The HumanNervous System

To complete our discussion of the brain, let’s travel down the neural axis. Sprouting off the back of the brain is the brainstem which controls many of our unconscious behaviors, including breathing, heart rate, digestive motility, hearing, facial movements and sensation, and eye movements. These behaviors are called unconscious because they do not require our attention to function. That does not mean they are not affected by our attention. We can focus on and influence our breathing rates, or heart rates, and our thresholds of sensory awareness.

The brainstem, like the brain and cerebellum, has left and right halves. It is about the size of both your thumbs side by side and is anatomically divided into the midbrain at the top, the pons in the middle, and the medulla at the bottom which blends into the spinal cord. All of this is encased by the skull and held into place by extensions of the dura mater (loosely translated as tough mother), which is the outer layer of the meninges.

The dura mater toughly attaches to the inside of the skull. In dissection, it is difficult to pull this layer off the inside of the top of your skull. The inside layer of the meninges, called the pia mater, is stuck delicately to the brain. In between, the arachnoid mater bungees, with spider web looking tethers, the dura to the pia.

Inside the brain, in large pools called the ventricles, cerebral spinal fluid is produced and flows into cisterns inside the brain and flows out of holes in this ventricular system to surround the brain. In essence, the brain is floating between this inner fluid and the outer fluid, reducing the effects of gravity. This cushions the brain to blows to the head, but only to a degree. Significant blows to the head will damage the brain at the site of the blow and cause the brain to sloosh against the inside of the skull, causing damage there. Concussions will alter perception and consciousness and can cause the consciousness/awareness system to shut down. Repeated concussive incidents can cause very long-term effects called traumatic brain injury. We will look at TBI and its effects on consciousness in a separate blog, as it deserves a closer look.

Let’s look a little closer now at the brainstem. You can see the gray matter cell bodies and the white matter tracts with the naked eye in dissection of the brainstem, as you saw last week, or by clicking the Nervous System link at the top left of this blog. Pools of motor neurons in the brainstem send their axons out of the central nervous system to most of the head and body. Bundles of these axons are called motor cranial nerves and provide innervation of both somatic and visceral (smooth) muscles of the head, neck, and digestive tract. Pools of sensory neurons collect sensory information from the head, neck, and body via sensory cranial nerves, and send that information to the thalamus.

These pools of neurons are called the cranial nerve nuclei, and do not passively transmit information to and from the brain. They perform conversions on the signals they receive, and these conversions can be very complicated. For example, the auditory nuclei in the brainstem add information about the location and intensity of sounds and share that information with the nuclei on the other side of the brainstem, contributing to the three-dimensional auditory experience. The brainstem also performs operations that regulate autonomic reflexes, for example, the baroreceptor reflex which uses input from receptors in the arteries to regulate our heart rate and blood pressure. Even considering these important functions of the brainstem, it is not a good candidate for the ghost, but without it we don’t have consciousness or behavior. It drives our basic life functions, and all vertebrate animals have one. Do all vertebrates have consciousness? Animal consciousness, and how we know about it, will be addressed soon.

The cerebellum, meaning small brain in Latin, hangs off the back and bottom of the brain, connected to the brainstem by three stalks. This neural organ is responsible for our muscle coordination, comparing the motor output (our intentions) with the sensory feedback (the result of those actions). A simple behavior, such as reaching out and touching a button, is a composite of hundreds of micromovements. The cerebellum adjusts and smooths out the motor output of each micromovement using the input of body sensation, including but not limited to vision. This tacking back and forth along a line of intention, like a sailboat in the water, is not visible in the intact system, but damage to the cerebellum causes a behavioral symptom called dysmetria, where the closer the patient’s finger gets to the button, the wilder the deviations become. This results in not being able to precisely point to and touch an object. The patient is conscious of the object and the patient is conscious of the results of internal processing. Again, the cerebellum is necessary for life, but probably not sufficient for consciousness.

The spinal cord comes out of a large hole at the bottom of the skull and runs inside and is protected by the vertebral column of bones and ligaments. Like the rest of the central nervous system, you can see the gray and white matter in the spinal cord. Unlike the brain, the spinal cord has gray matter on the inside and white matter surrounds it. The neural cell bodies and the bundles of axons have traded places. The gray matter, the neuron cell bodies, are organized into a winged structure looking much like a butterfly. The dorsal (posterior) wings, called horns, have sensory cell bodies in them, and the ventral (anterior) horns have motor neurons, specifically lower motor neurons. As we saw with the brain, we see motor functions coming out of the front (ventral aspect) of the central nervous system and the sensory information going into the back (dorsal).

Let’s think about consciousness in other animals. A spinal cord and brainstem are all that is needed to react to the environment. Animals like birds, reptiles, and fish can selectively direct their attention. These animals restrict their neural processing and behavioral responses by focusing on relevant stimuli and ignoring other stimuli. In other words, they have some control over access to neural processing. These animals have paleocortex, which includes structures like the hippocampus and the basal nuclei. Mammals have neocortex and primates and cetaceans (whales and dolphins) have a considerable expansion of the neocortex. Just as we see a developmental continuity of animal nervous systems, we should see a developmental continuity of consciousness. How could this be studied and how can we attempt to know that other animals are conscious, is discussed in next week’s blog.

Nature evolutionarily builds nervous systems on top of other, older nervous systems. The phylogenetically older structures are still there, and some have been adapted for new functions. Our basal ganglia work very differently from a bird’s basal ganglia, but they are built the same, and look the same in dissection and medical imaging. However, they are not connected the same way. Again, we find it necessary to move up a level of explanation and look at neural systems and how they contribute to our mind and behavior.

Let’s finish this discussion of the brain. We can organize the nervous system structures and their connections into functional systems, like the motor system, the somatosensory system, the visual system, the auditory system, the autonomic nervous system, and so forth. These systems tie together nervous system structures, mostly gray matter, with their connections, the white matter, and give us a better view of the causes of our behavior, and hopefully a better look at the foundation of consciousness. Let’s start with a basic organization into motor signals and sensory signals.

The motor signals, or commands, are an upper and lower two-neuron chain. The upper motor neurons cell bodies are in the frontal lobe, and synapse on the lower motor neurons in the spinal cord. An action potential is generated in the frontal motor cortex and travels down the axon to synapse on the lower motor neuron. Damage to upper motor neurons, for example in a stroke, usually causes the arms and or legs on the other side of the body and face to be frozen in a paralysis called spastic paralysis. The muscles are stuck in a position. Damage to lower motor neurons or their axons results in the target muscles to be limp, in a type of paralysis called flaccid paralysis.

Sensory information from the body goes up the cord to the brainstem, thalamus, and brain, in a three-neuron chain. Damage to the sensory tracts can result in a loss of awareness of the body, loss of touch, and loss of pain, altering our consciousness.

Other systems, like the visual system and the autonomic system, are combinations of these motor and sensory tracts. Our everyday behaviors come about by these tracts as well. Let’s look at one example that highlights consciousness and everyday behavior – our visual system.

The human visual system is really two systems, one built on top of the other. One is for detection of object forms and colors. The other system detects and processes motion of objects and low light vision. The information from these systems stays mostly separate throughout the visual stream from the retina to the brain. Information from the retina is highly condensed and organized by location before it ever leaves the eye. This information is also preserved throughout the visual stream. Stoke patients can be unaware of one half of their visual field. It no longer exists in the patient’s experience. They will draw all the numbers of a clock onto one half of the circle. They will ignore food on one half of their plate. Damage to different parts of the stream can result in the loss of awareness of motion. These patients are conscious, but they have lost their consciousness of parts of their awareness. We, just like these patients, assume that our experience is produced of whole cloth, but neurologically we see it is not. In the next blog we will look at the philosophical and scientific bases for understanding consciousness and the mind. Our mind and the minds of others.