The human nervous system Part 1 Forebrain

Our nervous system is responsible for controlling all the functions of the body. It is a super computer and demonstrates multitasking at its best.

It is divided into central and peripheral parts. The central part consists of the brain situated inside the skull and the spinal cord contained in the vertebral column.

The peripheral part in turn consists of the twelve pairs of cranial nerves and thirty one pairs of spinal nerves. These nerves innervate every part of the body receiving sensory information and also sending the commands from the brain.

There is another part of this system called the autonomic nervous system. This part is responsible for controlling the involuntary functions of the body like activity of the heart, lungs, maintaining blood pressure, secretion of the glands etc.

The brain sets man apart from other animals. While in all other animals it accounts for less than 1 percent of the total body weight, in man it accounts for 2 percent of the body weight (about 1300 grams).

The human brain is estimated to contain approximately one hundred billion nerve cells which are called neurons. It is covered by three layers called meninges which also cover the spinal cord. The outer most layer is called dura mater and is a tough fibrous membrane.

The middle one is called arachnoid mater and the innermost is called pia mater. The pia mater covers the brain. In between the arachnoid and pia mater there is a space called the subarachnoid space. This space contains a clear fluid called the cerebrospinal fluid.

This fluid fills the ventricles of the brain and is continuous with the cerebrospinal fluid surrounding the spinal cord. The brain is divided into three main parts-forebrain, midbrain and hindbrain.

Forebrain (prosencephalon)

This consists of two main parts-the telencephalon which consists of two large hemispherical structures (cerebrum) and the diencephalon.

The cerebrum when looked from above has ovoid shape broader behind than in front. A midline fissure divides the cerebrum into two hemispheres. This division is incomplete because the two hemispheres are connected in the middle by a mass of fibers called the corpus callosum.

Each hemisphere is in turn subdivided by fissures called sulci (singular=sulcus) into areas known as gyri (singular=gyrus). The outermost layer of the cerebral hemisphere which is made up of nerve cells is called the cerebral cortex (nervous tissue which contains bodies of nerve cells is also called gray mater while that which contains fibers of the cells is called white mater).

Each hemisphere has three surfaces-lateral, medial and inferior. The lateral surface is convex fitting into the concavity of the interior of the skull. The medial surface is flat while the inferior surface is irregular. It is divided into different areas called lobes by the sulci.

These lobes are associated with different functions. Four of these lobes can be seen on the surface of the hemisphere. They are frontal, parietal, temporal and occipital.

The frontal and parietal lobes are separated by the central sulcus. The lateral sulcus is situated between the frontal and parietal lobes above and the temporal lobe below. The parieto-occipital sulcus separates the parietal lobe from the occipital lobe.

Another lobe called the insula (meaning island) can be seen when the lips of the lateral sulcus are separated.

Each hemisphere contains a cavity called the lateral ventricle. The two lateral ventricles communicate with the third ventricle. The third ventricle is situated in between the two thalami (see below).

The lobes are associated with different functions. The frontal lobe is concerned with motor activities including speech and cognitive functions. The motor area is situated just in front of the central sulcus. It is connected with the sensory area of the parietal lobe for coordinated responses.

A part of the frontal lobe called Broca’s area controls the muscles responsible for making sounds. Damage to this area causes a condition called motor aphasia in which the person can understand speech but cannot produce meaningful sounds. Other areas of the frontal lobe are concerned with learning and memory.

The parietal lobe receives sensory input from the body. An area of the parietal lobe called Wernicke’s area is concerned with understanding of language. Damage to this area results in a condition called sensory aphasia in which the person will not be able to understand language but can produce sounds.

The occipital lobe is known as the visual cortex. It receives and interprets the visual impulses carried by the optic nerves from the eyes. The images are formed upside down on the retina and the visual cortex interprets them properly. It also shares the information with other areas for coordinated action.

The temporal lobe receives and interprets auditory impulses from the ears. It shares this information with the Wernicke’s area of the parietal lobe and the motor area of the frontal lobe.

The insular lobe is concerned with the regulation of autonomic nervous system.

The diencephalon consists of the structures around the third ventricle. It includes the thalamus (pleural=thalami), metathalamus, epithalamus and hypothalamus.

The thalami are large ovoid masses of grey mater situated one on either side of the midline and form the lateral boundary of the third ventricle. Each thalamus is about four centimeter long.

The thalamus is like a relay station. It sends incoming impulses from the peripheral organs to the appropriate areas of the brain. it sends fibers to all the five lobes of the cerebrum. It also receives fibers from the cerebral cortex.

The metathalamus consists of two masses of neurons called the medial geniculate body and the lateral geniculate body. The medial geniculate body receives and then relays auditory impulses to the auditory cortex. The lateral geniculate body receives optic fibers coming from the eyes and then relays the impulses to the visual cortex.

The epithalamus consists of the pineal body, habenula and the posterior commissure. The posterior commissure is a band of nerve fibers which crosses the midline. It is necessary for papillary light reflex.

The pineal body is a small gland situated in the midline. It secretes a hormone called melatonin. Its function is not fully understood.

The habenula is a triangular area containing a group of neurons called habenular ganglion. It is supposed to be involved in sleep wake cycle, stress response and reproductive behavior.

The hypothalamus is situated in the midline beneath the thalamus. It is involved in the regulation of many activities like eating, drinking, temperature control, sleep, emotional behavior and sexual urge.

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