The Brain's Lower-Level Structures
The brain's lower-level structures consist of the brain stem and spinal cord, along with the cerebellum. With the exception of the spinal cord, hese structures are largely located within the hindbrain, diencephalon (or interbrain), and midbrain. The brain evolved from back to front, such that the structures at the base of the brain, including the hindbrain and midbrain, are more geared towards basic body processes than those in the front.
The medulla oblongata sits at the transition zone between the brain and the spinal cord. It is the first region that formally belongs to the brain and is the control center for respiratory, cardiovascular and digestive functions.
The pons connects the medulla oblongata with the midbrain region. The pons houses the control centers for respiration and inhibitory functions. Attached to the dorsal aspect of the pons is the cerebellum.
The cerebellum (Figure 1) is a separate region of the brain located behind the medulla oblongata and pons. It is attached to the rest of the brain by three stalks (called pedunculi), and coordinates skeletal muscles to produce smooth, graceful motions. The cerebellum receives information from our eyes, ears, muscles, and joints about the body's current positioning (referred to as proprioception). It also receives output from the cerebral cortex about where these body parts should be. After processing this information, the cerebellum sends motor impulses from the brain stem to the skeletal muscles so that they can move. The main function of the cerebellum is coordination; however it is also responsible for balance and posture and it assists us when we are learning a new motor skill, such as playing a sport or musical instrument. Recent research shows that apart from motor functions the cerebellum also has some role in emotional sensitivity.
The midbrain is located between the hind- and forebrain. It displays the same basic functional composition as found in the spinal cord and the hindbrain. Ventral areas control motor function and accommodate tracts that convey motor information from the cerebral cortex. Dorsal regions of the midbrain are implicated in sensory information circuits.
The Diencephalon ("interbrain")
The diencephalon is the region of the embryonic vertebrate neural tube which gives rise to posterior forebrain structures. In adults, the diencephalon appears at the upper end of the brain stem, situated between the cerebrum and the brain stem. It is made up of four distinct components: the thalamus, the subthalamus, the hypothalamus and the epithalamus.
The thalamus is part of the limbic system (Figure 3) and consists of two lobes of grey matter along the bottom of the cerebral cortex. Lesions or stimulation to the thalamus are associated with changes in emotional reactivity. However, the importance of this structure on the regulation of emotional behavior is not due to the thalamus itself, but to the connections between the thalamus with other limbic system structures.
The hypothalamus (Figure 2) is a small part of the brain located just below the thalamus. Lesions of the hypothalamus interfere with several so-called motivated behaviors like sexuality, combativeness, and hunger. The hypothalamus also plays a role in emotion. Specifically, parts of the thalamus seem to be involved with pleasure and rage, while the central part is linked to aversion, displeasure, and a tendency towards uncontrollable and loud laughing. When external stimuli are presented (for example, a dangerous stimuli), the hypothalamus sends signals to other limbic areas to trigger feeling states in response to the stimuli (in this case, fear).
The Spinal Cord
The spinal cord is a tail-like structure that is embedded in the vertebral canal of the spine. The adult spinal cord has a length of 40 cm and weighs approximately 30g. The spinal cord is attached to the underside of the medulla oblongata, and is organized to serve four distinct tasks:
- to convey mainly sensory information to the brain,
- to carry centrally information generated in the brain to control peripheral targets like skeletal muscles,
- to control nearby organs via the autonomic nervous system, and
- to enable sensorimotor functions to to control posture and other fundamental movements.