The brain can be divided into three major regions: the hindbrain, the midbrain, and the forebrain. You can see where these regions are located in the brain by looking at Figure 1.
Figure 1. Structure and areas in the human brain
The hindbrain includes the cerebellum and two structures found in the lower part of the brainstem: the medulla and the pons. The medulla, which attaches to the spinal cord, has charge of largely unconscious but essential functions, such as breathing, maintaining muscle tone, and regulating circulation. The pons (literally “bridge”) includes a bridge of fibers that connects the brainstem with the cerebellum. The pons also contains several clusters of cell bodies involved with sleep and arousal.
The cerebellum (“little brain”) is a relatively large and deeply folded structure located adjacent to the back surface of the brainstem. The cerebellum is involved in the coordination of movement and is critical to the sense of equilibrium, or physical balance. Although the actual commands for muscular Imovements come from higher brain centers, the cerebellum plays a key role in the execution of these commands.
The midbrain is the segment of the brainstem that lies between the hindbrain and the forebrain. The midbrain is concerned with certain sensory processes, such as locating where things are in space. For instance, when a sound triggers a reflexive turning of the head, an area in the midbrain is at work. An important system of dopamine-releasing neurons that projects into various higher brain centers originates in the
midbrain. Among other things, this dopamine system is involved in the performance of voluntary movements. The decline in dopamine synthesis that causes Parkinsonism is due to degeneration of a structure located in the midbrain. Running through both the hindbrain and the midbrain is the reticular formation. Lying at the central core of the brainstemi, the reticular formation contributes to the modulation of muscle reflexes, breathing, and pain perception. It is best know however, for its role in the regulation of sleep and wakefull Activity in the ascending fibers of the reticular formats
ation contributes to arousal.
The forebrain is the largest and most complex region of the brain, encompassing a variety of structures, including the thalamus, hypothalamus, limbic system, and cerebrum. The thalamus, hypothalamus, and limbic system form the core of the forebrain. All three structures are located near the top of the brainstem. Above them is the cerebrum—the seat of complex thought. The wrinkled surface of the cerebrum is the cerebral cortex—the outer layer of the brain, the part that looks like a cauliflower.
The Thalamus: A Way Station
The thalamus is a structure in the forebrain through which all sensory information (except smell) must pass to get to the cerebral cortex. This way station is made up of a number of clusters of cell bodies, or somas. Each cluster is concerned with relaying sensory information to a particular part of the cortex. The thalamus also appears to play an active role in integrating information from various senses.
The Hypothalamus: A Regulator of Biological Needs
The hypothalamus is a structure found near the base of the forebrain that is involved in the regulation of basic biological needs. The hypothalamus lies beneath the thalamus (hypo means “under,” making the hypothalamus the area under the thalamus). Although no larger than a kidney bean, the hypothalamus contains various clusters of cells that have many key functions. One such function is to control the autonomic nervous system.
The hypothalamus plays a major role in the regulation of basic biological drives related to survival, including fighting, feeling, feeding, and mating. For example, when researchers lesion the lateral areas (the sides) of the hypothalamus, animals lose interest in eating. The animals must be fed intravenously or they starve, even in the presence of abundant food. In contrast, when electrical stimulation is used to activate the lateral hypothalamus, animals eat constantly and gain weight rapidly.
The Limbic System: The Seat of Emotion
The limbic system is a loosely connected network of structures located roughly along the border between the central cortex and deeper subcortical areas. Broadly defined, the limbic system includes parts of the thalamus and hypothalamus, the hippocampus, the amygdala, the septum, and other structures.
The hippocampus clearly plays a role in memory processes, although the exact nature of that role is the subject of debate. Similarly, there is ample evidence linking the limbic system to the experience of emotion, but the exact mechanisms of control are not yet well understood. Recent evidence suggests that the amygdala may play a central role in the learning of fear responses.
The Cerebrum: The Seat of Complex Thought
The cerebrum is the largest and most complex part of the human brain. It includes the brain areas that are responsible for our most complex mental activities, including learning. remembering. thinking, and consciousness itself. The cerebral cortex is the convoluted outer layer of the cerebrum. The cortex is folded and bent, so that its large surface area—about 1.5 square feet can be packed into the limited volume of the skull.
The cerebrum is divided into two halves called hemispheres. Hence, the cerebral hemispheres are the right and left halves of the cerebrum. The hemispheres are separated in the center of the brain by a longitudinal fissure that runs from the front to the back. This fissure descends to a thick band of fibers called the corpus callosum. The corpus callosum is the structure that connects the two cerebral hemispheres.
Each cerebral hemisphere is divided into four parts called lobes. To some extent, each of these lobes is dedicated to specific purposes.
The occipital lobe, at the back of the head, includes the cortical area where most visual signals are sent and visual processing is begun. This area is called the primary visual cortex.
The parietal lobe is forward of the occipital lobe. It includes the area that registers the sense of touch, called the primary somatosensory cortex. Various sections of this area receive signals from different regions of the body.
The temporal lobe lies below the parietal lobe. Near its top. the temporal lobe contains an area devoted to auditory processing, the primary auditory cortex.
Continuing forward, we find the frontal lobe, the largest lobe in the human brain. It contains the principal areas that control the movement of muscles, the primary motor cortex.
The portion of the frontal lobe to the front of the motor cortex, which is called the prefrontal cortex. is something of a mystery. This area is disproportionately large in humans, accounting for about 28% of the human cerebral cortex.