Complete information on the structure of human Hindbrain, Midbrain and Forebrain?

Unlike the spinal cord, the human brain is a very complex structure. As such, it is very difficult to know the different structures of the brain and the functions they carry out.

However, physiological psychologists have evolved some methods through which it is now possible to study some of the important structures of the brain and the functions they carry out.

Brain controls almost all the activities that we do, except of course the reflex activities, which are controlled by the spinal cord. Brain controls perception, learning, thinking, and different types of motor activities. Specific regions of the brain have been identified by scientists, which control specific behavior patterns. All the regions of the brain act in close cooperation with one another.

Now, let us examine these different regions of the brain as they control different functions. We shall limit our discussions to some of the important structures of the brain only.

We shall begin with the back of the head, where the spinal cord rises to meet the brain. Gradually we shall come to discuss the front and upper portion of the brain through the middle. The brain can be divided into three main parts: the hindbrain, the mid brain and the fore brain.

The Hindbrain

The backside of the brain is called the hindbrain. It has three sub divisions: medulla, cerebellum and pons.

Medulla:

It is also called as medulla oblongata. This structure is the most caudal (lowest) portion of the brain stem. It is a link between the brain stem and the spinal cord. 12 layers of nerves extending from the medulla connect some vital regions of the body; such as hearts, lungs, intestine, bladder, etc. Therefore, medulla controls such vital and autonomic functions such as respiration, circulation of blood, digestion of food, etc. It also plays a role in sleep, sneezing and coughing.

Cerebellum:

Cerebellum is also called as the ‘little brain’. It is a miniature version of the cerebrum. Its outer surface looks gray and the interior white. It has two hemispheres. It receives input from almost all the sensory areas of the body. Its vital function is to control body balance and posture, as it is connected with the static or the vestibular sense organ. Injury to the cerebellum may lead to lack of motor coordination, stumbling, and loss of muscle tone.

Cerebellum also controls biological rhythm or the perception of time. In other words, the biological clock is located in the cerebellum. Therefore, many of us can tell the approximate time without looking at the wall clock in the middle of night if we wake up form our sleep.

Some surprising new evidences have identified that the direct neural connections between the cerebellum and frontal parts of the brain are involved in language, planning, arid reasoning (Middleton and Strick, 1994). These connecting circuits are much larger in human beings than in other animals. These and other evidences suggest that the cerebellum may play a role in the control and coordination of higher mental functions as well as in the coordination of movements.

Pons:

The pons is a large bulge in the brain stem, lies between medulla and the midbrain, immediately ventral to the cerebellum. Pons is a Latin word, which means “bridge, but it does not look like one. It is so named because of the bundle of nerves that passes through it. The pons transmits information about body movement and is involved in functions related to attention, sleep, and alertness. It has important functions to carry out in dreams. The various cranial nerve nuclei in the pons play a significant role in feeling and facial expressions.

The Midbrain

The midbrain is located approximately at the center of the brain and above the pons. It is a tube-like structure, the outside of which looks white and inside gray. Through this tube, a fluid-like substance called cerebrospinal fluid passes which gives nutrition to the brain. Except this, it has sensory and motor pathways. It contains primitive centers for vision and hearing. It plays a role in the regulation of visual reflexes. The midbrain consists of two important structures. They are the tectum and the tegmentum.

The tectum (roof) is located in the dorsal portion of the mid brain. Its principal structures are superior colliculi and inferior colliculi. The superior colliculi are a part of the visual system and help in locating visual objects. The inferior colliculi are a part of the auditory system and process the auditory signals before sending them to the actual auditory center (temporal lobe) of the brain. The tegmentum (covering) consists of the portion of the midbrain beneath the tectum. Its function is the movement of the eye from one direction to the other. The midbrain contains an extension of the reticular activating system.

Reticular Activating System (RAS):

The RAS begins in the hindbrain and ascends through the region of the midbrain into the lower part of the fore brain. It is a network of neurons crossing each other. The RAS is vital in the functions of shifting of attention, sleep, arousal, and activity. Stimulation of the RAS causes it to send messages to the cortex, making us more alert to sensory information. Electrical stimulation of the RAS awakens sleeping animals. But if the RAS is damaged, the animal may not die, but will sleep forever. This kind of sleep is called as comatose or simply coma. Another important function of RAS is selective attention or filtering of information through learning. Thus, it allows important information to go for processing by higher centers of the brain while screening out less important information. Thus, while you are inside the class, you listen only to the lecture, even though a lot of noise is taking place outside the class.

The Forebrain

The forebrain is the most important part of the brain for it possesses virtually all the parts concerned with perception, coordination of behavior patterns including those of emotion, motivation, learning, memory, language and thinking. The major parts of the forebrain are thalamus, hypothalamus, the limbic system and the cerebrum.

Thalamus:

The thalamus sits at the top of the brain stem in the central core of the brain. It consists of an egg-shaped cluster of neurons. It is a structure of three different types of nuclei, which process different types of sensory information and send them to the appropriate regions of the brain. For example, the lateral geniculate nucleus receives sensory information from the eye and sends them to the primary visual cortex; the medical geniculate nucleus receives information from the ear and sends them to the primary auditory cortex; and the ventral-basal nucleus receives information form the somato-sensory sense and sends them to the parietal lobe.

Thus, the thalamus receives input from all of our senses except olfaction (smell), performs some preliminary analysis and then transmits the information to other parts of the brain. Therefore, it is called as the great relay station of the brain. Excepting this relaying function, the thalamus is also involved in controlling sleep and attention in coordination with other brain structures, including the RAS.

Hypothalamus:

The hypothalamus is a tiny collection of nuclei located beneath the thalamus. Even though, it is very small in structure (weighs about four grams), it exerts profound effect on our behavior. It regulates the autonomic nervous system. In other words, it regulates such activities like sweating, salivating, shedding of tear, changes in blood pressure, etc. It plays a key role in homeostasis that is the maintenance of the body’s internal environment at optimum levels. In other words, it controls body temperature, the concentration of fluids, the storage of nutrients and various aspects of motivation and emotion. When an organism is under stress, homeostasis is disturbed and processes are set into motion to correct the disequilibrium. For example, when we are too warm, we perspire; and if we are too cool, we shiver. The purpose in both the situations is to store normal temperature.

Portions of hypothalamus seem to play a role in the regulation of eating and drinking. Stimulation of lateral nucleus of hypothalamus causes psychological hunger, and the person takes more and more food. If damaged, the organism refuses to take food.

Similarly, if it is stimulated, the organism will go on taking more and more water, and if damaged, it refuses to take water. Ventromedial nucleus of the hypothalamus is the satiation center. If stimulated, the animal stops taking food and water. If damaged, this inhibition is impaired. However, it has been observed that if it is stimulated, the animal may take sweet food only.

The hypothalamus also controls the activities of endocrine system. Thus, it plays a role in reproductive physiology and sexual behaviour. The hypothalamus also plays a role in the sensation of emotions and our response to stress producing situations. Therefore, sometimes it is called as the brain’s stress center.

Limbic System:

The limbic System or the borderline system lies along the inner edge of the cerebrum. It consists of a group of structures that form a part of the old mammalian brain. The brain stem and the cerebellum are found in all vertebrates, but only mammals and reptiles have limbic system. It is a ring like structure having several other structures. Limbic system receives input from the olfactory sense, and as such responsible for olfactory perception. It is structurally interconnected with the hypothalamus. Therefore, it is also involved in the drives of hunger, sex, aggression, and some of the behaviors regulated by the hypothalamus. The three principal structures in the limbic system are amygdala, hippocampus, and septum. The amygdala is involved in memory, and certain basic emotions. The damage to the amygdala causes an animal to be less fearful and over curious, hypersexual, and more exploratory.

The hippocampus has been observed to play a special role in memory. The effects of surgical removal of the hippocampus have demonstrated that people can retrieve old memories but cannot permanently store new information.

In other words, damage to the hippocampus affects short-term memory. Destruction of another area of the limbic system, the septum, leads some mammals to respond aggressively, even with slight provocation.

Basal Ganglia:

The basal ganglia are a group of nuclei buried beneath the cortex in front of the thalamus. They are involved in the control of postural movements and the co-ordination of limbs. Basal ganglia produce most of the brain’s dopamine, which is a neurotransmitter. Degeneration of neurons in the basal ganglia has been linked to Parkinson’s disease in which the patient finds it difficult to coordinate different limbs. Excess production of dopamine leads to a mental disorder called schizophrenia.

The Cerebrum:

The cerebrum or the cerebral cortex is the crowning glory of the brain and is highly developed in humans than in any other animal. It is protected by the skull. Below the skull, there is a space filled with a fluid, which is gray in color and protects the cerebrum. The outside surface of the cerebrum is called the cerebral cortex. The cortex looks gray because it consists largely of cell bodies and unmyelinated fibers. The inside of the cerebrum beneath the cortex looks white as it is composed mostly of myeliated axons. The cortex has got many ridges called gyri throughout. In between the two gyri, there is a small depression called sulcus (valley) or fissure.

A very deep depression or fissure divides the cerebrum into almost two equal halves- the left hemisphere and the right hemisphere. The left hemisphere is connected with the right hand side of the body, and receives sensory information from right hand side of the body. The right hemisphere is connected with the left hand side of the body, and as such receives sensory information from the left hand side of the body. The two hemispheres are connected with each other by a thick fiber bundle known as corpus callosum.

Each of the hemispheres of the cerebral cortex is divided into four parts or lobes by two fissures. On the top middle of each hemisphere, the fissure of Rolando or central fissure runs from the top vertically towards the bottom. Another fissure, the fissure of sylvius or lateral fissure runs laterally in each hemisphere. The frontal lobe lies in front of the central fissure, and the parietal lobe lies behind it. The temporal lobe lies below the lateral fissure. The occipital lobe however is not clearly demarcated. For our purposes, we can say that it is located at the rear of the brain.

The frontal lobe is specialized for the planning, execution, and control of movements. The primary motor cortex, adjacent to the central fissure, contains neurons that participate in the control of movements. Electrical excitation to different parts of the primary motor cortex causes movements of different parts of the body. Because the cerebral hemispheres are connected with the opposite sides of the body, stimulation of the right primary motor cortex moves parts of the left side of the body. Similarly, stimulation of the left primary motor cortex moves the right side. In the motor cortex, the blood is represented upside down. For example, the face is represented at the upper part of motor area of the brain, trunk further down face and tongue.

The posterior (back) lobes of the brain, such as the s parietal, temporal and occipital lobes are specialized for perception and experience of touch, pressure and pain etc. The primary somatosensory cortex lies immediately close to the central sulcus, i.e., in the parietal lobe. This cortex re receives information from the somatic senses. Therefore, this area is specialized for touch, pressure and pain. The occipital lobe contains the primary visual cortex. It lies in the backside of the cortex. It receives visual inputs from the eyes. The temporal lobe contains the primary auditory cortex, which receives information form our ears.

Association Areas:

The areas of the cerebral cortex that are not directly concerned with sensory or motor functions are called as association areas. Each lobe is having an association area. The association areas are assumed to play a role in integrating the activities in the various sensory systems and transmitting sensory input into programs for motor output. In addition, the association areas are involved in complex cognitive activities such as thinking, reasoning, learning etc.

The junction of the three posterior lobes is known as the PTO (parietal, temporal, and occipital) area. In this area, the somatosensory, auditory and usual functions overlap. Persons who sustain injury to PTO area may have difficulty in reading and writing.

In many ways, the left and right hemispheres of the brain are structurally and functionally equal. However, they are not equal. For 96 percent of those people who are right-handed, the left hemisphere contains the language functions. For 70 percent of those people who are left-handed, the left hemisphere also contains language functions. For 15 percent of left-handed individuals, the right hemisphere contains language functions. For the remaining 15 percent of left-handed persons, both the hemispheres contain language functions. The dominant hemisphere with regard to language functions as described above contains Broca’s area and Wernicke’s area. Damage to either area causes aphasia- that is, a disruption of the ability to understand or to produce language. Broca’s area located in the frontal lobe and Wernicke’s area is located in the temporal lobe.

Corpus Callosum. The two hemispheres of the cerebrum are connected with each other by a band of fibers known as corpus callosum. Roger Sperry, the Noble Prize winner in 1981, damaged corpus callosum in a series of experiments and observed that the right hemisphere is specialized in the functions of memory for patterns of stimulations or images, and the left hemisphere specialized in analytical, logical and mathematical functions. However, both hemispheres act in close cooperation with each other.