Some effects of trauma to different parts of the brainstem
When thinking about the brain, most people envision the large cerebral hemispheres as shown in the video on the left below. These hemispheres determine our personality, our emotions, our ability to talk and interact with the world. Imagine these large, diffuse, brain areas sending information via fiber pathways down to the brainstem, the video on the right. There, the information is concentrated in a much smaller area. That is why a relatively small injured area in the brainstem can have devastating results. Many of these brainstem areas act as a relay station to different parts of the body and even back to the brain. If the relay station is damaged, the information cannot travel further. Functions we don't thing about such as breathing, our heart rate, filtering out noises occur in the brainstem. If the injury interferes with pathways linking the hemispheres and the brainstem, the result can be a deep coma, a vegetative state or even death. Understanding just a little of the functions of different parts of the brainstem can help to know what to expect with brainstem injuries.
Major sections of the brainstem and a couple of important adjacent structures, the thalamus and basal ganglia
At the top of the brainstem is an area of the brain called the thalamus (colored below in blue), another important relay station. Injury here can cause problems ranging from loss of sensation and motor function to lapsing into a coma. Below the cerebral hemispheres and above and around the thalamus are the basal ganglia, another structure important in a wide range of functions. Injury here can result in a Parkinson type syndrome.
On the left, the thalamus can be seen in blue on top of the brainstem. On the right, there is a lateral view of the color coded brainstem. Starting at the top: the Basal Ganglia associated with the thalamus is in green, (thalamus is not seen from this view since it is on the other side of the basal ganglia.) The Midbrain is purple, the Pons is red, and the Medulla is yellow.
The Basal Ganglia (Green)
The Basal Ganglia refer to three, deep gray matter structures deep within the cerebral hemispheres sitting on top of the brainstem and thalamus. Basal Ganglia hematomas, when associated with trauma, are seen with shearing injuries. These are usually the result of high velocity trauma such as a motor vehicle accident.
On the color coded brainstem illustration the basal ganglia is partially covered by the Insular Cortex, a deep section of brain of the temporal lobe.
The Midbrain (Purple)
The Midbrain connects the Pons to the large Cerebral hemispheres above. With trauma, different velocities relative to the large Cerebral Hemispheres and smaller Midbrain can result in distortion (tearing or stretching) of Midbrain fibers. Abnormal eye findings, extension (decerebrate) posture associatted with coma, abnormal respirations are common findings.
Cranial Nerves III and IV are associated with the lower Midbrain; these Cranial Nerves deal with eye movements. Cranial Nerve II is associated with the upper Midbrain and deals with vision. Injuries to the lower Midbrain can result in abnormal visual issues.
The Pons (Red)
The Pons is located between the Midbrain and the Medulla. Important origins of cranial nerves are located in this section of the brainstem (called nuclei). Multiple sensory and motor pathways travel in this part of the brainstem.
Traumatic injuries here result in minimal to no motor movement, pin point pupils and specific breathing irregularities.
Cranial Nerves exiting between the Medulla and Pons are CN VI (which affects eye movements), VII (which affects the muscles of the face) and Cranial Nerve VIII which has to do with hearing. Cranial Nerve V exits froms the side of the Pons, controls feeling in the face and the chewing muscles.
The Medulla (Yellow)
The Medulla is the lowest part of the brainstem, located between the pons and the spinal cord. It controls heart rate and breathing. Traumatic injury at this level is devastating.
Cranial Nerves coming from the medulla which can be injured include CN IX, X, XI, XII. For example, CN IX and X can affects swallowing, particularly the gag reflex. CN XI lets you shrug your shoulders and helps in turning your head. CN XII controls your tongue.