The 'emotional' nervous system
Emotion involves the nervous system completely. However, there are two parts of the nervous system that are especially important: The limbic system and the autonomic nervous system. The autonomic nervous system, together with the hypothalamus, regulates the pulse, blood pressure, respiration and arousal in response to emotional signals. When activated, the sympathetic nervous system prepares the body for emergency actions by controlling the glands of the endocrine system.

The Limbic system

The limbic system is a complex set of structures that lie above and around the thalamus, and just below the cortex. It includes the hypothalamus, the hippocampus, the amygdala, and many other nearby areas. It seems to be the main person responsible for our emotional life, and it has a lot to do with the formation of memories.

Hypothalamus

The hypothalamus is a small part of the brain located just below the thalamus on either side of the third ventricle. (The ventricles are areas within the cortex that are filled with cerebrospinal fluid, and connected to the fluid in the cord.) It is located within the two tracts of the optic nerve, and just above (and intimately connected with) the pituitary gland.

The hypothalamus is one of the busiest parts of the brain, and is mainly related to homeostasis. Homeostasis is the process of returning something to some "set point". It works like a thermostat: when your room is too cold, the thermostat transports that information to the heater and turns it on. The moment your room heats up and the temperature reaches beyond a certain point, it sends a signal that tells the heater to turn off.

The hypothalamus is responsible for the regulation of your hunger, thirst, response to pain, levels of pleasure, sexual satisfaction, anger and aggressive behavior, and more. It also regulates the functioning of the sympathetic and parasympathetic nervous systems, which means that it regulates things like pulse, blood pressure, respiration, and physiological arousal in response to emotional circumstances.

The hypothalamus receives inputs from various sources. From the vagus nerve, acquire information about blood pressure and distension of the gut (that is, how full is your stomach). From the reticular formation in the brainstem, you get information about the temperature of the skin. From the optic nerve, receives information about light and darkness. From unusual neurons that line the centers, it receives information about the contents of the cerebrospinal fluid, including the toxins that lead to vomiting. And from other parts of the limbic system and olfactory nerves (of smell), information is received that helps regulate food and sexuality. The hypothalamus also has some of its own receptors, which provide information about the ionic balance and the temperature of the blood.

According to one of the most recent discoveries, there seems to be a protein called leptin that is released by fat cells when we eat too much. The hypothalamus apparently perceives leptin levels in the bloodstream and respond with a decrease in appetite. It might seem that some people have a genetic mutation in a gene that produces leptin, and their bodies cannot tell the hypothalamus that they have eaten enough. Anyway, many overweight people do not have this mutation, so there is still a lot of research to do!

The hypothalamus sends instructions to the rest of the body in two ways. The first one is towards the autonomic nervous system. This allows the hypothalamus to have ultimate control over things like blood pressure, heart rate, respiration, digestion, sweat, and all the sympathetic and parasympathetic functions.

The other way in which the hypothalamus controls things is through the pituitary gland. It is neurologically and chemically connected to the pituitary, which alternately pumps hormones called release factors into the bloodstream. As you know, the pituitary is called "master gland," and those hormones are of vital importance in the regulation of growth and metabolism.

The Hippocampus

The hippocampus consists of two "horns" that describe a curve from the hypothalamus area to the amygdala. It seems to be very important in converting things that are "in your mind" now (in short-term memory) into things that you will remember for a long time (long-term memory). If the hippocampus is damaged, a person cannot build new memories, and lives in a strange place where everything he experiences simply fades away, even as the oldest memories before damage remain intact! This unfortunate situation is pretty well described in the wonderful movie Memento.

Amygdala

The amygdala is a mass in the shape of two almonds that are located on both sides of the thalamus at the lower end of the hippocampus. When it is stimulated electrically, the animals respond with aggression. In addition, if the amygdala is removed, the animals become very docile and do not respond to things that would have caused rabies before. Nevertheless, there are more things in it than anger: When it is extirpated, animals also become indifferent to stimuli that might otherwise have caused them fear and even sexual responses.

Areas related to the Limbic system

Under the hypothalamus, the hippocampus, and the amygdala, there are other areas in structures close to the limbic system that are intimately connected to it:

The cingulate gyrus is the part of the cortex located near the limbic system. It provides a pathway from the thalamus to the hippocampus, and seems to be responsible for the association of memories to odors and pain.

The septal area, which is located in the thalamus, has some neurons that appear to be centers of orgasm (one for boys, four for girls).

The ventral tegmental area of the brainstem (just below the thalamus) consists of dopamine pathways that appear to be responsible for pleasure. People with damage in this place tend to have difficulty getting pleasure in life, and often fall into alcohol, drugs, sweets, and gambling.

The prefrontal cortex, which is the part of the frontal lobe that is in front of the motor area, is also closely linked to the limbic system. In addition to apparently being involved in thinking about the future, making plans, and performing actions, it also seems to be involved in the same dopamine pathways as the ventral tegmental area, and plays a role in pleasure and addiction.

The autonomic nervous system

The second part of the nervous system that has a particularly powerful role to play in our emotional life is the autonomic nervous system. The autonomic nervous system is composed of two parts, which function mainly in opposition to one another. The first of these is the sympathetic nervous system, which starts in the spinal cord and travels to a wide variety of areas of the body. Its function seems to be to prepare the body for the kind of vigorous activities associated with flight or struggle, that is, with flight from danger or with preparation for violence.

The activation of the sympathetic nervous system has the following effects:
  • Dilate the pupils
  • Open the eyelids
  • Stimulates the sweat glands
  • Dilates blood vessels in large muscles
  • Constricts blood vessels in the rest of the body
  • Increases the cardiac rate
  • Opens the bronchial tubes of the lungs
  • Inhibits secretions in the digestive system
One of its most important effects is to cause the adrenal glands to release epinephrine (aka adrenaline) into the bloodstream. Epinephrine is a powerful hormone that causes several parts of the body to respond in the same way as the sympathetic nervous system. Once in the bloodstream, it takes a while to stop its effects. This is why, when you are angry, sometimes you take a little while before you calm down again!

The sympathetic nervous system also carries information, most of which concerns the pain of the internal organs. Because the nerves that carry information about the pain of organs often travel through the same pathways that carry pain information from more superficial areas of the body, the information is sometimes confused. This is called referred pain, and the best-known example is the pain some people feel in their shoulders and arms when they are having a heart attack.

The other part of the autonomic nervous system is called the parasympathetic nervous system. It has its roots in the brainstem and in the spinal cord of the lower back. Its function is to bring back the body from the emergency to that brought by the sympathetic nervous system.

Some details of the activation of parasympathetic activation include...
  • Pupil constriction
  • Activation of the salivary glands
  • Stimulation of stomach secretions
  • Stimulation of the activity of the intestines
  • Stimulation of secretions in the lungs
  • Constriction of the bronchial tubes
  • Decrease in heart rate
The parasympathetic nervous system also has some sensory capabilities: it receives information about blood pressure, carbon dioxide levels, and so on.

There is really another part of the autonomic nervous system that we do not mention very often: The enteric nervous system. This is a complex of nerves that regulate the activity of the stomach. When you get sick of the stomach or you feel butterflies when you are nervous, you can blame the enteric nervous system.