At the physician’s office, 55-year-old Mario presents complaining of weakness in his hands and feet. These symptoms have gradually progressed over the past couple of months. At first, he struggled to manipulate small objects like buttoning his shirt. Now he also often stumbles while walking and recently fell down the stairs. In addition, his family mentions that his speech has become slightly slurred. He denies any sensory symptoms, memory loss or any bowel or bladder complaints. Later that day, 43-year-old Donna comes in with difficulty walking. She has fallen several times over the last few weeks. Her past medical history is significant for HIV infection, which was diagnosed a few years ago. On neurologic examination, her pupils are small and irregularly shaped and do not react to light, but constrict with accommodation. The sensations of pressure, vibration, fine touch and proprioception are also reduced throughout the lower extremities. She has a wide-based gait and cannot maintain balance with her eyes closed.
Based on the initial presentation, both Mario and Donna have some form of spinal cord disorder. Okay, let’s talk about physiology first real quick. If we zoom in at a cross-section of the spinal cord, we’ll see that it is composed of both grey and white matter. Grey matter is found within the medial portion of the spinal cord and has two dorsal or posterior horns that contain cell bodies of sensory neurons and two ventral or anterior horns that contain cell bodies of motor neurons. In the center of the grey matter there’s a small cavity called the central canal which is filled with cerebrospinal fluid that provides nutrients and mechanical support. Surrounding the grey matter is white matter, which consists of the axons of various neurons and they are organized into tracts that carry information to and from the brain.
For your exams, there are a few main tracts to remember. First, there’s the spinothalamic tract which is an ascending pathway and it’s divided into two parts. The lateral tract carries sensory information for pain and temperature, while the anterior tract carries information for crude touch, or the sense one has been touched, without being able to localize where they were touched. For this to happen, a first order neuron, found inside a dorsal root ganglion carries sensory input from the skin to the dorsal horn of the spinal cord, where it synapses with the second order neuron. And that neuron ascends 1-2 vertebral levels and decussates or crosses to the opposite side of the spinal cord via an area of white matter called the anterior white commissure. The secondary neuron then ascends up the length of the spinal cord via the spinothalamic tracts, and eventually synapse with a 3rd order neuron located in the ventral posterior nucleus of the thalamus. This 3rd order neuron then sends its axon up to the sensory cortex of the brain, letting you know that there’s a sensory signal.