The Brain

The Five Major Divisions of the Brain

During early development, in the embryo stage of life, the brain begins as a fluid-filled tube, developing into the central nervous system. Three swellings occur at the anterior end of the tube and develop into the adult forebrain, midbrain, and hindbrain (J. Pinel, 2009, p. 64). The three swellings in the tube become the five major divisions of the brain before birth, consisting of the telencephalon, the diencephalon, the mesencephalon, the metemcephalon, and the myelencephalon. The forebrain develops the telencephalon and the diencephalon; the midbrain develops into the mesencephalon; the hindbrain develops into the merencephalon and the myelencephalon.

The Myelencephalon

The myelencephalon is commonly referred to as the medulla. The medulla “is a complex network of about 100 tiny nuclei that occupies the central core of the brain stem from the posterior boundary of the myelencephalon to the anterior boundary of the midbrain" (Pinel, 2009, p. 65). The medulla carries signals throughout the brain and body. It has a reticular formation and has a netlike appearance. The reticular activating system of the myelencephalon (medulla) plays a critical role in arousal. The functions of the myelencephalon vary greatly but do not play a role in language production or comprehension. Other various nuclei in the myelencephalon (medulla) are involved in a variety of other functions – attention, sleep, maintenance of muscle tone, and various cardiac, circulatory, and respiratory reflexes.

The Metencephalon

According to Pinel (2009, p. 66), “The metencephalon, like the myelencephalon, houses many ascending and descending tracts and part of the reticular formation." The pons are a structure in the metencephalon that create a bulge on the ventral surface of the brain stem connecting to the cerebellum. The pons and cerebellum create a division of the brain stem; the cerebellum is a complex structure containing millions of neural fibers and nerves. The cerebellum is an important structure in sensory and motor coordination that controls the nerves of the body. The cerebellum is not restricted to just motor coordination and sensory function. The metencephalon plays a role in controlling movements, adapting to changing conditions and language production.

The Mesencephalon

The tectum and tegmentum are two divisions of the mesencephalon, similar to the metencephalon. The tectum is the dorsal surface of the midbrain, “composed of two pairs of bumps, the colliculi (little hills)” (Pinel, 2009, p. 66). These divisions control the auditory and visual functions of the body. The tectum division, the colliculi, includes the posterior pair - inferior colliculi and superior colliculi. The inferior colliculi control the auditory functions and the superior colliculi control the visual functions. Because the lower vertebrates are entirely visual components, the tectum is often referred to as the optic tectum.

The tegmentum is ventral to the tectum, containing three structures - the periaqueductal gray, the substantia nigra, and the red nucleus. According to Pinel (2009, p. 67), “The periaqueductal gray is the gray matter situated around the cerebral aqueduct, the duct connecting the third and fourth ventricles.” The gray matter around the cerebral aqueduct controls the effects of reducing pain of opiate drugs. The black substance and red nucleus are important in sensory and motor functions.

The Diencephalon

The thalamus and the hypothalamus are two structures housed in the diencephalon. The thalamus sits atop the brain stem and contains a two-lobe structure. "The lobe sits on each side of the third ventricle, and the two lobes are joined by the massa intermedia, which runs through the ventricle" (Pinel, 2009, p. 67). The thalamus is visible on the surface, containing white layers composed of axons and nuclei which project to the cortex. Some are sensory nuclei and some are motor nuclei. The thalamus interconnects with the cortex, cerebellum, and other structures within the brain. The thalamus contains nuclei which transmit, receive, and process signals to the sensory cortex. The function of the thalamus includes attention and effects in perceptions.

The hypothalamus, located below the anterior thalamus, contains the pituitary gland, optic chiasm, and the mammillary bodies. The hypothalamus is important in regulating behaviors that are motivated. The pituitary gland, on the ventral surface of the brain, regulates the release of hormones in the body. The optic chiasm, located on the inferior of the hypothalamus, controls the optic nerves in the eyes. Also located on the inferior of the hypothalamus, behind the pituitary gland, are the mammillary bodies, a pair of spherical nuclei.

The Telencephalon

The telencephalon is the largest and complex division of the human brain; because it mediates the brain’s most complex functions, the telencephalon is divided into four parts which contain several other structures. The telencephalon is responsible for voluntary movement, interpreting sensory movement, and mediating “"complex cognitive processes such as learning, speaking, and problem solving" (Pinel, 2009, p. 68). The telencephalon is above the diencephalon (the midbrain) and contains the left and right cerebral hemispheres, which are separated by the longitudinal fissure. Each hemisphere is divided into four lobes by the lateral fissure and the central fissure: the parietal lobe, the frontal lobe, the temporal lobe, and the occipital lobe. “The cerebral hemispheres are covered by a layer of tissue called the cerebral cortex (cerebral bark), which is deeply convoluted (furrowed)” (Pinel, 2009, p. 68). The deep furrows in the cortex are called the fissures; the ridges are called the gyri. The frontal lobe is located at the front of the brain, the temporal lobe is under the temple, the parietal lobe is at the top and the occipital lobe is at the back of the head. Both the parietal and occipital lobes have overlying bones.

Each lobe, including the gyri, is functioning except the cerebral lobes, which are non-functioning lobes. According to Pinel (2009, p. 69), “The post central gyrus analyzes sensations from the body (touch).” The occipital lobe is responsible for visual input and the parietal lobes perceive objects and help direct attention. The cortex of each temporal lobe assists in hearing, language, memory, and identifies complex visual patterns. The frontal lobe is responsible for motor function and complex cognitive functions such as planning response, evaluating behavior patterns, and assessing potential behavior of others.

Most of the cerebral cortex is a six-layered cortex called the neocortex (new cortex). Four important characteristics of neocortical anatomy: (1) two fundamentally different kinds of cortical neurons, the pyramidal cells and stellate cells; (2) the six layers of neocortex differ from one another in terms of size and density; (3) many long axons and dendrites course vertically through the neocortex; (4) According to Pinel (2009, p. 70), “there are variations in the thickness of the respective layers from area to area (Brown & Bowman, 2002; Passingham, Stephen, & Kotter, 2002).” Layers that conduct sensory signals from the thalamus and the neocortex to the brain stem and spinal cord are extremely thick. The hippocampus is also part of the cortex but is not neocortex because it only has three major layers, not six. Located at the medial edge of the cerebral cortex is the hippocampus, which plays an important role in declarative memory, and learning a new language with articulation.

References

Pinel, J. P.J. (2009). Biopsychology (7th ed.). Pearson Education, Inc.

Rice University (2000). Langbrain – Language and Brain: Neurocognitive Linguistics. Retrieved October 21, 2011 from http://www.ruf.rice.edu