Brown-Sequard Syndrome

Brown-Séquard syndrome, first described by the physiologist Charles-Édouard Brown-Séquard, is a condition associated with hemisection or damage to one half of the spinal cord. The hemisection damages neural tracts in the spinal cord that carry information to and from the brain. This results in a loss of sensations like pain, temperature, touch, as well as paralysis or loss of muscle function in some parts of the body.

Now, if you look at the cross-section of the spinal cord, the white matter is on the outside and the gray matter is on the inside, and overall it looks like a butterfly. If we draw a horizontal line through the spinal cord, the front half is the anterior or ventral half, and the back half is the posterior or dorsal half. And the butterfly wings are sometimes referred to as horns; so we have two dorsal horns that contain cell bodies of sensory neurons and two ventral horns that contain cell bodies of motor neurons.  The white matter consists of myelinated axons which are separated into tracts that carry information to and from the brain. Think of them like highways for neural signals, where some highways carry sensory information to the brain and some carry motor information from the brain to the muscles.

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 pressure and crude touch--or the sense one has been touched, without being able to localize where they were touched. Second, there are two ascending dorsal column tracts- the fasciculus gracilis which carries sensory information from the lower trunk and legs, and the fasciculus cuneatus which carries sensory information from the upper trunk and arms. These tracts both carry sensations like vibration; fine touch, which is where you can localize where you were touched; and proprioception, which is an awareness of your body position in space. Finally, there’s the corticospinal tract which is a descending pathway that carries motor information from the brain to different muscles in the body and it controls voluntary muscle movement.

So, sensory information has to go through three neurons to reach the brain; kind of like 3 people in a relay race.  For example if you accidentally touch a hot pan, the sensation of pain and temperature is carried from the nerves in the skin of your fingers, through a 1st order neuron- which is a pseudounipolar neuron. A pseudounipolar neuron has a cell body from which only one axon comes out - and then that axon splits into two branches - a lateral branch that comes from the periphery and a medial branch that runs towards the spinal cord.  So, the nerve in the skin of your fingers is served by the lateral branch of a 1st order neuron which carries the sensation to the dorsal root ganglion. That’s where the cell body is located along with the rest of the cell bodies of other pseudounipolar neurons. From there the signal continues through the medial branch and into the spinal cord to reach the dorsal horn. Here, it passes off the sensory information “baton” by synapsing with a 2nd order neuron which sends off its own axon which ascends 1 or 2 spinal segments and crosses over to the opposite side to travel up the spinal cord 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 tissue damage.

Similarly, if you suddenly grab a vibrating cell phone, the sensation of vibration is carried from the nerves in the skin of your palm, via the fasciculus cuneatus- which is a collection of nerves all composed of first order neurons. The fasciculus cuneatus is contained within the dorsal white column, located posteriorly in the white matter of the spinal cord. The 1st order neuron then ascends up along the same side of the whole length of the spinal cord to reach lower level of medulla oblongata where it synapses with the cell body of a 2nd order neuron in the nucleus cuneatus. The 2nd order neuron then sends off an axon that crosses over to the opposite side of the medulla, and travels up to the ventral posterior nucleus of the thalamus to synapse with a 3rd order neuron. The 3rd order neuron then sends up an axon that carries the sensory signals to the primary somatosensory cortex of the brain, letting you know that your phone is vibrating.

Once your brain receives sensory information, it sends a motor signal down the upper motor neurons through the corticospinal tract in the midbrain and crosses to the opposite side at the medulla before continuing down the spinal cord. There, they synapses with lower motor neurons in the ventral horn, and the lower motor neuron axons then leave the spinal cord to innervate the muscles of  your arm and forearm, which allows you to lift your fingers and hand from the hot pan or pick your cell phone up. Hopefully this all happened quickly so you haven’t sustained major tissue damage and the caller hasn’t hung up.

In Brown-Séquard syndrome, a vertebral bone fracture or any kind of penetrating trauma like a gunshot injury or stab wound in the back can lead to the hemisection of the spinal cord; and this mostly happens in the neck or cervical region. Besides trauma, non-traumatic conditions like a spinal cord tumor may compress the spinal cord and lead to Brown-Séquard syndrome.