second model is a neuron with two long axons - the peripheral and the central branch. I will use this model for the neurons that transfer action potentials from the peripheral organs to the CNS.
B. An Overview of the Nervous System
A conceptual model we can use in order to organize the structure of the nervous system is presented in slide 5. First of all, we can divide the nervous system into a central part (the brain and the spinal cord) and a peripheral part (the nerves). The CNS can be further divided in a voluntary part (all the actions and reactions we voluntarily do) and an involuntary part or autonomic nervous system (all the actions and reactions our body makes without our "control"). The neurons of the CNS follow specific pathways in the spinal cord, which will be analyzed further. The peripheral nervous system (PNS) can be further divided into an afferent division (all the neurons that transfer action potentials from the peripheral organs to the CNS) and an efferent division (all the neurons that transfer action potentials from the CNS to the peripheral organs). The afferent division of the PNS transfers signals from the musculoskeletal system, the skin and the vitals. The efferent division of the PNS transfers signals to the musculoskeletal system, the glands, the smooth muscle, cardiac muscle and the vitals. So, they can be further divided as seen in slide 5. What is important for someone to realize is that these divisions connect with the pathways of the CNS mentioned before and follow specific routes which have been identified. Each route serves specific functions and it is these routes one is interested in when he/she tries to assess in a spinal cord injury.
C. The Major Pathways/Tracts of the CNS
C.1. The major tracts of the somatosensory system are the following
C.2. The major tracts of the motor system are the following
D. The Motor System and Reflexes
The motor system of our body is divided in two major anatomical parts: the upper motor neuron (UMN) and the lower motor neuron (LMN) (slide 9). The UMN consists of the tracts that “travel” from the brain in the spinal cord. The major tracts just described in C.2. are the tracts of the UMN. The LMN consists of the efferent neurons outside the UMNs. It is important to make this conceptual anatomical framework, because it will help us in the assessment part of a spinal cord injury. Another reason for making this demarcation is for introducing the reflex phenomenon that occurs in our body. The reflex is an involuntary, instantaneous reaction of our body as a response to a stimuli. So, it is a separate “tract” that does not follow any of the above mentioned tracts. In other words, it is a tract - called the reflex arc - that does not "have" to travel all the way up to the brain through the somatosensory tracts and then down to the organ through the motor system tracts. This phenomenon, as well as the LMN structure and function, is presented in slide 10-12.
E. Visualizing the Spinal Cord and the Tracts
Now that we have covered some of the tracts of the CNS and the divisions of the PNS, we can easily visualize them in the spinal cord. It is true that each of these tracts has a certain position in the spinal cord. A very nice step-by-step journey in the spinal cord is presented from slide 16-39.
F. Spinal Cord Injuries
For someone that has absorbed the above mentioned knowledge and is able to visualize the spinal cord, the tracts and the divisions, it should now be easy to understand and predict what is the effect of each spinal cord injury (SCI). A first grouping of SCIs is done based on the level of assessment. This means that , depending on which segment of the spinal cord in injured, a Physical Therapists wants to examine to symptoms of the injury at 3 different levels of the human body: 1. above the level of the injury, 2. at the level of the injury, 3. below the level of the injury.
> General principle
> motor and sensory functions are completely normal
> if afferent fibers destroyed at the point of entry
> anesthesia > dermatomes
> if reflex arcs and LMN destroyed
> flaccid paresis > myotomes
> if more than one segment > paralysis
> varying degrees of sympathetic dysfunctions
> anatomically intact, but lost communication with supraspinal centers
> if sensory information cannot reach the brain
> if motor commands from the brain cannot reach the isolated segments
> spastic paralysis
> reflex abnormalities
> dysfunction of pelvic organs and varying degrees of sympathetic dysfunctions
Another grouping of SCIs is based on whether the injury was complete or incomplete. In complete lesions, the entire segment of the spinal cord is damaged. In incomplete lesions, a part of the spinal cord segment is damaged. Below, you can find some general rules followed in each case.
> Complete lesions
> C4 >> diaphragm paralysis >> death
> C >> tetraplegia
> T, L >> paraplegia
> S > conus syndrome
> example C7
> biceps > normal (above)
> triceps > decreased (at level, myotatic reflex)
> abdominal > decreased (below, cutaneous reflex)
> patellar > increased (below, myotatic reflex)
> plantar > appeared (below, pathological sign)
> spinal shock
> directly after spinal cord injury > temporary cessation of all reflex activity below the level of an acute
spinal cord lesion
> only reflex arcs affected
> areflexia #> hyperreflexia (clonus and pathological signs)
> atonia > flaccid paralysis #> spastic paralysis
> autonomic disturbances
> paralysis and anesthesia remains
> slowly developing spinal cord transections do not cause a spinal sock
> for Incomplete lesions check slides 40-41. There you can see the effects of an SCI depending of which tract is affected
> also, check slides 42-50, where I guide you in a path of the effects of an SCI at C8-T1 in a step-by-step process
If you liked that and you are interested in going more in details check out this article on my blog. Or you can find more interesting articles and studying tips I have posted here.
This is an excellent review of introductory neuroanatomy. Great job Stelios and thanks for participating! As we all know, the nervous system is incredibly complex. While this presentation reviews the neuro system in general very well, there is a lot more detailed information that is too much to cover in just one post. So be sure to review your neuroscience books/notes if you need that detail. Also, check out Stelios' blog ilovephysicaltherapy.blogspot.nl/