The Forebrain

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The Forebrain surrounds the rostral end of the neural tube. Its two major components are the telencephlon and the diencephlon.

Telencephlon

The telencephlon includes most of the two symmetrical cerebral hemispheres that make up the cerebrum. The cerebral hemispheres Rae covered by the cerebral cortex and contain the limbic system and the basal ganglia. The limbic system and the basal ganglia are primarily in the subcortical regions of the brain _those located deep within it, beneath the cerebral cortex.

Cerebral Cortex

Cortex means "Bark" and the cerebral cortex surrounds the cerebral hemispheres like the bark of the tree. In humans the cerebral is greatly convoluted. These convolutions consisting of sulci (small grooves), fissures (large grooves) and gyri (bulges between adjacent sulci or fissures) greatly enlarge the surface area of the cortex, compared with a smooth brain of the same size. In fact two-thirds of the surface of the cortex is hidden in the grooves. The total surface area is approximately 2360cm (2.5 ft) and the thickness is approximately 3mm.The cerebral cortex consists mostly of glia and the cell bodies, dendrites and interconnecting axons of neurons. The cerebral cortex has a grayish brown appearance and it is called Gray Matter. Millions of axons run beneath the cerebral cortex and connects its neurons with those located elsewhere in the brain. The large concentration of myelin around these axons gives this tissue an opaque white appearance_____hence the term White Matter.

Different regions of the cerebral cortex perform different functions. Three regions receive information from the sensory organs. The primary visual cortex which receives visual information is located at the back of the brain on the inner surfaces of the cerebral hemispheres____primarily on the upper and lower banks of the Calcarine Fissure.(Calcarine means "spur-shaped"). The primary auditory cortex which receives auditory information is located on the upper surface of a deep fissure in the side of the brain ___ the Lateral Fissure. The primary somatosensory cortex a vertical strip of cortex just caudal to the central sulcus receives information from the body senses. In addition the base of the somatosensory cortex receives information concerning taste.

The region of the cerebral cortex most directly involved in the control of movement is the Primary Motor Cortex locate just in front of the primary somatosensory cortex. Neurons in different parts of the primary motor cortex are connected to muscles in different parts of the body. I like to think of the strip of primary motor cortex as the key board of a piano, with each key controlling a different movement. The regions of primary sensory and motor cortex occupy any a small part of the cerebral cortex. The cerebral cortex is divided into four areas or Lobes named for the bones of the skull that cover them, The Frontal Lobe, Parietal Lobe, Temporal Lobe, and Occipital Lobe. The frontal lobe includes everything in front of the central fissure. The parietal lobe is located on the side of the cerebral hemisphere, just behind the central sulcus, caudal to the frontal lobe. The temporal lobe just forward from the base of the brain, ventral to the frontal and parietal lobes. The occipital lobe lies at the very back of the brain, caudal to the parietal and temporal lobes.

Each primary sensory area of the cerebral cortex sends information to adjacent regions called the Sensory Association Cortex. Circuits of neurons in the sensory association cortex analyze the information received from the primary sensory cortex, perception takes place there and memories are stored there. The regions of the sensory association cortex located closest to the primary sensory areas receive information from only one sensory system. For example the region closest to the primary visual cortex analyzes visual information and stores visual memories. Just as regions of the sensory association cortex are involved in perceiving and remembering, the motor association cortex located just rostral to the primary motor cortex is involved in the planning and execution of movements. The rest of the frontal lobe rostral to the motor association cortex is known as the Prefrontal Cortex.

Although the two cerebral hemispheres cooperate with each other they do not perform identical functions. Some functions are Lateralized __located primarily in one side of the brain.

Corpus Callosum a large band of axons connects the two Cerebral Hemispheres.

The cerebral cortex that covers most of the surface of the cerebral hemispheres (including the frontal, parietal, occipital and temporal lobes) is called the Neocortex. Another form of cerebral cortex the Limbic Cortex is located around the medial edge of the cerebral hemispheres (limbus means "border"). The Cingulate Gyrius an important region of the limbic cortex.

The fornix is a bundle of axons that connects the hippocampus with other regions of the brain including the Mamillary Bodies, protrusions on the base of the brain that contain pats of the hypothalamus. Only part of the limbic system __the amygdale __is specifically involved in Emotions.

Basal Ganglia

The basal ganglia are collections of nuclei located deep within the forebrain, beneath the anterior portion of the lateral ventricles. Nuclei are groups of neurons of similar shape. The basal ganglia are involved in the control of movement. For example Parkinson's disease is caused by degeneration of certain neurons located in the midbrain that send axons to the caudate nucleus and the putamen.This disease consists of weakness, tremors, rigidity of the limbs, poor balance and difficulty in initiating movements.

Diencephlon

The second major division of the forebrain the diencephalon is situated between the telencephalon and the mesencephalon; it surrounds the third ventricle. Its two most important structures are the Thalamus and the Hypothalamus.

Thalamus

The thalamus (from the Greek thalamos "inner chamber") comprises the dorsal part of the diencephalon. It is situated near the middle of the cerebral hemispheres, immediately medial and caudal to the basal ganglia. The thalamus has two lobes connected by a bridge of gray matter called the Massa Intermedia which pierces the middle of the third ventricle. Most neural input to the cerebral cortex is received from the thalamus.

Projection axons are sets of axons that arise from cell bodies located in one region of the brain and synapse on neurons located within another region. The lateral geniculate nucleus receives information from the eye and sends axons to the primary visual cortex and the medial geniculate nucleus receives information from the inner ear and sends axons to the primary auditory cortex. The ventrolateral nucleus receives information from the cerebellum and projects it to the primary motor cortex.

Hypothalamus

The hypothalamus lies at the base of the brain under the thalamus. It is situated on both sides of the lower part of the third ventricle. Although the hypothalamus is a relatively small structure, it is an important one. It controls the automatic nervous system and the endocrine system and organizes behaviors related to survival of the species, such as fighting, fleeing and mating. The hypothalamus is a complex structure containing many nuclei and fiber tracts.

Note that the pituitary gland is attached to the base of the hypothalamus via the pituitary stalk. Just in front of the pituitary stalk is the Optic Chiasm where half of the axons in the optic nerves (from the eyes) cross from one side of the brain to the other. Much of the endocrine system is controlled by hormones produced by cells in the hypothalamus. A special system of blood vessels directly connects the hypothalamus with an important endocrine gland the Anterior Pituitary Gland. Most of the hormones secreted by the anterior pituitary gland control other endocrine glands.

Because of this function the anterior pituitary gland has been called the body's "Master Gland". For example the gonad tropic hormones stimulate the gonads (ovaries and testes) to release male or female sex hormones. These hormones affect cells throughout the body including some in the brain. Two other anterior pituitary hormones -- prolactin and somatotropic harmone (growth harmone) -- do not control other glands but act as the final messenger. The Posterior Pituitary Gland is in many ways an extension of the hypothalamus. The hypothalamus produces the posterior pituitary hormones and directly controls their secretion. These hormones include Oxytocin which stimulates ejection of milk and uterine contractions at the time of childbirth, and Vasopressin which regulates uterine output by the kidneys. There are produced by two different sets of neurons in the hypothalamus whose axons travel down the pituitary stalk and terminate in the posterior pituitary gland.