The HumanNervous System

Hi, my name is Dr. Robert Sorrells and I want to talk to you about the mind, the brain, and everyday behavior. Currently, I am an Emeritus Professor of Anatomy and have taught medical students, graduate students, and undergraduates. In the gross anatomy lab, I have removed hundreds of brains from cadavers, dissecting them with students and other faculty. I am writing this blog for anyone who wants to know more about the human nervous system and how it is involved in our behavior. Along the way we can talk about science, consciousness, and the impact of our behavior on our world.

Let’s think about our behaviors, the seemingly normal things that we do every day. While they are normal, they are far from mundane, particularly when you think about the underlying neural machinery. This machinery is biological. It is wet, warm, and pulsing. It can get injured, sick, and heal itself, to some extent. Our brain and nervous system produce our everyday wants, desires, beliefs, experiences, dreams, our consciousness, and our behaviors.

While the nervous system initiates, monitors, and controls our behavior, it is not ethically or morally responsible for our behavior. The environment is not responsible for our behavior, nor our genetics. Some behaviors are reflexive and certainly initiated by the environment, and what those behaviors are, what those reactions are, is shaped by genetics. The environment and genetics underdetermine our behavior.

We are responsible for our behavior, but how? The easier question is: How does the nervous system control our behavior? The harder question is: How is nervous system activity initiated and directed? What pulls the trigger and guides the behavior. If I am responsible for my behavior, who or what is the I?

Rene Descartes, the “I think therefore I am” philosopher, insisted that the mind and the body are different kinds of things. Gilbert Ryle, in criticism of Descartes’ dualism, is credited with the first use of the phrase, “the ghost in the machine”. The mind, according to this reductionist approach, is the living brain and can be understood in terms of the brain. In the language of philosophy of science, the mind can be reduced to the brain. We will talk more about the failure of reductionism in another blog, but for now, let me just say, the closer you look the less you see.

Behavior is a broad term. It can mean the response to a stimulus, a variety of actions performed by an agent in a certain environment, it can mean the actions we do each day, for example wake up, move, see, hear, feel, eat, communicate. This blog will look at the neural underpinnings of some of our behaviors, for example visual behavior, movement behavior, eating behavior, and other social behaviors such as language, memory, sex, and violence. Along the way, we will dip our toes into the neuroscience of thought, emotion, and imagination, and try and shine a light on the illusive ghost of consciousness. First, we need a basic understanding of the brain and the likely players in the production of our mind and behavior.

Our brain has two sides appropriately called hemispheres, that are sometimes slightly different in size. The left half of the brain controls the right side of the body, and the right half of the brain controls the left side of the body. The motor neurons in your brain that initiate movement of your right hand are in the left frontal lobe. The same pattern holds for incoming sensory information. Most of the sensory information coming from the right side of the body goes to the left side of the brain, and sensation from the left side of the body goes to the right side of the brain. This is important clinically. A patient with damage to the right side of her brain, for example a stroke, will have symptoms, usually called deficits, on the left side of her face and body.

For most people, the left and right hemispheres do different things. Each side of the brain has its own dedicated functions, and this phenomenon is called laterality of function. These functions, and which side of the brain they are on, are affected by genetics, such as sex of the individual, and by experience, especially injury. The left side of the brain, for ninety percent of right-handed males (and a smaller percentage of females), controls language, while the right side of the brain controls spatial processing (where things are, including our body, in three-dimensional space). Some left-handed men have the opposite pattern, with language represented in the right hemisphere. We know this because brain strokes on the left side for males almost always produces language deficits, while language can be preserved in women after a stroke. Brain imaging confirms this, as does experimental and clinical evidence.

To say that the brain controls language or spatial representation is too literal of a statement. If those parts of the brain are damaged, it is likely that those specific functions will be lost, temporarily or permanently. For example, Broca’s area, in the left frontal lobe, seems to be necessary for speech production. Damage there, and the patient has great difficulty communicating verbally, called Broca’s aphasia. The difficulty is in grammatically stringing together words. The syntactical component of the language is missing. Damage to the right lobe does not usually affect language, but rather leaves the patient with difficulty maneuvering in three-dimensional space. These brain regions are necessary for these behaviors, but are they sufficient?

The brain is the machinery, if you will, that produces these socially meaningful behaviors, or better yet, initiates them. Broca’s area is necessary for initiating spoken, syntactic speech. Its damage does not prevent other forms of communication, it does not affect the thoughts underlying the need to communicate. The patient can understand language and can utter emotional outbursts. Damage to another area, called Wernicke’s area, affects the ability to understand language. A sequence of neural events happens in these behaviors (and all behaviors) and we can map them out and name the pathways, for example, from Broca’s area to the vocal cords and the muscles of the tongue and mouth. We can talk about planning areas of the cortex that feed into Broca’s area, and other areas of cortex where modes of brain activity (for example sound representation, visual representation, touch representation) are combined into a new code. The question is: What triggers those initial neural responses? Why does the neuron fire?

The neural signals going from the brain to control the body are called motor signals, and the neural signals going up from sensory cells to the brain are called sensory signals. Additionally, the front of the brain (in front of the central sulcus) is for motor behavior, and the back part of the brain is for sensory processing. I tell my students, “Motor out the front and sensory in the back.” This is true of the spinal cord as well. Motor neurons are in the ventral (front) horn of the spinal cord, and sensory neurons are in the dorsal horn.

The signals are based on patterns of neurons firing action potentials that travel down their axons and triggering responses in downstream neurons. The story is a bit more complicated, but we will get into all that. Most of this neural information crosses in the brainstem or spinal cord producing contralateral effects, that is, effects on the other side of the body. This is the neuroanatomical reason why the left side of the brain controls the right side of the body, and vice versa. Roughly 10% of these signals stay on the same side of the body and brain, but this does not have significant effects behaviorally or clinically.

The nervous system, like the digestive system or circulatory system, is made of specialized cells. Neurons are the cells producing neural signals that are transmitted to other neurons or other parts of the body. Glial cells provide physical and physiological support to neurons. These cells are organized into units (circuits) and the units are organized into larger structures. We need to cover some of the specific neuroanatomy of these structures to be able to discuss their influence on our behavior. The next post of this blog will go into more details of the structures listed here. Eventually we will get to that ghost.

Much of this you might already know, but we need to get on the same page about the functions of these structures. The nervous system is divided into the central nervous system and the peripheral nervous system. The central nervous system is the brain, the cerebellum, the brainstem, and the spinal cord. The peripheral nervous system is the cranial and spinal nerves, and all the named nerves (e.g. the sciatic nerve) coming out of and going into the central nervous system. The brain is further divided into hemispheres as we have discussed, and lobes that have dedicated functions. The brainstem hanging off the bottom of the brain, is divided into the midbrain on top, the pons in the middle, and the medulla at the bottom of the stem, where it connects with the spinal cord. The spinal cord, like the brain, can be divided into white and gray matter areas, with subdivisions of those areas. Each of these areas and divisions has specific functions and specific input for our behavior. For example, the medulla controls the tongue via the hypoglossal nerve, from input from the frontal lobe facial motor cortex. The medulla also regulates our heart rate, breathing, and digestion. We will work out the details a bit at a time, while keeping our eyes on the purpose for knowing these details – understanding the mind, brain, and behavior.

We know a tremendous amount of useful information about the pieces and parts of these systems, including their function, their connections, their chemical messengers, and their dysfunction. We know this from a hundred years of clinical data, mapping behavioral deficits onto damaged nervous system structures. We know this from a hundred years of experimental data, including brain imaging, genetic engineering, and animal and computer models. I will bring this knowledge to bear as we move through these blogs. It is amazing to me how much we know. However, we know very little about how it all comes together into consciousness, into thought, into the richness and immediacy of our experience and behavior. This blog is my attempt to put some of these pieces together. Next week, we will go into more details of the brain, and then expand our view to look at behavior and the mind.