The Nervous and Endocrine Systems

What we call the nervous system actually has two main systems: the central nervous system (CNS), which consists of the brain and spinal cord, as well as the peripheral nervous system, which is further subdivided into the somatic and autonomic nervous systems.

The peripheral nervous system is made up of thick bundles of axons, called nerves, carrying messages back and forth between the CNS and the muscles, organs, and senses in the periphery of the body (i.e., everything outside the CNS). The central nervous system processes all incoming information, sorts it, evaluates risks, decides if information is important or irrelevant, interprets sensory signals such as sound, taste, or touch, activates memory, language comprehension, and more to interact with the world around us and to generate responses that are sent back out from the brain through the spinal cord.

The spinal cord has some immediate reactions to certain stimuli that we call reflexes, but mostly it serves as the conduit or path by which information from the peripheral nervous system is sent to the brain and vice versa. In the peripheral nervous system, the somatic nervous system (remember the word soma means “body” from our discussion of neurons) transmits sensory and motor signals to and from the central nervous system; it is how we control our “voluntary” muscle movements. For example, when you reach out to pick up your cell phone, the movement of your arm, hand, fingers, and so forth is under voluntary control through the somatic nervous system. In contrast, the autonomic nervous system controls the functioning of our organs and glands including involuntary muscle movement, usually things that we are unaware of most of the time like breathing, our heart beating, and the movement of the muscles that pushes our food through the intestines and is part of digestion and elimination.

The autonomic nervous system is also subdivided into the sympathetic and parasympathetic systems (see Figure 1). Sympathetic activation is related to energy use and maintaining homeostasis such as our body temperature; its major purpose, though, is to enable us to respond to emergencies or threats through fight or flight by mobilizing energy quickly. In contrast, parasympathetic activation is associated with normal functioning under relaxed conditions, sometimes referred to as governing “feed and breed” and then “rest and digest.” Although these two systems are often seen as opposites of each other, in reality they often work together, usually in sequence. For example, if you are late for class, the sympathetic NS may boost your ability to run, take in air more quickly, increase your heart rate, and so on to enable you to get to class faster; once you get to your seat, however, you would not want to maintain that level of activation. Your parasympathetic NS (think “parachute” or bringing you back down), then helps to slow your breathing back down, reduce your heart rate and heat levels, and enable you to resume normal functioning. Both systems are also actively involved in sexual activity and many other situations. You will likely hear reference to these systems in the modules to come.

Figure 1. The sympathetic and parasympathetic divisions of the autonomic nervous system have the opposite effects on various systems.

Working in conjunction with the nervous system is the endocrine system, which consists of a series of glands that produce chemical substances known as hormones (Figure 2). Like neurotransmitters, hormones are chemical messengers that must bind to a receptor in order to send their signals. In fact, some of the same molecules that constitute neurotransmitters are referred to as hormones when produced by the endocrine system. However, unlike neurotransmitters, which are released in close proximity to cells with their receptors, hormones are secreted into the bloodstream and travel throughout the body, affecting any cells that contain receptors for them. Thus, whereas neurotransmitters’ effects are localized, the effects of hormones are widespread. Also, hormones are slower to take effect, and tend to be longer-lasting.

There is a close relationship between the brain and the endocrine system, particularly between the hypothalamus, which has strong connections to the pituitary gland, often called the master gland because it regulates the functioning of other glands. Other important glands related to psychopathology include the adrenal glands, which release epinephrine (also known as adrenaline) and cortisol (one of the body’s major stress hormones); the thyroid gland, which is involved in energy metabolism and physical growth; and the reproductive glands (testes in males and ovaries in females), which not only produce eggs and sperm, but also estrogen and testosterone, both of which can impact mood and behavior.

Hormones produced by the endocrine system play a role in relation to many mental disorders. For instance, low levels of thyroid production often mimic major depression, and this is one reason that a blood test is a useful part of early assessment of someone presenting with significant depressive symptoms; the opposite is also true—excessive production of thyroid can mimic mania similar to that of bipolar disorder. The fight-or-flight system, described before in relation to the sympathetic nervous system could not function without activation of the adrenal glands and stress hormones.

The part of the adrenal glands that produces the stress hormone cortisol is heavily involved in what is known as the hypothalamic-pituitary-adrenocortical axis or HPA axis. An axis is a major pathway of neurons that connects to many locations; in this case, the hypothalamic-pituitary-adrenocortical axis (HPA axis) connects the brain and endocrine system and is central in the body’s response to stress or threat. Dysfunction of the hypothalamic-pituitary-adrenocortical (HPA) axis is implicated in disorders like depression, anxiety, and post-traumatic stress disorder. There is also evidence that overexposure to stressful events in early development, like chronic abuse or death of a parent, may cause the HPA axis to become overly sensitive to stressful events; sustained high levels of cortisol have also been shown to damage the hypothalamus, creating difficulties in regulating the stress response and leading to increased vulnerability for depression and anxiety later on in life. Again, this illustrates the complex interactions between environmental and biological events and processes.

Figure 2. The major glands of the endocrine system are shown.

Figure 3. The brain and its parts can be divided into three main categories: the forebrain, midbrain, and hindbrain.

Each cerebral hemisphere of the forebrain can be subdivided into four lobes, each associated with different functions.

Brain Lobes

The four lobes of the brain are the frontal, parietal, temporal, and occipital lobes (Figure 4). The frontal lobe is located in the forward part of the brain, extending back to a fissure known as the central sulcus. The frontal lobe is involved in reasoning, abstract thinking and planning, creativity, motor control, emotion, and language. It contains the motor cortex, which is involved in planning and coordinating movement; the prefrontal cortex, which is responsible for higher-level cognitive functioning including major inhibitory pathways of neurons that help us regulate our behaviors and reactions; and Broca’s area, which is essential for language production.

Figure 4. The lobes of the brain are shown.

People who suffer damage to Broca’s area have great difficulty producing language of any form. For example, Padma was an electrical engineer who was socially active and a caring, involved mother. About twenty years ago, she was in a car accident and suffered damage to her Broca’s area. She completely lost the ability to speak and form any kind of meaningful language. There is nothing wrong with her mouth or her vocal cords, but she is unable to produce words. She can follow directions but can’t respond verbally, and she can read but no longer write. She can do routine tasks like running to the market to buy milk, but she could not communicate verbally if a situation called for it.

One particularly fascinating area in the frontal lobe is called the “primary motor cortex.” This strip running along the side of the brain is in charge of voluntary movements like waving goodbye, wiggling your eyebrows, and kissing. It is an excellent example of the way that the various regions of the brain are highly specialized. Interestingly, each of our various body parts has a unique portion of the primary motor cortex devoted to it. Each individual finger has about as much dedicated brain space as your entire leg. Your lips, in turn, require about as much dedicated brain processing as all of your fingers and your hand combined!