Chapter 5. The Corpus Striatum, Rhinencephalon, Connecting Fibers, and Diencephalon
The corpus striatum, or "striped body" consists of the basal ganglia (basal nucleus) and the internal capsule. The basal ganglia is made up of neurons, so it is gray matter. The internal capsule is a group of tracts surrounded by myelinated axons, so it is white. Because the internal capsule runs between the caudate and lenticular nucleus of the basal ganglia, the group of structures looks striped. The area is called the corpus striatum (striped body). The blood supply is from the striata artery. This thinly lined artery is calledy the "artery of stroke" because it ruptures easily if blood pressure is too high.
The basal ganglia, which is the largest subcortical structure in the brain, is located near the thalamus. There are important extra pyramidal tract connections between the basal ganglia and the ventral lateral motor nucleus of the thalamus. The basal ganglia consists of the caudate nucleus and the lenticular nucleus, the latter consisting of the putamen and the globus pallidus.
The caudate nucleus is bounded on one side by the lateral ventricle and is divided into a head, body and tail. It contains endorphins, chemicals that, among other things, produce a positive emotional state.
The lenticular nucleus is also know as the lentiform nucleus. Lentiform means lens-shaped in Latin. It is located between the caudate nucleus and the island of Reil with its anterior aspect being attached to the head of the caudate nucleus. The putamen is the most lateral part of the structure. When the globus pallidus, the more medial part of the lenticular nucleus, is probed, sensations of thirst are produced.
Some anatomists consider the claustrum to be part of the basal ganglia.
The amygdala, which is involved in emotion, was once classified as part of the basal ganglia, but is no longer categorized in this way. It is part of the limbic system. It is attached to the tail of the caudate nucleus.
The subthalamic nuclei and the substantia nigra are both functionally related to the basal ganglia.
The internal capsule lies between the lenticular and caudate nuclei. It is a group of myelinated fiber tracts including axons of pyramidal and extrapyramidal upper motor neurons that connect the cortex to the cell bodies of lower motor neurons. Because so many axons are clustered within the internal capsule it is sometimes referred to as a bottleneck of fibers. This makes it a very bad place to get a lesion.The internal capsule ends within the cerebrum, just above the midbrain, but the axons that pass through it continue down through brain stem and spinal cord. They descend through the brainstem within two large bundles called the cerebral peduncles or cruz cerebri.
The striata, a branch of the middle cerebral artery, takes blood to the internal capsule. As explained above the striata is called the "artery of stroke" because it is very thin and prone to hemorrhage. It supplies blood to many important nerve fibers as well as the nuclei of the basal ganglia.
Despite its close proximity to the caudate nucleus and lenticular nucleus, the internal capsule is not part of the basal ganglia. As stated previously it forms part of the corpus striatum along with the caudate and lenticular nuclei.
The term "rhinencephalon, which literally means "smell-brain," is used by many anatomists to refer to the limbic system. Others make a distinction between these two terms considering the olfactory tract and olfactory bulbs to be part of the rhinencephalon but not of the limbic system. Broca felt that the limbic lobe on the medial surface of the hemispheres was the fifth lobe of the brain. The cortex of the limbic system is barely convoluted. It is phylogenetically older than the neo-cortex which is heavily convoluted (gyri and sulci). The limbic lobe consists of the hippocampal gyrus, cingulate gyrus, subcollasal gyrus, and septal region..
The limbic system is sometimes called the limbic lobe. This term is only descriptive; it does not imply that this group of structures is comparable to the lobes of the cerebral hemispheres which are neo-cortex. The limbic structures are much older.
The limbic system consists of both cortical and subcortical structures which are located on the medial, inferior surfaces of the cerebral hemispheres. Its components are phylogenetically related, being some of the most ancient parts of the brain.
The cortical areas classified as part of the limbic system include the hippocampus, the cingulate gyrus, and the subcallosal gyrus.
The hippocampus, is a gyrus found on the medial edge of the temporal lobe. It is named for its shape, as hippocampus means "sea horse." The hippocampus is very close to the basal ganglia and to the lateral ventricles. It is very important to memory especially in making new memories. It is an area where neurons die constantly and are replaced by new neurons. It is likely that this also occurs in other areas of the brain.
The cingulate gyrus is immediately superior to the corpus callosum. Although it is part of the limbic system it is cortical, albeit less convoluted than neocortex. Its role is a higher one than non-cortical limbic areas. A study by Dr. Mahzarin Banaji of Harvard University and and Dr. Liz Phelps of New York University using functional MRI demonstrated that when people see the face of a person of a different race the amygdala reacts with alarm and fear. The amygdala is much more primative than the cingulate gyrus. The functional imaging studies have shown the cingulate gyrus and part of the prefrontal cortex (high level cognition) quickly decide that there is no danger and give the all clear. According to Kluger, 2008 racism begins in the lower limbic system. You can take an on-line test to check your reaction to races and traits.
The subcallosal gyrus is immediately inferior to the corpus callosum.
The subcortical components of the rhinencephalon include the olfactory pathways, the amygdaloid bodies, the mamillary bodies, the dorsal-medial and ventral-anterior nuclear groups of the thalamus, parts of the reticular formation, and the septal region.
The olfactory pathway originates in the nasal area. As it passes posteriorly to enter the temporal lobe at the hippocampal gyrus the olfactory tract is immediately superior to the optic tract.
The mamillary bodies are also known as the mamillary nuclei. They are connected to the hippocampus, the thalamus and the fornix.
The septal region includes both the septum pellucidum, which is a double walled structure located between the corpus callosum and the fornix, and the septal nuclei.
The fornix is a group of fibers that arises from each hippocampus, and project to the contralateral hippocampus. It links the rhinencephalon to both the thalamus and the hypothalamus. It is connected to the septal nuclei, the mamillary bodies, and anterior nucleus of the thalamus.
The limbic system is involved in recent memory, emotion, motivation and reinforcement. According to Love and Webb (1992) responses mediated by the limbic system include pleasure, satiety, guilt, punishment, inhibition, wakefulness, alertness, excitement, and autonomic activity.
Based on the behavioral correlates of lesions to the limbic system, it may also be involved in cortical speech and language behavior. The characteristics of this involvement are not known at this time.
Lesions to the limbic system can also cause a variety of behaviors, including aggression, extreme fearfulness, altered sexual behavior, and changes in motivation. Damage specifically to the hippocampus can affect memory and emotion.
The cortex inhibits the activity of the limbic system. When this is compromised by alcohol or drugs limbic behavior occurs.
Lesions on the olfactory pathways can cause anosmia, which is a loss of the sense of smell.
An uncinate fit is an epileptic seizure that is preceded by an olfactory hallucination.
Connecting Fibers
There are three major types of axons, or nerve fibers, in the brain.
Efferent fibers take messages from the brain to the peripheral nervous system. They are almost always motor fibers. They consist of the pyramidal tract (volitional movement) and the extrapyramidal tract (automatic movement e.g dancing). The extrapyramidal tract includes the basal ganglia, the substantia nigra, the cerebellum,and the motor nuclei of the thalamus) McCaffrey, 2009)
Afferent fibers take messages from the periphery back to the brain. They are almost always sensory fibers.
Interconnecting fibers connect structures within the brain. There are two types of interconnecting fibers: commissural fibers, and association fibers.
Commissural fibers connect the hemispheres of the brain. The corpus callosum, the anterior commissure, and the posterior commissure are all composed of commissural fibers.
The corpus callosum, which is Latin for "large body" is the major group of commissural fibers. It is located some distance down inside the longitudinal cerebral fissure, the split that separates the hemispheres. It contains at least 200 million axons. Most of these fibers connect mirror image sites on the two hemispheres; axons might connect Brodmann's area 3 in the parietal lobe of the right hemisphere to area 3 in the parietal lobe of the left hemisphere. Not all of the connections follow this pattern, however. For example, area 17 in the occipital lobe is connected to areas 18 and 19 of the other hemisphere rather than to area 17.
One treatment for severe epileptic seizures is commissurectomy, an operation that severs the corpus callosum. Both Gazzaniga and Sperry conducted experiments on split-brain patients and noted differences between the functions of the left and right cerebral hemispheres. The right hemisphere appears to be involved in the intuitive, holistic processing of information and in spatial reasoning. The left hemisphere, on the other hand, seems to be more adapted for logic and analytical reasoning. Most importantly for speech pathologists, it was found that the left hemisphere plays a dominant role in the speech and language abilities of most people.
Of course, both hemispheres are apparently involved to some extent in any cognitive process.
The other two groups of commissural fibers are called the anterior commissure and the posterior commissure. Both are connected to the corpus callosum.
Many of the commissural fibers that connect the two temporal lobes pass through the anterior commissure. The anterior commissure also connects the temporal lobe to the amygdala and to the occipital lobe in the other hemisphere. The anterior commissure is used by neurosurgeons to locate the circle of Willis.
Almost all parts of the cortex receive commissural fibers. The "hand area" of the primary sensory strip is one of the few areas that does not.
Association fibers connect areas within the same hemisphere. The cell bodies of association fibers are the most prevalent type of neuron found in the brain.
Long association fibers connect areas that are located in different lobes of the brain. For example, the arcuate fascicules, which connects Broca's area in the frontal lobe with Wernicke's area in the temporal lobe, is composed of long association fibers. The term arcuate fascicules means "arching bundle" in Latin. Lesions to this particular bundle of long association fibers will cause conduction aphasia.
Short association fibers connect areas that are located in the same lobe. For example, the fibers which connect Heschl's gyrus with the auditory association areas are short association fibers.
Location and Description
The diencephalon consists of the thalamus,epithalamus (includes pineal gland), subthalamus and hypothalamus. Some sources classify the diencephalon as part of the brain stem. This is not the view of the majority and, for the purposes of this class, the diencephalon should be considered part of the cerebrum.
Both the thalamus and hypothalamus are located in the center of the brain at the level of the temporal lobe. They are very well protected in this area.
The thalamus is located below the caudate nucleus and the fornix and is medial to the lenticular nucleus. It is composed of two bodies which are separated from one another by the third ventricle, with one lying in each hemisphere. The two thalamic bodies are connected to one another by another part of the thalamus, the massa intermedia or thalamic adhesion, which makes up part of the ventricle.
The epithalamus includes the pineal gland and is involved autonomic functions.
The subthalamus is located ventral to the thalamus and is important for motor movement. It has connections to the basal ganglia, thalamus and brainstem.
The hypothalamus is a solid structure that is located immediately inferior to the thalamus. Part of it is also anterior to the thalamus. It forms the floor and part of the lateral walls of the third ventricle.
The thalamus has been described as the switchboard for the cortex. It receives information from the cerebellum, the basal ganglia and from all sensory pathways with the exception of the olfactory tract; it integrates the messages and sends them on to the cortex for further processing.
The thalamic bodies are composed of several different nuclei which are divided from one another by lamina or thin walls of tissue. The thalamic nuclei can be divided into four groups based on their functions; the specific relay nuclei, the association nuclei, the non-specific nuclei, and a subcortical nucleus.
Five areas of the thalamus are classified as the ventral nucleus complex. These include the lateral geniculate body, the medial geniculate body, the ventral posterolateral nucleus, the ventral posteromedial nucleus, and the ventral lateral/ventral anterior nuclei. All of the above receive sensory information and are considered specific relay nuclei. The ventral lateral and ventral anterior are motor relay nuclei
The lateral geniculate body is part of the visual information pathway. It receives information from the superior colliculus of the midbrain and then relays it to the visual areas of the cortex in the occipital lobe.
The medial geniculate body processes auditory information. It receives messages from the inferior colliculus of the midbrain and transfers them to the auditory areas of the cortex in the temporal lobe.
The ventral posterolateral nucleus or VPL is involved in the processing of somatosensory information. Messages come in from the spinothalamic tract and the medial lemniscus and are passed on to the somato-sensory cortex found in the parietal lobe. This nucleus mediates sensations of pain and temperature as well as proprioception.
The ventral posteromedial nucleus or VPM also handles sensory information. It receives input from the trigeminothalamic tract which it passes on to the somato-sensory cortex of the parietal lobe. Sensory information mediated by the trigeminal nerve is processed in this area. For example, information about toothaches is carried by this tract.
The ventral lateral and ventral anterior or VL/VA are motor relay nuclei of the thalamus. They process motor information. They get input from the cerebellum and the basal ganglia and send output to the motor and premotor cortex in the frontal lobe. As lesions here will affect motor abilities, knowledge of these nuclei is extremely important.
Association nuclei form connections between different areas of the thalamus. They are involved in the integrating and correlating processes that it performs. Three parts of the thalamus are classified as association nuclei. These include the pulvinar, the lateral posterior nucleus, and the dorsal medial nucleus.
The pulvinar receives information from other thalamic nuclei and from the superior colliculus. It sends output to association areas in the parietal, occipital and temporal lobes. The medial and lateral geniculate bodies are located on top of the pulvinar but are not a part of it.
The lateral posterior nucleus, or the LP, integrates input that it receives from other thalamic nuclei.
The dorsal medial nucleus receives input from the amygdaloid bodies, the hypothalamus, and from other thalamic nuclei. It sends information to the pre-frontal cortex. As this nucleus connects parts of rhinencephalon with one of the motor areas of the frontal lobe, it may be involved in the hypothesized connection between the limbic system and communication.
Two parts of the thalamus are categorized as non-specific nuclei. They are the intralaminar nucleus and part of the ventral anterior nucleus, a motor relay.
Information comes to the intralaminar nucleus from the basal ganglia, the reticular formation, and other thalamic nuclei. It sends output to many different cortical areas.
The ventral anterior and ventral lateral motor nuclei receive input from the basal ganglia and other structures as well as from within the thalamus and send it to the pre-motor ( planning/programming) and pre-frontal cortex of the frontal lobe. The ventral lateral nucleus is part of the motor feedback circuit between the cortex and the cerebellum.
One portion of the thalamus, the reticular nucleus, is classified as subcortical. This name does not refer to the location of the structure, as all parts of the thalamus are subcortical according to this definition. Instead, the reticular nucleus is labeled this way because it does not project to the cortex. Its input is from other thalamic nuclei and its output is also to thalamic nuclei.
The hypothalamus is considered a nodal point in the pathways mediating autonomic, emotional, endocrine, and somatic functions. It is involved in the following specific functions:
Release of some hormones from the pituitary gland.
Temperature regulation of the body.
Intake of food and water.
Autonomic nervous system pathways:
The hypothalamus is connected to reticular nuclei in the brain stem that relay axons that control autonomic motor functions. They are particularly involved with coughing and vomiting reflexes as well as the reflexes involved in expelling inspirated substances. The larynx spasms violently in response to food (liquid or solid) getting into the laryngeal glottis.Emotion:
The hypothalamus is connected to the septum and the amygdaloid bodies, which are part of the limbic system, via a group of fibers called the stria terminalis. Reciprocal connections exist between the hypothalamus and the thalamus, pituitary gland, brain stem, and the temporal lobe.The hypothalamus is composed of several groups of nuclei and regions, including the preoptic area, the supraoptic area, the paraventricular nucleus, the dorsal-medial nucleus, the ventral-medial nucleus, the lateral region, and the posterior region.
The preoptic area is located in the anterior portion of the hypothalamus. It is involved in temperature regulation of the body, including the dilation of peripheral blood vessels and sweating.
The supraoptic area is inferior to the preoptic area and just above the optic chiasm. It is connected to the pituitary gland and regulates water intake and output via control of the kidneys.
The paraventricular nucleus is also involved in water regulation. Both the supraoptic area and the paraventricular nucleus produce ADH or antidiuretic hormone.
The dorsal-medial and the ventro-medial nuclei are involved in the control and expression of emotions like rage, fear, and extreme anxiety.
The lateral region contains the apostat, which monitors the level of glucose in the blood and sends messages to the stomach provoking hunger when the blood sugar level drops to a certain level.
The posterior region is involved in controlling the body's temperature in cold environments. This area produces shivering by causing the muscles of the body to vibrate at a rate of between seven and thirteen Hertz. This area also controls sexual behavior.
Lesions in the hypothalamus may cause obesity, loss of the ability to control body temperature, or loss of interest in sex. Damage to the hypothalamus or to connections between the hypothalamus and the pituitary gland may cause diabetes insipidus which is far more serious than sugar diabetes, the more common form of the disease.