Brainstem
The most primitive area of the brain is the brainstem. It connects directly to the spinal cord and is central to the survival of all animals by controlling such functions as heart rate, breathing, digestion, and sleeping.
Ascending pathways in the brainstem, referred to as mesolimbic and mesocortical pathways, seem to play a strong role in modulating the emotional value of sensory material. These pathways project to areas of the cerebrum collectively known as the limbic system. The limbic system plays a crucial role in emotional status and psychological function using norepinephrine, serotonin, and dopamine as their neurotransmitters.
The role of these pathways in normal and abnormal mental activity is significant. For example, experts believe that the release of dopamine from the mesolimbic pathway plays a role in
psychological reward and drug addiction. The neurotransmitters released by these neurons are major targets of the drugs used to treat psychiatric disorders.
TABLE 3.1
Transmitters and Receptors
AMPA, α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid; NMDA, N-methyl-D-aspartate.
B O X 3 . 2 D e s t r u c t i o n o f N e u r o t r a n s m i t t e r s
A full explanation of the various ways in which psychotropic drugs alter neuronal activity requires a brief review of the manner in which neurotransmitters are destroyed after attaching to the receptors. To avoid continuous and prolonged action on the postsynaptic cell, the
neurotransmitter is released shortly after attaching to the postsynaptic receptor. Once released, the neurotransmitter is destroyed in one of two ways.
One way is the immediate inactivation of the neurotransmitter at the postsynaptic membrane.
An example of this method of destruction is the action of the enzyme cholinesterase on the neurotransmitter acetylcholine. Cholinesterase is present at the postsynaptic membrane and destroys acetylcholine shortly after it attaches to nicotinic or muscarinic receptors on the postsynaptic cell.
A second method of neurotransmitter inactivation is a little more complex. After interacting with the postsynaptic receptor, the neurotransmitter is released and taken back into the
presynaptic cell, the cell from which it was released. This process, referred to as the reuptake of neurotransmitter, is a common target for drug action. Once inside the presynaptic cell, an enzyme within the cell recycles or inactivates the neurotransmitter. The monoamine
neurotransmitters norepinephrine, dopamine, and serotonin are all inactivated in this manner by the enzyme monoamine oxidase.
Looking at this second method, you might naturally ask what prevents the enzyme from destroying the neurotransmitter before its release. The answer is that before release the
neurotransmitter is stored within a membrane and is protected. After release and reuptake, the neurotransmitter is either destroyed by the enzyme or reenters the membrane to be used again.
Hypothalamus
In a small area above the brainstem lies the hypothalamus, which plays a vital role in:
• Controlling basic drives such as hunger, thirst, and sex
• Linking higher brain activities, such as thought, emotion, and the functioning of the internal organs
• Processing sensory information that is then sent to the cerebral cortex
• Regulating the entire cycle of sleep and wakefulness and the ability of the cerebrum to carry out conscious mental activity
Cerebellum
Located behind the brainstem where the spinal cord meets the brain, the cerebellum (Fig. 3.4) receives information from the sensory systems, the spinal cord, and other parts of the brain and then regulates voluntary motor movements. It plays a crucial role in coordinating contractions so that movement is accomplished in a smooth and directed manner. It is also involved in balance and
Cerebrum
The human brainstem and cerebellum are similar in both structure and function to these same structures in other mammals. The development of a much larger and more elaborate cerebrum is what distinguishes human beings from the rest of the animal kingdom.
The cerebrum, situated on top of and surrounding the brainstem, is responsible for mental activities and a conscious sense of being. This is responsible for our conscious perception of the external world and our own body, emotional status, memory, and control of skeletal muscles that allow willful direction of movement. The cerebrum is also responsible for language and the ability to communicate.
FIG. 3.4 The functions of the brainstem and cerebellum.
The surface of the cerebrum is called the cerebral cortex. There are four major lobes of the cortex, each responsible for specific functions. For example, conscious sensation and the initiation of movement reside in the frontal lobe, the sensation of touch resides in the parietal lobe, sounds are based in the temporal lobe, and vision is housed in the occipital lobe. Likewise, a specific area of the frontal cortex controls the initiation of skeletal muscle contraction. Of course, all areas of the cortex are interconnected so that you can form an appropriate picture of the world and, if necessary, link it to a proper response (Fig. 3.5).
Both sensory and motor aspects of language reside in specialized areas of the cerebral cortex.
Sensory language functions include the ability to read, understand spoken language, and know the names of objects perceived by the senses. Motor functions involve the physical ability to use muscles properly for speech and writing. In both neurological and psychological dysfunction, the
factor in determining a diagnosis.
Underneath the cerebral cortex are pockets of gray matter deep within the cerebrum. Some of these, the basal ganglia, are involved in the regulation of movement. Others, the amygdala and hippocampus in the limbic system, are involved in emotions, learning, memory, and basic drives.
Anxiety disorders can be associated with abnormalities in the amygdala, which cause intense fear such as in panic disorder. Significantly, there is an overlap of these areas both anatomically and in the types of neurotransmitters involved. One important consequence is that drugs used to treat emotional disturbances may cause movement disorders, and drugs used to treat movement disorders may cause emotional changes.