Human Nervous System | forebrain, midbrain, hindbrain |

HUMAN NERVOUS SYSTEM.

The human has a centralized nervous system.

Central Nervous System:

The central nervous system of man consists of the brain and spinal cord. Both the brain and spinal cord are hollow. The spinal cord has a central canal and the brain has many cavities (ventricles) filled by CSF. Both brain and spinal cord are protected in three ways:

➤ Cranium: It is a part of the skull. It protects the brain and neural arches.

➤ The vertebrae of the vertebral column protect the spinal cord.

➤ Beneath the cranium, the brain and spinal cord are by a triple layer of meninges. The cerebrospinal fluid (CSF) is present between the layers of meninges. Its composition is similar to "blood plasma. It

bathes the neurons of the brain and spinal cord and protects the brain against the bumps and jolts.

BRAIN

The brain can be divided into the forebrain, midbrain, and hindbrain.

1.  Forebrain:

It is further divided into three functional parts, the thalamus, the limbic system, and the cerebrum.

(a). Thalamus:

It carries sensory information to the limbic system and cerebrum. The information includes sensory information from auditory (ear) and visual (eye) pathways. It also receives information from the skin and from within the body.

(b). The limbic system:

It is located in an arc between the thalamus and cerebrum. The limbic system works together to produce our most basic and primitive emotions, drives, and behaviors, including fear, rage, tranquility, hunger, thirst, pleasure, and sexual responses. The portion of the limbic system is also important in the formation of memories. The limbic system consists of the hypothalamus, the amygdala, hippocampus, and nearby regions of the cerebrum.

➤ Hypothalamus:

It through its hormone production and neural connections act as a major co-coordinating center. It controls body temperature, hunger, the menstrual cycle, water balance, the sleep-wake cycle, etc.

➤ Amygdala:

In the amygdala, a cluster of neurons produces the sensation of pleasure, punishment, or sexual arousal when stimulated. It is also involved in the feelings of fear and rage.

➤ Hippocampus:

It plays an important role in the formation of long term memory and thus is required for learning.

(c). Cerebrum:

The cerebrum is the largest part of the brain. It is divided into two

halves called cerebral hemispheres, These halves communicate with each other by means of a large band of axons, called the corpus callosum. Tens of billions of neurons are packed into this part. The outer region of the cerebrum is called the cerebral cortex. It forms folds called convolutions. These convolutions greatly increase its surface area. C.rubrum performs the following functions:

(i).  It receives sensory information. It stores some of this information in memory for future use.

(ii).  It directs voluntary movements.

(iii). It is responsible for thinking. Thinking is a poorly understood process.

(iv). The cerebral cortex contains primary sensory areas. These areas receive signals from sensory organs like eyes and ears. These signals are converted. into impressions such as light and sound. Nearby associated areas interpret this information.

(v)  This area is also involved in speech and also receives and interprets sensations of touch from all parts of the body.

(vi). This area is also a center for sending impulses to voluntary muscles. Thus it controls movements.

(vii). This is also involved in intelligence, reasoning, and judgment.

(viii). The left cerebral hemisphere controls the right side of the body, and the right cerebral hemisphere controls the left side of the body.

2. Midbrain:

Midbrain is reduced in the human brain. It contains an auditory relay center. It is also a center that controls reflex movements of eyes. Midbrain contains reticular formation. It is a relay center connecting hindbrain with the forebrain. The reticular formation is very important in screening the input information before they reach higher brain centers.

3. Hindbrain:

It includes the medulla, pons, and cerebellum.

(a). Medulla: It controls several automatic functions, such as breathing, heart rate, blood pressure, and swallowing.

(b). Pons: It is located above the medulla. Certain neurons in pons influence the transitions between sleep and wakefulness. It also influences the rate and pattern of breathing.

(c). Cerebellum: The cerebellum is important in coordinating movements of the body. The cerebellum guides for smooth and accurate motions. So it maintains accurate body position. The cerebellum is also involved in the learning and memory storage for behaviors. It is best developed in birds, which engage in the complex activity of flight.

SPINAL CORD

Medulla oblongata narrows down into an oval-shaped hollow cylinder called the spinal cord. It runs through the vertebral column. It is made up of a very large number of neurons. The cell-fibers and cell bodies of these neurons are arranged in a definite pattern. In cross-section, the spinal cord shows two areas:

➤ Grey matter: It is an inner butterfly-shaped grey matter. It contains a central canal. Gray matter consists of cell bodies and non-myelinated nerve fibers or tracts.

➤ White matter: The outer portion of the spinal cord is composed of white matter. White matter is made up of myelinated nerve fibers or tracts.

➤ Functions of the spinal cord:

(i) The spinal cord is the center for many reflexes

(ii) It acts as a pathway for conduction of impulses to and from different parts of the body and brain.